P Over D

Lift to drag ratio or L/D is the figure that many crucial aerodynamic criteria are measured by. When soon-to-be pilots study for the FAA written exam, they pore over diagrams of L/D curves and where the most efficient speeds are. They see how glide ratio, climb speeds and best endurance speeds are determined. They see how L/D varies with angle of attack and altitude. They see all this information presented before them and yet are only getting half of what they need to know operationally. The number we’ll be exploring today is the P/D, or payload to drag ratio.

First, what does L/D mean aerodynamically? Quite simple; it means that you have the lowest amount of drag for your given weight. Hence, lift (holding up your weight) over drag. If your L/D went from 10 to 5, it would mean that you are producing more drag for your given weight (whether it is parasite or induced drag depends on the type of speed change). Speaking in terms of the airplane itself, L/D is the determining factor for efficiency. Sailplanes have extremely high lift to drag ratios and are often touted as being the most aerodynamically efficient winged craft on the planet. Sailplane-type aerodynamic efficiency means that making 180 degree turns with no engine at 300 feet above the ground and staying aloft in thermals for hours is completely normal. On the other hand (and there always is another hand), the downside to measuring an airplane’s worth by its L/D ican limit its usefulness. While a sailplane can play in ridge lift for the better part of a day, it’s doing it at the expense of two very important variables: payload and speed.

P/D is the transportation equivalent of the aerodynamic L/D. While everyone talks lovingly of L/D, they almost never ask the question I always ask: “What are you lifting?” It’s similar to me telling a big rig driver that I’m more efficient because I get 100mpg on my moped and he only gets 6mpg. Sure I’ll feel like a eco-hero, but the missing part of the equation is what is my moped can carry and how fast. I weigh around 200lbs, or 0.1 tons, so we can assume that as the moped’s payload. The 18 wheeler on the other hand has a payload of roughly 48,000 lbs or 24 tons. My moped (wide open throttle and downhill with a tailwind) can hit 40mph while the truck can easily cruise over 70mph. What would you rather have deliver 24 tons of groceries to your store from a warehouse site 200 miles away? One truck that takes just under 3 hours, or 240 mopeds that will take 7.5 hours?

The same principle applies to aircraft. Most people buy airplanes for speed. They want to save time over driving or taking the airlines. They want to be on their own schedule. They sometimes just want to be able to say “I cruise at Mach 9.5” to their friends who all cruise at Mach 9.2. Very few people buy powered airplanes for a distinct lack of speed. There is also another camp that values payload over blistering speed. The ability to lift a lot of people or a lot of cargo is critical for airlines and charters in order to make a profit. It counts for general aviation as well since quite frankly, many of the standard “4 place” airplanes are really “2 people plus a couple bags” craft with respect to payload and full tanks. Many planes have been wrecked and too many people killed because pilots failed to fully understand the significance of this nebulous limitation.

Time to compare L/D to P/D. The Schwiezer SGS 2-32 glider has a reported L/D somewhere between 32/1 and 36/1. For it’s gross weight of 1340 lbs, it will be producing between 38 and 42 lbs of drag. Incredibly low numbers thanks in part to it’s low drag fuselage and high aspect ratio wing. This max L/D is achieved at a speed of 45 knots, which is very slow for transportation considerations and totally unusable for any powered airplane’s cruising flight. Assuming 2 large men weighing 250lbs each are on board, the payload is 500 lbs. P/D is between 13.9 and 15.6, which are the real numbers we want. For each pound of drag, the aircraft is carrying an average of 14 lbs of payload.

P/D ratios that high will not be commonplace in powered aircraft simply because they are much heavier in structure and can’t carry as large payload with respect to their maximum weight. A very heavy-lifter, the Boeing 747-400F has an average L/D of 17.5 at 35,000 feet and normal cruise speed, while maximum payload is listed as 248,000 lbs. Assuming an late-in-cruise weight of 700,000 lbs, drag would be 40,000 lbs and P/D therefore is 6.2. Closer to home for many of us, the Cessna 172 has an L/D in the region of 9.0. At 2400 lbs gross weight, the resultant amount of drag is 267 lbs. Assuming 3 people are on board who weigh 200 lbs each, the P/D works out to 2.25. Most light aircraft will end up reasonably close to this number, varying by the number of seats on board.

Why is P/D important to designers? Because while purist pilots may love airplanes, airlines quite frankly couldn’t care less about the aircraft itself as long as it does 3 things:

  1. Doesn’t crash randomly.
  2. Is reliable.
  3. Turns a profit.

If you wish to build military fighter/attack aircraft, the desires are slightly different:

  1. Doesn’t crash randomly.
  2. Is reliable.
  3. Carries a lot of weapons.

And if you want to build a business jet…well, you get the idea. The only category of aircraft where payload can be ignored somewhat is the personal pleasure craft, such as certain experimental and ultralight aircraft. They aren’t made to make money, they’re made to enrich the builder/owner’s life. On the other hand, if an airliner has a beautiful shape that inspires poets worldwide, sets speed records and adds alleged prestige to the company, that won’t save it from being cast aside in favor of something more pedestrian but economical (and yes, I was referring to that supersonic work of art, the Concorde).

The more an aircraft weighs empty equates to more structure and materials which directly leads to more cost. The more an aircraft weighs empty, the less it can carry in fuel and payload if it’s maximum takeoff weight is not also adjusted upwards. So not only will it cost more to buy, but it produces less per trip in terms of revenue. From an economic standpoint, an airplane that has lower than average aerodynamic virtues but exceptional carrying capabilities will be more desirable than a craft with a high L/D but limited payload. This explains the ongoing interest with composites to reduce airframe weight without sacrificing strength.

P/D is just another number that can help you analyze performance in a practical manner. For instance, if you’re looking to buy an airplane for personal use, consider who and what you want to take with you. A super efficient powered glider that has only one seat is pointless if your goal is to carry your family on short trips. Likewise, if you find an airplane that has a very high cruise speed but it requires you to lay flat like a Mercury astronaut, that won’t make the kids very happy, especially since there are no windows to see out of. Like everything else in aviation, its a balance with your needs as the fulcrum.

Next time, we’ll talk about all the “best” speeds and why they aren’t speeds.

Grounded: Why VLJs Will Not Work Until There Is A Shift In Design Philosophy

I’m about to make a lot of people angry. C’est ce que je fais.

I found an old article written by Austin Meyer the other day. For those of you who don’t know who that is, he’s the man responsible for me spending hundreds of hours in front of the computer, testing aircraft designs. He’s the creator, developer and programmer of the super-realistic simulation/design program, X-Plane. I’m not just saying it’s realistic because I use it or because I’m a fanboy. I say it because it’s the only simulation that allows the user to alter aircraft, airfoils, engines, and even the environment with such minute detail. There are a few quirks you have to cheat your way around, but for the most part, it’s very accurate. When I can pull up the TCDS sheet for a particular engine, find the N1 rotational speed, maximum interturbine temperature and flat rate setting, enter that data into X-Plane and have an engine that behaves almost exactly the same as the real one, that to me counts as realism.

Anyway, the article was written about Very Light Jets, or VLJs for short (VLJs have the same connotation as winglets and continuous descent approaches. That means in another 5-10 years, someone is going to make a technological breakthrough and VLJs will not only make sense but end up saving money, prompting people to flock to them and conveniently forget how vociferously they opposed them during the current gestation period). The article includes part of an email that Austin wrote when cancelling work on his own VLJ project known as the X-1 Cavallo. As Austin was progressing in the design work, he realized that there was no way to get performance that could surpass what a Lancair could do for a fraction of the cost. Wisely, he decided to abandon the project rather than wish for a radical and unlikely change in the laws of physics. He brought up several good points about why VLJs will never be as efficient as larger jets. In his examples, he compares a theoretical VLJ with the Airbus A380. His argument is not only correct but it illustrates the broken mindset aircraft designers have with approaching new technology at times:

This light jet plane is 1/10 as long, 1/10 as tall and 1/10 as wide. That should give a plane that is one one-thousandth the volume…one one-thousandth the weight…(and) one one-thousandth the thrust (to push one one-thousandth the weight). So, we have a plane that is like an airbus a-380, but with one one-thousandth the weight and thrust and fuel-burn, right? WRONG!!!!!!!!!!!!!!!!!!!!!!!! The frontal area and wetted area of our airplane is ONE ONE-HUNDREDTH THAT OF THE AIRBUS, NOT ONE ONE-THOUSANDTH!!!!!!!! Scale the Airbus down by 10x and you have one one-hundredth the frontal and wetted area, not one-one-thousandth! So, your new scaled-down plane has one-one-thousandth the thrust, but one-one-hundredth the (parasite) DRAG!!!!! So we have TEN TIMES THE DRAG PER UNIT THRUST!!!!!…So, if we managed to do everything as well as an Airbus A-380 scaled down, we would still only have one THIRD the speed and range!!!!!!!!!!!!!!!!!!!”

Finally someone brought up the four engine elephant in the room. Designers keep copying large jet designs for small jets applications and then scratching their heads when range is half of what it should have been, or when cruise speed is 100mph slower than the computer predicted. It may be acceptable to scale down a P-51 Mustang to a ¾ size homebuilt project. It is another thing entirely to try to scale down an A380 to VLJ dimensions. Which begs the question, why on earth would anyone try to scale down a 500 seat airliner to a 5 seat personal jet?

Financial And Emotional Reasons

Aviation is expensive. If you think it costs a lot to park your car at Whereyougoin International Airport for a week, think about what it costs to park an airplane on the other side of the fence. Aside from parking, there’s the very real cost of fuel, parts, training, annual inspections, and unscheduled maintenance. All of these conspire to make owning or operating an airplane a cash-heavy endeavor. And for those brave enough to design new aircraft, not only are there very high costs associated with the need for precision production lines and specially skilled workers, but tens of millions of dollars have to be spent to have a new design tested and certified by the FAA before even selling your first copy. With annual sales amounting to a fraction of what Ford or Honda would see in a week, the only way to break-even is to take the most low-risk/high-return approach as possible. And there’s no better way to reduce risk than to copy what has been done before. After all, if it worked great for all these other large aircraft, it has to work well for your small one too, right?

Unfortunately, scaling down good designs is not like reducing a photocopy. Sometimes it does not work well at all. As a kid watching planes at the local airports, I’d wonder why all the business jets looked like embryonic DC-9s. I’m sure the manufacturers’ marketing departments loved the up-sell comparison that those T-tailed jets naturally evoked from onlookers. As I got older and understood more about aerodynamics and psychology, the scale-down idea started to make sense. Take an image or shape that is instantly identifiable and positively associated with a group that seems desirable (in this case, the jet-setters) and apply it to your own design. Instantly people will associate your design with all the advantages, benefits and status boosts that go along with the larger aircraft. More than half the job of selling it is already done since potential customers will look at the Micro McDonnell MD-0.000080 and think, “Wow this is like a personal airliner! It even looks and sounds like the one I took from Chicago to Miami! I’ll take six!” (The associative up-sell also explains why nobody has ever tried to scale up a Piper Warrior to the dimensions of a 767).

The Reality Of People Who Aren’t Awed By Jets

But in reality, private airplanes are small, even those powered by jet engines. And to people who don’t fly for a living or for passion, they seem even smaller than they really are. A perfect example is the Embraer 145 regional jet. The Emb-145 is a commercial jet aircraft just under 100 feet long, just a few feet shorter than the original Douglas DC-9-10 (Back in the Douglas days, the Emb-145 would have been a regular airliner, today it’s a regional jetliner…everyone reinvents the wheel). As a Sunday afternoon GA pilot, I view the Emb-145 as a big airplane compared to what I’m used to.

Meanwhile whenever I ride in one, the same thing happens: A girl with too many carry-on bags steps aboard, glances around the cabin and immediately her face shows severe discontent and nervousness. She then turns to her 6’13 boyfriend (who somehow got a protein shake past the TSA checkpoint) and says “Why are we on this *expletive deleted* little airplane? There isn’t even a first class section! Oh God this is too tiny! I’m claustrophobic! I should never have let you plan this trip!” Meanwhile the massive boyfriend takes a sip of his Instamuscle shake and tries his best to place white noise where he hears her voice.

I didn’t make that up. I’ve seen it many times before and anyone who frequently flies on regional aircraft has seen some variant of it happen. The aircraft are called “Puddle-jumpers”, “Prop-jobs” (even with turbofans stuck on the back of the plane), and there’s always some derogatory comment about little airplanes and how their uncle read an article to them 20 years ago on how 19 seat airplanes are dangerous (even though this airplane was built in 2005 and has 50 seats). For better or worse, the public’s idea of riding in a jet aircraft is size “737 and larger”. If people complain about a 100 foot long airliner, they’re going to complain about a 40 foot long personal jet. It’s going to happen and it should be expected. Don’t think that because the engines happen to be jets and make a really cool shrieking sound, that your friends and family are going to want to crouch down and sit in the cabin for a 2 hour flight. Pilots and aviation enthusiasts will like it but regular people will not be enchanted like us. They’re going to see one thing: “Tiny airplane”. The only way to help alleviate that reaction is by designing the most spacious cabin possible. If people aren’t overtly uncomfortable, they’ll tend to not dwell on the cabin size. If they’re sardined in unable to move their leg, they’ll tell everyone how it was the most awful experience of their entire life.

Clean Sheet Required

If we are to follow conventional wisdom, reducing the cross-section of the fuselage is one of the fastest ways to reduce drag, which is already higher than it should be (or more accurately, what you wanted it to be). One has to either decrease surface area, or keep surface area the same but increase volume to maximize lift to drag (A high L/D ratio is only useful if I’m measuring the lift needed to support what I want to carry, not structural weight). Now we’re stuck. A relatively big cabin is needed so your passengers won’t cramp up and/or freak out, but you also need to reduce the drag and most likely the surface area as well. The only way to reduce either of these two variables is to take the drastic step of picking an entirely new and unconventional design for your aircraft. In other words, a straight wing and a T-tail won’t cut it.

This advice flies in the face of everything that makes economic sense in aviation. Building a scaled copy of something that is time-tested means you know it will fly and all the major parts will work (at least on the larger scale). Building something new means a lot more unknowns. Some people will say this is a safety risk, convinced that a new concept is going to have more problems than an existing one, but that’s just a catchall response to anything that looks different. Sure it’s possible that if you design a real turd of an airplane and don’t do any research, wind tunnel testing, CFD analysis, or component testing, that it may flip over just trying to taxi to the active runway. But any competent designer should be able to come up with an airplane of unconventional configuration that can safely complete a thorough test program. Building an unusual airplane doesn’t mean the wings are going to rip off the second you hit rotation speed. In fact if you build an airplane to do your intended mission instead of piggybacking off another designer’s intent, you’ll end up with a greater margin of safety (and just to keep egos in check, copying a classic design that has millions of hours in service doesn’t mean that your mini-version going to be just as safe or reliable).

Am I deriding the current status of the aviation manufacturing industry? Of course not. Just remember, before you start sketching something you think looks cool, write down what you want it to do, how far away you want to do it and how many people you want to take with you. Dassault and Cessna don’t pick a design because it’s cool or it looks futuristic. They build an airplane to do a particular job. Boeing and Airbus all build a product that fills a niche and gradually evolve it to fill more niches. Only when the product cannot be evolved anymore do they come up with a clean-sheet design. The fact is that airlines don’t care how cool or futuristic their airplanes look (as long as people don’t look out the terminal window and see a DC-6 dripping oil all over the ramp). The biggest concern for the airlines is that the airplane generates revenue. When independent designers come up with an airplane concept, many times they’re approaching it with a love or passion for aviation. This is wonderful and as someone who’s been in love with aviation for 30 years, I wish you all many more years of airborne joy. But the dirty truth is unless you’re building just one airplane for your own personal use, you need to be able to sell it. And just because you like the shape or it reminds you of something from a sci-fi book you read at age 8, or it’s a perfect replica of a Fokker F-28 in ¼ scale does not mean that anyone else cares. You’ll either never get the funding to build a prototype, or you’ll build the prototype, test fly it for a few months and realize that customers who placed deposits on your dream are withdrawing their orders based on the reality of how it performs.

What You Need Vs. What They Build

To be completely blunt with my own personal opinion (which borders on fact most of the time), to look at a large passenger jet and use that as the blueprint for your brand new VLJ or small jet design, is the biggest waste of your time, money and materials. Commercial jets are built to be mass-produced (compared to general aviation and business aircraft). Boeing and Airbus do not have time sit around a pottery spinning wheel and lovingly sculpt nosecones. They have orders that need to be filled immediately. Their aircraft are designed to be used thousands of hours per year and ridden by hundreds of thousands of passengers. The cabins, landing gear and engines will go through more cycles in a month than you may do all year. Any maintenance problems need to be solved either during a 40 minute turn or on an overnight. A 99% dispatch reliability is standard. They’ll be abused by weather, luggage loaders and even passengers (how those overhead bin doors stay intact I’ll never know).

Private aircraft do not see this kind of use. To build the exact same systems and features into your design in many cases is overcomplicating what could have been a perfectly safe and affordable aircraft. There is a difference between building robustness into your systems and cluttering up the airframe. Even when building in redundancy for flight critical items, there is no reason to copy certain aspects of airliner design. You won’t need a split rudder or inboard and outboard ailerons. You don’t need 7 generators, an APU and 3 different ways to lower the landing gear in a 5 seat airplane. Building a safe aircraft does not mean adding random multiple backups of any system that might fail in the next 10,000 hrs of operation just because that’s how airliners are built. It means picking the items that are critical to your aircraft remaining controllable, to you staying alive and ensuring that those have a graceful method of degradation.

That being said, everyone has to remember that any jet with current engine technology is undoubtedly going to be used up high with the airliners. Keep up with them or get out the way. Having a 350 mph airplane lollygagging in the path of 530 mph airplanes is going to create problems. Being able to keep people alive in the thin air above 30,000 feet is also important. Bad things happen fast when your only engine has a bleed valve malfunction and your cabin loses air pressure. Your aircraft is also traveling at a much higher percentage of the speed of sound (Mach number). You need a Machmeter because you can’t just let your indicated airspeed build up to the yellow arc while descending and think because the air is smooth, things are okay. Navigation equipment is going to be a substantial portion of the total aircraft cost. A simple handheld GPS and navigation radio won’t cut it when you’re on Q15 slipping between restricted areas. RVSM certification is also a must unless you plan to cruise at or above 43,000 feet (You technically can fly a non-RVSM aircraft in RVSM airspace if ARTCC’s workload permits it, but it’s kind of like riding a moped on the freeway. You know you’re not supposed to be there and bucking the system shouldn’t be the highpoint of your day). Regardless, of what it looks like, this aircraft has to have the same capabilities as the most basic airliner or else it has no business being up in the flight levels.

Shapes Of Things

So what type of shape will an efficient VLJ have? Personally, I’m a believer in blended wing body, and double delta designs. Are they the only options? No. There’s nothing written in stone that says these are the only two configurations that will work. Someone may very well come up with an unknown concept that makes everything else positively obsolete. But the blended wing and double delta do have certain advantages for a low-cost aircraft being flown by pilots who aren’t 80,000 hour steely eyed missile men. They’re low risk structurally, aerodynamically clean, have a lot of internal volume for fuel or passengers, and when designed correctly, are stall resistant and in some cases stall-proof. If the wing loading is kept low, they will not present any special handling characteristics and can tolerate extremely high angles of attack. Additionally, thanks to non-linear lift, the takeoff and landing speeds are equal to or less than the most docile business jets on the market. Even if you do screw up really bad and get the nose jacked up to 35 degrees AOA without doing anything about it, the airplane won’t snap roll or tip stall over on you. It will mush until you realize that the only way to recover is to get the nose down and accelerate. Again, this is if they are designed correctly and the CG is within the approved range.

Will a conventional design work too? Probably, but not the way you want it to. Straight wings are okay if you’re scared of anything swept more than a few degrees, but the issue of critical Mach number is something that needs to be considered. Your wing will be see supersonic flows a lot faster than your airplane as a whole will due to air having to accelerate around it. A straight wing has to be relatively thin in order to delay transonic drag rise to a velocity that allows a useful cruise speed. This can adversely affect stall characteristics and reduces room for fuel and landing gear inside the wing. A swept wing can be physically thicker and still delay drag rise, but they do evil things like tip stall and reverse ailerons if you don’t make certain aerodynamic alterations to them. A double delta wing gives you the best of both worlds in that drag is reduced often to Mach 1 or slightly beyond and they don’t cartwheel your airplane unexpectedly. So yes, a straight wing with a conventional tail can be utilized for a VLJ, but if a high cruise speed is desired, it may not be the absolute best option. There is simply too much drag generated and not enough thrust to overcome it. If you don’t believe me, show me a VLJ that has been designed in the last 10 years that cruises at Mach 0.75 or greater (ATG Javelin doesn’t count).

It’s Not That Bad

In the article, Austin continues, mentioning the high drag and slow relative speed of VLJs as an issue to be addressed. Although drag is high on some aircraft in certain configurations, I think it’s more an issue of engineers being too frugal on thrust requirements. There are a lot of problems with using tiny engines as if they were JT8Ds, a topic I addressed in one of my equation articles a while back. Austin gets the raw numbers right but then relates them in the way everyone thinks they should be used, which is part of the problem. It shows how the intricacies of turbofan design are often misunderstood and people end up with “jets” that barely perform better than turboprop twins, or even homebuilt piston planes:

“Jets do well because they have a high bypass ratio… teeny little turbines spinning at huge rpm driving giant, slow-turning fans, these teeny fast turbines give huge compression efficiency, these high bypass ratio fans give huge propulsive efficiency.. so we just scale it down, right? WRONG.”

“The thrust we get from air is the momentum-change: amount of air we grab times how much we accelerate it. The fuel flow we put into the air is the kinetic energy: amount of air we grab times how much we accelerate it SQUARED therefore, for any propulsion system to be efficient, it must take a LOT of air and accelerate it a LITTLE. Thus, all else being equal, the HUGE prop of a Lancair is inherently more efficient than the tiny compressor of a mini-jet. An internal-combustion recip engine gets the same compression ratio no matter how fast it turns. set the throttle to idle, take-off, cruise, descent, approach, or holding-pattern… it makes no difference: if the compression ratio of the engine is 7:1, you will get that compression ratio at all power settings: 7:1… the compression ratio is realized no matter how fast or slow the engine is turning… the piston still covers the same space in the cylinder, regardless of speed. The JET engine, though, must turn at 100% rpm to get it’s designed compression.. if the jet turns 1% less rpm than redline, compression is lost, and efficiency with it… the compression is caused by the dynamic pressure on the blades… 1% less speed on the blades is 2% less compression across them, with the resulting loss in efficiency. You can only run a jet on-design at 100% rpm… any speed less and the efficiency falls apart… no surprise that going to low power settings still involves huge fuel-flow… a jet engine at low power is losing compression! a jet engine at low power is like a recip engine that is losing compression and needs to have it’s pistons replaced!!!!!!!!!!”

As for Austin’s analysis of compression efficiency being critical down to 1%, that’s not exactly the way it works. First of all, different types of compressors have different advantages. Centrifugals are relatively lightweight and compress efficiently across a broad range of rpms but have a large frontal area. Axials work best in the higher rpm range but have a lower cross-section and can be staged for very high compression ratios. Secondly, compression efficiency is important, but it’s not the only part of your total efficiency. There is also thermal, turbine, fan, combustor and propulsive efficiency to be considered. While it is true that turbine engines have a design point where if particular criteria are met, the engine will be at its maximum efficiency, it is not always at “redline” (Redline in jets is different than redline in non-turbocharged pistons. You can overspeed N1 while still in the green for temps and you can overtemp while still in the green for N1. Changing altitudes, speeds and temperatures all have a say in what limitation is used. FADEC has made managing this easier for most new jets. The pilot can just click-click-click-click to whatever preset power is required and away they go without having to worry about melting a $750,000 engine).

Jet engines are designed around what is called an operating line, which shows maximum efficiency for various rpm settings. Designers know that jets are not going to be operated at 100% all the time so the engines are made to be as efficient as possible even when off the design point. Turbine aircraft do not become exponentially less efficient just from operating at reduced power settings. This explains why airliners and business jets can pull the throttles back and fly slower to extend range. Things such as variable stator vanes, bleed valves, active clearance control and in some cases adjustable exhaust nozzles all help to reduce the impact of running at a lower rpm. These devices also reduce the chance of surge and or stall, which occur when airflow through the engine is disrupted in some way.

There are a lot of factors that go into engine efficiency and it is very easy to pick one or two and conclude that those are the only ones that matter. Aside from the previously mentioned efficiency qualities, airframe integration is another critical factor. A poorly located engine that gets disturbed or turbulent airflow is going to see a loss of efficiency and possible problems with stalls or surges at certain flight attitudes. A long intake duct is going to sacrifice pressure recovery at low speeds, reducing static takeoff thrust. This is something to consider if takeoff performance is more critical than maximum speed.

The airframe itself is also a factor in how efficient an engine appears to be. A high-drag airframe with a very efficient engine may require more thrust at the same speed and altitude as a clean airframe with a less efficient engine because the high-drag airframe is forcing the efficient engine to produce more thrust. Is the efficient engine wasting less fuel even though it’s burning more fuel overall? Yes, it is making more thrust per pound of fuel than the less efficient engine. The less efficient engine is burning more fuel per pound of thrust but since it needs less thrust, its fuel flow is likely to be lower than the engine of superior efficiency. Everything depends on everything.

Don’t Cut Your Thrust Short

Misunderstanding of engine efficiency is in my view, probably one of the main reasons that many VLJs tend to be underpowered. The designers, in their attempt to have an engine that runs at exactly the design point of maximum efficiency, ended up with an aircraft that didn’t have enough excess thrust for other regimes of the flight envelope. Below is a quick glance of the thrust to weight ratio (actually shown as weight to thrust) of various VLJs. Some have a lot of excess thrust, some do not:

  • Eclipse 500: 3.30 to 1 (the most infamous VLJ, company went bankrupt, we all know the story)
  • Cessna Citation Mustang: 2.96 to 1 (the company does not refer to it as a VLJ, over 400 have been built)
  • Adam 700: 3.46 to 1 (very unusual twin-boom/twin-tail design, company shut down)
  • Cirrus SF-50 3.33 to 1 (probably the most well-known VLJ, new design in testing, over 400 orders have been placed)
  • Excel-Air Sport-Jet: 2.38 to 1 (aft fuselage mounted engine, in production/testing)
  • ATG Javelin: 1.97 to 1 (built as a 2 seat military trainer/private jet, company bankrupt)
  • Diamond D-Jet: 2.69 to 1 (fairly unusual mounting of its single engine in the aft fuselage with a single-boom tail, in flight testing)

While a 3 to 1 thrust to weight ratio may be normal for larger business jets and airliners, the major difference is that larger engines have more thermodynamic capacity and can thus retain takeoff power to much higher altitudes and or temperatures before having to reduce thrust to keep within thermal limits. Bigger engines can simply withstand higher stresses and temperatures than smaller ones. There is also the fact that heavier aircraft can have bigger variations in fuel or passenger loads with less total impact than smaller aircraft. Removing 3 people from a 200 seat airplane is 1.5% of the maximum capacity. Removing 3 people from a 5 seat airplane is 60% of the maximum capacity. I’ve been saying this for 15 years: small jets need big power because you can’t make yourself or your passengers any lighter.

Another concern with having limited excess thrust relates to hot and high conditions. Because of the smaller compressor and turbine, the engine will probably have a very modest flat rate temperature rating for takeoff thrust. Simply put, the turbine section is too small to provide enough thermodynamic cushion to compensate for above standard temperatures. This becomes critical when your maximum thrust at sea level is less than 1,500 lbs and your aircraft weighs 5,000 lbs. A warm day in El Paso or even a cool day in Denver may mean having to leave a lot of fuel or a lot of people behind. Heavens forbid if you’re just under max gross departing Addison at 3pm when it’s 105F outside (It’s been triple digit temps for the last 2 or 3 weeks here in Dallas. It’s so hot even the birds are walking). You’ll go right through the fence, over the train tracks and out onto Belt Line Road with the engines still giving their all, which obviously wasn’t enough.

Most manufacturers will list the takeoff thrust flat rate temperature on the brochures and flyers, but of equal importance is the maximum continuous thrust flat rate temperature (the following data is straight from the FAA TCDS database). For example the Williams FJ44-4A turbofan can hold its takeoff thrust of 3,621 lbs up to 79F for 5 minutes. However it can hold its maximum continuous thrust of 3,443 lbs only at 46F or less. The massive General Electric GE90-76B referenced earlier can hold takeoff thrust of 81,070 lbs for 5 minutes at 91F. It can also maintain a maximum continuous thrust of 75,430 lbs at 77 F or less. The FJ-44-4 can hold 95% of its takeoff thrust if the temperature drops 33F. The GE-90-76B can hold 93% of its takeoff thrust if the temperature drops only 14 degrees. The larger engine clearly has the capacity to hold more power to a much higher temperature or altitude. That’s one of the less obvious missing links in the VLJ performance equation. The bigger the engine, the more margin is available to accommodate non-standard conditions. The more excess thrust and the more you have to play with even if your thermodynamic capacity is not that great.

Efficiency = What You’re Doing/What You Wanted To Do

How jets behave and what bypass ratio they have is based on what you want them to do. For example, many people often mistakenly state as gospel that high bypass turbofans (hereafter, HBT) are more efficient than low bypass turbofans (hereafter, LBT). And that would be true if quantified with a particular condition, such as cruising around 530 mph at 37,000 feet. That testimony of efficiency would go right out the window if the condition were changed to 600 mph at 51,000 feet, or 250mph at 16,000 feet. HBTs are designed to be more efficient at lower altitudes and lower speeds than LBTs, but higher and faster than propeller driven aircraft. “Lower” in this case is a relative term since I’m referring to the mid 30,000 foot range and roughly around Mach 0.80 (530 mph). As you go faster, high bypass fans begin to lose efficiency. In Austin’s words, these engines have huge fans and teeny turbines which makes them efficient. Well yes, until you start going beyond their design range. The compressor (what actually does the compressing, not the turbine) of a HBT is not exactly “teeny”. But the cross-section is quite a bit smaller than the fan mounted in front. Because of this relatively small core, there is a significant drop in thrust at higher altitudes, specifically once crossing the tropopause (36,000 feet on a standard day). In large aircraft with massive engines, this is not as critical an issue since the reduced fuel flow compensates for whatever loss of fan efficiency there might be (again, everything depends on atmospheric conditions, length of trip, aircraft weight, etc). In small aircraft with equally small engines, it is a very big deal.

The best way to think of modern HBTs is a turbine driven propeller with a shroud around it. The Boeing 777, one of the best examples of modern efficiency, is powered by a pair of GE90-76B engines that spin their fans at only 2,465 rpm. That’s it! Even Cessna Skyhawks have a higher rpm limit on their propellers. The logic behind such slow turning fan is all tied to the large dimension of the fan, which is over ten feet in diameter. That’s a huge parcel of air being moved. In fact, over 3,000 lbs per second passes through the intake at takeoff power. In order to produce the advertised 80,000 lbs of thrust, that air mass doesn’t have to be accelerated very much. The closer the exhaust velocity is to the speed of the aircraft, the greater the propulsive efficiency. When Austin mentions the efficiency of the Lancair, he is referring to the propeller moving large blocks of air at a speed very close to the forward speed of the aircraft. This provides exceptional efficiency compared to a turbofan, as long as he’s not trying to fly it at Mach 0.78 and 33,000 feet where shock losses off the prop tips will erase any efficiency advantage previously enjoyed.

Tip shocks are why no propeller driven aircraft has ever been supersonic. The spinning propeller actually creates its own sonic shock waves well before the airplane itself ever reaches the speed of sound. You can hear this in the sound of certain WWII fighter planes. The sweet sound of a P-51 or a T-6 taking off is the crackle of supersonic shockwaves off the prop tips. Propeller efficiency is based on the intended speed of the aircraft it is attached to. No matter what that speed happens to be, the efficiency will drop substantially after tip shocks begin to form. Remember the big fan of the HBT spinning around? Those blades are just as susceptible to shockwaves as propellers are. While the fan tip speeds are often above Mach 1, the closeness of the engine cowling negates any losses. But there is an upper limit to this rorational speed, and there is the additional fact that no jet engine can accept supersonic airflow into their face. Because of this, HBTs see a similar degradation in performance as propeller aircraft but at higher ultimate speeds. This leads to most modern HBTs being optimized for flight in the upper troposphere/lower stratosphere at roughly 80% the speed of sound.

If one wishes to fly in the troposphere, having a lot of bypass air is a great idea. If one wishes to fly in the stratosphere, especially with a small engine, having less bypass air is a better choice simply due to air density issues. Bypass air is referred to as “cold” in that it does not go through the combustion chamber but instead bypasses it (hence word “bypass” you’ve been hearing). In terms of thrust, cold air is slow air and hot air is fast air. Large, slow-moving blocks of air do produce substantial acceleration at low altitudes, but as one goes higher, the air density decreases, making it harder for the same mass of air to be accelerated by the fan with the available power from the turbine. You can try to make the turbine generate more power by burning more fuel, but eventually, the temperature (EGT or ITT) will reach its limit and that’s all the thrust you’ll be able to get for those conditions. The exhaust velocity begins to taper off and settles around the mid-subsonic regime. Meanwhile, the low bypass counterpart has an extremely high exhaust velocity but of a smaller total mass. In other words, the LBT accelerates less air faster, resulting in the aircraft having to fly faster in order to get the required airflow at higher altitudes where the air is less dense, which can be done only because it’s not using its turbine power to turn a large fan (follow that one?).

To sum up the comparison, there are some things a HBT can do and some things a LBT can do. Don’t worry about what people say is “the best”. Get the raw data. Run the numbers. Pick whichever one works for what you want to do. “Everything else is rubbish”.

What can be done?

If a VLJ is going to fly at 500 mph or more, (which makes sense, otherwise why bother with having a jet in the first place), a designer can take one of the existing scaled-down business jet engines that is on the market like the JT-15D or FJ44 series. If one is determined to fly at 300 mph or less with a jet, a lot of bypass is required. Unfortunately, the small engines that do exist are basically scaled down business jet engines and have nowhere near the bypass required to make them efficient choices. So in addition to copying airframes, it seems that we’re also copying large engines and trying to force them to fit a role they really weren’t meant to do. This is totally understandable given the development and manufacturing cost of a turbine engine. Nobody in their right mind is going to lay out money to mass produce an unproven powerplant for a market that doesn’t even exist yet. Perhaps the answer lies in changing what we consider a jet engine to be. A turboprop is a jet engine that spins a propeller, yet 9 out of 10 normal people (again, not like you and me) see one fly by and say “It’s one of those little prop planes”. A HBT spins a big fan tucked away in a shroud, almost like a 40 blade propeller, yet everyone and their cousin says it’s a jet. Meanwhile, the aviation industry worries about the semantics of what kind of engine turns the propeller, or if the jet is high or low or medium bypass. We’re missing the point by a nautical mile.

The observation of the common individual should make us think: Is there some combination of a shrouded prop of smaller diameter than a turboprop but bigger diameter than a mini-jet that could work for an aircraft in the 300 mph range? Is anyone working on it? Gerry Merrill of Advanvced Propulsion Inc. has been working on a “low and slow” version of a turbofan for decades. He hasn’t really found any takers to seriously commit to such a concept and fund development of a series of full scale test engines. The unducted fan concept which was abandoned by the airlines may hold promise for general aviation since the smaller diameter would reduce tip speeds, thus reducing the noise level. There’s also exoskeletal engine idea studied by NASA’s Glenn Research Center which places the compressor and turbine blades on the inside of a rotating shell instead of mounting them to a rotating shaft. This may be yet another type of technology that makes the turbine more useful in the airspace below 28,000 feet. Emotionally for some, the idea of making a sleek and sexy jet “slow” may be too much to consider, even as they’re strapping engines that work best at 500 mph onto aircraft designed to fly at 300 mph.

What have we learned?

  • We trust in what is established and well known.
  • People in aviation have a passion unmatched in any other industry.
  • People who aren’t in aviation have completely different ideas of what flying should be like.
  • Imitation is the highest form of flattery until you can’t make range predictions.
  • Breaking from the herd is a good thing when it can be justified.
  • High bypass turbofans are not the be-all, end-all of turbine engines.
  • Piston-driven propellers are not the be-all, end-all of efficiency.
  • Everything you measure is relative to what you want measured.
  • Little details are what kill promising projects.
  • You can sell a dream, but not as easily as you can sell reality.

For want of a more useful powerplant, Austin cancelled work on his X-1 project. For all the other VLJs that have disappeared into the history books, a more useful powerplant or airframe configuration was needed years ago. But as long as engine and airframe designers keep scaling down what works in big applications and pointing the finger when a VLJ concept doesn’t work, we’re not going to get anywhere. People know how to build airplanes and people know how to build engines, that’s not the issue. We just don’t know how to take a gamble on an engine or airframe that makes sense for VLJs. Until then, the best thing for designers to do is to put as much thrust on their aircraft as possible and build an airplane to exactly what they want it to do, even if it means breaking the unofficial rules of design.

The best way to get to where we haven’t been is to arrive in something never seen before.

Comments, ideas, and “Who the bleep do you think you are” remarks go the box below.

Hangar Talk 1: Of Boats, Planes and Flying Wings

Standing in the hangar today, I was deeply engaged in a discussion with another pilot about the pros and cons of current aircraft design. We both agreed that designers have pretty much maximized the efficiency of the fuselage/wing method. Any further increases will come from add-ons like winglets, sharklets, alligatorlets, or whatever other things they come up with to slap onto the end of a wing. Therefore any radical increase in payload or range will have to come from a radical change in construction.

Before everyone throws down their copy of Controller and spills coffee in a rage at my upstart attitude, consider that almost all respected designers as recent as the 1990s regarded winglets as a fashion statement added by the marketing department. In 2007, the last true defender of “build the wing right the first time”, Dassault Falcon has finally and begrudgingly added winglets to their aircraft. It took a massive increase in fuel prices to force a change in perspective but everyone eventually came around. It will be much the same with a new type of aircraft design. Don’t worry, you’ll get used to it.

Lifting body/flying wing/blended wing body designs are among the most efficient shapes in the sky. All fighter jets from the 4th generation onward have lifting body tendencies. That’s why an F-4 Phantom’s turn radius makes F/A-18 pilots start laughing uncontrollably…the F-4 was a brute force airplane: Fuselage, intake/exhaust, wing, stabilizers and engines. The F/A-18 had and still has leading edge root extensions (shortened to LERX to prevent pilots from having to say “My plane has ‘stensions!”). At higher angles of attack, they begin producing powerful vortices that drift over the top of the wing and delay the stall far beyond what would be expected from looking at the airfoil’s lift curve graph.

McDonnell Douglas F/A-18F Super Hornet pulling into a square loop, showing the powerful LERX vortex generated from the increase in angle of attack.
(I know Boeing bought out McAir but it just doesn't sound right even today)

But there is part of the problem. In order for the LERX to work, it needs to be at a fairly steep angle of attack. High angles of attack and normal landing gear aren’t friends. The result is a long, ungainly set of gear so the aircraft can rotate to a useful pitch angle, or a set of regular gear that will require delaying takeoff to a higher speed, which negates one of the reasons for generating more lift in the first place. Also little tidbits like being able to see over the nose and not scraping the tail on the runway are important as well. These will have to be addressed before any alternative design can be taken seriously.

In the air, the problems begin to vanish as the advantages outweigh the disadvantages (depending on how fast, high and aggressively you’re flying). One major disadvantage is the volume requirement for flying wings. A pure flying wing encompasses everything inside the wing. In order for this to happen, the wing must be very large. Think YB-49 large. Otherwise, it will be a not-so-pure flying wing, which is not a bad thing (blended wing body or BWB). The B-2 Spirit stealth bomber really is a BWB as the cockpit and intakes protrude above the upper surface of the wing. The thickness to chord ratio would have been prohibitively large in order to fit the cockpit inside which would have reduced maximum speed. The really nice thing is that the “fuselage” on the B-2 does not need the normal streamlining since only a portion of it is in the airflow. The part that’s buried in the wing can be whatever dimension the designer desires.

Okay, back to the hangar conversation. My stance was that normal airplanes waste a lot of energy lifting parts they don’t need to lift. If I’m on a boat, I sit on the deck which is covering the hull. The hull is the only part that moves through the water. There is no heavy, non-buoyant passenger cabin supported by hulls on the port and starboard like poorly designed pontoons. Even catamarans keep their center sections out of the water (it would cause an incredible penalty in drag to have it actually in the water). And yet that’s how we build airplanes, with non-contributing sections in the airflow. The other pilot (who occasionally goes out in boats) understood immediately. We have to be able to put almost everything inside the wing, or make most of the fuselage into a wing in order to make it work more efficiently for us.

Vincent Burnelli, John Northrup and Charles Zimmerman laid the ground work in the early 20th century. Designers that followed have taken bits and pieces from their discoveries but few if any have been able to put them together in a way that was commercially viable, useful and safe for the civilian market. There is no reason that people can’t be flying around in a 4 seat airplane that burns 8 gallons per hour, lands at 50mph and cruises at 200mph. There is no reason why people can’t be surrounded by a steel safety cage much like the one utilized in Barnaby Wainfan’s Facetmobile to protect them in the event of an accident. We just have to readjust our perspective as pilots, designers and as people and ask what can be instead of what will never be.

What Are You Lifting?

Mention aerodynamic efficiency to most pilots and designers and you’ll probably hear the phrase “lift to drag ratio” within a sentence or two. Common aerowisdom dictates that for maximum efficiency, a high lift to drag ratio is required of an airframe. Of course, like most truths it is true and false at the same time. I’ll attempt to explain without going into a thesis level discussion as to why a high lift to drag ratio (L/D) is not the be-all-end-all of aerodynamic efficiency.

Long thin wing of a 757-200. Design is great for reducing drag on transcons, plus blended winglets provide ample space to advertise.

Now I’m going to I make designers start ripping out their hair and turning over their drafting boards…wait, it’s almost 2012…turning over their CAD mainframes. This is not exactly a law but it should be, and it will help many budding aircraft designers achieve their intended goals:

Any lift to drag ratio without a measure of what is being lifted is not operationally useful. 

(and in plain English)

Lift to drag is useless unless you’re lifting something useful.

Sorry, but I had to say it. Obsession over a high L/D is great if you want your airplane to look like a U-2. It’s not great if you want to be able to land it without pogo sticks under the wingtips. The first question you should be asking is “What do I want to lift?” More accurately, “What payload do I want to lift?”

The wing does not care how you divide up weight. If you build a 10,000lb airplane that’s creating 500lbs of drag at high altitude, you’ve got a 20:1 L/D ratio. That’s an excellent ratio, BUT…what are you lifting? If your aircraft empty weighs 7,000lbs and your useful load is 3,000lbs, it means you are experiencing a 6:1 load to drag ratio. If on the other hand your aircraft weighs 5,000lbs empty and your load is also 5,000lbs, your load to drag ratio goes up to 10:1 (designer Barnaby Wainfan describes this as an aircraft’s “transport efficiency” in his excellent article on low aspect ratio wings in the February 1997 issue of Flight Journal). Losing a bit of total L/D may be worth it to increase load to drag. Thus begins the tradeoff between light structure and increased L/D. Longer wings are heavier wings, with the attendant bending moments, possible zero-fuel restrictions and clearance problems for taxiways and hangars.

Yes, this high aspect ratio glider is efficient, but only at low relative speeds.

Yes, this high aspect ratio glider is efficient, but only at low relative speeds.

The other missing aspect is airspeed. While discussing the D-21 triple-sonic drone in his book “Aurora”, Bill Sweetman calls attention to its relative efficiency compared with a B-52. Using a modified version of the Breguet Range Equation, he multiplies the L/D of each aircraft by its cruise Mach. The B-52 with a 19:1 L/D and a Mach 0.85 cruise came out at 16:1. The D-21 with a 6:1 L/D and Mach 3.5 comes out to 21:1.

While this number (Mach to drag?) doesn’t do much since it ignores the D-21’s limited fuel supply compared with the B-52’s massive reserves and therefore does not give any useful information on actual range, it is interesting nonetheless. Personally, I’d use it as an early benchmark in the design process. If your intent is to go fast, a lower L/D is not as damning as one might expect, particularly since you’re “wasting” fuel for a shorter interval of time to cover a similar distance as a slower aircraft. This of course assumes you are not travelling supersonic where thrust specific fuel consumption begins to rise, further eating away at your endurance (but if your drag is low enough, you may still be able to recover range by the increase in speed…whew!). One way or the other, you’re putting either the slow penny or the fast nickel out your exhaust pipe.

I tried and probably failed miserably to get the point across. The point being that L/D is just one part of the equation when it comes to efficient design. If I had a choice between an airplane that had an L/D of 8:1 and a load to drag of 14:1 versus an airplane with an L/D of 18:1 and a load to drag of 5:1, I’d pick the 8:1 L/D aircraft 50% of the time…maybe.

Why? Because what I intend to do with the aircraft is what should drive its design. I’m not going to lie and say that high aspect ratio or low aspect ratio is better than the other because they’re not. I’m not going to say ignore L/D completely because that’s just stupid. But I do recommend that you look at your range, speed and payload requirements before committing to a design. Simply copying what comes out of Renton or Wichita doesn’t make much sense if you’re going to be doing something different than what they intended with their designs.

In summary, what did we learn?

  • Low aspect ratio aircraft tend to have lower empty weights and can be very strong structurally.
  • High aspect ratio aircraft tend to have higher empty weights and can require extra strengthening in extreme cases.
  • Lift to drag ratios are not fully useable until you know what fraction of the weight is payload/useful load.
  • An airplane with a high L/D and high aircraft empty weight has less transport efficiency (thanks Mr. Wainfan!) than an airplane with a lower L/D and lower empty weight.

Unfortunately, like most things in aviation, what I brought up here leaves a lot more to be addressed, like the role of wing loading, or integration of wings and fuselages. Or even why all my designs look like guitar picks (seriously, not all of them). We’ll continue soon with more on wing loading and how reducing it can improve high speed performance.

Dyke Delta N18DW at Oshkosh. Low aspect ratio double delta wing design very resistant to stalls and adds at least 200mph to every pass, at least in the spectator's mind.

Dyke Delta N18DW at Oshkosh. Low aspect ratio double delta wing design very resistant to stalls and adds at least 200mph to every pass, at least in the spectator’s mind.



Questions To Ask Before Designing An Airplane (#1-3)

What we’ll do in this series is ask the most important questions a designer should be asking before building an aircraft. To assist in the process, each time a question is asked, we’ll apply it to a group of fictional aircraft that are literally being made up as the articles are being written. This is to show that regardless of the powerplant, size or speed, the basics apply to every successful and safe aircraft.

The payload, range and cruise speed requirements directly affect how big an aircraft is.


The 3 most important questions are:

  1. What do I want to carry?
  2. How far do I want to carry it?
  3. How much time should it take to carry it.

Simple right?

Well, yes and no. These 3 questions open the pandora’s box of compromises, adjustments, and late-night napkin sketching that plague engineers. Since I don’t have a degree in engineering however, I’m not smart enough to be deterred the prospect of spending the wee hours with a stack of Marcal and a Sharpie (I actually enjoy it, most likely because my career isn’t at stake if I come up 300lbs overweight and $500,000 overbudget). Everytime one problem is solved, it is likely to create several others. Only by setting your performance limits early can the tradeoffs be minimized.

So what’s the simple part? The fact that you have full control over what you want your aircraft to do. If it can perform the mission or not within the bounds of Newtonian physics and the laws of Bernoulli is another story. But knowing that all aircraft performance figures stem from the initial 3 questions will make your life much easier when it comes time to run a weight reduction program. In fact, you probably won’t need to run the program at all.

Okay, onto dissecting the questions.

Question 1 simply asked what did you want to carry. This is the place where you’d write down what or who you want in the aircraft with you (I’m assuming that a pilot would be flying it, although if you’re building a UAV, the same principle applies). Once the basic paylod figure is created, the aircraft will start to take shape via raw numbers, not yet by appearance. Be realistic with the weights you assign to each passenger/crew. The FAA standardsare pretty ridiculous when one considers that the weights are supposed to include

How much stuff will you carry on a regular basis? Will you be able to trade passengers for cargo? Or cargo for fuel? Or fuel for a travel-sized guitar?

5-10lbs for clothing and 16lbs for personal items. Anyone who has seen the tremendous amount of carry-on luggage knows that this is way underbudget. For aircraft with fewer than 5 seats, the pilot-in-command can throw the passengers and their bags on a scale if they are concerned about being overweight or out of CG range since that is far less degrading to everyone involved.

For our purposes, we’ll assume 230lbs per passenger. This will represent a 200lb person with 30lbs of baggage. Yes it sounds high but if the actual passengers turn out to be lighter, you’ll end up with more performance, even though they’ll probably have extra baggage which negates the bodyweight advantage in the first place.

Aircraft A: 4 people. 230lbs x 4 = 920 lbs

Aircraft B: 50 people. 230lbs x 50 = 11,500lbs

Aircraft C: 40,000lbs of cargo. (230lbs x 2) + 40,000lbs = 40,460lbs

Note for Aircraft B that it is 50 people total, not 50 passengers or the number would have been 53 to account for flight and cabin crew. Aircraft C has presumably a 2 person flight crew.

So now you know how much weight you want to move. Now, where do you want to move it?

Question 2 asks how far did you want to carry the payload. This can be any range you want. If you are unconcerned with practicality or marketability there is no reason not to build a 2 seat piston single that can fly 4,000nm. There will be tradeoffs but it can be done. You could alternately fly 150 passengers 250nm via a turbofan twin. Your flight path will look like a giant parabola, but again it can be done.

Because you'll never hear "Now boarding Continental flight 179, nonstop service to anyplace that's 2163 miles away."

People don’t get in airplanes to tell others “I flew 1867nm today!”, they get in planes to go places. That 1867nm translates to leaving Dallas and ending up in Bermuda. It can also translate into taking off from Nashville and landing in Venezuela. Play the travel game. Pick your departure point and think of places you’d like to go to, or have to go to. It definitely helps if you end up facing a room full of non-aviation investors to say “Direct from Miami to Las Vegas.” rather than “1900nm plus IFR reserves.” If the room is full of aviation-savvy investors by all means, wow them with your knowledge of how NBAA reserves are calculated.

Airplane A: Dallas to Panama City Beach. 595nm

Airplane B: New York to Los Angeles. 2124nm

Airplane C: Charleston to Toulouse. 3746nm

The 3rd and final question for this post is how much time should it take to be moved the required distance. This is nothing more than division. Take the time you feel comfortable sitting (or think your passengers would tolerate being in one spot) and divide it by the distance to be covered and the result is the required cruise speed. If your range requirement was 4000nm over the course of 15hrs (maritime patrol missions often cover long distances and have attendant high endurance times) then you can set a normal cruise of 267knots. If on the other hand you picked 1700nm and wanted to spend no more than 2 hrs in cruise, you will come up with a cruise speed of 850knots. It’s completely possible to supercruise, but you will likely run into a fair amount of friction both from the aerodynamics and the government regulators who decreed that Thou Shalt Cruise No Faster Than Mach 0.95.

Airplane A: 3hrs to travel 595nm = 198knots

Airplane B: 3hrs 45min to travel 2124nm = 566knots

Airplane C: 8hrs 30 min to travel 3746nm = 440knots.

With only 3 questions you’ve already figured out what you want to carry, where you want to take it and how long it should take you to carry it. This was the easy part (I keep saying that). The next installment will feature our friends The Tradeoffs and The Compromises. They aren’t so bad once you get to know them.

Glass Cockpit Blues

The Square Elephant In The Cockpit

Original Date: June 3, 2009

I was observing on an instrument proficiency check in a Cessna 205 and noticed some things that really did stand out. The pilot undergoing the check was highly competent and ran very thorough checklists for all phases of flight. His VOR and ILS approaches were smooth and safe with limited deflection shown on the CDI that he corrected quickly. However the one instrument in the cockpit that caused the most trouble was the GPS. The instructor asked to see a GPS approach in Orange County. The PIC started pushing buttons to enter approach mode on the receiver. And the GPS promptly decided to ignore his request and do something else, like try to enter an approach for a VOR in the area (which to its credit, it gave a message saying “This is not an airport.”).

So the PIC said lets try a different airport, like Lincoln Park. The instructor said okay, enter the approach and fly the procedure. Again the same flurry of typing and head scratching ensued. By now the instructor is fiddling with the unit and flipping through operation checklists to see if there were any shortcuts to getting it to switch modes. After about 5 minutes he proclaims victory over the beast in the black box and then asks the PIC to enter the approach. The PIC tried several times but each time hit a key that ruined the string of info just entered. That or the wrong option was selected, giving us a flight plan to Aviano. All the while, I’m scanning for traffic and telling the potential student in the back seat next to me that flying is actually fairly easy, but operating the avionics is the thing that makes aces feel like aceholes.

We headed south back to Central Jersey Regional and by this time the PIC had figured out a way to get the GPS to accept the approach mode and left the flight plan mode alone for good. He flew a perfect GPS approach to runway 7, broke off and made a ridiculously soft landing. One of those landings where you have to remind the wheels that they’re supposed to start turning because we are in fact on the ground. After the flight, I talked to the potential student about the joys of general aviation, while the instructor spoke to the PIC about the flight. It was painfully clear that while GPS is a great tool (the map mode would have kept us from guessing where NYC’s class B began in case we couldn’t see ground references, but in that case you should be IFR anyway so it’s a moot point) and it can help you fly more efficiently.

However, if you are not completely comfortable using all modes of the GPS, you’re only getting a fraction of the benefit. Even more importantly, with your head down staring at the various modes on your receiver, you’re distracted from the primary task of flying the airplane. Granted this airplane had an autopilot and it had been used earlier, but the instructor wanted to see the PIC hand fly. The PIC got off heading and altitude far more often when messing with the unit than when he was just scanning the horizon. Granted, a person with an impeccable scan will be able to divide their attention perfectly, but the fact remains that you need to know exactly where the electrons are going before you start the engine.

What's it doing now? Direct ZELEN? I don't even know who ZELEN is!

If your GPS has home training software, use it. Don’t just hit the Direct button and stare at the map. That’s a waste of many thousands of dollars of capability. Practice going to a certain airport and then switching to an alternate. Know how the map orients itself and how to zoom in and out. If your GPS can output commands to an autopilot, do some local practice flights with it engaged in good weather. Basically using the full capability of any avionics needs to be second nature. Just as you can spin the numbers on the transponder without a second thought, so must be the operation of any nav gear.

In closing, a word to any avionics manufacturer who may be reading this (hey you never know). Please make your avionics big enough to use without having to train our fingers how to lock onto the right button while bouncing around in turbulence. Yes, panel space is always an issue but most owners would welcome a large knob that does the same thing in all pages (i.e. scan, change letters, change mode, etc), or large buttons that are spaced so that the bouncing finger doesn’t hit the wrong one. Yes, the “spider crawl” method does work but it freaks out passengers. Other than that one issue, I love the color maps and built in nav/coms. Anything to make the average Piper more like an A320….except for the J-3 Cub.
Let’s leave that one simple.


Virtual VFR and Pilot Safety

Don’t Worry, There Won’t Be Any J-3s On An ILS To Minimums

 Original date: August 2, 2011

"I'm sorry sir, these flat panel displays are for airliners only. Have fun with your morse code identifiers."

Warning: This blog is filled with aviation terminology that may be objectionable to land-locked readers.

This all started after reading Mac McClellan’s blog on head-up displays in light aircraft. I posted a response and one of my friends who happens to be an airline captain saw it and responded to my response (don’t you love the internet?). We’ve been going back and forth about the benefits of advanced technology for general aviation aircraft. Specifically, it was about synthetic vision and how it could create Virtual VFR regardless of weather conditions. His stance is that GA pilots don’t need super advanced instruments and information systems because it will make pilots fly into conditions they shouldn’t be in. My stance is that it will make those who take the time to learn how to use it much safer.

The sticking point in any field is that new technology that makes things easier is often seen as a crutch by those who did without for the majority of their lives. When GPS began showing up in aircraft, people said “What will you do if it all fails?” I would then point to their stack of navcoms, adf and loran receivers and ask them the same question. Stuff fails no matter how high tech or low tech it is. Dealing with failures is the burden of the pilot. The mean time between failures with modern electronics far surpasses any analog, transistor or vacuum tube based system that bore the generic label “computerized” in previous decades. Automatically that is a huge benefit not just for safety but for life cycle operating costs.

The other problem brought up during the initial GPS revolution was that people would forget how to navigate or look for other aircraft. That is a problem, not so much of the GPS but of people not knowing how to divide attention, especially in busy airspace. I remember several times with my instructor when we’d spot an airplane (or worse, get bounced from behind) I’d say “Did he even see us?”. To which Marty would always have a witty comeback like “Why don’t you get out and ask him. Think it’ll make a difference?”. I have no idea why the overtakers didn’t see us but a distraction is a distraction. I don’t care if its GPS, an ADF or some poor soul with headphones on listening for “dah-dit dah-dit” on the four course.

Flying in a general aviation aircraft, regardless of what we tell passengers is a more risky activity than driving on average. However the risks can be adjusted based on a pilot’s skill, comfort level and aircraft capability. Maybe a particular pilot doesn’t like flying in clouds, flies only for pleasure and operates an aircraft equipped with VFR only steam gauges. However this pilot wants to upgrade to a 3 tube EFIS system combined with a HUD. Should we deny them advanced navigation and weather information based on the assumption that he is going to suddenly start flying between level 5 thunderstorms? Should information-dense systems be the sole domain of the turbine fleet and business jets? If the light airplane pilot wants to fly a 300nm trip, is it fair to make them use less capable avionics, ostensibly to keep them out of trouble?  

There's a lot of information, but how easy is it to interpret under stress?"

Being able to navigate a couple hundred miles through a high-pressure system without super-duper graphics and satellite weather should not be too difficult for any pilot. A basic GPS or (gasp) a stack of VOR receivers can get you just about anywhere in the United States. But the cushion of safety for those who choose to learn everything that their super-duper system can do for them is undeniable. The objective for VFR pilots is to use extra information to stay away from weather (terrain shouldn’t be a problem since if they’re VFR they should be able to see it). To say they don’t need it because they’ll start flying into frontal systems is like saying that airline pilots shouldn’t have terrain avoidance systems because they’ll see where the ground is and fly into it.

IFR flight on the other hand is a more difficult situation because there are so many variables in the types of aircraft, the types of missions, and the weather conditions at any given place or time. There may be the person in the Cirrus who is cruising at 11,000 feet on top of a cloud layer and wants to know the exact position of the hills hidden beneath those clouds. Sure he can use an IFR chart and know that by staying above the MEA he’ll be fine but let’s use the favorite example of instructors: What if the engine quit? Synthetic vision cannot dead stick an airplane onto an open farmer’s field automatically, but it does give the pilot far more information in an emergency situation with regards to wind direction, terrain location, obstructions, etc.

Take a single-pilot King Air on an ILS on a scuzzy day. The pilot has approach charts that show what the decision height is, how far from the touchdown zone that will be, what the missed approach procedure is, etc. And since the pilot is IFR rated and trains in a simulator at least once a year, it should be no big deal. However, if there is a distraction, or a problem with the aircraft, a small mistake could be made. To the delight of lawyers everywhere, I will be completely honest: pilots do make mistakes (if you don’t believe me, ask the NTSB). The majority of accidents are not one massive brain-fart but a series of smaller errors that compounded until the snowball became an avalanche. By providing easy to interpret data, the pilot’s mind is freed to deal with any other issues that arise during times when the mind is approaching task-saturation. So now while dealing with a generator problem, a sick passenger, or just an unfamiliar approach, the pilot is able to see the image of where the runway should be and cross-reference that with the standard charts and data. This removes all doubt as to the aircraft’s location and where it will be in the next 15 to 30 seconds. Breaking the links in the accident chain should be reason enough for encouraging use of such equipment.

Information-rich technology is not for every style of flying. I admit, it would be odd to fly a Stearman with a HUD. And a Cessna 152 that is only used for $200 dollar hamburger runs (inflation hurts, doesn’t it) would not need an extensive weather suite and electronic IFR charts loaded into the system. Am I in favor of putting EFIS and HUDs in everything from 

Open cockpit EFIS

light sport to piston twins? Honestly it doesn’t matter what I think. If the pilot/operator feels that the technology will be a benefit to their type of flying, then full support should be offered for getting that equipment into their cockpits. I was in Woody Saland’s hangar a while back and was intrigued by the fact that his AirCam had synthetic vision EFIS, EICAS and an autopilot. Why would anyone want so much technology in an open cockpit airplane? Then it hit me: To make the task of converting numbers, radials and performance figures into an instantly interpretable view of what your aircraft is doing. With so much of your mental capacity relieved of that repetitive task, you can actually enjoy the act of flying.

The Case For Long Range Regional Jets

The Case For Long Range Regional Jets

A Practical Application


By Christopher Williams


The regional jet has become a dominant force in air travel in the last decade. While there are economic drawbacks to small jets on short routes, there are decided advantages on longer trips. Because of these advantages, it may be time to move towards long range regional jets. However, before an analysis of a long range regional jet is begun, it is necessary to examine why lower capacity makes sense in not only the traditional hub and spoke system, but in a direct flight system as well.

“Regional jet” is a fairly recent term for a fairly old concept. The Fokker 28 and McDonnell Douglas DC-9-10 would both fit into the category of regional jet (hereafter referred to as RJ) even though neither were consistently used in a manner congruent with current RJs. The modern class of RJs spans the gamut from the 50 seaters such as the Embraer ERJ-145 and Bombardier CRJ200, to the much larger 100 seaters like the Embraer E-190. They are generally powered by high bypass turbofans that have tremendous fuel consumption improvements over turbojets and low bypass turbofans that powered first-generation small jets. Due to their small size, they typically do not have multiple lavatories and baggage storage space is somewhat limited.

Larger airliners (often referred to as “mainline”) have the numbers advantage in terms of seat cost per mile, fuel consumed per seat per unit of distance, or any other statistic involving dividing a quantity by the number of seats. The more passengers that can be carried equates to a lower cost per passenger to operate the flight, which in theory should result in lower fares. RJs are at a disadvantage from the start due to their lower total capacity. Unless total operating costs are reduced by the same factor that the passenger capacity is, there will always be a disadvantage to operating with fewer total seats.

The disadvantage of larger airliners is their higher upfront purchase cost and their higher total operating cost. This is where RJs have the advantage from being smaller aircraft. The aviation consulting group Morten Beyer & Agnew refers to this as the “RJ Operating Cost Paradox”. Even though RJs cost less upfront to buy and less total per hour of operation, they still cost more per person and thus have a much smaller maximum profit margin. [1] This problem has led manufacturers to build larger RJs, like the aforementioned E-190 which boasts up to 114 seats and a breakeven capacity of 61%. [2]

But RJs are not the only aircraft that suffer from reduced capacity. The larger overall cost of operating a full size airliner at reduced capacity is even more of a problem for their operators, which is why so much effort is made to fill every seat on flights. It’s a lot like gambling. With small jets, you take a smaller investment and end up with a smaller return provided you fill your seats. With a big jet, you take a larger investment and end up with a much bigger return, again as long as the seats are filled. If the RJ doesn’t fill its seats, the total loss is still less than a larger jet in the same predicament. Win big, lose big.

Regional aircraft of varying sizes line up for takeoff at Denver International. Where mainline jets used to do more short to medium range trips, RJs have taken over the lions share of the workload.

How can it be possible combine the advantages of airliners and RJs? The answer may lie with what passengers want rather than what airlines want to give them. For an airline it makes financial sense to use the hub and spoke system to funnel passengers to a few mega-airports and then redistribute them to other smaller airports via RJ or commuter turboprop. For passengers it makes sense to go to an medium sized airport not too far from one’s home and fly direct to wherever they need to go. Unfortunately, unless people live near a large international airport, chances are good that they will have to switch planes at some point, even for a relatively simple trip between terminals of moderate size.

The hub and spoke system was first adapted to passenger aviation by Delta Air Lines in 1955 and later adapted to freight by Federal Express. [3] While this system works wonderfully for packages that have to arrive only by a certain time, say Monday at noon, it does not work well for passengers who want to get to their destination as soon as possible. A package does not care if it sits in a warehouse for 10 hours before being loaded onto a connecting flight to its destination. Passengers tend not to be as enthusiastic about such delays. Another reason for the popularity for hub and spoke had to do with the aerodynamics and performance of first and second-generation commercial jets. With their very high rates of fuel consumption at low altitudes, it was not wise to use them on short trips or at low altitudes. At the time, this had less to do with the price of fuel and more with getting useful range out of the aircraft. To fill the gap, turboprop commuter airliners were pressed into service to connect outlying airports to the major ones. While extremely efficient and fast, passengers still equated them with old lumbering piston airliners. Even though turboprops are technically jet engines with an exposed propeller instead of a shrouded fan, perceptions count for a lot.

RJs are jets through and through which makes passengers happy. But while pilots who may have upgraded from 4 and 6 seat general aviation or charter aircraft might think they’re big, the truth is many passengers still consider them small. And even though newer designs such as the CRJ900 are much larger than the original cadre of RJs, they still cannot compete with the sheer size of an Airbus A340, or Boeing 777. Size aside, the speed and ride quality are on par with small to medium sized airliners. Regional airlines and their supporting major airline counterparts could not buy enough RJs until the fuel crisis of the mid-2000s. At the point when fuel prices skyrocketed, the once beloved aircraft became very unattractive in the eyes of airline accountants. [4] Regional carriers wished for their fleets of turboprops back and many invested in newer designs like the Bombardier Q400 that rival jets with respect to ride quality and sound levels inside the cabin. On short trips, taxi, climb out, descent take up a major portion of a jet’s time and fuel. In some cases the actual time spent at cruise may be around 30 minutes, severely reducing whatever advantages the manufacturer may have designed into the airframe for that portion of flight. In a turboprop the maximum altitude is usually far lower, in addition to the fact that they use a lot less fuel at low altitudes making them more efficient on short trips.

Many people had assumed with the advance of RJs, so too would the ability to travel almost anywhere within reason in the United States without having to go through a hub airport. This never materialized and by 2007, flight delays had risen to a 13 year high as hundreds of RJs tried to occupy the same airspace and parking areas as hundreds of mainline jets. [5] By using RJs to fill in gaps in the hub and spoke system, airlines had unwittingly taken away the major advantage of low capacity jets and undermined their mainline fleets. With the lines blurred between RJ and mainline equipment, some low cost airlines such as JetBlue opted to use A320s for transcontinental and international routes and E-190s for shorter hauls. [6] Oddly enough, successful low cost carriers like JetBlue and Southwest Airlines fly on watered down versions of hub and spoke and use small airliners or RJs on all routes. It remains to be seen if current regional airlines will continue to serve as feeders for the majors or if they will venture off into the volatile world of low cost national operators. Major carriers, currently uninterested in their own RJ fleets, may change their position if that happens.

This next part of this paper will not deal with how to build such an aircraft in detail but will outline the criteria that will be required of the next generation of long range RJs.

Mainline jets, as mentioned, have a huge passenger advantage over RJs. This is also their Achilles heel if the flight is operating at reduced capacity. On red-eyes or on low demand long range routes, using a nearly empty Boeing 757 makes less economic sense than using a partly full Airbus A320. In that respect, using a partly full A320 would make less economic sense than using a nearly full long range RJ. For this to happen, the RJ would have to have transcontinental range with enough reserve to account for average yearly headwinds, missed approaches, a diversion to an alternate field in case of poor weather and any other situation that may stretch the endurance of the craft.

Airliners of all sizes only make money when they fly, and then only if paying passengers are riding inside. Empty seats do not make money which is why full airplanes equal happy shareholders. But passengers do require a bit more room and creature comforts than the average overnight parcel. It is for this reason that future RJs must have as much personal room as the smallest mainline jets. This will require advanced ergonomic design to ensure that the aircraft remains light enough to land at smaller airports, produces minimal drag in cruise but still allows people to stand up in the aisles or go to the bathroom without having to crouch. These same comfort features will be extremely important when considering that the next generation of RJ will have to be able to fly at least 2500nm, if not more.

Takeoff and landing are critical issues for any aircraft and commercial jets have a plethora of criteria to meet before every being certified to carry a single passenger. Airliners do not usually need an entire 10,000 foot runway to takeoff at normal weights, but the extra distance is required in case of an engine failure on takeoff. Below a certain speed (V1) the jet must be able to stop in the remaining runway. Above that speed the airplane must be able to accelerate to takeoff speed in the same remaining runway. For this reason, a Boeing 737 that might be able to become airborne in 3500 feet may require 7000 feet for regulations. This is the balanced field length that is listed as the takeoff distance for all commercial aircraft. Any reduction in takeoff velocity, any increase in acceleration or any combination thereof will go far in reducing balanced field length and thus open up whole new airport markets to airlines. This of course is if there is a demand for the service at those locations.

From takeoff to cruise the aircraft is in a climb. Usually this is not a constant uninterrupted climb as almost all airports have departure procedures (DPs) that require a pause in the ascent at certain points to help with traffic management. Once clear of the immediate area though, most jets are free to climb at their most efficient rate. A major problem for aircraft climbing has not been ability to climb, but excessive noise. Novel concepts for reducing power at strategic segments of the climb such as the Quiet Climb System by Boeing are aimed at making living near an airport quieter. [7] Another approach to the problem is to design aircraft with more excess thrust that allows for steeper climb angles that removes the source of the noise from the ground much faster. An added benefit for pilots and airlines is that the jet will arrive at cruise altitude sooner and begin its most efficient profile earlier in the trip. While excess thrust sounds wasteful, it is only used when required, thus allowing the engines to work at far lower power settings during normal phases of flight, prolonging their lifetimes.

Cruise speeds have been the same for almost all commercial airliners since the 1960s. RJs typically cruised slightly slower than their larger brethren. If the next generation of RJs cruised as fast as or slightly faster than mainline jets, a large advantage would be recognized when combined with the quick climb technique. Current state of the art includes winglets to increase effective aspect ratio of the wing, high bypass turbofans, vortex generators and laminar flow wing sections that all combine to reduce drag.

Descent in commercial aircraft is generally accomplished via flying a standard terminal arrival route (STAR) which is a series of waypoints and altitudes that aircraft follow to remain sequenced and separated on their way to a busy airport in all types of weather. An MIT led study using late night UPS flights into Louisville, KY helped develop the Continuous Descent Approach which saves fuel, time and reduces noise on the terminal approach phase of flight, which often begins over 80nm away. [8] Applying these techniques to an RJ that may even be capable of steeper descents simply means that the high speed cruise portion of flight can last longer and save even more fuel and time overall.

Finally, landing must meet other criteria much like the takeoff had to. Having a low approach speed, strong brakes, thrust reversers (very important on wet, slushy or icy runways as wheel brakes have reduced friction) and aerodynamic devices are all required to make the RJ land in a short distance consistently and safely. The latest in cockpit technology including heads up displays and velocity vector symbology will assist the pilots in using minimal power until the final portion of the approach and touching down exactly where they want to.

Even though flying is the glamorous part that everyone wants to talk about, no aircraft will fly unless it is serviced and repaired on a regular basis. An airplane that is a joy for pilots and passengers will be short lived with airline management if it consumes twice as much in maintenance costs as its contemporaries. Making it easy to work on is another very important aspect. Providing access panels, regardless of material used in construction of the aircraft is very important to maintenance personnel who may have limited time to get an aircraft turned around. Industry standards may apply here as most modern airliners have a 98% or higher dispatch reliability.

When one begins to think of RJs as actual airliners, possibilities for the restructuring of national routes begin to multiply. At this point, the advantages of point-to-point travel begin to make sense and airlines will have the freedom to place their fleet strategically at places that have the demand. If there are seasonal changes, the RJ can much more readily deal with the increase in demand, which may not be enough to warrant using a larger aircraft. It may become possible to fly from a low traffic airport such as Atlantic City, NJ directly to another low traffic airport such as Lansing, MI. How many people actually need to travel to certain city pairs will dictate how often the jets are used. In some cases it may be a once or twice weekly service. In others it may be multiple daily flights. Regardless, the longer segments of the new RJs will make it more economical to use even with a lower total passenger load.

But despite point-to-point being an intriguing idea for travelers, it is not likely that airlines will change their entire route structure overnight. It is therefore important that the new RJs can operate within the current hub and spoke system efficiently as well. This is perfectly feasible provided they are used for the segments that require them. For 65 people who need to travel 1000nm from a hub to an outlying city, they really are not concerned if their airplane is a Boeing 737-800, an Airbus A320 or a CRJ900. What matters to them is that they have practical options for departure and arrival times, that the aircraft is comfortable enough for the stage length and that the aircraft is safe. For pilots what matters is an aircraft that has enough performance to fly the assigned trips without undue effort, that its systems are intuitive and that it is safe. And of course airline managers want it to be inexpensive to purchase, operate and maintain. Having passengers, pilots and maintainers like it only helps to increase the overall value of the purchase.

The maturation of the RJ into an aircraft that is respected and loved by pilots, passengers and airlines is currently taking place. New procedures will have to be developed, new routes will emerge, and new pay scales will have to be developed as airlines integrate long haul RJs into their fleets of mainline jets. Pilots need not look at future RJs as almost-airliners or little airplanes. They will still carry passengers at a significant fraction of the speed of sound several miles above the earth. For those who enjoy flying but wish to spend more time at home with family, flying RJs can offer an attractive option to flying longer international routes. But crew pay must be fair in order to ensure the same caliber of pilot who is flying the Boeing 747-400 over the Pacific is flying the advanced RJ over the Midwest. Murphy’s Law does not care how big an airplane is and sharp individuals are required for all airframes. US Airways flight 1549 (Miracle On The Hudson) is perfect evidence of that.

In today’s economy it has become necessary to rethink everything. That includes how flights are planned, how aircraft are used, how we define what is major, what is national, and what is regional. But it should be remembered that classifications matter not, in the end only the efficient survive. 

As Bombardier CRJs line up at Philadelphia International, a Boeing 737 taxies past. Perhaps the future of commercial air travel lies in integration for maximum profit and passenger satisfaction.

Works Cited

1. Beyer, Morten & Agnew, Robert. “Morgan Stanley Conference-Regional Jet Update” http://dc228.4shared.com/doc/9I-TFT6r/preview.html

2. “Embraer 190 Specifications”: http://www.embraercommercialjets.com

3. “Delta Through The Decades.” http://www.delta.com/about_delta/corporate_information/delta_stats_facts/delta_through_decades/index.jsp Retrieved: Feb 6, 2010

4. Bachman, Justin. “Airlines Give Propellers Another Spin.” BusinessWeek, April 30 2008 http://www.msnbc.msn.com/id/24390211/page/2

5. Zibel, Alan. “Flight Delays Soar To 13 Year High.” The Washington Post, August 7, 2007 http://www.washingtonpost.com/wp-dyn/content/article/2007/08/07/AR2007080700583.html?tid=informbox

6. “JetBlue and Southwest Airlines Destination Maps”: http://www.jetblue.com/wherewejet/ 

http://www.southwest.com/travel_center/routemap_dyn.html?int=FOOTHOME_WHERE : Retrieved Feb 6, 2010

7. Friedrich, Jerry; McGregor, Daniel; Weigold, Douglas. “Quiet Climb System”. Aero, First quarter 2003: http://www.smartcockpit.com/pdf/flightops/flyingtechnique/25

8. Walton, Jim. “Continuous Descent Arrivals.” 2005 Boeing Performance & Flight Operations Engineering Conference: http://www.smartcockpit.com/pdf/flightops/aerodynamics/5  


Light Jet Thrust Formula (#1)

For determining minimum static thrust required for non flat-rated turbofan and turbojet engines.

As temperature and or altitude increases, the amount of thrust produced by an engine will subsequently decrease. On larger turbines, flat rating is available to supply rated thrust above sea level and ISA. For smaller engines that may not have the thermodynamic capacity to flat rate, the designer must take into account the degradation of thrust. Failure to do so will result in unsatisfactory performance, inability to meet expected book values and possible payload restrictions.

The simple fact is as engines get larger, the weight of passengers becomes less of a fraction of the static thrust. Those who wish to build cabin class transportation aircraft are probably going to find their optimum engine (or engines) in the 2000lb thrust class or larger. Use of small jet engines under 500lbs thrust is therefore likely to be associated with very light multi engine craft, experimental/test vehicles and unmanned aircraft. Development of small jet engines however, should not be deterred by market analysis that may not fully understand the different potential uses beyond business and high-end general aviation. Too often advances are cut short by the opposite nature of engineering and profit, which work on completely different time scales.

The urge to use as small an engine as possible should be avoided when building light jet aircraft for the simple fact that passengers cannot be reduced in weight. Smaller engines can operate closer to their optimal TSFC at cruise altitude, but like all of aviation, there has to be a tradeoff for other flight envelopes. Therefore if one wishes to have a 4 place jet with a 200 lb allowance for each passenger, using twin 400 lb thrust engines for a total of 800lbs will provide a 1:1 thrust to payload ratio. This number will gradually change as airframe, fuel, avionics and interior is added. By the time a weight of say 3200lbs is reached (1600lbs empty weight, 800lbs payload and 800lbs fuel…very minimal levels), the total thrust to weight has declined to 4:1. This figure is at sea level on a standard day. Take the same aircraft to Colorado Springs at 65 degrees and the thrust to weight balloons to 5:1. Each pound of thrust has to push over 5lbs of aircraft, thus increasing takeoff distance, time to climb, and fuel consumed over a given stage. Humidity and air pressure may also conspire to rob the engines of performance.

The following formula is very simple as it is meant to be a rough estimate to warn a designer of low thrust levels at a nominal altitude of 6,000 ft MSL and 65 degrees. This altitude also equates to approximately 0.785 with respect to air density at SL ISA. But we’re conservative and/or lazy so the easy to remember 0.75 ratio is the rule of thumb to remember. The thrust lapse rate is based on a low bypass turbofan engine (less than 3:1) and will vary with fan size, nozzle design and turbine temperature. Your engine will vary so consult the appropriate manufacturer tables when advancing beyond the initial planning stage of your design. By using a reduced thrust computation when setting gross weight, unpleasant surprises can be avoided while in the flight test stage.


Thrust to weight ratio 1 = A/C weight divided by thrust at sea level, ISA.

Thrust to weight ratio 2 = A/C weight divided by thrust at roughly 6000 feet MSL, ISA.

If you really want to scare yourself, multiply 0.75 by the static thrust of your selected engines (assuming they are not flat rated) and see what number you get!