Piston Propfan Proposal

Traditional Propellers

After a recent bout of studying piston engine aircraft performance, I’ve become convinced that advanced engines are not really necessary for higher cruise speeds. All evidence points towards propeller design as the missing link in light aircraft capabilities. Propellers for the most part have not changed since World War I. The most recent developments have involved scimitar shapes, swept tips and lightweight composite materials. However the basic issue of thrust diminishing with increasing speed has remained an issue. WWIScimitar

An aircraft with a fixed pitch propeller can be thought of as a car with only one gear. Fixed pitch props can be optimized for takeoff, cruise or a mixture of the two. The cruise prop will have lackluster takeoff and climb performance due to its higher pitch. The engine literally doesn’t have the power to spin the prop to its optimum speed at a static condition. Conversely, the takeoff or climb prop will have large amounts of thrust at low speed, but a limited top speed. The hybrid prop is a middle of the road compromise between the two.

High performance aircraft worAvengerHubk around this problem by varying the pitch of their blades to allow for maximum thrust at high RPM situations like takeoff, and maximum efficiency at lower RPM conditions like long-range cruise. This is the aeronautical equivalent of shifting gears in a manual transmission car. A system of flyweights and a governor allows the system to seek the pitch that maintains a selected RPM.

As an aircraft accelerates, all propellers make progressively less thrust, even though the engine is producing increasing amounts of power as the prop unloads. This situation has been considered unavoidable, even though it is very possible that there is a way around this dilemma. In theory, there is a way to achieve this, that is not complex and fairly easy to manufacture. To solve this problem, we will look to high bypass turbofan engines.

 

Turbofan Advantages

In the quest for speed, the piston engine has long been abandoned for turbine engines. With an extremely high power to weight ratio, long times between overhauls and superb performance at high altitudes, any turbine engine can be viewed as superior to a piston engine with regards to speed. Where they do no fare as well is in terms of fuel economy and purchase cost. While a new mid-size piston engine for aircraft will be in the $30,000 range, a small turbine engine can easily run 10 times that price.

Fuel consumption is also high for a pure jet, thus almost all new engines are either turboprop or turbofan variants. In the turboprop, a propeller is driven by the turbine via a reduction gearbox. In a turbofan, a large ducted fan is driven by the turbine. While aerodynamically speaking, they are very similar in operation, there are specific features in design that allow for the turbofan to perform at high subsonic speeds with incredible efficiency. The fact that the massive fan does not require variable pitch, yet produces most of the engine’s thrust from zero airspeed all the way up to Mach 0.90 indicates that their design should be studied in further detail for our purposes.CF6

A close look shows that a fan can be considered a nearly solid disk with slots for air to be pulled through (looking at a turbofan from the front, it is very hard to see behind the blades compared with the ease of looking past the 2 or 3 blades of a propeller). These slots are simply the spaces between the blades and vary based on the blade chord. The blades themselves can range in shape from simple twisted polygons to scimitar shaped with serrations for shockwave control. Blade twist is markedly more severe than the twist featured on a propeller due to the larger operational speed range. The cross-section is normally a circular-arc airfoil optimized for supersonic flow. These factors are important in creating a propeller derivative.

Propfan Ver 2.0?

Compare the fan on a turbofan engine with a traditional propeller. Whereas the fan may have over 30 blades, a propeller will have only 2 or 3. If properly balanced, the fan will operate with far less vibration than the propeller. It will also move a greater mass flow per second due provided that adequate horsepower is available to spin it. A propeller will have a larger diameter for a given thrust level than a similar turbofan. This allows a slower rotational speed, keeping tip speeds subsonic and providing higher propulsive efficiency. Supersonic tip speeds are not a concern for turbofans due to the duct eliminating tip losses. Finally, a fan can recover ram pressure as forward speed increases, whereas a propeller will not.

If we assume that our propeller will be a bolt-on replacement for standard 2 and 3 blade props, we cannot duct it. It also must not exceed the diameter or weight of the original prop and be simple to maintain. With these constraints in mind, a 6 to 8 blade design will provide a compromise between static thrust, ram recovery, and low enough noise without the issues of balancing that arise with increasing numbers of blades.

Starting with straight blades of roughly equal tip and root chord, we can introduce severe twist to allow the root to operate un-stalled at very high forward velocities at a rotational speed of around 2500 RPM, which is an average operating range for a piston engine. To ensure that noise is kept to acceptable levels, the blades must be curved and swept to reduce diameter without a commensurate loss of blade surface area. The sweep ideally would begin around the midspan of the blade, rather than near the tip as is typically done.

Those who remember the propfan studies of the late 1980s may make the connection that the aforementioned propeller sounds a lot like the UDF demonstrator. Aesthetically this is true but there are several significant differences. A propfan has much higher blade loading, variable pitch and is driven by a turbine. Our design has a relatively low loading, fixed pitch and is driven by a piston engine. The benefits are ease of construction, maintenance, and operation. For existing aircraft, the major advantages will be improved fuel consumption, faster climb rates, higher ceilings, lower noise and longer engine life. Obviously, cleansheet designs will have to be conceived in order to take full advantage of the possibilities.

Concerns

A potential issue with the use of these propellers is the relatively low RPM at takeoff. This reduces the amount of power that the engine can generate. Luckily, most existing aircraft have a narrow enough speed range that plenty of thrust can still be created even at reduced horsepower levels. As designs emerge that have a speed range greater than roughly 300mph, more work will have to be done on optimizing both ends of the speed spectrum.

With normally aspirated engines, a ram effect from the inner section of the propeller will delay loss of power at higher altitudes. Should forced induction through supercharging or turbocharging be used, care should be taken to ensure that dangerously high manifold pressures are not produced, especially at low RPM settings. The use of electronic ignition with variable spark timing is recommended to ensure that the engine remains out of detonation range at all times.

Structural integrity of the blades will be complicated by the compound curvatures. Materials that are resistant to impact, vibration and torsional stress are required for adequate durability and resistance to flight loads. Use of titanium or single crystal materials would provide the required strength at acceptable weights, but at the detriment of increased cost.

Conclusion

All of this is theoretical and has not been proven other than basic research and back-of-the-napkin sketches. Considering that experimental propfans were able to achieve a 30% reduction in fuel consumption over existing turbofans, the concept does hold merit. Our design may provide a similar boost to existing piston aircraft, but only if the thrust per horsepower ratio is improved over traditional propellers. Testing will begin with experimenting via computer simulations for the best designs and progress to physical models. Comparing our design to traditional propellers will indicate the potential efficiency gains. From there more complete analysis using the horsepower required and drag force of several common general aviation aircraft can be completed.

Supersonic Engines

(This is not meant to be an exhaustive text on gas turbine theory and application, plus I’m trying to refrain from being my usual aero-nerd self. Some things are not mentioned like combustion chamber design and the varieties of turbine one could opt for. I know! This is meant to focus on the really critical parts of cobbling together a supercruise engine.)

 

An adequate powerplant for supersonic flight is a massive sticking point for anyone hoping to develop a civilian transport. Efficiency at high speed without excessive fuel consumption is a balancing act that becomes more difficult the less one is able to spend on exotic materials. For a civilian designer faced with a budget that resonates more with FBO than DOD, we should explore what is required with regards to creating a supersonic powerplant for non-military aircraft.

The most important thing for any aircraft flying faster than the Mach is to have an exhaust speed which is equal to (in actuality, slightly faster) than the true airspeed. The drawback is very high noise levels from exhaust shearing (like I care) and very high fuel consumption for a given thrust setting (this I really do care about). The first solution most people would consider is to increase the bypass ratio. The complicated reality is that there is not a single bypass ratio that is optimal for all phases of flight, especially for an aircraft with a 750+ knot speed range. What may help at Mach 0.60 will probably hurt at Mach 1.15. Furthermore, by increasing the fan size or fan rotational speed, the core of the engine has to work harder and either give up thrust or increase turbine temperatures to compensate. Any increase in fan diameter has to be weighed against these factors.

We can rule out large bypass ratios of greater than 5, such as those used for modern airliners. The fan diameter is too large and creates too much frontal area drag. In addition, the massive volume of cool exhaust flow is not able to move fast enough to push the aircraft to supersonic speeds. Theoretically one could take a large diameter fan and spin it faster (many large civilian turbofan engines spin their fans at speeds below 3000RPM) but this would require a lot of extra energy from the core. On the other hand, a military style bypass ratio of 0.2 to 0.8 is not enough to produce the required TSFC for a civilian aircraft that cannot refuel in-flight or carry external tanks. Thus somewhere between a bypass ratio of 1.0 and 5.0 is the optimal choice for our engine.

Way too wide for our purposes, but perfect for the C-17.

Way too wide for our purposes, but perfect for the C-17.

On the front end, the fan pressure ratio affects the specific thrust and indirectly, speed of the air through the engine. Specific thrust is the thrust divided by the inlet airflow in pounds per second. Low bypass engines tend to have very high specific thrust values while large high bypass turbofans have a very low specific thrust. Civilian turbofans usually use one large fan whereas high performance military turbofans typically use 3 or more fan stages for this exact reason (as a reference, the F100-PW-229 has a specific thrust of 71.77, the F101-GE-102 ranks at 48.84 and the GE90-B4 comes in at only 28.78). The now “hardened” bypass air is able to move through the engine with enough energy (combined with the core flow) to provide a choked nozzle condition at the exhaust orifice (Mach 1 flow at the narrowest point). The flow can then be exploited by a variable system aft of the choke point to accelerate the air to the required supersonic speed (I have simplified so much that it annoys me, but otherwise, this article would go on for days).

We also must mention overall pressure ratio, which is the amount of total compression achieved by the engine/inlet combination. A lower pressure ratio means fewer exotic materials required in the hot section, a lower engine weight, less extravagant methods of cooling and as a result, a lower manufacturing cost. On the downside, the thrust level is lower for a given engine as compared to the same engine with a higher pressure ratio. Another negative effect is that a lower overall pressure ratio raises TSFC for a given thrust setting. Depending on the aircraft budget and expected operating environment, trading extra fuel burn for lower initial cost may be acceptable. A word of note is that as Mach number increases, TSFC increases as well. This may be offset by ram pressure recovery (mentioned later) but it is important to know that the TSFC at Mach 0.80 is not going to be the same at Mach 1.3.

The intake setup is important even though strictly speaking it is a part of the airplane, not the engine. As an aircraft with the proper fan pressure ratio moves faster, it is able to delay thrust decay and in some cases, reverse the process thanks to ram recovery. But this is if and only if the intake is designed properly. At low supersonic velocities, a simple normal shock inlet is sufficient to allow adequate pressure recovery at the fan/compressor face. As velocities increase past roughly Mach 1.5, the losses mount exponentially and thrust will degrade accordingly. A multiple shock inlet can reduce these losses significantly, however there may be issues with shockwave placement at off-design speeds. In the quest for low cost and low weight, a fixed position normal shock inlet is probably the best choice for a civilian supersonic jet. If one wishes to engage in what the military terms “carefree handling”, then intake design must be given far more attention to ensure that certain flight conditions do not lead to disturbed flow. Turbulent air at the fan/compressor face can lead to surges and stalls of varying severity.

If it's good enough for the Viper, it should be good enough for us...fixed normal shock inlets are lightweight, inexpensive and have no moving parts.

If it’s good enough for the Viper, it should be good enough for us…fixed normal shock inlets are lightweight, inexpensive and have no moving parts.

Finally, the most important piece of the puzzle is the exhaust nozzle. Ideally, a jet engine exhausts air at ambient pressure to produce a stable column of thrust. A given engine can force air out at a higher pressure than ambient, but this flow will simply overexpand, collapse in upon its now low pressure core and possibly re-expand. This is very inefficient and can be hazardous to the aircraft’s operation. To allow the higher than ambient pressure flow to expand under control so that it’s energy is translated aft rather than radially, a divergent section of nozzle is required. Every angle made with respect to the convergent and divergent sections has a particular Mach number and pressure ratio associated with it. Knowing this, for an aircraft to have maximum efficiency across a wide range of Mach numbers, a variable convergent-divergent nozzle would be required. However, a variable exhaust nozzle is extremely complex to build and requires a system to activate it (oil or bleed air in most cases). A fixed nozzle will have far less efficiency but an attendant lower cost.

Controlled by bleed air, these F100-PW-220 nozzles are very complex.

To recap, we are in need of a low bypass turbofan probably between 1.0 and 5.0 with a high fan pressure ratio, moderate overall pressure ratio, adjustable exhaust and fixed inlet. Starting at the front of the engine, we can assume a bypass ratio of 2.5 for no other reason than it’s halfway (and back-of-the-napkin wit). With this we are assured of an acceptable frontal drag penalty, while still having a fan small enough to stage if required. To keep core requirements within a reasonable range, a specific thrust range of 50-70 allows us to move air fast enough without taxing the core too much. An added bonus is that multiple fan stages can eliminate the need for a low-pressure compressor altogether. At the rear of the engine, the exhaust nozzle should be adjustable to a certain extent. A dual-position nozzle may be the best alternative to a fully articulated iris nozzle when cost and complexity is considered.

As of now, there are several civilian engines that qualify with minimal modifications and many that could fit the bill with more extensive changes. In the interest of rapid development, low cost and minimal risk to the aircraft, a minimal-change option is the best choice for a civilian budget. The Williams FJ44-2 and Pratt & Whitney Canada JT15D are both contenders for small aircraft. The medium to large designs could be well served by the Rolls Royce Tay 611-8, Spey 511-8 or Pratt & Whitney JT8D with very few changes (certainly for less effort and expense than a cleansheet design). While some of these engines are no longer in production, there are enough examples to support a test program and even limited run manufacturing of aircraft.

This is a classic chicken vs egg issue. Engines will not be produced unless there is an airframe that requires them and airframes will not be built unless there is a reliable engine available to power it. Somebody has to blink first.

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.

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.

Tr1/.75=Tr2

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!