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.

The Case For Long Range Regional Jets

The Case For Long Range Regional Jets

A Practical Application

　

By Christopher Williams

2/6/2010

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