The Long And Cautious Road To The Sky

There is an old saying that if you wish to make a small fortune in aviation, start with a large one. Aviation is littered with dreams that have turned into expensive and demoralizing nightmares. Many of the concepts made sense aerodynamically while others would have filled a market need and yet still did not succeed. This axiom holds true even for the current electric air taxi push. Even with the immense amounts of money being poured into this venture, that is no guarantee that it will flourish. With so many new variables at play, a conservative approach to goals will serve to help reduce the risk of failure. Here are a few aviation ventures that suffered delays, redesigns, bankruptcies and buyouts despite optimism, sound designs and in some cases, very high valuations.

Skycar

Dr. Paul Moller has worked on perfecting a vertical takeoff and landing vehicle named the Skycar for five decades. Intended to lift off from ones driveway, it was expected to be able to fly in excess of 300mph at altitudes above 30,000 feet using renewable ethanol as fuel. In 2003 it finally achieved tethered hovering flight.

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Moller Skycar in a tethered hover.

To date his company has spent in excess of $100,000,000 and despite many studies, patents and tests, the design has not progressed to even a pre-production phase. Part of this is due to the amount of capital required to move to the next stage of development. Despite the design’s fit with the current air taxi craze, future investments are unlikely due to the company’s long string of excessive delays and lack of progress.

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Dr. Paul Moller and aircraft he designed, (left to right) Neuera 200, Skycar M400 and Skycar M400X.

 

 

Eclipse 500/550

Eclipse Aviation was founded by former Symantec CEO Vern Raburn. His vision was a major part of a larger movement to create low-cost private jets that thousands of people would be able own and operate.

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Eclipse 500 aircraft in the colors of the defunct DayJet air taxi service.

The company touted several radical new construction techniques and features that would reduce the production cost of their Eclipse 500. Development was initially rapid until a significant problem with the new engines came to light. Without these specific engines, the aircraft would not meet its performance guarantees. Other issues with contractors, controversy surrounding FAA certification and financial issues led to bankruptcy in 2008, less than a year after delivering their first aircraft. It is the biggest financial failure in the entire history of general aviation. The company’s assets have since been acquired by several successive organizations that continue to construct and support the improved Eclipse 550 aircraft.

AW609 Tiltrotor

 

The Bell/Agusta (now AgustaWestland) 609 is a civilian answer to the military V-22 aircraft. As a tiltrotor, it featured the vertical takeoff and landing capability of a helicopter while retaining the speed of a conventional airplane. With the program beginning in 1996, testing and development proceeded at a cautious rate leading to a first flight in 2003.

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AW609 demonstrating its vertical takeoff and landing capability.

In the interim, the prototypes have continued flight testing and building the flight hours necessary to achieve FAA certification. Due to the craft’s unique nature, FAA certification categories had to be modified and in some cases rewritten, adding to the delays. An additional setback was the inflight disintegration of the second prototype resulting in the loss of both test pilots in 2015. The accident was attributed to the flight control software amplifying pilot control inputs beyond safe limits during a high-speed test dive.

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AW609 in horizontal flight mode.

 

Icon A5

Icon Aircraft was founded in 2006 with the intent of building light-sport aircraft that would appeal to the general public.

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Icon A5 in flight.

Two years after its founding, the company had a prototype recreational amphibian in the air. However, the realities of production and certification delayed the rollout of the first production model until 2014. The first delivery to a customer was in 2015 followed by an extensive pause while they redesigned their production facilities and methods.

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New Icon production facility in Vacaville, CA.

They are currently building aircraft at a rate less than a quarter of what was anticipated.

 

Optimism And Aerodynamics

I could list dozens of other civilian projects including the VisionAire Vantage, Adam 700, Piper Altaire, Diamond D-Jet, Aerion SBJ, LearFan, QSST, and Carter PAV. But that would amount to belaboring the obvious. By now, we were supposed to have access to supersonic business jets, personal air vehicles that fly themselves and private jets that cost a tenth of what some current models do. But despite very intelligent people working on the problem and literally billions of dollars invested, none of that has panned out as planned. There is nothing wrong with optimism, but history has shown aviation startups that hope and motivation must be balanced with data and demand.

All flying vehicles consume fuel and money, sometimes not in that order. The very nature of being suspended delicately in the third dimension demand a level of redundancy and safety that is not required while traveling on the ground. If your car has a problem you can simply coast to the side the road and wait for assistance. If your aircraft has a problem you have no choice but to get back on the ground. How soon and in what condition depends on the nature of the emergency. Designers must proceed with this sobering fact in mind at all times.

When a company announces its plans for a new product, there are significant differences between software and hardware. Rushing an application or software to market has never directly been responsible for someone’s death. In aviation, a rush to market can and has killed people with amazing predictability. In the tech world, three years is an eternity. For a flying vehicle that features several unproven methods, that same three year span is nearly childlike in its optimism.

Testing For Safety

Anyone who knows anything about science understands that you should only change one variable at a time and measure the effect. Smart aircraft designers similarly never test a new type of engine with a new type of airplane at the same time. In this situation, it appears that the designer is attempting to not only combine a new airframe and powerplant, but a new control system and new airspace integration/automation. Any single one of these challenges would be enough for a company to tackle. All of them combined may prove to be a minefield of delays. For a look at what has to be done from a purely operational standpoint, not considering any business model issues, one must consider all the constituent components of the vehicle.

This new vehicle will have a major hurdle in its hybrid electric propulsion system. While hybrid rotor systems have been proven on aircraft as far back as 1959 with the Fairey Rotodyne and more recently with the Airbus X3, electric helicopters and eVTOL vehicles are still in their infancy. This means that it will take more time and money to validate and certify an airworthy configuration since there are so many unknowns. The good news is that future aircraft that use the same type of propulsion system will not have to rewrite the rule book.

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Fairey Rotodyne

Flight controls are very important for the obvious reasons. So far, all illustrations of the most talked about air taxi concepts show no conventional flight control surfaces such as elevators and ailerons. Considering that during cruise flight the vertical lift rotors will be stowed and cannot be used for attitude control, it will be interesting to see exactly how they plan to control the vehicle. Again, seeing that whatever method they take will be unconventional, prepare for extended test periods while all of the proper redundancy is built into it.

The airframe itself may seem like the easiest component to build, but there are plenty of hurdles to clear with this piece of the puzzle as well. Vibration and flutter testing, occupant safety, crashworthiness and dynamic stability are all things that must be tested before certification is issued. In addition, the retractable vertical lift rotors will most likely be considered as separate systems. Therefore, the designer must prove that a failure of any unit to retract or deploy will not cause loss of the vehicle. 

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Lightning strike testing on a mockup.

It is very clear even from this relatively short list of design issues that assuming a short gestation period is not prudent. Throwing more money at a problem doesn’t always solve them. A history of being able to rapidly scale in other industries does not translate to building hundreds of flying vehicles in a few years, especially when the company has no prior experience in aviation. This is worthwhile technology that deserves the time and attention required to properly test it before rolling it out to the public.

Conclusion

Perhaps a lot of details are purposely being kept from the public. Maybe the propulsion systems, automation systems, flight control laws, and vehicle aerodynamics are all being tested concurrently. Maybe there’s more than what the media and general public has been shown in articles and press releases. Maybe there’s a secret squadron of prototype air taxis flying out of Edwards Air Force Base. Maybe there are special certification processes in the works that will allow fast-tracking of an entirely new class and category of aircraft. If this is the case, then all the words I have written mean absolutely nothing. If not, then we are all in for a longer wait than expected.

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Patience and precision is required at every stage of design and testing.

Perhaps the reason I’m so preoccupied with caution is because I don’t want an avoidable incident to occur where the public loses faith. Investors tend to be apprehensive about putting money into a venture when one of its test vehicles is on the news at the bottom of a smoking hole. Accidents may happen even with every precaution observed and meticulous planning. But to entice them by through optimistic blindness is inexcusable. There is no critical rush to get flying cars operational. We as humans have existed on this planet for a very long time without them. Adding a few years of development to ensure that they are released in a responsible and useful manner makes all the sense in the world.

 

 

Wing Loading: More Important Than You Think

In 2011 I said “We’ll continue soon with more on wing loading…”.

It’s 2015. I think you can see that I get distracted easily and persistently. In any case, the information I’m about to present to you has not and will not change. Use it to help understand why your airplane does what it does when you leave the boring confines of straight and level, or use it to help you design that superplane you’ve always wanted to build. Off we go into the underrated world of wing loading!

Wing loading and power/thrust loading are the two most telling specifications about an airplane. Most pilots go right to horsepower and start swelling with pride when numbers north of 300hp start appearing. The assumption is that a lot of horsepower equates to a lot of performance. This is a huge misconception. The total horsepower of an airplane is irrelevant unless you have another number to compare it with; the total weight. A 200hp plane that weighs 1500lbs is going to have way better acceleration than a 500hp airplane that grosses 8000lbs.

Likewise, wing size and thus the wing loading is critical for many of the aerodynamics qualities of a given airplane. A highly loaded wing doesn’t always equate to high speed. There is a delicate balance of wing loading and all performance parameters that matter to you. Absolute ceiling, stall speed, takeoff distance , landing distance, cruise speed, ride quality and turn radius are all tied to the wing loading of an aircraft. For our purposes, we’re going to ignore the effects of power/thrust loading and focus only on wing loading for the duration of this discussion. By pretending that adequate power is not an issue, we’ll have the ability to truly see the importance of the wing area to weight ratio.

ANGLE OF ATTACK

For any airplane to fly, there needs to be some positive angle between the wing and the relative wind. The higher the wing loading, the greater this angle of attack will be in 1G flight for the same wing at a particular speed (this is where the FAA safety advisory about heavy, clean and slow airplanes generating strong wakes comes from…more alpha = stronger vortex). One can think of wing loading as an energy budget for the wing. If the loading increases, level flight is going to cost more through either higher speed or increased angle of attack. No matter what, you’ve go to pay for weight.

 

HIGH SPEED

Conventional wisdom says that if I want to fly fast, I should have a small wing. This is not completely true; if I wish to fly fast, I need to have low drag. Wing size is related to speed only through skin friction drag, induced drag and in some cases, wave drag. The size of the wing itself is not the sole determining factor. That being said, it takes some creative airfoil design to allow for a big wing that does not produce large amounts of drag. The aft-loaded airfoils that first saw widespread use on the Boeing 757 and 767 series are examples of this type of design. Even with a relatively low sweep compared to earlier models, they produced far less drag per pound of aircraft even at high subsonic speeds.

 

SLOW SPEED

An assortment of flaps and slats allow this 757-200 to land at very slow speeds for an aircraft of its size.

An assortment of flaps and slats allow this 757-200 to land at very slow speeds for an aircraft of its size.

 

Conventional wisdom also says that if I want to fly slow, I should have a large wing. If the wing does not change shape, this is true. However, a small wing can become a large wing through the use of flaps, slats and slots. These devices change the camber and in the case of slotted flaps, the area of the original wing. Since nothing is free on this planet, the extra lift produced also produces extra drag. During takeoff and landing, the objective is to fly as slow as practical to reduce the amount of runway needed. For this reason, the flap/slat/slot solution is used on almost all airplanes to varying degrees. The best of both worlds is attained at the cost of complexity and cost.

 

RIDE

A small wing is advantageous for ride comfort and structural integrity at high speeds. With the increase in wing loading, gusts and turbulence have less impact on the aircraft due to its higher aerodynamic inertia. The best comparison is a ship cutting through 8 foot waves versus a little canoe being tossed around the same ocean. For an aircraft flying in turbulent regions or at high dynamic pressures (50 feet and the speed o’ heat), alleviating aerodynamic stress is a matter of keeping the plane in one piece. There may be reason to maintain a fairly large wing but create the same lift-curve effects through significant sweeping. An example of this requirement would include low level strike aircraft with a secondary air-to-air mission.

 

TURNING

Aircraft A vs Aircraft B and the difference in turning drag.

Aircraft A vs Aircraft B and the difference in turning drag.

Wing loading is very important during any type of maneuvering but for our discussion, we’ll focus on level turns. The moment an aircraft rolls into a turn, the angle of attack is increased. This creates more drag that has to be overcome with additional power/thrust. Obviously, the lower the angle of attack, the lower the turning drag generated. A low wing loading has the effect of reducing the induced and profile drag created during a turn. Using the chart, compare Aircraft A that requires 2 degrees alpha for level flight compared with Aircraft B that requires 5 degrees. As they both roll into a hard turn that demands an extra 10 degrees of alpha, Aircraft A will be turning with a total of 12 degrees alpha while Aircraft B will be at 15 degrees. It is quite clear which airplane will have the better turn performance with regards to drag.

 

CEILING

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 oft repeated maxim is that a long thin wing is best for flying at high altitude. The real variable at play here is wing loading, not necessarily aspect ratio. If a given aircraft requires a large angle of attack in order to generate the necessary lift coefficient, excessive amounts of induced drag will result regardless of the speed. An aircraft designed for low indicated airspeeds will probably have a high aspect ratio wing with minimal sweep while one designed for higher speeds will have some level of sweep and a much lower aspect ratio. Either way, the wing has to have a low enough loading to allow for a reasonable angle of attack.

 

INERTIA

For visualization, imagine a dumbell like you’d find in a gym. Pick up a 5lb weight, hold it up about 2 feet over your other hand and let it go. Chances are you can probably catch it quite easily. If you attempt the same feat with a 25lb weight, the only way to catch it is to let your hand drop down to eliminate the shock of it impacting your hand, if you can hold onto it at all. This is the same inertia effect that wings experience as their loading goes up. For this reason wing loading can be thought of as “inertia loading”. You may experience some of these effects in certain aircraft with highly loaded wings in the form of post-stall gyrations or uncommanded rolls.

 

GA DESIGN

A lot of modern GA wings can thank the P-51 for leading the way for laminar flow airfoil sections and high speed planforms.

A lot of modern GA wings can thank the P-51 for leading the way for laminar flow airfoil sections and high speed planforms.

Modern high-speed general aviation designs often have partial laminar flow, medium to high aspect ratio wings. These wings are very efficient at low angles of attack, resulting in low drag at high cruise speeds. They do have the drawback of less than satisfactory behavior at high angles of attack and post-stall. Usually these foibles are corrected with the use of leading-edge cuffs, vortex generators, and washout. In the case that the problems cannot be rectified totally, the aircraft will have limitations such as “spins prohibited” in the manual. If the aircraft encounters turbulence or maneuvers too hard, the stall margin can easily be exceeded resulting in a departure from controlled flight. How bad this gets depends on the aircraft’s design, center of gravity location, altitude and true airspeed at the time.

 

TRANSPORT DESIGN

Aft loaded “supercritical” airfoils allow commercial aircraft to use mildly swept wings without sacrificing subsonic performance. The airfoils delay shockwave formation on the upper surfaces, effectively raising the critical Mach number to a higher value. For this reason, a large wing is feasible but not often used in practice. Ride comfort and the fact that massive flaps are industry standard all point towards ubiquitous use of a highly loaded, high aspect ratio wing.

If a designer wishes to take advantage of smaller airports, utilize lower V1 and Vref values, and produce less drag at high altitudes (eliminate step climbs), a larger wing may be something to consider. The current trend is to utilize effective aspect ratio devices such as winglets or sharklets. These artificially raise the aspect ratio without actually adding any span to the wing. The result is that aerodynamic performance is improved markedly over much of the envelope, although not by the same degree that a larger wing would provide.

 

FIGHTER/TACTICAL DESIGN

F-4, F-22 and F-15E all with highly swept, low aspect ratio wings.

F-4, F-22 and F-15E all with highly swept, low aspect ratio wings.

The advent of thrust vectoring has made nose-pointing an accurate exercise even down to zero airspeed. This does not negate the need for a well designed wing since not all maneuvers will be able to take advantage of vectoring. For aircraft with a low level strike mission, encountering turbulence at high speed can seriously fatigue the crew and at worst, damage the airframe. A high wing loading, significant sweep or a combination may be required to keep the stresses within limits. For all other regimes of combat, a large wing is desirable for maneuverability, payload carriage, and high altitude performance. Due to the forces sustained during combat maneuvering, a larger wing in this case entails additional chord rather than more span.

 

This is just a short highlight of the importance that wing loading carries (punintentional) for all airplanes. I hope you’re more aware of how critical this number is for everything that you do in the air.

Designing A Dragon: Part 1

Since I’ve been working on a civilian supersonic aircraft since around 1995, I’ve amassed a large amount of data. Notice, when I say “working on”, I mean studying and testing on my free time at little to no cost. The project was originally nicknamed Dragon Wagon and was supposed to be a fighter jet that couldn’t fight. If that doesn’t make any sense, allow me to provide a little backstory.

Back in 1995, I saw the BD-10 fly at the Sussex Airshow in upstate NJ. Many things stood out to me, with the most important being the noise, the excess power and the ownership. The GE J-85 engine made prodigious amounts of noise, enough to wake neighbors three states away. The airplane was able to stand on its tail and climb nearly vertical. And most importantly, it was civilian built and civilian owned (meaning that in an alternate universe even I could own one). Needless to say, the BD-10 suffered developmental problems and never achieved the success that was hoped for. But the concept had set off an unstoppable juggernaut in my mind. Build an inexpensive non-fighting fighter jet for civilian pilots to fly and enjoy.

Owning a private fighter with 4th generation performance was and still is restricted by cost and legal issues. The massive expense of fighter jets is due to the fact that they are no longer just airplanes with guns, but weapons systems with wings. This is why there are so many WWII fighters flying around today and almost no 3rd or 4th generation fighters in private hands, even though Davis-Monthan AFB is littered with them. Remove the need to carry 350lb missiles, a Vulcan cannon, AESA radar, countermeasures and armor and suddenly a fighter jet loses a lot of weight and cost. Plus there would be no problem with demilitarizing systems or the State Department having a fit that a civilian is flying an airplane with hardpoints.

Armed (punintentional) with this knowledge, I set about designing a fighter that couldn’t fight. The first few design sketches looked like fairly conventional trainer jets, the most promising variant resembling the T-45. A low thrust to weight ratio was a goal (at most 1.4 lbs/lb st) with speed expected to be high subsonic and overall performance very respectable by any standards. However it wasn’t really what I wanted to create. All this changed once I happened upon an article in Aircraft Illustrated on the Facetmobile. It was a low aspect ratio lifting body with extremely light loading, didn’t stall, couldn’t spin and had a large AOA range. Plus it looked cool. Starting to research low aspect ratio and vortex lift, I abandoned my old designs and started to work on blended body designs.

The aircraft went from T-45 lookalike to a little arrow shaped airplane. The blended fuselage was designed around the occupants and the engine. The wing planform was at first a 70 degree single delta and later a double delta with a 70 degree leading section and a 50 degree aft section. Yaw control was achieved through a single large vertical stabilizer while pitch and roll is controlled with elevons on the trailing edge of the aft wing. The high sweep gave me the mathematical courage to increase the maximum speed from Mach 0.95 to Mach 1.4. At those sweep angles, the wing would be in subsonic flow throughout the entire speed range. I had no idea how to do a flutter analysis but did know enough that a higher aspect ratio on control surfaces would raise the minimum flutter speed if balanced properly (again, something to worry about in the distant future).

As for a powerplant, turbojet engines have small frontal area but also high fuel consumption (on the order of 1.0 TSFC for most power settings). Additionally, an afterburner was also out of the question. I love the noise but the fuel burn would have been far too high even with intermittent use. Thus a low bypass turbofan of lightweight and hight thrust was the only option. Engine selection should actually be credited to Cessna. I sent off for one of those deluxe information booklets on the Citationjet when it first hit the market (remember, this was before the internet and PDF files so waiting a week to get a big envelope from Wichita was a huge deal for a 14year old). It used a pair of FJ44-1 turbofans, an extremely compact, lightweight and efficient engine. A rough estimate in my mind quite literally went like this: “If I used only one engine, the Citationjet fuel flow numbers would be cut in half and with a smaller frontal area thanks to tandem seating, that half would be closer to 40%.” Later calculations with refined drag data showed that the 40% guess was pretty accurate.

I’ll stop there for now. Next time I’ll include original sketches and discuss cruise performance. In case you need motivation to read it, the initial performance numbers called for a stall at 50 knots, subsonic cruise of Mach 0.98, supersonic cruise limit of Mach 1.4, a ceiling of 50,000 ft and a climb rate of 30,000fpm.

See ya next time!

Delta Arrow Wings: Advantages For Civil Supersonic Flight

There is more than one way to skin a cat; this cat happens to be supersonic drag. There are many theories on how to achieve efficient civil supersonic flight, each with distinct advantages. One method that I have always favored is the simple and reliable delta wing. No complex construction, boundary layer control systems, or reliance on laminar flow. Just a big triangle without the need for high lift devices. There are tradeoffs but if your goal is to go supersonic without complexity, a good place to start is with the delta.

It is a fact that sweeping a wing delays supersonic drag rise and raises the critical mach number. It is also a fact that sweeping a wing with no special treatments will cause all kinds of hellacious stability problems at low speeds. While I could go into at least 4 pages of descriptions, case studies and NACA test data, I’m trying to write less like a mad scientist this year. So I’ll limit the following formulas to the basics in trying to get the point across.

An object moving faster than the speed of sound in air will produce a shockwave. The angle created by this shock cone (it’s a three dimensional wave) is dependent on the speed of the object. The faster the object, the smaller the angle created by the shock cone. With low supersonic Mach numbers, it is entirely possible to sweep a wing enough to contain it within the subsonic wake of the cone. The formula for determining the half-angle is:

1 / Mach#  = sin * cone angle

As stated, this is the half angle formula. To get what the entire cone would look like if drawn whole and not bisected, simply multiply the result by 2. For example, say that my aircraft is going Mach 1.3 and the wing is swept 54 degrees. With a cone half-angle of  50.3 degrees, my wing is definitely within the confines of the wake. This has a significant effect on reducing wave drag.

In addition to reducing wave drag, critical Mach number can also be reduced from sweep. Simply put, air is tricked into thinking that the wing has a longer chord and accelerates over the top of the wing at a slower rate. The formula for this effect is:

Vmach (cos * sweep angle) = Effective Vmach

This formula determines the Mach velocity over the wing when sweep is introduced. This may or may not be lower than the critical Mach number for that airfoil section. To determine what the new critical Mach number is when sweep is accounted for, the following formula is appropriate:

Mcrit / cos * sweep angle = Effective Mcrit

In effect, a highly swept wing can delay the critical Mach number to supersonic velocities. A straight wing with a relatively low Mcrit of Mach 0.7 would have an effective Mcrit of Mach 1.19 when swept to 54 degrees. This holds a lot of promise for reducing drag in the low supersonic speed range.

It’s not all free soda and candy for the swept wing. As mentioned before, there are very serious effects to consider aerodynamically. Swept wings stall at the tips first, creating unstable rolling and pitching moments during the stall. In other situations, pitch-up may occur when the horizontal stabilizer gets caught in the wing’s flowfield. Slow speed handling is degraded by spanwise flow, the same phenomenon that helps to reduce drag at higher speeds. Maximum lift coefficient for a given angle of attack is also reduced, leading to sometimes extreme pitch attitudes at slow speeds. There are other issues but these are the most critical to control and stability.

A straight wing that is swept may be troublesome, but delta wings have distinct advantages  that make them attractive for our purposes. The double delta design is a derivative that is configurable to nearly any range of speeds. The two main variants of the double delta are the “shovel”, with the low sweep segment in front, and the “arrow” with the high sweep segment in front. In either configuration, the forward segment produces low-pressure vortices that drift over the aft segment and delay the stall to a much higher angle of attack (non-linear lift). If properly balanced, a double delta will display highly favorable stability characteristics as well.  For this discussion, we will deal with the delta arrow variant since it is customized for supersonic speeds.

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LoFlyte test vehicle at USAF Museum in Dayton, OH. Designed for hypersonic waveriding flight, this basic design is applicable to low supersonic flight as well.

A delta arrow consists of a highly swept leading edge section and a less severely swept aft wing section. This ensures that the leading edge of the wing remains behind the shockwave at moderate supersonic speeds while retaining adequate lift reserves for slow speed operations. As mentioned earlier, swept wings have a reduced lift curve slope with the degree of sweep directly correlated to the reduction (provided airfoil section and thickness remain constant). This disadvantage becomes an advantage in supersonic flight. All aircraft experience a rearward shift in the center of lift which reduces maneuverability and increases drag. Rather than rely on large control surface deflections to correct this situation, the delta arrow’s forward wing segment provides the lift required with minimal drag.

Airfoil thickness is a strong modifier of total drag at high subsonic speeds. A thin airfoil has far less drag than a thick one, but in trade, it has a lower lift curve slope, and less space for structure and fuel. A way around this is to sweep a moderately thick airfoil so that the effective thickness is reduced while retaining actual space inside for the structure and fuel. This phenomenon is more pronounced with large amounts of sweep, so the volume inside the forward segment of a delta arrow is quite extensive.

Maneuvrability is closely tied to the wing loading and center of gravity location. A heavily loaded wing will have a larger angle of attack in 1G flight, reducing the amount of lift available for aggressive maneuvering, regardless of aspect ratio. A reduced angle of attack can be achieved by moving the CG aft but within certain limits. Locating the CG too far aft would render the aircraft uncontrollable, even with fly-by-wire. A forward CG will reduce the control response and increase static angle of attack, but enhance stability. Balance between the two extremes will be determined by the aircraft’s purpose and desired handling capabilities.

The reduction in lift coefficient for a given angle of attack is subject to the same cosine formula that was applied to the critical Mach number earlier. Therefore the formula is as follows:

Cl ( cos * sweep angle) = Effective Cl

Assume an aircraft requires a Cl of 0.2 to sustain level flight at a given speed. If the aft wing is swept at 54 degrees, the effective Cl would be 0.12, demanding the aircraft increase its angle of attack to create 0.2 Cl. An alternative is to reduce wing loading, reducing the required Cl and by association, angle of attack. This reduction in lift is beneficial for flying in turbulence as the reaction of the aircraft to disturbances will be markedly reduced. While some people may not consider turbulence reaction to be a reason to reduce Cl slope, those who fly low-level, high-speed profiles, especially over warm areas or near mountains may have differing viewpoints. Not all civilian designed turbine aircraft have to be business jets.

So what does this mean in plain English? It means that economical and safe civil supersonic flight is possible. The reason we have not been able to achieve this so far is that industry has been focused on improving efficiency of existing designs. To integrate these advantages, a radical departure in construction has to be undertaken. The wing must be blended with the fuselage to keep drag and weight to a minimum without sacrificing strength. Thrust to weight has to be increased to ensure adequate acceleration is available at high altitudes. Low wing loading will not only improve induced drag numbers while subsonic, but reduce the impact of sonic booms at higher speeds.

The aviation industry has been hearing for at least 15 years about proposed civil supersonic aircraft. In each case the designs were business jets. With the tumultuous world economy of the early 21st century, no builder or prospective buyer has seen fit to invest in such a jet for understandable reasons. To date, no one has proposed building a manned research aircraft of much smaller size to investigate actual performance, handling, environmental effects and integration with the current ATC environment. The cost of a purpose-built test aircraft would be far less than attempting to build a full size business jet requiring full Part 25 certification. Something to think about for the frugal mavericks among us.

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.

DykeDeltaOSH3

DykeDeltaOSH4

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.