Supersonic Now

When the Concorde was gearing up to fight off environmentalists, Lockheed and Boeing were in the middle of trying to construct a supersonic transport for the United States. Due to budget issues and noise concerns, the projects both were abandoned and we’ve spent the rest of our lives tooling along at the universal atmospheric speed limit of Mach 0.80. While I have no problem with people lobbying against having to live with 15-30 booms per day (for those who live under jet routes, it would have been very disturbing), I do have an issue with the “oh well, forget it” mentality that plagues our society after the passing of regulations. We collectively said forget it, nobody needs to fly that fast because supersonic flight was banned via federal law.

Or was it?

FAR 91.817 states that:

(a) No person may operate a civil aircraft in the United States at a true flight Mach number greater than 1 except in compliance with conditions and limitations in an authorization to exceed Mach 1 issued to the operator under appendix B of this part.
(b) In addition, no person may operate a civil aircraft for which the maximum operating limit speed MM0exceeds a Mach number of 1, to or from an airport in the United States, unless—
(1) Information available to the flight crew includes flight limitations that ensure that flights entering or leaving the United States will not cause a sonic boom to reach the surface within the United States; and
(2) The operator complies with the flight limitations prescribed in paragraph (b)(1) of this section or complies with conditions and limitations in an authorization to exceed Mach 1 issued under appendix B of this part.

 

Now, I for one would like it if FAA regulations were written for regular people instead of law students, but it doesn’t take many mental cartwheels to understand this rule. Nowhere in the entire rule did it say that you could not fly past Mach 1 under any circumstance. Further more, subsection (b)(1) has given operators a stipulation that allows them to break Mach. What the FAA has stated is that supersonic flight that creates a sonic boom on the surface is prohibited. Technically, if one were able to fly supersonic without creating an audible boom, it is allowed. That means you can imitate Chuck Yeager all the way from TEB to DFW provided that your boom does not reach the surface.

If my decidedly non-legal interpretation is true, why have we as a nation stopped all development on civil supersonic flight? Other than the persistent efforts of Aerion Corporation and a few attempts by Gulfstream, Dassault and Lockheed Martin, there has been very little interest in going faster. I shudder to think that manufacturers were limiting their efforts based on misinterpretation of a rule (or maybe they had it right and it’s my misinterpretation). What is more likely is that faster speeds like Mach 2.0 and Mach 3.0 were what designers wanted to achieve. Engine efficiency goes way up due to ram compression and range can actually be improved with an increase in speed (to a certain limit). Going that fast in the stratosphere will always generate an audible boom, so there was no point in conducting research. The end result is that by taking a government rule and connecting it with a physical rule (flight beyond Mach 1 will always produce some sort of boom), manufacturers in effect put themselves out of the supercruise business.

But for every law of nature, there is a workaround (you may not get exactly what you wanted but what you get is better than nothing). There is a way to go fast without scaring the crap out of citizens. It takes advantage of the different temperatures in our atmosphere and uses them as a muffler. For years, scientists, engineers and pilots have known about a phenomenon called Mach Cutoff. In layman’s terms, it is a certain Mach speed that if exceeded, will result in an audible sonic boom on the ground. Below this speed, no sonic boom will be heard by people on the surface. This speed varies based on air temperature, weight of the aircraft and of course the elevation of the ground one happens to be flying over. For practical purposes, most aerodynamicists use the range between Mach 1.15 and Mach 1.2 as a standard cutoff.

Hidden by obscurity, Mach Cutoff did not do anything to spur development of faster aircraft until fairly recently. In those intervening years there was also very little research in how to fly with a quiet, muffled or ground-level silent sonic boom. The conventional wisdom said that all sonic booms were created equal and that you couldn’t avoid or mitigate them in any way. However, in the late 1990s and early 2000s there was a resurgence in high speed flight testing. NASA tested a modified F-5 with what appeared to be a boat hull fuselage. They also tested an extendable spike in the nose of an F-15 to see if creating a series of weak oblique shocks would be less offensive acoustically. Aerion tested a natural laminar flow wing section on the bottom of an F-15 to study the effects of supersonic forces on a straight wing. It appears that there is at least some interest in going fast if it can kept quiet.

What does this mean for a well-organized and equally well-funded designer? It means that there is no reason that designer can’t build something fast today. Of course this is very easy for me to say sitting here with exactly $17.81 in my pocket and not a single airplane built to my name. But a person doesn’t need to run an aerospace firm to know that there is a lot to be gained from building a proof-of-concept vehicle. Investors like something they can see, touch and sit in better than CAD generated images. Other developers become inspired and improve upon another firm’s work. Sometimes breakthroughs are made when idiosyncrasies that simply cannot be predicted in computer models are experienced and rectified in a flight test program.

It’s 2015 and we are well into the future that was predicted when most of us were children. I don’t have to say that while some things are an improvement, other things are complete letdowns. Flying at the same speeds that we attained in the 1950s is not a limitation of physics or economics, but of our own desires. We’ve become complacent and comfortable with what is essentially six decade old technology. Sure we’ve refined it and eked out far more efficiency than we ever imagined, but is that it? Are we supposed to be excited over another 1% fuel savings? Are we supposed to look at an aircraft with awe because it features a self contained 5G network?

The last 60 years were nice, but it’s time to go meet the future. Let’s move beyond where we are to where we should be.

Is this as fast as we’ll ever go?

 

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.

 

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.

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.

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.

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.

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.

There Is Never* Too Much Airplane

* Let’s get the disclaimer out the way up front. When I say “never”, I’m not talking about your fringe elements. You know the guy who somehow managed to buy a functioning SR-71 for trips from Tamiami to Beale. Nor am I talking about the person who has 50 hours in a Cessna 152 and buys a P-51D because he’s “always wanted one”. I’m ruling these aircraft choices out because they are pretty rare. This article is meant for the pilot who wants to upgrade from a trainer to a faster/larger aircraft in the same class and category and is intimidated by the change in cruise speed or engine size.

Part 1: Speed

“Speed is relative. If he’s doing 600 and you’re doing 600 what’s the difference? Zero.”     ~ Brigadier General Robin Olds

When sitting around hangars, the talk occasionally brings up some guy who allegedly went from a Cherokee to an Arrow and got so far behind the airplane that he was just contacting departure at the same time he was beginning his initial descent to the destination. And the talk will progress to how complex the new airplane is and how much power it has, which inevitably leads to someone belting out “It’s the same old thing, it was waaaaaay too much airplane for him.” At which point I sigh and hold in what I really want to say.

What I really want to say is “That’s nonsense.” Simple and to the point. Sure there may be some truth in not taking someone who had an intro flight in a Skycatcher and suggesting that they do the rest of their training in a T-38. But the chances of that happening are very, very remote. It doesn’t help pilots to keep them in slow airplanes for hundreds of hours anymore than it helps to teach them how to fly in said T-38. How can I say this with a straight face? Go to the FARs and look at Part 23.49c which regulates stall speed for all single engine aircraft (we’re ignoring twins in this discussion). No aircraft can stall above 61 knots in landing configuration unless an equivalent level of safety is demonstrated with respect to occupant restraint. This stall speed important because it’s a benchmark for any certified single engine aircraft.

Example: Bill Generic wants to upgrade from a Cessna 172 to a Piper Saratoga. Aerodynamically, it’s not a huge leap. In terms of systems of course, it is a completely different aircraft and if Bill doesn’t get transition training, he has a few cotter pins loose. But once Bill finishes training and goes out on his first solo cross countries, he may end up blasting past intersections or call on the wrong frequency because he’s not keeping up mentally with his progress and there’s no instructor/mentor next to him to help coach. The easy way for Bill to fix all that is to simply pull the power back. No he won’t drop out of the sky. No he won’t be hanging on the edge of a stall. Since all singles are created equal in that they cannot exceed 61 knots Vso, it means you can cruise any single between 100 and 120 knots without any superhuman effort at all.

Will the faster airplane like loafing along so slowly? It may not handle exceptionally crisp and depending on how far back Bill has to pull back the power, the engine may start getting warm if he tries this at 2000 feet over Death Valley. But done with agreeable weather conditions, this is a great way for him to get used to progressively faster speeds. It may take 3 or more trips for he becomes comfortable spanning the gap between 110 and 170 knots. Or he may be fine after one leg of continuous increases in speed. But the important part is that he can adapt at his own pace. The throttle doesn’t control power, its controlling time.

The ability to control time means that while doing transition training and even afterwards, a pilot can get used to the checklist items and internalize procedures. Your aircraft upgrade may have a turbocharger or oxygen system or cowl flaps or even extra fuel tanks. More time gives you a cushion for making sure these systems are working the way they should and that you understand why they’re doing what they’re doing. Waypoints passing too quickly and your map (some of us aren’t totally paperless yet) can’t get turned to the proper side fast enough? Pull back the throttle/time controller and slow things down.

Landing and approaches are another place where the 61 knot rule levels the playing field.  Surely you practiced rectangular patterns at altitude, so making the base and final turns shouldn’t be an issue but all the same, give yourself more time in the form of a slightly wider and longer pattern. On downwind, fly whatever was recommended but 90-100 knots is a safe range (1.47 to 1.63 times Vso) where you have some wiggle room in case your aircraft bleeds speed quickly when flaps or gear come down. If that happens, use the time controller in reverse…push the throttle up so that the bleed rate slows and you have more time to adapt to the aircraft’s idiosyncracies. The standard 1.3 Vso approach speed for the 61 knot aircraft comes out to 79 knots (just say 80). Flying final at 80 knots is an easy and safe target if for some reason everything the instructor taught in transition leaked out of your ears. If you’re being vectored for an approach and the controller wants you to hold 170 knots to the marker and you aren’t comfortable doing that yet, tell them you are unable. They don’t need to know why and if they do ask, mention something about monitoring a fluctuating sonidecimeter indexer. By the time they figure out you’re full of it, you’ll be making the second turnoff.

Part 2: Power

Power is relative too. In my view, too much is made of horsepower being the primary indicator of performance. The FAA categorizes any aircraft with over 200hp as high performance. Because of this, you will often hear the same people who griped about “too much airplane” complaining that a certain airplane had way too much power. While there are cases when this is true, 90% of the time you can never have too much power. High performance is what you make of it, what you wish to measure and what you’re used to.

For a real life example, take a Rockwell Commander 114 and a Pitts S-2C. Both use a 540 cubic inch, 260hp engine and both are technically high performance, but that’s where the similarities end. The Pitts can fit 2 people very snugly and climb at 2900 fpm. The Commander can fit 4 in leather-bound comfort and climb at just under 1100 fpm. The Commander has a 1000nm long-range cruise while the Pitts is limited to 247nm. We could go on and on but the point is that power gives you radically different performance based on what you want to do with it. The Commander is intended to carry people places, the Pitts is made to go up, have some fun and come back within a relatively short time. Which one is high performance? Depends on what you want to do. Flying an advanced aerobatic sequence complete with a pull-push-pull humpty and snap rolls on a 45 downline? The Commander is going to be a letdown. Need to take 3 people nonstop from Teterboro to Nashville in IFR conditions? The Pitts would not be a good choice. What if payload is the determining factor in what you count as high-performance?  Looking at a much larger aircraft, the Cessna Caravan, has 675 shaft hp from its turboprop engine, but because of its rugged design the aircraft max cruises at 186 knots. By comparison the Cirrus SR22T has less than half the power (315hp) but max cruises at 214 knots. But while the Cirrus can only hold 4 people, the Caravan can carry up to 14. If lifting payload is the performance factor your type of flying depends on, the Caravan is the clear winner. And what about your own experience? What you have flown in the past and what you are flying currently has a lot to do with this perception. If you had a PC-12 and are downgrading to a Commander, you may not consider it to be high performance by comparison. If you are coming from an MX-2, then maybe the Pitts isn’t such a wild ride. But if you’re upgrading from a Diamond DA20, you’re most likely going to be wowed by either aircraft. Performance is just as relative as power.

Perhaps the regs should focus on acceleration, climb rate and cruise speed instead of power for a definition of high-performance. This is probably what the FAA intended people to focus on all along but we ended up getting fixated on the semantics of 200 vs 201 hp. Here is an example of why horsepower can be a dead-end. Take an airplane with a pretty anemic power to weight ratio of 16 to 1, meaning there are 16 pounds of aircraft per horsepower. With a 100hp engine, this fictional airplane will weigh 1600 pounds. But what happens when we scale everything up by a factor of 3? The horsepower explodes to 300…definitely high performance by federal standards. But the weight also balloons to 4800 pounds. And in case you didn’t notice, the power to weight didn’t change either which means acceleration and climb is going to be pretty similar (provided the wing loading remained constant). Glancing back at our earlier examples, we’ll compare the difference in power loading for the Commander and Pitts. The Commander has a 12.5lb/hp loading while the Pitts has 6.6lbs/hp. That’s a massive difference in the amount of weight each horsepower has to work with and why the performance figures are so radically different for the same horsepower engine.

Part 3: The Moral

Yes, upgrading in aircraft will take time and money. Get transition training, talk to pilots and owners of the same aircraft first. Learn the little quirks that aren’t written in the operating manual. Study the systems and know little things like how to start the engine. Nothing makes a passenger reevaluate their friendship with you like watching you fumbling around with throttles, mixture knobs and primers on the ramp and saying “It normally doesn’t do this.” Things like starting the engine are and knowing how the fuel system works are pure study items. The things that you can’t learn from a book are your own comfort zones. The only way to learn that is by actually doing it in increments.

Hearing people repeat over and over how hard it is to keep up with fast airplanes and how so much horsepower will ball you up like a Bf109 in a crosswind does not boost confidence. But remember that they’re just repeating what they’ve heard. If flying high performance aircraft was so hard, nobody but astronauts would be doing it and the last time I looked, there’s plenty of non-astronauts in Meridians, Stationairs, Bonanzas and the like. It will just take a little getting used to. Be kind to yourself and don’t expect to go full-tilt right out the gate. Remember, nobody goes 220mph out of turn 4 at Talladega their first time in a race car either.

F/A-18F Super Hornet “Mini-boom”

Thunder Over The Boardwalk 2009 practice show. No, he did not go supersonic, but bank angle plus control surface deflection caused a small sonic boom to form. For those who have never heard one, it sounded like a gun fired at close range. Since it was only a miniboom, there was not a “double crack”, only the single pop and then the jet noise. I could get into a very long lecture about transonic airflows, ambient temperature and g-loading but I’ll sum it up as fn awesome!