They were rare in general aviation airplanes until 2014 when the FAA simplified the requirements for installing them. A proliferation of aftermarket AOA systems followed, ranging in price from around $300 to more than $3,000. I don’t know how widely these devices have been adopted, nor do I know whether any study has been made of their impact on the GA accident rate.

Despite its well-known shortcomings as a stall-warning device, the airspeed indicator remains the only AOA reference in most airplanes. It has the advantages of being a mechanically simple system, intuitive, and familiar. Speed is an everyday experience, while angle of attack, for most pilots, remains in the realm of the theoretical.

Theoretical or not, I think, to start with, that we could improve the terminology. “Angle of attack” is really a proxy for something else, namely “the amount of the maximum lift available that is currently in use.” So it would be more meaningful to speak of a “lift indicator,” “relative lift indicator,” or “lift fraction indicator.”

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One of the advantages of thinking in terms of lift fraction is that almost all of the important characteristic speeds of any airplane—the exceptions are the nonaerodynamic speeds, such as gear-and-flap-lowering speeds—fall close to the same fractions of lift regardless of airplane size, shape, or weight. Best L/D speed is at around 50 percent and 1.3 V_s at exactly 60 percent. Stall, obviously, is at 100 percent. A lift gauge is universal: It behaves, and can be used, in the same way in all airplanes.

A few years ago, in a column titled “A Modest Proposal,” I suggested demoting the hallowed airspeed indicator to a subsidiary role and replacing it with a large and conspicuous lift indicator. I borrowed the title from a 1729 essay by Jonathan Swift, the author of Gulliver’s Travels, in which he satirically proposed that poverty in Ireland might be relieved if the populace were to sell its manifestly too numerous babies to be eaten by the rich. My appropriation of Swift’s title was meant to suggest that I considered my proposal was about as likely to be adopted as his.

At the time I wrote my article, I was not yet aware of a 2018 paper by a team led by Dave Rogers, titled “Low Cost Accurate Angle of Attack System.” Using a simple underwing probe and electronic postprocessing, Rogers and his group achieved accuracy within a fraction of a degree of angle of attack with a system costing less than $100. That’s more accuracy than you really need, but better more than less.

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The low cost is made possible by the availability of inexpensive small computers— Rogers’ team used a $20 Arduino—that can be programmed to do the math needed to convert the pressure variations read by a simple probe into usable AOA data. Processing is necessary because the airplane itself distorts the flow field around it and makes it all but impossible to read AOA directly with a vane or pressure probe situated close to the surface of the aircraft. Besides, configuration changes, like lowering flaps, alter the lifting characteristics of the wing.

Designing an accurate system is only half the challenge, however. There is also the problem, perhaps even more difficult, of how best to present the information to the pilot. Little agreement exists among current vendors. Some presentations use round dials, some edgewise meters, some various arrangements of colored lights or patterns of illuminated V’s and chevrons resembling a master sergeant’s shoulder patch.

In 1973, the late Randy Greene of SafeFlight Corp. gave me one of his company’s SC-150 lift indicators for my then-just-completed homebuilt, Melmoth. The SC- 150 used a rectangular display with a moving needle. There was a central stripe for approach speed flanked by a couple of dots for climb and slow-approach speeds, and a red zone heralding the approach of the stall. The probe that sensed angle of attack was a spring-loaded, leading-edge tab, externally identical to the stall-warning tabs on many GA airplanes.

Apparently, some people mounted the SC-150’s display horizontally, but that made no sense to me at all. Given that I wanted it vertical, however, Greene and I did not see eye to eye about which end should be up. Greene was a jet pilot used to a lot of high-end equipment (SafeFlight made autothrottles, among other fancy stuff, for airliners). He understood the device as a flight director—as you slowed down, the needle should move downward, directing you to lower the nose.

I, who despite having acquired in my younger days a bunch of exotic ratings, am really just a single-piston-engine guy, saw it as analogous to an attitude indicator and thought that as the nose went up the needle ought to do the same. Greene saw the display as prescriptive; I saw it as descriptive.

Recently, Mike Vaccaro, a retired Air Force Fighter Weapons School instructor, test pilot, and owner of an RV-4, wrote to acquaint me with FlyONSPEED.org, an informal group of pilots and engineers working on (among other things) practical implementation of a lift-awareness system of the type described in Rogers’ paper. The group’s work, including computer codes, is publicly available. Its proposed instrument can be seen in action in Vaccaro’s RV-4 on YouTube. 

The prototype indicator created by the FlyONSPEED group mixes descriptive and prescriptive cues. Two V’s point, one from above and one from below, at a green donut representing approach speed, 1.3 Vs, the “on speed” speed. The V’s are to be read as pointers meaning “raise the nose” and “lower the nose.” An additional mark indicates L/D speed. G loading, flap position, and slip/skid are also shown on the instrument, along with indicated airspeed.

Importantly, the visual presentation is accompanied by an aural one. As the airplane slows down, a contralto beeping becomes more and more rapid, blending into a continuous tone at the approach speed. If the airplane continues to decelerate, the beeping resumes, now in a soprano register, and becomes increasingly frenetic as the stall approaches. Ingeniously, stereo is used to provide an aural cue of slip or skid—step on the rudder pedal on the side the sound is coming from. The audio component is key: It supplies the important information continuously, without the pilot having to look at or interpret a display.

This system—it’s just a prototype, not a product—is pretty much what my “modest proposal” was hoping for, lacking only the 26 percent-of-lift mark that would indicate the maneuvering speed. Irish babies, beware.

Now I just have to figure out what we’ll do with all those discarded airspeed indicators.

This column first appeared in the Summer 2024 Ultimate Issue print edition.

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In one of his books, Austerlitz, the title character goes flying at night with pilot friend Gerald Fitzpatrick in a “Cessna.” He describes the mesmerizing sight of the familiar constellations overhead. Now, looking up at the stars from an airplane is an entrancing experience, but no one ever had it in a Cessna.

The corresponding photograph, though somewhat distant and blurred, is clearly not of a Cessna but of a small twin-engine, twin-finned airplane that does, however, have a transparent canopy. I got the explanation for this apparent authorial fumble from a Swiss friend: Among nonpilots in Germany, “Cessna” would simply mean a private airplane, no particular brand.

The twin was actually a Miles Gemini, an airplane brought into being, like the original Beech Bonanza, by the anticipated postwar explosion in demand for personal air travel. It had four seats and was equipped with two 100 hp engines of the inverted in-line variety, housed in those nice narrow cowlings that many British and French aircraft of the 1930s and ’40s had. One of its unusual features was a big external airfoil flap.

Despite the flap, however, the published stalling speed of 35 knots cannot have been a calibrated airspeed—45 is more plausible.

Whatever its real landing speed, the fictional Gerald Fitzpatrick crashed fatally in his Gemini. His friend Austerlitz gloomily comments that this was bound to happen, since he was so fond of making sightseeing flights in the south of France.

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Novelists just won’t give private planes a break.

I wondered how the 3,000-pound Gemini would do on one engine. Late designer John Thorp, contemplating a trip to Europe with his wife, Kay, once propped up a couple of small Lycomings in front of his two-seat Sky Skooter. His friend George Wing, creator of the ubiquitous Hi-Shear rivet, happened to walk in, and thus was conceived the Wing Derringer.

Wing was not taking any chances on O-235s, however. The two-seat Derringer, with 160 hp O-320s, could definitely climb on one engine. The question of how a twin with 100 hp engines climbs on only one was answered, however, by the Champion Lancer, whose woeful single-engine performance was, like Sir John Falstaff, a cause of wit in many men.

Like many other early aviation enthusiasts, Frederick George Miles began in the 1920s as an amateur builder. Miles then started manufacturing small airplanes and eventually turned out a series of products that recalls, in its variety and inventiveness, the career of another homebuilder-turned-professional, Burt Rutan. Like Rutan, who started the Rutan Aircraft Factory with his then-wife Carolyn, Miles found a business partner in his remarkable wife Maxine, nicknamed Blossom, who, in addition to being his beloved, was a pilot, aeronautical engineer, stress analyst, and businesswoman.

In some respects, the paths of Miles and Rutan were different. Miles made airplanes for military and commercial use. Rutan, after leaving the homebuilt plans business that had launched his career, mainly produced one-off prototypes and never certificated any of his designs. (Beech ruined the Starship, he complained, in the process of certificating it. Beech engineers naturally took a different view of the matter.) But the two shared a wide-ranging versatility. Some designers, like Thorp and Dick VanGrunsven, turn out incremental variations and improvements on a basic theme.

With Miles and Rutan, you never knew what might come next. In Miles’ case the variety may have been due in part to his employing other designers, whereas Rutan designed all of his airplanes himself. Both men mastered the art of fast prototyping: Scaled Composites, the company Rutan founded, exploited foam-cored composites for that purpose; Miles’ medium was resin-bonded wood.

Miles’ greatest commercial success came during the pre-World War II years. He developed a number of training and transport airplanes and manufactured them in large numbers for the Royal Air Force. His efforts to produce a fighter were less successful. A 1940 prototype of a small wooden “emergency” fighter, proposed to stop the gap in the event that Hurricane and Spitfire production were hampered by German bombing, had a bubble canopy and a stock Merlin “power egg,” and looked just like a miniature Hawker Typhoon. Despite fixed landing gear, it rivaled the Hurricane in armament and performance, but it was never produced, mainly because the anticipated emergency did not materialize.

During the war, Miles produced a design remarkably similar in conception to Rutan’s first homebuilt. Like the VariViggen, Miles’ original Libellula—Latin for dragonfly—had a single pusher propeller, low wing, and high canard. The configuration was supposed to solve several problems associated with shipboard fighters, but the British Admiralty didn’t bite. A second version, this one with a high wing and low canard, was conceived as a bomber, with the idea that the tandem wing arrangement would provide an unusually large CG range. That airplane also ended up on the scrap heap.

The little Gemini twin, the one illustrated in Austerlitz, was a commercial success, as was a side venture the resourceful Miles got into: ballpoint pens. But the most striking Miles design from the wartime period was something completely different.

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The M.52, born in 1943, is said to have been the offspring of a ridiculous error. An intercepted German communication referred to the 1,000 kph speed of one of the jets then being developed. Someone failed to perform the conversion, and the belief took root that the Germans were perfecting a 1,000 mph airplane. Inevitably, the British felt they needed to follow suit, and Miles Aircraft earned the contract. (If it isn’t true, at least it’s a good story.)

The result was a 5-foot-diameter cylinder with thin, straight wings and a then-unprecedented, and prescient, powered all-flying stabilizer. Air for its centrifugal-compressor jet engine came in through an annular intake surrounding a shock cone, à la the MiG-17 or SR-71. The pilot sat inside the shock cone. In retrospect, the design looks sound except for its lack of area ruling, and it could probably have gone supersonic, given sufficient thrust. But in 1946, with the first prototype nearly complete, the U.K.’s Air Ministry suddenly canceled the project.

The abrupt cancellation, which was never persuasively explained, fueled a persistent notion among British airplane buffs that their government had abjectly bowed to U.S. insistence on being the first to “break the sound barrier.” Indeed, the Bell X-1 rocket aircraft, which did so in 1947, was being developed at the same time as the M.52.

However, the M.52 may have been shelved simply because of the distinct possibility that its still-unproven afterburning turbojet might not be powerful enough to propel it past Mach 1 in level flight—let alone to 1,000 mph.

This column first appeared in the May 2024/Issue 948 of FLYING’s print edition.

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“Wing warping,” as this approach was called, was satisfactory for very slow airplanes, but faster ones required more rigidity, and by around 1908 or 1909 the idea had arisen of replacing part of the trailing edge of a wing with a hinged, controllable flap. Actually, a prescient Englishman, Matthew Boulton, had patented the idea in 1868, when airplanes were still a thought experiment. His invention had been forgotten, however, by the time real airplanes came into being. Despite the early invention of the aileron, wing warping continued to be used, even on some fighters, as late as 1916.

That a hinged trailing-edge flap would have the same effect as warping the entire wing is obvious to us, because we have seen it in action. But it cannot have been quite so obvious then. The evolution of airplanes in the United States suffered from the Wrights’ unfortunate attempt to establish a monopoly on flight by patenting the very concept of lateral control. Litigation over that ambitious claim held back aeronautical development in America for years while it raced ahead in Europe.

The function of an aileron, or any hinged trailing-edge surface, is commonly explained in ground school by simple analogy to, say, a door opened on a windy day. The wind hits the deflected surface of the aileron and pushes on it. If the aileron is deflected downward, the wind pushes it upward, and if it’s deflected upward, the wind pushes it downward.

This explanation, while intuitively appealing, fails to capture what is really happening. The majority of the lift generated by an unstalled airfoil is always concentrated near the front, and moving a trailing-edge flap up or down changes the flow conditions at the leading edge. An aileron deflected downward impedes air passing below the airfoil, and as a result, the dividing line between air passing below the airfoil and that passing above it moves aft. More air now passes over the top, the velocity of air rounding the leading edge increases, and the pressure there is correspondingly reduced. In other words, the lift change that results from deflecting the aileron is not confined to the aileron itself. It affects the entire area ahead of the aileron as well.

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The effect of the aileron, like that of wing warping, amounts to a change in angle of attack. This understanding helps clarify what is happening in a steady-state roll. Why does an airplane with deflected ailerons settle at a steady-roll rate rather than roll faster and faster? It’s because the rotation reduces the angle of attack of the up-going wing and increases that of the down-going one. The change, which is opposed to the change caused by aileron deflection, is not uniform. It is greatest at the tip, where the rotational velocity of the wing, relative to the forward velocity of the airplane, is greatest. When the change of angle of attack due to rotation, integrated across the entire wing, is equal in magnitude to that resulting from deflection of the ailerons, the airplane is in equilibrium about its roll axis and rate of roll stops increasing.

Rate of roll is one of the “wow” numbers associated with a high-performance airplane. When we read that a T-38 or an A-4 rolls 720 degrees per second, we are amazed and wonder how the pilot knows which way is up. Arguably, roll acceleration—how quickly you get from zero to, say, 90 degrees of bank—might be more important in air combat maneuvering.

Roll rate is not a single number, however, as it increases with speed. Preferring a criterion that is independent of speed, engineers often refer to “peebee-over-toovee”—a (more or less) constant value represented by the ratio “pb/2V,” where “p” is the rate of roll in radians per second (a radian is 57.3 degrees), “b” is the wingspan, and “V” is the true airspeed (in the same units as the wingspan). Thus, for example, an airplane with a roll rate of 70 degrees per second (deg/sec), a wingspan of 35 feet, and a forward speed of 300 feet per second has a pb/2V of 0.071 radian.

In physical terms, that means that if the airplane flew past you while rolling, the path of its wingtip, in profile, would be at an angle of 0.071 x 57.3, or about 4 degrees to the flight path. That is called the “helix angle,” because the rotating wingtips form a double helix, like DNA.

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The rolling helix angle is theoretically a constant for any given airplane, determined by wing planform, aileron design, and various other subtler factors. In principle it allows you to predict an airplane’s roll rate at any speed. Things don’t quite work out that way, however, because at high speed deflecting the ailerons makes the whole wing twist, counteracting the ailerons themselves, and cables stretch, preventing the ailerons from deflecting completely. Still, helix angle remains a convenient criterion, at the very least for setting a minimum acceptable standard for rolling performance.

A 1941 National Advisory Committee for Aeronautics report set that minimum value at 0.07 for transports and bombers and 0.09 for fighters. According to some published figures with which pilots who flew the airplanes are bound to disagree, the Spitfire Mk.V and FW-190 were fast-rolling airplanes, and the P-40 was not far behind. The FW-190 rolled 151 deg/sec at 226 knots, the Spitfire 150 at 176, and the P-40 134 at 315. The P-47 rolled a mere 71 deg/sec at 250 knots, the P-51B 98 at 260, the P-38 78 at 260. The corresponding pb/2V values are 0.118, 0.163, 0.082, 0.060, 0.072, and 0.084 respectively. The T-38 scores around 0.26.

Neither helix angle nor rolling acceleration fully expresses the quality of lateral control experienced by the pilot. That has more to do with effort, linearity of response, and presence or absence of “hysteresis,” or slop. Pilots used to single out the ailerons of Bellancas for praise, but pb/2V had nothing to do with it. It was really all about the ailerons’ smooth, frictionless, instantaneous response, low forces, and lack of free play.

The fact that pb/2V is theoretically constant for a given airplane has a couple of corollaries. One is that a larger span results in a lower roll rate. Another is that roll rate in degrees per second, taken alone, is misleading.

The Sopwith Camel, one of the deadliest fighters of World War I, rolled at a mere 40 degrees a second. But if you judged it by its pb/2V of 0.083, it was equal to the P-40 and superior to the P-47 and P-51.

This column first appeared in the March 2024/Issue 946 of FLYING’s print edition.

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RAF started in the pre-digital age, when the tools of the aeronautical engineer were still slide rule and drafting paper. Around 1980, however, Rutan bought an Apple 2 computer. When the salesman asked him whether he would like a second 160 kilobyte floppy disk in addition to the one supplied with the computer, he declined it, saying that he would certainly never fill even the first disk. Some time later he proudly showed me a Corvus hard drive for which he had paid thousands of dollars. It had the unimaginably vast capacity of 10 megabytes.

I too had acquired a computer, but not a printer. So when I developed a rudimentary program for generating fuselage cross-sections, I delivered the results to Rutan in the form of snapshots of my computer screen, developed and printed in my basement darkroom.

During one of our Wednesday lunches at Reno’s Cafe, Rutan mentioned that some fellow in the Midwest had written to offer to design airfoils for him. At that time, most designers picked their airfoils from catalogs developed by the National Advisory Committee for Aeronautics in the 1930s. I suspect that Rutan may even have designed some of his with a French curve and draftsman’s spline, relying on the time-tested principle that they ought to be round in front and pointy in back. This Midwestern fellow said he could do better, using a computer that he had built and codes he had written. Thus did digital simulation come to RAF.

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Eventually this Midwestern fellow turned up in Mojave, and it was there that I met him. His name was John Roncz. He was cherubic: plump and pink, with a boyish voice and air of innocent candor. He was, I would soon learn, in addition to his coding and analytical skills, sweet-natured and generous and a formidable classical pianist. His day job, rather incongruous, involved running a metal stamping business he had inherited from his father.

Thereafter our paths would cross from time to time, and at a certain point they converged. A pilot himself—he once glided a Commander 112 to an airport landing after a night engine failure—he had by then left the metal stamping business and become a full-time computational aerodynamicist— that is, someone who uses computer simulations in place of wind tunnels to discover the properties of airplanes and their various parts. He was using a computer program called VSAERO for his analyses of complete aircraft.

It happened that I and a partner, Dave Pinella, were selling a package called PSW that bundled a program I had written for defining airplane geometries with Pinella’s programs for analyzing the digital models and displaying the results. Our analysis code, Cmarc, was descended from VSAERO, and their input “decks” were sufficiently alike that Roncz and I could conveniently collaborate. My main contribution was turning complicated geometries into digital models digestible by VSAERO and Cmarc.

Though we seldom met in person, we corresponded copiously and became good friends. When I needed an answer to some difficult question for an article— such as, how much power would be required to hover a 7,000-pound, four-rotor electric air taxi in ground effect?—I would email Roncz, knowing that a reply would come to me within hours. Roncz’s contributions to aviation were many and significant. He created airfoils for Voyager, the Beech Starship and other airplanes, including mine, and designed several complete airplanes. He also designed the wing sail for a victorious America’s Cup boat and odds and ends such as windmill blades and race car downforce wings.

He had a merry sense of humor and would name his airfoils with funny acronyms. The laminar profiles for my second Melmoth were SODA (Stamp Out Drag Airfoil) and POP (Peter’s Other Profile). When Rutan needed an airfoil with extra trailing-edge thickness for a complex Fowler flap, Roncz produced OSPITE (Olympic Swimming Pool in Trailing Edge). A STOL project yielded GOLA (Gobs of Lift Airfoil). Although his work involved scrutinizing mountains of numbers, Roncz was not an obsessive drudge. He laughed often. Beset by intermittent maladies and amorous tragedies, he had an entirely separate life into which he would disappear from time to time and in which I suspect he found more satisfaction than he did in his fluid-dynamics work.

In this other life, Roncz was a medium. He would regularly visit Arthur Findlay College in England—“the real Hogwarts,” he called it—to practice his spiritualistic skills in sessions in which he would stand before an audience and, as he described it, say whatever came into his mind. Invariably some astounded stranger would confirm that what he had said was true, and what’s more, there was no possible way he could have known it.

Roncz was quite aware of the incongruity between his two lives and said he was just as puzzled as anyone else about how “it” worked—“it” being his weird ability to hit so many invisible nails on the head. He wrote a book, An Engineer’s Guide to the Spirit World: My Journey from Skeptic to Psychic Medium, part autobiography and part case study, in which he matter-of-factly laid out his dealings with the “spirit world” without making any attempt to provide a scientific-sounding explanation for it.

Being on the knuckle-dragging level of spiritual evolution myself, I could never view Roncz’s mediumistic side other than with a skeptic’s condescending amusement. But he didn’t mind. For him, it was a lived experience, and my doubts were like those of a shut-in who questions a visitor’s assertion that outside the sky is blue.

Roncz died in September of cancer at the age of 75. I talked with him a little before his death and suggested that he come knock on the wall of my house once he was comfortably ensconced in the spirit world. He laughed and said he hoped he would.

I have not yet heard the knock. But his airfoils keep me aloft, and whenever the raccoons are dancing on the roof, I think of him.

This column first appeared in the January-February 2024/Issue 945 of FLYING’s print edition.

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Most sublime, however, are the turkey vultures—ugly, roadkill-gobbling, baldhead, wryneck creatures, but calm and silent and steady in their flight like no others. When they are airborne, their repulsive heads are small and inconspicuous; you see mainly the broad black wings skating with effortless grace, tip feathers spread like a musician’s fingers strumming an inaudible air.

How many times, as I sat or worked on the deck behind the house, has one of these birds materialized and swept past me at eye level, skimming over the ridge between one canyon and the other. Stillness in motion, it would fly perfectly level, now and then minutely adjusting a feather or two in concession to the restless air. It seemed to be propelled by nothing at all; nothing, at least, other than its own inscrutable will. As it crossed the western edge, an upwelling breeze from the distant Pacific would buoy it gently upward, a favor it took as its due. The bird seemed to occupy a universe in which motion could occur without friction.

I suppose each pilot must find in some single manifestation of flight a quintessence that resonates more than any other. For some it may be speed, or wild aerobatics, or the sound of a Merlin or formation of radials, or the sweet glint of sunlight on a liner inaudibly high in a sky of pure blue. For me it is, and always has been, the act of gliding. The glide of the vulture radiates the truth that makes all flight possible. That truth spoke, over millennia, to every human who imagined being able to fly and most intelligibly to the few who set about to do it and actually succeeded.

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I was not alone in being thrilled by the mere sight of something gliding. The collective “Ooh!” evoked by the long flight and smooth landing of a sheet of paper hastily folded while the teacher was out of the room revealed a general susceptibility to its charm. Ten-cent balsa airplanes—the kind whose flat wings and tail slid into slots in a two-dimensional fuselage imprinted in red with insignia and a pilot—would loop if flung with sufficient force and then settle into a series of scallops, but their achievement was not crowned until they had settled, straight and flat, to the ground, slid, and then spun half-round as a blade of grass caught a settling wingtip.

Decades ago, I bought a glider kit from a hobby shop. A product of Easy Built Models of Lockport, New York, it was called “Super Soarer.” It had a 6-foot gull wing and a cruciform tail, and its tall fin had an elegant semielliptical shape. I built it, but then put off weighting its nose and flying it. It was such a handsome thing that I feared what a bad landing or encounter with a tree might do to it. After a long delay, I hung it from the ceiling of my office, where it slowly oscillated until its translucent tissue became brittle, and dust darkened the fuselage’s doped balsa skin.

I was not always so fastidious. When our son was in second grade, his class took a camping trip to a desert park called Anza-Borrego, east of San Diego. Parents went along, and I took with me a new, smaller glider kit, mainly to keep myself occupied but also in the hope that I might interest one or two of the kids in the ancient art of building balsa-and-tissue flying models. I doubt I succeeded in that, and, even if I had, any curiosity I might have aroused would soon have been extinguished by the banning, in the subsequent years, of clear dope, the replacement of silkspan by mylar, and the general erosion of human patience.

Kids would drift by the picnic table on which I had set up my little airplane factory and then drift away when they realized how long glue took to dry. Only after a couple of days did the assembled parts even begin to look like an airplane. On the last day, however, the glider was finished. Moving to a remote place free of both cactus and children, I test flew it, adjusting the nose weight, rudder trim, and energy of the launch until it achieved just the long, flat descent that delighted me.

There was a steep, rocky hillside beside the campsite. In the early evening, I let it be known that I was going to fly the glider, and I climbed the hill, zigzagging among the boulders and bushes while holding the delicate airplane up as if I were in chest-deep water. The children milled around below. The air was still and dusk was gathering. Fifty or 60 feet above them, I turned, paused long and significantly, and then sent the glider forth into the twilit air.

It went out straight at first, then slowly turned to the left and flew parallel to the foot of the hill. The throng of children chased it, their mingled voices rising up to me. It led them along, sometimes dipping toward their outstretched arms and then bobbing upward again on vague bubbles of rising air—a tiny, bright, tantalizing dancer. It seemed to foresee its fate, as I did, and held it at bay as long as it could. But then straining fingers touched it, and it vanished into the little crowd.

By the time I had picked my way back down the hillside in the failing light, the throng of children had partly dispersed. A little group came to me with what was left of the glider. One wing was broken but still retained much of its shape. The tissue cover was punctured in several places. Half-apologetic, half-hopeful, they handed it to me. “Will you fly it again?” one boy asked. “No,” I said. “You only fly once.”

A few days later, one of the parents gave me some snapshots. Two were of me high on the hillside, the third of the glider in flight. With open sky all around and the gibbous moon above, the little airplane sailed happily in its element. Invisibly sustained, remote, ethereal, it seemed to be savoring the few seconds that remained before the end of its final and fatal glide.

This column first appeared in the December 2023/Issue 944 of FLYING’s print edition.

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Recent years have seen a general drift toward longer spans and higher aspect ratios. The Beechcraft Bonanza has a span of 33.5 feet and an aspect ratio of 6.2; the Cirrus SR22, which might be seen as today’s Bonanza, has a span of more than 38 feet and an aspect ratio of 10.1. The trend is generally toward greater aerodynamic efficiency, partly in response to fuel costs and partly because the increasing use of turbocharging leads to higher cruising altitudes, where longer wings are more at home.

The two airplanes I’ve built, Melmoth and Melmoth 2, are (or were—the first Melmoth was destroyed in an accident long ago) broadly similar, with low wings, T-tails, bubble canopies, retractable gear, and the same 200 hp Continental 360 engine and Hartzell constant-speed prop. The first Melmoth was aluminum, with 2+1 seating; the second is composite and seats four. Both were built with long-distance travel in mind and have lots of internal tankage: Melmoth’s wing and tip tanks held 155 gallons; Melmoth 2’s completely wet wings hold 142 gallons. The two Melmoths, with the same engine, propeller, empty weight, and cabin cross-section, differ significantly in one aspect: wingspan. The first began life with a wingspan of 23 feet and went through 21-foot and 28-foot iterations before its eventual demise. Melmoth 2 has a wingspan of 36 feet but only a little more wing area—106 square feet to the first Melmoth’s 93. (For comparison, the wing areas of most commercial four-seaters range from 145 to 180 square feet.) The first Melmoth’s aspect ratio was 5.75; Melmoth 2’s is 12.6.

Span and area are entangled with one another in the sense that structural strength and stiffness (not to mention space for retracting landing gear) require a certain wing thickness, and that in turn implies a minimum chord (the distance from leading to trailing edge), because airfoils shouldn’t be too thick. So you can’t just increase span willy-nilly without at some point having to increase chord and area as well. However, increasing the wing area, which was originally selected to permit a certain landing speed at a certain weight, adds drag and makes the airplane heavier.

Increased wingspan—other things remaining the same—rewards you with better efficiency and climb rate, and improved high-altitude performance. The first Melmoth had a maximum lift-drag, or L/D, ratio of about 11.8 and a “Breguet range”—a fictional, greatly exaggerated number that ignores takeoff, climb, and varying engine efficiency and assumes that you always fly at a low and ever-decreasing ideal speed—of 3,000 nm. Melmoth 2, with half again the span, has an L/D ratio of 17 and a Breguet range of 3,600 nm, despite carrying 8 percent less fuel. Rate of climb is less strongly influenced by span than L/D and range are, but Melmoth 2, climbing at 1,800 fpm at full power and a typical weight of 2,200 pounds, betters the original Melmoth by about 20 percent.

Note that I said “half again the span” and added nothing about aspect ratio. That is because, contrary to widespread belief, aspect ratio actually does not enter into it. Aspect ratio is generally thought of as the quintessential measure of efficiency, but if you could double an airplane’s wing area (thereby halving the aspect ratio) without increasing its parasite drag, the L/D ratio and Breguet range would remain the same. But you can’t increase wing area without increasing drag and weight, and that’s why aspect ratio becomes important: It’s a measure of how little wing area you can have with a given span.

Curiously, and I think unexpectedly for most pilots, altitude also does not enter into it. You might intuitively suppose that thinner air would make the airplane more efficient, but in fact neither the maximum L/D ratio nor the maximum range is affected by altitude.

You will object that at 8,000 feet you will go faster, with the same fuel flow, than at 2,000 feet. True. But that is because your indicated airspeed is lower. If you flew at the same indicated airspeed and fuel-air ratio at both altitudes, you would find your fuel flow is greater at the high altitude. The reason is that drag at a given indicated airspeed is the same at all altitudes, but the power required to overcome it is proportional to the square of the true airspeed, not the indicated airspeed. At the bestrange speed, the miles per gallon is at a maximum, however, and is unaffected by altitude except to the extent the engine’s efficiency might vary at different settings of manifold pressure and rpm.

• READ MORE: Do Flaps Make Lift?

“Best range” and “best efficiency” are not seen in normal flying. Under actual cruising conditions, Melmoth 2 is not that different from the original Melmoth. The reason is that maximum L/D and the Breguet range assume speeds that are quite low—around 40 percent above the clean stalling speed—and remote from those we actually use. At real-world speeds, 65 percent or 75 percent power, the differences shrink. Melmoth 2 will cruise at 170 knots at 12,500 feet using about 8.5 gallons an hour—about 60 percent of rated power; the first Melmoth would burn about 9.6, around 70 percent power, at the same weight and altitude. So you see that despite a 50 percent improvement in best L/D, the practical benefit of the longer wing is much smaller.

When I designed the first Melmoth, I was strongly influenced by John Thorp and his T-18 homebuilt, whose wing I copied almost exactly. Thorp, who also designed the original rectangular-wing Piper Cherokee, used to say that low aspect ratio wings perform better than theory would lead you to expect, and he was adamant there was no reason to taper the wings of any airplane weighing less than 12,000 pounds. When I designed Melmoth 2, however, I was more influenced by Burt Rutan’s derisive observation that if I intended to fly long distances, I had certainly chosen the wrong wing to do it. Aesthetics, too—hence the long, slender, tapered wing of Melmoth 2, of which Thorp might have disapproved.

For efficiency—the least fuel burned for the most work done—a large wingspan is necessary. But Melmoth 2’s long wing cost it the rollicking roll rate I enjoyed so much in the first Melmoth. Melmoth 2 rolls more like an Airbus. Sometimes, I think I would pay for the extra fuel just to have the rolls back.

This column first appeared in the November 2023/Issue 943 of FLYING’s print edition.

The post The Importance of Wingspan Can’t Be Underestimated appeared first on FLYING Magazine.

Alongside and below me flew a Strizh. My Russian dictionary, a relic of college days, translates strizh as “martin” or “sand martin.” Another source offers “martlet,” which is certainly incorrect. Some kind of bird, at any rate. But this strizh was not a bird. It was an airplane of a special kind that the Russians called an ekranoplan.

The nomenclature is unsettled. Some call them WIG ships, wingships, GEVs (ground effect vehicles), or surface skimmers. Their defining feature is a cruising altitude measured in fractions of a wingspan. Their reason for existing is that a wing flying very close to the surface enjoys a large reduction in drag.

Despite considerable government support, the Russian ekranoplan program came to nothing. A few of them still molder on the shores of the Caspian Sea like the crumbled monuments of ancient empires. Ground effect, however, is alive and well.

Pilots did not fail to notice, back when many flying machines had scant excess power, that when heavily loaded they might climb a few feet above the ground and then refuse to go any higher. A 1934 survey of the ground-effect literature by Maurice Le Sueur (translated from the author’s urbane and personable French into one of the National Advisory Committee for Aeronautics’ less arid technical notes) cites commentary on the phenomenon from as far back as 1912.

By Le Sueur’s time, and even a decade earlier, it was well known that ground effect became noticeable at half a wingspan or so above the surface. But the image of a “cushion of air” commonly used to characterize it was misleading because it implied that ground effect had something to do with supporting the airplane. In reality, the thing it principally affects is drag, not lift. The maximum lift the wing can produce is not appreciably changed by ground proximity, although the stalling angle of attack is reduced.

• READ MORE: Technicalities: Faster than a Boat

The kind of drag that ground effect reduces is induced drag, which is an unwelcome but inevitable byproduct of lift.

If you imagine an airplane in a vertical dive, or rolling along with its weight entirely supported by its wheels, all of its aerodynamic drag is what is called “parasite drag.” Parasite drag increases with the square of speed, and it is what principally limits the maximum level speed of an airplane.

Induced drag works the other way: It increases as the airplane slows down. Near the stall, induced drag may be 75 or 80 percent of the total drag of the airplane. But when a wing is flying very close to the surface, its induced drag may be reduced by as much as 50 percent.

When you land a tailwheel airplane in a “three-point” attitude—that is, the two mains and the tailwheel touching at the same moment—you are at or very close to the wing’s maximum lift coefficient, or, in other words, its stalling speed. The procedure for making a full-stall landing is to approach the touchdown point with a certain amount of excess speed then to flare and continue to raise the nose and hold the wheels off as speed bleeds away. The thing that causes you to slow down is drag, and ground effect makes the deceleration take longer by reducing the drag. The airplane “floats,” not because of extra lift or resting on a cushion but because the aerodynamic braking has diminished.

The floating tendency is most noticeable on clean, low-wing airplanes with fixed landing gear (because retractable gear serve as drag brakes). High-wing airplanes are somewhat less susceptible to floating, but still, the difference is a fraction of a fraction. A high-wing Cessna’s wing is only a few feet higher than a Mooney’s, and those few feet represent only a small part of either airplane’s wingspan.

Floating while landing is usually just a nuisance, although, abetted by an obstinate pilot, it occasionally ends in an overrun. The really hazardous thing is the converse—floating while taking off.

In this case, by “floating” I mean getting airborne in ground effect at a speed at which the airplane cannot climb without its help. Again, the impression is the “ground cushion” is providing extra lift, but in fact it is reducing the power required to fly. The airplane has enough excess power to remain airborne as long as it remains in ground effect, but when it tries to climb higher, its drag increases, eating up the excess power and leaving nothing for further climb. It can neither climb nor accelerate. The only out, terrain permitting, is to descend again, as low as possible, in hope of taking advantage of ground effect to gradually accelerate. Unfortunately, not all airports are surrounded by miles of flat, unobstructed, gradually descending lawn.

Assuming the airplane will actually have enough excess power to climb once it is out of ground effect, the wiser takeoff procedure is not to get airborne as soon as the airplane feels as if it can but to keep it on the ground for as long as possible, accelerating toward its best rate of climb speed.

The circumstances that lead into the ground effect trap can be several: high density altitude (particularly if the airplane is not turbocharged), gross or over gross weight, a short runway, and rising terrain.

What is true of airplanes is also true of helicopters. Under conditions of high weight or density altitude, a helicopter may be able to hover close to the ground but not far from it. (It can climb away, however, by first gaining forward speed.) Although this phenomenon is usually explained in terms of the familiar and appealing “cushion of air,” it is really a matter of the power required to spin the rotor at the required angle of attack—in other words, it’s again about induced drag.

For airplanes, ground effect comes into play only during certain brief parts of a flight. Not so for wingships. One of the inconvenient things about them is that the faster they go, the less they gain from ground effect. Thus, one benefit they seek to enjoy—efficiency—diminishes in proportion as the other benefit they seek—speed—increases. Airplanes solve this problem by climbing to where both lift coefficients and true airspeeds can be high at the same time. Wingships, bound as they are to the surface, can’t have it both ways.

This column first appeared in the October 2023/Issue 942 of FLYING’s print edition.

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(Cameron being James Cameron, the film director and undersea explorer who was an early and outspoken critic of the Titan project; Melmoth 2 being my homebuilt airplane, whose wing structure consists mainly of carbon fiber composites. An unnamed third party in the drama is Stockton Rush, the designer of Titan and, like me, a self-taught amateur engineer.)

Until the investigation of the accident is completed, we will not be certain why the submersible failed. But in case others wondered whether Cameron’s comments apply to carbon-fiber composites in aircraft…

Deep-sea submersibles are normally made of metal; carbon fiber construction was an innovation. Its high strength-to-weight ratio looked attractive for a craft that was expected to be very strong and yet able to float.

Carbon fibers are filaments two to four ten-thousandths of an inch in diameter—closer to spider web than human hair. They have a higher tensile strength than the strongest steel—around 500,000 pounds per square inch—and a compressive strength about a quarter of that.

You may wonder how such a slender filament can have compressive strength at all. Isn’t that like talking about the compressive strength of a rope? But if you take a sufficiently small segment of a filament, say only five or 10 times as long as it is thick, it is no longer a rope but a rod. And a rod can obviously support some amount of compressive load—that is, pressure applied to opposite ends.

We know from experience that if you push too hard on the ends of a long, skinny pole, it buckles. A 6-inch-long piece of uncooked spaghetti will bend outward and break if you press on its ends. A half-inch-long piece will hurt your fingers, but it won’t break.

You could make the longer strand carry a greater load if you tied it to a rigid support every half inch, so that it couldn’t buckle. This is the principle on which fine fibers in an epoxy matrix carry loads. Fibers are glued together to make a thick column within which each filament is continuously stabilized by all the others.

Compressive members are critical because if composite structures are going to fail, they are much more likely to do so in compression than in tension. Compressive components in airplanes, such as the upper portions of wing spars, are made of bundles of parallel fibers, not of the woven stuff found in fancy sporting goods. For the wing spars in Melmoth 2, I bonded multiple precured strips of unidirectional material that had been lab tested and found to fail in compression at 60,000 pounds per square inch. (Composites laid up in my garage and cured at room temperature do not attain the theoretical maximum strength of the material.) The way they would fail is significant: The surface fibers would pop out of the laminate. In other words, the force tending to bow the fibers outward exceeded the ability of the epoxy to hold them in.

This can happen within the laminate as well. Any imperfection—in epoxy, a tiny void, a speck of foreign material, a kink in a fiber bundle—can trigger “microbuckling” and minute delamination that grow with repeated loading. When Cameron spoke of carbon-fiber laminates failing insidiously over time, he was referring to this gradual accumulation of small cracks. Unlike cracks in metals, which usually appear on the surface and can be detected by visual or dye penetrant inspection, these are buried and detectable only when they have become quite large and then only by special testing equipment.

Titan’s pressure vessel, designed to withstand the 5,500-pounds-per-square-inch pressure where the Titanic wreckage rests, was an eight-foot long, 65-inch, carbon-fiber cylinder with hemispherical titanium end caps. At depth, each cap pressed on the cylinder’s ends with a force of around 18 million pounds. The aggregate pressure on the cylinder’s reportedly 5-inch-thick walls was more than 100 million pounds. If you assume a compressive strength of 125,000 psi and ideal fiber orientations, the hull would have had, when new, a factor of safety of around two. Because fiber orientations were probably not ideal, however, it was most likely lower than that, say 1.5 or even less.

Airplanes, too, use a safety factor of 1.5 over and above their highest anticipated aerodynamic loading. However, airplanes seldom, if ever, actually experience extreme loads, and if they do, it is only for very brief periods. A composite submersible, on the other hand, experiences extreme loading on every descent. Furthermore, it spends long periods at that loading, during which epoxy-to-fiber bonds can progressively deform and break. Indeed, people who had made previous dives in Rush’s composite submersibles—he designed several—report having heard cracking sounds, which must have concentrated their attention wonderfully.

An airplane undergoes a vast amount of strength and flight testing before being certificated and put on the market. The performance of its structure continues to be monitored, and the normally large number of examples in service gives the individual owner or passenger the comfort of knowing that if a problem occurs, it will probably happen to someone else first.

Titan was—and always would be—an experimental prototype. Notoriously, it was unclassed—the nautical analogue of uncertificated—because its creator, attempting to wrest the moral high ground from his critics, insisted that outside scrutiny puts a brake on the precious spirit of innovation.

Indeed, he was right. It does. Burt Rutan waxed eloquent on the unfair burden the required protection of desert tortoises placed upon the development of SpaceShipOne. But a brake is needed. Innovators are often people who are ambitious and headstrong, qualities that give them energy but may also blind them to the ethical implications of their actions. And this was not just innovation for its own sake. Rush was selling tickets.

Titan had made several previous deep dives and was evidently strong enough when it was new. But it apparently deteriorated with each prolonged imposition of enormous pressure. Composite airplanes, using similar materials and analogous construction techniques, are designed in the context of a huge accumulation of statistical data on loads encountered in flight—their character, strength, and duration—and on the behavior of composites in the flight environment. Composite airframes may have a service life limit, but none has yet been firmly established. Indications are that if one were established, it would be in the tens of thousands of hours.

Although the sample size falls short of statistical significance, Titan’s service life limit appears to have been more like a day or two.

This column first appeared in the September 2023/Issue 941 of FLYING’s print edition.

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It is the Bell XP-77. With a 100-square-foot wing of 27.5-foot span, it resembles a homebuilt, but one whose somewhat massive snout conceals an inverted Ranger V-12 engine of 520 hp. Two prototypes were built. Conceived to be fast, agile, and cheap, it managed only two out of three, topping out at a sluggish (for a fighter) 330 mph. According to Wagner, this was a disappointment because it was supposed to make 350, but it was actually remarkable the prototypes managed to go as fast as they did, considering they lacked superchargers. The project ended ignominiously. One prototype was destroyed when the pilot bailed out of an inverted spin. A second became a “gate guard” at some random Air Force base and eventually deteriorated into firewood.

Yes, firewood—because, as an offspring of one of the U.S. armed services’ occasional brief flings with nonstrategic materials, the XP-77 was built out of wood. I revisited it recently after happening to read an essay about wooden aircraft in World War II. The author, professor Eric Schatzberg, contends wood construction was viewed differently, and succeeded or failed, in different countries—he focuses specifically on Britain, Canada, and the United States—because of unconscious cultural biases that colored the way people perceived successes, failures, and the qualities of different airplanes.

Wood was the primordial material of airplane construction. Although Anthony Fokker—influenced by his chief designer, Reinhold Platz, who was a former welder—adopted welded steel tubing for his fuselages in 1916, by and large the airplanes of World War I were made of wood and fabric. Some were surprisingly sophisticated. The German firm Albatros, for instance, made beautifully streamlined, compound-curved fuselages of molded plywood in 1917. By 1918 Fokker had produced a wooden wing for the D.VIII fighter that would have looked perfectly in place a decade later on a Lockheed Vega. The preference for wood construction, particularly for wings, continued into the 1930s. Wood was a familiar material, inexpensive and easily repaired. Woodworkers were readily found or trained, and, as the importance of drag reduction came to be appreciated, the smooth, stiff surfaces of wooden wings were valued.

Wood and aluminum were, in some ways, interchangeable materials. The tensile strength of aircraft aluminum is 10 times that of Sitka spruce, but spruce has a tenth the weight of aluminum. (Sitka spruce, from the western forests of the U.S. and Canada, is the wood of choice for aircraft not because it is especially strong but because it is readily available in long, knot-free planks with straight grain.) But wood had some arguable advantages. Shaping and assembly of wood parts does not require exotic tools. Wood, unlike metal, does not fatigue. The great drawback of metal construction, before the advent of reliable adhesives, was its reliance on tens of thousands of rivets, which a pair of workers must install one at a time. Even apart from the labor saving, bonded joints had the advantage of avoiding the stress concentrations produced by rivets and the opportunities for hidden corrosion offered by overlapping metal joints.

There were metal-skinned airplanes in the 1920s, such as the Ford and Fokker Trimotors, but they were horrible, corrugated things. The Douglas DC-1—followed closely by the DC-2—with its smooth aluminum semi-monocoque construction and superb streamlining, was as stunning an innovation in 1933 as the SR-71 was in 1964. In the United States, Schatzberg argues, metal was perceived as synonymous with progress and modernity. Wood seemed to be a quaint material more appropriate to sailing ships. The hostility to wood, on grounds of supposedly poor durability, moisture absorption, and lack of uniformity, was sometimes quite palpable, but, Schatzberg suggests, was owing to an unconscious aversion to adjusting methods and expectations to the peculiarities of the material. When James “Dutch” Kindelberger, CEO of North American Aviation, complained to General K.B. Wolfe, head of the production division at Wright Field, the Army Air Corps’ testing center, about an Air Corps requirement for wooden fuselages for AT-6s, Wolfe did not hold back. “I would just like to just push a few of these [wooden] jobs out into the training crowd and let them see what they are up against,” he fumed. “We are just making a lot of trouble for ourselves on this wooden program.”

The British mindset was quite different. When the Air Ministry was considering de Havilland’s proposal of a fast, light bomber built of wood, the objections raised were to its lack of defensive armament, not to the material. De Havilland’s argument that the twin-engine Mosquito’s speed and range would protect it against German fighters eventually prevailed and proved to be correct.

• READ MORE: Technicalities: Piggybacks and Parasites

It may be that, apart from any cultural proclivity for wood as a raw material, the British acceptance of wooden construction had its roots in the remnants of an artisan culture that persisted in Europe well into the 20th century but was vanishing in the United States, thanks largely to Henry Ford. Wood construction, because of the dimensional instability of the material, often required some hand work preparatory to final assembly. The British willingness to employ files and sandpaper extended beyond woodwork. British Merlin engines were assembled with a similar acceptance of imprecision, and when the Packard company, contracted to produce Merlins in the U.S., received the British blueprints for the engine, it judged them unusable by American workers, who expected parts to be identical and fully interchangeable, and to fit perfectly on the first try.

It’s easy to grow sentimental over the special beauties of fine wood construction, be it in a desk, airplane, or old Chris-Craft boat. The most ambitious example to emerge during the war, however, aroused no such feeling and was also probably the most compelling proof that wood was a freak visitor from the past. It was Howard Hughes’ monster “Spruce Goose,” for a long time the world’s largest airplane. I climbed through it once, while it was still locked in a tin hangar at Long Beach, California. Its cavernous fuselage was built of molded panels of heat-cured, resin-bonded plywood, a process pioneered by American Virginius Clark in the 1930s. Standing at the base of its vertical fin, my eyes (but not my feet) following a ladder into its murky recesses, I thought of medieval bell towers and their twisting, claustrophobia-inducing stairs. Wherever I looked, however, I did not think about the future.

This column first appeared in the August 2023/Issue 940 print edition of FLYING.

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Well, OK—you can just stretch a dumb joke so thin.

Q—uppercase here because it’s starting a sentence but normally lowercase—is the symbol aerodynamicists use for the dynamic pressure, or impact force, of moving air. It is the thing captured by your pitot tube and registered on your airspeed indicator—just a different way of expressing indicated airspeed. Max q is the peak dynamic pressure experienced by a rocket as it ascends through the atmosphere. It is an important milestone during the ascent because rockets have to be as light as possible, and so their structures are engineered to withstand no more aerodynamic stress than necessary.

We have all watched the launches of large rockets. At first they rise slowly, balancing acrobatically on a slender finger of flame. As they burn fuel and become lighter, they move faster and faster. While they ascend, the air through which they pass becomes less and less dense. At some point between liftoff, when the dynamic pressure is infinitesimal because they are hardly moving, and arrival in empty space, when it is infinitesimal again because there is no air, the product of speed squared and air density reaches a maximum. This is max q, and it defines the structural strength of the rocket. During a typical Falcon 9 launch, max q occurs at an altitude of around 10 miles, a minute or so into the flight. In order to keep max q below the design limit, many rockets throttle back their engines at some point to allow them to gain more altitude before resuming maximum acceleration.

A typical value for max q is around a third of an atmosphere, or 4 or 5 pounds per square inch. To get a feel for what this means, you can make use of the convenient fact that q at sea level is approximately 25 pounds per square foot (psf) at 100 mph. It’s proportional to the square of speed, so at 200 mph it’s 100 psf, at 300 mph it’s 225 psf, and so on. Five pounds per square inch is 735 psf, which would correspond to a speed of 540 mph (470 knots) at sea level. The rocket reaches max q at an altitude where an indicated airspeed of 540 mph means a true speed of 1,000 mph or more. (Coincidentally, an SR-71 flying at 65,000 feet at 2,000 mph would also have an indicated airspeed of 540 mph.)

Different rockets experience different values of max q, depending on their intended rate of acceleration and trajectory. Trajectory matters because rockets do not aim straight up for long. The orbital velocity they aim to achieve, somewhere above 17,500 mph, is a horizontal velocity, not a vertical one, and so the flight path of a rocket is a compromise between wanting to get above the denser portions of the atmosphere as quickly as possible and wanting to devote as much of the energy available from fuel as possible to accelerating horizontally rather than vertically.

Dynamic pressure is of obvious importance in aerodynamics. When the Wright brothers were trying to figure out how to fly, one of their preoccupations was determining the correct value of what was then called “Smeaton’s coefficient” after John Smeaton, an ingenious civil engineer of 18th century England whose work with windmills and waterwheels required him to find out how much pressure could be exerted by moving fluids. The Wrights concluded, correctly, that the value Smeaton had arrived at was too high by half. Besides, it was valid only near sea level. Once airplanes started getting up a few thousand feet, Smeaton’s single coefficient became obsolete and was replaced by a mathematical function that took into account the variation of air density with altitude.

Merely knowing that air exerts a pressure of 25 psf at 100 mph, and that its pressure is proportional to the square of speed, tells you a lot about actual airplanes. The lift and drag coefficients you hear about are just numbers you multiply by the dynamic pressure to get an actual force. For instance, if a book tells you that an airplane with a simple unflapped wing stalls at a lift coefficient of around 1.35, you can estimate that at 60 mph a wing of 120 square feet will support about 1,460 pounds. (Hint: q at 60 percent of 100 mph is 25 x 0.6 x 0.6, or 9 psf.)

The drag coefficient works the same way. The terminal velocity of a falling object, be it a skydiver or a spent rocket booster, is reached when its drag is equal to its mass. Thus, terminal velocity for a falling object represents a sort of max q in reverse.

Here is a challenge for the arithmetically nimble. Suppose you know that a certain finned object has a drag coefficient of 0.03 based on frontal area. Assuming that its drag coefficient is tripled on its way through the “sound barrier,” could it, if dropped from a sufficient height, surpass the speed of sound at sea level, which is 761 mph? How is it affected by making it larger, while preserving its shape? What would its girth be, if it were made of iron? Don’t you miss the word problems you used to get in school?

Max q imposes the maximum aerodynamic drag experienced by the rocket. One way to reduce the drag penalty might be to launch in thinner air, using an airplane to lift the rocket the first 10 miles or so to put the densest portion of the atmosphere behind it. While a rocket engine cannot lift a mass exceeding its thrust, the lift of a wing exceeds the thrust needed to propel it by a factor of at least 10 or 15. Air launch was the method Burt Rutan used to launch SpaceShipOne—but that was a suborbital flight, accelerating only to Mach 3.

For launches to orbit, overcoming gravity demands far more energy than overcoming drag. Elon Musk has argued that the energy savings from air launch is only around 5 percent, and that he would rather make his rockets 5 percent bigger than build and maintain an enormous carrier aircraft. As the old Packard car ads used to say: “Ask the Man Who Owns One.”

This column first appeared in the July 2023/Issue 933 print edition of FLYING.

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