Definition: Flight airspeeds greater than the average, especially speeds close to the maximum attainable speed for the era.
Significance: The utility of civilian or military aircraft is always enhanced by increases in practical flight airspeeds. Much of the historic progress in aviation has revolved around solving the aerodynamic, structural, power, and heat problems associated with ever increasing speeds. Races and speed records, with their promise of prize money or prestige, have often stimulated individuals and governments to advance the art and science of high-speed flight.
As the first humans to make a controlled, powered flight in 1903, Orville and Wilbur Wright held the first unofficial speed record, at 30 miles per hour. Official speed records, though, are those that have been authenticated by the rules of the Federation Aeronautique Internationale (FAI), formed in Paris in 1906, and these speed records begin with Alberto Santos-Dumont’s 26 miles per hour in 1906, increasing to 83 miles per hour by 1911, 119 miles per hour in 1913,192 miles per hour in 1920,469 miles per hour on the eve of World War II in 1939,606 miles per hour with the first jets in 1945,1,526 miles per hour with second-generation fighter aircraft in 1959, and 2,194 miles per hour by the SR-71A in 1976.
Early High-Speed Flight
Initially, flight speeds were limited primarily by the lack of lightweight power plants, not surprising in view of the fact that development of the gasoline engine was still in its infancy. Thus, the first airplanes required a very large wing area and the most efficient structure was the bridge-based biplane, but its large size and attendant struts and wires generated a great deal of drag. By 1909, Glenn H. Curtiss was able to take his draggy biplane to first place in the first Gordon Bennett closed-circuit race at Rheims, France, with a speed of 47 miles per hour, mostly due to his development of a 50-horsepower V-8 engine that bettered the Wright brothers’ original 12-horsepower engine.
Invention of the relatively lightweight, reliable, air-cooled rotary engine, in which the crankshaft is bolted to the aircraft and the cylinders and propeller rotate, led to the next great increment in high speed, culminating in a winning 124.5 miles per hour by a special Deperduss in monoplane racer in the last prewar Gordon Bennett race in 1913.
World War I led to improvements in structures and power plants but negligible increases in speed because of the emphasis on climb rate and maneuverability, which favored biplanes.
After World War I, the Schneider Trophy race for seaplanes inspired great advances in engines. The liquid-cooled engine assumed prominence because of its low frontal area. By 1927, Reginald Mitchell’s 900-horsepower Supermarine S.5 had established an absolute speed record of 282 miles per hour; it was the last time a biplane would win the race. The Supermarine S.6 retired the Schneider Trophy with an uncontested win at a speed of340 miles per hour in 1930, with 2,300 horsepower available from its supercharged, liquid-cooled Rolls-Royce V-12. These racers, however, were impractical aircraft because they used low-drag skin radiators to dissipate the tremendous heat from their liquid-cooled engines.
The winning Rolls-Royce V-12 engine in the S.6 had been inspired by the Curtiss V-12 engine that had powered the Curtiss CR-2 biplane to first place in the Pulitzer race of 1921 (and later to an absolute speed record of 198 miles per hour) and by the CR-3 that won the Schneider Trophy in 1923 at 178 miles per hour. In the United States, however, by the mid-1920′s, water-cooled engines were taking second place to newly developed, reliable, air-cooled radial engines that were much lighter and much more suited to commercial applications, but presented a great deal more frontal area and accompanying drag. Charles A. Lindbergh’s historic New York-to-Paris flight in 1927 was made possible by a 220-horsepower Wright Whirlwind radial engine, for example.
The monoplane route to higher speeds, now that brute power had accomplished about all it could do, was shown by the features of the unheralded, trouble-prone Dayton-Wright RB-1 racer of 1921: a small unbraced (cantile-vered) wing that used flaps to yield acceptable take off and landing speeds, a closed cockpit for less drag, and a retractable landing gear. With a low-drag NACA cowling for its radial engine (developed by the National Advisory Committee for Aeronautics or NACA, the predecessor of NASA), the 450-horsepower TravelAir Mystery Ship won the 1929 National Air Races in Cleveland, besting all the military biplanes. James H. “Jimmy” Doolittle set a land plane record of 294 miles per hour in 1932 in the small-winged, bottle-shaped GeeBee R-1, using an 800-horse-power Pratt & Whitney radial engine. By 1939, however, Lockheed had flown the aerodynamically clean prototype of its P-38 twin-engined Lightning to a top speed of 413 miles per hour, and civilian aircraft were forever out of the race for the highest speeds.
In World War II, piston-engined fighters reached speeds of around 500 miles per hour, but only at altitudes above about 20,000 feet, where their supercharged or turbo-charged engines could take advantage of the less dense air. The liquid-cooled P-51H Mustang reached 487 miles per hour at 25,000 feet using water/methanol injection and high-octane fuel. An experimental version of the Republic P-47 Thunderbolt, using an 18-cylinder, air-cooled radial engine, reached a speed of 504 miles per hour at 34,450 feet. The Goodyear F-2G, developed from the Chance Vought F-4U Corsair, could reach speeds close to 500 miles per hour. Nonetheless, two facts threatened to forever prevent higher speeds: propeller and wing compressibility (Mach) effects.
The Sound Barrier
Propellers were becoming less and less efficient as their tips approached the speed of sound; the air would break away from the tips and form ear-splitting shock waves. Worse, planes and pilots were being lost when the airflow over the wing approached the speed of sound. Because air is speeded up over the top surface of a wing to produce a lower air pressure there and thereby generate lift, the speed of sound is reached at that point before (sometimes well before) aircraft speed reaches sonic speed (Mach 1). Shortly after a local airspeed of Mach 1 is exceeded, a shock wave is formed where the air suddenly has to be slowed back to subsonic speeds; because the pressure waves that inform the air that it must change its speed or direction cannot propagate into the region ahead of this point, the shock wave represents an extremely narrow region perpendicular to the wing surface where the pressure and density and temperature of the air greatly increase. Shock wave formation not only greatly increases the power requirement, it also causes the airflow to break away from the wing at that point, producing effects very similar to the low-speed stall created when the wing is at a high angle relative to the oncoming air.
At less than 70 percent of the speed of sound (Mach 0.675), a speed easily reached in a dive, the P-38 became uncontrollably nose-heavy as the wing lost lift and the horizontal tail surface lost its downward force; dive flaps were added to the sides of nacelles to save future pilots. Of U.S. fighters, the P-51 Mustang suffered the least from compressibility, thanks to its laminar-flow wing with the thickest point well back from the leading edge, but its pilots were warned that in high-speed dives, uncontrollable violent porpoising preceded a loss of altitude of 10,000 feet or more, at which point a recovery might be possible, because the Mach number decreased as the air temperature increased. The British Spitfire, with its very thin wing section, was eventually dived successfully to Mach 0.9, but it still was not at all clear that controlled supersonic flight would ever be possible.
The propulsion problem was solved by the invention of the turbojet engine by the British and Germans, an engine which obtains thrust by taking in air and using it to burn fuel, with the byproducts exiting to the rear at a much higher speed. The thrust generated is equal to the rate of change in momentum (the product of mass and speed) generated by the engine. The jet engine is most efficient in the less dense air at altitudes above 20,000 feet.
Rocket engines produce even more short-term thrust than jet engines, for their weight, and were used for the earliest transonic and supersonic flights. (Transonic flight is flight for which there is mixed subsonic and supersonic flow, approximately Mach 0.8 to Mach 1.2.) By 1944, the German rocket-powered Messerschmitt Me-263B had reached a speed of 703 miles per hour and the much more practical, jet-powered Messerschmitt Me-262 had reached 624 miles per hour.
Higher speeds required better aerodynamics, including a recognition of the advantages of the swept-back wing and solutions for its disadvantages. Adolf Busemann, in 1935, first published the finding that a swept wing permits a wing to be effectively thinner because the chord (width) of the wing is greater than for a similar unswept wing. It spreads the lift and the cross-section of the wing over a greater percentage of the fuselage, reducing the suddenness of the drag rise and the pitch-down tendency. However, the spanwise flow on a swept wing also tends to cause the wingtips to stall first at low speeds or when maneuvering, making the ailerons ineffective and producing a violent pitch-up tendency. The swept wing also suffers from a Dutch roll (coupled yaw and roll) tendency, which can be serious enough to destroy the aircraft. The stall problem can be treated by using high-lift devices (slats) on the leading edge of the outer wing panels and by chordwise plates (fences). The Dutch roll tendency can be treated by a gyroscopic-based yaw damper, as well as aerodynamically. In May, 1948, the swept-wing, jet-powered North American F-86A Sabre jet achieved an official world speed record of 670 miles per hour.
Supersonic Flight
The sound barrier, however, was still to be breached. The United States chose the bullet-shaped, rocket-propelled Bell X-1 to make the attempt. (Bullets were known to reach supersonic speeds in flight, but they had to spiral for stability and they did not try to use lift to stay in the air.)
With only 2.5 minutes worth of rocket fuel, the only available route was to use an air launch from a modified B-29 bomber and glide to a landing afterward. This was possible only because the United States had California’s clear skies and vast Muroc Dry Lake (the present Edwards Air Force Base) for landing. On October 14, 1947, test pilot Charles E. “Chuck” Yeager reached Mach 1.06 at 43,000 feet and the first human-generated sonic boom was heard. He glided back at 250 miles per hour, approached at 220 miles per hour, and landed at 190 miles per hour.
The Russian MiG-19 was the first fighter capable of supersonic flight in level flight, followed shortly by the Republic XP-91 and North American’s F-100 Super Sabre. The secret was obtaining a short burst of extra thrust by dumping raw fuel directly into the exhaust (after the turbine), a practice called afterburning in the United States and reheat in Great Britain. However, the Super Sabre also had to be cured of a new disease: inertia coupling. With a long, heavy fuselage supported by short, light wings, an aircraft has roll inertia (resistance to changes in roll around the nose-to-tail axis) that is much less than its pitch and yaw inertia, and rolling motion can induce a pitching moment that sends an aircraft into a disastrous tumble. Additional tail and wing area solved the problem for the Super Sabre.
The FAI set new rules for speed records at high altitudes and Great Britain quickly claimed an absolute speed record inMarch, 1956, whenits Fairey Delta 2 (FD.2) flew at 1,132 miles per hour. The official speed record as of 2001 was held by Lockheed’s SR-71 Blackbird (a reconnaissance aircraft capable of over Mach 3 at 100,000 feet altitude); in 1976 it averaged 2,194 miles per hour, but probably could have gone even faster.
Transport aircraft have followed in the path blazed by research and fighter aircraft. The Douglas DC-2 almost won the London to Melbourne race in 1933 against specialized racing aircraft. The Boeing 707 was the first successful jet transport, going into service in the late 1950′s. The French/British Concorde began Mach 2 airline service in 1976; it uses a highly swept delta planeform with sharp leading edges generating vortices that greatly enhance lift at low speeds (vortex lift).
Hypersonic Flight
Hypersonic flight (greater than Mach 4 or 5) is the new frontier and in this flight realm, heat is the primary foe. Even on the Concorde, skin temperatures of 260 degrees Fahrenheit are reached and the fuselage lengthens by 9 or 10 inches in flight. On the titanium SR-71, temperatures reach 600 degrees Fahrenheit.
The North American X-15 showed that a rocket-powered research aircraft could reach hypersonic speeds; by 1967 the X-15 had flown to 354,000feetandMach6.7, but it had also been seared by 3,000-degree-Fahrenheit temperatures. Ablative (sacrificial) coatings were used; they melt away at high temperatures while absorbing and dissipating the heat.
After the Soviet Union sent Yuri Gagarin into Earth orbit in 1961, beginning the race to the Moon, spaceflight, rather than higher-speed atmospheric flight, became the next U.S. challenge. It was followed by space stations and the space shuttle, the latter of which must attain just the right speed for its orbital height and then, with braking rockets, reenter the atmosphere at about 140,000 feet and Mach 6.7.
The X-43, making its first flight in 2001, is an uninhabited hypersonic research aircraft. It remains to be seen whether it will make hypersonic flight a regular occurrence.
