Planes Race Past Speed of Sound; Bump in Floor Shows How
June 1948 Popular Science

June 1948 Popular Science
June 1948 Popular Science Cover - Airplanes and Rockets[Table of Contents]

Wax nostalgic over early technology. See articles from Popular Science, published 1872 - 2021. All copyrights are hereby acknowledged.

Chuck Yeager broke the sound barrier in his Bell X−1 (aka Glamorous Glennis) airplane on October 14, 1947, over the Mojave Desert. Control reversal in the transonic realm (transitioning from subsonic to supersonic speeds) is a phenomenon caused, per most authoritative sources, from the pressure wavefront around the aircraft transitioning from entirely in front of the airplane to some point aft of any leading portion of the airframe. That includes the fuselage nose, and wings and empennage leading edges. Airflow can transition from laminar to turbulent at various distances, thereby altering the aerodynamic forces on the fixed and moving portions of the surfaces. Control reversal can also occur due to control surface deflection causing a twist in the fixed surface which opposes the input intention. Britain's Spitfire exhibited such behavior in high speed dives, and even the human-powered Gossamer Condor reportedly had control reversal due to a flimsy airframe structure to the extent that the solution was to reverse the direction of aileron deflection in order to obtain a proper response. Supersonic airframes are rigid enough to mitigate the flexure problem.

Planes Race Past Sound; Bump in Floor Shows How

During tests, air speed over wing is measured by pressure openings on bump and in vertical plate - Airplanes and Rockets

Model wing is installed, below, on turntable. It has 6.8-inch span and is swept back at 60° angle. During tests, air speed over wing is measured by pressure openings on bump and in vertical plate, 10 inches high, that can be seen in photo at the top of the opposite page.

By squeezing air in a wind tunnel, engineers help solve the problems of flight through the transonic region.

By Andrew R. Boone

For years pilots and engineers have worried about what would happen to an airplane that entered the terrifying region bounded by the speed of sound. Under that speed, an airplane was safe; above that speed, an airplane would be safe. But at that transonic speed the cockpit controls might "reverse," and a plane do just the opposite of what it was supposed to do. Transonic forces might even shake a plane apart.

Now they know that little need happen at the speed of sound that doesn't happen normally in flight at other speeds. In fact, the government research agency, the National Advisory Committee for Aeronautics, refers freely to "the transonic research airplanes now flying."

The scientists have discovered what to do to an airplane to make it fly successfully into and through the mysterious transonic region. They found out by building a bump in a wind tunnel to produce an air speed equal to that of sound, and testing the behavior of model planes and parts in this blast.

Without the bump, air blown through the tunnel flows smoothly at speeds below that of sound; but, just at that speed, it "chokes" between the walls and the test model. With the bump, air moving slightly slower than sound is squeezed up to transonic velocity for a fraction of a second just where the tunnel narrows - and where the test model is placed. Beyond the bump, the air slows up again before it has had time to choke the tunnel.

Model is polished before a high-speed test run - Airplanes and Rockets

Model is polished before a high-speed test run to remove surface irregularities whose air drag would be magnified many times when readings of instruments in the wind tunnel are translated into stresses on a full-sized aircraft flown at the corresponding speed.

Model is assembled for test at transonic speed in wind tunnel with bump - Airplanes and Rockets

Model is assembled for test at transonic speed in wind tunnel with bump. Wing shown here, made of machined brass, has 5-inch span and 28-degree sweepback. Maple fuselage, shaped like that of P-80 Shooting Star jet fighter, is reinforced by aluminum strip along its base.

One miniature model, made wholly of metal, began disintegrating at the speed of sound - 763 m.p.h. at sea level. But this was a planned disaster. By shattering its wing, Lockheed Aircraft's scientists, working at California Tech's Cooperative Wind Tunnel in Pasadena, proved that this particular design could never endure supersonic flight.

Other designs can and will. This too has now been proved. The logical design for the airplane assaulting the transonic barrier is as smooth as a torpedo.

Air 'Boils' at Speed of Sound

"In the past" we have had trouble in the speed-of-sound region because air gets flustered at that velocity," says Hall Hibbard, Lockheed's chief engineer. "It acts like boiling water. But air is stable both below and above that speed."

"We have had trouble because air flows at different velocities over different parts of an airplane. A plane may be flying at 600 m.p.h., but the air must flow 763 m.p.h. or faster to get over some projection - like the cockpit cover. It's the parts of a plane, not the entire structure, that run into trouble."

Miniature wind tunnel patterned accurately to one-hundredth of an inch - Airplanes and Rockets

Miniature wind tunnel, patterned accurately to one-hundredth of an inch on the big one at Caltech, served as a working model in which these Lockheed engineers perfected bump that produces transonic air speeds. Experimental bump, of brass, is visible inside its mouth.

So the trick is to design a plane that will disturb the temperamental air as little as possible. It will take extra-clean fuselages to pierce the transonic wall. It is also reasonable to expect, on the basis of these experiments, that these airplanes will need swept-back wings. Here's why this is so:

On the conventional Lockheed Shooting Star jet fighter. air drag suddenly goes up around the wing at 625 m.p.h. Things get worse so fast that this resistance is almost eight times as great at the speed of sound as it is at 625 m.p.h. But sweep back the wing 50° and the air doesn't start to act up until 742 m.p.h., and the resistance at sound's 763 m.p.h. is only twice as great. This means that a conventional Shooting Star would require 55,000 hp. to smash through transonic resistance; one with a wing swept back 50°, only 22,400 hp.

Wings will be thinner too, the wind-tunnel tests show, but they must be rugged. Otherwise, they may twist under pressure of the aileron and thus apparently reverse the effect of the controls. When this danger is avoided by proper design, the transonic barrier is reduced to a momentary loss of control due to air turbulence. And designers hope to get around even that by altering the tail.

"Tails present the same problems as wings," explains Paul W. Theriualt, Lockheed's wind-tunnel chief. "They are actually small wings. There will be a tendency to place the tails higher and farther from the wings, to get them away from the wake and turbulent air. Tails will probably have more sweep than the wings, so that the compressibility effect will be felt first by the wing. During that critical initial period, over in a second or two, the tail will control the airplane."

Still secret is the exact wing shape that keeps control past the speed of sound.



Posted March 9, 2024