Faster-than-Sound Effects

Many a layman was struck by an odd item of news about the giant new B29. To avoid certain mysterious "supersonic" effects on the propeller tips, the whirling of its 16-foot-long propellers had to be geared down to one-third of the speed of revolution of the plane's engines.

Whether or not they could explain it, the phenomenon itself was an old story to airmen. They have known for some time that mankind has caught up with the speed of sound and is being catapulted by the airplane into a weird, high-speed unknown which plays some very strange tricks.

Diving airplanes have been reported as reaching supersonic-speeds* of 840 m.p.h. Veteran airmen raise their eyebrows at these reports, knowing that at such speeds speedometers are unreliable. But there are authenticated records of peculiar happenings at plane speeds of 500 to 600 m.p.h., which approach the velocity of sound: Examples:

P: The rudder and elevators sometimes lock tight, sometimes flap violently, as if buffeted by Niagaran rapids.

P: Pieces of cowling and window panels, supposed to be many times stronger than any load that could be put on them, are torn off like paper.

P: Though the pilot pulls hard on his stick, the plane has little lift, comes out of a dive very slowly.

P: Sometimes, despite a surplus of power, a plane in a dive can go no faster and may actually begin to lose speed.

Bullets and Planes. All these phenomena are the result of what engineers call air "compressibility" (a kind of super air resistance). The compressibility of air determines the speed at which sound travels. That speed varies according to temperature; at cold, high altitudes sound slows down. At sea level, sound moves at about 760 m.p.h.; above 35,000 feet, at about 660 m.p.h. At these speeds, air molecules bunch so solidly and resistance becomes so great that sound waves reach their speed limit.

Bullets and airplanes are similarly impeded. When they hit air at high speed, the air bunches up in front of them like the hill of water against a ship's bow. The compression creates "shock waves"--turbulent air formations whose impact enormously increases air drag. A bullet can be driven through these waves by explosive power, but no one has yet designed an airplane engine and propeller combination powerful enough to overcome them.

Shock waves become an aviation problem long before the plane itself reaches the speed of sound. Reason: local changes in air speed on the plane's surfaces. Air molecules, hastening to get out of a plane's way, speed up as they flow around the curved surfaces of the plane's wings and propeller. The wider the curve, the faster the air travels. This accelerated air, moving faster than the plane, may reach supersonic speeds and create local shock waves, known to airmen as "compressibility burble." Designers have reduced this hazard by giving wings and propellers thinner leading edges; these are now shaped more like a knife than a teardrop. But the biggest advance is the jet-propelled plane. The part of a plane that first feels supersonic effects is its whirling propeller, whose tips reach a very high speed. By eliminating the propeller entirely, the jet-propelled plane raises the airplane's speed ceiling by 100 m.p.h.

Physicists have yet to answer the fascinating question of what effect, if any, sound itself has on a plane's speed. Why do shock waves hit a plane at the speed of sound? Why not sooner--or later? The physicists' best guess: sound waves signal ahead to air molecules to get out of the way of a moving object; at supersonic speeds the object outraces the warning, runs smack into groups of unwary molecules.

* Supersonic, coined to describe sound waves inaudible to the human ear, is now also commonly used to mean faster than sound."

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