Demon friction
Evan Zabawski | TLT From the Editor October 2017
Going fast can be such a drag.
(X-1 courtesy Wikimedia.)
‘The resistance of a wing shoots up like a barrier against high speed as we approach the speed of sound.’ (Chart courtesy of NASA.)
“THERE WAS A DEMON THAT LIVED IN THE AIR” opens the narrator in 1983’s The Right Stuff. “They said whoever challenged him would die. Their controls would freeze up, their planes would buffet wildly, and they would disintegrate. The demon lived at Mach 1 on the meter, 750 miles an hour, where the air could no longer move out of the way. He lived behind a barrier through which they said no man could ever pass. They called it: the sound barrier.”
The term refers to a seemingly physical barrier encountered as an aircraft approaches the speed of sound, at which point it experiences sharply increased drag, buffeting and loss of lift and control. Its origins stem from a misinterpretation of an explanation by British aerodynamicist William F. Hilton, of his high-speed experimental work, to a reporter in 1935. While pointing to a plot of airfoil drag, he said, “The resistance of a wing shoots up like a barrier against high speed as we approach the speed of sound.”
At the time it was known that projectiles like cannonballs and bullets could exceed the speed of sound, but it was unknown if an aircraft and its pilot could withstand the forces transitioning from subsonic to supersonic speeds (the transonic speed range). In 1945 the Bell Aircraft Co. was contracted to build three aircraft to be used to obtain flight data within this range; the aircraft were originally designated XS-1 for Experimental, Supersonic but later changed to X-1. The Bell X-1 was called a “bullet with wings” since its design was based off a .50-caliber bullet.
The Right Stuff depicts Capt. Chuck Yeager piloting the aircraft as if it were on a maiden flight. In reality, test pilot Jack Woolams had already flown 10 glide flights before Chalmers Goodin took over. Goodin completed 26 more test flights in two different X-1s, some under power, before Yeager began his trials. Yeager then completed 10 incrementally faster flights in the X-1 with serial 46-602, nicknamed Glamorous Glennis, after his wife.
During the eighth flight, the plane stopped responding to elevator controls, which analysis revealed was due to a shockwave forming along the elevator’s hinge line. Aeronautical engineer Jack Ridley determined that the elevator need not be used at transonic speeds but, rather, pitch control could be realized by adjusting the entire horizontal stabilizer. This design became known as the “flying tail,” a design incorporated into all supersonic aircraft.
Yeager’s 13th X-1 flight, and the program’s 50th flight, took place on Oct. 14, 1947. The X-1 was dropped from a modified B-29 Superfortress, after which Yeager lit the four rocket engines and began a steep climb. In his West Virginian drawl, he radioed, “Had a mild buffet… jes the usual instability,” and once he reached Mach 0.96, he radioed, “Say Ridley, make a note here… elevator effectiveness regained.” The flying tail worked!
After achieving Mach 0.98, the Machmeter needle fluctuated and jumped off the scale. Personnel on the ground both heard and felt a sharp crack. The voice on the radio said, “Say Ridley, make another note will ya? There’s something wrong with this ol’ Machmeter… it’s gone kinda screwy on me.” Following a quick exchange, Yeager could be heard chuckling to himself. After the sound barrier had been broken, the X-1 aircraft would go on to fly 107 more times.
A few years later, wind tunnel experiments helped develop the transonic area rule, which prescribed that the cross-sectional area of the fuselage, from nose to tail, be smoothed out by pinching the fuselage over the wings. This design led to significantly reduced drag at transonic speeds and removed the last major hurdle to supersonic travel.
Evan Zabawski, CLS, is the senior technical advisor for TestOil in Calgary, Alberta, Canada. You can reach him at ezabawski@testoil.com.