Despite over a century of development since the Wright brothers first developed their own surprisingly efficient design, has propeller technology already reached its limits?
By: Ringo Bones
Believe it or not, the Wright brothers not only managed to make the first working heavier-than-air craft but also a surprisingly efficient propeller that made it possible to fly in the air with relatively little engine power. Even though other aviation pioneers that came before the Wright brothers managed to take off, albeit briefly, into the air with the heavier-than-air craft of their own design, it failed sustained powered flight largely because the propellers used are highly inefficient.
After the Wright brothers developed a suitable airframe and a gasoline engine light and powerful enough to theoretically take it into the air, designing an effective – as in efficient – propeller proved perplexing. The brothers recognized a salient point: that a propeller is really a wing or airfoil moving in a spiral course. Just how it worked, however, baffled them. “With the machine moving forward,” they later wrote, “the air flying backward, the propellers moving side-wise, and nothing standing still, it seems impossible… to trace the… reactions.” It took months, but in the end they had formulated and built an efficient propeller, and on December 17, 1903 at Kitty Hawk all was ready for the final test.
The efficiency of the original Wright propeller was a marvel for its day: it could translate 66-percent of its engine’s rotational energy into forward thrust. After more than a century of research in aeronautics, the best of today’s airscrews achieve about 85-percent efficiency. Does this mean that we can no longer design more efficiency into a typical airplane’s propeller this day and age?
From a historical perspective, the limits of propeller technology were probably reached around a decade before Frank Whittle build his very first working jet engine. As piston engine performance advanced, it became necessary also to improve the thrusting device, the propeller, which had been a major source of trouble since the pioneer era of aviation. Until the 1920s, all propellers were made of wood. In wet weather they were likely to absorb water, and if one blade absorbed more than the other, the propeller became unbalanced, setting up a tremendous vibration in the airplane. At the time, propellers sometimes flew apart in the air. If one blade flew off and the other remained, the resulting imbalance could – and often did – tear the engine out of the airplane.
The more powerful the engines became, the faster propellers had to turn. This produced very high tip speeds. As with any whirling mechanism – be it a propeller, automobile wheel or merry-go-round – speed increases with distance from the hub, since the outer rim or tip must move a greater distance during each revolution. High tip speed brought on potentially destructive vibrations.
Hoping to avoid the defects of wooden propellers, planemakers tried aluminum. But aluminum metallurgy was still at its infancy back in the 1920s and aluminum propellers were subject to cracks and pitting and occasionally, one would shear off in flight. The first steel propellers, tested in the early 1920s, frequently caused trouble, sometimes before they leave the ground. In 1921, Frank Caldwell, a propeller specialist, subjected an early steel propeller to twice its rated power on an electric testing device. It appeared to withstand the strain beautifully, so it was mounted on a stationary airplane engine in a laboratory. The propeller was revved up to its full power – at which point a blade broke off, sliced through an instrument control board, passed between the heads of two technicians, flew up a flight of stairs and out through the roof. The engine was reduced to rubble.
Years of testing and experimentation, particularly directed to reducing the vibration inherent in propellers turning at high speed, led to more reliable designs and manufacturing techniques. Propeller failure virtually ceased to be a serious problem.
But those who, during the 1920s, looked ahead of to the day when aerodynamic research would make possible high subsonic and even supersonic velocities concluded that even the best propeller has a limited future. They saw that the piston engine had a power potential of perhaps 5,000 horsepower, and this would increase the problem of tip speed.
Since propeller tip speed is faster than the airplane’s forward speed, the propeller tips of most planes flying at 450 miles per hour would have their propeller tips rotating at supersonic speeds. At such speeds, the thrusting efficiency of the propeller is reduced. Clearly it was time to investigate a source of thrust free of the propeller’s limitations and thus paving the way for the development of the jet engine.