Some time ago I wrote about the relationship between thrust and horsepower, and the question of why one is used to describe the output of pure reaction engines and the other that of engines driving propellers. I argued that the reason was historical. The great majority of engines-steam, gasoline or what have you-have been designed to bring about some sort of mechanical rotation. The force driving a rotating shaft, multiplied by the speed of rotation, provides a straightforward way of measuring the rate at which a machine can do work, which we call its horsepower.
How horses got involved in all this is another matter. A human athlete is capable of brief bursts of about one horsepower, and continuous output at about a third of one. I would have thought that a normal horse, being much larger than a human being and having eaten fewer Twinkies, should be capable of putting out much more than one horsepower; but the Oxford English Dictionary avers that horses possess only three-fourths of a horsepower. I invite readers to contemplate this koan-like paradox at their leisure. At any rate, we apparently owe the term horsepower, which came into use around the start of the 19th century, to James Watt. Watt developed the first practical steam engine, and in modern times his name has replaced the horsepower as a measure of engine power wherever the metric system is used, which is to say, almost everywhere in the world except here. The watt is a relatively puny thing, however-without umbrage, I hope, to the excellent Mr. Watt-and it takes about 750 of them to make a single horsepower.
What I did not touch upon before, however, is why jets perform differently from props, and why they are the preferred powerplants for fast aircraft.
When you bolt a propeller to a rotating shaft, be it driven by a reciprocating engine or a turbine or what you will, you have a device for producing thrust-a reaction engine, just like a jet. The propeller sucks air in from ahead and shoots it out behind, and the momentum imparted to the air that passes through the propeller finds its equal and opposite reaction in the forward impulse imparted to the airplane. The propeller, however, has a very large diameter in comparison to the amount of power it absorbs. The increment of velocity in the slipstream may be surprisingly small-a few knots-because the amount of air passing through the propeller is so large. A propeller seven feet in diameter advancing at 200 knots chews its way through more than 13,000 cubic feet of air per second. Since sea level air weighs about eight-hundredths of a pound per cubic foot, that's 1,000 pounds of air per second. To overcome the drag of a clean small airplane at 200 knots-about 400 pounds-requires accelerating 1,000 pounds of air by very little.
The efficiency of propellers is high-as much as 85 to 90 percent of the power put into them ends up doing useful work-because the velocity of the slipstream is so low compared with the velocity of the surrounding air. As a general rule, when producing thrust by accelerating gas backward, it is more efficient to add a little speed to a lot of gas than a lot of speed to a little. Jets, at the opposite end of the spectrum, impart a lot of velocity to a small cylinder of exhaust, and are less efficient.
