BACK TO BASICS
PROPS I: BASIC PROPULSION
One of the first things to get taken for granted when flight instruction starts is the propeller. As unlikely as it might seem at first, that small metal toothpick spinning around up there is actually capable of transforming the thrashing of the engine into usable thrust. And once you've seen it in action, the prop can easily become just one more mechanical device that works--kind of like the wheel: easy to use, not necessary to understand. (While we're at it, why not mention the remaining list of aviation improbables: Those wings hold it up? That arrangement of control surfaces maneuvers it? And, most impossible--starting and stopping a piston 2300 times a minute is a workable way to make horsepower?).
The fixed pitch propeller attached to the front end of the average general aviation airplane has changed little in concept since 1903. Cast in the shape of an airfoil, its primary purpose is to provide thrust. It pushes air backwards, and in reaction moves the aircraft forward. Basic examination reveals that the prop tips always move through the air at a higher speed than the sections near the hub, so the angle of attack at which they meet the relative wind doesn't need to be as great to produce the same lift. This gives the reason for the twist built into all props, ensuring a high angle of attack at the slowest, central portions, and a decreasing angle out toward the tips.
In addition to producing necessary thrust, the work done by the propeller in pushing air backwards serves to limit engine RPM. To see how this principle works, imagine a propeller designed with a very high angle of attack: the prop would operate under a heavy load and push a great deal of air around, but peak RPM, even at full power, would be quite low. Takeoff and climbout performance would be terrible, because limited RPM means limited horsepower.
Conversely, imagine a prop built with a very low blade angle: it wouldnt push much air backwards, but would allow for high engine RPM, thanks to reduced workload. The beneficial effects would be immediately apparent: increased RPM would produce increased horsepower, and the airplane would accelerate and takeoff more quickly. Taken to the extreme, very low blade angle might actually allow RPM to exceed engine redline, making it necessary to avoid full throttle settings in order to protect the engine from serious harm.
But, you might say, my engine and fixed pitch prop never even approach redline during takeoff and climbout, and there appears to be a great deal of room for flattening out blade angle to allow higher RPM (and more horsepower) without endangering the engine--a flatter prop should give me better performance.
On takeoff, this is undoubtedly true. Most general aviation airplanes are equipped with fixed pitch props that allow a maximum of 2300-2400 RPM during takeoff and climbout, despite engine redline limits of 2550 to 2700. Redline is therefore an abstraction, available only by putting the aircraft in a shallow dive at full power.
The full importance of all this becomes evident when you realize that if redline is an abstraction, so is horsepower. The full rated power of any engine depends upon its reaching full rated RPM; failure to do so reduces horsepower substantially. In practical terms, this means that although a 172 engine might theoretically be able to put out 160 hp (at 2700 RPM), you will get nothing close to that if you can only turn 2300.
Since the propeller is the limiting factor to RPM development, it should be clear that the average fixed pitch prop puts full power out of reach at the time you need it the most.
Why make props work this way? The answer next month.