Isn’t forward motion created by moving more air?
In: Engineering
The length of an airplane propeller is the compromise of a lot of things, many of which have already been mentioned. The big ones are:
– Propellers, like wings, have induced drag and parasitic drag. Induced drag will go down with a larger diameter, but parasitic drag will go up. Going very large (helicopter size) will be counterproductive for an airplane, but for the reasons below most airplanes have propellers that are smaller than what would be aerodynamically ideal.
Larger propellers are:
– Heavier (for strength)
– Require taller landing gear (for clearance to the ground)
– Require a gear reduction — With piston engine airplanes, the propeller is often bolted directly to the engines crankshaft. Thus the propeller spins at the same RPM as the engine. Longer propellers need to spin slower (given the same engine power). To spin a larger a propeller, a gear reducer would likely need to be added. This is done sometimes, but adds weight and cost — it’s a compromise. Note: Turboprop drives already include a gear reduction, so this reason doesn’t apply to these.
So why do helicopters use such large blades? They need to be able to hover. Airplane designs often put a lot of emphasis on efficiency at cruse speed — and they definitely can’t fly at zero air speed. Helicopters sacrifice a lot of efficiency in cruise to be able to takeoff and land vertically. The large rotor is necessary for this zero air speed operation, but isn’t great for flying at speed.
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