Isn’t forward motion created by moving more air?
In: Engineering
An airplane pushes itself forward by pushing air backward. It can move a little bit of air slowly, or a lot of air fast. It takes less energy to get the same thrust if you move a lot of air slowly, *but* if the air comes into the propeller moving faster than it leaves, then your plane is actually going to be slowing itself down.
So a big propeller moving air slowly is way more efficient, *if* the plane is moving slowly. Helicopters don’t move up or down very fast, so moving a lot of air slowly is ideal. Airplanes move pretty fast, so they *have* to move air fast too, and a small propeller generates enough thrust because the air it moves is fast. A bigger propeller would be less efficient, moving extra air.
The ground provides a natural limit to the length of propeller blades in a fixed-wing aircraft, even when stationary. If there is any chance of the plane bouncing, for example in a bad landing, you want even more ground clearance. A propeller striking the ground automatically results in having to disassemble the engine to check for any damage – easily >$10.000 worth of damage even in a small plane.
Helicopter propellers aren’t designed to ascend and descend as rapidly as a plane has to move horizontally. The longer the propellers, the more stress is being put on the propellers from air resistance.
One thing to keep in mind is that the helicopters massive rotors are nice but the extra length isn’t as useful as you think. The tips of the rotors can’t go faster than the speed of sound. Therefore, an airplane with short propellers can spin those propellers at a much higher RPM than a helicopter.
Helicopters also have this problem in that the blade(s) on one side of the machine are moving with the forward movement of the helicopter and the blade(s) on the other side are moving against the forward movement of the helicopter. The speed of the helicopter needs to be limited so the advancing side remains below the speed of sound and the retreating side isn’t slowed to a stall. Helicopter blades are extra big because this is pretty inefficient.
A fixed-wing aircraft has wings that provide lift, given sufficient airspeed. The propellers only need to contribute the airspeed. The helicopter’s rotors need to be large enough to lift the aircraft.
There are some hybrid, known as “tiltrotor,” aircraft, like the V-22 Osprey, that have very large propellers so that they can be aligned differently to serve in either role.
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.
Longer propeller blades would hit the ground while on the runway for takeoff/landing, unless mounted higher up than the nose of the plane.
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