Different Rotors and Propellers for different Purposes

All rotors and propellers are not the same, in fact, they can be very different depending on their purpose.

Without going into every detail, in general, a large, slowly spinning rotor is ideal for situations in which high torque and low speed are required, similar to first gear in a car. Hover flight, for example with a helicopter, is a typical example of this. In hover flight the speed is zero, but the torque that has to be produced, or the thrust, is immense, as the spinning rotor has to lift the entire aircraft. Large slowly spinning rotors are ideal for this purpose as it is far more energy efficient to move a large mass of air slowly, than to move a small mass of air more quickly in order to produce the same thrust.

Big rotors are good for high torque and low speed applications. Small rotors are good for high speed low torque applications.

On the other extreme, small and fast spinning rotors are ideal for high speed low torque applications, similar to 7th gear in a race car. The thrusters, propulsors or propellers of an aircraft are a typical example. While a large propeller produces torque and can hence accelerate an aircraft quickly, it cannot achieve high speeds because it is simply not chopping through the air fast enough. Another effect of a large rotor is that simply because of its size it offers a lot of resistance to the oncoming air; the larger a propeller or jet engine are, the more drag they cause, further reducing the attainable speed. A small quickly rotating propeller on the other hand moves air quickly and hence allows for a lot of speed and low drag.


These two types of rotors/propellers closely relate to the VTOL problem. VTOL and hover are classic applications demanding a large slowly spinning rotor because low disc loading leads to high hover efficiency, while fast forward flight demands the exact opposite, a small rapidly spinning rotor. Helicopters emphasize hover efficiency over speed, thrust vectoring systems emphasize speed over hover efficiency while tiltorotors make a compromise in between.

SkyCruiser and SkyProwler on the other hand use separate systems for VTOL and for forward flight, employing the switchblade mechanism to switch between the two, and so get the best of both worlds, at a modest price in additional weight.