What is the benefit of more, smaller rotor blades over fewer, larger ones?
I thought that the area of air that the blades worked on scaled with pi-r-squared, which favours fewer larger rotor blades, as seen on most helicopters?
I thought that the area of air that the blades worked on scaled with pi-r-squared, which favours fewer larger rotor blades, as seen on most helicopters?
I can think of some significant downsides though.
1. This is fixed pitch, not collective pitch. That means any change in the attitude of the aircraft requires one or more sets of rotors to change their speed of rotation. This means that you need top notch electronic control systems, gyros etc. to mediate between pilot input and what the aircraft does. These electronic systems are sensitive and fragile and degrade with time. And when they let go, it's spectacular.
2. A loss of any one of the 8 motors is a Jesus Christ, yank the ballistic parachute moment. This is likely a hull-loss event as there will be all sorts of load-bearing parts with the job of absorbing the opening shock of the parachute. These will at minimum require inspection prior to the aircraft being certified airworthy again.
3. related to 2 above, this aircraft has no capacity for autorotation whatsoever. A traditional rotary-wing helicopter with collective pitch can autorotate safely in to a controlled landing in the event of an engine failure. If you lose an engine in this, and your ballistic chute tangles or doesn't deploy properly, you are proper fucked.
It's cool, and quad-copters are fantastic cargo and gp platforms. But I would never fly in one.
But not in the event of a propeller or pitch-control failure. Also probably single propeller failure in this quad will mean "we lost 1/8 of power, we must land". Also if you lose gasoline engine, you have battery backup for 5min.
The reason all the cheap drones use 4 or more engines is that they can individually be very simple, with the motor directly driving a rotor. With no swashplate to alter blade pitch the mechanism is very simple. Short blades are easy to make for a required strength and stiffness, and the torque of electric motors is sufficient to rapidly change rotor speed to change lift for steering. The control system is simple. However efficiency is terrible, both in terms of lift, and maneuvering.
The energy to change blade speeds constantly to maintain attitude is a significant cost. But having low rotational inertia, frequent changes are required, especially in windy conditions. As blade diameter increases (and hence inertia) the resulting energy costs to change speed increase too, as does the size/power/weight of the motor.
Coaxial counterrotating rotors, as here, have two further advantages:
1. they're much more efficient, as less energy is converted into (useless) air rotation.
2. in forward flight, one side of the rotor "sees" higher relative speed, thus extra lift, while the receding side sees less lift. That's cancelled out by the counterrotating rotor above.
Generally, larger diameters are good for hover performance and better autorotation, though autorotation is not really a factor for this design.
EDIT to add: Note that the total disc size of one large rotor is of course the same as the one of 4 smaller ones with half the radius.