Propeller RPM drops and plane velocity

[Stewcumber]

Hopefully a clever person knows the physics behind this..

I made a simple "car", with 4x large propellers each on 2x counter rotating small electric rotor engines. I set it to full power and the blades immediately spin up to 460m/s, and the car gradually increases in velocity. Reload, set it to about 10% power and the blades still spin up to 460m/s. However, once I hit about 90m/s velocity, the blade rpm begins to fall. Why is this?

Is it something to do with drag on the blades? When my velocity is 0, (pretty much) all of the torque is spent rotating the propellors against stationary air. When I'm moving at 100m/s the torque is also fighting against air that is pushing the blades perpendicular to the torque at 100m/s... so less RPMS per unit of torque.

Anyway I ask this as I think this phenomenon is what limits the maximum velocity of my car/plane whatever. I seem to have a max velocity of about 115m/s with 2x rotor engines and 130m/s with 4x rotor engines. Perhaps if I have 3x or 2x blades (not 4x) there's less drag but also less thrust, so I would need more engines or bigger engines...

I think I need to experiment but I was hoping someone would know the science behind it?

[Just Jim]

Without seeing the actual craft, my first guess is it's your propeller pitch. Starting off you want it at a somewhat shallow angle, but as the craft begins to accelerate, you will hit a falling off point, yes. But you can increase the propeller pitch once you're moving to compensate, and, and it will speed you up even more
I know I'm not explaining it well, and I apologize
This video has been posted a few different times now on the forum. I found it several weeks ago, and it explains everything about prop pitch and all that 1000% better than I can
 

 

 

Also, on my planes, I bind the propeller authority limiter(s) to the translate F/B axis, which allows me to change the pitch on the fly... literally

I hope that helps

[Jestersage]

Aside from drag (and maybe related), I do find that if your motor cannot overcome either the drag or torque, it falls.

I was experimenting a motor that originally have the midsize propeller. switch it to the largest prop and suddenly it didn't work on the same standard-size motor, and thus require me to switch to the larger motor.