Off season FRC quadrocopter build

[EricH]

Originally Posted by ferret_guy

parts. The only problem with it is the props, is that the prices make me sad . My idea was to simply fly it very low after performing extensive static testing and whatnot. However by changing the props as EricH suggested the result was unable to lift off, could you provide more info on this?

The props... I ran the simulator with basically the settings you had, but a heavier frame and smaller props (no hover) and then the same size props (short flight).


Speaking from the fixed-wing world, the right prop, especially for high weight, is a tough balancing act. You're looking for thrust, but you get that from three factors that are all integral to the propeller or the engine: RPM, Diameter, and Pitch. Too high of a pitch, and the RPM suffers due to drag (engine doesn't quite have enough torque). Too large of a diameter, and the motor might not have enough torque to turn it fast enough. Either one can really hurt performance.

Now, if your motor is fixed (ours had to be a certain type), you can get a few props of various sizes and do some testing. You're primarily looking for thrust, which can be measured with, say, a skateboard, a mount, and a fish scale and a stake, but how much power the engine is exerting can also be handy (but needs a dynamometer to measure).

You need enough thrust to lift off using 4 motors; you want some runtime in the air... It's a tough problem, and the props are extremely critical. No substitute for testing. (And yes, good props are expensive. I think props for our testing could run $20 or more a pop, and we used cheaper ones.)

[mman1506]

I don't want to be a dream crusher but to get the proper rpm to run a reasonable propeller(Cim tops out at about 3000 rpm under load) you would need a gearbox which would add more weight to the system.

To put it into perspective each motor on most quads have the same power as a cim and even with there lightweight, high discharge Li-Po batteries and electronics they still need 40-50% throttle to fly. To even make a cim copter hover you will be looking at using 90-100% throttle which won't leave any headroom for stabilization making a very unstable platform if it flies at all. Maybe if you used a LiPo battery and replaced the cRIO with a lighter controller it would fly but just barely.

[EricH]

Originally Posted by mman1506

I don't want to be a dream crusher but to get the proper rpm to run a reasonable propeller(Cim tops out at about 3000 rpm under load) you would need a gearbox which would add more weight to the system.

A quadcopter's motor loading is nothing like an FRC robot's motor loading. The closest approximation would be a shooter wheel with no shots going through. The CIMs should be running around 5000 RPM for quadcopter use. (This is lower than the free speed, but not by anywhere near 2000 RPM lower.)

I would say a gearbox might be helpful--I want to say a 1:2 or higher to get closer to a nitro engine--but I think there's something that would be even more helpful. It's called a BaneBots RS-775-18. You're allowed 4 of them in FRC, and they run reasonably high power and definitely high speed if you take the planetary gearbox off. They're similar power to a CIM, but aiming more on the speed side than the torque side, which is better for propeller usage, and they're lighter, which is better for flight usage. An RS-550 might also work.

[mman1506]

Originally Posted by EricH

I would say a gearbox might be helpful--I want to say a 1:2 or higher to get closer to a nitro engine--but I think there's something that would be even more helpful. It's called a BaneBots RS-775-18. You're allowed 4 of them in FRC, and they run reasonably high power and definitely high speed if you take the planetary gearbox off. They're similar power to a CIM, but aiming more on the speed side than the torque side, which is better for propeller usage, and they're lighter, which is better for flight usage. An RS-550 might also work.

+1 RS-775 would be much better suited. One of the few large brushed quad rotors I've seen used motors close to RS-775. I still seriously doubt that you will be able to pick up a 17AH battery, my 2100 watt hexa is not able to lift that amount while staying stable