Hey Alberto,

Actually the design intent here is to take advantage of two specific features of Contra Drives.

1/.. The lack of shaft bending moments due to the contra rotating propellers.
2/.. The need for rubber mounts due to the blade passing frequency of the spinning props.

The lack of bending moments means that, for the most part, the only forces on the system are substantially axial, and a large diameter motor rotor shaft isn't needed to resist bending moments.

Also, using rubber mounts in the system lowers the first resonant natural frequency of the mounting system to an rpm that is about 60% or so below the max rpm of the props so that the Drive passes through it as the throttle is advanced. This has the effect of practically eliminating the stiffness of the mounting system and isolating the airframe from the effects of unbalance when the Drive is spinning at max speed.

What this means is that any unbalances in the system will just cause the Drive itself to shake and any vibration forces that result will be isolated from the rest of the structure.

Also, rubber mounts in the system change the boundary conditions for any beam vibrational resonances so that the system is now somewhat close to a free state, as opposed to a state where the Drive is rigidly clamped to the airframe. This raises beam vibrational mode considerably, and allows us to use a smaller diameter motor rotor shaft, which then offsets the added weight from the rubber mounts.

We are running this Drive right now, and it's working great. It is spinning very true, and vibration free.

The gears and the bearings also look like they'll last forever, and we are adding a second front bearing to the motor to take axial loads, so this shouldn't be a problem either.

One problem we are having is with the thermal performance of the 1512 motor that we are using. I am worried that heat rise with this motor is going to be too high for typical flyers, and we may have to upsize it to a 1515 motor to make it more robust.

One of the benefits of this kind of design is that it is very easy to swap different motors in an out. The Drive itself is going to be rated for 4500 W, so if we get any dramatic new advance in battery technology in the next few years, we can use more powerful motors to take advantage of it, and the drive can remain unchanged.

Brenner ...