Efficiency of a backdriven hypoid ring and pinion

[Rene123]

I am considering a Dana 44 ring and pinion for use in a UAV helicopter. While in autorotation the rotors which are attached to the ring gear will backdrive the pinion. There is a spag before the transmission but the ring and pinion must remain attached to the rotor since some flight critical systems are picking up power there. My question is how much drag will backdriving the pinion create? I know this is complex but I am only looking for some general guidelines. For example I have read a hypoid is 94%-96% efficient in the normal drive direction. This is dependent on pinion offset and ratio. I am guessing a small pinion offset (1.5 inches) and a low ratio (3:1) has greater efficiency in both directions, but I would like to hear any comments on this.

Thanks,
Rene

 

[Metalguy]

Hypoids are not as efficient as a spiral bevel gear set--they have a lot of sliding. If you're after efficiency, forget them.

 

[Slowzuki]

The coast efficiency should be comparable, if you prefer you could pick up a reverse rotation housing (ford front d44) so drive is on the coast side of the gears.

The gear oil generally represents the largest loss in the axle.

The D44 is a fairly good choice as it is not as offset as other axles (like you mention). Ford 9" are very offset.