Dave, here are some interesting links:
Wedge Roller Traction Drive Unit
This traction drive transmission idea combines power transmission and sprag clutch functionality, which has helicopter potential, would be very nice for your application.
Different Wedge Roller Traction Drive
This unit also explores the wedge rollor traction drive idea (used on an electric motor) with a fixed reduction ratio of 6:1. The sprag clutch function is also incorporated.
As a product
Here are the above concepts as products.
1982 traction drive study - 137 hp, 14:1 reduction, gas turbine
You have to buy this one.
Aerospace quality traction drives
Abstract of study - 240 HP fixed ratio traction drive transmission
Click on "Citations" for more information.