Figure 1: Two ``05'' cobalt motors: the left is geared, the other is
direct-drive. Adding the gear drive yields 1/2 lb more static thrust.
Reduction gear drives are a common feature in electric flight, and are slowly becoming more common in the gas/glow world as well. They are employed to allow our engines/motors to spin larger props at fewer RPM's. What's the point of doing that? Three gains: 1) more torque, 2) more thrust, and 3) better propeller efficiency.
A good example is the Ace Cloud Dancer. A healthy glow .40 will pull it very well, but the plane wants to fly FAST and has the tendency to slow quickly during vertical maneuvers. At the same time, powering the 'Dancer using a geared electric motor with the ``power'' of a strong .25 will yield nearly vertical performance! How is this possible? Well, the .40 is spinning a 10x6 at maybe 12,500 RPM (with a thrust speed of 80 MPH and static thrust of 3.5 lbs), while the ``25'' electric motor is swinging a 12x8 prop or larger at about 8,000 RPM (a thrust speed of 55 MPH and static thrust of 4.5 lbs). The slower, more powerful thrust from the larger prop couples better (i.e. more efficiently) to the natural speed of this airplane. The geared motor won't allow the plane to fly as FAST, but the torque and static thrust are much higher than with the 2-stroke - the plane will ``pull'' through vertical maneuvers smoothly. They often fly much better, and with less power! I've seen 1/4-scale Piper Cubs fly nicely on geared (electric) 40's swinging big 18x10 props (at about 5,000 RPM). It's impressive.
Just some food for thought for some gas and glow engines on those Big Birds, and for anyone thinking about electrics. Incidentally, my trusty old Skyvolt does NOT use a gear drive (not enough prop clearance). I shudder to think what it'd be like with one... grin