ORIGINAL: Dennis Flora
The tail motor turns much faster with the spur gear than it would in DD form, alot faster. Without the spur gear gearing it down, the stock TM probably wouldnt have enough power to swing the rotor fast enough. Thats my opinion anyway...
I think the stock motor would actually turn the rotor "too fast" and let me state (before you jump to conclusions) if it could. The purpose of the spur gear would be not only to slow down the motor, but enable it to move the larger mass of the rotor. I agree the motor would likely not be strong enough to use DD without reducing rotor size by about 2/3. And then the RPM would be so high it probably wouldn't move enough air to be effective. But hey, I work for a structural engineer, not mechanical.