View Single Post
Old Aug 04, 2005, 11:54 AM
Jeffery is offline
Find More Posts by Jeffery
And You're Not
Timbuktu, Mali (Happy?)
Joined Oct 2002
6,514 Posts
And here's another post regarding the effects large props have on an airframe making rapid attitude changes (if it's ripping the firewall from your plane, it's safe to say the amount of force needed to accomplish the attitude change if fairly large)

Originally Posted by GWRIGHT
Looks good,.. I really like the idea of really high voltage and low current. Should be nicely efficient.

As a side note on mounting. I've done some bizzare things with gearing to use REALLY LARGE props with relatively low power levels, like 20 to 24 inch props with 600 to 1200 watts input. Even at the lowest end of the scale in power, I had to use a rear motor support. It wasn't due to the power,or size and weight of the motor, but rather the gyroscopic forces on the prop. When flown pattern style,.. no issue at all, so you're probably just fine,..however, when doing any high rate pitching or yawing maneuver with some power on, it was very easy to loosen things up,.. actually to the point of ripping a 1/4" ply firewall out while in the air on one plane. That plane had a 1/4" birch ply firewall.A 3 inch by 2.5 inch machined aluminum plate (1/8" thick) was added to the front, so the C50 acro bolted to it with the 8 bolts into the big acro gearbox, then the plate bolted by 4 bolts to the 1/4" ply firewall. This was done to distribute the mounting forces further appart (3X2.5 is actually the bolt spacing, not the total aluminum plate size). That setup was only 900 watts input power at the time, with a 24X12 prop. Doing some high rate pitching maneuvers (tumbling), the ply firewall broke into 3 pieces and the powerplant departed the plane. After repairs, an 1/8" light ply brace was added to the rear of the motor, so it's a couple inches behind the firewall, creating a much more robust mounting. Power is up to 1300 watts now using more cells, still the 24X12, and everything stays in place. The big props tend to go into "whirl mode" when pitching/yawing quickly and the vibration is tremendous. hinged props (folders) tend to absorb most of these forces and eliminate the problem, but nobody makes really large folders. Off topic sort of,.. but just wanted to mention my experiences in case they might benefit someone.
Jeffery is offline Find More Posts by Jeffery
Reply With Quote