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Old 01-06-2010, 05:17 AM
ShoGinn's Avatar
Join Date: Aug 2009
Location: Colorado Springs, CO
Posts: 22

I think your post is great and I liked your table on watts/lb
My father and I are working on converting an old 1/4 scale cub and I used just the 50w/lb rule and ended up going to finding at 75w/lb

We will be choosing roughly a 1100watt output with an 18x10 prop on a ~340kv motor from Scorpion thanks to Lucien.

Originally Posted by aramid View Post
The battery won't always deliver its nominal voltage when it's under load, but that's the only place you'll see a voltage drop. Losses in the ESC and in all the wiring from the battery to the motor shouldn't even be measurable with consumer multimeters - if you were really dropping from 11.1 to 9.1 across the ESC, that would mean the ESC would have to dissipate (2V * 40A) 80 watts of waste power - that would require an enormous heatsink (think about trying to keep an 80 watt incandescent light bulb cool enough to touch).

The actual reason your estimate is so low is that Kv is only a valid measurement on an unloaded motor - no prop attached. Once you add a load, the motor will slow down even if it's still getting the same voltage.

I'm a bit concerned about your power system, though. You definitely have enough speed - 7300 RPM with an 8-inch pitch is a 55 MPH pitch speed (7300*8/1056), which is just about perfect for a plane like the Telemaster. You're definitely underpowered, though. The normal rules of thumb for power into the motor are as follows:

<50 W/lb: Unflyable
50-75 W/lb: Able to sustain flight from a hand launch. Climbs will be very gentle and full power will be required for much of the flight.
75-100 W/lb: Able to perform ground takeoffs and perform basic aerobatics. May need to dive for speed before some maneuvers.
100-150 W/lb: Capable of nearly any maneuver from level flight.
150-200 W/lb: Capable of sustained vertical climbs and "3D" aerobatics.
200+ W/lb: Hahaha!

Even for a big, lazy Telemaster, I would aim for at least 100 W/lb. Even if you just want to float around at minimum speed, having that extra power in reserve means you can get out of trouble easily - takeoff is simpler, stall recovery is simpler, and aborted landing approaches are stress-free. The extra power also means you can try big, graceful loops and hammerhead turns, both of which are great fun in that kind of plane.

Many kit manufacturers still haven't gotten caught up with lithium batteries and brushless motors, and many still suggest very inadequate power systems for their planes. Don't listen to Hobby Lobby's suggestion. Just remember - if you add power by going to a larger prop, keep an eye on your pitch speed. If your pitch speed drops below about 40-45 MPH, you'll probably have issues, even if the total power is high enough.
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