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Old 08-12-2011, 03:25 PM   #7
Rockin Robbins
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On two cell batteries you're only going to get around 700 grams of total thrust with any 7" prop, only 660 with a 7x5 APC E prop. With a 1500 gram (minimum) plane that's a huge problem. At that point it doesn't matter if it will fly or not, you'll have great problems getting off the ground.

With the 7x5 prop you're only going to draw 17.2 watts per motor for a total of 68.8 watts, let's call it 70 and be optimistic. If we figure 100 watts per pound is adequate (you're not going to 3D a four engine whatever, are you?), your 3.3 lb plane at 70 watts is.........(the envelope please).......21.2 watts per pound. Not acceptable for Del Monte.

There's not enough pitch available on a 7" prop to pull you out of the quagmire. If there were your pitch speed would be a hundred miles per hour. Your thrust would be even less than with the 5" pitch prop. So there's no way to make 7" props work with 2 cell batteries. I suspect the same is true of 3 cell batteries. There's just too much gap between capability and requirement.

You need to work backward. First choose the prop you want to use, then attempt to match up available motors to see if you can make something work. The SuperTigres aren't going to cut it. You're going to end up with very high KV motors to have any chance at all. You need scads of RPMs and a prop designed to deal with that kind of stress.

Denny will now be by to hawk his beloved Hacker motors. You'd better listen because you are talking a high stress environment here. My crystal ball sees your wallet shrinking! But your long term most economical decision will be to spend what you have to. The Supertigres won't Hack it. (had to say that)
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