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Author Topic: EMBREO, OPTIMUN RUBBER WEIGHT  (Read 206 times)
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Thurman
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« on: January 19, 2021, 04:29:24 PM »

Any thoughts or formulas would be helpful in determining the best est. flight times based on rubber weight of this 50 sq. in. class.

Thanks!
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Bredehoft
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« Reply #1 on: January 19, 2021, 08:11:46 PM »

All you have to do is hit 2 minutes, since that is the max flight time.

There are many ways to go about this.  And there are many factors involved.

Indoor vs. Outdoor is also a critical question.

--george
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lincoln
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« Reply #2 on: January 20, 2021, 01:22:23 AM »

If the model can do 2 minutes in dead air, it might be beaten by one that can do 2 minutes in a little bit of sink. The optimum motor for sink will be fatter than the optimum motor for dead air. Ditto for rough air. When I fly a model in scale, I use a fatter motor than in an event that restricts the motor weight. That's fairly easy, since I have a rubber stripper. If you don't have one, that constrains you. If you're stuck with 1/8", you may want a shorter motor for a bigger prop. You can always adjust the prop instead.

I suspect that the longer the motor is, the more the cg can shift as it unwinds, though braiding might help. If the cg will move, then it might help to make the tail bigger.

I don't know how to allow for the efficiency of the prop in different phases of the flight. I imagine this is more important when the model climbs steeply, as opposed to a cruise climb.

If the center of the motor is behind the cg, you'll have to use more nose weight with a heavier motor.

A model that uses a fat motor may have to have a stronger and/or stiffer fuselage, making slightly it heavier unless it was somewhat overbuilt in the first place. A model that uses a heavier motor may have to be a little stronger in places that hit things.

Having written all that, we can say that the power required for steady flight will be proportional to the weight^1.5. Actually, because a heavy model will have higher Reynolds numbers, it should require slightly less power than this predicts based on a light model's performance. The energy available, of course, is proportional to the weight of the rubber.


My intuition suggests that 40 percent of the model weight might be a good place to start, but of course that depends on the factors I've mentioned.

I've flown a bunch of other types, but my only experience with embryos was helping to proxy fly one of Chet Bukowski's. From that I know that a good embryo can have spectacular performance. We had a max every time, but lost it in the corn on the last flight and couldn't get it into the flyoffs.

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