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Old Jan 19, 2013, 09:13 PM
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Again, minimum power does not occur at best L/D. It happens at a considerably lower speed, in sailplane terms called minimum sink rate speed.

Light weight will decrease the power requirements, but hurts the best L/D at model scales.

Larger wing area doesn't help L/D, except that the longer chord will have a higher Re. L/D is independent of wing loading, except for the Re effect of a higher wing loading flying faster, and increasing the Re. Minimum power depends on the span loading, not the aspect ratio - lower the weight per unit span, either by increasing the span or decreasing the weight, and you will decrease the power required.

A big span and a low zero lift drag coefficient are what you want, except because of Re effects there is an optimum AR for a given Re.

Wing planform has an effect - you want a near elliptical lift distribution, not necessarily an elliptical wing.

Good low Re airfoils, appropriate for the Re at each wing station help quite a lot. Dr. Drela's AG series are very good. Camber changing flaps can increase the performance at other speeds than best L/D.

Larger will always fly better than smaller, all else being equal.

Low drag of all the other components is very important.

If max L/D is really your goal, you want to copy a very good sailplane in the size you want. The AG airfoils and excellent sailplane designs are on the CR site:

http://www.charlesriverrc.org/articles.htm

Power system efficiency is a huge variable. selecting the right motor and propeller for the speed and power required can make a huge difference to the overall efficiency.

Full scale airplanes such as the U2 had different mission requirements (very high altitude, high Mach number) at a different Re. They won't be optimum for low Re and different requirements.

Kevin
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