Thread: Discussion Figuring out L/D ratio and drag coefficients View Single Post
 Jan 19, 2013, 05:03 PM naive realism Joined Jan 2007 3,851 Posts Minimum power required will be at a slower speed than best L/D speed. Minimum sink speed will be the airspeed for for minimum power required, which will be very close to stall speed. L/D gets much worse at low Reynolds number. The small size and low speeds of models means the Re is quite low, and the L/D will be lower than full-size airplanes. Your model is quite small, so the Re will be low. It looks like you have a constant chord wing, so the efficiency at slow speeds will be a bit lower than a tapered wing. Even so, the best L/D should be much higher than 2:1. How did you measure the thrust? If you measured static thrust, that will not be the same as the thrust when flying. Depending on the prop and motor characteristics, the thrust should be much lower as the prop unloads with speed. I'm also not clear how you measured the amperage? Without measuring the in-flight watts, you will only be able to get a very rough idea of the power from how much you have to put back into the battery. You have also ignored all the power system losses. The watts into the motor probably only end up 50% thrust or less. The motor efficiency may be 60% or less at low power, the ESC efficiency is lower at low power settings, the propeller efficiency may be anywhere from 30% to maybe 60%. If you multiply all the power systems efficiencies together, it is the amount of power that actually gets converted into thrust may be 20%. Measuring airplane performance is not easy, and I think the method you tried to use has big errors and has missed a lot of the power losses. Kevin Edit: Here are some good articles on how to do performance measurements on a full-size airplane: http://www.eaa.org/experimenter/arti...09-08_drag.asp https://engineering.purdue.edu/~andr...-46372-872.pdf It isn't easy! Last edited by kcaldwel; Jan 19, 2013 at 05:41 PM.