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Old Jan 19, 2013, 04:39 PM
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Figuring out L/D ratio and drag coefficients

Hey all,

I recently did some flight tests for an aircraft I built to try to figure out the minimum thrust required to keep it in the sky. If I'm not mistaken this minimum-throttle speed is L/D max. Here's my issue, though. My airplane is fairly streamlined, it's basically a wing, two very skinny booms, a small tail section, and a flat, short, streamlined pod suspended off the wing that carries the battery/engine. Very flat, smooth airplane. Here's my issue though-

Like I said, I did several flight tests at minimum throttle to determine flight time (and thereby amperage). I then took the averages of all the flights, and then measured the thrust produced by that amperage - the results left me scratching my head. At absolute minimum power for flight, the motor is producing 245g of thrust. The weight of the aircraft, however, is 470g. That's where I'm confused. See, when I divide out all the other factors to get the drag coefficient, it's something crazy high like 0.32 or something. I thought I did my math wrong, but this checks out. The L/D ratio for this this is about 2:1 - horrendous compared to even some of the "draggiest" aircraft out there.

Like I said, this is a fairly streamlined aircraft, and has far less of a frontal profile than something like a cessna 150 (Which has a L/D ratio of 7:1 and a drag coefficient of ~0.03). So I'm just scratching my head trying to figure out why my airplane has a drag coefficient 10x higher than modestly-streamlined general aviation aircraft. I realize induced drag also plays a part in this - the aspect ratio is about 6:1, which means the plane isn't so great at the back of the power curve. But a tenfold increase in coefficient? What could be the cause of this?

Even a flying squirrel has a higher L/D ratio than this! LOL!

Here are the specs for aero purposes:

AGW: 470g

Wingspan = 30"
Chord = 5"
Area = 150"
A/R = 6
Coefficient of lift at 0 AOA = ~0.3
CG is at 20% MAC


Regards,

-DE
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