What is the L/D penalty of outside mounted GoPros?

On Saturday, January 3, 2015 2:15:41 PM UTC-8, wrote:
I knew I could catch a smart fish with that bait. Thanks! I try not to fly around too much at 60 knots but cruise more in the summer months around 85+ knots...I said try.

Please check my math and reasoning on this: I went to an online calculator and it said 85 knots at 15k alt is ~24.4 lbs/sq ft. Therefore, about 2.5 lbs of drag. Looking at the polar of a 27 at 85 knots, you are about 39/1 at that weight and speed...I think. So you would then be 36.5/1 with one camera or a drop in performance of around 6.5%.

I was seriously considering a camera on the tail and one on the wing. If both together add up to at 13% penalty, that is pretty harsh to impose upon yourself when attempting longer 750km - 1000km flights. If these calcs are correct, looks like the external cameras will be for the soft weather local flying days only.

Thanks!
Bruno - B4



Very roughly: at 48:1 flying at 1000 lbs, total drag is 20.8 lbs. At 60 knots, dynamic pressure is 12.2 lbs/sq ft. Camera is roughly .1 sq ft, adding 1.2 lbs drag, increasing your total to 22 and reducing L/D to 45.5. That is probably pretty worst case (drag coefficient probably less than 1, not in free stream, etc.). A tiny bit of streamlining on it could improve that a great deal I would think.

When you are doing these calcs do not correct the air density for altitude (unless you also correct your airspeed to true). The airspeed indicator shows dynamic pressure directly. So the dynamic pressure at 85 knots IAS @ 15K is the same as the dynamic pressure at 85 knots IAS @ sea level. Or if you are using true density at 15K then you must use true airspeed - you will find the result to be the same. Dynamic pressure is about 0.5 * .002378 * (airspeed in ft/sec) at sea level standard conditions. Your drag at 85 knots at 15K will be more than you estimated.