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

On Sat, 03 Jan 2015 14:15:39 -0800, brunovassel 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.

Did your calculation include drag from turbulence set up by the flat back
of the camera?

What would be the result of adding a streamlining cone with an A/R of 5
to 10 to the back of the camera, and maybe extending it down onto the
wing or tailplane surface to get rid of interference drag under it?

You could make one pretty easily (cheaply too) by carving and sanding
white or blue insulation foam to shape, and then filling the surface with
spackle to make it smooth before finish sanding, painting and polishing
it.


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martin@ | Martin Gregorie
gregorie. | Essex, UK
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