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Old Jul 23, 2012, 06:48 PM
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Adding weight to a bird, what should I know?

So, I've been thinking about an idea. I'm sure all of you have seen videos from planes, all from the same perspective- camera mounted on the nose just behind the prop. What I want to do is build a small mount near the tail, and a few inches above the fuselage of the plane. This should result in being able to see the canopy, part of the wings, and of course what's in front of the plane.
My concern is adding weight aft of the CG. This will obviously cause it to be tail heavy, which I plan on countering by adding weight to the nose. Are there any general rules regarding adding weight? I'm flying a Parkzone P51D Mustang BL, which I believe could easily take on an additional payload, but I really am only guessing. I'll be using an 808 #16 camera, and building the mount most likely from carbon fiber rod and other lightweight components. It'll basically be a perch for the camera to sit upon.
So what aspects am I missing? Is this going to be as simple as I hope or will this become a terrible nightmare?
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Old Jul 23, 2012, 07:38 PM
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Weigh the camera, and measure the distance from where you plan to mount to the CG. Multiply the two, then divide by largest distance you will be able to plant the weights from the CG; this will give about how much weight you will need to add, plus some to offset the mount. Still think the aircraft can carry that much weight?
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Old Jul 23, 2012, 08:19 PM
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Let me see if I got this right.

Weight of camera (plus 5g for approx. mount weight) = ~23g
Distance from camera mount to CG = 14"
Distance from CG to ballast location = 8"

23*14 = 322
322/8 = 40.25g

So, I'd need to add roughly 40-41g weight to the nose to counteract the camera?

I could also move the mount to approx 10.5" aft of CG, and move the ballast to about 8.5" fore of CG, which would need only ~29g of ballast. At least according to your formula, granted I'm understanding it correctly.
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Old Jul 23, 2012, 09:33 PM
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Correct. Think of it as lever with fulcurum point at the cg. Moment = weight x distance from fulcurum. Moments on both sides of fulcurum need to be equal for the lever to be balanced.

(Mass in front of cg) x (distance in front of cg) = (mass behind cg) x (distance behind cg)
solve for (mass in front of cg) = ((mass behind cg) x (distance behind cg)) / (distance in front of cg)
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Old Jul 23, 2012, 10:39 PM
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So, your original location would call for about 64g total additions; Parkzone shows the flying weight is 710g, so you'd be adding nearly 10%. That may make it fly like a pig, and land really hard. Shifting the camera forward drops that to about 53g; still enough to impact the flying qualities, but maybe more manageable. Got flaps for landing?
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Old Jul 23, 2012, 11:21 PM
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No flaps, unfortunately. The weight is quickly adding up, and even though 10% additional weight seems minimal, I can see how it would affect the flight. I once tried strapping my cellphone to the plane to take video which weighed 155g, it flew like hell and could hardly stay in the air. Perhaps my plans of a tail mount have been debunked, at least with this particular plane anyways.

If there are any other ideas to achieve a unique perspective to RC flight, I'd love to hear it.
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