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Finding the COG?

Old 01-27-2004, 05:29 AM
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Scratcher
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Default Finding the COG?

Hey fellas, new guy here. Could someone please tell me the correct way to find the center of gravity of an airplane? My current understanding is that the COG is the center point which balances the aircraft equally. But how is this changed - by shifting weight? And what is the best configuration? Better to have weight forward of COG than behind? Also, am I correct in saying the COG is completely different in a pusher prop being the COG is toward the back of the airplane? Sorry for the numerous questions, just break down COG in simple terms for the new guy, Thanks!
Old 01-27-2004, 06:41 AM
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FHHuber
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Default RE: Finding the COG?

Ideal way to get the center of gravity (CG) in the correct location:

You've got an ARF assembled ready for the engine and radio. Strap on the wing.

use rubber band(s) to affix the engine (muffler attached) to the engine mount.

set the RX battery on the fuselage between the wing and engine.

place fingers under the wing at the location indicated on the plans for the CG. (appx 4 inches to either side of the fuselage) and lift the model. If it tilts to the rear... you need nose weight... if it tilts forward you need tail weight. move the battery to correct. (hopefully this can correct it without the battery being on the engine or behind the wing)

Maybe you will need to move the engine forward or back on the engine mount a bit...

When that works... mark the engine mount with where you will need to drill it. and bolt the engine on. Foam pad the battery in place inside the aircraft as close to where it ballance the plane as possible.

Put the wing back on... and start setting the servos on the plane and reballancing. whent he servos make the plane ballance correctly... you know where you want them. (except you want them just a little forward... as the pushrods will shift the CG back a little)

Mount the servos and install the pushrods.

Now... rebalance AGAIN... and shift the battery to its final location.

You should be able to ballance the aircraft with no lead ALMOST every time with this method.

****

The CG is not different based on where the engine mounts... just the configuration of the wings and stabilizer can change where inrealtion to the wing center of lift the CG will need to be. the engine can be on the tail... or in the nose and the CG will need to be in the same place if the wing and tailplanes are not changed. (there was a test done with 2 planes where this was proven... A Pattern design was build exactly by plans... then another of the same kit modified just enough to put the engine at the tail.)

CG needs to be the same if the engine is running, or dead. You change it only to change how aerobatic or stable the plane will be. (and then there's a very small range that is safe)
Old 07-20-2012, 06:32 PM
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Default RE: Finding the COG?

Thank You, very helpful.
Old 07-30-2012, 10:10 AM
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Default RE: Finding the COG?

I would add to the above post that, for combustion engines, the measurement of the balance point should be done for the condition at which the plane will be most tail heavy.

As fuel is consumed during flight, the CG will move aft for a conventional model.
There is certain point for the CG behind which the plane becomes unstable (known as tail-heavy).
In order to ensure that that point is never reached when the tank gets empty, those models are balanced with empty tanks.

At the beginning of each fly they will be nose-heavy, which is not ideal, but always better than ending up with a tail-heavy and uncontrollable model by the end of the flight.

All the above applies in reverse for models that have the tank aft the CG: they should be balance with a full tank.
Old 08-02-2012, 10:20 PM
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Default RE: Finding the COG?

Note that the first two posters have not logged in to RCU for 8 years. So they may not show up to take part any longer.

For newer folks reading in there are also some good online CG location finders that are free to use. You'll find links for them in the sticky threads at the top of this forum's title listings.

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