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HomeOff Topics › C of G help please
02-05-2008 06:26 PM  10 years agoPost 1
sparky666

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uk

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I have got a small park fly glider 2 foot in the length, wing span 3 foot, the wing has a width of about 5inch could some one please give me an idea of where the C of G should be ,about, from the leading edge.
Just an idea so it doesnt stall on first flight,

Many thanks.

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02-05-2008 06:28 PM  10 years agoPost 2
SSN Pru

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Taxachusetts

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a good rule of thumb is that the CG should be about 1/4 to 1/3 of the way back (chord wise) from the leading edge. 1/4 of the chord will give you a really good starting point if you don't know where it should be. find tuning based on flying afterwards will dial it in.

Go a little at a time.

For your 5" chord plane, put the CG about 1.25" back from the leading edge.

You could possibly start it out about 1" from the leading edge in which case the plank will likely be nose heavy but easily controllable in pitch.

Id hazard a guess that if you put the cg about 1.75-2 inches back, you would be tail heavy and have a hard time controlling it in pitch without expo or quick thumbs.

I assume you know how to check the plane for the right CG?

Stupidity can be cured. Ignorance is for life!

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02-05-2008 06:37 PM  10 years agoPost 3
SteveH

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Texas

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umdpru's data on CG point is good for powered planes, but gliders typically use a CG that is farther aft. I would start at 1/3 the chord back from the leading edge.

The government cannot give you anything without first taking it from someone else.

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02-05-2008 06:53 PM  10 years agoPost 4
sparky666

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uk

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many thanks for your help, very kind of you.

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02-05-2008 06:55 PM  10 years agoPost 5
SSN Pru

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Taxachusetts

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umdpru's data on CG point is good for powered planes, but gliders typically use a CG that is farther aft. I would start at 1/3 the chord back from the leading edge.
An airfoil is an airfoil it doesn't matter whether it's attached to an aircraft that is powered or not. The reason gliders use a farther aft CG is for performance reasons. The stab's don't need to do as much work becuase the CG location is helping to negate the pitch down of the airfoil in flight. this equates to less drag and better duration.

I would be conservative at first. You decide

Stupidity can be cured. Ignorance is for life!

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02-05-2008 06:59 PM  10 years agoPost 6
Rob_T

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Start with the CG at 30 to 35% of the mean chord and then test fly. A glider is most efficient (actually so is any plane) when it is rigged with the CG as far back as possible to get close to neutral stability.

The classic way to check this is the dive test. Trim the glider for level flight and put it into a dive (about 30 degrees) then release the controls. If the nose rises up quickly you have a lot of positive stability and can move the CG back. If the nose stays down (and the dive does not recover) you have neutral stability, and if the nose drops and the dive steepens you have negative stability. Most pilots find the workload of flying a glider rigged at neutral stability to be too high so they move the CG slightly forward so the glider will gradually pull out of the dive.

Also a lot depends on experience. A beginner will prefer a CG further forward than an expert.

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