center of gravity
 

im knew to flying and bought a glider of a friend and iv flown it a couple times and it struggles to stay in the air i think its tail heavy how do i find the center of gravity and move it to where it needs to be to fly?

center of gravity
 

swain etc

A model which is tail heavy will be excessively twitchy on elevator and will tend to tend with its tail low and be generally 'wallowy' - sorry, can't think of a better description. As the model already exists (as opposed to being at the design stage) the simplest way forward is to add a little noseweigtht (if you are sure you have a tail heavy condition) climb to 200-300 feet and put the nose down into a gentle dive with down elevator. After a few seconds return the elevator to neutral. If the model continues in its dive with no sign of pulling out or (even worse) has the dive increase in steepness the model is tail heavy. Keep adding nose weight until when the elevator is neutralised the model gently returns to level flight after a few seconds.
If the model rears ack up as oon as the elevator is neutralised it is nose heavy - remove nose weight.
Mike

center of gravity
 

Try balancing the plane by picking up the plane with only your index figers (one holding each side of the wing, with the tips of your finders pointing upwards). The plane should balance at about 30% of the wing width, from the leading edge of the wing. If it balances behind this point, add weight to the nose until it does. If it balances in front of this point, add weight to the tail until is does. This should setup your plane for a very slight nose heavy condition, a good starting point.

Now, having said all that, my advice only goes for a traditional glider. If you have a flying wing (Zaggi, etc.) or the wing is tapered backwards (like a jet) then you need to balance the plane at a completely different place.

- Q.

center of gravity
 

Mike- I'm pretty sure that a tail heavy plane will want to pitch up, while a nose heavy plane will want to pitch down. At least, I've always heard an aircraft that wants to pitch down described as nose heavy.

center of gravity
 

Mike- I'm pretty sure that a tail heavy plane will want to pitch up, while a nose heavy plane will want to pitch down. At least, I've always heard an aircraft that wants to pitch down described as nose heavy.<<

Daniel

This is a common misconception - but that is what it is - a misconception. Here's why - but remember that it applies specifically to the test routine I described (see later):

If the aeroplane has the CG too far forward then for it to fly straight and level it will be carrying an excessive amount of up trim compared to what it would need if the CG was correctly positioned for its aerodynamic set-up. Put it into a dive with down elevator and the speed increases. Increased speed equals increased lift and drag. Release the down elevator and you now have an aircraft with excessive lift (a simplified explanation but adequate for what we are discussing). Hence it immediatley (or nearly immediately, depending upon how far too much forward the CG is) zooms up.
Reverse applies when the CG is too far back (always assuming it is flyable at all!)

Perhaps you are thinking about the fact that in normal flight a model with a CG too far back can (not necessarily will) tend to drop and drag its tail whilst a model with a CG too far forward can tend to drop its nose. There is also the problem with a too forward CG that the elevator effectiveness can be compromised to the extent that at low speed (i.e. the landing approach) there will not be enough elevator power to effect a proper flare, even thought the elevator power was OK at higher speeds.
The difference here is that you are not introducing the increased speed compared to normal level flight.

You don't have to believe the words - it is very easily bemonstrated with a hack model and a bit of removeable weight :)

Mike

center of gravity
 

thanks for the info i did balance the plane and it was like 50% of the chord i ve added weight till it was about 28% but i havent had chance to fly it though. and the wings are swept back like a jet a bit and the last third or so of the tips are angled up about 15 to 20 deg.