I'm thinking about converting a plane to have retracts, but I think the wheels would be way to close to the CG, so I got thinking.
I know this would take some design work, but...
What about mounting a weight on some type of track/slider, attached to a servo. So that when landing or taking off you could move the CG back a little with say the flap knob on the Tx. Then once up in the air, move the CG up to the regular position.
Might also be fun on a 3D plane.

I don't know, maybe the weight needed would be too much.

Seems like it would be to much weight..

Sounds like the complications of doing this would outweigh the benifits. [8D]

This is a rather silly idea.

Nonetheless, it's something that I have thought about on several occasions

But rather than use a weight on a slider (an idea I cast aside early in the thinking stage), I was thinking of having a balloon partially filled with fluid at each end of the plane connected with a tube.

Then have some sort of squeezebag mechanism that could squeeze the fluid from one balloon to the other.

Why do a lot of people try to complicate things ?

Balance your plane with the gear retracted, which is the way you will normally be flying. When the gear is down, MOST retracts will either move the wheels forward slightly or they will stay in the same position relative to the CG. If the wheel moves forward (MOST retracts ), this will make the plane slightly nose heavy. While this will make the plane a little sluggish, it also makes the plane more stable, which IMHO is good for takeoffs and landings

It would seem to me that the CG wouldn't change at all. The retracts are using the same hinge location no matter if they are up or down. It shouldn't matter to the CG.

Try it for yourself. Pick up a 10 pound dumbell and let it hang by your side. Then rotate your extended arm forward, lifting the weight. Even though your shoulder (the hinge point) doesn't move, you will feel your cg shift forward. The CG is definitely affected by the location of the mass, not where it's connected.

Well, whether or not the CG would change depends on the type of retract, or rather the direction of retraction.
If the retracts move in towards the fuse (or out), then the CG won't change, but if the retracts rotate and move forward (or back) then the CG will change.

Mike,
Ballons and squeeze bags? Why not lead shot in a tilting box or tube? (With hatches to prevent unwanted changes of course.)
Dennis-

ok, not exactly cg, but close... couple years ago I saw a show on discovery that showed a guy who converted a small 2 seater to have a pivoting wing. basically instead of a normal wing like planes usually run, this one was only attached at 1 pivot point at the center of lift. supposidly it was impossible to stall since the front of the wing would dip if it encountered a stall... anyone up for incorporating that into rc?

They laughed at the Wright Bros too. I say give it a shot. If it doesn't work, you have gained experiance, if it does, you may want to contact Bert Rutan.

If I'm reading this correctly, it sounds like you want to move the CG BACK when you land and FORWARD when you are in flight? This is reversed from what you would want to accomplish. Moving the CG back during slow speed flight will make it harder to handle. Moving the CG backward to accommodate improper gear location is not the right solution. I think the bottom line is if you can't get the gear in the right place relative to the CG, don't convert it to retracts.

I was basically thinking out loud. Thinking that I would want some weight aft of the gear so it doesn't nose over.
But I think your right Chuck.

Campy said it all -- don't worry about trivialities--- balance for the normal in-flight configuration & go flying. There will be more trim change (nose down) from the gear extension, than the change (if any) resulting from a CG shift due to gear movement.

Minn,
Just use a battery powered reverseable pump and two fuel tanks filled with water if you want to shift weight that way.

I've thought of using a jack screw arragement to slide a weight back towards the tail (small weight, long travel) to make a plane really tail heavy for some fun in the air.

As for dealing with retracts, if you use light wheels, the CG shift isn't really that much, and usually results in the CG coming forward when the gear is down, which is a good thing anyway, as mentioned above.

generally as has been noted it will even out due to gear drag causing trim changes as well. moving the CG around has been done on full size planes such as the B-1 Bomber due to the swing wing and the fact that fuel consumed in flight changes CG.(were talking thousands of pounds here)but its a dicy business if not done corectly.

I was always under the impression that Hanno Prettner used to do this in his pattern ships (in the '80's) to make his plane "snap" more crisply--- (Dunno where I read/heard this-)

A servo pushed a weight backward a bit to give a more aft cg-