Dynamic CG
 

I always looking for something to hook up my 7th or 8th channel too, and here is somethig I just thought of.

What if I placed my battery (I fly electric, but this would also work with the receiver battery in a Glow/Gas plane) on a track so that a servo can move it fowrad or aft in a controlled fashion. In this way I could setup my plane with forward CG for takeoff and landing and rearward CG for aerobatics.

Has anyone tried this and if so how did it work out?

-Q.

RE: Dynamic CG
 

We've discussed variations on this in the past, but I've never heard of anyone successfully implementing it.

There is a real-life equivalent...
When I used to skydive, we would routinely stuff 4 or 5 skydivers and a pilot into a (gutted) Cessna 180 with a lot of fuel. It wasn't unusual for the pilot to ask us all to move as far forward as possible during takeoff... not to reassuring, but I'm still alive.

Those were the good old days... Sometimes (since the plane was gutted, and the control cables were exposed) one of us would yank on the control cables (at a safe altitude) just to "check the pilot's reflexes".

RE: Dynamic CG
 

Maybe a driving screw would be good since all the weight of the battery would not be directly on the servo on the case of a snap roll or anything violent. It would be slow though. Just imagine if you get some glitches and appart from having troubble controling your plane, you could get a really rearward CG for landing.

RE: Dynamic CG
 

Technically, it should work... but keep in mind that the balance on the elevator will change with a dynamic CG configuration (think of pitch changes on a hang glider to visualize this).

The ideal situation will be when the CG moves from a slighly downward force on the tail to a slightly upward force when the CG is moved backward (this should keep drag to a minimum). An all-moving horizontal stab would be best.

You could do this in a glow/gas powered craft by locating the gas tank far forward and carefully balancing it so that when the gas tank is empty, the CG moves back no farther than the rear CG limit. In this configuration you would need no servo control, just careful planning! The bad part of this is that your CG is constantly changing.... so the aircraft dynamics are constantly changing as well. This type of setup would also require that the fuel be a significant percentage of the aircraft weight so that this "slide" in the CG is significant (this may be very difficult to figure out).

So, doing this with an electric craft would be much much simpler!

There are a few full size craft that needed a great deal of pilot input during flight to balance out the aircraft. The reason for this was because the fuel tank could not be located near the CG due to structural limitations, etc. And no, I can't think of which aircraft had this problem/feature off the top of my head.

Good luck!

RE: Dynamic CG
 

I like the jack-screw idea, but don't like the slow response. I want to move the battery (a sygnificant contributor to an electric plane's wieght) around a little, but not drastically.

I shudder to think of a very rearward CG that cannot quickly be moved forward to save the plane.

I actually want to move the CG aft until the plane diverts, then bring the CG forward and see how the plane reacts (with no control input). It should be an interesting thing to experience.

RE: Dynamic CG
 

Do you guts think that moving the CG forward when deploying flaps (30-45 degrees) could be of benefit?

RE: Dynamic CG
 

That sounds like that could be allot of fun. I think you could really get some strange maneuvers with a extreme aft C.G. Maybe some sort of track with a cable pulley system would be quick enough. Just make sure the battery does not come loose or loose it's connection obviously. lol Let us know how you make out... sounds like fun.

MT

RE: Dynamic CG
 

You could try this:

Make the aircraft CG fall behind the normal limit, until it would become unstable. To then counter the inherent instibility, install a cheap gyro on the elevator servo.

I have been doing this for a few years. Makes for real interesting flat spins and flys good in high wind.

Schmleff