FighterBird873

Has anyone ever tried to rig up a variable CG? Maybe a movable weight mounted on the pre-flight CG, and use a servo to shift it fore and aft. How sensitive is the CG, would a 1oz mass moving plus and minus 1/2!QUOT! make a difference? I figure it would be easier to figure out how tail heavy you can make the plane while you're still two mistakes high. Instead of dual rates, you'd have dual CG's. Comments and criticism welcome.

Redback

Just played around with some numbers:

Seems to me that for a plane in the 5 - 6 lb region a 1 oz weight would need to move a long way to make any significant difference to the CG.

The mechanism needed to move the weight would be a significant penalty also. Best way is probably to fit two fuel tanks and pump fuel around. Problem when you run out of ballast though!


Terry

the-plumber

IIRC the gents who tooled around in rider-scale Vigilantes didn't much care for their 'variable CG'. Seems like that's why the A-5s got turned into photo birds.

MinnFlyer

My Feedback: (4)

I like my idea... have a piece of tubing running the length of the fuse with a balloon at each end. then with some sort of pump or "Squeeze" mechanism, move fluid from one balloon to the other

Richard L.

My Feedback: (24)

Want to experience variable CG? Fly a Corsair, Hellcat, or P-40. Drop the wheels and the CG shifts forward. Retract the wheels and the CG shifts back.

twinman

My Feedback: (2)

Well, it seems I have made most of the mistakes in this sport and tried to do the others....and this is one.
Even though I did it on a single engine plane ( Please do not tell the guys in the twins forum) it was an ugly stick, it did work.
I admit that hovering has always not been my strong point,,,,,,,,so...why not get the CG to move back......A LOT.
All that practice stuff just burns fuel!!!
How to do, if you have a few loose screws and a plane you are willing to try this on.
Get a 1/4" thick x 1.5" wide x 18" or whatever length you need, long piece of plywood. Make a "Sled" frame to wrap around the long plywood and attach a four ounce ( or whatever) lead bar to it.The idea is to allow this "Sled" to slide up and down the plywood.
Put a pulley on one end and a "Sail Wench" servo on the other. These are the servos used to control the wind sails on RC sail boats.
They are completely controlable in their 3 revolutions according to transmitter input.
Rig up the fishing line to the sled, around the pulley on one end, and the winch servo on the other end.
Set the end points on the transmitter for this channel to not over move the "Sled". Suggest put the operation on a switch, verses a rotary knob.
For take off, set the Sled to normal CG,,,,,,,,but for hovering. ..HIT THE SWITCH...and watch the tail fall like a rock........uh ...did I mention to try this away from the flight line and four mistakes high......and that you have enough power to hover and pull out? DO!!
Remeber that it takes 1-3 seconds for the weight to move front to rear, so do not expect to get out of trouble instantly.
Does it work.....you bet and is good hovering practice.
Crazy to try......You bet,,,,,,,but what do I know,,,,I fly twins.
Have fun.
Twinman

Deadeye

Really? I'm not sure I understand how the CG can shift, when the LG is anchored to the same point?

Richard L.

My Feedback: (24)

I guess you are not familiar with those planes. Look at the landing gear of the F4U Corsair, for instance. When the wheels are down, they are under the wing leading edge and ahead of the CG. When the wheels are up, they are back near the wing trailing edge and behind the CG. Those are heavy wheels. I should know because when I retract the wheels, the plane becomes slightly unstable and twitchy.

BillS

TM you must be an engineer.

Bill S

Richard L.

My Feedback: (24)

The LG on these planes do not retract inward toward the fuselage like on the P-51 Mustang. Instead, they rotate and retract rearward into the wing.

Attached are three pictures showing the side view and bottom view of the F6F Hellcat. The side view shows where the wheels are when they are down. The bottom view shows where the wheels are when they are retracted. As you can see, when the wheels are down, they are in front of the CG. When the wheels are up, they are behind the CG.

Deadeye

Roger that, I understand how they retract. But would you help me understand how it can affect the CG when the landing gear is anchored on the same point? When retracted, the gear is indeed back, but the mount doesn't change. Isn't that where the weight is, on the mount? I just don't get it. Thank you.

Richard L.

My Feedback: (24)

Most of the weight is in the wheels, just like the weight of a pendulum. The mount doesn't change, but the weight shifts from one point to the next. You might be thinking too hard. The CG is affected by weight and how far it is away from the CG (moment arm). It doesn't matter if the anchor point doesn't move. The strut acts as a moment arm. When the wheels are up into the rear of the wing, they want to tilt the nose of the plane up. Put your plane on a balancing stand and put a pair of wheels on the leading edge of the wing. Now move the wheels back toward the trailing edge of the wing.

Deadeye

Ah, now I get it. I wasn't considering the strut. Thanks for clearing that up.

RCKen

My Feedback: (9)

Hope this helps.

twinman

My Feedback: (2)

Bills,
I don't know about the engineer title..Just poor old country boy.........who is bad about going where sane people should not go...Note, I only fly twins or three or four and one five engine planes. It is called Twinsanity!!
This little idea was before I caught the twin craze. You should see what I have tried with gyros!!
Hint....twin engine plane...rudder not in prop blast....separate channels on the engines....Gyro on the engines to aid hovering!!
Told you twins make you nuts..Wonder what I did with that assembly....have a Ultra Sport 1000 with two engines and empty fuse..Got to go!!
The one good thing about this hobby is that I KNOW I WILL WALK AWAY FROM THE CRASH!! Tee Hee.
CG shift is normal for most planes that retract to the rear. If you think HellCats and Corsairs are bad...try the P-38, P-61 with three heavy landing gear and sometime brakes, that ALL rotate to the rear!!
Can you say balance with the gear up!! Let us not also not consider the effect of aerodynamic drag on the landing gear down verses up,,,just to keep it interesting! I usually insert a program for some down elevator when the gear goes up on my P-38's due to drag going away. Yes, I said DOWN elevator, due to wind resistance. One problem with this...if you set the gear movement to scale like speed, the radio commands down elevator immediately upon hitting the retract switch...haven't figured how to get around that one!!
Good Luck,
Twinman

joeb102072

My Feedback: (9)

In stead od adding weight to the plane I had the general idea of making a sled type set up maybe slotted so when you move it sort of snaps into place as you slide the weight forward and aft. But instead of lead or outside source why not use your battery pack. Given the weight of your battery you wouldnt have to move it far. Down side is when you want ot change CG you have to land and change it. Which might not be a bad idea anyway.

twinman

My Feedback: (2)

If I could suggest, go back to my post on using a sale winch. Completely adjustable in the air and more important.;.RESETABLE
Twinman