Correct CG location?

[stan]

I installed new landing gear which weighs more than the stock hardware. The Cub is now nose heavy and probably needs some CG adjustment. Anyone know what the CG location should be for the Super Cub? I have looked and I can't seem to find it anywhere. Thanks.

[jt01]

Welcome Stan, You will find that Wayne had a similar situation and took some time to figure it out. You can read his trials on this site in "controllability trouble" also a while back I posted how to make a simple tool to fine your cg in tips and tricks also on this site.Good luck and happy reading.
Send some pics of your landing gear mod,we all love to see what others are doing to their Cubs.

Jim


Re: Controllability trouble w/450
« Reply #4 on Mar 6, 2007, 8:22pm »

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Did some crunching tonight and this is what I have learned thanks to everyone's help/suggestions here and a few other sources. This may get lengthy.

First when I added the CF rods to stiffen wing I did two things to adversely affect the wing.

One, the wing is not symmetrical from side to side. Looking aft to forward the the wing left of the fuselage has dihedral and the wing to the right of the fuselage is quite flat.

Two, again looking aft to forward the wing right of the fuselage is 3/16th's of an ounce heavier than the opposing side. The 3/16ths of and ounce is measured at about 3 inches in from the wing tip.

To eliminate this whole wing mess I bought a new one.

Secondly I took Jim's suggested reading and enlightened myself about Center. Of. Lift. and it's correlation to Center. Of. Gravity.. After this read which was quite interesting I did the math and this is what I came up with.

- C.O.L. is 1.78125" from leading edge of wing.

- My ideal C.O.G. would be between 1.78125" and 1.50"from the wing's leading edge.

- My actual C.O.G. is 2.56" from leading edge. This plane is so aft heavy it is alarming.

Armed with the knowledge above I will be making some changes to meet C.O.G. and try this all over again.

[stan]

Thanks for the info. I will try to set it up with CG 1.5 to 1.75".

[jt01]

Should work fine.The further forward you have it ,the easier it will handle

[cjg]

As stated, the center of lift (COL) is 1/4 of the way back from the front edge of the wing.

The center of gravity (COG) should be on or a bit forward of the the COL.

I mark the COL on the underside of my wing about a foot out from the fuselage on each wing. I then use two pencils with eraser head tips to lift the plane on those marks. I move the pencils forward or backwards from the COL until the plane is level. This is your COG.

By trial and error my SC flies best when the COG is about 1/8" forward of the COL, or right on the COL (not behind it). My experimenting confirms what our friend wrote earlier.

I too was alarmed to find my plane grossly tail heavy - proably due to some epoxy on the tail and larger tailwheel and landing gear. It makes for wild flying, and I just couldn't get the thing to come down to a smooth landing. It simply floated up near the ground, then stalled.

A very slight nose heavy flight makes landing and turning easier. Too much nose heavy and the plane is sluggish in turning. Again, as stated, about 1/8" forward COG makes the SC a great flier.

I use a bit of plastic tack (modeling clay would work) and add it to the inside front of the cowl until the COG is where I want it. I ended up adding about an ounce and a half of pastic tack as I remember - a ball a bit smaller than a golf ball.

[stan]

cjg said:

I use a bit of plastic tack (modeling clay would work) and add it to the inside front of the cowl until the COG is where I want it. I ended up adding about an ounce and a half as I remember - a ball a bit smaller than a golf ball.

That's a pretty big wad of clay. Dosen't all that extra weight create a problem?

After I added the new heavier LG, the CG seems to be in the correct place. Perhaps the stock plane was tail heavy to start with. Now I am just concerned about the extra weight. I'll find out tomorrow when I try to fly it (weather permitting).

[cjg]

It does have an overall heavier feel in flight, but more stable. Landings and flying in breeze is much easier. It does take a bit more throttle. I used to be able to cruise level flight at mid throttle with stock battery, now I nudge it just past it.

Overall it flies better with the weight in the nose. I may have overestimated the size of the ball - it is plastic tack which is lighter than clay. I do know ths cowl is about 1.5 ounces from weighing it.

[itchycods]

Stan, after modding my cub, I had to add a big castle nut I had in my garage to the motor mount. I was concerned about adding weight as well, and I looked hard at the fuse to see if I could actually take away weight from the tail to correct my CG, but I decided not to, because the cub's achilles heel is the tail section about where the control rods exit. Seems like that's where they break pretty often. How did your flight go?