How to properly balance for CG?

[tpwpny]

The plans for Kit 302 (Cessna 170) indicated where the CG is, but what is the best way to balance the plane once assembly is done? Hang it from a string? Hold each wingtip lightly against each forefinger? I have seen videos where someone just lays their finger across the bottom as a fulcrum.... (of course!) Any advice appreciated...

[Bill Gaylord]

I hold the model upside down on my finger tips, over a soft surface like a bed. The problem is that the CG is often a bit forward of the thickest chord, so you have to make sure that it is really balancing level, at the spot where it's sitting on your finger tips. I try to get my finger tips on the formers, which helps alleviate that problem.

[davidchoate]

The whole CG thing has me confused. I put it so the nose is a bit heavy, and remember if itd FF or RC, then what? Ask David Lewis. I agree its better to sart nose heave ,and fine tune from there.

[davidchoate]

Its weird, but I will sometimes hang it by a single thread of string at the exact center and stare at it hanging for a few weeks before putting on the last piece of covering. sometomes.

[davidchoate]

Its weird, but I will sometimes hang it by a single thread of string at the exact center and stare at it hanging for a few weeks before putting on the last piece of covering. sometomes.

[Bill Gaylord]

In general, I believe most Guillow's airfoils are created so that the center of pressure is fairly far aft, compared with some other sections. They realize that applying nose ballast to an already heavy model will further detract from flying ability. I believe the plan CGs are also shown aft for that same reason, being a trade off between being a bit tail heavy, versus not flying well. For ff, the lower weight will tolerate a more tail heavy condition. For r/c, after building enough of these, a builder gets a pretty good feel of how to build and where to install components, in order to set the CG with minimal to no ballast. Often that requires locating the gear and battery forward. My current Cub build was nose heavy prior to painting, but again after building enough of these, it should be pretty much spot on, with the added aft paint weight.

[David Lewis]

The optimum CG location depends on the horizontal tail volume and the airfoil, and you dial it in through flight testing. If, when the airplane goes into a dive, it pulls out quickly, it's probably nose heavy. If it doesn't pull out (or tucks under) it's tail heavy. You are looking for a gradual pull out -- except in those cases where the stabilizer is a little on the small side, you may be forced to go more nose heavy than usual.

[davidchoate]

Is this because the larger tail is producing lift? As on FF models to keep the weight added to nose to achieve balance down by the airfoil of the vertical stabilizer creating lift, and giving the effect of a lighter tail?

[tom arnold]

No, not really. This is a another case in aerodynamics where what seems intuitively obvious is misleading. Every conventional airfoil has the center of lift behind the center of gravity---it is just way physics makes it. This creates a forward tumble on an airfoil. This is countered by a horizontal stabilizer that pulls the tail down and it all balances out and the airfoil flys level.That also means your horizontal tail must either be angled down slightly or have a bit of "up" elevator to accomplish this. If you take weight off the nose of this nice gliding model (i.e. move the CG back), you need less and less of "up" elevator. Finally, you reach a point in which the elevator is providing only a razor's edge of balance to that tumble and the model will never pull out of a dive and go anywhere the nose points. It is very squirrely at that point and you will never be able to trim it out. If you keep moving the CG back, you will have to give it "down" elevator to keep it from just doing a falling leaf out of the air but that quickly will be not enough and the model is just unflyable. Just try it with a sheet balsa model for a demonstration. The only way moving the CG back works is to keep making the horizontal stabilizer bigger and bigger until finally the stab approaches the size of the wing and then it is called a Tandem Wing and it obeys different laws of physics for trimming for flight and is beyond a scale model we fly. The best way to find the CG of a model if it is not called out on the plan is to place it just a smidgen AHEAD of where the main gear touches the ground in a tricycle gear aircraft or slightly BEHIND the main gear for a taildragger. Bend the elevator up in increments for the best glide during hand tosses.

[David Lewis]

Some models have a relatively large horizontal tail volume, the CG is around 40% to 60% of the wing chord, and the stabilizer contributes to wing lift (operates at a positive angle of attack). This increases the maximum lift coefficient and glide ratio.

[cliffm]

TPWPNY I just stick a small pin in each wing tip where the plans say c.g. is and set it up on a couple of your favorite 16oz. beverage cans. My favorite thing for ballast is #4 shot from a shotgun shell. Remember the further away from the c.g. you put it the less weight you'll need to balance.