Hawaiian Hawker

Aloha1
I saw a few post where peeps said you turned the plane upside down to check the CG. Is the for real or what?

Gray Beard

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On low wing planes yes. Not for planes like trainers or bipes.

SeamusG

Em - and how about a mid-wing? Now, is that a setup or what?

BTW Gene - thanks for the idea of doing a CG test before covering!

Oh yea - I have a problem with determining "what's friggen nose up or nose down" when I'm balancing the plane on my finger tips. One technique that I have found simple, cheap and effective is the use a string level to make sure that the plane is level. I place the level on the horizontal stab and shim the plane on my bench using whatever you have available. This sets the plane at level "flight" because most of the planes have the stab set a 0 (zero) degrees incidence (aka - level). Then when I put my fingers under the wing and lift I watch to see which end drops - front drops - nose heavy. This works whether your plane is upright or inverted.

Yea, I've tried a bunch of "devices" including dowels with erasers and the Vanessa rig. This is pretty simple.

JohnBuckner

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HH with your high wing trainer as you know you can simply place your fingers under the wing and arrive at a point where the airplane will hang level and this is how we check where the CG is.

Now though what will happen when we turn that high wing airplane upside down and try to find that point? we will not be able to find that point simply because the airplane is top heavy. In other words it just will not balance and is too unstable to find the point.

When we have a low wing airplane we can find that point (in most cases) balancing it on our fingertips with it upside down, But agine now if we try to find that point with the low wing right side up it will be top heavy and too unstable to find that point.

John

jetmech05

On a mid wing it would depend on where the weight is...if the weight is above or below a line running the length of the fuselage.....if the weight is above the line then you'd balance upside down...bottom line is you want the weight down

Lnewqban

This article explains the same as above, and it has good illustrations:

http://www.rcuniverse.com/magazine/a...article_id=601

stevenmax50

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Great post.

Gray Beard

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A true mid wing should be able to go either way but how often do you find a model that has the wing perfectly set at the datum. That's one of those common sense things that creates that thinking stuff. Knowing how I feel about having to think I have often CGed mid wing planes first upside down then right side up just to see what happens. They are usually off one way or another. Now comes that thinking stuff. I scrathc my head while looking at the plane then decide how I would like to do it. Usually upside down and slightly nose heavy.
I didn't read LNs thread but I'm sure it's a lot more detailed then what I do.
To date I have only had one plane that the CG was off but I knew about it in advance, I'm talking a bunch!! It was a giant scale Bipe I scratch built from plans and I measured the CG shown on the plans against my old stand by of 25% of the wing cord. The plane was way tail heavy if I went to the plans. My advanced instructor had me go to the forward mark of the plans stating the designer knew where it should be. He may have but the guy that inked the plans didn't. I always check the CG location of plans built planes before I fly after that. The CG calculator on the net works great so I use that too.

Charlie P.

I balance with fingertips (or knuckles with veryheavy gasmodels). I am of the opinion that the optimal balance point can only be determined by flight characteristics - not the other way around. It is just a rough benchmark based on the designer's estimate or experience with the specific model(s) he flew.

I enjoy flat spins, so I tend to balance further back. But I am also pretty mindful of my fuel and usually land with 1/4 tank remaining or more. I've seensome pilots boast about how far back they keep theirbalance point - like it's a badge of honor - and then pop beads of sweat trying to dead-stick a squirrely model in after runningout of fuel. Stall atail-heavy model at low altitude trying to stretch a glide and you'll be brushing up onyour "I was hit" and "the windgot me" excuses.

MinnFlyer

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Charlie makes a good point - Remember that the recommended CG is just a ball-park figure. Once you fly it, you should determine if you want to adjust it

Gray Beard

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Pretty much gets adjusted again every time. I build them so the battery pack can get shifted quite a ways back. I too love the flat spin and stunts that start inverted. Tail heavy I don't do exactly but I do CG at neutral then move the CG as requires a little at a time until I get it where I want it. I always have students start with the CG just a bit on the nose heavy side though then move it as they get used to flying the plane.

stevenmax50

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Balancing my Tiger 60 (both of them) has always been a little bit of a pain.  They are soooo tail heavy.   I have never balanced it upside down.  I am gonna check that out saturday when I have some help other than my wife.