somewhere...

[Bootlegger]

         Somewhere I was told or read how to determine the c/g location on a scratch built model so as to have a good idea as to what length of nose and tail moment so the model will be close to balanced without having to add balast.
    Does anyone have suggestions ??    Thanks a lot

[Tim Wescott]

http://exp-aircraft.com/library/heintz/stabilty.html

If you're not afraid of the math...  It'll be conservative, but conservative is always a good starting point.  When I design for RC I aim for his numbers with no weight in the tail, then figure that I can add excitement later with a little bit of lead or brass.

Otherwise find a model with a similar tail volume coefficient and use their balance point from MAC.  Of course, you still have to do the math to find TVC and MAC.  Dang...

[Chuck Feldman]

It is hard for me to see how this can be done with math. But those who do it that will say I am wrong. A long time ago there was an article or two on designing stunt ships. What it said in its most basic form was to measure the distance between the LE and the spinner back plate. Then you use that measurement between the flap and the leading edge of the stab. But I could be wrong about that. It might have said to measure from hinge line to hinge line. If I was going to do what your talking about I would get out a set of Nobler plans and measure the moments of it. You know nose length and hinge to hinge. Using these dimensions you will be in the ball park. The reason the math part is wrong for me is I do not know how to do it. Besides that it is subjective. The weight of the wood finish etc are all unknown. I think that is why so many ships are an evolution of several models. This I do know. The Cg should be at the thickest airfoil section at the wing root. Forward of this is stable aft of it is unstable. Another thing to remember is that the finish process ads weight to the aft section of the model more than the forward part. I hope by jumping in here and making some statements others will also jump in and agree or disagree and the end result will be good information for you.


Chuck Feldman

[Tim Wescott]

It is hard for me to see how this can be done with math. But those who do it that will say I am wrong.

I won't disappoint:  You're Wrong!!! :-)

Actually, I misread his post -- I thought he wanted to know where to put the CG on a ship that's built, not how to design the ship so that, once built, it'll be right.

A long time ago there was an article or two on designing stunt ships. What it said in its most basic form was to measure the distance between the LE and the spinner back plate. Then you use that measurement between the flap and the leading edge of the stab. But I could be wrong about that. It might have said to measure from hinge line to hinge line. If I was going to do what your talking about I would get out a set of Nobler plans and measure the moments of it. You know nose length and hinge to hinge. Using these dimensions you will be in the ball park.

... and you'll just be building a Nobler with different aesthetics.

The reason the math part is wrong for me is I do not know how to do it. Besides that it is subjective. The weight of the wood finish etc are all unknown. I think that is why so many ships are an evolution of several models.

Not so much subjective, as subject to guesswork.  You can estimate the weight of the finish, and for the rest of it you can monitor where your center of gravity is going to be as you build.  I've done pretty good that way -- with some spectacular flubs along the way.

Just find the point where you want the CG, calculate all the moments around that point, and make sure they add to zero.  It's easy as pi (calculating pi, that is), except where you estimate the finish weight wrong...

This I do know. The Cg should be at the thickest airfoil section at the wing root. Forward of this is stable aft of it is unstable.

Half of that is half wrong, but the last half is right.

Where the CG lands depends on the planform of the airplane -- that's what the article that I quote goes into.  Most stunt planes are designed so that the CG should be somewhere around 25-30% back from the leading edge of the wing, and most stunt wings are designed with the thick point there, too.  But that relationship is largely coincidence.  Use a funky airfoil with the thickest point ahead of or behind the 25% point and you'll still need the CG to be around 25% -- it won't change at all with the airfoil change.  On the other hand, make the stabilizer really, really big and you'll be able to move the CG back (in fact you'll have a wider range of good CG locations).  Rip the stabilizer off and you'll still have a plane that can be made stable and fly -- think flying wing, or combat ship.  Build a canard, and the CG will be in space, between the two wings.

Etc.

(make your stunt ship so weird that the CG doesn't land somewhere fairly close to the 25% point, and see if you have a chance to make it fly nice...)

Another thing to remember is that the finish process ads weight to the aft section of the model more than the forward part. I hope by jumping in here and making some statements others will also jump in and agree or disagree and the end result will be good information for you.

I check the CG before finishing, to see if I can lard up the tail or if I have to make it really light.

[FLOYD CARTER]

I don't know an easy way to determine CG when all you have is plans!  You would have to know the weight of every component fore and aft of the proposed CG, hard to do unless you calculate the weight of every piece of balsa, stick, paint, etc.  If you could do that, then the sum of moments behind the proposed CG must equal the sum of moments ahead of the proposed CG.  Whoops!  Too many variables and unknowns.  What we normally do is estimate, based on assumed weights, and then hope for the best, and then making corrections with added lead ballast.

Floyd

[Chuck Feldman]

  There you go Gil. Standby I think this may get even more interesting.. OK here is another one for all to consider. Most of all the old ships are nothing more than different looking Noblers and some actually aren't that different. Even the latest ships have to trace there designs back to Georges genius when he struck up on the Nobler design. I do not know a lot about aerodynamics and I wonder if anyone really does. I do read material that is not full of math and formulas. Now let me make another statement. Our models (the good ones) have neutral stability. What that means is that the model goes where you aim it and will not do anything to recover the attitude of the plane from the position you put it in. Kind of like a fighter plane. They require a control input to move to a new position.  

Chuck Feldman

[Tim Wescott]

I don't know an easy way to determine CG when all you have is plans!  You would have to know the weight of every component fore and aft of the proposed CG, hard to do unless you calculate the weight of every piece of balsa, stick, paint, etc.  If you could do that, then the sum of moments behind the proposed CG must equal the sum of moments ahead of the proposed CG.  Whoops!  Too many variables and unknowns.  What we normally do is estimate, based on assumed weights, and then hope for the best, and then making corrections with added lead ballast.

If I build a plane and weigh every bit of balsa that goes into it I build a lighter plane.  Why?  Because I weigh it, I compare it's weight to what it should be if it were 10lb/ft^3, and then I decide if I want a heavy bit there or a lighter bit.

I'm still working on nailing the balance, though.

[john e. holliday]

I am going to jump into the fire.  From day one I always read that starting point for CG is 25% of the wing chord from the leading edge or the high point of the airfoil.  I have flown more planes without ever checking for CG until it has been flown.  If the plane is real touchy the CG is too far aft or the controls are set up too fast.  If the plane will not turn the CG is too far forward of the controls are too slow.  I have cut into planes to slow down the controls with the CG at the point shown on the plans.  Need to go look at the plans but, did George show the CG on a Nobler?  Now that I am confused, what is CG?

[Tim Wescott]

I am going to jump into the fire.  From day one I always read that starting point for CG is 25% of the wing chord from the leading edge or the high point of the airfoil.  I have flown more planes without ever checking for CG until it has been flown.  If the plane is real touchy the CG is too far aft or the controls are set up too fast.  If the plane will not turn the CG is too far forward of the controls are too slow.  I have cut into planes to slow down the controls with the CG at the point shown on the plans.  Need to go look at the plans but, did George show the CG on a Nobler?  Now that I am confused, what is CG?

The Top Flight kit of the Gieseke Nobler shows the balance point on the spar, which is both the high point and just about the 25% point.

[john e. holliday]

Thanks,  brain fade again.