Here is a technique for balancing your models which are just too big to use your fingers. I think this should be a sticky.



Here is a link for determining where it should be to verify if your instructions or plans have it right as well. http://adamone.rchomepage.com/cg_calc.htm
[HR][/HR] Is this how you balance your planes??? Read on!!!

I acquired this method of balancing your giant scale plane from none other than our very own Wayne Geffon. It's probably one of the simplest methods for balancing a giant scale bird I've ever seen, thanks Wayne!!!!!



This method is how I go about checking CG on a plane that weighs too much to put on a CG machine or lift.

Have the plane all assembled RTF minus fuel. Find a place where you can put the nose of the plane up against a wall. Take a level and prop up the tail wheel till the fuse is sitting level. Your gonna have to take the hatch off in most cases to set the level on.

Once the plane is sitting level, mark on the ground the two points where the main wheels are with masking tape and draw a line right where the wheels contact the tape. Then make a mark where the center of the tail wheel sits in the same manner. Its gonna be on whatever you propped the tail up with. (I use magazines)
Its also a good time to measure from the wall, how far back the recommended CG is. Save that number for later.

What you are looking for at this point is the distance to each wheel from the tip of the spinner. (which should be touching the wall at this point)
I use a long straightedge to get the distances

Lets say you come up with theses numbers:
LH main - 22 inches
RH main - 22 inches
Tail wheel - 70 inches

Now get a scale and place it under each wheel individually. You will have to re-adjust the other wheels to keep the plane sitting level. Add more magazines.
(If you have three scales you can get the weights at the same time but make sure the plane is level)
Remember to put the hatch on after you level the plane before you weigh each wheel. You gonna wind up with three weights. One for each wheel.

Lets say you come up with these numbers
LH main - 9lbs
RH main - 9lbs
tail wheel - 5lbs

Now its just a simple math problem to find where the CG sits right now on the plane.

Weight x ARM (distance) = Moment
22x9=198
22x9=198
70x5=350

Now add the total of all the moments and the total of all the weights.
Weights 9+9+5 = 23lbs
moments 198+198+350 = 746

Then divide the total moments by the total weights.
746 divided by 23 = 32.43

That number 32.43 is "in inches" how far back from the tip of the spinner to where the plane balances right now.
Compare that to the distance that you measured earlier to the recommended CG location.
If you are off one way or another adjust items then recalculate the weights. The distances (or ARM's) aren't going to change so you already have those numbers.


It may sound confusing and a lot of work but it really only takes about ten minutes to do it.

This is generally how full scale planes are balanced. You cant go lift one of them by your fingers or stick it on a CG machine.

Try it......

Originally written by Wayne Geffon

Yep that is how big planes do it. What is nice is if you know where you have to add weight, like the back of the firewall, to get the CG right you can calculate how much you need. Big airliners like 747s put load cells at the jacking points, calculated in the same ARM(distance) X WEIGHT= MOMENT formula, but because jacking makes the plane bounce and it has to be jacked so high, the hangar is designed to drop the floor leaving the plane in the air.

So the first post shows a way to find the correct CG of a plane. Does it work for bi-planes? Or just one wing aircraft?

You may have to determine your average chord for you biplane. This is simple. Level out your plane, and stand a large square up against the L.E. of your top wing. Then measure from the square, to the T.E. of the bottom wing. This will give you the average chord of your bipe. Think of it as looking straight don onto your airplane. The width of the wings that you see, is the chord width you use to calculate the chord. Hope this made sense.

This method will work for any airplane and you dont have to find the MAC of anything.

What you need to know before hand, is the CG range in relation to your Datum. In the OP's post, the Datum was the tip of the spinner (or the wall), so ALL measurements need to be referenced from this point.

If your CG point is 4" aft of the Wing LE and the wing Le is 12" aft of your Datum, then the math you get from weighting each individual wheel and its arm (the distance from the datum to axle C/L) should come out to a final arm of 12" if your CG is perfect.

I have a spreadsheet i made up and plan to do a CG how-to video once i get another odd/large model done.

I just made one of these.

Simple, cheap, easy, works perfectly!

http://creswellrcflyers.org/Text/Vanessa_CG_Machine.pdf

http://iflyrc.hypermart.net/CG%20Machine.htm

That doesnt work for every model though.. Like big heavy models with thin trailing edges an no fuselage

I had been thinking the same thing regarding using a Vanessa rig for my CI with its wire trailing edges.

Great information Vertical Grimmace! This is the way I have done this on my large models and it works great!

Bob

+1 - That's how I balance mine, though I use three scales to make it a little quicker. I started with an article written by a local RC/full scale pilot who also happens to be an aeronautical engineer: A Method for Locating the CG on Model Aircraft

All my numbers go into a Google sheet. The calculations tell me where the plane is in reference to the specified CG.

Yes I have used the 3 scale method and it works well. I was asking if there was a way to use the formula from VG's first post to determining where the CG of a bi-plane should be.. Thanks to VG, I understand how to use it now. I shall give it a try. Thanks.