BALANCING GIANT AIRPLANES

[rede2fly]

hey guys dont want to sound like a idoit here but, what balancer are you using to balance airplanes that are say around or over 20 lbs?? all i have is the great planes balancer and i dont trust it to cary that much weight. plus it isnt very user friendley when trying to balance bigger planes. whats out there??

[Pale Rider]

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[Wayne]

I posted this a long time ago on RCU..
It works!

This method is how I go about checking CG on a plane that weighs too much to put on a CG machine. This is generally the same way full scale planes are balanced. You can’t go lifting a 747 by your fingers!


First, have the plane assembled as RTF, but 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 until the fuselage is sitting level. You will probably have to take the hatch off in most cases to find a suitable area to set the level on.

Once the plane is sitting level, place masking tape under the area where the main wheels are touching the surface and draw a line where the wheels contact the tape. Make an index mark where the center of the tailwheel falls in the same manner. This mark will be on whatever you propped the tail up with. (I use magazines). It’s also a good time to measure from the wall, to the point where 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

As an example, let’s say you come up with theses numbers:
LH main – 22”
RH main – 22”
Tailwheel – 70”

Now, place a scale under each wheel individually. You will have to re-adjust the height of the other wheels each time to keep the plane sitting level. Add more magazines as necessary. (If you have three scales you can get the weights at the same time but make sure the plane is level)
Don’t forget to re-install the hatch before you weigh each wheel. You will wind up with three weights, one for each wheel.

Let’s say you come up with these numbers
LH main – 9 lbs
RH main – 9 lbs
Tailwheel – 5 lbs

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 in-lbs
22x9=198 in-lbs
70x5=350 in-lbs

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) arent going to change so you already have those numbers.

[rede2fly]

thanks wayne. i knew i was saving my 15 year old mags for something. thats a lot to take in, after only reading once. think ill have to print this out

[Wayne]

It looks worse than it is.
Pretty simple to do.

[rede2fly]

	Quote: Originally Posted by Pale Rider
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hey pale rider why did you pull your post?? i didnt have a chance to go look at it.

[digger44]

I downloaded this spreaadsheet from some where but for the life of me I can't remember. Havn't tried it yet but it is basically the same method that Wayne described above except total weight of the model is needed. At the time I found this, those that used it said it worked well. Any way, it might be worth a shot. I will be attempting this myself in a week or so.

[rede2fly]

thanks digger ill take a look at it

[BigDuke1]

WAYNE...... That is great. I took my A&P (Airframe & Power Plant) a lot of years ago. That really takes me back. You are spot on.... Thanks Jay P.

[Wayne]

Jay,

As you know, I didnt make it up.
Just passing it along.

[reyn3545]

Most of the larger planes will tell you to start out with the plane balanced on the wing tube, so just take the canopy off, grab the tube (lightly) and lift.

Some obviously won't have wing tubes (warbirds) or will balance at different points... the above weighting method works great.

[showme3d]

If the balance point is the wing tube. I take a string and make a loop on both end. Put the loops over both sides of the wing tube (with tube thru plane) put the wings on and all other parts (canopy, cowling, etc). Lift the plane off of the ground by the string, should be pretty easy to see if it is balanced.