radfordc

My Feedback: (14)

After seeing the new Xicoy CG machine ( http://www.ultimate-jets.net/product...tal-cg-machine ) I decided that I could do about as well using the method that is used to do Weight and Balance on full size planes and at a much lower cost.

I purchased three digital scales from Walmart's web site. The small one is for up to 11 lbs, and the big ones go to 18 lbs. These should suit weighing a 40 lb airplane. The total cost was less than $50.

Start by leveling the plane and lining it up with a reference mark and then make three measurements: distance to main gear, distance to tailwheel (or nose wheel), and distance to wing LE. I used the edge of my work table as the reference and dropped a plumb bob down from the tip of the prop spinner. I measured: Main gear = 18.75; Tail wheel = 58.4; LE = 16.75

With the plane still level, zero the scales and put them under the wheels and measure the weights. I got: TW = 36.9 oz; R Main = 145; L Main = 146.9 Total weight of plane = 328,8 oz (20.5 lb)

For the CG here is the math:
1. Calculate tail moment: tail distance x tail weigh or 58.4 x 36.9 = 2155
2. Calculate main gear moment: main distance x sum of L and R main gear weight or 18.75 x 291.9 = 5473
3. CG distance from reference point is total moment divided by total weight: 5473 + 2155 = 7628 CG = 7628 / 328.8 = 23.2
4. CG distance back from LE: 23.2 - 16.75 = 6.45"

The manual calls for the CG to be 6.375" back....I figured this was close enough and sure enough the plane flys great.

acerc

That is how I do it only I have a spreadsheet that does the math for me.

Wizard61

My Feedback: (7)

This might be a dumb question, but on setting the CG on say a P-40 or Corsair where there is a significant shift in weight as the gear rotates and retracts rearward, my question is how do you account for that shift in weight with this method? Most people calculate the CG with the gear retracted. So how would you balance an aircraft like that?

Thanks,

James

SWORDSN

My Feedback: (13)

That's a good ? Not a dumb one

radfordc

My Feedback: (14)

Balancing a P-40 with the gear up would require making support stands to set on the scales. You can zero out the weight of the supports before setting the plane on them. You would still need to have the plane level and you would use the distance from the support stands to the reference point for your calculations.

radfordc

My Feedback: (14)

Here is how to do a Skyshark P-40. Make the supports that go under the wings and tail. Decide where the supports will fit and make the measurements back from the tip of the spinner. In this case: Main supports = 21.1"; Tail support = 53.1"; LE = 19.7".

Place the supports on the scales and zero the scales. Place the plane on the supports where you made the measurements. Record the weights and do the math.

Main moment = 21.1" x 325 oz = 6857.5 Tail moment = 53.1" x 33.7oz = 1789.5 Total moment = 8647

CG = 8647 / 358.7oz = 24.1" back from tip or 4.4" back from LE of wing.

Top_Gunn

My Feedback: (6)

With a Vanessa rig you can find the balance point of any plane, big or small, upright or inverted, with gear up or down. And you can make one out of stuff you probably already have. And when you're done, you can raise it up to the ceiling, so it doesn't need storage space. I do sometimes bump my head against it though (low ceiling in the basement).

radfordc

My Feedback: (14)

I agree with the Vanessa rig....I've used mine many times. The Xicoy CG system is the new electronic method of doing a weight and balance, but it costs $300. I did this at less than $50.

Also, the Vanessa rig is subject to some potential problems. I would hate for the rope to break with a 20 lb plane hanging in the air. And, the Vanessa doesn't provide the weight of the plane....you still have to have a scale.

Wizard61

My Feedback: (7)

OK Radfordc. That makes sense to me now. Thanks for posting the method.

James