jchorak

My Feedback: (9)

I have seen it here before but, cannot seem to find it. Wanting to the ratio of tail weight to nose weight when balancing an aircraft, such as if an ounce is added to the tail, how much weight to the nose is needed to balance?
Thanks,
JC

GREG DOE

Weight and balance is a calculation based on "moments" The moments are the distances measured from a "datum" The datum can be anywhere. On many general aviation airplanes the firewall is the Zero datum, and everything is measured from that point. Having said all that, there can be NO rule because every airplane is different. Some scale war birds have a short nose moment in relation to their tail moments Most of our sport models have longer nose moments, and longer tail moments Electric motors are lighter then equivalent glow, or gas motors, so electric airplanes often have longer noses. To cite an example; If you had a model with a 10 inch nose to center of gravity distance, and a 30 inch measurement from the CG to the tail it would be a 3 to one ratio. In your example, one ounce of tail weight would require a three ounce nose weight to counter act it. The problem is every airplane, model or full scale, is different I'm currently working on an electric racer that has a 28mm nose to CG measurement, and a 46mm CG to stab leading edged measurement, so the ratio of nose weight to tail weight would be 1.642.

khodges

My Feedback: (1)

What Greg mentions is spot-on, every plane is different.

Your plans, or instructions, should note where the CG is located. Generally, it is somewhere between 1/4 and 1/3 of the mean chord back from the leading edge of the wing. Set the complete UNFUELED model on some sort of stand that will allow it to rest on the CG point. If it is nose heavy, add weight to the tail until it balances. The further back on the tail you add weight, the less weight you will need, but it is better to add a bit more weight about halfway between the balance point and the tail than a small lump at the tail, or spread the balance weight along an area rather than all in one spot. Just try to use the least amount you can, to achieve balance. If tail heavy, add weight as far forward as you can, usually on the firewall. You would prefer the plane a bit nose heavy, but never tail-heavy. Try to locate the fuel tank as close to the CG as possible, so the balance doesn't shift much as the tank empties. If the plane is electric, you can use battery placement to help balance the plane.Try to make any added weight useful.

jchorak

My Feedback: (9)

Working with a club member on 60% Hempel cub, trying to reduce weight in the tail to shed some of the nose ballast to come in under the 125 lb. weight limit. May have it.
Balancing with scales/measurement method.

tedsander

Ummm, at it's simplest, IF you have an existing model AND you want to keep the balance point in the same position, you'd do it in terms of "inch-ounces" (or gram-millimeter, or any other weight/length units you want to use).
Measure from the balance point to the weight to be removed. Weigh the weight. Multiply one by the other. So if it is 2 oz, and it is 15 inches, that would be 15*2 = 30 oz/in. removed
If the other distance is 6 inches, then divide the result above (30) by the 6. So 30/6 = 5 oz have to be added to compensate.

A little googling of "Aircraft Weight and Balance" will give all sorts of permutations of the above, the FAA has a number of docs on how to calculate. Even how much to add/subtract from one point or another to move the CG a given amount.
You're only limited by your equipment - how accurate you can measure the distances and the weights. Tiny free flights need very accurate high precision scales. Giant scale planes not so much, or may exceed the capacity of whatever scales you have.
I frequently balance my large planes by weighing the wheels and doing the math...

jaka54

??? You just add weight and use "cut and try metode " ! No advanced calculations are needed to balance a model airplane!.
All airplanes are balanced (Cof G ) at approximately 25-30% of the wing cord. You place your fingers there (Cof G) and if the plane is front heavy you move the radio gear rearward.
Adding lead or any other weight is only used as a last resort.

GREG DOE

Ted, and Jaka, You are missing the point. JC is not asking where the CG should be, or whether to put weight in the nose or the tail. He's trying to figure out how to reduce the overall weight, to get below 125 lbs. So his problem is how much of the total weight would have to be removed from the nose, and the tail, and still maintain the correct CG.

tedsander

Actually, he asked "... if an ounce is added to the tail, how much weight to the nose is needed to balance?"

So if his ounce added to the tail is 25" from the CG position (before the ounce was added) 1x25=25
If then his location to put the weight in the nose to counter balance is 10" ahead of the original CG, he would need 25/10 or 2.5 ounces in the nose (ie 10x2.5 = 25, the same as the answer for the tail addition)
Just for illustration - if he wanted to add at a spot 2" ahead of the CG to counterbalance the added tail weight, he'd need 25/2 or 12.5 oz at that location, which would work instead of the further forward addition.
Yes, overall weight would increase by the sum of both additions. And the CG would not move.

His later post (after my replay) noted he was actually trying to reduce overall weight, but same math relationship still applies. Using the numbers above, remove one ounce in the tail, means removing 2.5 up front to keep the CG the same. But it all depends on the actual distances from the CG.