I need help- Center of Gravity!!!!

[Ultralightfreak]

:eek:Ok I was reviewing the a sample of Center of Gravity and got stuck on this question:

So the datum point was selected as 60 inches in front of the wing leading edge. The distances aft of the datum point for various locations on the airframeand all removable items (pilot fuel radio) was recorded.

So the nose wheel was 36lbs
Moment arm from the datum point 30.5 innches
Balance moment = 1098 inch-lbs

Right wheel 100lbs
moment arm= 91 inches from the datum point
9100inch-lbs

Left wheel 102lbs
moment arm= 91 inches from the datum point
Balance moment = 9282

So my total weight is 238 lbs and my balance moment (total) is 19480inch-lbs
So I did the division (Total Moment/Total Weight) and I got 81.8 inches from the aft of the datum. After that I subtracted 60 inches (The distance aft of the datum of the wing leading edge) from 81.8 inches and got 21.8 as the C of G behind the leading edge of the wing. Now to find % MAC you have to: Divide; C of G location aft of wing leading edge/mean aerodynamic chord of the wing. So I am reading am calculated 21.8 inches/54 inches...? My question is where did I get the 54 inches from. I have recalculated and subtracted...I did as much as I could but I require some help to move on. I know it may sound confusing but this is what the Condition Manual shows.

[Glen Dewsbury]

Is 54" the wing cord? If yes then C of G calculates at 40% of cord.

Glen

[Ultralightfreak]

Oh I remember now!
For each aeroplane the % Mean Aerodynamic Chord differs...For a Mirage it is 54 inches for...a Quicksliver it could be X number of inches thanks Glen!:)

[draco049]

I am not sure,but on a gyro we just do a hang test.Can this be used on a fixed wing?:rolleyes:

[Glen Dewsbury]

A hang test can be used to determine CofG on any object including fixed wing aircraft but does not give sufficient information to make changes later by calculation. A change in weight between pilots in a single seat aircraft where the pilot is not at the CofG can be significant. It can be convenient way to get a quick result and find if the aircraft CofG is within range if you do not have a scale on hand.

[tigermb44]

it is not clear if you are talking about center of gravity or weight and balance

In order to do your center of gravity you need to weigh your aircraft and do your calculations. It seems this is what you are doing as you are providing the weights of aircraft as weighed from the wheels.
The manufacturer should have provided the information to allow you to calculate your center of gravity and the center of gravity range which is permissible.

weight and balance, this is usually a preflight task where you check the gross weight of the aircaft and make sure it falls whithin the max gross allowed and the load is positioned in such a way as the c/g range is not exceeded.

if you are trying to do a weight and balance from scratch thats a little different
mb

[kitfoxflyer]

What kind of Aircraft is this ?
Have you done your measurements according to manufacturer

Also
It looks as if you are expecting the empty plane to be within the w&b envelope, but this rarely happens in a light plane. You now have to plug in the weight and location of the crew and do that calculation to see if you can get into the CG range. Do all of this with the tanks empty to be certain that if you run dry the plane could still be controlled, and so that if you stalled the plane you would be able to recover. Then fill the tanks and do the calculations again to be sure that the plane will be in the envelope at takeoff.

[Ultralightfreak]

Kitfoxflyer, there was an example in one of the books that I found on how to find the center of gravity in an airplane...I asked several people and then FINALLY came up with an answer that I understood (meaning: it wasn't hard to understand). I now understand how to find and calculate the C of G of any airplane....well I may be a little naive on my last statement but the truth is that I now know how to find the C of G.

Thanks anyways, I will take your advice whenever I need it:)