Found this nugget on the old forum, is this method accurate aka accurate compared to the plans method of finding the cg?
Simple method for finding airplane cg
In Response To: Weight and Balance / CG calculations. (Tom Sampson - Portland, Oregon)
I like to use the following method:
1.) Rotate the propellor to a level position (3 o'clock - 9 o'clock). Then push the plane up against a vertical wall so that the propellor spinner just touches the wall, with the fuselage perpendicular to the wall and the plane resting level on all three wheels. Put some blocks under the tailwheel, so the fuselage is in level-flight attitude.
2.) Put scales under each wheel (if you have only one scale, make sure you put blocks under the other two wheels to keep the plane level as you weigh one wheel). Write down the weight resting on each wheel.
3.) Measure the distance from each wheel (the center of the wheel) to the wall. Make sure the spinner is still touching the wall as you do this. Write down the distance from each wheel, to the wall. I always write it in inches, like 38-3/4" or whatever.
4.) PILOT WEIGHT: Weigh yourself in full flight gear, and write it down. Then measure as best you can, the distance from the wall to a point about 6" in front of your belt buckle where you would sit in the seat. Write this distance down.
5.) FUEL WEIGHT: Multiply the number of gallons you want to carry, by six pounds per gallon, to get the weight of the fuel. Then measure from the center of the tank, to the wall. Write down the weight and distance.
6.) Keep doing this for any other add-on items, such as the sleeping bag you occasionally stuff into the turtledeck or the video camera you hang under the wing. Weigh it, and measure how far it is from that wall, and write them down.
7.) Add all the weights together (not the distances), to get the TOTAL WEIGHT of everthing you're going to take off with.
8.) Also, for each thing you weighed, multiply its weight by the distance it is from the wall. Do this for each wheel, the pilot, the fuel, etc. If you fly with only pilot and fuel (and the plane of course), this will give you five numbers. Or six if you included the video camera. These numbers are called the "moment arms" of each thing.
9.) Add all the moment arms together, to get the TOTAL MOMENT ARM.
10.) Divide the TOTAL MOMENT ARM (from Step 9) by the TOTAL WEIGHT (from step 7). This will give you a number, which is the distance from the wall to the Center of Gravity of the entire plane with pilot, fuel, and whatever else you added in.
Make sure all your weights are in pounds, and all distance are in inches! Or else you'll come up with really bizarre numbers, and you'll find that the CG is somewhere in the next county.
Did that come out at all clearly? Or did I succumb to Engineer-ese again?
Here's an example:
(Step 2.) Plane is empty, with 1-1/2 gallons fuel left in the tank. I put a milk crate under the tailwheel to hold the fuselage level. Left main wheel has 130 lbs. sitting on it. Right main wheel has 132 lbs. Tailwheel has 34 lbs. It's a fat UL. So sue me!
(Step 3.) With the spinner touching the wall, the left main wheel (center of the axle) is 38-1/2" from the wall. Right main wheel axle is 38-3/4" fom the wall. Tailwheel axle is 168" from the wall.
(Step 4.) With my ski jacket, normal clothes, helmet, headset (they weren't in the plane when I weighed the wheels), and long stylish silk scarf, I weigh 186 lbs. When I sit in the seat, my belt buckle is 88" from the wall. Take off six inches, and the distance is about 82".
(Step 5.) The 1-1/2 gallons still in the tank, got figured in when I weighed the wheels, so I will not count it here. But I want to take off with a full tank, so I need to figure in 3-1/2 more gallons, which is the number I'll use here. Distance from the center of the tank to the wall, is 77".
(Step 6.) That sleeping bag, which wasn't in the plane when I weighed the wheels, weighs 6 lbs. And where it sits in the turtledeck, is 114" from the wall.
(Step 7.)
Left wheel weighs 130 lbs.
Right wheel weighs 132 lbs.
Tailwheel weighs 34 lbs.
Pilot weighs 186 lbs.
3-1/2 gal fuel weighs 21 lbs.
Sleeping bag weighs 6 lbs.
----------
TOTAL WEIGHT: 509 lbs. at takeoff.
(Step 8.)
Left wheel weighs 130 lbs., and is 38.5" from the wall. Moment arm = 130 x 38.5 = 5,005 inch-pounds.
Right wheel weighs 132 lbs., and is 38.75" from the wall. Moment arm = 132 x 38.75 = 5,115 inch-pounds.
Tailwheel weighs 34 lbs., and is 168" from the wall. Moment arm = 34 x 168 = 5,712 inch-pounds.
Pilot weighs 186 lbs., and is 82" from the wall. Moment arm = 186 x 82 = 15,252 inch-pounds.
3-1/2 gal fuel weighs 21 lbs., and is 77" from the wall. Moment arm is 21 x 77 = 1,617 inch-pounds.
Sleeping bag weighs 6 lbs., and is 114" from the wall. Moment arm is 6 x 114 = 684 inch-pounds.
(Step 9.) Left wheel moment arm 5,005 in-lb
Right wheel moment arm 5,115 in-lb
Tailwheel moment arm 5,712 in-lb
Pilot moment arm 15,252 in-lb
Extra fuel moment arm 1,617 in-lb
Sleeping bag moment arm 684 in-lb
---------------
TOTAL MOMENT ARM 33,385 in-lb
(Step 10.) Divide TOTAL MOMENT ARM by TOTAL WEIGHT: 33,385/509 = 65.6"
The Center of Gravity (CG) is located 65.6" away from the wall, when all the things you weighed, are in the plane where they belong.
With the spinner still touching the wall, use the tape measure and measure back from the wall, and make a small mark on the fuselage, 65.6" from the wall. That's the place!
Hope this helps,
Steve Maher
ondeck35@hotmail.com
|
Logged
