how do you figure the center of gravity on aircraft?

Answer 1

The center of gravity is most commonly near the thickest part of the wing.....this assumes a "conventional" layout i.e. motor in front of wing, big wing in front, fuselage, small tail in back. It sounds like that is what you have built. For an easy flying model, the center of gravity should be placed 25% to 33% back from the leading edge of the wing. Simply put- If the wing is 4" wide(chord) from front to back, the entire model should balance 1" to 1.3" back from the leading edge of that wing. If it is farther back than that, the model will be unstable at best, and unflyable if too far back. If the balance point is farther forward, it may not have enough elevator authority to keep the nose up or level while flying slowly and will like diving into the ground! Remember that these measurements are used with the aircraft as it will be flown, with all equipment and batteries etc. on board. If it does not balance where it should at first, try moving the stuff around to change it(the flight pack will have the biggest effect). I suggest starting out with the C of G at about 25% to 28% back from the L.E.(1" - 1.1" in my example 4" wide wing) What is the width of your wing?
Good luck with your new creation!! I love building my own designs, just wish I had more time to do it.
Hope this helps.

Answer 2

For the empty plane: Pick a datum. Use the factory-specified one if available. If not, the firewall is often used, but if you use the prop bolts, all the numbers will be positive.

Measure the horizontal datance to each wheel and the weight on that wheel. A tail-dragger's tailwheel should be lifted into level-flight attitude.

Multiple each weight and distance (gives answers in inch-pounds.

Add them up.

Divide by the empty weight. The answers is in inches and gives the distance from the datum to the COG.

Example:
Left main 30" x 540 pounds = 16200 in-lbs
Right main 30" x 528 pounds = 15840 in-lbs
Tail wheel 211" x 45 pounds = 9495 in-lbs

Total moment = 41535. Total weight = 1113

empty COG = 41535/1113 = 37.3 inches behind datum.

For COG while flying, take the COG from above (often provided by the manufacturer or rental club) and multiple by empty weight.

Add the "moment" (distance x weight) for fuel, cargo, and each passanger to the moment for the empty plane.

Divide by the gross weight. That gives you the COG as loaded. If the COG is not within specs, DON'T fly. Move something or leave something/someone behind.

Answer 3

Many commercial planes have weight sensing built into the landing gear so that they know the center of gravity before attempting takeoff

Answer 4

There is alot of calculating involved. I guess it is more or less the geometricall "midlle" of fuselage and wings. However engine placement can shift the centre of gravity as well as payload.

Answer 5

WEIGH THE AIRCRAFT BY PUTTING A SCALE OR WEIGHT DEVICE UNDER EACH WHEEL. THEN USE SIMPLE FORMULAS TO CALCULATE CG.
SUM OF FORCES IN VERTICAL DIRECTION = ZERO
GIVES TOTAL WEIGHT OF PLANE

SUM OF MOMENTS ABOUT SOME POINT = ZERO
GIVES RELATIVE LOCATION OF POINT THROUGH WHICH FORCE OF TOTAL WEIGHT HAS TO ACT IN ORDER TO BALANCE EQUATION