FAA Private Pilot Helicopter (PRH)Helicopter Aerodynamics and SystemsHard
A helicopter's approved lateral CG limit is ±2.0 inches from the centerline. The empty helicopter weighs 1,500 lb with a lateral arm of 0 inches. A 170-lb pilot occupies the left seat at a lateral arm of -12 inches, and 300 lb of fuel is carried on the right side at a lateral arm of +10 inches. What is the resulting lateral CG, and is the loading within limits?
- ACG is 0.49 inches left of centerline; within limits
- BCG is 2.44 inches right of centerline; out of limits
- CCG is 0.49 inches right of centerline; within limits
- DCG is 1.55 inches right of centerline; within limits
Show answer & explanationAnswer & explanation
Correct answer: C. CG is 0.49 inches right of centerline; within limits
Total weight = 1500+170+300 = 1970 lb. Moment = (1500×0)+(170×-12)+(300×10) = 0 - 2040 + 3000 = 960 lb-in. CG arm = 960/1970 = 0.487 in, or about 0.49 inches right of centerline, well within the ±2.0-inch limit.
Why the other options are wrong
- A. The sign is incorrect; the net moment is positive (right side), not left.
- B. This exceeds the limit but does not match the correct total moment calculation.
- D. This value results from an arithmetic error in the moment or weight calculation.
Lateral CG Calculation
Lateral center of gravity is found by dividing total moment (sum of weight × lateral arm for each item) by total weight, then compared to the aircraft's lateral CG limits.
- Lateral arms are measured left (-) and right (+) of the helicopter centerline
- Moment = weight × arm for each item, summed algebraically
- CG = total moment ÷ total weight
- Must fall within the manufacturer's approved lateral CG envelope
Memory trick: Add up left and right moments like a seesaw balance.