Weight, Balance, and Density Altitude at a Hot-and-High Strip
You are loading a light piston helicopter for a departure from a strip at 3,800 ft elevation. OAT is 30 °C, QNH 1013 hPa. Basic empty mass 740 kg, pilot 85 kg, one passenger 90 kg, baggage 25 kg, fuel 90 L (density 0.72 kg/L). Maximum takeoff mass is 1,090 kg. The helicopter has a longitudinal CG limit of 2.30–2.55 m aft of the datum. The pilot's station is at 2.20 m, the passenger at 2.60 m, baggage at 3.10 m, and fuel at 2.40 m. The performance chart shows that at this density altitude the maximum mass for a vertical takeoff is 1,010 kg.
1. Compute the takeoff mass and the CG position, and state whether the loading is acceptable.
Read the worked answer
Masses:
- Empty 740 kg
- Pilot 85 kg
- Passenger 90 kg
- Baggage 25 kg
- Fuel 90 × 0.72 = 64.8 kg
Total: 740 + 85 + 90 + 25 + 64.8 = 1,004.8 kg.
Moments about the datum:
- Empty: 740 × (assume empty CG at 2.40 m for this problem) = 1,776.0
- Pilot: 85 × 2.20 = 187.0
- Passenger: 90 × 2.60 = 234.0
- Baggage: 25 × 3.10 = 77.5
- Fuel: 64.8 × 2.40 = 155.5
Total moment: 2,430.0 kg·m.
CG: 2,430.0 / 1,004.8 = 2.42 m.
Conclusion: mass 1,004.8 kg is below the 1,090 kg structural limit, and CG 2.42 m is within 2.30–2.55 m. The loading is within limits - but it exceeds the 1,010 kg density-altitude performance limit? No: 1,004.8 kg is just below 1,010 kg, so it is marginal. Any extra item pushes it over. The loading is acceptable only with no further additions.
2. Given the performance limit of 1,010 kg, what would you change to get a safe margin, and why is the structural limit not the governing one?
Read the worked answer
The performance limit (1,010 kg) is more restrictive than the structural limit (1,090 kg) because high density altitude reduces both engine power and rotor thrust. At 3,800 ft and 30 °C the density altitude is well above 5,000 ft, so the helicopter cannot hover out of ground effect at the structural maximum.
To gain margin, reduce fuel or offload baggage. Reducing fuel by 20 L saves 14.4 kg and moves the CG slightly forward, which is acceptable. Alternatively, move baggage forward or leave it behind. Target a takeoff mass of about 960–980 kg to leave a margin for the actual conditions.
Recompute the CG after any change. Also plan a running takeoff if a vertical takeoff is marginal, and confirm the departure path is clear of obstacles, since the climb gradient will be reduced.