Power to weight ratio
The power to weight ratio is an indication of the performance potential of a helicopter.
As helicopters get bigger their power systems can generate more power, but unless the power to weight ratio improves the helicopter's performance (ability to maneuver and climb) will remain the same.
While it's commonly called a ratio, it's not really as the value has units and is not just a pure number.
- A typical 450-sized electric helicopter may weigh 800 grams (0.8Kg) and have 400 Watts of power available, giving a power to weight ratio of:
- 400 / 0.8 = 500W per kg
- A T-Rex 600e weighing in at 3kg may have a motor capable of generating in excess of 2000W, giving a power to weight ratio of:
- 2000 / 3 = 667W/kg
- and thus far better performance, assuming that power can be well used by the rotor system.
- A Compass Knight 3D powered by an OS 50 SXH produces 1.8hp (1.3kW) and weighs in at 4kg all-up, giving a power to weight ratio of:
- 1300 / 4 = 325W/kg
- For comparison, the AH64A Apache weighs in at 8000kg and has 2 turboshaft engines rated at up to 1490kW each, giving a power to weight ratio of:
- (2 * 1490000) / 8000 = 372.5W/kg
- In practice, the transmission is probably not rated to cope with even this amount of power, at least not for any length of time.
- Another comparison, the Robinson R22 weighs in at 417kg loaded. powered by a Lycoming piston engine derated to 93kW (more in more recent models), giving a power to weight ratio of:
- 93000 / 417 = 223W/kg
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