Pounds thrust to horsepower

[barit1]

Smokey, there is a place to measure shaft HP in a high-bypass fanjet that makes some sort of sense, but you'd have to be privy to some internal figures to work the sums.

Recall that one HP is 33000 ft-lb of work per minute. In rotational terms, think of a winch with a one-foot radius. One revolution of the winch pulls the rope 6.28 feet, and if the load is 33000 lb., the torque is 33000 lb-feet.

So (example 1) - if the shaft turns 1.00 rpm, with a torque of 33000 lb-ft, the amout of work done in a minute is 33000 x 6.28 ft-lbs = 6.28 HP.

Scaling back this particular problem to example 2, If the shaft turns 1.00 rpm, with a load of 33000/6.28 lb on the rope, the torque is 5252 lb-ft, and the work rate is 1.00 hp.

ERGO: Shaft HP = RPM x Torque (lb-ft) / 5252 (stick that in your personal QRH!)

SO: if you know the torque on the fan shaft, and you know the fan rpm (derived from the % gage), you CAN calculate the horsepower driving the fan. And at static conditions, it's ROUGHLY the same number as the static thrust (lbs.) of the engine.