Thrust to power

[kmpillai]

Hi,
An IC engine is running with a propeller. Is there any way to estimate the power output of the engine from the thrust and rpm.

KMP

 

[GregLocock]

Not accurately. You could reverse engineer the prop design to give you a table of torque to rotate vs thrust, and then do it.

The usual way is to measure the torque produced by the engine, which can be accurate to within about 1%.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[rb1957]

i agree that it's difficult to Know the thurst developed yb a prop.

if you did, then P = Thrust*V (V= airplane velocity).
alternatively, P = Torque*rpm.

alternatively, you Could relate the airplane velocity to the propeller advance ratio (i think that's the term, dependent on the pitch setting of the prop, how far does the propeller (and the plane) advance (along the line of flight) as it rotates); so that V = rpm*k (k = ft of forward motion per propeller revolution) and then fudge the units back to ft/sec !

 

[DavidLeyman]

Oh, dear. This comes up often.
The old 'D' Licence guys used to do measurements of airflow corrected to ISA SL. This was on engines post major servicing that were bolted to the back of a lorry (truck) and pointed into the wind - for which a graph was available. They would then do a judgement (note "judgement") of the thrust obtained from the propeller. The power output is determined by the rpm of the propeller at the time and then corrected to ISA SL so that it can be determined if that engine has passed the tests done on it.
BUT!
The essential thing here is that the engine achieves the correct rpm for the ambient conditions. All this time we are considering only fixed pitch propellers.
VPP are done the same way but set to maximum rpm (Note: NOT fine pitch).
On a test bed the thrust of a jet engine can be converted into power equivalent by a factor of 2.6. If you measure the thrust from a turbopropeller engine (about 10% of total output) and convert it into HP then add it to the HP at the propeller you will get ESHP = Equivalent Shaft Horse Power. OR: 40,000 lb of thrust is about 15,000 HP at the fan.
(Notice 'lb' and NOT 'lbs'. This comes from the Latin 'libra' the plural is librae' so there is no 's' at the end. 1 lb or 3 lb - no 's'!)