View Full Version : EPR vs Thrust produced

7th May 2008, 11:15
A question....

The 732 I fly has either JT8 - 15 or - 17's. The -17 produces more thrust.

IF you have say 2 EPR on a -15, is it the same amount of thrust as 2 EPR on a -17 , or is it less?? As far as I know it will be the same as it is purely a pressure measurement, the -17 can just produce more pessure differential.

Thanks a mil

7th May 2008, 13:31
To keep it simple, a setting of 2 on the EPR gauge is not the same for the two engines. Just look at the trim tables for the two engines at the same OAT. The readings( operating parameters ) are different and that is because the output is different for each engine. Higer on the -17 as would be expected.

7th May 2008, 13:59
As far as I know it will be the same as it is purely a pressure measurement

I think that you also have to consider flow area in order to accomodate mass flow rates differences for the same pressure.

7th May 2008, 15:08
The -15 and -17 are the same dimensions so im not sure the area would come into it in this case,

Thanks for the replies

10th May 2008, 08:46
Now I'm not a a boffin with these things as I do not fly gas turbines. But the EPR of two is always the same no matter what. The reason is that pressure measured at the PT2 probe would be 2 x PT1 probe so it would be the same for all engines. The thrust would be derived from the acceleration induced into the intake air stream which is accelerated by the combustion chamber. So the reason the -17 engine has more thrust is purely due to it working with more fuel burned in the combustion chamber.

10th May 2008, 13:55
Gas turbine aero will tell you this:

If in fact EPR = 2.0 on both engines, and the -17 produces more thrust, then the exhaust nozzle area of the -17 must be greater than the -15 (i.e. more mass airflow through the nozzle)

And to pump the greater mass airflow, more fuel will be burned.

On the other hand, if the two nozzles are identical, then the EPR's are not exactly the same (even though the calibration of the gages is "trimmed" so they look the same).

N1 Vibes
11th May 2008, 00:55
maybe it's easier to illustrate by comparing it to another engine type - different nozzles and all.

- RB211 524 - 1.58ish EPR - 50-60k lbs thrust
- Trent 500 - 1.4ish EPR - 53k lbs thrust
- JT8 - 2ish EPR - 20k ish lbs thrust


- Ferrari - 100mph = 2,000rpm/400bhp
- Mini Cooper - 100mph = 3,500rpm/150bhp


the rim
11th May 2008, 13:15
ni vibes you have hit the nail on the head mate its not the size of the dog in the fight but the size of the fight in the dog