View Full Version : Very basic EPR question.

24th Oct 2007, 09:14
Just a quicky, I cant find any ref for this and have done a search etc.

The numbers represented on the EPR gauge. If you have 1.85 EPR set, does that mean the jet pipe pressure/integrated pressure is 1.85 * the compressor inlet pressure? IE a ratio of 1:1.85 - seems a little low.
Never used EPR only N1.

Capt Pit Bull
24th Oct 2007, 09:37
seems a little low

Try quantifying it, you'll feel better. Back of an envelope approximation:

Say we have an engine with a fan diameter of 5 feet. Thats a radius of 30 inches.

pi R squared for the fan area gives about 2,800 square inches.

Atmospheric pressure at sea level 14.7 psi. EPR of 1.85 means the prssure differential is 14.7 *.85 = 12.5 psi.

Pressure x Area = Force = 12.5 * 2,800 = 35,000 lbs of thrust.

Pretty fair?

Swedish Steve
24th Oct 2007, 11:05
Remember that RR use IEPR. Integrated EPR when they measure the EPR in the jet pipe and the fan outlet and add them together in a little box. So EPR is the total pressure out the back, not just the jet pipe.

N1 Vibes
24th Oct 2007, 11:36
almost there. IEPR because on the RB211 engine the pressure from the 'back end' of the engine is integrated/combined with combustor pressure. So you've got integrated pressure (P.int) and fan pressure (P.fan), actually a mixture of LP turbine pressure (P.E) and combustor pressure (P4), though I'm not sure of the exact ratio of these pressures. The equation for IEPR on an RB211 is as folows:

Integrated Pressure (P.int)/Fan Pressure (P.fan) = IEPR

N1 Vibes

24th Oct 2007, 13:11
Thanks Capt P-B and others, appreciated.

Swedish Steve
24th Oct 2007, 13:53
N1 Vibes
OK this is from the RB211-524G MM 77-11-00
Signals from the LP compressor exhaust pressure (fan outlet)(P1.3) and gas generator exhaust pressure (P5) are supplied from rakes to the multiple connector where they are integrated and the resultant pressure (PINT) then moves to the FAFC.
LP compressor entrance air goes to the P2/T2 probe and flows to the FAFC.
The FAFC supplies EPR to the flightdeck.
The multiple connector is a tube with two different sized baffles in it, one baffle for P5 and one baffle for P1.3.
Combustor pressure is not involved in EPR.

24th Oct 2007, 14:55
Ok another one for you: Why does EPR need to be set by 80 kts?
I would think, because of the Ram rise effect. Above 80 kts the EPR reading will not be as accurate as below???:confused:

FE Hoppy
24th Oct 2007, 16:23
Yes because of RAM effect the inlet pressure increases and therefore the TO EPR can no longer be used as an indication of take off thrust. If you were to look closely you will see the EPR roll back a tad. The GA EPR takes this RAM increase into effect.

24th Oct 2007, 19:50
Capt PB. Your "back of envelope" formula work out pretty well for a P&W JT9 and a RR RB 211..................but, in calculating the area, we have disregarded the fairly high percentage of intake area blanked by the spinner.
which, one would have thought, should be subtracted ?
Why do the sums work out ?