Engine Pressure Ratio

[Boddster]

Hello All

I would like to know what happens to the EPR of of an engine when the aircraft goes into a dive, when the throttle setting remains contant at say 75% N1 There are no autopilots/throttle engaged and there is no FADEC on

Any help would be appreciated Regards

Chris

[aerostude]

EPR is equal to the ratio of stagnation pressure at the turbine nozzle to the stagnation pressure at the compressor inlet.
Considering the compressor inlet. The dynamic pressure will go up as the aircraft accelerates but the static pressure will go down so the stagnation pressure remains constant here. However, the mass flow of air going through the engine will surely increase?? which will produce more thrust and therefore the pressures in the turbine must surely change??! So thinking outloud, I would expect the EPR to increase.

Looking forward to the feedback.

[Boddster]

That was my way of looking at it,

In a dive your airspeed will go up thus the RAM air pressure will increase so the pressure at P2 (compressor inlet) will increase causing a rise in the EPR, but.......

when the velocity of the air increases, as in a dive, the pressure of the air would drop at P1 thus causing a drop in EPR

[Four Good Engines]

Boddster,

If you are referring to Bernolli when you say that 'the pressure drops as the speed increases', remember that the air is stationary and the aircraft is travelling through it. As mentioned above the ram effect coupled with the mass flow which also increses, will cause the EPR to increase.

[Iz]

Boddster, I think I replied to your question in the 767PIC forum :)

But you and FGE are both wrong in thinking that EPR will increase in a dive due to the ram effect. 2.00 EPR means that the pressure at the AFT of the engine is twice as high as in the engine inlet. So if the pressure in the engine inlet increases, EPR will drop. Whether or not it will drop below 1.00 (ie. the pressure in the inlet is higher than at the exhaust end), I'm not sure, it's possible.

Iz

[Cough]

I have seen EPR fall below zero in high speed descents before. What we are describing above is similar to the EPR falling on take off due to ram effect.

C........ough {ed for sp)

[This message has been edited by Cough (edited 28 April 2001).]

[CCA]

On a roller powered -400 on an idle descent you can expect to see 0.95 EPR and the faster you go the more it drops.

[Ignition Override]

If this understanding were needed for a rating or company test, is it considered by "the authorities" more important to know the right answer, or to be able to keep the plane's speed less than Vmo/Mmo, while having altitude awareness? If so, does this knowledge prepare the pilot to be better prepared to control the airplane?

Just curious as to whether certain knowledge helps us in the cockpit, when one is not a factory or test pilot.

[Iz]

IO, is that the point? I was recently baffled by the lack of info found in my new Boeing manuals. Not saying that that's a bad thing, not at all. But it doesn't hurt if you're curious and want to know more about something does it?
I personally never really appreciated whenever someone answered my question with the term "PFM" ;)

[fly4fud]

Remembering the A310 (what a good aircraft!), the EPR indeed went below 1.0 during descent

------------------
... cut my wings and I'll die ...

[Iz]

http://airliners.net/open.file?id=86431

As you'll see on this picture, this B744 has 0.75 EPR during the descent.

[FE Hoppy]

Don´t be confused by the decimal point.
On my a/c (l1011) we have (I)EPR guages.
I= Intergrated because we have LP air mixing with HP air at the engine exhaust.(Hp air is Ip air further compessed before anyone asks).
T/O power should be set before 80 kts after which the power will roll back due to intake ram px.
eg max t/o epr(isa day) .643= 1.643*intake pressure.
inflight t/o epr(go around power) .631 =1.631*intake pressure.
the 2 power settings are the same ie throttle setting and fuel flow. the speed is the only thing that changed.
if you were to set the former above 80 kts you would be overboosting the engine.
In the descent above 350 the min epr the fms will set is .122epr reduceing to .087epr below around 310 then .0 epr below around 280.
this is for bleed air management and sub idle spool speed at high level.
when the throttles are at idle in the descent the epr indicated varies with tas but is usualy between .01 and .04

When the thrust management system indicates an epr value it does not show a decimal point. ie 122, 087.
but one thing for sure, te pressure at the back is always more than the pressure at the front. if not then the fuel guages would be going up not down,(think about it)

nb this is my longest post in three years so I hope I´right.

[Iz]

In the 757, the least EPR I've seen during an idle descent is 0.97. The engines are RR RB211-535E4s. Seems to me like they probably have some sort of ram-effect compensation since I've never seen what you described in the L1011. If that's the case though, it would appear that pressure in front of the fan is higher than at the end. I don't see why it isn't possible.

[FE Hoppy]

I think you might find that the 0.97 you see is 1:1.097 ratio. The effect of ram on the compressor is simply to offload the turbines. But there is a minimum speed that the engine can accelerate from. This is set and controled by the FFR and is a higher speed than when windmilling.
This being the case, any fuel being used by the engine will impart power to the turbine thus driving the compressor and increasing the pressure of the air entering and so exiting the engine.
If the presure at the front were to be higher that the rear the fan would act as a turbine and the turbine as a fan so what happens to the energy imparted on the air due to combustion?

[gas path]

Hoppy
The 535e4 epr is a ratio of Pf and P1 only. It isn't an integrated EPR and there is no P8 rake.
For the Tristar 22b 524b4 the IEPR is(PF+P8)/P1
p.s. Hows your thrust reversers?

[FE Hoppy]

Much better now,
The jet went to Marshalls for an A and they did what TAP couldn't. FIXED IT.