EPR basics

[Jamie320200]

I've never used an EPR and would like to know the following:

Is it front over rear or vice versa?

If it is front over rear this would explain (I think) why the ram effect on take off increases EPR (as I'm told it does). But how can it be that you set 1.2 or 1.3, I would have thought that the pressure at the front was less than the rear and hence the ratio would be less than 1.0 !?

Please help.

[Checkboard]

EPR is defined as: (Low Pressure Turbine Exhaust pressure) ÷ (Engine Inlet Pressure)

[flipside]

And the ram rise???

[Ignition Override]

Checkboard: I must make an embarassing admission. For all the numerous... years I've flown fanjets, nobody has been able to explain to me just what sort of engine stress is caused by exceeding EPR limits, unless it is some connection between N1 and N2 limits, maybe a combination?

Although this is purely academic and is not required knowledge at this airline, or in Navy Reserve squadrons, from a realistic cockpit perspective (knowing that we do our best to constantly adjust the EPR for the limits, but don't pull it back at our company during takeoff even if EGT is in the yellow arc), it would be interesting to finally understand it.

[black diamond]

anyone got a reference for info about EPR?

[Jamie320200]

Well I am still confused.

However, after being told by many that EPR rises in the later stages of the take off roll and subsequently blowing this question in a job interview I just found a pilot who seemed quite certain that the opposite is true!

Apparently it does drop as the basic ram rise theory suggests.

Dam, why isn't there a good reference for this!

If anyone knows one please reply.

Thanks.

[bsevenfour]

From my observations on the flight deck operating fanjet airplanes EPR falls during the takeoff as you accelerate.

Also to back this up I quote from the operating manual of a Boeing 747-200. In it there is a section that details why take off thrust must be set by 80 kts.

"Above 80 kts EPR will decrease due to the ram air effect. Do not compensate for this change."

As the aircraft accelerates the ram air effect increases the inlet air pressure and since EPR is exhaust pressure divided by inlet pressure the EPR reduces.

[bsevenfour]

BIK

As I said in my post my observations have been based on the different Boeing types I have flown over the years.

As far as your question on fuel control units yes I guess it's entirely plausible that a fuel control unit could be produced to counteract this reduction. While I could see the possible merits in a control unit that maintained a constant EPR why would you design a fuel control unit that actually increases the EPR on a take off roll ? Wouldn't this system increase the stresses on an engine as your speed increased ? (not a very good combination).

It would make more sense to me to allow for the reduction in EPR in your take off performance calculations rather than
A> Go to the bother of designing a fuel control unit that is going to increase EPR as the take off roll continues and
B> Design a system that will increase the stresses on the engine during the more critical phase (high speed) of an already critical procedure (take off).

[lomapaseo]

From Ignition Override

>Checkboard: I must make an embarassing admission. For all the numerous... years I've flown fanjets, nobody has been able to explain to me just what sort of engine stress is caused by exceeding EPR limits, unless it is some connection between N1 and N2 limits, maybe a combination?

Although this is purely academic and is not required knowledge at this airline, or in Navy Reserve squadrons, from a realistic cockpit perspective (knowing that we do our best to constantly adjust the EPR for the limits, but don't pull it back at our company during takeoff even if EGT is in the yellow arc), it would be interesting to finally understand it.<

Generally speaking its easier to understand the redline limits on N1, N2 and EGT regarding engine stress. However there is also a limit on burner pressure which the fuel control tracks quite nicely.

The only problem that I can ever recall directly attributable to over EPR was the SAS Double Ice ingestion and the AA MD80 double engine woodchipper at Hartford where the extra pressure in the cmpressor coupled with other damage managed to light off the titanium blades in the compressor.

In both cases the EPR signals were confused by significant flow loss out of the damaged fans and an overboost throttle command at the same time.

[Jamie320200]

On ya cobbers, she'll be right now.

[EI Pilot]

:confused: Don't know anything about those fancy engines with fans at the front, but FWIW, I was always taught that our aft fan CF700s would burn themselves up before you could blow them up with too much EPR. The instructors told us that the EPR limit was just to keep the Vmc as published should one of the noisemakers quit on T/O or G/A.