View Full Version : PW Engines using EPR or N1


HEALY
9th Sep 2007, 13:58
Hi

Just need to clarify a couple of points about engines and the associated primary indications for power setting.

RB211 - EPR
All TRENT series - EPR

GE - N1
Pratt & Whittney - N1?



Willit Run
9th Sep 2007, 15:32
Pratt and Whitney is usually EPR

Dutch74
10th Sep 2007, 16:15
Having flown all three, definately prefer GE's idea of getting rid of EPR. Absolutely useless guage. Good for setting power but if you want to know how your engine is doing, look at N1, N2, EGT & FF. Never EPR.

Crossbleed
12th Sep 2007, 03:51
Isn't EPR the only true indicator of thrust being produced?
As told to me by some checkie or other...
Having said that, I spend more time looking an N1.

lomapaseo
12th Sep 2007, 21:51
Having flown all three, definately prefer GE's idea of getting rid of EPR. Absolutely useless guage. Good for setting power but if you want to know how your engine is doing, look at N1, N2, EGT & FF. Never EPR.

Well if you would add in a pressure measurement or two (EPR is an example) then I absoultely agree with you. But how many pilots can really discern how much thrust they are getting by interpreting N1, N2 and EGT. Yes, I agree that there are some charts for a normaly functioning undamaged engine, but I doubt that you can tell anything about thrust for a FOD fan in a pilots seat by reading the gages you cited. I would recommend looking at airspeed and deducing thrust loss when you've got a damaged fan (N1 & N2 and EGT out of whack) and no EPR reading

barit1
13th Sep 2007, 03:07
If you're going to use EPR to set thrust, I much prefer R-R Integrated EPR (core + fan). P&W uses core EPR only, and its long-term accuracy (EPR vs actual thrust) suffers as the engine wears in (seals, shrouds, etc.)

lomapaseo
13th Sep 2007, 12:27
If you're going to use EPR to set thrust, I much prefer R-R Integrated EPR (core + fan). P&W uses core EPR only, and its long-term accuracy (EPR vs actual thrust) suffers as the engine wears in (seals, shrouds, etc.)

agree

I've seen mixed flow EPR readings (long duct fan air mixing with the core in the tailpipe) getting badly screwed up with some fan FOD damage leading to a control system overboosting the core. but life is just a bunch of trades :hmm:

Shot Nancy
13th Sep 2007, 12:47
Having flown A320s with N1 (CFMs) and EPR (V2500s) and 744s with N1 (CF6s) and EPR (RB211s), I reckon I can comment.
I donít care who makes the donk or how many spools it has but EPR is a waste of time, just ask the Air Florida guys.
With N1, move the thrust levers and you get the N1 you command. In an EPR bird N1/N2 etc will vary to provide the EPR commanded.
A manual thrust approach in a N1 powered aircraft is a smooth easy affair, in an EPR bird it is a bit wishy washy.
Other than that push forward on thrust levers and trees/waves go past fast, pull back and they go past slower.

barit1
14th Sep 2007, 14:38
IIRC, the MEL for the DC-10-40 (JT9D) permitted dispatch with one EPR inop, and called for matching TO N1 on the affected engine with the others, with appropriate cross-check of FF. (There are more details I'm sure...)

The 747 probably has something similar.

N1 Vibes
15th Sep 2007, 03:53
When I worked in the shop test cells, where you get the true static thrust reading, working RB211's, with the FADEC control. You would see at least a 1% diff, if not more in N1 speed, between engines at the same take-off and max-continuous thrust. This demostrated that each fan was individual in it's performance, and speed required to attain the true thrust. Yes, EPR is also a 'moveable feast', but RR believe it less of a moveable feast than N1.

If you were running to an N1 and the efficiency of the fan meant you were 1% or more lower on actual thrust, then it may only be 600lbs.

BUT, remember that all these engines have a 'trimmer' system, be it EPR/N1, bringing them closer than 600lbs to the true thrust.

Brgd's

N1V

Junkflyer
15th Sep 2007, 18:49
The 74 classic may dispatch with an epr inop using n1 a you say, also we must use max power for take off under that mel.

virgo
15th Sep 2007, 21:23
N1 is only confirmation of power if there are no compressor anti-surge bleed valves fitted.The RB211 and JT9 has them and the GE hasn't.
On the RB211 and P&W JT9 you could have a bleed valve(s) stuck open and although you're achieving the appropriate RPMs the engine would not be producing anything like take-off power - a large percentage of the mass airflow is being dumped overboard.
The only TRUE measure of thrust produced is a measure of the pressure rise measured from the intake and fan/turbine exhaust.
Because of this, if you're dispatching with an EPR gauge u/s, you try to reduce the potential problem by not using graduated power and sometimes imposing an RTOW penalty

barit1
15th Sep 2007, 23:37
virgo, it's a bit more complicated.

GE & CFM engines DO have bleeds (called bypass doors) - and they should open only at low power to dump excess booster (or LP Compressor if you will) air to prevent a booster stall.

But if the doors leak, their discharge goes into the fan duct, ADDING to fan nozzle flow. Thus at a given N1, thrust should be HIGHER than normal.

The catch is that with less core airflow, the LP turbine probably won't be able to spin the fan up to speed without hitting some other control limit.

411A
16th Sep 2007, 00:21
Hmmm, many different opinions.

One wonders..why not look in the respectice aircraft AFM and see exactly what the manufacturer says?

Instead of pontificating, and many times getting it wrong...:}

And yes, RB.211's, EPR is the primary thrust setting instrument.
If you doubt this, why not call up a Roller tech rep and ask him...the answer you recieve might be enlightening, IE: something might indeed be learned.

Perish the thought....:ugh::ugh:

FCOC
16th Sep 2007, 09:18
removed due to a better explanation being given in the next post

N1 Vibes
16th Sep 2007, 09:38
EPR Command - is the EPR the autothrottle of the a/c requires to maintain an a/c speed

EPR Trimmer - Function to trim the engine so that at a given EPR, at a given OAT at a given Baro that all engines produce the same thrust. It is NOT there to restrict the engine to below test-cell thrust levels.

N1 - On the RB/Trent is used as the alternate, not the primary thrust setting parameter

EPR - is a ratio of the air pressure measured behind the fan OGV's at the front of the engine and in the LP turbine exit at the rear, therefore it has a pretty close relation to thrust. If you cut 2" of the end of all your fan blades or the fan attrition lining was damaged/worn, but the fan was turning at 100% N1 speed, would you still be getting the required thrust? No. If the fan was dirty and therefore less efficient, then it's compression would be less efficient, therefore pressure will be lower, which EPR would show. Since the engine would have to work harder overall to make the same pressure, this would be seen on ECM trending by the engine speeds gradually trending up in relation to EPR.

Hopefully this illustrates. :)

lomapaseo
16th Sep 2007, 13:47
N!Vibes

EPR Command - is the EPR the autothrottle of the a/c requires to maintain an a/c speed

EPR Trimmer - Function to trim the engine so that at a given EPR, at a given OAT at a given Baro that all engines produce the same thrust. It is NOT there to restrict the engine to below test-cell thrust levels.

N1 - On the RB/Trent is used as the alternate, not the primary thrust setting parameter

EPR - is a ratio of the air pressure measured behind the fan OGV's at the front of the engine and in the LP turbine exit at the rear, therefore it has a pretty close relation to thrust. If you cut 2" of the end of all your fan blades or the fan attrition lining was damaged/worn, but the fan was turning at 100% N1 speed, would you still be getting the required thrust? No. If the fan was dirty and therefore less efficient, then it's compression would be less efficient, therefore pressure will be lower, which EPR would show. Since the engine would have to work harder overall to make the same pressure, this would be seen on ECM trending by the engine speeds gradually trending up in relation to EPR.

Hopefully this illustrates.

Good explanation for the RB, but how about PW the subject title of this thread?

barit1
16th Sep 2007, 13:55
I won't disagree with N1 Vibes about IEPR, but there is a bit more to the picture of N1 as a thrust measurement.

In typical airline service, the fan suffers some degradation, from simple dirt and insect accumulation to outright physical airfoil damage to tip clearance erosion. All of these tend to reduce fan airflow (and thus fan thrust) at a given N1.

But the above is accompanied by similar deterioration in the core engine, and this shows up as increasing EGT over a period of time. This means the core nozzle velocity increases, and it is producing MORE thrust at a given N1.

The net effect is the two tend to cancel each other, and the total thrust/N1 relationship changes very little. The effect on takeoff performance will be within the normal day-to-day statistical variation.

So - is the supposed advantage of IEPR as a thrust measurement worth the added weight/expense/maintenance burden of an EPR system? The certification rules require a tachometer on each independent rotor, so removing the N1 gage on an EPR machine is not an option. Why not use it for thrust setting?

The proof is left to the student. :)

virgo
16th Sep 2007, 19:42
barit............see earlier answer, reference malfunctioning bleed valves