EPR vs N1
 

I wonder If anyone can tell me why RR persist with producing engines with EPR as the thrust setting parameter? The only reason I ask is that either on the 744 or 777 that are showing their age (some) the target EPR setting often means the n1 spools are considerably out giving that horrible prop out of phase beat effect which is becoming complained about by passengers and is fatiguing and stressful for crew when on Long haul ops. Taking the a/t out is not recommended on the 777 and I can now only liken it to a form chinese water torture !!

I did never like EPR; N1, I understand.

EPR or TPR, give me N1 any day!

Having seen the sometimes rather different N1 indications for equal EPRs on the RR Tay 620 and 650, I have to draw the conclusion that EPR tells the truth about engine power output while N1 allows a rough estimate only. During takeoff/climb, obviously the engines were adjusted to produce equal EPR, but during cruise and above a certain... IIRC Power lever angle, N1 was automatically synchronized to avoid cyclic beat.

Actually, if a little thread drift is allowed: With an engine using N1 as power indicator, is there some degradation allowed for when it comes to calculating performance tables?

The Tay engine (F70/100) required a TOGA takeoff every Monday morning to see whether the engine would deliver the required EPR, and therefore thrust, without busting any limit - be it N1, N2, EGT or whatever. If the engine hit any limit before showing the required EPR, a severe engine damage was assumed and a shutdown ensued. This was up to the flight crew; everything beyond this simple monitoring was a job for maintenance.

The difference in thrust during cruise with synchronized N1 was not really noticeable - generally, the rudder trim was set after takeoff with the engines still delivering equal EPR (unsynchronized N1) and did not need to be touched again for the remainder of the flight. May well be though that the tail-mounted engines with their short arm play a role in this; on types like the 744, the situation may be different. The additional drag caused by this is consequently likely negligible.

Solely relying on EPR, without the crosscheck of N1 can be a fatal trap. Several T/o accidents can be contributed to the inadvertent miss setting of power at T/O., by using EPR alone.

I like to know what RPM the fan is at.

You only need a blocked EPR probe for e.g., and there goes the EPR accuracy, right there.

Why not just use N1 as the main power parameter.

On a turbofan engine, the best measure and control of thrust is by fan speed (N1). This is because all of the air is pumped by the fan rotor, whereas the core rotor only handles a portion of the airflow. EPR or core speed is affected by various parameters and is not an optimum manipulated variable to set fan speed.

This doesn't work very well after bird strike damage or inlet damage. Lots of speed not so much flow.

My recommendation is to monitor aircraft speed vs pitch as well.

Both methods have their advantages, disadvantages and vulnerabilities (is that a word?) that can lead to the available thrust being lower than what might be expected from the indications.

One specific accident where N1 was of no use at all was the forced landing of OS111 (Fokker 70 OE-LFO) at Munich 2003. The Accident report is available in German language only, as far as I know.

This flight had the bypass duct clogged up by improperly attached ice impact trays that came loose in extreme icing conditions and got stuck in front of the fan stators. High N1 values together with next to no EPR ensued, as nearly no noticeable thrust was produced any longer, although the engines were still running. While this may have been a freak accident, it still shows that N1 only indicates how fast the shaft is spinning, not how much thrust is actually resulting.

So with two imperfect methods of measurement, I would tend to prefer to get as much information as possible on the engines state. Observing the two power plants show not only equal EPRs but also symmetric, plausible N1 and other values is a better indication of everything being in order than only seeing the symmetric, plausible N1 and other indications.

It all boils down to philosophy though...

At least on some engines I've come across, EPR is measured by multiple probes, not only in the core exhaust but also in the bypass duct or fan exhaust.

(For optimum propulsive efficiency the total pressure in the hot stream is close to that in the cold stream.)

For an N1-rated engine, the question of airfoil deterioration and its effect on thrust is often asked. The response I've heard from GE (CFMI) goes like this:

Gradual airfoil deterioration (e.g. sand ingestion) does cause a small loss of thrust (maybe 1 or 2%). However, during this period, there is also deterioration in the core compressor and turbine, resulting in higher EGT. The effect here is to increase thrust at a given N1.

So the NET result is less fan thrust, and more core thrust, the two approximately cancelling each other. :cool:

Tell that to the guys that plopped into the Potomac River (esp page 48).

Granted more modern engines would be less-vulnerable to a single blocked probe.

I flew 747 Classics with almost all the variations of the engines that were made.
With the Rollers and the Pratts I'd always calculate the takeoff & maximum EPR's and also on the card write in the corner the target N1. It wasn't company procedure but I figured it might save my rear end one day. Fortunately I never had to find out.

Definitely used to hear it as a pax on the MD-80 at the back. And the 727 had it as well. Could just barely feel it at the front of the -200 and would adjust N1 in cruise.

That's because EPR is king during takeoff, at least on my Autothrottle machine. For takeoff, EPR is set by the system, not the N1s. If one engine was a little "better" than the other, it's N1 would be less and so you'll get out-of-sync harmonics/beating. At climb and cruise power however, N1 is king; one engine is picked as the master and the other is slaved to it using N1 as the slaving parameter = no beating.

Your right that epr maintains steady symmetric thrust for t/off and N1 is king in the crz, but EPR is used to set all flight segments on the RR RB211 and trents, so it will symmetrically match epr right across the f or 2 engines resulting in differing n1's on each engine hence the beat in the crz. It is very subtle but when some sod points it out to you, the brain latches onto it and it drives me to despair eps when on the 777 you aren't allowed to take the a/t out, an incredibly poor design for modern jet engines for pax/crew comfort

Dunno about you lot but I'd prefer N1/N2 as my "engine power"
instruments.

N1 can be likened to RPM and N2 to MaP - i.e. what work the
engine is really doing. The CFMs of the 737-300 I was pretty
much at home with.

Nowadays EPR is something that the V2500 engine maker says
I have to set. Yes its accurate for TO but referencing that gage I
draw no feeling of what actual elbow grease the engine is doing.

I fly both N1 and EPR variants and prefer N1 for several reasons:

- There is no easy "ballpark" estimation of what EPR should be for a given phase of flight, as it changes markedly with temperature & altitude.
- On the old generation of EPR-controlled engines, 1.0 meant no net thrust; who knows what it means on the RR Trent, for example?
- Engines that synchronise on N1 are much quieter that those that use EPR as you don't get that horrible phasing vibration in the cruise. Probably better for the life of every other component in the aircraft, too.
- N1 is directly coupled to something physical whereas EPR is derived, so more liable to error/miscalculation.
- One less gauge required.

I suppose when everything is working, EPR can be more accurate but as virtually nothing else about the aircraft or conditions (weight, wind, surface state, humidity, etc.) is known to the same level of accuracy, it's rather meaningless. Gross errors are what get you...

Soon to be history?
 

Rumour I heard was that future RR engines will be controlled to N1 rather than EPR. Don't know if anyone can validate that as I am no longer persona grata in the controls fraternity!


PS two spoolers rather than three, too.

iEPR = Thrust
N1 = half the information needed to work out thrust.