More RR EPR confusion

[jtr]

Flew with a guy (on a check dammit) who asked me why, in an idle descent (on a A330 with rollers) the EPR reads less than 1.0.
I said ... `cause in effect, work is being done on the engine, as opposed to the engine doing work. i.e. the air is turning the engine iso vv, hence P5 (at the front) is experiencing more P then P2 (at the back), and EPR = P2/P5

He said... `Actually, it`s because outlet pressure (P2) is measured only in the hotstream duct (which I agree is true), and is compared to the entire coldstream (P5)

So I said... `Yes sir, of course sir, may I have another please sir` And lowered head, placed tail between legs etc.

As I see it, both answers are correct, but is one more correct than others?

Disclaimer> Any incorrect Pn labelling (where n in an integer b/t 1 and 6) please forgive, you know what I mean.



[This message has been edited by jtr (edited 20 September 2000).]

[CCA]

Rolls use Integrated Engine Pressure Ratio (IEPR) which uses Intake pressure (P2.0/T2.0) vs Fan exhaust (P1.3) AND Hot stream exhaust (P5.0).
(P1.3 + P5.0)/P2.0 or P INT/P2.0
The P2 probe is at the top of the inlet.
The P1.3 probes are on the A frames, Upper left, lower right and the Interservices fairing.
The P5s are on the turbine bearing support struts.

Have a look here for photos http://www.pprune.org/ubb/NonCGI/Forum3/HTML/001256.html

This is all in relation to the RB211-524G/H(T) and I'd assume the Trent 700 uses the same system.

I think your correct.

[This message has been edited by CCA (edited 21 September 2000).]

[jtr]

Thanks for the relpy. I may may have thrown a curve ball with my explanation, (or should I say my incorrect labelling) of the various P sources. What I am trying to find out is if his explanation of EPR<1.0, or mine, or both, are/is correct.

As a complete aside, I am not aware of a P1.3 coming into play on the Trent, but as you are aware they are pretty much the same thing design wise. Will have to ask a Gingerbeer next time I get the chance.

And yes, if asked, what measures compressor inlet pressure?, I do say... that probe thing that hangs down at the top of the inlet. As you can see I always get my P`s screwed up.

[This message has been edited by jtr (edited 20 September 2000).]

[gas path]

RR Trent
EPR is P20 in front of fan (probe P20/T20 visible at 12 o'clock in intake) and P50 LP turbine bearing suport vanes (5 off rakes)
EPR=P50/P20
So at idle in flt. ram pressure can be greater than the exhaust pressure so indicated EPR will be less than one.
Stationary at idle exhaust pressure will be greater than intake pressure so EPR will always be greater than one usually around 1.02.


[This message has been edited by gas path (edited 20 September 2000).]

[This message has been edited by gas path (edited 20 September 2000).]

[casual observer]

CCA:

I think the IEPR that you're referring to is simply the ratio of the exhaust pressure (P50, as indicated by GAS PATH) and fan inlet pressure. The RB.211-524G/H and G/H-T has, what R-R called, integrated exhaust nozzle. (CFMI called it long-duct mixed flow.) That's why the EPR is referred as IEPR. Thus, on the Trent 700, I figure the IEPR is used. But on the Trent 500 and 800, I think EPR is used. BTW, I believe the PW6000 will use N1 instead of EPR.

[773]

What possible benefit can an engine gain by having an "Integrated Exhaust Nozzle". Can't be that great or all the manufacturers would have it.

[jtr]

ditto for EPR altogther.

Only flown rollers and CFM`s. How many major manufacturers use it? None `cept RR?

[CCA]

Casual Obs, I'm afraid your incorrect it's true the G/H has an intergrated nozzle but Rolls has been using Intergrated EPR on early model engines such as B,C, and D4 plus the 22B's on the Tri Star which don't have intergrated nozzles.
P INT is P1.3(cold) + P5.0(hot) and P1.3 are located on the A frames and interservices fairing just aft of the fan (see pictures).

[casual observer]

773 and jtr:

High bypass engines for large commericial aircraft applications that have integrated exhaust nozzle include: the RB.211-524G/H (747/767), Trent 700 (A330), V2500 (A320 family/MD90), BR715 (717), CFM56-5C (A340), and the still-in-development PW6000 (A318). As you can see, four of the six have R-R heritage. The advantages of this configuration are improved SFC, thrust, and reduced noise. The disadvantages are increased weight and drag which usually wipe out the gain in improved SFC. The three-spool design of R-R's RB.211 and Trent is more compact, hence, these engines in general weigh less. Thus, it's easier for R-R to afford having the heavier integrated exhaust nozzle deisgn. (But the CFM56-5C, BR715, and PW6000 are two-spool engines). That's a major reason why few American-made engines have long-duct mixed-flow configuration.