liamksa
Posts: 301
Joined: Mon Oct 29, 2001 1:13 pm

Typical Engine RPMs - Idle / Starting

Mon Sep 22, 2003 4:56 pm

Gday folks

I've got a couple of questions re: engine starting and RPM. I have a source which says the following:

- Addition of fuel - 15% of max. rpm
- Self-sustaining - 35% of max. rpm
-Idle - 55% of max. rpm


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Photo © Bevin Shively



1. I always thought 55% was a high N1 for idle and the photo above shows 68% N2 and 26.3% N1. So do the numbers I have listed above the photo actually refer to N2 speeds?


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Photo © Enrique Puche Aznar



2. The above photo shows 91.2% N1 and 95.7% N2 so does the N1 'catch up' to N2 as engine RPM increases?

3. And lastly, what are some typical N1, N2 or EPR values @ idle of common engines eg: JT8D, JT9D, CF6, RB-211, CFM56, GE90 etc...

Any info is appreciated, thanks

Rob.
 
CX Flyboy
Posts: 6030
Joined: Sun Dec 26, 1999 6:10 pm

RE: Typical Engine RPMs - Idle / Starting

Mon Sep 22, 2003 7:40 pm

Off the top of my head, a quick check that I do, after start is that on the RR 777s, the N1 is in the high teens, N2 in the 20s, and EGT is mid 300 degrees. Don't have any figures here with me though.
 
darkblue
Posts: 227
Joined: Thu Sep 11, 2003 10:27 pm

RE: Typical Engine RPMs - Idle / Starting

Mon Sep 22, 2003 11:48 pm

1.) First of all, many engines are controlled at idle using N2 instead of N1 or EPR (at least the ones I'm familiar with). So the numbers you have listed probably are the N2 speeds.

2.) At idle, the engine is not really interested in producing thrust, but it does need to operate at a high enough power to keep the burners lit and to satisfy the aircraft bleed/HPx requirements. At this condition, the HPT extracts most of the energy available in the flow leaving very little work left for the LPT. This is why N2 is so much higher than N1 at idle. At high power, EGT is much higher and therefore N1 will in fact "catch up" to N2.

3.) CF6 engines generally operate at idle with N1 around 23% and N2 around 62%. I'm not familiar enough with the other engines to quote any speeds.

DB
 
Guest

RE: Typical Engine RPMs - Idle / Starting

Tue Sep 23, 2003 4:38 am

JT9D idles at about 58% N2... and 30% N1.
CF6 idles about 60-62% N2... cannot remember N1, prob close to 30%...
xxx
In the 747 we have two different "idle" modes: flight idle and ground idle, we get a higher flight "idle" value (for faster spool up) whenever the flaps are selected in the landing range positions (25 or 30 flaps).
xxx
Happy contrails  Smile
(s) Skipper
 
cdfmxtech
Posts: 1319
Joined: Wed Jul 05, 2000 11:37 am

RE: Typical Engine RPMs - Idle / Starting

Tue Sep 23, 2003 6:37 am

- DELETED -

The message you were about to post is too short and probably not of any higher value to the topic at hand. You should think long and hard before posting a message in this forum and make it detailed and a valuable addition to the topic discussed.

[Edited 2003-09-22 23:38:26]
 
cdfmxtech
Posts: 1319
Joined: Wed Jul 05, 2000 11:37 am

RE: Typical Engine RPMs - Idle / Starting

Tue Sep 23, 2003 6:37 am

With the CFM56-3s used on the B737 Classic Series, an easy way to remember the Main engine parameters (N1, EGT, N2, EGT) is 2,4,6,8 in that order. It is in that order because that is how the instruments are laid out from top to bottom.
N1 = Approx 25%
EGT = Approx 440 Deg
N2 = Approx 60%
FF = Approx 780 pph

These numbers are off the had of course

On the CFM56-7s used on the B737 Next gens, 2,4,6,6. The eng has a smaller fuel flow at idle than the classic engines.
 
AAR90
Posts: 3140
Joined: Fri Jan 21, 2000 11:51 am

RE: Typical Engine RPMs - Idle / Starting

Tue Sep 23, 2003 10:04 am

On the CFM56-7s used on the B737 Next gens, 2,4,6,6. The eng has a smaller fuel flow at idle than the classic engines.

No kidding! They've really got it sipping fumes.

N1 = 20%
EGT = ~410 deg
N2 = 59%
FF = 600 pph
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