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Trent_800
Topic Author
Posts: 136
Joined: Thu Jan 03, 2002 11:29 pm

Why No RPM Display For Big Jets?

Sat Jan 05, 2002 8:29 pm

In airliners you have all the requierd engine readouts NI, N2, N3(on RB211s) EGT, EPR, Fuel flow and Vibration but i'm not sure why they dont have a Fan RPM readout. Surly the more information the Pilot has about how the engine is performing the better.
Just a thought.

Cheers,
Trent_800
 
SailorOrion
Posts: 1960
Joined: Tue Feb 06, 2001 5:56 pm

RE: Why No RPM Display For Big Jets?

Sat Jan 05, 2002 8:33 pm

N1 is the fan rotation speed

SailorOrion
 
Trent_800
Topic Author
Posts: 136
Joined: Thu Jan 03, 2002 11:29 pm

RE: Why No RPM Display For Big Jets?

Sat Jan 05, 2002 8:34 pm

but how can it be when it shows a percentage?
 
Trent_800
Topic Author
Posts: 136
Joined: Thu Jan 03, 2002 11:29 pm

RE: Why No RPM Display For Big Jets?

Sat Jan 05, 2002 8:40 pm

What i mean is why show, say for instance 92% N1 instead of 6,800rpm N1 (these figures are just for demonstration purposes)
 
XFSUgimpLB41X
Posts: 3961
Joined: Fri Aug 25, 2000 1:18 am

RE: Why No RPM Display For Big Jets?

Sat Jan 05, 2002 11:58 pm

Mainly just becauset the numbers are so big.... 100% N2 is usually somewhere in the range of 40,000 RPM... thats a big number to put on a little guage.
Chicks dig winglets.
 
Minuteman
Posts: 260
Joined: Sat Aug 05, 2000 1:01 am

RE: Why No RPM Display For Big Jets?

Sun Jan 06, 2002 1:09 am

It's easier to remember "don't go over 100% N1 for more than ninety seconds" instead of "don't go over 5,420 RPM on this engine, 5,480 on another..."
 
seagull
Posts: 334
Joined: Sat Jun 30, 2001 5:58 pm

RE: Why No RPM Display For Big Jets?

Sun Jan 06, 2002 3:15 am

Big jets don't get up to 40k, that's a turboprop number. However, in answer to the question, the scale has nothing to do with anything, it is just arbitrary. In fact, you can exceed 100% N1 or N2 in normal operations on many engine types!

Using the % just standardizes it with different engine types. Airbus simplifies the flap settings this way as well, and it wouldn't be a bad idea for the other companies to do the same, which have a different number for what, operationally, is the same thing? Flaps 1, Flaps 2, Slats Extend, all these are essentially the same thing to the pilot as far as what they accomplish, why not just call it all the same thing?
 
JETPILOT
Posts: 3094
Joined: Tue May 18, 1999 6:40 am

RE: Why No RPM Display For Big Jets?

Sun Jan 06, 2002 11:15 am

You're all wrong....

Jets dont have RPM indicators because RPM has no significance with changing air densities.

Engine power production at 3,000 RPM at 20C is different than the engine power production at 3,000 RPM at 85C.

What a pilot is interested in is what power level the engine is operating at. That is supplied by % of N.

Percent of N is a constand power regardless of ambient air pressure.

RPM is a good indicator of power in an automobile where temp and pressure are relatively constant.

Notice the "relatively".

Anyhow thats the story as far as RPM goes.

JET
 
JETPILOT
Posts: 3094
Joined: Tue May 18, 1999 6:40 am

RE: Why No RPM Display For Big Jets?

Sun Jan 06, 2002 11:17 am

previous should read:

Engine power production at 3,000 RPM at 20C is different than the engine power production at 3,000 RPM at 25C.

I was thinking in F..... Dummy

JET
 
Minuteman
Posts: 260
Joined: Sat Aug 05, 2000 1:01 am

RE: Why No RPM Display For Big Jets?

Sun Jan 06, 2002 12:10 pm

Are you saying N1 is a fabricated value, not based on only a scaled tachometer indication?

Percent of N is a constand power regardless of ambient air pressure. (I assume you mean density?)

I agree that 3000 RPM at 20°C and 25°C will produce different thrusts (assuming isobaric conditions), but I thought that's why those multi-dimensional lookup tables for takeoff thrust settings (based on weight, pressure altitude, temperature, etc.) were necessary ... because different conditions require different engine speeds? Choose a takeoff weight and you've still got a range of thrust settings that depend on the ambient pressure and temperature.
 
411A
Posts: 1788
Joined: Mon Nov 12, 2001 10:34 am

RE: Why No RPM Display For Big Jets?

Sun Jan 06, 2002 1:38 pm

...and if we look back a few years, the Boeing 707's with RR Conway engines had only RPM to set power, no EPR gauges.
RPM is very important due to blade length and their propensity to rub, ie: blade creep.
 
jsuen
Posts: 204
Joined: Tue Aug 14, 2001 2:36 pm

RE: Why No RPM Display For Big Jets?

Sun Jan 06, 2002 2:48 pm

N1 and EPR are dependent on ambient conditions. The A380 will have direct thrust readings for that very reason.
 
H. Simpson
Posts: 933
Joined: Sat Mar 10, 2001 7:30 am

RE: Why No RPM Display For Big Jets?

Sun Jan 06, 2002 3:02 pm

They don't put RPM because of it's large number basiclly, I used a Atr-42 panel in my A300 in flightsim and the RPM gauge go way high(about 10 spins in 3 seconds) before it stablize.
 
JETPILOT
Posts: 3094
Joined: Tue May 18, 1999 6:40 am

RE: Why No RPM Display For Big Jets?

Sun Jan 06, 2002 3:44 pm

It just so happens...and its my preference.... to set climb power at 92% N2 in the 727.

The EPR will incease or decrease for ambient temp but % N2 will remain constant throughout the climb.

There are exceptions to the rule. If yo ugo through a temperature inversion in a climb the N2 will decrease.

If you look at the max climb thrust EPR chart and work it backwards it always works out to approx 92% N2.

TO power in the 727 is always approx 98% N2. After you set the EPR which is always different for every condition you glance down at do and check the N2 and if everthing is ok the N2 will be at 98%.

%of N is a constant power setting. RPM isnt.

JET
 
FDXmech
Posts: 3219
Joined: Sun Mar 19, 2000 9:48 pm

RE: Why No RPM Display For Big Jets?

Sun Jan 06, 2002 7:13 pm

Percent of RPM, either N1, N2 or N3 IS directly scaled to actual RPM.
You're only as good as your last departure.
 
Trent_800
Topic Author
Posts: 136
Joined: Thu Jan 03, 2002 11:29 pm

RE: Why No RPM Display For Big Jets?

Sun Jan 06, 2002 7:26 pm

So what IS the region of RPM for a Trent (for instance) at full power.
 
FDXmech
Posts: 3219
Joined: Sun Mar 19, 2000 9:48 pm

RE: Why No RPM Display For Big Jets?

Mon Jan 07, 2002 1:11 am

I don't know about the RR Trent but for the CF6-80-C2.
...........................................................................
GE CF6-80C2
100% N1.......3280 RPM
100% N2.......9827 RPM
...........................................................................
Redlines
117.5% N1....3854 RPM
112.5% N2...11055 RPM.
You're only as good as your last departure.
 
airplay
Posts: 3369
Joined: Sat Oct 25, 2003 1:58 am

RE: Why No RPM Display For Big Jets?

Mon Jan 07, 2002 1:12 am

Some (older) turbine powered aircraft had RPM gauges. Viscount and HS-748 for instance.

Many newer turboprop engines use %Np for propellor speed.

JETPILOT,

With respect to stages of turbine engines, "N" corresponds to rotational speed ie:RPM which is normally expressed as %N but could (and has been) just as easily expressed as RPM.

You are right that engine power production is different at different ambient conditions, but if your guage says "85% N1" it represents the same RPM whether the airplane is at -60C at 40,000 feet or +40C at sea level.

The reason percentage is used is to make operation (and maintenance) simple. Who the heck wants to remember hundreds of values of fairly large numbers?

Like always though, when it comes to airplanes, "never say never". There may be exceptions to this in some highly integrated EICAS equipped airplanes that I'm not familiar with. Does anyone have information about N1 indication on an A320 for instance??

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