Re: Do airliner engens run at approximately constant speed, varying only torque?

From: (Robert Dorsett)
Organization: Netcom Online Communications Services (408-241-9760 login: guest)
Date:         17 Jul 95 04:29:33 
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In article <airliners.1995.1061@ohare.Chicago.COM> (Robert Westbrook) writes:
>   As far as I know, high bypass turbofans measure their thrust output
>through the N1 (rpm of the fan) gauge, as opposed to EPR (engine pressure
>ratio, which compares the static pressure of the tailpipe to static
>pressure of the inlet, which I suppose could be considered ambient), which
>is the best measure of thrust for medium bypass engines like the JT8, or
>pure turbojets.

The 777, which has probably the highest highest-bypass engines around,
sets thrust through EPR (to *three* decimal places, no less :-)).  EPR is
a standard convention for thrust management, for reasons which have already
been delved into extensively in this newsgroup.

One manufacturer that still uses N1 extensively is GE, with some variations
of the CF6.

>RPM will of course increase when the throttle is put to
>firewall, and the fan turning faster is the primary source of thrust in
>fan engines.

To answer the original question, since N1 ratings are high, anyway, one will
not see as much variation as with a piston engine.  An 18-30%  variation
between flight idle and takeoff thrust would be typical on the JT8D-15.

Robert Dorsett                         Moderator, sci.aeronautics.simulation