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

From:         simonlc@ozemail.com.au (Simon Craig)
Organization: OzEmail Pty Ltd - Australia
Date:         17 Aug 95 04:58:45 
References:   1 2 3
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>  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. 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. I don't think there is any tourquemeter to be found on a
> modern high bypass engine, since all (or most) of the torque goes toward
> spinning that fan.

This is not entirely true of our engines at Qantas.  Whilst the CF6 engine
measures power by N1 rotor speed, both the JT9 and RB211 types use EPR as
an indication of thrust.

Simon.