[INVESTIGATING] Engine flight idle same as ground idle

[some1]

The engine has the same idle RPM on ground and in high speed flight. The idle RPM only goes slightly up with altitude. 

 

This is very unusual (if not improbable) for a jet engine. Normally the ram air during flight causes compressor to spin faster even at idle. Also the fuel management system is often set up to increase the idle RPM in flight, to avoid flameouts and reduce the time it takes for the engine to spool up to high power. 

 

Better check with your SME how it is in the Harrier. 

 

28.5% on the runway.

 

 

28.5% at 470 kts. 

 

[Wisky]

just out of curriosity, did you manage to recover from screenshot 2?

[some1]

For Science!

[PeneCruz]

Minimum Idle is 28.4% at ISA and goes up 1% per 1000 ft of pressure altitude starting at 1500ft, It's listed as a limitation. So I think you will find the engine has no flight idle setting hence why tactical descents are flown at 65% to ensure sufficient acceleration.

[some1]

So it rises with a slight change in pressure from altitude but it does not rise from a huge change in pressure from ram air at 450 kts? Makes little sense. 

[PeneCruz]

DECS does that stuff, It also has a sub idle mode for ground ops IRL, (not in the Sim) used in slippery conditions. this is probably the equivalent of other engines ground/flight idle logic. just guessing as the philosophy is not documented. It also has a whole heap of IGV tolerance restrictions that indicate something funky going on in general.

 

Just found this EFC failure:
If the engine remains stable following EFC control loss the rpm will increase in a descent at constant Mach number
and decrease during deceleration at constant altitude.

 

And this regarding the P3 limiter input
The P3 limiter (combustion chamber pressure limiter) vents the DECS P3 air pressure signal to the atmosphere when
P3 pressure exceeds the limit value. The drop in the DECS P3 air pressure signal causes the FMU to reduce fuel flow
thereby reducing engine internal operating pressure preventing possible engine overpressure resulting in structural
damage. The engine is most likely to operate on the P3 limiter during low altitude, high airspeed conditions with cold
ambient temperatures (ISO standard ambient temperatures and below). P3 limiting may begin as airspeed exceeds
475 KCAS (standard day) and may be noticed by the pilot as rpm fluctuations on the order of one to three percent
rpm. These fluctuations will occur at a rate of two to three per second. Reducing the throttle slightly will cause the
fluctuations to stop.

Edited July 6, 2021 by PeneCruz

[AlphaJuliet]

Investigating