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Old 1st Jan 2013, 16:08
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barit1
 
Join Date: Feb 2005
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That sounds like gas generator energy and not necessarily a significant relationship as a comparative indicator of idle propulsive thrust
Correct, nonetheless significant.

There is an operability parameter known as acceleration fuel margin. It is the difference between the fuel required to hold a steady-state speed (say, ground idle) and the capacity of the engine to accept more fuel at that speed. For a successful accel, this must always be a positive margin, preferably a healthy margin.

Now, load up the gearbox with SHP extraction (hydraulics and BIIIG gererators), and bleed out a lot of pneumatic load; The steady-state fuel required will increase, eating into the accel fuel margin.

If we were to adjust ground idle downwards, the steady-state fuel flow will decrease (but not much), but the accel limit line (related to stal/surge margin) will creep downward more rapidly. All this time, idle thrust is decreasing, but so is the ability to accelerate.

At some point we'll find the beast will refuse to budge in response to the throttle!
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