What is the difference between ground idle and flight idle settings? Is it pitch or engine rpm that changes? If it matters, this is in reference to a Mitsubishi MU-2.
Thanks.
Well I fly a variable pitch propellor aircraft, and the way the prop changes pitch & rpm in those two positions are different. In ground idle, the blade angle is controlled by the throttle to aid in taxiing, but in flight range the prop keeps a constant rpm in flight settings, thru changing propellor pitch.
Also, ground idle is pretty flat, so that you don’t put too much stress on the brakes or the chicks when starting or shutting down the engine. .
Given the nature of turbine engines I would probably guess that most all turboprops use a constant RPM prop setup.
Okay, I can understand ground idle, but what is “flight idle”? Is it high enough rpm to pull the airplane through the air fast enough to maintain lift over the wings and allow straight and level flight?
A quick google on this showed that the “Flight Idle” is a bit higher than ground - the idea being that it was a few seconds quicker to full power from “flight” than from “ground”.
This would come in handy for either thrust reversers or go-arounds (aborted landing requiring another take-off).
It’s basically the minimum engine speed that allows it to be safely self sustaining while supporting whatever loads are on it. There are also requirements for jet engines to be able to accelerate to full power within a certain time, I’m not sure if the same acceleration requirements apply to turboprops.
If the idle speed is too low, when you apply power there is a risk that the addition of fuel will make the combustion section burn hotter without a corresponding increase in engine speed. Decreasing or stagnating rpm with increased combustion temps is termed “bog down” and will lead very quickly to engine failure as the temperature runs away.
In flight the loads on the engine are normally higher due to running hydraulic pumps, generators, and airsupply requirements for air conditioning, pressurization, and airframe and engine ice protection so the minimum idle in flight will be different to the minimum idle on the ground.
As mentioned above there is also a change in the way the power levers control the engine/prop on a turboprop when below the flight idle gate (beta range). This is not directly related to flight idle vs ground idle though as some engines have a flight beta range where prop blade angle is controlled directly by the power levers in flight. The dash 8 has a small amount of flight beta available.
http://en.m.wikibooks.org/wiki/Jet_Propulsion/Engine_ratings#Flight_Idle
Some good info there though it is specific to jets but a lot of it is applicable to turboprops.
Thanks Richard and others.
That makes sense.