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boacvc10
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Idle thrust - questions

Mon Jan 24, 2022 8:00 pm

Can a propeller plane be configured for idle thrust by adjusting pitch of the blades? for ground startup? For descent?

If a jet engine is at idle thrust, does the thrust level have to be adjusted for ground, air, altitude?

When parked is idle thrust just the engine turning over ay low RPM? I have seen up close jets spooling up to get started at the ramp. Or when pushedback they have low thrust and sometimes spool up to slow down and stop, but is the TUG not supposed to do that?

So what is "idle thrust" in practical terms?
 
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fr8mech
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Re: Idle thrust - questions

Mon Jan 24, 2022 8:52 pm

boacvc10 wrote:
So what is "idle thrust" in practical terms?


Probably not the technical definition, but one that can be used is: the minimum speed or power setting an engine can run and be stable. I guess you can add the provision that it can carry a minimum load, but starting to get more technical.
 
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fr8mech
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Re: Idle thrust - questions

Mon Jan 24, 2022 9:01 pm

iPad went a little screwy.

boacvc10 wrote:
If a jet engine is at idle thrust, does the thrust level have to be adjusted for ground, air, altitude?


The Electronic Engine Control manages the the various environmental and aircraft inputs to control idle speed. On older engines, the jet fuel control or hydraulic-mechanical unit or fuel metering unit or fuel flow governor would do a decent job of managing idle speed.

There is a “flight idle” setting, that is higher than the “ground isle” setting in order to provide faster response in the air.

boacvc10 wrote:
When parked is idle thrust just the engine turning over ay low RPM?


A jet engine turns at about 65% N2 (N3 for the RB) for idle speed.

boacvc10 wrote:
I have seen up close jets spooling up to get started at the ramp. Or when pushedback they have low thrust and sometimes spool up to slow down and stop, but is the TUG not supposed to do that?


Not sure exactly what you’re asking here. Depending on company policy, engines can be started prior to push, during push or after push. Some other factors may play in, but the engines have to be running eventually.
 
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zeke
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Re: Idle thrust - questions

Mon Jan 24, 2022 10:15 pm

boacvc10 wrote:
Or when pushedback they have low thrust and sometimes spool up to slow down and stop, but is the TUG not supposed to do that?

So what is "idle thrust" in practical terms?


The acceleration of the engine to idle just as pushback finishes is a coincidence, I normally like to time the starts so the engines have started by the time the push is finished. We can release the tug and get going. It’s the most efficient for fuel burn.

Idle thrust is the thrust lever at idle.

Turboprops can be zero thrust, ie engine at idle and propellers still fully feathered, or even stopped with a prop brake.
 
RetiredWeasel
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Re: Idle thrust - questions

Tue Jan 25, 2022 12:44 am

boacvc10 wrote:
Can a propeller plane be configured for idle thrust by adjusting pitch of the blades? for ground startup? For descent?




The only turboprop I flew was the OV-10 Bronco, and as we shut the engines down after a sortie, we put the props into 'flat' pitch. This provided the least air resistance when starting the engines for the next sortie and almost zero thrust in that position. Feathering also provides zero thrust, but more air resistance when starting them. Turbo-props differ though in their power train connections.
 
GalaxyFlyer
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Re: Idle thrust - questions

Tue Jan 25, 2022 1:12 am

PT-6s shutdown and start in feather, the Garretts on the OV-10 started in flat pitch. I think only free turbine designs like the PT-6 have a hotel feature.
 
Tristarsteve
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Re: Idle thrust - questions

Tue Jan 25, 2022 5:12 pm

There is a “flight idle” setting, that is higher than the “ground isle” setting in order to provide faster response in the air.


Modern big jet engines now have an approach idle. The engine stays at ground idle until it is close to landing, then accelerates to approach idle. Approach idle is designed to achieve the time to take off power in the case of a baulked landing.
When I was at work this was 6 seconds from approach idle to 95pc of take off power.
On a big fan engine, approach idle is quite a lot higher than ground idle, so to save fuel the aircraft descends with the engines in ground idle, then they accelerate to approach idle when the flaps go down to a landing setting.
 
Woodreau
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Re: Idle thrust - questions

Tue Jan 25, 2022 6:28 pm

boacvc10 wrote:
Can a propeller plane be configured for idle thrust by adjusting pitch of the blades? for ground startup? For descent?



The turboprop I flew had PT-6 engines. During normal operation the governor adjusted prop pitch to maintain a specific RPM. Normally 1700rpm for takeoff and landing. In cruise a lower RPM setting that I don’t remember.

There isn’t a way to choose a specific pitch angle for the prop blades except for feather.

As soon as you released the brakes the plane would accelerate and you would need brakes to keep the plane from flying down the taxiway

Turboprops have a beta setting to reverse the prop. If you went into beta there was a setting where the plane would remain motionless with the brakes off. Nudge the power lever forward the plane would start rolling forwards, nudge the levers back you could stop and start the plane rolling backwards. The movement wasn’t very much less than a few millimeters of power lever movement. So there was a fine line where the prop was on the governor and not generating any thrust in beta.

Beta is not used in flight only on the ground.



boacvc10 wrote:
So what is "idle thrust" in practical terms?


The current aircraft I fly has a ground idle, flight idle, and approach idle, as well as a reverse idle for when the thrust reversers are deployed on the ground.
 
boacvc10
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Re: Idle thrust - questions

Wed Jan 26, 2022 12:41 am

Thanks everyone for the discussion so far. Learning a lot.
 
bigb
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Re: Idle thrust - questions

Wed Jan 26, 2022 3:54 pm

Various types of different Idles.

Ground Idle

Flight Idle

Approach Idle - allows for quicker engine response/reduced spool time for go-around/toga thrust to be apply or higher bleed pressure for Anti-Ice operations.
 
Yikes!
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Re: Idle thrust - questions

Mon Feb 07, 2022 12:47 pm

Woodreau wrote:
boacvc10 wrote:
Can a propeller plane be configured for idle thrust by adjusting pitch of the blades? for ground startup? For descent?



The turboprop I flew had PT-6 engines. During normal operation the governor adjusted prop pitch to maintain a specific RPM. Normally 1700rpm for takeoff and landing. In cruise a lower RPM setting that I don’t remember.

There isn’t a way to choose a specific pitch angle for the prop blades except for feather.

As soon as you released the brakes the plane would accelerate and you would need brakes to keep the plane from flying down the taxiway

Turboprops have a beta setting to reverse the prop. If you went into beta there was a setting where the plane would remain motionless with the brakes off. Nudge the power lever forward the plane would start rolling forwards, nudge the levers back you could stop and start the plane rolling backwards. The movement wasn’t very much less than a few millimeters of power lever movement. So there was a fine line where the prop was on the governor and not generating any thrust in beta.

Beta is not used in flight only on the ground.



boacvc10 wrote:
So what is "idle thrust" in practical terms?


The current aircraft I fly has a ground idle, flight idle, and approach idle, as well as a reverse idle for when the thrust reversers are deployed on the ground.


"Beta" range on a turbo-prop is defined as "power lever control of propeller pitch angle." There are actually in-flight situations where this can occur and is part of normal operations - the PT6 is a good example. While the propeller(s) is/are still in a very positive pitch angle, reverse while not on the ground is usually inhibited by mechanical means. In the "old" days of the Dash 8, this was not the case. Similarly on the Twin Otter. There were many instances of intentional (believe it or not) and accidental uses of in-flight reverse operations hence the modern mechanical inhibitors.

Many turbo-props (PT6, Garrett) can employ "blade latches" which allow the engines to be shut down in a perfectly flat pitch. On the PT6, this is particularly useful on float plane ops where, if normally started in feather, the transition to forward pitch would render the aircraft difficult to control during the transition. So the engines are started in flat pitch. On the Garretts, if the engines are shut down without engaging the latches, the propellers would feather. Starting a Garrett in this condition guarantees a fried engine during a start attempt. So normal shutdown of the Garrets was fuel cut-off, then as RPM decreases with the engine's fuel shut-off, the power levers would be hauled back to the full reverse setting to allow the latches to deploy before the props had a chance to feather due to the reduced oil pressure.

More trivia - hope it's useful!

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