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Compared to a [modern] turbofan/turbojet, why is the throttle response slow in a turboprop?

A constant speed propeller mounted on a piston engine responds much quicker.

Is the underlying reason mechanical (e.g. size of combustion chamber), or electronic (e.g. ITT protection)?

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    $\begingroup$ I'm guessing a lot of it has to do with the addition of the prop, gearbox (if equipped) and connection equipment being a large mass to spool up causes a noticeable lag in command. $\endgroup$ – Ron Beyer Jun 26 '16 at 18:56
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    $\begingroup$ I wonder if there isn't a direct implication of reducing the rotation speed and increasing the torque (compared to a turbofan) on the effort required by the turbine to spool up. $\endgroup$ – mins Jun 26 '16 at 19:33
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    $\begingroup$ Am I missing something? a constant speed, variable pitch propeller driven by a turbine engine is very responsive. More throttle causes an immediate pitch change because it is a very small movement and so it happens now. The engine and prop both are always at 100% rpm; there is no engine or prop spool-up. So all that happens is the blades change pitch and bam, instant thrust. Turbine, shaft, and prop rotate together via gears and splines. There is no clutch, duh, and it's not an an automatic transmission like a car. $\endgroup$ – radarbob Jun 27 '16 at 5:17
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    $\begingroup$ @radarbob, well, most turboprops have free power turbine, which means the core (high-pressure turbine driving the compressor) is independent of the power (low-pressure) turbine driving the propeller. $\endgroup$ – Jan Hudec Jun 27 '16 at 7:41
  • $\begingroup$ Excuse me, where did you get the impression turbofans have faster thrust response than turboprops? Turbofans generally have slower thrust response than turboprops. $\endgroup$ – Jan Hudec Jun 27 '16 at 7:43
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Turboprop engines typically have a core which includes a compressor and turbine that rotate as a unit (although an engine might have more than one such "spool"). The gas generated by the core drives a "power" turbine which, via a gearbox, drives the propeller, which by the way has a non-trivial rotating mass. The core and power sections rotate independently of each other. So, upon a commanded power increase, the core will first spin up, throwing more gas at the power turbine which will then speed up. The delay is therefore due to the rotating mass of the propeller and the fact that it rotates independently of the engine core.

For what it's worth: turboprop engines are just a specific application of a turboshaft engine - where the energy of the gas produced by the core is captured through a power turbine to drive a shaft, which could be used to power anything - propeller, helicopter rotor, electric generator, etc.

I should add that there are exceptions to the above, like the Garrett TPE331 which have only a single shaft.

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  • $\begingroup$ Are you talking about free-turbine? $\endgroup$ – mins Jun 26 '16 at 22:25
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    $\begingroup$ @ymb1 As I described, a turboprop typically contains more than one turbine, each on its own shaft. One turbine is part of the core, responsible for driving the compressor, and another turbine on its own shaft (not mechanically connected to the core turbine) drives the propeller. $\endgroup$ – Anthony X Jun 26 '16 at 22:59
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    $\begingroup$ The TPE-331 series are geared. That is, you can trace a physical touch-point between each component from the tip of the spinner to the rear bearing seal (practically speaking). They spool up much faster than their PT6 free-turbine counterparts, so much so that pilots consider them to have "power on demand." There is very little lag (almost none) with TPE series turboprops. The prop governor response is virtually instant, or should be, unless there is a problem with the various governor interconnects. $\endgroup$ – acpilot Jun 27 '16 at 1:04
  • $\begingroup$ Turboprops are used with constant speed propellers. So the power turbine does not need to spin up, only the core. And while the core and power turbine are not connected mechanically in most engines, the pressure conditions still mean the core spins faster at the same power setting if you keep the power turbine spinning faster via fine prop pitch. $\endgroup$ – Jan Hudec Jun 27 '16 at 7:47
  • $\begingroup$ @JanHudec Do turboprops use FADEC? Don'tthey have the same problem of surging if the throttle increases too quickly $\endgroup$ – TomMcW Jun 27 '16 at 22:28
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This is a feature of turbine engines, due to the need of the gas core to 'spool up' ie increase speed before the power levels increase. It used to be a serious problem in turbine aircraft; newer engines are much better and have much faster response times to power inputs from the flight crew.

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