Ground idle takes about 5% of the total rpm, flight idle takes about 85% of the total rpm. Why is so much of the engine power wasted before it gets to flight idle?

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    $\begingroup$ Are you talking about a turboprop? $\endgroup$ – John K Jan 12 at 4:45
  • $\begingroup$ It is not clear what you are asking about! Please give more information about which aircraft you are referring to. Also, the questions you ask in the title and the body seem to be asking for something different. $\endgroup$ – Bianfable Jan 12 at 8:30
  • $\begingroup$ I have voted to reopen, because the comment above provides the clarification being sought. $\endgroup$ – Penguin Jan 16 at 10:32
  • $\begingroup$ @Penguin What does the claim that "so much of the [is] engine power is wasted" mean? $\endgroup$ – David Richerby Jan 16 at 21:42
  • $\begingroup$ @DavidRicherby. They are saying, that because if flight idle is at 85% rpm, all the power developed at lower rpms is “not used” because flight idle is the lowest throttle setting. Obviously, this is a bit misleading; 85% rpm does not mean the power produced is 85% of max power, and the power produced at lower rpms still helps produce the power provided at 85% rpm, is it does have a benefit, and hence is used. $\endgroup$ – Penguin Feb 12 at 10:37

Because flight idle is used in flight, and hence needs to provide enough thrust (or power) to be useful, such as sustain level flight at some parts of the flight envelope, under certain conditions (e.g. low vehicle weights). On the other hand, ground idle only needs enough power to enable taxing.

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    $\begingroup$ If that was the case, it wouldn't be called "idle". $\endgroup$ – John K Jan 12 at 4:57
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    $\begingroup$ Flight idle does not maintain level flight. $\endgroup$ – Ron Beyer Jan 12 at 4:57
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    $\begingroup$ The one aircraft that I flew that had both ground idle and flight idle was the 727-100. The purpose of flight idle was to keep the engines spooled enough that if you needed to go from idle to more power, you could do it in a reasonable about of time. For example, if you needed max power to go around. As I remember, in the development they found that it took a full 7 seconds to go from ground idle to max power. That was not acceptable, hence the flight idle. $\endgroup$ – Terry Jan 12 at 7:05
  • $\begingroup$ That makes sense. Thanks. $\endgroup$ – David Jan 13 at 7:35
  • $\begingroup$ @RonBeyer. OK, maybe not. But the essence is, flight idle is used in flight, ground idle is not. $\endgroup$ – Penguin Jan 14 at 10:46

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