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Forgive my poor English skill and my entry-level understanding of aviation stuffs. I'm currently a student pilot.

As I understood, the propeller slip kind of represents the propeller efficiency in some way, and when slip gets greater, the more thrust is "wasted", in another word, they are not being used on moving the aircraft forward. So why they are doing that way? It's confusing me.

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  • $\begingroup$ Hi there. What is your source for this information. $\endgroup$
    – Jpe61
    Apr 10, 2022 at 9:11
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    $\begingroup$ @Jpe61 It's on my textbook, and chinese pilot license exam as well.(I'm a chinese btw). Now I'm not sure if this whole concept is correct XD. $\endgroup$ Apr 10, 2022 at 10:26
  • $\begingroup$ Well, your english is just fine, do not worry about that. As I have not seen this claim myself (or just can't remember, my readings on prop theory are ancient), I can only speculate: I think this is because at high power settings the the propeller simply loses more and more "grip". While the propellers does create more thrust, it is highly inefficient in this condition. There are two ways to produce high thrust: accelerate a little air a lot, or accelerate a lot of air a little. Propellers use the latter method. $\endgroup$
    – Jpe61
    Apr 10, 2022 at 11:02
  • $\begingroup$ It makes sense, Thanks a lot! I finally get a grasp on this :D @Jpe61 $\endgroup$ Apr 10, 2022 at 13:25

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The concept that the propeller is "slipping" is just another way to express the idea that the propeller is meeting the air at a non-zero angle-of-attack. It's logical that for a given speed of motion of the prop blade through the air, the greater the angle-of-attack-- i.e. the greater the slip-- the more the thrust, as long as the angle-of-attack is not so high that the prop blade is stalled.

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This is actually a very good question in that it can be interpereted in two ways for aircraft:

  1. The acceleration point of view

  2. The steady state "cruising" point of view.

From a standing start, increasing rpm means more turns per meter. Technically, until the plane speeds up, it is "slipping" more and producing more thrust.

When cruising at constant speed, you want as little "slip" as possible. In other words, lowest rpm per unit thrust. This, of course, is a function of the propeller being at its best AoA for greatest efficiency when peak thrust is not needed.

This is why complex aircraft "flatten" their prop blades for takeoff and reduce rpm for cruising.

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  • $\begingroup$ Got it! Thanks a lot! $\endgroup$ Apr 10, 2022 at 13:25
  • $\begingroup$ Here is a good definition of "slip" jargon relative to propellers. $\endgroup$ Apr 11, 2022 at 0:14

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