This is a common explanation:

A tailwheel airplane is on the takeoff roll and the tail comes up, the propeller disc rotates forward. That's akin to someone pushing on the top of the disc. The force results 90 degrees ahead, or on the right side, pushing the nose left.

I do not understand the term "90 degrees ahead".

In my mind, in this example, we are viewing the propeller spinning clockwise. If a force is applied at the top (12 oclock), then the resulting gyroscopic force on the right (3 oclock) should be called 90 degrees after. If the force is applied at 12 oclock, spines 90 degrees and then a resulting force at 3 oclock, why is that called before instead of after?

  • $\begingroup$ Seeing as the statement you cited actually made no reference at all to the word "before", there may be grounds for closing this question as in need of clarification. $\endgroup$ Jan 22, 2021 at 15:13
  • $\begingroup$ This is just a semantic discussion on what "ahead" and "after" means in the context. If you're describing it as a future action/result, "blade moves ahead 90 degrees" from the vertical is one way of looking at it, and that's how I would interpret the block quote. If you're describing it as a past event, you might say the blade moved to a position 90 degrees after the vertical. In either case, the action started at 12 o'clock provides the resultant force at 3 o'clock; ahead, behind, whatever. $\endgroup$
    – John K
    Jan 22, 2021 at 17:46
  • $\begingroup$ Another problem with this question is that the title appears to be asking a question about the mechanisms of the physics (which might take a PhD-level understanding to answer, or at least a lot of math), while the body makes it clear that it is actually just a question about the meaning of words. I'll try to help by adding quotes around "ahead", but I suspect the question will be soon closed. $\endgroup$ Jan 23, 2021 at 1:33

1 Answer 1


If I say "my brother was born ahead of me", you might reasonably assume that my brother is older than me. That his birth took place at an earlier point in time.

You seem to be assuming that "ahead" refers to the position that any given blade of the propeller occupied at an earlier point of time. I.e., a position displaced opposite to the direction that the propeller is rotating.

But if you are driving a car and your passenger says "the light's green, go ahead", would you take that a suggestion to go backwards? Or to go forwards?

The meaning of "ahead" in the passage quoted in the original question is correct and reasonably unambiguous-- it means forwards along the direction that the propeller is rotating.

Note that the original passage did not contain the word "before".

  • $\begingroup$ Thank Quiet Flyer, I believe you interpreted my misunderstanding correctly and have given me the context to reconcile the description with my understanding. $\endgroup$
    – Fun-Fly-AZ
    Jan 24, 2021 at 13:36

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