# Why does a force on a propeller act 90° "ahead" due to gyroscopic precession?

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?

• 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. Jan 22, 2021 at 15:13
• 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. Jan 22, 2021 at 17:46
• 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. Jan 23, 2021 at 1:33