I already know that this is because of P-factor. But I don't know how to draw a picture or explain this simply?

  • 2
    $\begingroup$ Wikipedia on P-factor has sufficient picture to give a clue why. And it depends on the direction of propellers rotation: it may be left, right or even none (for counter-rotating props, like on PA-44 or P-38). $\endgroup$
    – Zeus
    Mar 22, 2018 at 23:09

1 Answer 1


The "critical engine" (as it is called in this context) is based on the direction of propeller rotation and the "arm" length from the descending blade to the aircraft's center of gravity.

Here is a picture and explanation (source: Skybrary)

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  • $\begingroup$ Was there a reason why the P-38 had both engine rotating top side away from the fuselage? It would seem to be no advantage in doing this whatsoever. $\endgroup$ Mar 23, 2018 at 3:44
  • $\begingroup$ @CharlesBretana Sounds like a new question. $\endgroup$ Mar 23, 2018 at 3:48
  • $\begingroup$ I would think that being a military aircraft rather than a civilian one, the trade-off is handling over stability. The outer lever arm increases the moment and as a result the rate of turn. this is much more important when you are being shot at vs heading to the local fly in for a serving of heart attack on a plate. $\endgroup$ Dec 13, 2021 at 4:13
  • $\begingroup$ @RowanHawkins: The P38 along with many other military and civilian airplanes have "counter-rotating" props that along with a few advantages did not have the disadvantage of a "critical engine." $\endgroup$
    – RTO
    Dec 13, 2021 at 4:26
  • $\begingroup$ @757toga nothing I said disagrees with that. The question is about a civilian plane. The P-38 has counter rotating props which places both moment arms further from the center axis -- the fuselage. This gives them a larger moment arm and results in a tighter turn. The civilian ones as noted above have the moment closer to the fuselage which instead increases stability. The question asked in the comment above is why the choice was made between inboard and out board lever since they felt they were equivalent. From a critical engine stand point they are. From a handling standpoint they are not. $\endgroup$ Dec 13, 2021 at 4:40

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