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If an airplane with two propellers has both propellers with the same rotation direction (say, both the propellers rotate counter-clockwise as seen from nose to the tail), will be the four left-turning tendencies still be generated?

Airplane with two propellers

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Yes, for the most part.

  • Definitely P-factor will still exist and provide a yaw torque in the expected direction. This is why twin-engined aircraft have a "critical engine".

  • Engine torque will still produce a roll torque in the expected direction.

  • Gyroscopic precession will still produce a yaw torque in the expected direction when the aircraft pitches.

  • The yaw torque from the spiralling slipsteam interacting with the vertical fin-- which is normally quite significant in conventional single-engine aircraft-- is the one effect that may be be greatly reduced in a twin-engine airplane of the usual configuration, since the fin is not located in a strong part of the circulation behind either prop.

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  • $\begingroup$ Do you have a link how a proper way to calculate the tendencies? My intuition also said that there should be also tendencies. $\endgroup$ Commented Nov 9, 2019 at 3:10
  • $\begingroup$ @AircraftLover-- no I don't $\endgroup$ Commented Nov 9, 2019 at 13:03
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    $\begingroup$ I agree with you regarding the spiraling slipstream for aircraft with a single vertical tail. For a twin with a dual vertical tails set behind the engines, the spiraling slipstream would be similar to a s single engine with a single tail. $\endgroup$
    – Gerry
    Commented Sep 17, 2020 at 21:11
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No it can not work that way.That is the begining of acceleration in airplane then it will begin to move one side.

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  • $\begingroup$ As quiet flyer has pointed out, p-factor, gyroscopic precession, and torque will all still be present and act on the aircraft as expected. Think of a front-wheel, a rear-wheel, and a four-wheel drive car. All three will lift the nose/front of the car and lower the rear/tail of the car if slam on the gas at a complete stop like in a drag race. $\endgroup$
    – Dean F.
    Commented Sep 17, 2020 at 23:40
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No, because they turn on different axis which does not coincide with the longitudinal axis of the airplane. Having two propellers turning the same way on the same axis, it would enforce the effect.

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    $\begingroup$ How does the separation of the axes make the torque disappear? This seems to contradict classical mechanics. $\endgroup$ Commented Jan 24, 2019 at 14:52
  • $\begingroup$ As the engines rotate clockwise seen from a pilot view (or anti-clockwise from the nose to the tail view), when the engine 2 rotate, will it not hil the vertical stabilizer which will make turn-left tendency? $\endgroup$ Commented Jan 25, 2019 at 1:50
  • $\begingroup$ As long as my deduction comes from observation and not from a mechanical engineer degree, both engines outflow will hit the vertical stabilizers one on each side of the longitudinal axis, having not the torque applied to one of the axis won't let the airplane turn, in my humble opinion, one or both the engines running, since the torque they would apply is not on the longitudinal axis of the plane. $\endgroup$ Commented Jan 28, 2019 at 9:01
  • $\begingroup$ Seems that I have missed this discussion. I apologize. $\endgroup$ Commented Mar 1, 2019 at 2:49
  • $\begingroup$ As I said, if both the propeller are rotating counter-clockwise (seen from nose to the tail), then in my opinion, propeller 2 will generate wind spiral, which it will still hit the vertical stabilizer, then make the airplane forced to turn left. $\endgroup$ Commented Mar 1, 2019 at 2:53

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