Consider two-propeller airplane as picture below (picture 1 (the above) and picture 2 (the below)). Which one is the most effective configuration to produce thrust power, stability, structural design, etc if still any?

  1. Picture 1 (above): the bigger angle of attack (due to parallel to the gravitation) is meet near the fuselage, in between the two propeller's axis.
  2. Picture 2 (below): the bigger angle of attack is outside the two propeller's axis.

Need physic analysis to answer this question.

Propeller rotation


1 Answer 1


TL;DR: The first one.

It all comes down to P-factor.

enter image description here

Airplanes fly at a positive angle of attack, so as you can see, this AoA for the wing translates to positive AoA for the descending blade. Vice versa, the positive AoA for the wing is negative AoA for the ascending blade. I.e., the descending blade has more thrust than the ascending blade under AoA.

Now it is pretty obvious why you want the descending blade on the inboard and ascending blade on the outboard, because in a single-engine-out incident this creates a less yawing moment to the otherwise arrangement.

However, life does not always turn out as we want it, as the engineers at Lockheed learned the hard way when testing their XP-38. They had gone through the trouble of using handed engines and props, making the starboard engine turn counterclockwise and the port engine clockwise. Now the fighter appears to be perfect: no torque, no gyro, a minimum yawing moment in a single-engine-out situation. But the XP-38 had pitch control issues which were later found out to be caused by the downwash created by descending inboard blades hitting the wing section between the boom and the central nacelle, creating a disturbed airflow trapped between the twin booms and finally affecting the elevator. In the end, they had to switch the engines' positions so the P-38 ended up with two critical engines.

  • $\begingroup$ Can you give physic/mathematical calculation, please with both configuration? What is the aspect affected so the the first configuration is better than the second. $\endgroup$ Jan 26, 2019 at 5:52
  • $\begingroup$ @AirCraftLover what calculation do you need? the inboard blade has more thrust so the overall yawing moment is smaller than otherwise $\endgroup$ Jan 26, 2019 at 6:06
  • $\begingroup$ I still can not get the reason why the inboard descend is better than the other one. That the reason why ask mathematical or physic calculation to see what make them different between the first option and the second one. Apologize for my lack of my understanding. $\endgroup$ Jan 26, 2019 at 6:23
  • 1
    $\begingroup$ @AirCraftLover let the inboard thrust be T, outboard t. let the hub of the prop be D away from airplane's central line. let the distance from the hub to the aero center of each blade be R. inboard descending configuration has yawing moment (D-R)T+(D+R)t, inboard ascending (D-R)t+(D+R)T. which is larger? $\endgroup$ Jan 26, 2019 at 7:53
  • $\begingroup$ Give me some moment to understand. $\endgroup$ Jan 26, 2019 at 10:26

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