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It seems that having counter-rotating twin engines would be a natural choice to avoid the unwanted yaw seen in single engines and yet there are several twin-engine aircrafts that that aren't counter-rotating. Why is that?

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    $\begingroup$ Cost-savings. It is easier to install the exact same model engine on the aircraft than two separate model engines. It also allows owner / operators the ability to find and use a loaner engine easier than airplanes with counter-rotating props. IMO. I don't know how difficult it is to switch the direction of rotation on the engine. $\endgroup$
    – wbeard52
    Oct 5, 2022 at 2:59
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    $\begingroup$ Your assumption that only “several” twin engine aircraft don’t have counter rotating propellers is incorrect. MOST twin engine aircraft don’t have counter rotating propellers. $\endgroup$ Oct 5, 2022 at 3:07
  • $\begingroup$ @MikeSowsun Do you know why most twin engine aircraft don't have counter-rotating propellers? $\endgroup$
    – Tfovid
    Oct 5, 2022 at 7:18
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    $\begingroup$ It is only cost savings because having one engine turning the opposite way complicates manufacture and maintenance $\endgroup$ Oct 5, 2022 at 13:25
  • $\begingroup$ @Tfovid - re "Do you know why most twin engine aircraft don't have counter-rotating propellers?" - because they have no propellers? Seriously though, nothing in body or title makes it clear the question is limited to aircraft with propellers, might want to consider an edit. $\endgroup$ Oct 6, 2022 at 14:20

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Engines are mounted slightly canted downward to eliminate the P-factor in cruise, and the rudder has to be sized to compensate for yaw due to asymmetrical thrust, the P-factor on take-off is tiny compared to that, so the pilot has no problem compensating it. And the pilot has to use the rudder for other reasons, like compensating for cross-wind, anyway.

The P-factor does play role in the engine-out situation, as it makes the outward-rotating engine produce a bit more yaw. But an outward-rotating engine also causes less rolling moment toward the other engine, see e.g. Why is an outward spinning propeller better in theory on the North American F-82 Twin Mustang?, which improves the engine-out handling in other ways. And it just takes making the rudder a bit bigger.

According to this answer to Do turbine engines on multi-engine aircraft rotate in opposite directions to offset torque?, the main reason for counter-rotating engines is actually stall characteristics. But still, that can be fixed by other means like appropriate wing twist.

On the other hand, having to stock two different types of propellers and gearboxes (the engine can spin the same way and have to rotation reversed in gearbox on one side), and the additional risk of mounting them wrong, are a huge complication to maintenance. Aircraft already cost a lot to maintain. So if a slightly bigger tail and wing twist make the design controllable enough, including with engine out, without having to deal with counter-rotating propellers, that's the design that gets chosen.

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  • $\begingroup$ Continental used to make reverse-rotation versions of a few of their engines, specifically for light twins. All that's required (assuming gear, rather than chain drive for the camshaft) is a different camshaft and starter motor (plus different propeller, of course). Cost was barely any more than the standard rotation version (lower volume cam and starter production). Errors are a much bigger factor than cost in why more reverse rotation engines don't exist. $\endgroup$
    – Zeiss Ikon
    Oct 6, 2022 at 11:09
  • $\begingroup$ @ZeissIkon, note that I did not mention manufacturing. It is indeed mainly a maintenance problem. $\endgroup$
    – Jan Hudec
    Oct 6, 2022 at 12:41
  • $\begingroup$ In fact I will just do it; roll back if objectionable. $\endgroup$ Oct 6, 2022 at 15:51
  • $\begingroup$ (Deleting lengthy explanation of tiny edit; see your inbox if wish to read it ;) ) $\endgroup$ Oct 6, 2022 at 16:39
  • $\begingroup$ (Or did you mean each engine was canted slightly to right (if props turn CW)? If so, better edit my edit. Sorry and thanks-- ) $\endgroup$ Oct 6, 2022 at 17:44

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