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In this question the Wikipedia article of the F-82 Twin Mustang is quoted. In the third paragraph of the article it says:

In this arrangement both propellers would turn upward as they approached the center wing, which in theory would have allowed better single-engine control.

There was a comment in the other question that questioned this statement:

according to that article, an outward-spinning engine configuration is better in theory. i don't see how this would be. in a single-engine scenario, wouldn't the working engine torque push the non-working engine side down?

A spinning propeller should also produce more lift on the wing section behind it as it is increasing the airflow over that section. Forgetting about the torque for a second, this would already push the other wing (with the failed engine) down. [EDIT: As answered in the comments, the lift generally has way more effect then the torque.]

So what exactly makes it more favorable to have a outward-spinning configuration in theory?

Also: Does this only apply to this aircraft, or is it valid in general?

Why does it say that the outward-spinning engine configuration is better in theory? What am I missing?

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  • $\begingroup$ This is actually answered in the answer to the question. The vortex generated by the propeller increases lift on the upswing side because the angle of attack is effectively increased. Because lift is generally larger than thrust and thrust larger than torque, this change is generally greater than the torque. YMMV, though. $\endgroup$
    – Ville Niemi
    Commented Mar 12, 2015 at 11:26
  • $\begingroup$ @Ville_Niemi Okay, so if I got that right you are saying that it doesn't really matter if there is any more torque as the lift has much more effect anyway. But why is an outward-spinning engine config better in theory then? O.o Thanks though, will edit the question a bit... $\endgroup$
    – Maverick283
    Commented Mar 12, 2015 at 11:35
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    $\begingroup$ the more i researched the other question, the more i think this "theory" is bunk. $\endgroup$
    – Erich
    Commented Mar 12, 2015 at 14:19
  • $\begingroup$ @erich Jupp, I really just want to double check that here. Would be interesting to get a more in depth explanation about this sentence as well $\endgroup$
    – Maverick283
    Commented Mar 12, 2015 at 14:25
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    $\begingroup$ also relevant to ask: better in theory in general? or better in theory for this particular design? $\endgroup$
    – Erich
    Commented Mar 14, 2015 at 13:22

2 Answers 2

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The theory is actually quite simple and straightforward. It should go like this:

In one-engine flight, the asymmetric prop position will pull the aircraft into a sideslip. The sideslip-induced rolling moment will then proceed to roll the aircraft such that the airplane rolls over the dead engine - something every multi-engine pilot learns to avoid early, because it will be hard to get the airplane back to level flight in this configuration.

If the single operating propeller will give the center wing an increased angle of attack due to the propeller swirl, and the outer wing a lower angle of attack, the prop produces a rolling moment which works against the sideslip-induced rolling moment, and should make it easier to avoid rolling over the dead engine. Engine torque will work against this moment, but the induced rolling moment of the wing is stronger, leaving a net moment which rolls the aircraft over the operating engine.

This should apply to all multi-engined aircraft. My reasoning why it didn't work for the F-82 can be read here.

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Honestly, I never cared about the minuscule amount of lift generated on the wing by the turning propeller. It's not the lift you need to worry about; it's the angle of attack. When I flew multi-engine prop planes, there was a reason that one of the engines was considered the critical engine (ie the one that most affects the handling of the aircraft) A descending propeller blade creates more thrust than an ascending one (p-factor). And since you want the thrust to be as close to the front or back of the center of gravity as possible to prevent yawing around the CG, you'd prefer to have that descending blade as close to the middle of the aircraft as possible. Torque can play a part with a big engine (like the old Merlins), but the p-factor usually tends to be the bigger issue. Peter is right, you do tend to roll into the failed engine, which can be pretty bad if you don't or worse can't stop it (angle of attack again). You're already doing bad things to the aerodynamics of the aircraft operating on one engine.

I don't know how the aerodynamics of this specific aircraft were, but asymmetrical thrust is asymmetrical thrust no matter the aircraft. The last thing you want to do is stall like that.

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    $\begingroup$ combined with peter's answer, the aircraft will both roll and yaw in the direction of the dead engine. in your opinion, the yaw is the more adverse factor? $\endgroup$
    – Erich
    Commented Mar 16, 2015 at 23:24
  • $\begingroup$ The yaw is the one that's going to hurt you. The roll is what will make the aircraft stall so that the yaw hurts you. The yaw will usually be more pronounced, but you'll also want to address the roll before it becomes too pronounced. I always taught that you provide a couple of degrees of bank into the operating engine to try to offset the roll. $\endgroup$
    – Shawn
    Commented Mar 17, 2015 at 15:05
  • $\begingroup$ To clarify just a little bit more, bank increases the load factor on the aircraft and increases the angle of attack (thus the stall speed) of the the high wing. Yaw is what turns a stall into a spin. Granted, you're stalling the wing that's still got the operating engine, but you're probably not far from stalling the other wing too. Plus, again with the off-center thrust, any type of stall will be bad. $\endgroup$
    – Shawn
    Commented Mar 17, 2015 at 15:11
  • $\begingroup$ I used to teach in a Piper Tomahawk, which had propellers that spun in the same direction, so it had a critical engine. There was a very noticeable difference between shutting down a left engine vs a right engine. I also flew a Beech Duchess, which had counter-rotating props. There wasn't a lot of difference between either engine, though the yaw was still very pronounced. $\endgroup$
    – Shawn
    Commented Mar 17, 2015 at 15:14
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    $\begingroup$ And lastly, I flew a KingAir (and its variants), which had some of the best single-engine performance of any prop-driven aircraft I've ever flown. Granted, it had all sorts of systems to help with an engine failure, but the PT-6s on that thing are fantastic engines. You could lose an engine coming off the runway and barely notice except for all the red lights that start flashing. :-) $\endgroup$
    – Shawn
    Commented Mar 17, 2015 at 15:19

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