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According to Wikipedia, on the Douglas DC-4E...

[...] the four 1,450 hp (1,080 kW) Pratt & Whitney R-2180-A Twin Hornet 14-cylinder radials were all mounted with noticeable toe-out, particularly the outer pair.

(Emphasis added)

Why would the engines be mounted toe-out, instead of straight in line with the direction of flight? Is this practice common today?

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    $\begingroup$ The first thought that comes to my mind is a better thrust line in an engine out situation. However, toe-out would be counter productive in such a scenario, I should think. Technical specs seem to show 2.5° and 3° toe-out for the inboard and outboard engines respectively. $\endgroup$
    – J W
    Dec 14, 2016 at 15:33
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    $\begingroup$ I think we posted at the same time! $\endgroup$
    – Dave
    Dec 14, 2016 at 15:38

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According to this source

The engines were toed out slightly to improve engine-out handling; three tailfins were used to ensure that the aircraft could keep flying more or less straight if both engines on one wing went down.

I belive it was done to keep the thrust more equal in the event of an outboard engine loss, i.e. they are inline with the direction of flight when an engine loss occurs. Keep in mind that at the time these planes were designed engine failure was a [very]common thing and the mentality was a bit different than it is now. If a little bit of toe out mounting took a speed toll but made the aircraft far more controlable in the event of an engine loss it was considered worth while.

In modern multi-engine propellor aircraft counter rotating propellors (left side and right side props spin oposite directions) are generally employed to fix this problem and avoid the critical engine issue. I do belive that some single engine aircraft still do employ a slightly canted engine position for this reason.

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    $\begingroup$ But why does the toe-out improve such engine out handling? $\endgroup$
    – J W
    Dec 14, 2016 at 16:04
  • $\begingroup$ Puts the thrust line closer to the center of mass $\endgroup$
    – pericynthion
    Dec 14, 2016 at 16:35
  • $\begingroup$ Which way would the engine be canted in a single engine plane to combat asymmetric thrust in an engine-out situation? $\endgroup$
    – FreeMan
    Dec 14, 2016 at 16:46
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    $\begingroup$ Is there any thrust in single engine engine-out situation? $\endgroup$
    – vasin1987
    Dec 14, 2016 at 16:55
  • $\begingroup$ @vasin1987: No, but now the turning tendencies normally present from torque, P-factor, etc. are gone. $\endgroup$
    – Fred Larson
    Dec 14, 2016 at 18:52

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