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I stumbled upon a very unusual Russian amateur built ultralight:

One engine, two propellers configuration

My first impression is that it adds another useless layer of complexity and moreover it may involve some safety issue: I particularly think of the entry into a in case of one propeller failure.

What is the rationale behind it? The pros and cons?

enter image description here

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    $\begingroup$ I think Wilbur and Orville did this first. $\endgroup$
    – Fred Larson
    Dec 8, 2014 at 16:42

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It looks as if they rotate in opposite directions. This eliminates prop torque, which should improve handling, especially at low speeds.

On the other hand, you are right with your impression that it adds complexity, but at least it is not completely useless.

Asymmetric prop failure is indeed an issue and will markedly reduce handling qualities, and for that reason both props are close together, so if one is left, the yawing moment should be manageable if the craft flies fast enough.

Putting a single prop in the center would present it the messy flow coming from the unfaired cockpit and engine, so placing them left and right should improve inflow conditions. They are still close enough so the tail is in the prop wake and will have improved effectiveness at slow speed and on the ground. On the other hand, the outriggers will create additional drag, and it is hard to say if this arrangement will have performance benefits.

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  • $\begingroup$ Does the number of propeller have something to do with efficiency too? $\endgroup$
    – menjaraz
    Dec 8, 2014 at 9:37
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    $\begingroup$ Also: 2 small pusher props rather than 1 large pusher = (1) less vertical space needed in airframe, (2) props in cleaner air? $\endgroup$
    – RedGrittyBrick
    Dec 8, 2014 at 9:59
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    $\begingroup$ To me it looks like they are driven off the same pulley and neither of the belts crosses between the motor and the prop, So to me it looks like they rotate in the same direction. $\endgroup$
    – ratchet freak
    Dec 8, 2014 at 10:40
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    $\begingroup$ @ratchetfreak: Yes, when looking again I am not so sure anymore. But driving them in the same direction would give away the main benefit of this arrangement. $\endgroup$
    – Peter Kämpf
    Dec 8, 2014 at 11:21
  • $\begingroup$ There's also the point that a propellor is hugely unlikely to fail compared to an engine failure in a two-engine-two-prop design $\endgroup$
    – Jon Story
    Dec 9, 2014 at 14:17
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Basically it allows the benefits of multiple propellers with a single engine. At low powers increasing the number of engines decreases power-to-mass ratio. Additionally, the builder might simply have had only one engine available or affordable.

Removal of prop torque, as mentioned in the accepted answer, is a clear benefit. Probably relevant for this particular ultralight, having larger propeller area at limited height. Generally having larger propeller area for power improves efficiency and reduces noise. In the design in the image the height is especially limited. Also not having the propellers in the same line with the body probably improves air flow slightly.

I think the limited height available in the design is the actual main reason.

It is slightly more mechanically complex than single prop, which increases weight and chance of failure.

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  • $\begingroup$ From the title I thought we'd be looking at pictures comparing a Corsair and something with contras; height requirements. The duel props on this one strike me as the way to get more torque out of the engine. $\endgroup$
    – Mazura
    Dec 8, 2014 at 12:46
  • $\begingroup$ @Mazura Good point. I already mentioned having only one engine available as possible reason, designers engine choices might have been limited in more ways than just the number of engines. $\endgroup$
    – Ville Niemi
    Dec 9, 2014 at 7:43
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While the currently accepted answer would be valid in terms of eliminating prop torque had that been the setup, a close look at the photograph shows both propellers rotating in the same direction (counterclockwise from the point of view of the viewer) - the pitch of each blade is clearly visible.

I would suggest that in this application, the two propellers are used to give the benefit of an equivalent larger overall propeller (greater propeller area), without the additional ground clearance or vertical space requirements - a larger propeller puts through more power for the same engine RPM.

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