When looking at existing tiltrotors, most of the designs use two propellers, sometimes four. helicopters (and light planes), on the other hand, generally use only one propeller.

Have there ever been designs or proposals using a single rotor (or contra-rotative rotors on the same axis) instead? If so, what advantages and drawbacks did they have? If not, are there specific reasons why this configuration is always ignored?

The most obvious one would be that the engine would sit right on the cockpit, taking its place or at least blocking visibility. But we can imagine a tiltrotor UAV where this would not be an issue.

For the needs of this question, tail-sitters like the Convair XFY Pogo are not considered tiltrotors, as the entire craft changed orientation instead of only the engines.

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    $\begingroup$ How is this different from a helicopter and how would it counter torque? $\endgroup$ – AEhere supports Monica Jul 5 '19 at 12:10
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    $\begingroup$ I feel the current phrasing invites opinion-based answers $\endgroup$ – Federico Jul 5 '19 at 12:33
  • $\begingroup$ @AEhere The difference with a helicopter is that it could switch to horizontal flight, presumably using wings. Countering torque may be a significant drawback, and explaining why it would be difficult would probably have its place in an answer. $\endgroup$ – Eth Jul 5 '19 at 12:55
  • $\begingroup$ @Federico How should I rephrase it to make it less opinion-based? $\endgroup$ – Eth Jul 5 '19 at 12:55
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    $\begingroup$ @Eth I agree with Federico's point here because as it stands you are vaguely describing a non-existent aircraft and asking what the pros and cons would be in operation (never mind development), without defining the design to be at least believable. Therefore, each respondent can make their own interpretation of how it should look (counter-rotating helicopter with tilting? vectored thrust? with or without cyclic control on the rotor?) $\endgroup$ – AEhere supports Monica Jul 5 '19 at 13:04

Such an aircraft can't exist.

Helicopters don't work with a single rotor, they need at least two of them to counter torque. There are several popular designs, including conventional, coaxial, intermeshing and tandem rotors.

All tiltrotors operate as tandems in helicopter mode. It's the only setup where both rotors can be placed at the nose, tail or wings. In all other designs, at least one of the rotors needs to be mounted in the middle, above the center of gravity, where it can't be tilted without crashing it into something.

  • $\begingroup$ So it is impossible to use another scheme than tandem rotors for a tiltrotor? Also, at which point does a plane rotor becomes too big and needs to counter torque? $\endgroup$ – Eth Jul 8 '19 at 9:01
  • $\begingroup$ @Eth - Lift needs to be aligned with the center of gravity, otherwise the craft will tip over. If you can't mount a rotor in the middle, the only remaining option is to mount two or more rotors evenly spaced around the center - a tandem, quad- or hexacopter is born. Also, all rotors need counter torque, but in fixed wing aircraft there's rudder and ailerons for that. $\endgroup$ – Rainer P. Jul 8 '19 at 10:59
  • $\begingroup$ At which point does the propeller becomes too big for an aircraft to counter its torque? I suppose such tiltrotor could use tip-jets, counter-rotating propellers or maybe NOTAR, but it doesn't seem worth it. $\endgroup$ – Eth Jul 8 '19 at 11:30
  • $\begingroup$ @Eth - You still have the problem that a single propeller has to be mounted in the middle, where it can't be tilted. $\endgroup$ – Rainer P. Jul 8 '19 at 11:38

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