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Tandem-rotor helicopters, such as the Boeing CH-46 Sea Knight and CH-47 Chinook, are helicopters with rotors distant by more than 10 m.

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Photo by Cpl. Ryan Carpenter on SNAFU

It seems unlikely a single rotor could sustain the force moment created by the failure of the other rotor. On the other hand it seems to me it would be difficult to have one rotor in autorotation and one powered.

If this is possible, how would the pilot fly the helicopter? If it's not possible, why?

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  • $\begingroup$ If its a structural failure, survival is unlikely. If its a gearbox failure, as the answer says, you can probably autorotate down. $\endgroup$
    – Ron Beyer
    Oct 2, 2016 at 1:45
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    $\begingroup$ @ROn My Ops O (circa 1982) is one of the few people I know who auto'd a 46 down, and lived, when the aircraft broke in half due to such a failure. The two up front survived, nobody else did. $\endgroup$
    – KorvinStarmast
    Oct 3, 2016 at 2:07
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    $\begingroup$ @KorvinStarmast I'd love to read the accident report on that one. Either way I meant rotor structural failure, not airframe. $\endgroup$
    – Ron Beyer
    Oct 3, 2016 at 2:25
  • $\begingroup$ @RonBeyer Possibly the only survivors of an inflight blade loss I was unable to find the mishap my OPSO was in, IIRC it happened during the mid/late 70's). $\endgroup$
    – KorvinStarmast
    Oct 3, 2016 at 12:16

1 Answer 1

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It's been a few decades since I crewed CH-46s, but the situation you're describing is called a de-sync and it's a catastrophic failure. The forward and aft rotors are linked by a driveshaft that drives the forward transmission and synchronizes the rotors. In a de-sync the intermeshed rotors will collide and the aircraft will tear itself to pieces.

Also, setting aside for a moment the consequences of a de-sync, the flight controls for both rotors are linked so it would be impossible to flatten the pitch to autorotate just the front rotor head. I suppose it would be mathematically possible to keep the rotors in sync and match the rate of deterioration in the rotor speed, but the odds would be extremely low, and the chances of recovering from any altitude beyond a few feet would be near nil.

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  • $\begingroup$ @ymb1 It is slightly higher, but not enough to clear each other as the blades flex. The rear rotor is raised to prevent it from striking the fuselage when the cyclic is tiled forward. $\endgroup$
    – TomMcW
    Oct 2, 2016 at 18:18

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