Helicopters are under a constant torque due to the action-and-reaction forces caused by the rotation of the rotor(s). In the tail rotor and tandem rotors configuration, the airframe is experiencing tension on one side and compression on the other. A weak airframe would be twisted by the strong torque produced by the rotors.

Are opposite sides of a helicopter airframe reinforced differently do due with the difference in stress?

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    $\begingroup$ This is an unusual, yet very interesting question! $\endgroup$ – FreeMan Feb 24 '17 at 21:31
  • $\begingroup$ Looking at the pictures of a Bell 47 I can't see any asymmetric structure. $\endgroup$ – Ron Beyer Feb 24 '17 at 22:29
  • $\begingroup$ @TomMcW Ah, hmmm. I'll delete, fix and re-add. Thanks. $\endgroup$ – Simon Feb 25 '17 at 19:42
  • $\begingroup$ I can only speak directly on 6 models. R22, B206, Westland Wessex, Puma, Chinook and Schwiezer 269. All of those are strengthened with the normal stringers, ribs and panels on both sides. The power output of a typical tail rotor is in the order of 50-100 hp. This doesn't need special strengthening but this is probably a good enough sample of different types to conclude that most, if not all, do not use special strengthening to counter torque. $\endgroup$ – Simon Feb 25 '17 at 19:42

As far as i know, the helicopter airframes are not reinforced asymmetrically due to this reason. Note that the helicopter has to yaw in both ways- so there is no point in reinforcing only one side.

The helicopter airframes are not weak- they are designed to withstand the full envelope of expected loads. Also, it is easier to design and manufacture a symmetric helicopter structure.

That said, I've personally seen at least one case where the torque has affected one side of the helicopter more than the other locally (especially in tail fin area), though the effects were not considered serious enough to warrant a design change.

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    $\begingroup$ But isn't that yawing is accomplished by reducing or increasing slightly the tail-rotor torque that balances the main-rotor torque? $\endgroup$ – DJohnM Feb 25 '17 at 2:56
  • $\begingroup$ @DJohnM In a hover, the forces are in balance. You can push a hovering helicopter around with your hands. $\endgroup$ – Simon Feb 25 '17 at 6:01
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    $\begingroup$ In balance, so the copter isn't turning, but the two forces are trying to bend it? $\endgroup$ – DJohnM Feb 25 '17 at 6:48
  • $\begingroup$ @DJohnM My point is that we are not talking about massive forces trying to tear the machine apart. A typical light helicopter, with the weakest fuselage, is expending perhaps 10-15hp from the tail rotor at the end of a long arm. Asymmetric strengthening is simply not required. $\endgroup$ – Simon Feb 25 '17 at 9:22

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