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tip jet

Some helicopters use "tip jet" on the rotor blades tips such as the McDonnell XH-20 Little Henry and Hiller YH-32 Hornet.

How do they cancel the rotor torque (often canceled by the tail rotor)?

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    $\begingroup$ There is a small amount of torque reaction, but it's in the opposite direction, with the rotor, and it's simply from friction drag of the hub bearings - the rotor is trying the pull the body around with it, however small. In a hover in dead still air, bearing drag may be enough to cause a very slow yaw in the direction of the rotor's rotation, but there is usually enough air blowing past the vertical rudder surfaces just from downwash to cancel it out. Of the designer can impart a passive compensating moment by tilting the vertical surfaces slightly. $\endgroup$
    – John K
    Commented Mar 24, 2020 at 17:15
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    $\begingroup$ What torque are you referring to? $\endgroup$
    – Harper - Reinstate Monica
    Commented Mar 25, 2020 at 7:00
  • $\begingroup$ Oh that just looks like too much fun... $\endgroup$
    – T.J. Crowder
    Commented Mar 26, 2020 at 16:35

4 Answers 4

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This is a basic physics question, involving Newton's third law of motion (For every action, there is an equal and opposite reaction.) When a centrally mounted engine applies force to turn the rotor, the equal and opposite reaction creates torque on the fuselage. With a tip jet, the force is applied by the jet shooting its exhaust perpendicular to the blade, so the reaction is that the blade moves in the opposite direction.

So the answer is that tip jets don't CANCEL the torque, they simply don't create the torque in the first place.

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    $\begingroup$ Actually, they DO create torque, but in the opposite direction, due to friction within the bearings. Several orders of magnitude smaller than with a 'traditional' helicopter design. $\endgroup$
    – MikeB
    Commented Mar 25, 2020 at 10:39
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    $\begingroup$ @MikeBrockington yes, but more relevant than bearing friction is probably that the rotor downwash has a significant vorticity component, which will tend to swirl the fuselage with it. $\endgroup$
    – leftaroundabout
    Commented Mar 25, 2020 at 13:18
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    $\begingroup$ @Mike Brockington: Sure, but since this is a physics-based answer, I'm assuming an ideal frictionless rotor bearing :-) $\endgroup$
    – jamesqf
    Commented Mar 25, 2020 at 17:15
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    $\begingroup$ @jamesqf I understand that you want to talk about spherical chickens, but this forum is Aviation, not Physics, so the question remains of how does this design of helicopter control its direction - if it is facing North, and wants to fly South, how does it achieve this? $\endgroup$
    – MikeB
    Commented Mar 27, 2020 at 10:15
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    $\begingroup$ For the small amount of friction & downwash torque, would the tail perhaps have a control surface that tilts left and right? It would make sense for yaw control also. $\endgroup$
    – jpa
    Commented Mar 27, 2020 at 12:55
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Think of the aircraft engine as one isolated system and the rotor as another isolated system. In its simplest terms, torque is the force required to move mass in a circular motion. Torque is caused by the engine providing power to the rotor shaft which moves the mass of the blades. The torque is the interaction of the stationary engine trying to move the stationary rotor blades in a circular motion. If you disconnect the engine from the rotor (for instance via a clutch), you eliminate the torque. The rotor is allowed to spin freely.

Tip jets remove the torque being supplied by the engine via the shaft because the engine is no longer providing power to the shaft. The reactive force of the jets is moving the mass of the isolated system of the rotor. The rotors mass is being moved without the need to receive power input from another outside system moving it in a circular motion.

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    $\begingroup$ This is true even with tip-jet rotors driven by air compressed by an engine and blown up the rotor shaft, BTW. Germany flew at least one such helicopter during WWII. $\endgroup$
    – Zeiss Ikon
    Commented Mar 24, 2020 at 17:29
  • $\begingroup$ @ZeissIkon I think you just described a lawn-sprinkler :-) . $\endgroup$
    – Carl Witthoft
    Commented Mar 27, 2020 at 12:27
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    $\begingroup$ @CarlWitthoft Never seen a lawn sprinkler with an internal engine pumping ambient water, but I get your drift... ;) $\endgroup$
    – Zeiss Ikon
    Commented Mar 27, 2020 at 12:29
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Think of 2 scenarios: in one you are holding a rocket shaped projectile that is inert and you throw this projectile using your arm. In the second scenario you hold a real rocket, which is then launched.

In the first scenario you throw the projectile, and you have to counter the twisting motion of your arm because your arm is imparting a force. In the second scenario you don't have to take any action because the rocket does the work of moving the projectile instead of your arm.

A regular helicopter is like the first scenario, the engine pushes on the rotor, so the rotors push back. A tip jet is like the second scenario as the tips push themselves.

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Ejection of the tipjet exhaust streams in different directions creates a reactive torque on the rotor. This causes the rotor to spin up until aerodynamic drag exactly opposes the jet reaction. So in a way you can say that tipjet helicopters cancel the rotor torque through aerodynamic drag.

But no torque is applied to the rotor through the rotor hub. Its bearings may create a tiny amount through friction, but that is negligible.

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