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I am now reading books about helicopter aerodynamics. And I have one question about blades flapping. If you push forward on the cyclic, the swashplate will tilt forward and will set the maximum pitch angle on the retreating blade, and this blade will flap up while going over the tail of the helicopter, and this is the reason for the rotor tilt forward in forward flight.

But in the dissymmetry of lift explanation, the advancing blade is making more lift, so the advancing blade will flap up while going over the nose, and that is opposite of the previous flapping movement, and causing the rotor to tilt backward. So what is the true path of rotor blade in forward flight?

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The dominating force on the blade is centrifugal, by huge margin, so it really always moves in a plane, with the changes in lift just tilting that plane relatively slowly to the rotation.

The reason for flapping hinges or flex element or teetering is to allow the fuselage to follow the rotor tilt at its own pace without overstressing the mount.

Now both effects you mention are true at the same time, and they cancel each other out. As you push the cyclic forward, lift is increased on the retreating side, which tilts the rotor forward (with the fuselage following) and the helicopter accelerates. That increases the lift on the advancing side, and that in turn stops the forward pitching motion.

This is good for controllability, because it means the helicopter will settle on some forward speed corresponding to the displacement of the cyclic control rather than keep pitching forward.


The ‘flapping’ term is perhaps a bit misleading. It's better to view the rotor as the primary component, a gyroscope that maintains its orientation like any other free gyroscope due to conservation of angular momentum, and any changes in lift, whether caused by cyclic input or by speed differential between the sides, cause it to precess.

The fuselage is just hanging underneath. In some helicopters there is some torque carried by the hub, but in teetering rotor helicopters, and that's common design in light helicopters, it's literally just hanging there like a pendulum.

Now the swashplate is designed so that when the rotor is not in line with it, the blade pitch is manipulated so that the rotor tries to re-align itself with the swashplate. This design ensures that the rotor won't continue to precess until it hits the helicopter even when there is no torque transferred by the hub.

Except you have to add the effect of differential lift, and that means during forward flight, the rotor will actually spin tilted somewhat aft relative to the swashplate, with the difference increasing with forward speed.

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  • $\begingroup$ So you want to say that initally main rotor tilts in the directon of flight. and blade flaps up over the tail, but while accelerating, advancing side is making more lift, and blade flaping movement change, but maintain that tilted position based on cyclic displacement ? $\endgroup$
    – cirica
    Jun 25, 2023 at 18:57
  • $\begingroup$ @CiricAleksandar well, yes. When the lift changes due to cyclic and speed difference cancel out, the rotor will maintain its orientation due to conservation of angular momentum like any other gyroscope. $\endgroup$
    – Jan Hudec
    Jun 25, 2023 at 20:15
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So what is the true path of rotor blade in forward flight?

Simply the sum of the two movements that you described.

  1. The rotor must be tilted forward so that its thrust possess an horizontal component which pushes it forward; the more the cyclic is pushed forward, the more the rotor tilts forward;
  2. anyway, due to asymmetry of the lift, the rotor tends to tilt backward, partially counteracting the previous effect; the faster the helicopter flights, the more the rotor tilts backward.

So if you push the cyclic forward from hovering: the rotor plane tilts forward $\rightarrow$ thrust tilts as well forward generating a forward thrust $\rightarrow$ helicopter accelerates forward $\rightarrow$ due to the asymmetry of the lift and the presence of the flapping hinge, the rotor begins to tilt backward partially compensating for the forward tilt due to the cyclic input $\rightarrow$ acceleration decreases until the equilibrium of 1. and 2. is reached $\rightarrow$ acceleration has decreased to zero $\rightarrow$ helicopter keeps on moving forward at a constant speed.

Obviously if you want to accelerate further forward the effect 1. must overcome the 2. Anyway at a certain point you can no more push the cyclic forward: you've reached the top speed of the helicopter.


Short typo correction:

If you push forward cyclic, the swashplate will tilt forward

if you push the cyclic forward the swashplate will tilt to the right (considering a helicopter with a main rotor rotating counterclockwise when seen from above)

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  • $\begingroup$ Thank you for this nice explenation. So you want to say that initally main rotor tilts in the directon of flight. and blade flaps up over the tail, but while accelerating, advancing side is making more lift, and blade flaping movement change, but maintain that tilted position based on cyclic displacement ? Are you sure for about swashplate tilting, because of the advance angle on pitch link ? $\endgroup$
    – cirica
    Jun 25, 2023 at 19:54
  • $\begingroup$ Actually if you push cyclic forward, the swashplate will tilt forward. The lift decreases on the right, so the blades reach the lowest point forward, but that's because the linkages ride the swashplate ahead of the blades. This is necessary for introducing correct negative feedback in the tilting motion—the linkages are rigged so the rotor aligns itself with the swashplate, any other configuration would result in it wobbling around. $\endgroup$
    – Jan Hudec
    Jun 25, 2023 at 20:06
  • $\begingroup$ @CiricAleksandar: yep, you have understood correctly. Obviously "initially main rotor" is made "tilt in the direction of flight" via the longitudinal cycling input. $\endgroup$
    – sophit
    Jun 25, 2023 at 20:38
  • $\begingroup$ So if I u understood right. Pushing cyclic forward cause blade to flap up over the tail because of the increasing lift, and this also produce new plane of rotation based on new swashplate position. And then after change of flaping montion, plane of rotation is tilted forward because of the swashplate position, and flapping is like it is discribed in the dissimetry of lift, and also tilted a bit to the retreating side to compensate inflow role. $\endgroup$
    – cirica
    Jun 25, 2023 at 20:44
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    $\begingroup$ @CiricAleksandar: That really takes up more than a comment to answer, I'd suggest you to open a new question if you want. $\endgroup$
    – sophit
    Jun 26, 2023 at 20:09

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