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I watched this video of an RAF Chinook taking off vertically and transitioning to forward flight.

I'm curious if the method for increasing lift to move forward, controlled by the cyclic in a single rotor if I'm not mistaken, is different when you have two rotors along the length of the aircraft. My untrained assumption is that to lift the rear of the helicopter and start moving forward, you would increase power or the collective on only the rear rotor.

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    $\begingroup$ May just be pitch changes on both rotors $\endgroup$
    – Pheric
    Apr 8, 2020 at 22:08

2 Answers 2

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The cyclic (dubbed control stick for the CH-47) takes care of that, not separate collectives (there's only one collective for both rotors per seat).

enter image description here

For forward flight, the pilot moves the control stick forward; the thrust of the aft rotary wing is increased while the thrust of the forward rotary wing is decreased, causing the helicopter to move forward. Since there is a large range of longitudinal control, high forward speed and extremes in center-of-gravity travel are inherent.

Source: CH-47 familiarization manual (see pages 55-56; PDF; 40 MB)

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  • $\begingroup$ Very good doc(pdf) but where is the citation(pages ?) $\endgroup$
    – L'aviateur
    Apr 10, 2020 at 8:08
  • $\begingroup$ From pilot point of view, it is cyclic, but mechanically it is differential collective and the description clearly says so. $\endgroup$
    – Jan Hudec
    Apr 10, 2020 at 13:44
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First

-collective variation = intensity

-cyclic variation = direction

The CH-47 Chinook is a heavy maneuver and assault helicopter manufactured by Boeing. Its two tandem rotors (set of horizontal rotors mounted one in front of the other) are counter-rotating and are controlled synchronously in stationary and in translation, thus their torque effect is canceled out by their opposite direction of rotation. It is equipped with two turboshaft engines with a power of 2,850 hp to 3,750 hp,(help to moving forward) with a maximum weight of 18.5 tonnes. Photo below we see that the rear rotor is raised relative to the front rotor. chinook

an asymmetry of the lift between the 2 rotors implies a tilting along its pitch axis and causes the device in forward flight.the tandem rotors are very bulky and not very maneuverable, which constitutes their main defect. Note also that the lengthening of the fuselage makes them more sensitive to strong wind

So it's mostly like an helicopter with one rotor( cyclic variation to move forward) but a few different.your hypothesis is not really right because the two rotor are connected and are controlled synchronously in stationary and in translation(same speed) The main MGB gearbox is interconnected with the shafts of the two turbine engines. A seven-section shaft connects the main gearbox to the front gearbox, and a two-section shaft connects the main gearbox to the rear gearbox. The oil cooling fan is driven by the rear gearbox.

Source:https://www.google.com/url?sa=t&source=web&rct=j&url=https://tel.archives-ouvertes.fr/tel-00709015/document&ved=2ahUKEwjH3qT05droAhVM4YUKHfc5DtAQFjAEegQIBhAC&usg=AOvVaw1SPfruh9nYtNK5IvBNd6v4&cshid=1586416929818

https://www.google.com/url?sa=t&source=web&rct=j&url=https://www.lavionnaire.fr/HelicoAnticouple.php&ved=2ahUKEwjH3qT05droAhVM4YUKHfc5DtAQFjAGegQIARAB&usg=AOvVaw1YZG69MlCkIC6sLwiWFvb8&cshid=1586416929818

Longitudinal control of tandem rotor helicopters is accomplished by differential thrust of the forward and aft rotors creating pitch attitude changes of the fuselage about its lateral axis.

Lateral control is accomplished by the lateral tilt of both rotor discs, which causes the fuselage to roll about its longitudinal axis

chinook

Directional control is accomplished by differential lateral tilting of both rotor discs, which causes the aircraft to turn about its vertical axis.

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  • $\begingroup$ Helicopters don't tilt the rotor. The cyclic control varies lift of blades in different positions, which shifts the centre of lift sideways. Since the blades are free to flap up and down, the plane of rotation does tilt a little, but that is an effect, not mechanism, and the shaft does not. $\endgroup$
    – Jan Hudec
    Apr 10, 2020 at 13:55
  • $\begingroup$ Wrong: The pilot has three commands: The lifter manipulated by the feet which controls the yaw axis, i.e. the pitch of the anti-torque rotor The broom or cyclic handle which controls the axes of pitch and roll Finally the general or collective pitch located to the left of the pilot who controls the engine and the pitch of the blades. Flight controls and their actions The principle of anti torque. $\endgroup$
    – L'aviateur
    Apr 10, 2020 at 14:12
  • $\begingroup$ For the cyclic the tray is tilted! $\endgroup$
    – L'aviateur
    Apr 10, 2020 at 14:13
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    $\begingroup$ That image is wrong! It correctly shows the swash-plate (indicated by the dotted red arrows), but not it's effect. It does not tilt the rotor head. Rather the scissor links change pitch of the blades. Both cyclic and collective work (on single-rotor helicopter) by moving the same swash-plate. Collective moves it up or down as a whole, cyclic tilts it. For correct explanation, refer to wikipedia. $\endgroup$
    – Jan Hudec
    Apr 10, 2020 at 19:16
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    $\begingroup$ Now of course in tandem-rotor helicopter the pilot has the same controls, but they are connected differently, since forward pitch command instead of tilting both swash plates forward instead raises the aft and lowers the forward, while side pitch still works by tilting both swash plates sideways. But the rotor heads don't tilt; the blades change pitch—and flap somewhat, so the plane of rotation does change, but as a secondary effect due to the helicopter lagging behind the rotor. $\endgroup$
    – Jan Hudec
    Apr 10, 2020 at 19:18

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