How does the Chinook fly?

Answer 1

I will answer this in simple terms and not include the AFCS (Automatic Flight Control System) that is required to smooth out inherent problems with the tandem rotor design.

The obvious starting point is the lack of Tail Rotor that conventional helicopters have. With two counter rotating rotor discs, each cancels out the torque of the other, and so no requirement of a tail rotor to keep the aircraft from spinning uncontrollably.

The pilot inputs are the same (Cyclic Stick, Collective, Yaw Pedals) and produce the same movement of the aircraft but are translated to the rotor discs in a complicated way.

Collective, or thrust as it can be known, is simple. The collective lever is pulled up and both rotor heads are fed with a mechanical linkage from the lever to pitch all blades to produce equal lift.

Roll is simple too. As the cyclic stick is moved left or right, both rotor discs will tilt in the same direction to roll the aircraft to the desired side.

Yaw is linked with Roll in the case of the Chinook, and as such both discs will tilt but in opposing directions. I.e. right pedal forward will cause the forward disc to tilt right, and the aft disc to tilt left, hence rotating the aircraft about the central vertical axis. It is possible to pivot the aircraft about either the forward or aft rotor heads by using a combination of pedal and left or right cyclic (but that requires a physics explanation)

Pitch is the most different to a conventional helicopter. Moving the cyclic stick forwards or backwards does not cause the rotor heads to both tilt forward or backward as you might imagine. Pitch outputs are linked to the collective outputs, in which I mean one rotor disc has more collective pitch applied and the other has less. I.e. cyclic stick forward causes the forward rotor disc to decrease in collective pitch (therefore dropping the nose of the aircraft down) and the aft disc to increase in collective pitch (therefore lifting the tail up) which results in pitch up or down.

That is the basic principle of Chinook flight theory. In more depth, it is necessary to say that although there is a sophisticated AFCS system, the aircraft can fly without it and does, and all flying controls are mechanically (hydraulically boosted) linked to the rotor heads.

Because of the complicated way the pilots inputs must be output to the rotor heads, the channels (Pitch/Roll/Yaw/Collective) are mixed mechanically. So as Roll and Yaw both have a tilting effects on the rotor discs, they are mixed to give one common output channel; and Collective and Pitch share the same effect on the discs and so are also mixed to give one output channel.

Further to all of this, a few facts about how the Chinook flies:

Both engines have gearboxes that attach to a central Combining Transmission, which then feeds the engine torque forward and backwards to a Forward Transmission and Aft Transmission, which each spinning the heads above them.

The Transmissions (or gearboxes) are linked therefore and in sync, so the counter-rotating blades inter twine each other and will never collide.

A common misconception is that one engine powers one head and the second the other. Both engines drive a central transmission, and it is possible to fly on one engine (although with enough weight, the aircraft would lose altitude or alternatively the remaining engine overheat)

Hydraulic pressure is provided by pumps driven by the forward and aft transmissions. Two flight hydrualic systems therefore. Again, it is possible that if one fails, the other will suffice. There is a third hydraulic system, the Utility Hydraulic system, which, should both Flight Hydraulic systems fail, can transfer its pressure into the Flight Hyd system.

All pilot inputs are hydrualically boosted via these Flight Hyd systems. The Util Hyds system powers things such as the rear ramp, centre cargo hook opening and closing, Power Steering, steering locks, hoist and winch, and engine starter motors.

The AFCS system allows for Pitch, Roll, and Yaw stability and eases the workload on the pilot to be constantly correcting aircraft attitude. It also allows for Height Hold (Radar Altimeter or Barometric Alitimeter), Heading Hold, Airspeed Hold, Controlled Turns, and more comlicated tasks such as levelling the fuselage during hover and forward flight, amongst other things.

I could go into depth and on forever, but the basics are easy to grasp once you get to know them.

Chinook Engineer.