How does Helicopter Cyclic/Collective Mixing work?

[Raulboy]

Hey all,

 

I am attempting to build a lego technic helicopter with full cyclic/collective/directional control, but I'm running into a snag-

 

Cyclic and collective mixing for both airframes was glossed over when I went through flight school, and there is no reference material that I've found that explains how it works. I'm not sure I could use an explanation for my current airframe from our MTPs due to its complexity, but if I don't get an answer soon I guess I'll have to.

 

Does anyone have a down-to-earth explanation for cyclic/collective mixing, and diagrams or videos to go with it?

 

I'm familiar with everything else- aerodynamics, everything above the mixing assembly etc.

[UH60L-IP]

Can I be the first to ask, "What are you talking about?"

 

You went through flight school, are now trying to build a helicopter out of Legos, your current airframe (whatever that may be) is too complex and you therefore need a down to earth explanation of cyclic/collective mixing.

 

Cyclic mixing - changes the pitch of the blades in a cycle

Collective mixing - changes the pitch of the blades collectively (at the same time)

 

Is that down to earth enough for you?

 

Hope it helps.

[iChris]

Hey all,

 

I am attempting to build a lego technic helicopter with full cyclic/collective/directional control, but I'm running into a snag-

 

Cyclic and collective mixing for both airframes was glossed over when I went through flight school, and there is no reference material that I've found that explains how it works. I'm not sure I could use an explanation for my current airframe from our MTPs due to its complexity, but if I don't get an answer soon I guess I'll have to.

 

Does anyone have a down-to-earth explanation for cyclic/collective mixing, and diagrams or videos to go with it?

 

I'm familiar with everything else- aerodynamics, everything above the mixing assembly etc.

 

A simple mixing box maintains the cyclic angle by mixing collective input with right lateral, left lateral, and fore/aft inputs into a summing bellcrank.

 

 

The disk angle is maintained constant as the collective is moved, as you can see from the red summing bellcrank, the right lateral, left lateral, and fore/aft move simultaneously with collective movement.

 

Edited March 18, 2014 by iChris

[Raulboy]

 

A simple mixing box maintains the cyclic angle by mixing collective input with right lateral, left lateral, and fore/aft inputs into a summing bellcrank.

 

 

The disk angle is maintained constant as the collective is moved, as you can see from the red summing bellcrank, the right lateral, left lateral, and fore/aft move simultaneously with collective movement.

 

Thank you; that's what I was looking for. It's kind of difficult to wrap the mind around though... Why doesn't the collective affect how much cyclic displacement is available or change the ratio of cyclic input?

[iChris]

 

Why doesn't the collective affect how much cyclic displacement is available or change the ratio of cyclic input?

 

The mixer prevents the collective input from affecting the cyclic input as they move together or in relation to each other. With the overall arrangement of various types of push-pull tubes, adjustable rod-ends, pivot points, and angled bellcranks, almost any range of movement is obtainable.

 

When the collective is pulled up, the swashplate will move upward and the cyclic control movement (A, B, and C) that was already present will move the same amount in the same direction. This will move the swashplate up parallel to its original position. As the collective is lowered, the swashplate will lower, staying parallel to its prior position. In both cases the ratio is maintained.

 

Maintenance manuals have complete sections on cyclic and collective adjustment and rigging.

 

Edited March 19, 2014 by iChris

[Eric Hunt]

There is more to it - when you pull the collective up, there is also a forward cyclic input.

 

Try it with the machine shut down - take off all frictions, lightly feel the cyclic as you raise the lever, feel it move forward.

 

Same with hydraulics off in a 206, collective up, cyclic forward, lever down, cyclic back. One explanation I heard was that if you find your hydraulics have failed and want to pull pitch to get out of there, it helps you get some forward speed as well. Probably horsefeathers, but it fits. However, this cross-coupling makes it tricky to hover hydraulics-off, so most schools and manuals advise to make a shallow running landing to minimise any control inputs and cross-coupling from the mixing system.

[Raulboy]

There is more to it - when you pull the collective up, there is also a forward cyclic input.

 

Try it with the machine shut down - take off all frictions, lightly feel the cyclic as you raise the lever, feel it move forward.

 

Same with hydraulics off in a 206, collective up, cyclic forward, lever down, cyclic back. One explanation I heard was that if you find your hydraulics have failed and want to pull pitch to get out of there, it helps you get some forward speed as well. Probably horsefeathers, but it fits. However, this cross-coupling makes it tricky to hover hydraulics-off, so most schools and manuals advise to make a shallow running landing to minimise any control inputs and cross-coupling from the mixing system.

Good to know! We never did hydraulics off in the TH-67(206), unfortunately. Also, the Apache has a hydromechanical flight control system, so we can't move the controls without the APU running or the rotors turning.

[iChris]

On 3/22/2014 at 5:20 PM, Eric Hunt said:

There is more to it - when you pull the collective up, there is also a forward cyclic input.

 

Try it with the machine shut down - take off all frictions, lightly feel the cyclic as you raise the lever, feel it move forward.

 

 

On 3/23/2014 at 1:27 AM, Raulboy said:

Good to know! We never did hydraulics off in the TH-67(206), unfortunately. Also, the Apache has a hydromechanical flight control system, so we can't move the controls without the APU running or the rotors turning.

 

That feedback into the cyclic channel is a result of how most flight control linkages attach to the swashplate. There are normally three control rods that manipulate the movement of the swashplate. The two lateral inputs are across from each other and create a pivot axis perpendicular to the single fore and aft input. When the aircraft is shut down on the ground or with rotors turning, without hydraulic servo assist, there is a small amount of feedback as the collective is moved up and down.

 

As the collective moves up, the summing bellcrank, right lateral, left lateral, and fore/aft rods move up simultaneously with collective movement. However, in the case of the fore/aft linkage, the initial movement acts in the path of least resistance by moving the friction free cyclic forward, since there is nothing opposing it. However, since the lateral inputs are opposite each other, they oppose (balance-out) similar lateral cyclic feedback. When the collective moves down, any friction or drag in the fore/aft linkage could initially cause a slight aft cyclic movement. However, in both cases, this mechanical feedback is of little or no consequence with hands on the controls.

 

When the hydraulic control boost system is operational it service as an irreversible hydraulic system, which isolates the pilot from feedback forces of the main rotor by the use of servos. When the hydraulics fail on an aircraft that normally requires hydraulics, both mechanical and aerodynamic flight loads combine to make control difficult if not impossible in some cases.

 

This applies to most light single engine helicopters.

 

 

 

 

 

 

Video: http://youtu.be/cVNBC9EDOcU

 

 

 

Clip Descriptions: 



 

Clip 1 Collective down; Cyclic moved fore & aft



 

Clip 2 Collective down; Cyclic moved laterally 



 

Clip 3 Collective moved up & down; Cyclic held in desired position



 

Clip 4 Collective moved up & down; Cyclic allowed to move freely with feedback



 

Clip 5 Collective moved up & down; Cyclic is again held in desired position to counter feedback

Edited February 23, 2020 by iChris

[Rotorhead84]

Can I be the first to ask, "What are you talking about?"

 

You went through flight school, are now trying to build a helicopter out of Legos, your current airframe (whatever that may be) is too complex and you therefore need a down to earth explanation of cyclic/collective mixing.

 

Cyclic mixing - changes the pitch of the blades in a cycle

Collective mixing - changes the pitch of the blades collectively (at the same time)

 

Is that down to earth enough for you?

 

Hope it helps.

 

That awkward moment when you try to look like a hardass on the internet and end up looking dumb instead.