# How does a helicopter phasing unit work?

I'm currently distance studying to become a B1.3 technician and the course material mentions a phasing unit in the control system, but the descriptions and images are very abstract and I'm having trouble understanding how it works. I've looked for other sources of information about the subject but have found next to none.

To my understanding the swashplate wants evenly distributed loads acting upon it, and the phasing unit somehow allows for this to happen. However, I don't understand what the phasing unit looks like, or its working principles, and to further confuse me; it can apparently be placed anywhere between the controls and the swashplate according to my coursebook.

Edit: I apologize to those who have responded; I should have uploaded an image from the start to be more clear.

• I’ve worked with quite a few different helicopters and have never seen anything called a phasing unit. Are you just asking about cyclic pitch?
– Jim
Mar 19, 2022 at 1:01
• Same here. Have written simulation physics models for about 20 helicopters, and have never heard of a phasing unit. Mar 19, 2022 at 1:16
• Is this the control mixing unit? aviation.stackexchange.com/questions/67322/… Mar 19, 2022 at 1:33
• There is a phenomenon called Phase Lag that is related to lead/lag hinge design, which has an effect similar to Precession Lag but varies at different angles less than 90 deg, whereas Precession Lag is always at 90 deg. It sounds like the machine in question has Phase Lag issues that require active compensation in the control linkage, and there is a controller with actuators of some sort in the linkage. Mar 19, 2022 at 4:26
• @JohnK Yes there is phase lag in the blade flapping and lead/lag, usually fixed by placing the swash plate actuators at appropriate positions. So what would the phase unit actuators actuate? If it’s a mixer unit then yeah, we know what we’re talking about Mar 19, 2022 at 6:43

As mentioned in the comments of the question, the function in question is normally performed by a control mixing unit (see this related question), also called control mixing or controller mixing. The principle is quite simple. Given the three actuators of your swashplate, you have to "map" the control inputs from the pilot's stick (so roll, pitch and collective inputs) to the three actuators given in your picture, such that the swashplate tilts or translates as requested.

Three examples:

• The simples case is, when the pilot pulls on his collective, then all three actuators have to move down.

• When the pilot gives a roll-right-input the desired output is that the swashplate tilts backward (seen in direction of flight). Therefore, in your diagramm actuator 1 has to move up, and actuator 2 and 3 have to move down a bit.

• When the pilot gives a pitch-forward-input, the desired output is that the swashplate tilts to the left (seen in direction of flight). Therefore, in your diagramm actuator 1 stays constant, actuator 2 moves up a bit and actuator 2 moves down a bit.

Additionally such a mixing unit also automatically mixes inputs for example by automatically applying a rudder input whenever the collective is pulled.

It seems to me, that you textbook differentiates between these two actions. Mixing (e.g. automatically applying yaw when collective is pulled) is defined as mixing, and "phasing" is the act of distributing the mixed inputs to the servos spaced 60° apart.

As a reference look at this diagram of the control linkages of a Sikorsky S76, and observer that the mixing is performed in the same step as the "phasing".

Personally I have never heard of this, although I am involved in computerized helicopter control quite a bit. I would put such a function under "mixing". My opinion is that normally one does not differentiate between these two parts.

• Your explanation makes a lot of sense. Thanks for your response. Your link just points to this page however, so I can't see the question you're referencing. One thing I'm not sure about is the phasing unit and mixing unit being the same. The book talks about the mixing unit as well, as a completely separate thing. Mar 19, 2022 at 15:11
• @Smith: I corrected the link, but it is just the one from the previous comment. Concerning the mixing unit, could you take a photo of that page too? Perhaps we can work out what exactly the difference is. Mar 19, 2022 at 15:18
• Sure. Here are the next two pages in the book The mixing unit is only briefly mentioned, but there's a picture showing both mixing unit and phasing unit in a control system. Hope this clears it up. Mar 19, 2022 at 16:36
• Hm, it looks like I should update my answer. From what I can tell, the phasing unit (as the name indicates suboptimally) uses the previously mixed inputs and then allocates these to the three actuators. It seems to me that your textbook differentiates between mixing (e.g. yaw is applied automatically when collective is applied) and what they call phasing (distribution of the mixed inputs to the actuators...) Mar 19, 2022 at 17:09
• I am pretty sure the rotor always tries to align itself with the swashplate, so when the pilot gives a right-roll command, the swash-plate tilts right. Due to gyroscopic effect the blade pitch does indeed have to be raised forward (ccw rotation), but that's why the control horns ride the swash-plate ahead of the blades they control. If the rotor responded to swashplate tilting forward with tilting right, it would easily overshoot and exceed its flapping limits, because there is little or no torque back on the shaft. By matching the swashplate it won't tilt further until the body catches up. Mar 21, 2022 at 8:46