More and more recently introduced machines are using some form of moving swashplate system incorporating Cyclic/Collective Pitch Mixing (CCPM). They can take many forms and the essential mixing can be accomplished mechanically, electronically, or by a mixture of both.
In order to understand just what is involved in a moving swashplate system, let's first explain how a fixed swashplate works. In its simplest form, only two inputs are required (Fig.1). One link tilts the swashplate in a for/aft direction and the other in a lateral direction. The centre of the swashplate is a large ball joint which is fixed in position on the main shaft. On the recently introduced 'Hornet' electric helicopter, the swashplate is fixed in its position by the links going up to the flybar. Some systems use more than two inputs which serves to steady the swashplate and give more precise control (Fig.2). This arrangement is normally only found on the higher priced competition machine.
The essential point here is that a fixed swashplate system uses some other way of changing the collective pitch of the rotor blades. A moving swashplate moves up and down to control the collective. For all practical purposes, CCPM means 'moving swashplate'.
If a swashplate is to move up and down with pitch variation and still give satisfactory control of the cyclic inputs to the blades, it must be held firmly in any given position. This requires a minimum of three inputs, or operating links. These can be spaced at 90 degrees to each other (Fig.3) or at 120 degrees (Fig.4).
90 degree spaced inputs are normally arranged so that there is one on each side and one at the front (or rear). The inputs at each side move in opposition to each other for lateral cyclic control (aileron), and the front input moves to give fore/aft cyclic (elevator) control (Fig.5). All three links move together for collective control.
There is, of course, no reason why there should not be two linkages at front and rear, moving in opposition, for elevator control and a single linkage at one side for aileron control. However, the writer is not aware of any model which has used this system.
When the inputs are spaced at 120 degrees, there are two arrangements which can be used. One (Fig.6) has one link at the front and links at either side, similar to a 90 degree system, but the side links are further to the rear. The actual control inputs for this system are rather more complicated, since the cyclic inputs cannot be totally separated.
For example, when the front link is moved up and down for elevator control, the whole swashplate will move up and down unless the two side links are moved a smaller amount in the opposite direction to compensate (Fig.7). The two side links will still operate the aileron input as before.
Once again, the same linkage can be used with the single link at the back and the side links fitted a little to the front of the side location (Fig.8).
Another variation on this particular set-up is as shown in Fig.9. Here the single side link will give lateral control, with some opposite compensation from the other two, while the fore and aft linkages will move in opposition to each other for elevator control. However, this is not a common arrangement.
The final variation, which gives even more support to the swashplate, has four links at 90 degrees to each other (Fig.10). Here both axis are controlled by a pair of links moving in opposition, with all four moving together for collective control.
The manner in which these swashplate movements are communicated to the blades varies from model to model and does not concern us here. The basic points to remember are:
1). The fixed swashplate tilts in the direction that the helicopter is required to move. Collective pitch control is effected by some other means.
2). The moving swashplate has two functions. It tilts in the direction that the helicopter is required to move, and it moves up and down to control the helicopters height.
There are many ways of operating a moving swashplate via the three radio functions involved - aileron, elevator and pitch. Until recently all of these were entirely mechanical, with any mixing being performed by means of some form of mechanical mixer. The advent of increasingly complex radio equipment with various mixing circuits incorporated in the transmitter has made some of the existing systems simpler and also paved the way for more complex systems. For example, the 120 degree swashplate systems described above would be virtually impossible to achieve mechanically. It has also made the adjustment of such systems much simpler.
Fig.11 shows the mechanical system used on many of the Morley machines. Note that each input is controlled by a completely separate servo, which makes adjustment fairly straightforward. While some interaction is inevitable, careful design, as here, can reduce this to a minimum. This is a 90 degree system.
Another mechanical system is found in the various versions of the Heim mechanics. While this is a four input 90 degree system, only the two lateral inputs are used to raise and lower the swashplate for pitch control (Fig.12). This means that only these two inputs need to be mechanically mixed. This is normally achieved by means of a 'rocking servo' mixer (Fig.13).
Note that there is some interaction between collective and elevator due to the swinging arm carrying the elevator bellcrank. This was slight and never caused any problems. It could be mixed out with a modern system.
The Heim method of actuation is very easily converted to a hybrid electronic system by substituting the mechanical mixer for an electronic type commonly used for 'V' tail models. Here the aileron and pitch inputs (say A and B) from the receiver are fed into an electronic circuit which mixes them together and gives two outputs. One of these outputs is the sum of the two inputs (A+B) and the other is the difference (A-B) (Fig.14). When this system was popular, some radio manufacturers incorporated it into their transmitters and referred to this as '180 degree CCPM'. Few modern systems have this.
One problem with this method is that electronic mixers are usually arranged to reduce the final servo movement to avoid over driving the servo when the two inputs are added together. The only solution here is to use long servo arms.
The latest, entirely electronic, systems have all of the circuitry in the transmitter and each of the swashplate inputs (3 or 4) is controlled by a separate servo. For optimum results, these should be mounted below the swashplate and connected by a short, direct, link (Fig.15).
There are many advantages to this layout. Many of the connections of previous set-ups are eliminated, as are many bellcranks or levers. It is, therefore, lighter and more compact. However, it will almost certainly be necessary to use long servo arms to obtain sufficient movement and this does involve some exaggeration of any servo deficiencies.
There are certain limitations involved in the use of electronic mixing systems. Early types suffered from the fact that increasing the throw of one of the mixed inputs would automatically reduce the other. For example, if the pitch and aileron channels were being mixed, increasing the aileron throw would reduce the pitch throw.
The latest equipment does not suffer from this problem, but care must still be exercised when two channels are mixed. In the example above, full aileron throw and full pitch throw together could drive the servo well beyond its normal limit. This can produce a very non-linear output or even interaction of the controls.
In this situation, the mixed servos must have their throw reduced via the travel adjustment facility and then, if necessary, longer servo arms will be needed. Remember that you cannot get something for nothing, there has to be a snag somewhere. There is a definite limit to how much travel any servo can give. If you exceed this there will be problems somewhere.
The advantage of electronic mixing with a modern radio is that everything will be adjustable and, with patience, you can get everything set to its optimum.
This material originally appeared in 'Setting Up Radio Control Helicopters' by Argus Books. It also appears in updated form in the latest version of 'Flying Model Helicopters', by Nexus Books.
