Collective cyclic pitch mixing
From RC Helicopter Wiki
Collective cyclic pitch mixing (frequently shortened to CCPM) refers to a variety of methods used to control the swashplate on a helicopter where collective pitch control is effected by the swashplate moving up and down the main mast. Other methods include having the swashplate at a fixed position on the mast, and using a linkage running up the mast to control collective pitch.
CCPM can be divided into methods where the control inputs are combined mechanically on the helicopter (mCCPM) or electronically, either by the transmitter, or electronics on the helicopter (eCCPM). There are advantages and drawbacks to both methods.
In mechanical mixing (sometimes referred to as single servo mixing), at least three servos, each controlling only one of the natural axes of the helicopter (aileron, elevator, or collective pitch) have their movement combined together mechanically to control the swashplate. The cyclic servos simply cause the swashplate to tilt as appropriate, but the collective servo has to cause the swashplate to rise and fall; this is accomplished either by allowing the collective servo to physically move the cyclic servos and hence their outputs, or by using a series of bell cranks to move one end of the cyclic linkages up and down in unison.
- + Simpler to set up
- + Simpler electronics
- + If one servo fails, the remaining axes will work as expected
- -- Typically has more linkages, and therefore more control slop
As mechanical CCPM is, from the point of view of the transmitter, indistinguishable from any other mechanism where each servo controls a single axis, CCPM is usually used exclusively to mean electronic CCPM.
In electronic CCPM, the servos are connected (often with just one straight linkage) to points around the circumference of the swashplate. The necessary control movements are then synthesized by coordinated movement of all the servos together; for example, collective pitch inputs will require all servos to move together by the same amount. This mixing is accomplished either by a CCPM-capable transmitter, or less commonly by electronics on the helicopter, such as the CSM CycLock. There are a number of eCCPM swashplate layouts, the most common being 3 servos at 120 degrees and 3 servos at 90 degrees.
- + Simpler mechanics
- + More direct links, therefore more precise control with less slop
- + Flight loads tend to be spread over all servos
- -- More complicated electronics needed
- -- Harder to set up.
- -- Controls can cause minor interaction with each other if not precisely set up
- -- Even when set up perfectly mechanically, differences in servo speeds can cause control interactions
- -- If a servo fails, it will affect all axes
Despite these drawbacks, the reduced complexity and increased precision of electronic CCPM systems means that it is becoming increasingly popular, especially on small electric helicopters.
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