The swash mix values are transmitter settings used with helicopters with electronic CCPM swashplate control to specify the direction and amount that the swashplate is moved by for the cyclic and collective controls. The swashplate type specifies which sort of CCPM mixing to use, and needs to be set first.
When using a helicopter with electronic CCPM mixing, the normal swashplate controls of cyclic and collective no longer have individual servos corresponding to each channel. No one servo moves when an elevator, aileron or collective input is made; instead the output is synthesized by the coordinated movements of all the swashplate servos. In this way it is similar to elevon or flaperon mixing in the fixed wing world; where the servos can move together for one function, and apart for the other. Unlike elevon mixing, where the control throws are usually set by configuring dual rates, with CCPM mixing we have a special menu.
The transmitter 'knows' the relative amounts to move the servos by for each control by the swashplate type specified. The servo end points are per-servo and so no longer specify the throws of the individual controls (but can be used to help eliminate CCPM interaction). Servo reversing is used to get the servos working together correctly; if they are set for correct operation on collective inputs, then the swash mix values can correct any other issues.
To specify how much (and in which direction) cyclic and collective controls should move the control surfaces, the swash mix values (also called swashplate parameters, swash AFR and a variety of other names) are used. The three values specify symmetrical throws for the aileron, elevator and collective pitch virtual servos. The values are set appropriately by using a pitch gauge and adjusting the value until the appropriate amount of collective or cyclic pitch is seen at full control deflection. If the control works in the wrong sense (i.e. pushing the cyclic forward causes the swashplate to tip backwards), then the sign of the value can be changed from positive to negative (or vice versa); you may need to scroll through zero, or one of the transmitter's function keys may swap it. If servo reversing is used to get the servos working together for collective inputs, then the swash mix values can correct any other issues.
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