Dual rates are a feature of programmable transmitters that allow one or more controls to have less (or more) control authority at the flick of a switch. With the dual rate selected, the servo deflection will appear to have been reduced in a manner similar to reducing the servo's end points. Often they are selected by the same switch that enables exponential (which keeps the channel's overall range but decreases (or increases) sensitivity around the control's center point).
A dual rate is given as a percentage of the normal full control defection. High rates generally give a more nimble feeling to an aircraft, while low rates make the model more docile, but reduce overall maneuverability. Sometimes different dual rates can be specified either side of a control's center point.
Typically dual rates can only be specified on aileron, elevator and rudder controls; collective pitch 'dual rate' can be specified by setting up alternative pitch curves and throttle curves that are then selected using the flight mode switch (dual rates can sometimes be selected using this switch too, depending on your transmitter). Dual rates apply to the control inputs, rather than the servo outputs---this becomes obvious when CCPM mixing is used. Any trim or subtrim is applied separately to the channel output, and so will not be changed by using a dual rate.
It is often pointed out that controls can be made 'softer' or 'dumbed down' simply by specifying smaller servo endpoints or CCPM parameters. However, dual rates allow changes to be made easily and quickly to the sensitivity of controls, without risking changing the rest of the helicopter setup, as may happen when changing servo endpoints.
One way the helicopter can be set up is to use a 100% dual rate to set up the helicopter, ensuring that no binding occurs when the controls are manipulated to their full extents, and then set up the dual rates tailored to your level of flying. As your flying improves and you need more aggressive response from your helicopter, you can increase the extents of the dual rates safe in the knowledge that as long as you don't exceed 100% you won't introduce any binding.
Alternatively, you can use a dual rate to slow the helicopter's responses down for those days when you are not feeling up to throwing the helicopter about aggressively, and just want a gentle hover about.
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