Special functions commonly found in sailplanes

Model Airplane News, Aug 1998

Aileron->flap mixing

Two aileron, two flap servos required.

Servos work opposite for roll (aileron) function, together for flap function.

Gives higher maneuverability.

Aileron differential function

Two aileron servos required.

The down-moving aileron moves down less than the up-moving aileron moves up.

Different up and down travel helps coordinate turns and reduces "adverse yaw" tendency, making turns more efficient (less drag).

Reverse differential (more down than up) used for better response with airbrake.

Aileron->rudder mixing

Uses existing aileron and rudder servos.

Reduces "adverse yaw" tendency and coordinates turns.

When aileron commanded, rudder moves at same time.

Allows coordinated turns, scale-like flight.

Airbrake (butterfly/crow) function

Two aileron and one or two flap servos required.

Aileron servos go up, flap servos go down.

Controlled by airbrake (throttle) stick.

Allows steeper yet slower landing approaches; precision landings.

Use up-ailerons only if no flaps.

Optional elevator delay stops trim changes.

Launch function

Uses two aileron, one or two flap servos.

Entire trailing edge droops for higher launches,

Includes elevator preset to maintain trim.

Switch-activated.

Speed or cruise function

Uses two aileron, one or two flap servos.

Entire trailing edge raised slightly for low drag at higher cruise speeds.

includes elevator preset to maintain trim.

Switch-activated.

Camber function (proportional)

Uses two aileron, one or two flap servos.

Entire trailing edge rises or droops proportionally.

Controlled by side lever.

Can improve low- or high-speed flight.

Elevator->camber function

Dual flaperon servos or single flap servo.

When elevator pulled, entire wing trailing edge droops at same time.

Allows tighter pylon turns, smoother thermalling.

Glide-path control is referred to by various names (crow, butterfly, spoilerons) but they mean the same thing: a way to move the ailerons and flaps in opposition to each other so that there is both more lift (reducing the sailplane's speed) and more drag (making its glide angle steeper). These motions, along with elevator compensation, allow you to make very steep approaches and precision landings. The newest radios also provide an elevator delay function, which allows the flaps to "catch up" with the elevator to prevent trim changes. Glidepath control can also activate increased rudder coupling for smoother landings.

Reverse differential is used when you have the ailerons raised for glide-path control. This function commands the ailerons to move more down than up, so there's no loss of control authority when the ailerons are raised.

Flap->aileron mixing makes the entire trailing edge of the wing act together, allowing full-span camber control. Camber control (Figure 3) is used to change the shape of the model's airfoil, which makes it have less drag in different flight regimes. The best sailplane radios (in this author's opinion) have a side lever conveniently located near the fingertips when you hold the transmitter. Others use an inconvenient knob on the front of the transmitter case (these can be programmed to put the camber control on the airbrake stick).

Elevator->camber mixing is used to make very sharp and rapid pylon turns and can be set to only occur after the up-elevator passes a certain threshold; this is called "snap flaps."

Launch and speed presets are functions that command the wing's servos (and often the tail as well) to have a different neutral position, which is offset from the normal neutrals. The offsets are selected to provide better flight performance. For launch, the wing servos are all drooped to improve the sailplane's lifting capability while on tow. The speed mode reflexes (moves up) the trailing edge for less drag at higher speeds. Both modes provide elevator offsets so your model is in trim. The launch and speed modes are commanded by a switch.

Futaba* calls the launch mode "START." In addition to all these functions, advanced computer radios have what's called "flight modes." Flight modes typically combine sets of servo offsets, control throws and programmable mixers. Here's what we mean by flight modes:

Launch mode. Here, the ailerons, flaps and elevator are moved to offset positions for best climb. The rudder may have a high percentage of coupling to the ailerons for easier towing. Lots of differential is used because the ailerons are already drooped.

Normal mode. The wing controls are trimmed for minimum sink or best glide angle. Some differential is included, as is a small amount of rudder coupling. Camber control may be used to control the droop in the ailerons and flaps for slightly improved thermalling.

Speed mode. The wing is commanded to have reflexed airfoils across its span for less drag at higher cruise speeds. Differential and rudder coupling are both disabled, as they are not needed. Elevator is coupled to camber for tighter turns. Flap motion matches ailerons 100 percent for faster roll rate. Higher control throws are also commanded.

Landing mode. Glide-path control (airbrakes, butterfly, or crow) is enabled. With airbrake stick, ailerons rise and flaps drop, allowing steep precision landing approaches. An elevator delay function (if available) is used to control pitching when activated. Reverse differential (more down aileron than up, for improved roll control) is commanded, since the ailerons are already raised. Rudder coupling may be added for landing.