XP3/4/5 Control Setup

XP-3 Control Setup and Camber Flight Management by Mark Drela

To get the best performance out of your aileron XP-3, it is necessary to finely adjust the camber, or flaperon position, to the flight condition currently being flown. In addition, it is highly desirable to have the ability to set large downward flaperon deflection for airbraking. This gives precise landing control which is critical for most HLG contest tasks.

A fairly capable computer radio is necessary to program in all the required controls and mixing. I use an RD-6000.

Standard controls — aileron, elevator, rudder

Most US fliers use the Mode-2 basic control setup: Aileron and Elevator on the right stick, and Rudder on the left stick.

The following control throws work well: Aileron : -25 mm up, +15 mm down; Elevator: -20 mm up, +20 mm down; Rudder: +/- 35mm

Adjust these throws and/or add expo as you see fit.

Note: On the XP-3, 1.0 degree of flaperon deflection conveniently corresponds to 1.0 mm of trailing movement at the wing center, so the aileron movement is -25 / +15 degrees.

Camber control

A) Using a two-position switch:

  1. Full reflex (-2 degrees).
    Flaperon is lined up with the unhinged bit of the wing at the center, and bottom of wing is flat across the hingeline. Gives minimum drag at high speed. Used for launch, fast upwind penetration glide, and fast roaming for lift in windy conditions.

  2. Moderate camber (+2 degrees).
    Flaperon’s trailing edge is dropped 4mm at the wing center. This is midway between the max-L/D position and minimum-sink position. Used when circling tightly in lift, when slowly roaming for lift in low wind, and for max flat-air duration.

B) Using a three-position switch:

  1. Full reflex (-2 degrees).
    Same as above. Used for launch, and fast upwind penetration glide.

  2. Neutral (0 degrees).
    Flaperon’s trailing edge is dropped 2mm at the wing center, and top of wing is locally flat across the hingeline. This is the best-L/D position, used for roaming for lift at an intermediate speed.
  3. Full camber (+3 degrees).
    Flaperon’s trailing edge is dropped 5mm at the wing center. This is the minimum-sink position, used when circling tightly in lift, and when trying for maximum duration in dead air.

C) Using a slider:

Slider limits correspond to settings 1) and 3) in the three-position switch setup above.

A slider obviously gives the finest camber control, although the two-position switch setup is nearly as effective in practice and easier to manage in flight.

Whatever setup you use, it is important that the switch or slider be easily accessible when your hands are in their normal flying positions on the radio, since you will be moving it very frequently. Being able to sense its position by touching it is helpful.

Airbrake control

Large downward flap deflection for airbraking is best controlled by the left throttle stick. The endpoints should be programmed as follows:

  1. Throttle stick all the way forward: No flap deflection.
  2. Throttle stick all the way back: Flaps down 25-40 mm, depending on how much maximum airbraking action is wanted. It is suggested that you increase the maximum deflection until roll control starts to degrade unacceptably. Mixing lots of rudder into the aileron stick helps to maintain roll control at large flap deflections and thus allows stronger maximum braking.
  3. In practice, the position of the throttle stick is continually varied to get the desired amount of airbraking as the glider comes in for the landing.

Elevator mixing

Downward flaperon deflection effectively increases the decalage which causes a pitch-up tendency. It is highly desirable to cancel this by mixing some down-elevator into the camber and airbrake controls. The proper amount of mixing depends to some extent on the chosen CG position. Start off with about 3 mm of down elevator with full airbrake deflection. Then adjust so that the glider doesn’t change pitch trim appreciably when camber or airbrakes are deployed.

Aileron to Rudder mixing

It is recommended that a hefty amount of rudder deflection be mixed into the aileron control stick. This will maintain some degree of roll control when full airbrake deflection is used, as described above. The reason is that the rudder always has consideable roll power via the generous dihedral, even when the ailerons are mushy during braking. A suggested first setting is 15 mm of rudder deflection when full aileron is applied.

Lots of rudder mixing will prevent the ability to do clean axial rolls, so you’ll want to reduce or eliminate this mixing when flying on the slope.

Launch preset

It is necessary to program one of the switches to apply some up-elevator during launch. The switch must of course be accessible when holding the radio with the non-throwing hand. I use the left gear switch on my RD-6000, and set it to actuate the elevator via a P-Mix. How much launch up-elevator is needed greatly depends on the chosen CG position — a forward CG may need little or no up-elevator, while an aft CG may need quite a lot. Start with perhaps 2 mm of up elevator.

Some outside rudder preset may also be used to get a straighter launch. The XP-3 should need little or no rudder preset if a good launch technique is used.