4 Axis Flying

4 Axis Flying
4 axis flying is another French contribution to the radio controlled soaring world. Most pilots are familiar with the more common 2 and 3 axis concepts, in which the elevator, ailerons and rudder (for 3 axis planes) provide pitch, roll and yaw control of the aircraft. In 4 axis flying, the additional axis of control comes from the use of dynamically controlled flaps or flaperons that allow the pilot to instantly deploy a variable degree of camber or reflex.

4 axis control is effected via a mix that allows control of the wing's trailing edge via the throttle stick, in this case referred to as the "flap stick". Pushing forward on the flap stick raises the trailing edge, reflexing the airfoil proportional to stick deflection. Pulling backwards on the flap stick lowers the trailing edge, cambering the airfoil proportional to stick deflection. When the stick is at the center of its travel (the "half throttle" position), it returns the trailing edge to neutral position. The flap stick gimbal is often modified to add spring centering to facilitate using this mix, and while this is highly recommended, it is not mandatory (and may not be practical if the radio is to be used for powered aircraft as well).

Combined with a suitable airfoil and large control surfaces with plenty of movement, 4 axis flying allows a degree of control quite different from the more traditional arrangements for camber and reflex manipulation. Because you can instantly switch back and forth from camber to reflex and/or neutral (and vice versa) you can give the plane exactly what it needs, when it needs it. You also have extremely precise and dynamic control over exactly how much trailing edge deflection is deployed.

NOTE: The following drawings represent a Mode II setup - rudder & flaps on left, ailerons & elevator on right.



Some aircraft have a pitch response when reflex and camber are applied. If needed and/or desired, elevator compensation can be mixed in so that the plane will have no pitch response when the trailing edge is manipulated. Conversely, some pilots forgo the compensation, with the result that, with some aircraft, they can make the plane loop and turn simply by pulling or pushing on the flap stick–a bit different but surely fun!

As a landing aid, 4 axis flying is perhaps one of the best systems available. A typical 4 axis landing approach might employ reflex to kill the wing’s lift and bring the plane down through the rotor quickly, then switch to camber just before touchdown in order to slow the descent speed to a crawl and initiate the landing flare. Once the plane is slowed and just a few inches above the ground, the camber is removed and the plane completes the landing flare and settles gently onto the ground.

Aerobatics with a 4 axis setup
Aerobatic gliders with symmetrical airfoils should always setup using snapflap mixing (trailing edge is cambered when up elevator applied, and reflexed when down elevator applied). This is helpful for extending the performance of a symmetrical airfoil, and in fact many of the so-called "double cusp" glider airfoils, such as the MG05 and TP42, are designed around always-on snapflap mixing.

However, the downside of snapflap mixing is that the camber applied by the snapflap mixing will make it harder to initiate a snap or spin than if the trailing edge were neutral, as the camber (or reflex for negative figures) will make the wing more dificult to initiate a stall.

A 4 axis setup allows the pilot to override the snapflap mix by apply reflex or camber as appropriate to temporarily negate, and even reverse, the effect of the snapflap mix. This is quite helpful and highly worthy of consideration by any aerobatic pilot who uses snapflap mixing. See the diagrams below for further explanation.

NOTE: The following drawings represent a Mode II setup - rudder & flaps on left, ailerons & elevator on right.







Of course, snap rolls and spins are not the only aerobatic figures which can benefit from 4 axis mixing. The possibilities are limited only by a pilot’s talent and imagination. Some other examples include the following:
 * Hammerheads: Just prior to initiating the rudder kick at the top of the figure, apply full 4 axis reflex. This will immediately cause the wing to stop flying and enhance the effectiveness of the yaw input.
 * Loops: Deploy additional camber (beyond whatever you get from your snapflap mixing) to make a loop extra tight. Conversely, apply full camber (or reflex if inverted) and make an extremely slow loop with no fear of stalling.
 * Fly backwards: In sufficiently strong lift conditions, you can deploy enough camber to make the plane fly backwards in a controlled hover. If you start to climb too much, deploy reflex and descend. You can almost bounce the plane up and down like a basketball!

Setup tips for 4 axis flying

 * You’ll generally want to get as much travel as you can on your flaperons so that you have the widest range of possibilities for your 4 axis setup.


 * Plan on running at least 30-50% exponential to soften up the center feel on your surfaces so that the plane remains responsive but is not difficult to fly.


 * You may or may not want to include deadband on the flap stick, but it is not a bad idea.
 * Spring return is a very nice feature, but not mandatory.


 * It’s easy to slowly work into flying 4 axis style. With time and practice, manipulating the flap stick will become as automatic as the other controls, and you may find that you really don’t enjoy flying unless you’ve got full “4 axis” control!