4 Axis Slope Aerobatics

What is “4 axis” flying?

4 axis flying is, to the best of this writer’s knowledge, 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 the use of a return spring on the throttle or flap stick. Pushing forward on the flap stick raises the trailing edge, reflexing the airfoil proportional to stick deflection. Pulling backwards on the stick lowers the trailing edge, cambering the airfoil proportional to stick deflection. Releasing the stick returns the trailing edge to neutral position.

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 images present a Mode II setup – Flaps & rudder on left stick, aileron & elevator on right stick

Flap stick centered, trailing edge is neutral; the plane will fly straight and level
Flap stick up, trailing edge reflexed; the plane will either descend or bunt depending on elevator compensation
Flap stick down, trailing edge cambered; the plane will either climb or loop depending on elevator compensation

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 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 are often 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.

However, the downside is that the camber applied by the snapflap mixing will decrease the reflex of the wing to the inside of the snap or spin, making it harder to initiate the maneuver than if the trailing edge were neutral, as the wing will be less likely to stall. A 4 axis setup can completely alleviate this undesirable circumstance and so is worthy of consideration by any aerobatic pilot who uses snapflap mixing. See the diagrams below for further explanation.

Snap roll control inputs & surface movements with no mixing enabled. Note that both the stalling wing (left) and the unstalled wing (right) have equivalent amounts of control surface deflection.
Snap roll control inputs & surface movements with Snapflap mixing enabled but no 4 axis reflex compensation. Note that the stalling wing (left) has very little reflex and the unstalled wing (right) has so much deflection that it is probably generating more drag than lift. Together, these will make the snap roll slower and more difficult to initiate.
Snap roll control inputs & surface movements with both Snapflap and 4 axis mixing enabled. Note that thanks to the 4 axis reflex compensation input at the flap stick, the stalling wing (left) has much more surface deflection than the unstalled wing (right). This will make the snap easier to initiate as well as more rapid and violent. With less 4 axis compensation applied, the pilot could also have replicated the surface deflections of the standard non-mixed setup.

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% negative 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.
  • Definitely setup your flap stick with some kind of return spring system – trying to manually center the stick without a return spring will get very old, very fast. In a pinch, try wrapping a rubberband around the transmitter case, looped around the flap stick at either end.

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 (like this author) that you really don’t enjoy flying unless you’ve got full “4 axis” control!

Have fun and best of luck!


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