Aileron Differential

The secret to coordinated turns

In a recent trip to the flying field, I flew my giant-scale Sopwith Camel and Extra 300. Even though they are quite different, both require aileron differential. For these particular models, the down-aileron induces greater drag on the wing than the same amount of up-aileron, and this causes the aircraft to yaw outward from the turn. Without aileron differential, most airplanes require a certain amount of coordinated rudder to prevent—or at least minimize—adverse yaw. Adverse yaw is present if the tail of the model drops in a turn or it is quite challenging to roll in an axial fashion. The addition of aileron differential will help correct these tendencies and, in the case of the Extra 300, will also contribute to axial rolls. Let’s talk about aileron differential and coordinated turns. Setup should really start on the building bench before the actual flight tests. Proper program mixes are also important things to take advantage of. Let’s get started!

PROPER CONNECTIONS

With respect to aileron differential, it is important to mention that one can mechanically change the placement of the clevis (or ball link) that is attached to the servo arm to mechanically incorporate aileron differential. The more we advance the servoarm neutral position, the more the differential will increase on the ailerons. With advanced computer radios, however, the easiest way to incorporate aileron differential is to apply a certain percentage of differential in the Differential program. Always review the capabilities of your radio system before accomplishing the same task with multiple programmable mixes.

Assuming that your model is equipped with a receiver that allows each servo to plug into a separate receiver port, do so. If trying to decide the proper wiring configuration, examine your radio system. A radio like

the Spektrum iX12 features multiple wing and tail configurations, which cover most conventional airplanes. My Camel, for example, uses two servos, so I choose a Dual Aileron configuration in the Aircraft Type function. Similarly, the Extreme Flight Extra 300 has two servos per aileron and uses a Four Aileron wing type. Selection of a given wing type will assign each servo to a separate port on the receiver, where the pilot can individually control the center and endpoints of each servo and take advantage of any matching capabilities that a radio may offer.

Whenever multiple servos are used on a single surface, please make certain that each pair of servos (i.e., both right aileron servos and both left aileron servos) do not bind with the other to ensure that they work in unison. If this is not the case, this not only can lead to servo failure but also will result in unequal travel at different points within the aileron and will “twist” the aileron surface.

Before attempting the first flight, confirm that no values are assigned within the Differential program and use an angle meter, like the AnglePro II 5-in-1 Digital Throw/Incidence Meter, or (at a minimum) a simple ruler to obtain the same up- and down-aileron travel. If the travel is not the same, you will be constantly searching to find the right differential value and will likely be unsuccessful. Knowing for certain that up- and down-aileron travel is the same and assuming that you are content with the general “feel” of your model and the centerof-gravity position, you can now head to the flying field.

COORDINATED TURNS WITH THE SOPWITH CAMEL

On a model like the Camel, adverse yaw occurs during a turn and a turn will not be coordinated unless rudder input is applied. Interestingly, a pilot of a full-scale Camel would apply left rudder while turning left or right, and the amount of rudder input given would depend on the speed of the aircraft and engine rpm. Thank goodness the model pilot does not have to worry about a few of these characteristics, but a few items are similar and can be corrected.

As a simple test, climb to a moderate altitude and apply aileron input in one direction to initiate a turn. Pay particular attention to the nose, and if it appears to yaw in a direction outward from the turn (e.g., a left bank yields a right yaw tendency from the model), aileron differential is required. Access the Differential program, assign the function to a three-position switch, and enter different differential values for two of the settings, keeping one switch position standard where no differential exists. Take to the skies and bank the aircraft, being sure to try all switch positions. Adjust the amount accordingly, and once satisfied, record the differential value, eliminate the switch, and activate the differential so it is “on” without being linked to a switch.

While some pilots, including me, prefer to apply rudder manually at all times, others prefer to decrease their workload by using a programmable mix. Over time, inexperienced pilots will realize the need to use the rudder control surface as much as the aileron and elevator, and as with all things in life that are practiced, rudder implementation will become second nature. On the other hand, you can implement a mixer to apply a certain percentage of rudder deflection automatically when aileron input is given. Either option is completely acceptable and should be considered.

A radio system like the Spektrum iX12 features a default “aileron-to-rudder” mix (AIL > RUD). To test this mix, I recommend that you assign the mix to a two-position switch, where the mix is active in one position and not in the other. On the iX12, simply select the switch, select the position in which you wish the mix to be active, and assign a value in the Active position. Remember, though, that too much aileron and rudder travel at the same time can result in a snap roll, positive or negative, depending on the direction of the control-surface deflection. Start off with small mix percentages, and take note of the flight result.

This type of mix, however, may not be appropriate in certain instances. If you are performing a roll, for example, one may desire no rudder deflection with the mixer when full aileron is given. In this case, you would need to implement an “aileron-to-

AILERON DIFFERENTIAL, COUPLED WITH RUDDER, WILL RESULT IN A REALISTIC AND COMPLETELY COORDINATED TURN.

While this beautiful Sopwith Camel and this Extra 300 are quite different in many ways, they also share numerous similarities. One such parallel, as you will see over the course of this column, is aileron differential.

In the most common form, aileron differential simply means that one will make the down-aileron movement less compared to the up-aileron movement.

I prefer to assign my differential values to a three-position switch so that I can try different settings during a single flight to observe any subtle differences and tune my aircraft quickly. Once content, I will eliminate the switch where the differential amount will be active at all times.

During the initial steps of setting up an aircraft, select the proper wing configuration to take full advantage of certain features, like aileron differential.