Aileron Differential: The secret to coordinated turns

During a 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 servo-arm 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 center-of-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-rudder” mix that uses a “curved” mix type. This type of mix allows you to adjust the mix from point to point over the full range of the aileron movement. You can command, for example, precise rudder deflections to coordinate the nose of the aircraft in the turn when small amounts of aileron are applied. When larger amounts of aileron are applied to execute a roll, however, the mix amount can return to 0% so that no rudder is automatically applied, resulting in a clean roll with no automatic yaw input—well, as clean of a roll as a WW I model is able to perform.

If you find that your particular radio system does not feature this mix by default, explore the programmable mixing capabilities of its system. If applicable, access the programmable mix function on the controller and assign the aileron channel as the master channel and the rudder channel as the slave channel. This means that when aileron input is commanded by the pilot, a mix amount, which is customized by the pilot, is automatically given. For newcomers, this is a great feature to have as it teaches the need for rudder to result in a coordinated turn but simplifies the process.

AXIAL-ROLL FUNAMENTALS WITH THE EXTRA 300
On a pure aerobatic model, you will test and tune the differential amount in a different manner. A lot of servo power is required to prevent flutter and to maintain the required deflection for maneuvers. Before we start adding differential percentages, test for adequate aileron-servo power. I prefer to perform rolls on vertical uplines and compare them to the roll rate found while performing rolls on a downline. Roll rates should be the same. If the roll rate is faster while travelling upward, you are experiencing what is referred to as “blowback.” This term simply means that the servo power is not enough to obtain consistent performance from the aircraft, given a certain deflection and airspeed. Either increase your servo power or improve your geometry by reducing the servo-arm radius and/or increasing the distance that the control-horn pickup is from the hinge line. A final option, of course, is to either replace your servos with higher-torque servos or to simply add another servo per control surface. When roll rates are the same whether you are travelling up or down, you are ready to tune your model with the proper differential percentage.

A few different methods exist for actual flight tests. Some pilots prefer to pull to a 45-degree upline into the wind and apply full-aileron input in one direction and note how the aircraft rolls. If the model wanders in the direction of the roll, too much down-travel exists in the ailerons, and if the model wanders to the opposite direction of the roll, too much up-travel exists. I like to climb to an extremely high altitude when no wind is present and push to a vertical downline. I then apply full aileron in one direction and take notice of the spinner. If the roll is not axial, I increase or decrease the differential amount until the model is rolling perfectly.

FINAL THOUGHTS
As we have seen, different types of models require that you test for aileron differential in a different manner. Whether you’re trying to decrease your workload by using a programmable mix or by mastering the rudder, there is no one right way to do it. As I have stressed time and time again, everyone has a personal preference. Aileron differential, coupled with rudder, will result in a realistic and completely coordinated turn.

Always remember to seek the advice of fellow experienced giant-scale enthusiasts for assistance in an effort to build confidence or if you’re unsure of how to implement a certain mix or differential percentage. Use this column as a basis, and most important, enjoy every part of this great hobby and seek out every learning experience possible.

Text & Photos By John Glezellis