Proper trimming and programming for a better-flying plane
This column will provide a basic overview of a model’s mechanical setup and also touch upon a few ways in which you can leverage your computer radio system. The goal is to eliminate inconsistent trim performance, adverse yaw, and certain pitching tendencies that occur when a given control surface is deflected. To do this, you must first understand how to analyze your airframe and the process with which to test and address each issue. Let’s begin!
THE PROPER FOUNDATION
Every decision you make throughout the build will have an impact on the aircraft’s overall performance. On the mechanical front, these choices include but are not limited to the proper selection and installation of all servos and their respective linkages in addition to having the correct wing and tail incidence angles. While programmable mixes can help, they are often considered “Band-Aid” fixes. Let’s see why.
The servos on an airplane must have a sufficient gear train to prevent failure, torque to prevent what is called “blowback,” and precise centering abilities. Blowback is when the servo loses holding power as pressure is applied on the surface. To illustrate this, while you may see 45 degrees of control-surface deflection on the ground, once in the air and at a reasonable flight speed, the surfaces may only move a fraction of that amount! For ailerons, it is impossible to tune the differential amount when the servo has inconsistent travel points from flight to flight and while traveling at different speeds. If this is the case, blowback might be the cause, and you won’t be able to precisely tune your model to eliminate the yawing or pitching tendencies noted earlier. Several online spreadsheets as well as a few online torque calculators allow you to enter basic information to determine whether your servos have enough torque to prevent blowback. Additionally, you should connect servos in a way that supports differential. It is quite common for many sport models to use either a single servo to control the ailerons or two servos, with the use of a Y-harness, to connect them to a single port on the receiver. If you do this, you won’t be able to program the differential.
Next, make certain that the linkages are sufficient for your aircraft. In a push/pull configuration, I prefer to use heavy-duty aluminum servo arms that allow me to use a titanium Hangar 9 Pro-Link secured with Du-Bro’s heavy-duty 4-40 ball links. If your biplane has a slave aileron that is connected to the master aileron by a pushrod, make sure
Over the years, advancements like the use of a single-axis gyro and stabilization systems like Horizon’s AS3X have made flying more enjoyable. But no matter what type of aircraft is being flown, each model may exhibit a trait that you may not desire or—even worse—fear! It’s important to recognize that there is no substitution for proper trimming and programming practices.
there isn’t any slop within the attachment points as well as within the servo, servo arm, and control horn. Similarly, make certain that the linkage will not flex under a simulated load. Never use fully threaded pushrods unless they are installed in a carbon-fiber or similar sleeve with metal nuts at each end to firmly secure the sleeve in place.
With the servos and linkage installed and properly connected, the next step is to program each servo. While it may appear obvious that control-deflection angles must be correct, I cannot begin to tell you the amount of times I have flown an aircraft and found that the right and left ailerons differed with respect to their maximum travel amounts. Using a digital angle meter is beneficial, but at a minimum, the use of any measuring device is sufficient. Be sure the deflections are identical before you program the differential so that you know exactly how much deflection you have and that each percentage change in the differential function obtains an accurate deflection from the desired control surface.
Some manufacturers offer aileron-deflection amounts that incorporate differential. Set the maximum travel points within the adjustable travel volume (ATV) or endpoint adjustment (EPA) functions so that travel is equal and matches the maximum amounts given by the manufacturer. Then, use the differential program to tune all settings. Never change the ATV or EPA settings to result in the desired differential amount as severely different percentages will change how the servo responds to a given command.
Testing for differential will differ between different model types. With a slow-flying scale model, many sport pilots typically turn by applying a touch of aileron input followed by up-elevator. If they don’t use differential, the increased drag on the down aileron will result in a yawing tendency in an adverse direction to the desired turn because a greater amount of drag exists on the wing opposite the turn. With an aerobatic model like the 70cc Mamba bipe, I prefer to perform two different tests. First, I climb to a high altitude, pull the throttle to idle, and push to establish a 90-degree downline. I then apply full left aileron input and note any tendency for the aircraft’s nose to move. Another test is to apply maximum power, pull to a 45-degree upline, and apply full left aileron input to perform a roll or two. If the nose wanders to the left, you need to reduce the downward-moving aileron with differential; similarly, if the nose wanders to the right, reduce the amount of up-aileron travel with differential. The goal is to perform a roll, or multiple rolls, so that the aircraft will stay on the same vertical climb. If you prefer to perform the 45-degree climb test, limit the number of rolls performed. Airspeed may drop over time, and depending on the power-to-weight ratio of your aircraft, this will have an impact on what differential value you feel is necessary.
Typically, you want more up-aileron movement compared to downward movement. Different hinge methods decrease the amount of adverse yaw, but while most center-hinged airplanes need differential that results in having the top aileron move more than its downward deflection, some composite airplanes that have top hinges need reverse differential
(the aileron will travel more down than up as the control-surface area differs from the top surface to that of the bottom surface). You can mechanically change the placement of the linkage attachment point to incorporate aileron differential into an aircraft. The more you advance the servo-arm neutral position, the higher the differential effect on the ailerons. With an advanced computer radio, the easiest way to incorporate aileron differential is to apply a certain percentage in the “aileron differential” program.
With the assumption that the construction of the aircraft is structurally sound, with
no excessive twist on the control surfaces, it is time to begin the fine-tuning process. For the plane to fly well, you must ensure proper incidence angles for the wing(s) and the horizontal stabilizer. All reputable kit manufacturers state the proper incidence angles for their models. If this is not the case, it is important to seek advice. Typically, you will begin with the incidence angle between your wing and stabilizer so that they are parallel to one another and that the engine is also aligned. Then, take to the skies to trim the model.
With a pure aerobat, you should be able to perform a vertical dive in which the aircraft tracks completely vertical. If it demonstrates any pull or push, however, increase or decrease both the wing or stab incidence angles. With a
PROGRAMMABLE MIXING BASICS
Advanced radios offer programmable mixing capabilities that can prove beneficial in different ways but may also serve only as Band-Aid fixes. Common mix types are linear or a multipoint curve mix. Linear mixes allow for a fixed maximum amount, but a curve mix lets you adjust the mix from point to point over the full range of the movement for a given control surface. You can command, for example, precise rudder deflections to coordinate the nose of the aircraft in the turn when you apply small amounts of aileron. When you apply larger amounts of aileron to execute a roll, though, the mix can return to 0% so that no rudder is automatically applied and the plane performs a clean roll with no automatic yaw input.
Some transmitters offer a default “aileron-to-rudder” mix. In this preassigned programmable mix, rudder is automatically applied with any aileron deflection to keep the aircraft in a coordinated turn. This mix is beneficial for those pilots who may find it challenging to apply rudder input. If your radio does not feature this mix, it is quite easy to accomplish. The “master” channel is the control surface that is being deflected, and the “slave” channel is the control surface for which you want the automatic input. Always assign the mix to a switch, if needed, or have it active throughout the entire flight. As with all changes, modifications should be made in small increments so that you can fly the model and note the result.
Aerobatic enthusiasts may notice a pitching tendency whenever the model performs a four-point roll or knife-edge flight whenever rudder is input. If the airplane pushes to the undercarriage, use the “rudder-to-elevator” mix. In this case, a small percentage of up-elevator is needed. You want the aircraft to remain neutral and not to push to the undercarriage when you add rudder. Similarly, if your airplane also rolls while in this attitude, use a “rudder-to-aileron” mix.
Different centers of gravity will also change the overall mix percentage required. Some pilots, however, prefer to use “throttle-torudder” mixes to correct for an incorrect engine thrust angle or a “throttle-to-elevator” mix to tend to an incorrect incidence angle. Take the additional time to trim the aircraft, obtain the correct amount of right and up or down thrust, and enjoy your model’s pure characteristics.
FINAL THOUGHTS
In closing, it is important to understand not only your ability as a pilot but also the capabilities of the aircraft that you are flying and how you can address certain undesirable flight characteristics. While electronic advancements like using a single or three-axis stabilization system may prove beneficial, they are never a substitute for poor assembly. For consistent flight performance, you need a properly built model that incorporates the items discussed here. Use this column as a basis to improve your understanding of proper setup techniques, and most important, enjoy every part of this great hobby and seek out every learning experience possible. biplane, you may notice that your control trims change in flight, especially in roll. Wing rigging, if applicable, can loosen in flight. Similarly, an interplane strut may develop some slop or something similar, as can the gear train of a servo. Address all items, which include tightening the wing cables, if applicable, and monitor any servo or linkage slop.