[Headline: Be prepared for competition day]
As a pilot gains experience and progresses up the competition ladder, he or she quickly learns that every point counts. Attention must not only be given to perfecting a particular sequence, but also the aircraft’s setup. If you are too focused on correcting any odd tendencies that an airplane may exhibit because of a lack of proper trimming efforts, the result will be downgrades. This will affect how you place in competitions.
I will discuss basic setup techniques to better an aerobatic model’s flight characteristics. I will examine how a pilot can find the center of gravity (CG) that best suits his or her needs, how to trim a model for basic “hands-off” flight, and how to adjust the engine’s thrust angles. I want you to learn how to decrease your workload so you can remain focused on competition day.
General Aircraft Setup Techniques
Before taking to the skies, a pilot must evaluate the airframe. To begin, verify that all control surfaces are moving equally to one another (one elevator half to another, one aileron to another, etc.). If one elevator travels a different deflection amount than the other, the aircraft may drift in what appears to be yaw when performing thrust-angle tests.
To eliminate any uncertainty in deflection, use a digital angle meter such as the AnglePro from Hangar 9. As you are examining control surface deflection amounts, ensure that the clevises for all control surfaces attach to the corresponding control horn directly over the hinge line. Doing so will allow for equal travel in both directions without requiring different percentages from a computer radio’s adjustable travel volume function.
Basic Trimming and Proper CG
On an aerobatic platform, you should always maiden your aircraft with the CG that was recommended by the manufacturer. Although the balancing process is tedious on larger airplanes, it can be simplified by using the EZ Balancer II from Southwest Systems. The EZ Balancer II has been tested for airplanes that exceed 100 pounds, so it is perfect for all common competition aerobatic airframes.
When the airplane is in the air, the first step is to trim it at full throttle. The aircraft must be able to fly straight and level without pilot commands.
I prefer to check the CG in flight by rolling the airplane from upright level flight to inverted at maximum power, assuming that the aircraft isn’t overpowered. For a “sequence-prone” model, the airplane should pitch slightly downward, requiring a touch of down-elevator. If the airplane pitches upward, weight should be applied to the front of the aircraft. Similarly, if the airplane excessively pitches downward, weight should be added to the tail.
For temporary fixes, I use Self Stick Weight from Hangar 9. With a total weight of 6 ounces, you can apply the weight to the aircraft in 1/4-ounce increments.
After the weight is added and the pilot is content with how the aircraft behaves, it is time to balance the airplane and see where the new balance point is located. When that measurement is found, remove the weight that was added to the airframe and move components such as the receiver battery and/or batteries to a different location to obtain the same CG. Secure the newly relocated components and get the aircraft in the air again!
The goal is to have an aircraft that performs to the pilot’s liking with no visible pitch trim. The model’s behavior may change depending on the power setting.
If an airplane is tail-heavy, for example, down-elevator trim will be required for hands-off upright, level flight. If the airplane is rolled from upright to inverted, this trim, aside from the CG, will cause the airplane to climb. Similarly, these elevator trim amounts will impact knife-edge flight because they will cause the airplane to pitch.
Start by checking that the engine’s thrust was set according to the manufacturer’s recommendations. The winds must be calm for the initial flight tests so the airplane’s nose won’t lean into the wind.
Align the aircraft so it is parallel to the runway, apply maximum power, pull to a vertical upline, and release all control inputs. Pay particular attention to the aircraft’s nose. If it drifts to the left, you need to add right thrust. If the nose drifts to the right, you need to add left thrust. Note what the airplane exhibited in yaw and prepare to check for upthrust/downthrust.
To check for upthrust and downthrust, fly a line parallel to the runway and into the wind at maximum power. Then quickly reduce the throttle to idle and observe the model’s tracking. If the airplane pitches downward, downthrust is needed. If the aircraft pitches upward, upthrust is needed. Making this change can be as simple as adding a washer or two between the engine and the firewall.
After the engine thrust has been changed, ensure that the movement of the throttle servo remains unchanged. Depending upon how much the thrust is altered, the end points for the throttle servo and/or choke servo (if applicable) may also change.
After the thrust angle has been altered, fly the aircraft and check for upthrust and downthrust. Add more thrust if needed. Again perform the CG tests to ensure that the aircraft is behaving as desired and that the elevator trim is close to a neutral state.
SWB Manufacturing offers a few unique engine motor mounts for various Desert Aircraft, DLE, and 3W engines that incorporate different amounts of right thrust into the mount. For example, SWB Manufacturing offers a mount for the Desert Aircraft DA-150, DA-170, and DA-200 engines that is available with 0°, 2.5°, 3°, and 3.5° of right thrust. Although a washer or two may be needed to fine-tune the aircraft, using one of these mounts may save the builder time because it eliminates the need to make a spacer out of laminated pieces of plywood and to use a belt sander to obtain the correct amount of right thrust.
Knowing how to properly tune an aircraft—although it’s time-consuming—is mandatory if a pilot wishes to perform each flight with consistency. A flier should only think about the sequence he or she is to perform and how to correct for any weather that is present on competition day.
Always remember to learn from each experience, ask fellow skilled pilots for advice, and enjoy this fine sport!
Until next time, fly hard![dingbat]
SWB Manufacturing Inc.
International Miniature Aerobatic Club (IMAC)