Friday, October 20, 2017
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Aerobatic Trimming

One important but often overlooked aspect of flying precision aerobatics is setting up your aircraft trimming it. This isn t trimming in the general sense of adjusting flight controls to hold level flight, but rather the small adjustments you make to your airframe and radio setup to help your model fly as precisely as possible. While this may seem to be a bit of overkill for a new aerobatic pilot, I find that a nicely balanced and precisely set up model makes learning precision aerobatics much easier, and it really isn t all that hard to accomplish. In this article, we ll discuss the why of aerobatic trimming. A companion online feature at www.flyrc.com details the specific techniques and processes for programming two popular radio systems the Futaba 9C and the JR 9303.

SETTING CG
Adjust the CG so that at a neutral elevator position the model descends slightly during inverted flight. If the model descends abruptly, adjust the CG aft. If the model climbs while inverted, adjust the CG forward.

THE TRIMMING PROCESS

For starters, let s assume that your model is straight and square. If not, correct those areas that you canwe ll live with the rest. Your model should have the appropriate dihedral, wing and tail incidences, and thrust line. It should also be set to the manufacturer s recommended CG. Let s give the manufacturer the benefit of the doubt in the design and adjust from that starting point.

Before we get too far along, I think it is important to point out that trimming your model for aerobatic flight is a process with a distinct sequence. I d love to be able to lay claim to the process I ll outline, but in reality, the primary architect of what I ll cover is Peter Goldsmith, of TOC and IMAC fame. Many pilots, while trimming their model for aerobatic flight, tend to either jump ahead or omit steps from the trimming process. Unfortunately, by jumping ahead, their trimming efforts can cause more problems than they solve. Each setup item has an effect on the next, so their order, naturally, becomes important. We ll break the process into the following unique steps.

1 Center of gravity

2 Thrust line

3 Aileron differential

4 Sideslip coupling

One item to note: this article is intended for setting up most sport aerobatic models. It isn t, however, intended as a competition setup guide. We won t cover setting wing and tail incidences, as those aren t adjustable on most small sport aerobatic models. Everything I discuss here is appropriate for setting up sport aerobatic airplanes from your common aerobatic park flyer all the way up through giant scale models.

CENTER OF GRAVITY

When settling on a particular CG for precision flight, a good starting point is the manufacturer s recommended CG position. I have found, however, that many models recommended CG positions are very, very conservative, as are their recommended control throws, so fine-tuning is generally appropriate.

For discussion purposes, the position of your model s CG affects primarily the pitch axis of the model. A forward CG results in a more stable yet less responsive model. An aft CG will result in a more pitch-sensitive and less stable model, while extreme aft CG positions can make the airplane over-responsive and even uncontrollable. Our goal is to balance those characteristics, creating a stable yet responsive aerobatic machine.

Let s start from a level inverted line. Flying level inverted, neutralize the elevator control. Your model should descend ever so slightly, requiring only a very small down elevator input to maintain level flight. If the model descends abruptly, adjust the CG aft and repeat the test. If the model holds perfectly level inverted flight or climbs inverted with neutral elevator, adjust the CG slightly forward. For precision aerobatics, I recommend a slightly forward CG, allowing the instantaneous snap and spin recovery required for precision aerobatics. This is different from an ideal 3D CG position. For 3D flight, I prefer a perfectly neutral model one that maintains perfectly level flight both upright and inverted.

A good secondary CG test is 45-degree up-lines with a half roll mid-line. During the upright portion of the 45-degree up-line, you ll likely be holding only a slight up elevator input. After the half roll to inverted, the down elevator input should only be slightly more down elevator than the amount of up elevator required for the upright. Once you ve got a CG you re comfortable with, the next step is adjusting your model s thrust line.

THRUST LINE

Adjusting your model s thrust is the one change that most modelers overlook when setting up their model for precision aerobatic flight. In fact, I find most modelers simply bolt the engine to the firewall, match the cowling to the spinner location, and settle for whatever thrust line that happens to be never again considering that the thrust line may not be perfect for how they want the model to perform. As a part of our trimming sequence, I advocate that you test the thrust line, adjust it if necessary, and then adjust the cowling to match the appropriate thrust line. On some models, realigning the cowling isn t possible, in which case I simply accept the fact that the spinner is offset from the cowl slightly. I d rather have a precise flying model than one that only looks like it flies well!

I adjust the thrust line in two separate steps. We ll start with left/right thrust. Most models have clockwise rotating propellers when viewed from the cockpit and, as a result of spiral slipstream from the spinning propeller, the model tends to yaw to the left under power. The common remedy is to adjust the thrust line to the right slightly. Most modern model manufacturers build some right thrust into their motor mounts, so let s start with what the factory gave us. If your model doesn t have any right thrust, I d recommend two to three degrees of right thrust as a starting point.

I test the left/right thrust line by flying a vertical up-line at 3/4 power, paying close attention to the nose of the model for any yaw divergence from the vertical line. Make sure that when setting the vertical line that you re actually vertical. Any residual yaw on the vertical line, caused by the pitch up from level, will disguise the required thrust line change. So pull to vertical, set the line, and then neutralize any rudder inputs. If the model yaws to the right, reduce your right thrust. If the model yaws to the left, increase the right thrust. I find that most models need around two to three degrees of right thrust to adequately compensate for spiral slipstream.

Adjusting up or down thrust may also be necessary. It isn t uncommon for models with low wings and a high thrust line to require up thrust, while models with a high wing and lower thrust line often require down thrust. A good test for the vertical thrust line is, again, to fly vertical lines at a steady thrust setting of around 3/4 throttle. If the model pitches toward the canopy, add some down thrust angle. If the model pitches toward the landing gear, add some up thrust. In most cases, the up/down thrust changes are less than a degree in all. The actual thrust line setting can be confirmed by flying other lines, including level lines, 45-degree lines, and inverted lines. With thrust changes, the model should maintain a constant flight path. No thrust line is perfect at every airspeed and attitude, but a reasonable compromise is achievable.

AILERON DIFFERENTIAL

With the CG and thrust lines set, the next step is to make your model roll axiallyas if on a stringby adjusting aileron differential. Barring any manufacturer specific direction, I generally recommend symmetrical throws as a good starting point. Some models, however, just won t roll axially with symmetrical throws and aileron differential is a great solution.

SETTING AILERON DIFFERENTIAL

Setup perfectly, your model should roll perfectly axially. To test, fly vertical down lines while rolling. Compensate for any barreling of the roll with aileron differential.
 
As a result of adverse yaw from the ailerons, some models require aileron differential to roll axially. If the vertical roll barrels, reduce the travel of the aileron that descends below the wing. Be sure to test in both directions as they are often different.
 

I find the best way to test for axial roll characteristics is on vertical down lines. With the throttle closed, establish a down line and begin a constant roll. Note any barreling of the roll. Perfectly trimmed, the model should maintain a straight vertical down line while rolling axially as if on a taught string. To correct for a barreled roll, start by reducing the throw of the down aileron for that roll direction incrementally. I find that small changes in differential, often just a couple of degrees, are enough. Also, don t forget to test the roll in the opposite rolling direction, as it isn t uncommon for the two rolling directions to be slightly different.

One thing to note: I recommend testing for axial rolls only on vertical down lines, not on level lines. The reason for this is that on vertical down lines there is no up or down, so any non-axial rolling tendencies noticed on the vertical line relate only to the rolling characteristics of the model, not the effects of gravity. Also, unlike upright level flight where the wing is at a slightly positive or negative AOA, vertical lines are close to zero AOA. At zero AOA, we can truly assess whether the model needs differential to roll axially.

SIDESLIP COUPLING KNIFE-EDGE FLIGHT

Sideslip coupling, more commonly known as knife-edge coupling, is the last trim adjustment I make. Some pilots start making this adjustment right away, but I find both CG and thrust line have an effect on sideslip coupling, so setting those first and then coming back to sideslip mixing makes the most sense. Any adjustments to the sideslip coupling on your model are done through mixing functions on your transmitter. In reality, all we are doing is mixing in a certain amount of elevator or aileron with rudder deflection to compensate for sideslip coupling.

Before we get too far into the discussion, I think it s important to clarify a misnomer specifically that knife-edge coupling only occurs during knife-edge flight. In reality, if a model displays knife-edge coupling, the same coupling exists anytime the model is flown in a sideslip condition. So what s a sideslip condition? Uncoordinated flight would be the technical answer to that question, but in layman s terms, sideslip flight occurs virtually every time you deflect the rudder in your modelwith the lone exception of coordinated turns. Next time you re at the field, take a model with known knife-edge coupling issues and aggressively deflect the rudder while in level flight. If your model pitches to the landing gear and rolls with the rudder in knife-edge flight, it will also pitch to the gear and roll with the rudder from level flight with rudder usage.

The best test for setting your models thrust line is the vertical up line. If the model tracks left, add more right thrust. If the model tracks right, reduce the right thrust. Set correctly, your model should be able to track a perfect vertical line for several hundred feet with only minimal corrections.
 

The easiest test to determine the need for a sideslip mix is to simply apply rudder from level flight. If the model pitches or rolls, you ll need to apply a mix with the rudder channel as the master and the correction as the slave. Most aerobatic models tend to pitch to the landing gear and roll one direction or the other. I generally start by mixing out the pitch coupling first, then focus on roll coupling. As a secondary test, fly knife-edge lines both directions and fine tune your previous adjustments. While I ll discuss the technical aspects of setting sideslip mixes in a future article, it s important to note that you should never try to make any adjustments to your computer radio while flyingdon t ask: it has been attempted before with less than stellar results. I ve found better luck drafting a flying buddy for scribe duties during each flight to accurately record impressions and make adjustments after each flight. In most cases, I can accurately tune sideslip mixes in less than 10 flights.

One item to note: sideslip coupling is rarely identical from left to right. With one wing high in knife edge flight, the model may pitch to the landing gear slightly; and with the other wing high, it may pitch to the canopy. Simply adjust your mixes to match your particular model s needs. Finally, I recommend you set up your sideslip mixes to be active during all phases of flight. For precision aerobatics, there are very few instances in which the mix would hamper the precision of your flight, as sideslip coupling applies any time the rudder is used.

CONCLUSION

I ve heard many competition pilots describe the trimming process as a never-ending circle. Each additional change you make can force modifications to previous changes. I find, however, that most sport aerobatic pilots require only one trip through the steps outlined here to dramatically improve their model s precision aerobatic flying traits.

By following the basic order of adjustments outlined here, you will be making the model fly more precisely on its own. This means less to think about, allowing you to focus on flying each figure as precisely as possible. Some look down on electronic mixing. Others widely accept mixing as reasonable and necessary to achieve the best possible flying model. I find it to be very helpful and, from my perspective, any adjustments you can make to help your model fly precise lines, roll axially, and maintain clean knife-edge passes is time well spent. Don t forget to visit www.flyrc.com where we dig into the how-to of electronic mixing with the Futaba 9C and the JR 9303 transmitters. Till next time, remember: aerobatics make the world go round.