Trimming from the Ground Up: Pitch

Trimming from the Ground Up: Pitch

Originally by Dean Pappas for Sport Aviator

You can learn a lot from watching what happens at the flying field on a Sunday afternoon and even more from the beginners. You learn what the basic flying skills really are and, most importantly, you see the beginners struggling with their trainers’ shortcomings. So much hard-earned experience goes into building a well-behaved RC airplane, more goes into installing the mechanical and electronic systems, and even more goes into adjusting or trimming for best flight performance. The purpose of this article is to make it easier to gather that knowledge and experience.

When we refer to “best flight performance,” we don’t mean making your trainer perform like a P-51; we mean getting your model to perform its intended “mission” as well as it was designed to. For a trainer that mission is to be well behaved, predictable, and have solid control, especially during takeoff and landing.

The mission of sport and Scale airplanes is similar to the following—with some additions, depending on the type of model. It would be good for a Scale airplane to be well behaved while performing any maneuver that is typical of the prototype. For the sport flier it would be nice if the airplane’s predictable behavior helped him or her “look good” while enjoying the sport.

On the other hand, many airplanes have what we often call a “personality.” That’s code for “It ain’t quite right but I’ll live with it.” Sometimes experienced fliers do not even realize they’re living with a model’s undesirable quirks; either their skills are good enough to cover for it or maybe they have never had their hands on a dead-honest airplane. It can be an eye-opening experience! Students don’t have those skills yet, and they have no basis for comparison at all; and that can be a problem.

That, in a nutshell, is why we are here: to learn that you don’t have to live with it. We can make it better and your flying will benefit at all skill levels, from beginner to highly competent. Most important, as a student your learning curve can be shortened if your airplane is working with you rather than against you.

The Kinds of Problems to Be Fixed: Your Model’s “Personality Problems”

The list of common trim problems is not that long. It doesn’t have to be because any problem can make flying your airplane difficult. Multiple problems usually add up to more than the sum of the individual parts. There is often more than one cause for a particular problem, and we must figure out where to attack.

1) Poor aileron control response (especially at low airspeed) and directional trim that changes at different airspeeds make accurate flying difficult. These two problems can make it unnecessarily hard to learn to land. It’s tough enough for a student to learn left from right while on the landing approach, but if the airplane tends to deviate to one side and then the control you use for correction becomes sluggish, you have the beginnings of a panic situation. This is supposed to be fun, and we just don’t need panic situations!

2) A tendency to veer off in one direction (usually the left) when climbing or when full power is applied adds an unnecessary workload during takeoff. Combine this with poor aileron control response, and you have another potentially unsafe combination.

3) If your airplane drastically changes pitch trim with changes in throttle and airspeed (meaning it’s either climbing or diving without elevator input), it’s a problem that can lead to a loss of airspeed and control at the wrong time. This can combine with both of the preceding to create even bigger problems. Depending on the airplane’s mission, we often intentionally set it up to climb with full throttle (but not too steeply), to maintain level flight at cruise power (maybe a bit more than half throttle), and to finally descend at a gentle glide slope (with enough airspeed for good control) at a fast idle.

4) This next problem is closely related to the preceding problem. If the airplane does not settle into a predictable glide slope when the throttle is reduced, this can add to the pilot’s workload during final approach and landing. A proper glide has a predictable sink rate that is just steep enough to maintain adequate airspeed for good control, but it is not so steep or so fast that it makes it hard to get the airplane to settle to the ground in the flare.

The flare is that last portion of the landing, in which up-elevator is added to almost stop the descent rate and bleed off the last bit of excess airspeed. This makes the model touch down in a three-point attitude if it is a tail-dragger or with the main gear first and the nose wheel an inch off the ground in the case of a tricycle-geared model.

If the glide is too shallow, the airplane will mush along with the nose up and with low airspeed, leading to poor directional control authority. This often leads to the problems in item 1. You will often find experienced pilots landing a particular airplane “hot,” or fast, every time because the model has a controllability problem at low speed.

The Pitch-Control Balancing Act

Predictable control is a balancing act. There is a balance of forces always at work to make the airplane fly straight and level, to climb, and to descend. When the forces are not precisely in balance, the airplane will be changing pitch—either nosing up into a climb or dropping into a dive.

The dominant forces are aerodynamics, gravity, and engine thrust. That’s not much of a surprise, is it?

This explanation will not be entirely rigorous, but we do want to give you a feel for how these forces juggle so that the kinds of adjustments we make later will make sense. For almost all “normal” airplanes the horizontal tail holds the tail end of the airplane down. The wing makes lift, and the act of making lift creates a nose-down torque. This is for two reasons, the first of which is that for stable flight (again, for almost all normal airplanes) the center of gravity (CG), or balance point, is in front of the wing’s center of lift.

The second reason is that as the wing bends the passing air downward, it can be said to rotate the airflow; therefore, the air imparts an opposite, nose-down rotation to the wing and the airplane to which it is attached. Although it’s simplistic to put it this way, the wing pushes down on the passing air and the passing air pushes up on the wing.

Along with this nose-down torque, which is a by-product of making lift, add the nose-up effect of the horizontal-stabilizer incidence angle and the level-flight trim position of the elevator. Ideally the elevator should be straight, as compared to the horizontal stabilizer, but sometimes it is necessary to trim the elevator up or down a bit.

Finally, there is the small nose-down torque caused by the engine downthrust. That effect is changed by the engine’s throttle setting; at idle the trim force caused by downthrust is nil, while at full throttle it can be important. This makes downthrust an important part of the pitch-trim balance “see-saw.” Look at the diagram showing pitch see-saw and the diagram showing incidence angles and downwash.

There is also a balance of forces in roll or from side to side, but we’ll cover that later.

Pitch Trim

In the list of preceding problems, items 3 and 4 were devoted mostly to pitch issues; we’ll start there.

First we should tend to a few details of the sort that are best taken care of at home, in the workshop.

To begin with, make sure the balance point, in the fore and aft direction, is where the plans or instructions indicate. If the plans show a range of positions, as they should, shoot for somewhere in the forward half of that range. We call that a “nose-heavy” CG. The ideal balance point is not a well-defined location for a particular airplane design. It can vary a bit depending on the flying for which your airplane is intended. It also depends on the all-up weight, the size and location of the fuel tank, and small differences in building or assembly.

A quarter of a degree difference in the incidence angle between the wing and horizontal stabilizer in your airplane compared to the designer’s can change the ideal CG location. For that reason, most designs show a CG range. As the CG moves aft from the initial nose-heavy position, the airplane becomes less stable in pitch. This is not necessarily a bad thing; excess stability makes an aircraft more sensitive to airspeed changes and makes it less maneuverable.

On the other hand, if the model is too tail-heavy it tends to have a short life! Instability, or even near-instability, causes many crashes. As an airplane gets close to tail-heavy, the first sign is that elevator control gets touchy. When a model is set up at the aft end of its CG range, the elevator control will usually be more powerful. But if it gets jumpy, or the airplane feels as though the elevator trim is inconsistent, you are flirting with tail-heaviness.

For more advanced sport airplanes with semi-symmetrical or symmetrical airfoils, an important factor in where the CG belongs is inverted flight. If it takes too much down-elevator to fly inverted, the model is likely nose-heavy. If it takes no down-elevator, or even climbs sometimes, it is definitely tail-heavy. A jumpy elevator is a sign of near-disastrous tail-heaviness.

If your airplane always seems to run out of elevator authority when it comes time to flare for landing, it could be a sign of nose-heaviness. That is not the only reason for this problem, but we’re mentioning it at this point for completeness’ sake.

Checking the CG

To find the balance point, you need to hang the airplane from somewhere above its three-dimensional CG. All that really means is that if your airplane has a high or shoulder-mounted wing, you can hold it up using one finger on each hand under the wing. If you have a low-wing airplane, you may find it easier to do this with the model upside-down. The trick in checking the balance point by hand is to place a thumb under the wing at the same place on both sides. For low-wing airplanes, do the same upside-down. A piece of tape, on both sides at the CG location shown on the plans, helps you place your thumbs evenly. Make sure to place both fingers the same distance back on each wing panel, and move back and forth until the airplane hangs level.

A typical safe starting point for almost any airplane is if the CG is placed at 25% of the mean aerodynamic wing chord (MAC). The farthest back the CG usually gets on a typical trainer is 33%, or one-third, of the MAC. See the graphic for an illustration of this. Flying wing and tailless models typically fly with the CG at 15%-20% of the MAC. On a constant-chord wing, the 25% point is exactly one-quarter of the way back from the leading edge (LE) to the trailing edge (TE). Most trainers are designed with constant-chord wings.

Once you have found the starting balance point, move equipment if necessary to make the airplane balance properly. When the balance point is incorrect, the first thing that typically gets moved is the battery pack for the radio. Most often the battery has to be moved forward under the tank to move the balance forward. If that isn’t enough, you may even consider using a heavier, larger-capacity battery. After all, nickel and cadmium are useful heavy metals, and lead is just dead weight.

If you must add nose weight, place it as far forward as practical so that less is necessary. The weights that mount to the crankshaft are not generally recommended. If, on the other hand, your airplane is nose-heavy to start with, it is slightly easier to move the battery and receiver aft. The receiver is relatively fragile in a crash (and expensive, compared to the battery), so keep the receiver behind the battery! If you must add tail weight, place it as far aft as you can, on the fuselage, because less will be necessary.

Take a good look at your airplane to make sure the wing and stabilizer are mounted exactly as described on the plans. You are looking for incorrect incidence angles, which could force you to counteract them with excessive amounts of elevator deflection.

One more thing: Make sure the elevator trim on the transmitter is centered and the elevator control surface is straight. That will require a control-linkage adjustment. You don’t want to run out of trim-lever movement because you didn’t set the elevator straight to begin with. That goes for all the other control surfaces too!

Going Flying

Most trainers are designed to climb at full throttle and fly in level cruise at a power setting just above half throttle without having to change the elevator trim. On takeoff your test pilot will take this into account and wait until the airplane is throttled back to cruise power before making any fine elevator-trim adjustments for level flight.

Now, the importance of knowing that the elevator was straight with the trim lever centered will become apparent. As you first put trim into the airplane, you already have some idea of what you are dealing with. Does it need up or down from the ideal, and roughly how much? That’s better than waiting until after landing to look and see that all that furious wiggling of the transmitter trim lever was just to get things straight!

Pitch Flight Testing

Now that the airplane is trimmed for level cruise, let’s do a couple simple tests. Smoothly advance the throttle to full. Without making elevator corrections, but still keeping the wings level with minimal, smooth aileron control inputs, watch the climb that results.

Is the climb too shallow and fast? This might be ideal for an advanced sport airplane, but for a trainer you want a solid climb with adequate airspeed.

Is the climb too steep? Watch to see if the climb is so steep that the airspeed has decayed.

Is it difficult to promptly correct the wind’s effects? If so, that is a sign that the airspeed is too low because of the steepness of the climb. In that case, you can do one of two things: make the airplane less speed sensitive by moving the CG aft and adding down-elevator trim or add more downthrust. If the airplane climbs too shallow, you would do the opposite.