Trimming Your Model Sailplane

Trimming Your Model Sailplane
by Sherman Knight

Trimming your model aircraft is simple. However, most pilots are convinced that because instructions from the manufacturer are in written form, the information is accurate and can’t be changed under any condition. In all the model aircraft that I’ve built, I’ve only had one aircraft where the CG was correctly located on the plans. Until you overcome the belief that if it’s in written form, it must be correct, trimming will never become easy.

There seems to be a prevalence among model manufacturers that to ensure stability, you need to have a forward CG. However, if you wish to increase performance, the CG must be moved aft. We’ll go into this in detail later in the article. The important thing here is to understand that at best, the instructions that came with your model are simply a place to start. A willingness to move your CG and change the throw on your control surfaces will result in a better performing airplane.

Let’s start with the aircraft itself. If the aircraft is twisted or the control surfaces are out of alignment, the trimming of the aircraft will simply consume more time and you will never obtain the best results.

Many model airplane instructions provide a sequence for construction. Let me simply recommend that four separate components of the aircraft should be built, independently of one another, before they are assembled in the finished project. Those four basic components are: the wings, the fuselage, the fin and rudder, and the stabilizer/elevator.

First, let’s start with the wings. Place both wing panels bottom to bottom with each other. Each wing panel should be exactly (exactly!) the same length. If they are not, they need to be modified so that the lengths are equal.

If your wings have bent up wing tips or are polyhedral in nature, place them on your work bench, trailing edge to trailing edge. The angles should match exactly. Build one wing first, then match the second wing to it. Simply using a measuring stick or a compass is not accurate enough.

The fuselage. The wings may be attached to the fuselage in many different ways. If your fuselage has an integrated fin (typical fiberglass fuselage) then you need to ensure that the vertical fin is perpendicular to the wings. Do not try to align the wings with the fin until all the internal controls are permanently mounted and the rudder post is installed. If you have a top mounted wing, the saddles may have to be modified. If you have a shoulder mounted wing with a joiner rod, you may have to modify the joiner rod hole through the fuselage or the alignment pin. Once completed, (even after painting) the fin can still be aligned by placing hot towels or heating the fuselage boom with a heat gun and twisting the boom to align the fin. Be careful not to overheat it, or you may cause some permanent damage.

The stabilizer or elevator should be mounted last. Now that the fin is square to the wings, you need to ensure that the stabilizer is also square to the wings. I’ve seen instructions that show placing a 90E triangle between the fin and the stabilizer. I can’t think of a quicker way to induce a problem.

The stabilizer must be square with the wing, not with the fin. Use the wing as the reference point and eyeball it. From a distance of 25’ or 30’ away, looking straight down the fuselage is the best way to obtain that perfect symmetry.

Now that you have all the various parts of the airplane square with one another, it needs to be balanced. Add the appropriate nose weight to obtain the center of gravity indicated in the plans. Do not add additional nose weight or glue the weight in place. It’s a very safe bet that the manufacturer has located a CG forward of its optimum performance position. I don’t know why they do this, but they all do.

Now you need to balance the aircraft laterally. The aircraft needs to be placed in such a position so that it can rotate around the center axis of the fuselage. If one wing tip wants to consistently crash to the table top, you need to add weight to the opposite wing tip. It is important to perform this step. A laterally unbalanced aircraft will require aileron or rudder input which will result in unequal flying surfaces. These unequal flying surfaces will require that you change or modify the trim of your aircraft every time its speed changes. In other words, a little right aileron trim may be necessary to keep the airplane from rolling to the left at low speeds. (i.e., the airplane wants to roll to the left because the left wing is heavier than the right) However, if you put the nose down and pick up speed, the airplane will now want to roll to the right as soon as the unbalanced control surfaces over come the heavier left wing.

These types of difficulties require constant pilot input. Reduction of this unnecessary stick movement is one of our goals. In other words, an airplane that can fly consistently over a wide range of conditions with little stick movement is much easier for a pilot to control than one that requires constant attention.

Before you take the airplane to the field for the first time, standing behind the airplane, make sure that all the control surfaces move in the same direction as the control sticks on your radio. If you set up your airplane while standing in the front looking toward the tail, a common problem may occur. It’s typical to get the ailerons moving in the right direction, but when standing in the front, it’s also typical to get the rudder mix backwards. Stand behind your airplane, not in front of it, to make sure that all the control surfaces work in the right direction.

Charge all your batteries up, take your flight box along with glue, tools and anything else necessary that you might have to use to modify the airplane at the field, and let’s go flying.

It doesn’t matter how many times you’ve broken in a virgin airplane, that first flight is without a doubt one of the most nerve wracking experiences you can go through. It is well worth the time and the effort to have somebody along who can help you out. Hopefully, someone experienced.

Once you get to the field and everything is appropriately assembled, it’s time for your first test flight. Find somebody who can throw your aircraft for you. It doesn’t have to be thrown hard, just hard enough to reach flying speed. Not only that, but you don’t want to throw the airplane up or down, but simply level, at the horizon. The purpose of this initial trim flight is twofold. The first is to provide the proper elevator trim. The second is to provide the proper amount of roll. Keep throwing the airplane until both roll and elevator provide for a nice smooth and stable flight. If you throw the airplane hard and it pitches up immediately after leaving your hand, it’s probably nose heavy. We’ll spend more time on that later.

The aircraft is now set up so it can be launched on a winch or high start. However, before you do, hang the airplane upside down from the tow hook. Use the end of the winch line or high start line. The upside down airplane should hang tail down (just a little is all that is necessary). If the airplane hangs tail up and wants to slide off the tow ring, then the tow hook has been placed too far aft and must be moved forward. Do this before your first launch. Nothing is more exciting than trying to calm a wild aircraft on launch.

Once the aircraft is launched, turn and head directly into the wind. Trim the elevator so the airplane will fly as slow as possible.

Put the airplane in a shallow dive (30E) for about two seconds. Pull the elevator stick all the way back and loop the airplane. The aircraft should loop and then return to the exact same spot that the loop started. If the airplane wanders to the left or right, first check to ensure that the rudder trim has been neutralized – not on the radio, on the airplane. If the rudder trim is neutral, and the aircraft has been properly laterally balanced, you either have a twisted wing (OOPS) or a left and right wing at different angles of attack (double OOPS). If you’re flying a built-up wing, you can take the twist out of the wing by simply reheating the covering, twisting the wing into its new shape, and reshrinking it. Composite aircraft can usually be unwarped by placing them in the fold of an electric blanket, raising the temperature and twisting the wing back into shape.

On shoulder mounted wings with alignment pins, it’s easy to misalign the angle of attack of the two wings. Typically, the modeler will align the root of the wing with the wing saddle on the fiberglass fuselage. It’s been my experience that these fiberglass saddles are not created equally on both sides of the fuselage. In the field, you may actually have to modify the location of the alignment pin in the fuselage so that the wings have the same incidence angle to each other.

An airplane with a warped wing or different incidence between the two panels will result in an aircraft that will turn one way significantly better than the other. If twisted enough, you will wind up with an airplane that will snap roll in one direction and want to fly out of the turn by itself in the other.

Matching the angle of attack on each wing half is one of the most important steps that you can perform. If you have the necessary equipment, make the incidence change in the field before doing any other trimming. I recommend gluing your alignment pin permanently in the wing root. In the field, if you have to modify the angle of attack, the hole for the alignment pin must be relocated. This can be done easily in the field. Overdrill the alignment pin hole, coat the pin with jelly, place some thick CA in the alignment pin hole and reattach the wing (the jelly will keep the CA from bonding to the alignment pin or wing root).

Now that the aircraft is flying straight and level, without a tendency to wander to either side and turns equally well in both directions, it’s time to adjust the center of gravity. It’s typical to have to add lead to the nose of a model sailplane. It behooves you to move the servos, battery pack, and receiver as far forward in the nose of the aircraft during construction as possible to minimize this additional weight.

Whatever you do, don’t permanently glue all the lead in the nose of the aircraft before you go to the field. You need to insert any nose weight in a way so that it can be removed easily in the field. You also need to install it in the airplane in a way so that it will not move around on its own. It is extremely important that the weight not shift during flight.

Every model I’ve ever flown, but one, required the removal of nose weight. Now that we’ve got the plane flying straight and level, we need to launch it again. It’s now time for the effective (but controversial) dive test. Again, place the model in a shallow dive (30E) and after two seconds, let go of the elevator stick. If you built and balanced the model at the point indicated on the plan, the model will pitch up after you let go of the stick. If the aircraft pulls up all by itself, the aircraft is nose heavy and nose weight must be removed. I know this sounds backwards to many of you, but the reason is actually quite simple.

If you look at your model airplane from the side, the center of gravity is that point at which if you were to place a fulcrum, the airplane would balance level. The tail surfaces of an airplane typically produce lift but in a downward direction (opposite direction from the wing). It is this downward force which counter-balances the weight in the nose of the airplane and the pitching moment of the wing. However, as an airplane increases its forward speed, downward lift created by the stabilizer/elevator increases. However, the weight in the nose of the sailplane remains the same. As the aircraft velocity increases, the center of effort moves and the airplane wants to pitch up (pull out of the dive) all by itself. It’s actually much more complicated than this, but a discussion concerning the moving of the CG relative to the center of lift would take a whole book.

The goal is to obtain an aircraft which no longer pulls out of the dive. In other words, one that has a neutral or almost neutral elevator.

After the airplane lands, take some of the nose weight out of the airplane. After taking nose weight out of the airplane you must add down trim to the elevator. Re-launch the airplane, turn into the wind, re-trim for slow flight, and re-execute the dive test. The plane will continue to pull up but not as abruptly as it did before. Land, remove additional nose weight and add additional down elevator trim, and re-launch. You will have to keep performing this test until the aircraft no longer pulls out of the dive on its own. One BB at a time is too little. Try taking weight ¼ oz. at a time. This usually equals 1 to 2 clicks of down trim.

No doubt, some of you are now thinking about those articles you’ve read where an aft CG produces an event known as “pitch instability” or “tuck under”. As the CG of the aircraft moves back, the elevator of your aircraft becomes more efficient. In other words, it requires less control throw to obtain the same pitch movement (climb or dive).

Many new pilots don’t like this increased pitch sensitivity, so they add weight back to the nose of the sailplane. You can accomplish the same result by simply reducing the control throw at the elevator. If you have a radio with dual rate functions, just reduce the amount of control throw. The same goes for the computerized radios. If you have a simple 4-channel radio without dual rates, move the clevis at the elevator control horn all the way to the outboard position. If that’s not enough, move the clevis at the servo arm as close to the center hole as possible.

If your model has a full flying stabilizer, the trim adjustment of the stabilizer to obtain the proper incidence in the dive test is relatively easy to do. On an aircraft with a separate stabilizer/elevator, this could be much more difficult. You may need to re-trim the stabilizer so that after the dive test, the elevator and the stabilizer are perfectly matched (i.e., no up or down deflection on the elevator). Shim the leading or trailing edge of the stabilizer as necessary to obtain the neutral elevator so that in the dive test, the airplane does not pull out of the dive on its own.

Here in the Pacific Northwest, an extremely popular novice aircraft is the Great Plane Spirit. We’ve trimmed hundreds of these airplanes at our field. To give you an idea of just how far off some of the manufacturers’ information may be, performance is enhanced significantly by performing the following:

1. Build and balance the airplane per the CG on the plans.

2. Remove 22 oz. of nose weight.

3. Reduce the elevator deflection by one-half.

4. Double the rudder throw.

5. Move the tow hook all the way to the forward most location.

Two and one-half ounces of nose weight is a lot of weight in a 2 meter sailplane. Not only that, but its removal significantly increased the thermal performance of the airplane.

To return to the trimming of the original airplane, we now need to complete its trim by providing the appropriate amount of control throw.

The first item to be adjusted is elevator throw. Determine the tightest thermal turn you wish the plane to perform. While in the turn, your elevator stick should be nearly bottomed out. If you have a lot of additional stick throw, it’s unnecessary and will never be used. Reduce the amount of control throw still further on the airplane, and eliminate the additional stick movement. You will find, that by softening the elevator input, the sailplane is more gentle and easier to fly at a distance.

The next issue deals with aileron differential. It’s important that the ailerons move up further than they move down. If the plans don’t call for differential, a good place to start is with twice as much up as down deflection.

Launch the aircraft and disable any rudder mix that you may have. Roll the airplane left and then roll it right. If the airplane rolls along the fuselage center axis, without yawing left or right, your differential is just right. If you roll left, but the airplane yaws left (nose down), you have too much differential. If the airplane rolls left and yaws to the right (nose up), you need to increase your amount of differential.

Once the proper amount of differential is obtained, then turn on the rudder mix. If your differential is correct, it won’t take much rudder mix at all (the rudder mix on my Prism is less than 3/8” at full aileron deflection).

Too much rudder mix can actually stall the inboard wing in a turn. Many may find this difficult to believe, but if you have an airplane that tip stalls easily, try reducing the amount of rudder mix and increasing the aileron differential. That tip stall may go away.

Soft controls result in a gentle flying airplane. A gentle or smooth flying airplane thermals better. It’s just that simple. If you have more control throw on any surface than you absolutely need, you have too much. To give you an example, the other day I trimmed out a fellow pilot’s airplane that was “twitchy.” Although it was twitchy, it was very flyable. At a distance, where it was difficult to see, it became nearly unflyable. After some experimentation, we reduced his elevator throw from 100% to 30%. Not only that, but his radio had exponential rates and we bumped his exponential rates up to 35%. The airplane now became extremely smooth, very easy to fly, and with an aft CG, it thermalled like a bandit.

The dive test referenced above is not the end-all to trimming the CG of your airplane. It is merely a good place to start. If you read all of the airplane magazines, you’ll soon discover that the dive test can be relatively controversial issue. However, if you want to get your CG in the ballpark quickly, it’s the best and most expedient way to accomplish it. Your elevator will become more sensitive, so simply reduce its throw.

Finally, the tow hook. The last and final item to trim. As the CG is moved aft, down trim must be input to maintain slow speed flight. Through out this exercise the tow hook remained in the same place. To obtain the steepest climb possible, the tow hook must be moved back at least as far as the CG was moved. The tow hook only needs to be ahead of the CG, and not by much. An eighth of an inch is plenty.

I hope the above information provides some useful guidelines. But remember, that’s all it is, a guideline. Don’t be afraid to experiment and to try alternate setups. Try those alternate setups, and continue to modify them until the airplane actually begins to fly worse. Then simply back up a little bit.

It’s unfortunate, but a common statement I hear from many fliers is, “It’s flying good enough, I don’t want to touch it.” There’s a big difference between “good enough” and “excellent performance.” There’s no reason why every airplane in your inventory shouldn’t be an “excellent performer.”

A sailplane that flies perfectly out of the box doesn’t exist. If you expect yours to, you will be disappointed. Be patient and don’t be afraid to make those necessary trim changes. You will be glad that you did.

I’d like to thank the members of the Seattle Area Soaring Society for showing me these useful tips over the last several years. Without their help, guidance and support, my airplanes would not fly as well as they do.

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