Trimming from the Ground Up: Directional Controllability

Originally by Dean Pappas for Sport Aviator

In this article, we will approach trimming a plane for directional controllability. We will begin with roll forces. When an airplane is properly trimmed and balanced from side to side, the center of gravity (CG) is in the middle and both wings lift equally in straight and level flight. When an airplane is imbalanced, one wing must lift more than the other, making the roll balance airspeed-sensitive and adding asymmetric drag to one side of the airplane.

The Problem: Roll Control

If the airplane is heavy on one side, it will tend to roll that way when in level flight. Because the source of this force is gravity, it does not change with airspeed. The lift of the left and right wing panels can also cause an airplane to roll, but airspeed can affect the degree to which this is a problem.

The solution to keeping the model balanced at all airspeeds is to have the weight of the aircraft balanced from side to side and to make sure both wings gain and lose lift in exactly the same way as airspeed changes.

Here are some possible causes of airspeed-dependent lift imbalance:

  • Aileron hinge-line gaps. If air can go through the aileron hinge lines, it will. That represents a loss of lift, and the leakage is an often unpredictable function of airspeed, angle of attack, “G” loading, and aileron-control deflection or trim. That means the leakage is seldom balanced from side to side. The leakage often gets worse at high angles of attack, such as in a climb. The airplane will turn to that side.
  • Imperfect airfoils. Tiny differences in airfoil shape from side to side (especially the rounding of the leading edges (LE’s)) can require that the ailerons be trimmed to counteract. The aileron deflection and airfoil shape will have different airspeed characteristics, so the trim will be upset as the airspeed changes.
  • Wing warps (even subtle ones) will require the ailerons to be trimmed to counteract, and these two also vary with airspeed. The warp usually maintains its influence at very low airspeeds better than the aileron deflection.
  • If the ailerons are trimmed to one side to counteract a problem caused by the rudder trim not being centered (or a crooked fin!), the balance between these control surfaces will change with airspeed. We call this condition an “aileron vs. rudder cross-trim”.

Let’s go into more detail about cross-trim. We typically trim the ailerons to make the model fly straight at cruise speed. One of the hallmarks of a stable aircraft is that the application of rudder control will yaw and roll the airplane in the same direction.

If the rudder trim is slightly off one way, the ailerons will have to be trimmed the other way to make the model fly in a straight line. We usually do this trimming at cruise speed. The balance gets upset at low airspeed (such as in a climb or glide). The rudder normally predominates at low airspeed.

Balancing the Wings

It is surprising how far off-balance many airplanes are. The muffler alone can do that; many are close to a half pound in weight and maybe 4 or so inches from the center of the airplane. If there are one or two heavier sheets of wood in one wing panel than in the other, the resulting imbalance can be severe.

When that happens, you have a difference in the required lift from one wing to the other. At a high speed, this imbalance can easily be counteracted with a tiny bit of aileron trim. That’s usually how we set the transmitter trims in our airplanes: while we are in level flight at cruising speed.

At landing speed the imbalanced wing weight doesn’t change, but the aileron and rudder effectiveness do, so the model starts to wander off to the heavy wing.

That’s the why of it; now for the how.

Supplies needed:

  • String or thin wire
  • Sturdy place on the ceiling to tie the plane to
  • Weight (See instructions for more information)
  1. Tie a string to the bare crankshaft and tie it to a nail in one of the rafters above a clear area on the floor.
  2. Run a piece of string or thin wire under a rudder hinge, approximately halfway up the rudder, and lift the tail by the wire coming out of both sides. You can get the most sensitive measurement of side-to-side balance by picking the correct hinge. If you start at the top, a large imbalance will only cause the model to tilt a bit. As you move down, the balance becomes more sensitive, and if you pick a hinge that is too low on the rudder, you won’t be able to get the airplane to balance at all. It will just flop over one way or the other.
  3. Move up one hinge from there and balance the model by adding weight to the high wingtip until it balances properly.
  4. Find a way to keep the weight from falling off. Everything from stick-on lead tire balancing weights to finishing nails stuck in the end of the tip-block has been used. If you feel like patching the covering job on the wing, feel free to put the weight inside the wing. It looks better!

Time to Go Flying Again

In trimming for good directional control we have two main goals:

  • Trim the (now sealed) ailerons and rudder so that the model is not cross-trimmed and flies straight at all speeds.
  • Achieve predictable aileron response at all speeds—especially slow. The two critical flight regimes are the steep climb right after takeoff and the critical low-speed turns used to line up with the runway for landing and to counteract wind on final approach.

Aileron and Rudder Trim

When an airplane is cross-trimmed, it behaves differently turning left vs. turning right. Let’s say the model has the rudder offset to the right. The ailerons will have to be trimmed left while in cruise flight in order to fly in a straight line. In fact, the aircraft will be crabbing to the right in straight flight. The same sort of thing happens when a car has the rear axle bolted in crooked.

When this airplane is turned to the left it will tend to hang its nose “out of the turn” and may even constantly tend to roll back to level flight. When turned to the right, this model will tend to “wind into the turn” and even try to roll over into a spiral dive.

You already know the test to detect a cross-trim: make left and right turns, always using the same bank angle, and adjust the rudder trim away from the direction of turn that winds in. Every time you adjust the rudder, go back to trimming the ailerons for straight and level flight. You’ll have to go back and forth a few times to get it right.

When you think you have it right, try a long glide at idle power as a fine-adjustment test. Set up with the airplane flying straight into the wind, and repeat the hands-off glide test a few times if there is any kind of wind out. If the model wanders off to one side, tweak the rudder trim to correct and re-trim the ailerons again.

Any difference between this test and the turn test is generally caused by subtle wing warps or other assembly issues. You’ll have to accept any difference that remains between left and right turns, although nine out of 10 times the glide and turn tests agree.

Your aircraft is now really trimmed to fly straight!