Saturday, May 7, 2011

Trimming from the Ground Up – Part Three


When the airplane is balanced from side to side, the CG is in the middle and both wings lift equally in straight and level flight. When the 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 compared to the other.

IN the previous installment of this “Trimming From the Ground Up” series I wrote about improving the ground handling during takeoff and improving the controllability of the model in the critical seconds after liftoff. Right-thrust and downthrust adjustments figured prominently.

In this installment I will approach the largest subject: directional controllability. I saved the best for last!

In the original list of airplane personality problems presented in Part 1, the first two items were devoted to directional control problems. As with the pitch discussion we started with two months ago, there is a balance of trim forces in roll as well as in yaw. Let’s address the roll forces.

Roll-Control Balancing Act: There are fewer actors in this balancing act than in pitch. There is the wingtip-to-wingtip weight balance. 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 other players on the see-saw are the lift of the left and right wing panels. (See the Roll See-Saw diagram.)

The solution to keeping the see-saw 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; that actually takes a little effort. Following are some possible causes of airspeed-dependent lift imbalance.

1) 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.

2) Imperfect airfoils. Tiny differences in airfoil shape from side to side (especially the rounding of the LEs) 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.

3) 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.

4) 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 cover 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.

Suspend the model at the front with a string or wire tied to the crankshaft, and lift the tail with a string under a rudder hinge. Select a top hinge for a less-sensitive balance and a lower hinge for greater precision.

Back Into the Workshop! There are a few things we need to do before we leave the workshop to make life easier at the field. As we did in the section on pitch, we will fiddle around in the shop for a bit. However, almost all of this could be done at the field if you don’t mind wasting daylight on a flying afternoon.

Let’s cover side-to-side balancing. First let’s balance those 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 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: in cruise-speed level flight.

For some of us, cruise speed is at full throttle. No problem; I like to go fast too! 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. I like to suspend the entire airplane from the crankshaft and from one of the rudder hinges. (See the lateral balance photo.) It is important to balance the entire airplane—not just the wing—because of the influence of things such as the muffler or engine hanging out one side.

The way I do it is to 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. Then I 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.

Move up one hinge from there and balance the model by adding weight to the high wingtip until it balances properly. Then find a way to keep the weight from falling off, and you are finished.

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!

Sealing the aileron hinge line is often done with clear, flexible tape. Wrap a piece of tape long enough to run from hinge to hinge around a credit card, sticky side out, and jam it into the underside of the aileron as far as possible. Trim the loose tape, and voilà!

Sealing the Aileron Hinge Line: Sealing the hinge gaps is a biggie; it ranks right up there with balancing the airplane from side to side. Serious aerobatic types don’t even take the model out of the workshop before doing this. (At least they are not supposed to!)

Don’t get the idea that this is a high-tech technique. It is one of the simplest things in the world to do, and it can fix all kinds of problems.

There are a couple different ways of doing this, the first of which is the old-fashioned method. This is not really a way to fix the gaps, but rather to eliminate them. Old-fashioned cloth hinges and their cousins sewn hinges don’t have gaps, so all you old-timers out there were doing it right 40 and 50 years ago—before the hardware manufacturers made hinging easier for all of us.

The modern cousin to this hinging method is sometimes used on park flyers and small models weighing 4 pounds and less. This technique can be done with tape or iron-on covering.

Short lengths of covering are ironed together, sticky side to sticky side, with roughly 1/8 or 1/4 inch of overlap. The pieces are ironed to the top and bottom of the fixed surface, in an alternating fashion, and each piece is fed through the hinge gap in an “S.”

After a little work with an iron, you have a gap-free hinge. It’s light, simple, and economical. I don’t recommend this for larger models. (See the Iron-On “S” Hinge drawing.)

Many of us use an iron-on plastic covering for at least the wings and tail feathers. Even with trim schemes that cut across the hinge lines or color changes from fixed to moving surfaces, we can do a pretty job with the same covering material.

To make a seal that does not tighten and sag when the controls are moved, we have to make an “S” seal as with the hinges above. You can even use different colors in each half of the “S” bend to match the colors on the top and bottom of the airplane.

The beauty of the “S” seal is that it does not tighten and bind the control surface—even at 3-D control throws. Clear iron-on covering can also be used if there are too many color changes near the hinge line.

For painted models you need to seal with clear tape. I like to use a pliable clear-vinyl window-sealing tape. I used to buy 3M part number 117, but a walk down the appropriate aisle of the local home-improvement megastore presented a variety of brands. This stuff sticks tenaciously, provided the surface underneath is clean.

To apply the seal, cut a credit card-sized piece of 1/32 plywood. Make it just long enough to reach from hinge to hinge. Wrap a piece of the tape, sticky-side out, around the card and keep it taut with your fingers.

With the aileron bent up against the stop, stuff the edge of the card as deep into the underside of the hinge line as you can. Stick the tape to the wing and aileron by rocking the card, and leave the free ends. With a sharp knife, cut the free ends off just inside of the corner of the beveled edges. (See the two tape-seal photos.)

Why do we seal the aileron hinge line? To answer that we have to review a bit of theory. We don’t need Bernoulli or any of that fancy stuff; airplanes fly because the wing pushes down on the air and the air pushes back up against the bottom of the wing. The purists out there are screaming about this oversimplification. That’s okay.

The high-pressure air on the bottom wants to leak upward through the aileron hinge gap. The effect of high-pressure air leaking out from under the wing, through the gap between the wing and aileron, is bad. Sometimes it is really bad. (See the hinge-line leak drawing.) This leakage causes a loss of lift and hampers good roll control.

An old friend I lost track of many years ago had a Piper J-2 Cub. You could stick your fingers and palm right through the aileron hinge-line gaps.

The J-2 was slower than molasses in January and had pitiful aileron response during a stall. At airspeeds only a few mph faster than stall speed, the ailerons worked backward! If overused they could force the airplane to drop into an unwanted spin entry. That’s the way the Cub was designed!

Pilots who trained on this airplane decades ago were taught to use rudder as the primary roll control during near-stall conditions. In those days spin training was necessary just to get a private pilot’s license.

Back to the Cub. Yellow duct-tape seals on the ailerons (they had to be yellow, didn’t they?) improved the cruise speed by a whole 4 mph, and the ailerons worked all the way through the stall. That is abnormal for any Cub! It also briefly put the airplane in the experimental category.

Aileron seals have no bad effects that I am aware of. They can actually have good effects such as saving servo power, preventing flutter, and making the airplane behave better during takeoff and landing.

The problem of aileron hinge-line leakage gets worse when the airspeed is low and the angle of attack is high, and it gets even worse when aileron is drooped. High angles of attack result from pulling “G”s or from flying slowly. As the angle of attack increases, the leak worsens.

The leak is further worsened when you apply aileron control. Picture the left wing as you roll into a right turn. (See the drawing.) The depressed aileron forces the air downward so that the local air pressure is even greater. The leaking air squirts out as a “sheet” that eventually breaks up and joins the airflow past the wing.

Until it breaks up, that sheet of air looks like an aileron pointed the wrong way. It’s not made from wood, but it is real.

Let’s put this together. Your model is climbing steeply just after takeoff, and you push right aileron to start a turn. The left aileron goes down and the right one goes up. The sheet of air leaking on the left wing gets worse, and you have an airplane with the right aileron going up and the left aileron going—well, the wooden aileron goes down, but the aileron made from a sheet of air goes up at the same time.

As a result, the left wing has a big drag brake on it. That doesn’t help when turning right!

This yaw in the opposite direction of the desired roll is called adverse yaw, and it’s bad. Sealing the gaps gets rid of the leakage problem and reduces (but not eliminates) adverse yaw. It also makes the ailerons more powerful, so you can reduce the aileron throw and still get the same control effectiveness.

Time to Go Flying Again: In trimming for good directional control we have two main goals, the first of which is to trim the (now sealed) ailerons and rudder so that the model is not crosstrimmed and flies straight at all speeds from slow to fast.

The second goal is to 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: I should point out at the start that this topic overlaps the right-thrust adjustment discussion. There was no straightforward way to get a handle on both subjects at one time, but we will combine the tests and adjustments at the field.

When an airplane is crosstrimmed 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 in cruise flight to fly 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 crosstrim: 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. Everytime you adjust the rudder, go back to trimming the ailerons for straight and level flight. As are many other trimming adjustments, it’s an iterative process and 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 retrim 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. Landings can be prettier, and more effort can be put into that picture-perfect three-point flare rather than fighting to keep the model from veering off the runway.


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