OFF THE BOARD by: John Thompson
The proud builder beams over his creation at the club meeting, as members gather round to take a first look. “It flew great right off the board,” the builder proclaims proudly. If he’s telling the truth, our builder is an expert builder– and undoubtedly a lucky one at that.
“Right off the board “ is a model building cliche that means the plane was in perfect trim on its maiden flight, with no adjustments necessary. Skeptics have proclaimed this statement a myth, maintaining that the performance of any plane can be improved upon after its maiden flight.
Trimming a control line model is the equivalent of fine-tuning the TV, or focusing a camera, or editing a manuscript. We’re going to take a plane that flies and make it fly better. We’re going to strive for optimum performance, though the more demanding fliers will never be satisfied–that’s what keeps us building!
We can go as far with trimming as our own demands for performance take us. Expert precision aerobatics fliers will continue making minute adjustments to airplanes that appear, to a casual observer outside the circle, to be absolutely perfect. Atmospheric and seasonal conditions, the alignment of the planets and the current prime interest rate all seem to have effects on performance that these experts can perceive. For many others, just getting the plane to fly more or less straight and stable is an elusive goal.
Modern control line precision aerobatics planes are built with many adjustable features. In fact, many planes intended for casual “sport” flying–as well as planes for other types of competition–now are being designed with some of these features, including adjustable tip weight compartments, movable leadout guides, etc.
Let’s take a look at some control line model airplane trim problems from a basic level, as they are encountered by the novice or sport flier.
We’ll assume that the plane has no obvious major problems and in fact makes a successful maiden light–successful at least in the sense that it takes off, stays aloft until the tank is empty and lands safely. Now let’s assess that first flight and begin considering adjustments. It may take several flights to begin to approach acceptable flight characteristics. Adjustments should be temporary and reversible. Don’t do anything you can’treverse until you’re sure you know what’s causing the problem. And only try one trim adjustment at a time, so you can determine the effect of each change.
• Was the plane stable in level flight? You should be able to fly the plane level without effort, but it should respond when you ask it to. The issue here is balance.
If the plane is difficult to fly level–it wants to go up and down without your input, or is simply too sensitive (experienced fliers refer to it as “nervous” or “twitchy”)–then the plane probably is a bit tail-heavy. Try adding some nose weight, such as a heavy prop hub, and fly again.
On the other hand, the plane may be too stable; it labors to turn, even with plenty of control surface travel. This suggests noseheaviness, and a little tail weight should be tried. It takes much less weight in the tail to move the CG aft than in the nose to move it forward–don’t overdo it, or you’ll risk an unflyable plane that may crash. Add a little weight, fly, and adjust further if necessary.
• Did the plane respond appropriately to control inputs? This question is related to the issue of stability, but is not identical; you may have to consider both issues simultaneously. We’ll assume that the plane is stable in level flight and appears willing to respond to control inputs.
The plane should react immediately, positively and smoothly to control inputs, without unwanted movements. If it seems to react too quickly, to turn more sharply than you want or turn farther than you want–bobbing at the end of a loop, for example, rather than leveling out at the bottom–you have oversensitive controls. The problem could be at the airplane end or at the handle end. The appropriate solution could be a matter of choice, depending on the degree of the problem.
Narrowing the spacing between the handle’s leadouts reduces control response to arm movement. However, if you have a favorite handle that you want to continue using, the best solution is to solve the problem at the airplane end. Many novice fliers start with too big a handle. I recommend starting with a line spacing of no more than 4 inches at the handle; this seems to be about right for the common sport/stunt plane. Bigger handles can result in overcontrol. The best solution to handle questions is one of the several excellent adjustable handles now on the market.
At the airplane end, oversensitive control response is likely to be a result of some combination of the settings of the bellcrank or the pushrod setting in the bellcrank, flap horn and/or elevator horn. Ideally, all of these settings can be changed; the bellcrank could pose a problem if it is hidden inside the plane.
Oversensitive response is often the result of an improper selection or setting of the bellcrank. As a general rule, I always arrange the controls so that a fairly large arm movement is needed to move the controls through their entire range. This usually means a relatively large bellcrank, with the pushrod in the hole nearest the pivot. A smaller bellcrank and the pushrod farther out from the pivot results in a control system that requires much less arm movement and therefore a much finer touch by the pilot. There is also less margin for error.
Assuming you can’t change the bellcrank situation, you can reduce control response by moving the pushrod to holes in the elevator and flap horns that are farther away from the control surfaces, reducing the amount of elevator and flap travel. This should tame the airplane. If necessary, you may also be able to substitute larger horns.
If you have the opposite problem–the plane, while not nose-heavy, does not turn quickly or tightly enough–you need to make the opposite adjustments, increasing elevator travel and possibly widening handle spacing.
Another common control-related problem is stalling. This can be caused by an excess of control surface travel, a common construction mistake of novices. Any elevator deflection above 45 degrees is excessive and likely to cause stalling. Many planes will perform just right with much less travel; racing and combat planes often are set up with only a few degrees of travel in each direction. If the plane is slowing or stalling in turns, reduce the control surface travel and try again. You may find that the plane turns very well but maintains its speed and lift.
One of the most common control response problems is unequal turning; the plane turns tightly in one direction and too widely in the other. This is a simple adjustment that can be made at the elevator and/or flap horns if you have built the plane with adjustable control features such as Kwik-Links. Simply turn the link in the direction needed to adjust elevator travel in the desired direction. Some planes will need more deflection in one direction for equal turning in flight. If the adjustment appears to be ineffective examine the pushrod setup; it may need a guide to prevent flexing, which can cause lack of response in one direction.
• Did the plane fly with one wing higher than the other? There are two possible problems here, and it’s fairly easy to tell which one is the culprit. It’s either a matter of wingtip weight or a wing warp.
Unless the problem is so extreme that there’s a risk of a crash in making the maneuver, fly the plane inverted and examine the alignment of the wings. If the wing that was low when upright is now high when inverted, you have a warp. This is fairly easy to correct if you used an iron-on covering; just twist the wing in the desired direction and reshrink the covering to hold the new shape. A silkspan/dope wing may require steaming, or trim tabs for alignment. Flapped planes can be trimmed with adjustment–”tweaking”–of the flaps to correct the warp.
If the wing that was low when upright remains low when the plane is inverted, it’s more likely a tip weight problem. If the outboard wing is low in both altitudes, there’s too much weight; if it’s high, there’s not enough. You want to use the minimum weight necessary–remember that tip weight is mostly an aid in takeoff and has limited value once the plane is flying. Too much weight can cause the outboard tip to drop in maneuvers.
• Was there appropriate line tension? Some of the problems discussed above can affect line tension, particularly the warp and tip weight situations. If those have been corrected and line tension seems to be a continuing problem, you need to examine what is referred to as “line rake.” Simply put, this is the point at which the leadouts exit the inboard wingtip.
An exit too far forward in relation to the center of gravity causes a lack of tension; an exit too far back can cause excessive tension or other problems, such as a yawing of the plane to the outside of the circle. If the plane has adjustable leadouts, move them forward or aft as needed to correct the problem. Search for the minimum effective line tension–enough to keep the plane tight enough to respond to the controls while not so tight as to cause other performance problems.
Some fliers try to enhance tension with rudder or engine offset. These are both poor methods of providing line tension. A properly trimmed plane, with proper line rake, should have good tension with little or no engine or rudder offset. Both of these–particularly rudder offset–are most effective when least needed.
Unless the line tension problem is extreme, it will be most noticed in maneuvers. There is no substitute for building a plane straight and light. An overly heavy or under- powered plane will have line tension problems in maneuvers no matter what trimming measures you take. Same goes for a plane with a serious warp or alignment problem. Speaking of alignment, there is one trim difficulty that may defy all of the above adjustments; it’s what modelers refer to as “hunting.” This means that the plane oscillates up and down without any control input. It’s not nose-heavy or tail-heavy, but it won’t fly level.
This can be caused by an alignment problem of the wing, elevator, stabilizer and flaps that may be hard to chase down and correct. Another possible cause can be a too-precise control system, one in which there is no play whatsoever. One easy thing to try with a hunting plane is to simply loosen up the pushrod connection to the elevator horn–enlarge the hole in the horn so that there’s a little free movement of the elevator. This has been observed to tame hunting planes dramatically.
These ideas should correct most basic trim problems. We’d be interested in hearing from individual fliers on their particular trim techniques or anecdotes about how trim problems were solved.