Newcomers
32 Hollandbush Ct., St. Charles MO 63376
WHEN WAS the last time you used a trammel? Before you rush to Merriam-Webster's Collegiate Dictionary, let me provide a litany of definitions.
- "A net for catching birds or fish;
- an adjustable pothook for a fireplace crane;
- a shackle used for making a horse amble;
- something impeding activities, progress, or freedom;
- an instrument for drawing ellipses;
- a compass for drawing large circles;
- any of various gauges used for aligning or adjusting machine parts."
Who can deny that this is an interesting blend of meanings for one little old noun? I don't catch fish or birds, nor do I hang a pot in the fireplace, so the first two definitions are out. I don't have a horse, and I'm not looking to impede progress, so gadgets used to draw, measure, or align are what I'm going to discuss!
I'm not certain when I first heard the word "trammel" — or "trammelling" — applied to aligning the various parts of a model, but no matter what you call it, it is a vital step in assuring that a model flies properly.
The process starts during the construction stage. In past columns I have discussed how using badly warped wood creates a situation where producing a straight finished piece becomes very difficult. This is especially true for wing spars and leading and trailing edges. The harder the wood, the more likely the final item will be deformed.
In some cases it is possible to correct bows or bends in these pieces. By thoroughly wetting the piece and firmly pinning it down, sometimes you can correct the problem. However, pinning the wood so it is straight may not do the job.
More often than not, you will need to bend the wood slightly beyond straight, to a bend in the opposite direction. When it's dry, the resulting "give" toward the original bend will tend to produce a straight piece.
How much should this compensation bend be beyond straight? There's an element of "guess!" The bottom line may be that the piece has multiple bends and kinks, and the time and energy needed to create a straight piece just isn't worth it! Go buy one that is straight!
In this day and age, we are fortunate to learn that most kit manufacturers have solved many of our problems by creating straight, properly formed components.
They have accomplished this by using interlocking tabs and notches, which tend to "manage" the parts into a fixture or trammel in the construction process.
One area where this is most noticeable is the temporary tab sometimes found on wing ribs. During the building process, these tabs force the components into the correct alignment.
In some models they automatically induce a small amount of washout in the wing. This is a condition in which the trailing edge is progressively raised toward the wingtip, helping to reduce tip stall — a frequent "airplane killer," especially on a landing approach.
Once the wing panel is completed, these tabs are cut away.
Another help is a full-depth, notched spar that accepts matching notched ribs. One step beyond employs notched leading and trailing edges. All of these help you produce a better model.
There will be a time in the construction process when you must assemble the various components you have built. In most cases, this involves joining wing panels and tail surfaces and mating them to the fuselage so they are correctly aligned.
A number of things happen almost at once at this point; you are taking into consideration not only the alignment to the fuselage, but their alignment to other parts. I use the following steps.
- Check each component (wing panels, fin and rudder, stabilizer, and elevator) for warps. Do this as a repeated process during and after construction.
The same can be said for covering. When you apply the covering, you can induce a warp during the shrinking process. This is especially true with smaller, lightly built models.
Most trainers do not use wing panels that are completely sheeted, but wings of that type require considerable care during the sheeting process. Once balsa sheeting is securely attached to itself, ribs, and spars, warps will be almost impossible to straighten without harming the glue joints.
One very important step in the alignment process is joining the wing panels. The builder's focus is often set only on making certain that each panel is not warped and the amount of dihedral. The alignment of one panel to the other is sometimes lost in the process.
If the panel-building was accurate, there shouldn't be a problem. If it wasn't, the entire wing can be compromised.
To check this, anchor the wing center-section to your building board and measure from the board to the leading and trailing edge at each wingtip. The distances should be the same.
- Once you are certain everything is warp-free, begin the process of attaching the tail surfaces to the fuselage.
Kits will generally have precut notches or saddles to receive the parts. When you are ready to attach the stab, pin it to the correct mounting area.
The precut area may result in the stab being slightly off in horizontal relationship to the fuselage. This happens easily when you join the fuselage sides at the tail. The smallest fraction of an inch will cause the stab to lean, but trimming or adding a shim will clear that up.
Check to see if the stab is aligned at right angles to the centerline of the fuselage by finding the center of the firewall (where the engine is mounted) and placing a T-pin in the top edge of it. Tie some string (or something that doesn't stretch easily) to the pin. Align the string as best you can by eye, using a centerline from the leading edge to the trailing edge.
Holding the string with two fingers, measure to a reference point at the tip of one side of the stab. I use the outermost edge where the elevator is attached to the stab.
Once you have that measurement, move the string — holding that spot — to the other stab tip. Work the stab back and forth until those distances are equal.
Draw lines on the bottom of the stab where it meets the fuselage side. Remove the stab, apply adhesive, and replace it on the fuselage using the lines as a guide. Make certain you use an adhesive that does not set too quickly; you want some time to recheck both alignment characteristics of the stab.
The vertical fin goes on next. Make certain it is aligned with the centerline of the fuselage and is at right angles to the stab. Use a 30°-60°-90° triangle to check that the fin is square to the stab. If there is a rounded or triangular piece of wood to support the fin, you may need to come up with a piece of the triangle at the 90° angle to clear it.
There are also trammels made specifically to perform this task (check the hobby store). (You may want to take a definition for trammel.)
- Once these items are securely attached, it's time to attach the wing.
The laser- or die-cutting of the fuselage sides probably set the wing saddle cutouts. The alignment issues are handled in much the same manner as those of the stab: a little shifting, sanding, and shimmying will take care of the horizontal angle of the alignment with the fuselage and stab.
Carefully sighting down the fuselage from the nose and tail will allow you to spot any misalignment. One visual trick is to stand six feet or more away from the model while it rests on the table. Move your head until the fuselage appears to be in a straight vertical and watch the alignment of the wing and stabs — they converge in your line of sight.
Another way to check is to measure the distance from a flat surface — such as your building board — to the wing tip and to the stab tip on one side. Repeat for the other side. The sets of measurements — wing tip to wing tip and stab tip to stab tip — should be equal.
If your building surface is not large enough to accommodate this, try using your garage floor. Poured concrete floors tend to be level; they typically slope toward a drain and are not finished in a flat plane.
The other step for the wing is to align it laterally with the tail. You can accomplish this in a couple of ways. You can place a pin on the fuselage centerline near the tail and measure it as you did the stab. The only problem is that you come up with a much greater angle in the measuring process because of the shorter distance.
My preference, since I assume the stab is correct, is to measure from the same reference point on the stab tip to a similar point on the trailing edge of the wing. I typically use the point at the tip where the aileron attaches to the wing.
At this point, the major components should be aligned.
- There are two other areas of concern, and both are harder to measure: the incidence of the wing and stab and the thrustline of the engine.
The average kit builder usually relies on the kit manufacturer to establish the proper incidence line for the wing and the stab. These are generally established by the cut of the saddles. Incidence is the angle at which each of these components set relative to the fuselage, thrustline, and one another. Some plans may not provide you with this information.
Depending on the type of airfoil, general configuration, and several other factors, the incidence will vary considerably. If you look at the wing saddle of your trainer and the leading edge of the wing appears to be higher than the trailing edge, it probably is — and should be!
A way to check incidence is by using a trammel called an incidence meter. Several companies sell this device. One brand uses a needle-type pointer that reads in degrees. Another uses a projected beam of light to achieve the reading. These meters employ sliding pieces on a bar that engages the wing or stab leading and trailing edges.
To understand how to use these meters and set the incidence, you need to know two things: the number of degrees of incidence planned for the model and a datum or thrustline on the fuselage. By creating a level, fixed thrustline, you can check the wing and stab and their relationship to the fuselage.
Another helpful check can be made with the incidence meter on the wing. By starting at the wing root next to the fuselage, you can move it toward the tip, stopping at several points to note any change in the degree reading. If the model uses a washout, there will be a change toward the wingtip.
- Checking the thrustline or engine, if needed, is the most difficult step.
If some right and/or downthrust is required, the kit will often be designed so the firewall or engine mount provide it.
In some cases a commercial incidence meter will allow you to make this measurement by attaching it to the engine propeller shaft.
You can also take care of at least the right thrust by mounting the propeller on the engine, pinning a pin on the fuselage centerline near the tail, and measuring as you did with the wing and the stab.
If the distance on the right side is less than the left when the blades are horizontal, the engine is aimed to the right (which is typical). How much is a different concern. The two or three degrees often used doesn't translate into a big linear measurement difference.
With most kit-built and ARF (Almost Ready to Fly) trainers, these alignment measurements are not nearly as important as for a high-performance model. The key element is getting used to doing this, because a properly aligned model flies better.
The fact that the incidence or thrustline is off won't show up as a bad thing on a trainer, but it certainly will on a high-performance model.
Now go to The Home Depot and ask the experts where they keep their trammels. That will make their day!
RBA
Transcribed from original scans by AI. Minor OCR errors may remain.
