Newcomers

Newcomers

Box 40, St. Peters MO 63376

READERS MAY remember a children's nursery rhyme that speaks of a man who lived in a crooked house and walked a crooked mile. Hopefully he didn't build a crooked model! Lots of us have built crooked models, and strangely enough, that's also how they fly—crooked! Of course, the models may not fly that way for any extended length of time.

I've written about alignment before; the method is often referred to as trammeling: the process during which you assemble the parts of the model (tail, wing, etc.) so that they are in proper alignment.

What I am referring to this time is the warps and twists that often sneak their way into your models. It's very easy to overlook these faults through a lack of focus, and it is kind of like the saying, "You can't see the forest for the trees." You work diligently for the proper wing, stabilizer, or fin alignment, but fail to see a profound twist in one wing panel. Such a condition can be fatal to the model!

Causes of warps and twists

How do these twists and warps work their way into the structure? There are a variety of possibilities.

• You may be using a building surface that isn't flat, or has a twist to it. It's no surprise that the finished product will reflect the twist.

• The warp or twist could be introduced through dissimilar wood density or hardness.

• If long pieces such as spars are themselves warped or curved, you will be creating strains in the structure that manifest into problems after construction is complete.

• The wood itself may season, or in some cases pick up moisture, causing warping problems. You only need to watch the life of your home to know what happens during a period of time. Sometimes this occurs from season to season.

How to avoid creating a crooked fuselage or wing

Consider the following:

• Make certain that the building surface is flat and straight. You can accomplish this by using long straightedges to check the level qualities of the surface. You can sight down the surface from the edge, or level one edge with a carpenter's level and then check all of the other parts.

• Carefully check the wood you use. If there are warps in spar material, you may be able to straighten them by wetting the wood, inducing a bend in the opposite direction, pinning it down, and allowing it to dry. The overcompensation of the original bend in the opposite direction may result in a straight piece.

Bear in mind that this process requires a degree of guesswork and generally some experience to achieve good results. The harder the wood, the more difficult this may be.

• Match the density or hardness of the wood. This is particularly true for the sheet stock used in fuselage sides. If one piece is medium or soft and the other is hard, you may create a "banana" shape when you attempt to draw the sides together at the tail section. You may have it correct on the building board, but when you pull it up, the strong side prevails!

In the name of economy, modelers often refuse to discard wood from a kit and replace it with wood selected at the hobby shop. That's false economy! To go ahead and use poor wood may well result in a poor-flying model!

Covering and induced warps

The concern for the creation of warps doesn't end with building a straight, uncovered piece. You still have a chance to "do the Twist." It matters little whether the covering material is tissue, silk, plastic film, or whatever. Your efforts to achieve a tight, drumlike surface may also induce warps. The lighter the basic construction, the more likely this is to occur.

The biggest factor in preventing warps is to be careful as you shrink the material. There's no substitute for experience! Every material we use to cover framework has a different shrink rate. Even with plastic film, you will find different degrees of shrinkage, though two products may look the same.

The heat level applied to the material will also be part of the control factor. With tissue or silk, the shrinking agents will probably be water and/or "dope." By dope I mean a chemical paint, either butyrate- or nitrate‑based, that will shrink the material and waterproof it.

There are three processes that control the tightness of the covering:

1. The original stretching that occurs when applying it.
2. The action of the water.
3. The shrinkage that occurs from the paint action.

It should be noted that open structures are not the only structures subject to warps. Even solid-sheet tail surfaces may be warped if you are not careful.

Fixing a warped surface

Say you’ve covered the model; lo and behold, one wing panel (or both) looks a little like a propeller blade. What do you do?

• If it’s a sheeted foam-core or fully-sheeted built-up wing, the problem is rather significant. It may be possible to cut long, spanwise slits in the sheeting and then twist it into alignment. By lightly sanding the surface while it’s in that straightened position and applying thin cyanoacrylate (CA) glue to the slits (that are now filled with balsa dust), you may be able to salvage the wing. The emphasis is on may!

• For a built-up wing covered with plastic film, you can induce an overcompensated twist in the opposite direction of the warp. This will most likely produce wrinkles in the material. Using the heat source you used to apply the film, reshrink the material. It may take a little playing around with, but you should be able to create a narrow, warped panel. The warp produced in your originally straight panel was, in most cases, induced by an uneven application of heat.

I haven’t mentioned the actual fabric materials that are shrinkable. In some ways, they act like the plastic film. Fabric materials do have a significant shrinkage potential in many cases, and a very light structure may not stand up to the amount of pressure they are capable of producing. Don’t overlook fabric materials; they are very durable and can add considerable structural integrity.

Is misalignment ever desirable?

Here is an interesting question: Is there ever a time when you would actually want a situation of misalignment or warping?

The answer is yes.

If you have an opportunity to watch Free Flight (FF) models, either indoor or outdoor, you will notice what appears to be some really sloppy workmanship. You see stabilizers set at unusual angles, motorsticks with funny bends, wing panels with funny twists.

This is not sloppy workmanship. This is done for a specific purpose. Unlike Control Line (CL) or Radio Control (RC), many Free Flighters are faced with some interesting challenges.

It must be remembered that any "trimming" (adjustments) of surfaces must happen before a FF model is launched. Once the model leaves the hand, it’s too late! For powered FF models, the power level may tend to change; in a rubber model (for instance) the power level diminishes as the rubber motor unwinds. This results in a change in aerodynamic factors acting on the model.

Even an engine‑powered FF transitions to a glider at some point. If you watch a FF model climb under power, it may tend to spiral one direction, but will glide in the other direction. The sometimes-not-too-subtle angles or twists used in the model compensate for these factors.

CL models use various methods to help them fly. The most noticeable surface adjustment is a rudder or fin displaced to the outside of the circle. This tends to help keep the lines taut. There may also be outward thrust built into the engine mount. The most interesting design technique is to employ a slightly larger wing panel on the part toward the center of the circle. This compensates for the outboard wing panel actually traveling a greater distance than the inboard panel.

The lateral center of gravity (CG) also shifts by adding weight to the outboard wingtip. All of these, plus some others related to the bellcrank and leadouts, are used to make the model fly right. The bottom line is that if the construction is sloppy and there’s a warped wing panel, it may still be a crash looking for a place to happen!

For persons involved in RC, you will find a bit more latitude in what the traffic will allow with misalignment or warps. Those nice little trim levers on the transmitter really help (or partially help) you overcome faulty workmanship. This is not to suggest that you should allow warps and twists to enter the scene. Do your twisting elsewhere!

There are some purposeful cases of misalignment or twists that you may find in RC models. One that is used is right thrust and downthrust applied to the engine. This is often employed to overcome the aerodynamic forces resulting from the turning propeller and the fact that many trainers use high‑lift, flat‑bottomed airfoils. If the plans call for right and down, then do it!

Washout in both wings may be used in other cases. This is a condition where the wing panels are purposely twisted a small amount so that the trailing edge of the wing is raised ever so slightly as you sight along the panel toward the wingtip. The purpose of this is to prevent the wingtip from stalling or stopping flight (lifting).

Washout is more often used on models with tapering wing chords than those with constant chords, though it has been used in some kits as an extra measure to help prevent stalling. In those cases, the kits generally have a built‑in method to induce this minor twist during construction.

You will see many crooked models that fly. They certainly fly well enough to serve as trainers. But remember that allowing warps and twists to enter the picture simply adds one more factor to complicate your flying life.

Build carefully, and check your model before, during, and after you fly it. This applies long after, alas! As the airplane ages (and hopefully it will), you will notice changes. And if you don’t believe that, then look at some old pictures of yourself. I’ll bet you’ve picked up a warp or two!

MA

Transcribed from original scans by AI. Minor OCR errors may remain.