To Repair or Not to Repair?

[from the AMA National Newsletter, November 2001]

by Phil Bayly

Concept: All of us have crashed our share of model airplanes. Following any crash, the immediate question that follows is whether or not to repair the monster that, at that moment, we wish we would never see again. Nothing is more ugly, and painful than seeing a remaining bunch of balsa parts that have just littered up the place and is beyond recognition. You may also have your pride overrun with embarrassment and negative emotions playing their part in the scene. As a diversion to your frustrations, your next thought may be to determine what caused the accident. But alas, it is an exercise of futility to find a way to exonerate ourselves from the blame since we did it through building or flying error.

Following this evaluation, most will quickly come to the conclusion to “junk it all” after salvaging the engine and hardware for the next airplane that will be “better than ever.” We've all been there and done that! At least I know I have. I also have observed too many flyers as they rushed to judgment and totally junked their crashed airplanes right on the spot, whereas it would have taken them only a few hours to completely repair the damage. All that said, let's look at some of the logic that usually goes on here.

Logic and Process: First, we know our planes are originally built from a bunch of small parts anyway. Therefore, we can easily see that a crash simply results in a bunch of smaller parts than with what we originally started to put together again or replace. That makes it sound a lot less tragic, doesn't it? We also know that we spent a lot of time developing our building skills to be able to fit an airplane’s parts together exactly right without any gaps in the joints. Beyond it looking better, it means those well fit glue joints will bond together with greater strength, especially if CyA glues are used.

Now let's look at some facts. With a typical crash, most breaks in the wood should occur at places other than at its joints, unless the joints are poorly fit. More on that later, but otherwise, we would only need to glue the separated joint(s) again for an easy repair. Have you ever noticed how perfectly those broken pieces of wood fit back together again? Using our best craftsmanship skills, we could never achieve a joint fit like they will fit together. It is perfect! All of the separated wood fiber ends fit snugly into every mating crevice. Therefore, the first principle to accept for our repair consideration is: following any crash, gather up all of the parts. This is not to just be an good guy and keep the flying field clear of debris. It is also to have a basis for “crash analysis” and to retain all options for repair.

There are two things that may be lost during your repair. One is a “new looking appearance” and the other is that the center of gravity (CG) usually moves aft a bit from the extra glue and wood being added in the process. Awareness of both of these typical occurrences can usually, minimize the amount of each. So, rule two is, get everything out of sight until you are cool again and can better assess the repair option.

Now let's deal with “crash assessment.” I examine the details at every crash to learn something more about construction. That includes other people's crashes, too. The question to always ask is why did the break occur where it did instead of someplace else, or maybe not at all? Following enough crash inquiries, you may find a pattern emerging. You may begin to better realize where you had excess wood in your construction as well as where more or harder wood was needed. Gussets, hardwood, grain lines. balsa density, and glue joints are just some of the considerations that must be taken if the “next one” on the horizon is to be better. This introduces the third principle: assess the damage with an eye toward improved design and construction layout, as well as to complete the repair.

The Repair Process: Now let's look at the repair itself since there are definitely some techniques and approaches that can help accomplish the repair of these crashed critters.

1. Following the “crash analysis” we should know how and if each part fits back together in its original location.
2. Plan a schedule of events that must occur to get everything back together, including the parts or areas that will need replacement or additions, such as doublers.
3. Pay primary attention to any repair required on load bearing items such as spars or motor mounts. They may require replacement if they broke off squarely. Doublers are suspect for final strength but don't under estimate the strength of a good doubler, or progressive tripler. Consider 1/64-inch ply for the first layer.
4. Minimize the tail heaviness that typically occurs from the extra glue and wood that is added.
5. Progress from the inside outward.
6. Keep the wing straight with jig studs projected upward from a flat building board.
7. Wing rib damage (crunched out areas) can be replaced by overlapping and gluing sheet balsa to the remaining rib parts as necessary, front and back and contoured to streamline the covering surface after it’s in place.
8. Broken wing spars need doublers for necessary strength. I prefer thinner doublers on all four sides to box in the damage and to ensure strength in all dimensions.
9. Fuselage breakage repair is easily done with 1/64-inch ply over the entire broken area with good overlap. There may be a minor bump on the surface that will hardly show through filler and painting.
10. Motor mount breakage is the most serious. Replace if possible. If not, glue in broken mount part(s) and overlap and glue them and the remaining mount, top and bottom with 1/8-inch ply. Go through the firewall if necessary. Then re-drill the mount holes. I also add bolt(s) through the mount stud to not rely solely upon the glue. This modification alters the thrust line by the ply thickness and may also require a tank adjustment if the engine is vertically mounted.
11. Following all wood repairs, you may recover and paint or MonoKote® as necessary. A lot of strength results from the covering, so do not skimp here. Double covering is always an excellent option for additional strength.

Conclusions: Repairs are easier to accomplish than you first think. If you are unsure about a crashed airplane, begin with the hardest part. if it goes okay, you may continue with confidence. if not, this is the time to concede, not before. Good luck on your new adventure.

from Tangled Lines
Tampa Bay Line Flyers Controline Model Airplane Club
Phil Bayly, editor
Tampa Bay FL