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Written by Mark Wilkins
Giving a Pup a second chance
Feature
As seen in the March 2017 issue of Model Aviation.


Using a sharp paring chisel, carefully carve away the old balsa longerons. Take care to scrape away old glue because it will compromise the new joint.


All that was salvageable from the author’s old Pup fuselage was the nose section, which held together well.

It happens. We’ve all been there, flying blissfully around our field, hotdogging and doing every stunt in the book. The time slips by, flight after flight, and a nagging voice in the back of our mind tells us to check our flight battery pack.

The sky is so blue and the wind is so light—just one more flight … or perhaps we are flying late in the day and the aircraft silhouette is such that we lose orientation, or perhaps it is a simple case of dumb thumbs. Whatever the reason—or lack thereof—the result is the same. The airplane goes down.

With a heavy heart, you gather the pieces and tell yourself that it is not that bad and it can be fixed. The good news is that it can, and sometimes you’ll end up with an airplane that is better than it was before.

After a model crashes, it’s a good idea to set it aside for a few days or even weeks. This does a few things. The emotional aspect and discouragement fades (you finally stop calling yourself an idiot!), and you are able to approach the repairs with a fresh, positive attitude.

Now, take a good look at the wreck. You’ll probably notice things now that you missed when you brought it home from the field. Keep an eye out for areas of torn covering that can indicate fractured framing that isn’t readily visible. Check all of the major joints—e.g., firewall, wing roots, places where two wing panels are joined, etc. You’ll develop a list of repairs and get a better sense of what’s involved.


New basswood longerons were fitted into place against the turtledeck and other framing for alignment.


A new side panel was built over the original plans using basswood longerons. A partial wing saddle was incorporated into the side panel, designed to be joined to the original wing saddle on the forward portion of the fuselage.


The old meets the new: a close-up of the half-lap joint between the original and new lower wing saddle.


The aft ends of the fuselage were cemented using epoxy. The crossmembers are not fitted between the fuselage side panels.

Most repairs aren’t that difficult and knowing when to rebuild and when to discard is what matters most. Before undertaking an involved repair task, ask yourself a few questions.

Did the airplane really fly so well that a potentially lengthy rebuild is justified? Are the repairs within your ability? Are spare parts—be it die-cut formers or entire airframe components—available for my aircraft?

If it is a favorite ARF model and it’s the middle of the flying season, sometimes it is simply easier to buy a new fuselage, wing panel, or set of landing gear if your budget allows. If the accident occurs at the end of the flying season, well, you have found a winter project! Such was the case with my ⅙-scale Balsa USA Sopwith Pup.

Prelude to a New Fuselage

This airplane flew well and I desperately wanted it back in the air as soon as possible. The wings were basically in okay shape, but the fuselage aft of the engine/fuel tank area was gone. I always keep the plans for the models I build, so I pulled them out and prepared to build new fuselage side panels.

Before doing this, I needed to remove any trace of the old longerons, which I accomplished using a small paring chisel. I used a scrap of new longeron material to test fit the slot where the new fuselage longerons would mate with the intact nose section and turtledeck.

I made sure to scrape away all of the old glue to ensure a good bond. When I was satisfied, I fitted the new longerons into place to check the alignment.

Additionally, I removed all of the servos, the fuel tank, and the receiver from the fuselage nose section and set them aside. While I waited for glue to dry, I checked the flight components for damage and replaced them as necessary. Often when a model makes a sudden stop (crash), the control surfaces are whipped in one direction or another. Be sure to check these servos, especially the all-important elevator servo(s).


The new aft turtledeck stringers were cemented onto new formers. Check to be sure these stringers are straight by sighting down them. Also check them from the sides to ensure that there are no humps or hollows.


Test fit the lower wing to the new wing saddle.

A New Fuselage

The side sections were built over the plans the same way as the original fuselage was built, but this time I substituted basswood longerons (spruce would be even better) to make a sturdier fuselage. A new partial lower wing saddle was built into the side panels and would join the nose section by means of a half-lap joint. After the sides had completely dried, I carefully fitted them to the old nose section to check and adjust the fit. When I was satisfied, I epoxied them into place.

When everything was dry, the emerging fuselage was carefully positioned over the top view of the plans and pinned securely to ½-inch foam core beneath the plans. Next, the two sides were carefully brought together using squares to ensure perfect alignment relative to the plans. Crosspieces were fitted and cemented into place, checking the fuselage frequently to ensure that no twist was being introduced.

Aft turtledeck formers were drawn onto tracing paper, which was in turn rubber cemented to cardstock and cut out, yielding templates to make new formers. The formers were made from medium balsa and cemented into place.

If you don’t want to bother with this, some companies such as Balsa USA offer replacement die-cut balsa or plywood sheets for roughly $5 each. Find the instruction sheet with the formers on it and make a phone call. Hard balsa stringers were positioned with pins in the formers, checked to ensure that they were straight, then cemented.

Tail Skid and Gussets

The original tail skid was a prebent piece of wire supplied in the kit. It worked well and lasted a long time. Because I was rebuilding the fuselage anyway, I decided to give the Pup a shock-absorbing tail skid setup with a bungee. I made two light plywood plates through which an axle was passed.

The tail skid was made from basswood and a brass bushing was epoxied through it. I slid the finished unit into place. An axle was passed through the two plywood plates and the tail skid’s bushing and was secured. You can bend one end to form an L and secure the free end with either a wheel collar or a soldered washer.

A plate with a hole drilled in it was epoxied to the end of the fuselage to serve as an anchor for the bungee. As a reinforcing measure, I made plywood gussets from 1/32-inch aircraft plywood and glued these to all of the joints inside the fuselage. Where applicable, these were notched to receive the crossmembers.


A new tail skid was made from scrap basswood and light plywood. A plywood plate with a hole bored through was epoxied to the inner rudder post and serves as an anchor for the tail skid bungee.


Aircraft plywood gussets or backing plates were glued over all of the major joints in the fuselage framing.


The fuel tank and servos have been returned to the fuselage.


The fuselage is being covered with Linen Solartex.

Checking the Fit of the Tail Feathers and Bottom Wing

After the fuselage has been completed, it is time for a reward: checking the fit of the tail feathers and lower wing. The basswood lower wing mounting plate (through which the nylon bolts were passed) was intact; however, the portion of the old wing saddle to which it was attached was destroyed. I needed to carefully fit the old mounting plate into new notches in the new wing saddle.

Fortunately for me, I had measured and cut the notch accurately and the wing lined up well when the distance between the wingtips and the end of the fuselage was checked. If need be, nibble away the notch until your lower wing and plate are perfectly aligned, then liberally epoxy it into place. West System or 2-Ton epoxy would be my choice.

After the lower wing has been successfully reinstalled, check the alignment and fit of the stabilizer and rudder relative to the lower wing. Shim or adjust as necessary to ensure that the stabilizer is parallel to the lower wing, and the rudder and fin are perpendicular to the stabilizer/elevator assembly.

Reinstalling the Linkages, Servos, and Receiver

With the fuselage completed, reinstall the servos, pushrods, fuel tank, and receiver. I decided to install the satellite on my Spektrum 6210 receiver for added insurance. I also relocated the receiver further aft; I like to keep my receiver well away from the servos to prevent interference. The satellite was mounted with its antenna perpendicular to the run of the main receiver antenna, per the instructions.

Finishing the Fuselage and Reassembly

With the gear securely reinstalled, I covered the finished fuselage using Linen and Olive Drab Solartex. After I completed that, I reinstalled the engine, aluminum cheek, and top cowling panels. Roundels were painted on the fuselage using acrylic paints, and the entire unit was thoroughly cleaned and given two coats of Minwax spray polyurethane for protection from glow fuel.

The wings and tail feathers were reattached to the fuselage, paying particular attention to the alignment of the lower and upper wing. With biplanes you want this alignment nearly perfect. Forcing the wings into alignment will often cause an unwanted twist in the wings.


New British roundels were painted on the fuselage using masks and acrylic paints.


The Olive Drab Solartex aft turtledeck covering was carefully applied and trimmed to fit, and then the airframe was reassembled.

Test Flight

The test flight went off without a hitch and this Pup has shrugged off its shame and returned as the queen of the skies that it truly is. This repair took approximately a week, working part time, and I was able to incorporate improvements such as the basswood longerons, working tail skid, and receiver satellite.

This is perhaps the best way to view a repair—if you’re going to go to the trouble of doing it, you might as well build in some improvements while you’re at it.

—Mark Wilkins

markcwilkins@gmail.com

SOURCES:

Balsa USA

(800) 225-7287

www.balsausa.com

Minwax

(800) 523-9299

www.minwax.com

4 comments

Good to see a repair to a big biplane.

I was a little disappointed that the title of this article didn't really reflect the actual content. My impression is that the author basically rebuilt the fuse from plans he originally used to build the model.

I would have liked to see an article describing how to "repair" an ARF without any plans. Repair the fuse after firewall broke away. Repair a wing after some of it was broken into bits. Repair the landing gear mount after a hard landing. Repair a cracked canopy. These are repairs most folks in the hobby face all the time.

I'm not into modern foam planes, but how about an article on how to "repair" some common crash results found in foam-based planes. Not just glue it back together, but how to replace areas that have been totally crumbled. I'm sure you get the idea.

When I get a new kit, especially if it is no longer made, I take the plans to a local office supply store and have two copies made. This allows me to keep the original and have a set to cut up during the building process.

Also, I have used light pine for spars if spruce is not available. I have found that it is as strong and the little extra weight does not matter in the type of planes I build.

I am impressed with your visionary outlook. Never say die. I noticed your pellet fuel bags. You’re an old fashion guy. Your approach to the event is typical of the 60’s, when I built my first radio from an article in RCM. 27 MHz led to a lot of rebuilding. That was part of the experience. No ARF’s, just a box of sticks. You start with a box of sticks and you end up with a pile of sticks. Great job!

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