Academy of Model Aeronautics






After The Crash: Fixing A Tower Trainer 40 ARF






As a novice wannabe RC pilot, I have suffered two fatal plane crashes in my short flying career. The crashes were due to radio failure: the communication between the transmitter in my instructor's hands and the receiver in the aircraft failed and not even my experienced instructor was able to save the planes.

Experienced pilots told me that crashes happen to all pilots sooner or later. So, I was prepared, mentally, for a crash, especially after my instructor saved my aircraft numerous times--when I placed the aircraft in jepordy during my training sessions. Some aircraft crashes result in a total loss of the plane; other crashes result in varying degrees of damage that can be repaired.

My first crash, a Great Planes Easy Sport, resulted in a total loss on June 6, 2009(Figure 1-2); my second crash was in November, 2009, and resulted in fixable damage. I decided to fix the damaged trainer, a challenging project considering my building experience, small workspace (Figure 3), and tool inventory. My last model building was done nearly 50 years ago, and building materials and techniques have changed drastically since then. I considered fixing the trainer to be a great learning opportunity for building and repairing my RC aircraft.

Figure 1: Easy Sport Before The Crash
Before The Crash

Figure 2: Club Member Bob Beatty At The Crash Scene
After The Crash

Figure 3: My Small Work Bench
My Small Work Bench

Fortunately, I belong to an RC club, Sanderson Field RC Flyers, and club members helped me throughout the project. Collecting the necessary tools and materials and effecting the repair was done over a period of several months; an experienced modeler with the necessary tools and materials could probably have completed the repair in a few hours.

The Damage

The damaged trainer was a Tower Trainer 40. The nose section needed a complete rebuild, as did the wing mounting and other areas of the fuselage, such as the main landing gear area. The covering also needed lots of repair work. Fortunately, except for some trailing edge damage, the wing was in good condition.

Repairing The Nose Section

Early in the project, I decided to replace the standard clamp-type engine mount with a Great Planes .46-sized engine mount. This would require plugging old mounting holes and drilling new holes for the Great Planes mount.

My first step was to remove the old balsa from the bottom and top of the nose section. After drilling the necessary engine mounting holes and installing blind nuts for the engine-mounting bolts, I re-epoxied the existing engine firewall into place. I also added a 1/16" thickness of plywood to the inside sides of the engine compartment to stiffen the front end of the fuselage.

A club member drilled the engine mounting holes in the engine mount using his drill press and I tapped the holes with a 6/32 tap: I used 6/32 socket head cap screws to secure both the engine mount and engine. After coating the interior of the engine compartment with a thin coat of epoxy (diluted with 99% isopropyl alcohol), I bolted the new engine mount to the firewall (Figure 4). My next step was to remount the fuel tank.

Figure 4: Great Planes Engine Mount
Great Planes Engine Mount

I repaired the fuel tank cradle and added some flexible tubing to help dampen engine vibrations (Figure 5-6). On a tip from a club member, I used flexible tubing from a sling-shot repair kit purchased at my local Walmart sports department. Before installing the fuel tank, I painted the compartment with a thin coat of epoxy for fuel proofing. I also used silicon adhesive around the front of the fuel tank to further isolate the tank from engine vibrations and seal the tank compartment from any engine compartment fuel leakage. I covered the top of the fuel tank compartment with a hatch made from 1/8" plywood and recovered the bottom of the compartment with new balsa.

Figure 5: Fuel Tank Cradle
Fuel Tank Cradle

Figure 6: Installed Fuel Tank
Installed Fuel Tank

New Landing Gear

Since the landing gears, both nose and main, were damaged, they had to be repaired or replaced. I decided to replace the main 5/32" wire landing gear with a more durable "Super Strength Landing Gear" by DU-BRO. The landing gear is made from a shock absorbing composite material and will not bend out of shape as the wire gear did. In addition, I planed to use larger wheels for both the main and the nose gear: I decided to use 3-1/4" Dave Brown "Lite Flite Wheels." I felt the landing gear upgrade would make the trainer more durable as I learned to fly--and would be a good setup for our grass field, too. Also, with the upgrade, I thought the plane would track better on the ground and, because of the added ground clearance, lessen the chance of damaging a propeller during a rough landing.

In order to accomplish the upgrade, I first replaced the original main landing-gear strong point with a wider 3/8" plywood base for attaching the DU-BRO gear. Underneath the plywood, I reinforced the strong point with 1/2" triangular balsa. I used four 6/32" socket head cap screws to securely mount the DU-BRO landing gear to the fuselage. The landing gear can be painted and I plan to use a red spray paint for this sometime in the future.

For the nose gear, and to level the aircraft longitudinally, I bent a DU-BRO "5/32 Universal Nose Gear Wire." One problem with wire landing gear these days is that commercial landing gear comes with a plating that increases the wire diameter slightly. This creates a problem when using it with commercial parts, such as wheel collars, engineered for 5/32" wire. I found I had to use a #20 drill bit to enlarge both the wheel collars and also the engine-mount nose-gear holes so everything would fit. The results of the upgrade are shown in Figure 7 (that's an 11-6 prop in the image).

Figure 7: New Landing Gear
New Landing Gear

Rebuilding The Servo Tray

The servo tray also had to be replaced. Although some might consider this an overbuild, I used a piece of 1/4" plywood for the tray. First, using some card stock, I made a template of the tray (Figure 8) and cut out the tray with a jigsaw. The image indicates the holes I first drilled in the plywood in the cut-out corners. I then use the jigsaw to cut between holes. Finally, I drilled out holes for the servo screws with a 5/64" drill bit. The final servo mount, with servos, is shown in Figure 9.

Figure 8: Servo Tray And Template
Servo Tray And Template

Figure 9: Servo Tray And Installed Servos
Servo Tray And Installed Servos

Fixing The Wing

Only a little CA was needed to repair the wing. However, a new arrangement was required for the wing positioning dowels. I epoxied a block of balsa to the bottom leading edge of the wing and epoxied two positioning dowels into the block (Figure 10). I built a new plywood former to accept the dowels and epoxied it into the fuselage (Figure 11). In the images, the cut-off original dowels are visible in the leading edge. This arrangement made for a much more secure wing than the old, weak arrangement. Figure 9 shows the DU-BRO wing mounts used for the wing hold-down bolts.

Figure 10: Wing Dowel Block
Wing Dowel Block

Figure 11: Wing In Place
Wing In Place

Removing The Old Covering

At this point, I decided to remove the old fuselage covering. Since I had never done any covering and did not have any of the necessary tools, I visited club members to learn as much as I could before ordering tools and materials. Eventually, I removed the covering and then removed any leftover adhesive with Methyl Ethyl Ketone (M.E.K.). Figure 12 shows the fuselage after removing the old covering. Once the old covering was stripped, I could clearly see other areas that needed to be re-glued or strengthened.

Figure 12: Fuselage Almost Ready To Cover
Fuselage Almost Ready To Cover

One area that needed to be strengthened was the tail. The stabilizer and vertical fin were not mounted securely. I decided to sheet the surfaces with 1/64" plywood and reinforce the joints to the fuselage with 1/4" triangular balsa. Previously, I had to balance the trainer with a good deal of lead weight attached to the tail, so, I calculated that the added strengthening could usefully replace some or all of the lead weight: in fact, when the repairs were completed, the plane balanced almost perfectly. Before sheeting, I cut away the solid-balsa rudder and elevator: these would be replaced and re-hinged later.

Figure 13 shows the nearly completed sheeting job. I used thin CA to attach the first side of each surface: I could reach all contacting surfaces through the open side. In order to attach the second side, I first attached the trailing edge and allowed the CA to thoroughly set. Next, working towards the leading edge, I applied thick CA to about 1/3 of the remaining contacting surfaces, held the sheeting down until the CA set, and moved onto the next 1/3. The thick CA allowed me enough time to quickly position the sheeting before the CA set. I used an Xacto with a #11 blade to trim the excess sheeting. Where necessary, I used additional thin CA around the edges of the sheeting. After sanding the edges of the stabilizer and fin, the sheeting blended nicely into the edges. When the sheeting was completed, I added the 1/4" triangular balsa reinforcement.

Figure 13: Sheeting Nearly Completed
Sheeting Nearly Completed

Covering the Fuselage

After filling holes and gaps with either Hobbylite balsa colored filler or DAP spackling and sanding everything smooth, I was ready to begin covering with red and white monokote. Starting with the fuselage bottom (Figure 14), I covered the fuselage (Figure 15). After covering the fuselage, I judged the job to be a #10 job: looks good from 10 feet or more. My second job should be better. Note: the work surface for covering is my "remote workbench," the kitchen counter--with permission of the boss.

Figure 14: Covering The Fuselage Bottom
CCovering The Fuselage Bottom

Figure 15: Covering The Fuselage
Covering The Fuselage

New Elevator and Rudder

I cut out a new rudder from 1/4" balsa; I used 3/8" trailing-edge stock for the new elevator. Hinging the elevator to the stabilizer was not too difficult. I had to reposition the hinge slots on the old stabilizer because the old hinges were still in the wood: I used an Xacto knife with a #11 blade to cut the new slots. I slotted the elevator leading edge to match. Because of the more limited space to reposition new hinges on the rudder, and on the advice of a club member, I elected to use Hayes Live Hinges, which needed to be epoxied into small holes. In order to bevel the leading edges of the elevator and rudder, I built a small bevel-sanding jig (Figure 16). The bevel jig worked well for the soft balsa rudder but the hard trailing edge stock needed to be razor planed to the approximate bevel before final sanding on the jig. After the beveling, I covered the surfaces with red monokote and then attached the rudder and elevator to the stabilizer and fin.

Figure 16: Bevel Jig
Bevel Jig

VoltWatch2

It was necessary to replace the plywood windshield, and I decided to mount the VoltWatch2 here. I use a VoltWatch2 to monitor the receiver battery--a low battery level can lead to crashes! I cut a small slot in the windshield and mounted the VoltWatch2 under the slot. I covered the slot with a small piece of clear monokote donated by a club member. Installed in this way, the LEDs of the VoltWatch2 are protected and the battery level can be easily monitored (Figure 17).

Figure 17: VoltWatch2 Installation
VoltWatch2 Installation

Figure 18 shows the finished fuselage after two successful test flights on March 30, 2010.

Figure 18: Finished Fuselage
Finished Fuselage

Special Thanks

I would like to thank the members of SFRCF, my RC club, who helped me with this project. Special thanks are due to members Bob Beatty, Richard Robb, and Bob Andrew for their help, advice, and tips.





 I hope you enjoyed this article,
Royce Tivel


Royce Tivel

 Royce Tivel
rtivel@selectdigitals.com







Select Digitals Home Page


Valid HTML 4.01 Transitional