|Mar 24, 2009, 08:03 PM|
T-Hawk boom repair
Having flown T-Hawks for a few years now, in various configurations, I have come to learn that the most fragile part of the plane is the boom, just before the tail feathers, where the control rods exit to connect with the elevator and rudder. Seems that most T-Hawks, when they crash, are in "lawn dart" mode at the time of impact. The inertia of the tail feathers assembly is enough to cause the boom to flex at this location, resulting in a snapped boom, bent push rods, and no way to fly the plane until a boom replacement is made. The replacement is more difficult than one would think, having to remove the original (or 2nd, 3rd, etc) and then run the control rods, antenna through the new boom. The process is further complicated now that www.readytoflyfun.com has closed it doors (for the moment ). I decided to experiment with some aluminum tubing, to act as a long sleeve over the fractured area. The taper in the original fiberglass boom (on standard T-Hawks, not the trainer) is sufficient to prevent sleeving the entire boom. I wanted the aluminum sleeve to fit tightly without having to be oversized so as to fit the larger diameter of the stock boom where is connect to the fuselage pod. The procedure was basically this:
1.) Removed tail assembly, with fin and stabilizer intact. I disconnected the control rods from the control horns.
2.) Removed any ragged edges from the broken boom tubing.
3.) Prepared an aluminum sleeve, using a K&S 3/8" x .035 tube. Stock number is #3033, purchased from a local hobby store.
4.) To make the repaired boom the same length as the original, as measured from the fuselage pod to the horizontal stab leading edge, I found a 7" length would suffice. I wrapped a piece of blue painter's tape around the original boom, at the location where the aluminum sleeve was tight against the taper of the original boom. Sliding the 7'" length of aluminum to this taped point would assure the same overall length of the repaired boom.
5.) I noted the location of the original control rod slots on the original boom and marked the aluminum tube accordingly, actually making the slots slightly farther apart. The cut the slots into the aluminum tube using a Dremel tool with a cutoff wheel bit. Then, I removed any ragged edged in the slots, opening the slots wider, using a flat file.
6.) After confirming which control rod was for rudder and which was for elevator, I slid the aluminum sleeve onto the original boom, watching for the control rods to appear in the slotted areas. An Exacto knife with a #11 blade was used to help fish the control rods up and out of the slots.
7.) Before epoxying the aluminum sleeve to the boom, I twisted the aluminum tube as needed so as to provide good clearance of the control rods through the slots in the tube. Then, I pulled the tube back slightly, applied epoxy to the area of the original boom where the blue tape was wrapped, and finally slid the aluminum tube to that position.
8.) When the epoxy had cured, I temporarily installed the tail feathers assembly in the rear of the aluminum tube. To keep the tail assembly from rotating freely and making it difficult to get a good alignment of the horizontal stab with the main wing, I squeezed the rear tip of the aluminum tube to get a tighter fit of the tail feathers assembly as (and the short portion of original boom) were being fitted to the tube. I applied some epoxy to the original boom piece and then slide it into the aluminum tube, adjusting for proper alignment of the stab with the main wing.
9.) After the epoxy cured, I reconnected the control rods to the rudder and elevator. I use a split rod arrangement, held together with a wheel collar. This allows for mechanical adjustment of the rudder and elevator without having to make an adjustment to the servo arms.
10.) The aluminum tube caused the plane to become tail heavy, a definite no-no for the T-Hawk. Some weight was added in the nose to provide a neutral balance. I normally prefer a slightly nose heavy plane, but I found the neutral balance worked find for this highly modified T-Hawk.
I did a lengthy range check at the flying field. I found there was no interference problems with the metal boom and control rods. Then antenna runs though the boom and out the end, as is the stock configuration. The T-Hawk flew quite well, so I consider this repair/modification to be successful.
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