DJA Chrysalis Aluminum Tail Boom Modificiation|
(I just gotta be different...)
Here is how I converted my Chrysalis hand-launch glider (HLG) from a conventional balsa fuselage to use an aluminum tail boom, for several reasons. I will try to clean up the wording and add some pictures when I get a chance, but it ought to explain the process.
I started with a standard DJ Aerotech Chrysalis HLG, built with a V-tail, lightening holes in the tail, and with the narrow fuselage. I have a bolt-on wing, as described on my general Chrysalis page.
A crash caused by throwing it into the back of my head (DOH!) on the fourth flight caused the entire rear fuselage to explode into a bunch of little pieces. The longerons survived, and served to keep the tail nicely attached to the rest of the airframe while I took it back to the shop. This was enough for me to decide that I was gonna change things when I made the repair. Incredibly, a friend captured the very moment that lead to this little adventure in aerospace engineering!
Photo courtesy Steve Merchant
After an unsuccessful experiment with a kite-spar fiberglass tube that quickly shredded itself by twisting, I finally broke down and went with an aluminum arrow shaft. It may not be the lightest thing in the world, however the ship still flies very well and I have NO concerns about strength!! I have yet to weigh it, but I believe that the overall weight of the fuselage is probably only marginally more than with the original balsa fuse. If I did it again I could do it a lot lighter. Given the fact that most of my soaring is either slope-soaring at the beach where lift is not a big problem, more speed is usually a good thing and durability is vital, this is fine. The rest of my flying is just fun flying, so again a slightly-heavier-than-ideal ship really isn't terribly noticeable given my own abilities. Overall I am quite happy with this setup.
This is a cheap project. You can probably do it for about $5-15 if you already have the glue and other standard model stuff laying around.
- One aluminum arrow shaft. I went to Sports Authority and found the lightest one. It is labeled Hunter's Specialties Superlite XX75, made by Easton USA (number 241 I think, sorry it's now obscured behind some fiberglass cloth) was the lightest. It cost about $5.
- Some scrap blue or white foam, a block 4" x 1.5" x 2.25" is about right. It's OK if you glue thinner pieces together with foam-safe CA or thinned epoxy.
- A fair bit of light fiberglass cloth, half-ounce per square yard or similar is good (1-2 oz. cloth would probably be OK too, use one less layer.)
- A bunch of those small plastic coffee-stirrer straws that are about 1/8" diameter. These actually can be hard to find retail, unless you can get a shop keeper to give them to you... which is how I got mine.
- One regular soda straw.
- 1/16" plywood... NOT lite-ply garbage though. You don't need much so scraps may do you here.
- Small music wire for pushrods, .025" or .030" size is about right.
- Either thin foam-safe (no-odor) CA glue -or- finishing epoxy resin (NOT the 5-minute stuff, but slow stuff for laminating like Hobbypoxy "Smooth & Easy" or similar.)
- Slow or medium CA glue
- Scotch tape
- Normal hobby tools: X-acto knife, scissors, sandpaper, ruler, marker, pliers, etc. A tube cutter would be good but I got by with the knife for that J
Apologies for the slight fuzziness. You can see the basic layout of the inside of the fuselage in this photo though. From left to right (aft to nose), note the yellow fiberglass tailcone, rear former, antenna (barely visible, exiting the rear former and running to the lower left where it's attached to the tip of the stabilizer), rear wing hold-down mounting plate (it forms the top of a box with formers vertically on either side), another former, launch peg plate (with contoured, balsa-laminated finger pegs on either side), two FMA HS-80 servos, and the forward hatch (red.)
I'll assume that you have either crashed your Chrysalis and damaged it like I did, or you are willing to hack it up to try this. It would be even easier, albeit pretty similar, to build it this way from the start. Just read and understand the directions before you do anything. Usual disclaimers about no guarantee, it might not work for you, this is probably a bad idea, your mama dresses you funny, etc.
- Cut off the rear fuselage about 1/4" behind the back of the rear former (the one at the TE of the wing.) Cut off the longerons here as well. You may wish to leave perhaps a bit more hanging out to absorb hangar rash and provide more to fit into the tail cone we'll make later... but really, it's not critical as long as you have a bit hanging out on the bottom and sides.
- You need to make two new formers out of the 1/6" ply to hold the tail boom. These should be the same width as your fuselage, whatever width you built (mine is the narrow fuse and is 1.5" wide.) The height should be 7/8" for one and 15/16" for the other. Draw a vertical centerline on the formers, then you need to cut out a tight-fitting hole for the boom to pass through. On each former, it should be the same distance off the bottom surface. On mine, the boom is 3/8" diameter, and the center of the holes were placed 3/8" from the bottom of the formers (providing 3/16" clearance from the bottom of the hole to the fuselage floor.) This is not terribly critical, what counts is that they are both the same distance off the fuselage floor to provide a level boom, that they are lined up exactly on the centerline, and they are low enough to clear your wing attachment mechanism. Make sure the fit of the boom through the holes is tight.
- The taller of the formers will replace the pre-existing former at the TE of the wing, and rest on top of the bottom fuselage sheeting. If you prefer, there should be little problem with gluing it instead to the back of the existing rear former, after cutting out the center of the original one to within about 1/6" or 3/16" of each edges. I used the fuselage sides that were sticking out beyond the rear former to glue in 1/6" longeron scrap pieces to the corners to provide additional strength. After this, cut off the fuselage side and bottom to match with the 1/6" scrap.
- The small former goes in the inside fuselage, the exact spacing is non-critical but I put mine 3.5" ahead of the rear former. Again, bracing the corners with 1/6" longeron scrap is a good idea to increase strength. Just be sure it's square with the fuselage side and vertically. You may wish to use CA to glue these two formers in, then use a tiny bit of extra epoxy resin from the tail cone step later to really make the joint solid... remember, watch the weight, a little dab will do ya J
- Cut the tail boom to size. My overall tail boom length is 24.25". You want it to extend about 1/6" beyond the front of the front former you installed and about 20.5" behind the back of the rear former. After cutting to size, rough up the rear 1.5" of the boom and around the areas around where it will contact each former, using rough (80-grit) sandpaper. This is to ensure that the glue will hold well. The ink used to paint the arrow shafts is tougher than acetone and regular sanding will strip, so you likely won't want to sand the whole boom down!
- The block of foam will now be used to form a tail cone behind the rear former. You need to create a foam block measuring about 4" long by 1.5" high by 2.25" wide (OR, a bit wider than the width of your fuselage). You need to add a 1/16" soft balsa sheet inserted about 3/4" back from the front face of the block (the one that is up against the rear former.) This balsa sheet will become a former of sorts to help hold the tail cone in place properly. You need to drill a hole through the block to accept the tail boom... the remaining piece of tubing from trimming the boom is extremely helpful here. The block should be oversized, so you don't have to worry about perfect alignment. Aim for having a snug fit, but don't make it difficult to slide on and off.
- Once you get a hole through the block, insert the boom into the fuselage and slide the block over the tail boom. Use a pen to mark the outline of the fuselage on the front face of the block. Then proceed to carve and sand the block to form a tail cone. Smoothly taper it from the rear former back to a circular cross-section, about 1/6" out from the arrow shaft and 4" behind the rear former. Just try to get it nice and smooth and symmetrical, the exact shape is not terribly critical. Make it just a hair (1/64" per side) smaller than the outside of the fuselage at the front edge of the block.
- Once that's all sanded to a nice shape and you wipe off the sanding dust, measure out enough light fiberglass to go around the foam block with a bit of an overlap. Lay out the fiberglass on a sheet of glass or on saran-wrap pinned flat to your table. Mix the finishing epoxy and pour a bit on the cloth, then use a credit card or business card to thoroughly spread it. IMPORTANT: once the cloth is wet, squeegee off as much resin as possible!!!! If it's wet, it's got enough resin to do the job, and anything else is dead weight. Apply this to the foam, and smooth out the bubbles until it's nice and smooth. Allow to cure. If you use CA for this, just stretch the glass and apply the CA a section at a time. Use the leftover piece of arrow shaft to hold the part while you work on it.
NN NOTE: DO NOT WORK WITH EPOXY RESIN WITHOUT GLOVES AND DECENT VENTILATION!!!! NN I can speak from experience that epoxy allergies are BAD BAD BAD!!!
- Repeat two or three times. Between each layer, use sandpaper to sand the whole surface lightly and remove any fiberglass strands. Leave some excess cloth hanging off the ends unless it is wrinkled and separated from the foam. For the final layer, you may wish to leave a LITTLE more resin on the cloth to fill some pinholes, but if you don't it's no problem either. Trim the edges of the glass skin, and sand the excess resin off the entire piece. Try to get a nice, smooth, sanded surface. Place it on the tail boom and trial-fit it to the back of the fuselage, trimming the fuselage or the front face of the foam (as little as possible!!!) as necessary to make for a nice clean fit on the rear fuselage.
- You may finish the tail cone with water-based "polyurethane" acrylic. DO NOT, repeat, DO NOT try to remove the foam from the inside!!! The only place you MAY need to do this is along the front face, to provide clearance for the reinforcements to be applied to the boom/former joint. I used acetone to remove the front half of the foam, and had to add 3 more layers of glass to those areas as a result. The foam really doesn't weigh much at all anyway (especially white foam) and it adds a lot of stiffness to the part.
- You now get to assemble crude but very lightweight and sufficient pushrod sleeves out of the coffee stirrers. Take the stirrers and make a tiny (1/16" at most) slit in one end of each one. Then take one, slide it OVER (not inside) the other one to form a longer one. Hold this to a third stirrer, placing the joint towards the middle of the third one. Wrap a single layer of scotch tape around the joint and the adjacent stirrer. Repeat this process on both sides until you've built up a dual-tube guide for the pushrods that is at least 4" longer than the tail boom. Make sure they stay in as straight a line as possible. Insert the music wire pushrods to make sure they slide freely. The assembly will seem quite flimsy but that's fine, it merely needs to prevent major flexing of the music wire.
- Now for the interesting part. Cut five 1/2" sections off the regular soda straw. These will act as lightweight but effective spacers to hold the pushrod guides to the side of the tube. Every 8" or so along the coffee-stirrer assembly, loosely tape one of the soda straw pieces on, in the same direction at the tube and all on the same side. These will act as springy spacers to push the guides to the bottom of the arrow shaft and hold them in place, glue is not required as there isn't much load to deal with. Don't do this on the part of the guide that will end up right at each end of the tube, those will be inserted after the guides are in place. Place a little extra tape around the pushrod guides at the end that will become the rear pushrod exit to hold them together securely, and trim them to be even. Add some tape for the section that will be just inside the other end of the boom as well.
- Now, carefully insert the guides into the arrow shaft from the aft end. Just take care to keep it aligned to one side (the bottom) with as little twist along the length of the tube as possible. You should have a couple of inches sticking out from the front of the arrow shaft, while the rear end should be flush with the end of the shaft. Put a guide into the tube at each end of the shaft. After inserting these, check that the music wire goes smoothly through both guides, and secure the end spacers with a small drop of CA.
- Insert the arrow shaft into the fuselage and position it so that the pushrod guides are towards the bottom and it is aligned fore and aft correctly. Glue it in place with the medium or slow CA at both the front and aft formers. Then, take some small pieces of light fiberglass cloth and wrap them around the tube on either side of the formers, pressing them into the "corner" of the boom/former junction. This may be easier while the thick CA is still slightly sticky. Use either thin or thick CA to put onto the glass cloth, or alternatively you may wish to use some epoxy resin if you want more working time.
- Place the tail cone onto the boom. Make sure the newly-added boom/former reinforcements don't keep it from fitting, if so, trim just enough foam around the boom to provide clearance. Tape the cone in place for now.
- My tail feathers were in the V-tail configuration, although it shouldn't be too hard to figure out a good scheme for a conventional tail version. I actually cut apart the stabilizers and re-joined them after the initial crash in order to get rid of some "goo" from the original mount on the balsa fuselage, which shortened them very slightly. I left the elevators attached as there was no great need to separate them only to re-attach them later. Join the two halves at the proper angle, and be sure to add a well-fitting half-round fillet between the sections on the top from the leading edge to the rear of the fixed portion. Then, on the bottom, sand the bottom of the joint to be flat (take the "knife edge" off of it) or even better, wrap sandpaper around the scrap arrow shaft tube and carefully sand a matching groove along the bottom of the joint. Then using the scrap arrow shaft for a guide to alignment, glue some 1/6" longeron scrap on each side parallel to the join line. These will align the whole tail with the boom, so take your time and tack-glue them until it's right before gluing them in place permanently. You may wish to round the outer corners off a tad, but it's not vital.
- The stabilizer will be mounted to the tail boom such that the tail boom ends 3/4" ahead of the hinge line. mark this point off on the elevator, or you may even glue a TINY scrap stop at that position. Place the stabilizer on the boom in roughly the right place, and insert one of the pushrods. Bend the pushrod to go through the control horn, on mine this involves about a 20 degree angle before the Z-bend to go through the pushrod hole. Check that there is free movement of all the bends through the entire range of ruddervator travel on both sides.
- Tape a flat stick (popsicle stick) in place on each side of the elevator so that it holds the ruddervator in line with the stabilizer. Then, with each pushrod in place in the ruddervator control horn, and with the entire tail held in the proper position, figure out where the Z-bends in the pushrods will be on the servo end. Be sure that you have plugged in the servos and neutralized the trims to center them before you do this! It helps to mark the bend points with permanent marker so you can cut off most of the excess pushrod and make it easier to work with. Once you have laid the servos in the fuselage and are confident that they will properly actuate the pushrods without binding, make the Z-bends. Be sure to orient them properly with respect to the other end! I ended up actually gluing a piece of coffee stirrer onto one of the servos to act as an additional pushrod support due to excess deflection under load. The more of each pushrod guide you can leave leading out of the front of the boom without binding, the better you are. Be sure to push on the pushrods by hand while holding the control surfaces in place to get an idea of the deflection, and add any intermediate braces (coffee stirrer sections) that are necessary to support it. Do not mount the servos yet... take the front Z-bends out of them.
- Now, put the rear Z-bends into their respective control horns, and attach the tail assembly to the boom. To align it properly (important!) you need to attach the wing fully. Then sight from the rear of the plane, looking directly down the centerline of the aircraft, and make sure that each tip of the fixed portion of the tail appears to just touch the trailing edge of the wing on both sides. You will probably end up guesstimating the right place, then tack-gluing with a couple of small drops of thin CA. Then look to see how far off you are, free the tail and adjust it in the right direction. OR if you have two people you might be able to align it and tack-glue it while holding it in the right place. Just take your time to get this step right. Once you get the alignment correct, use thick CA to glue the tail securely onto the boom, and allow to dry.
- While that dries, take a 1.5" long piece of pushrod wire and bend it into a tail skid. This will protect the control horns from damage. Bend a quarter-circumference curve on one end using the scrap arrow shaft for a template. Then run it straight back from the curve (along the length of the tube) for about 3/8", then bend it down at about a 45 degree angle. Bend a nice little loop in the end to keep it from poking a hole in something or somebody.
- Once that is done, wrap a single piece of fiberglass cloth reinforcement from the bottom surface of the stab about 1/6" outboard of the longeron scrap you used for alignment, and work it around the boom to the same position on the other side. Laminate it using either CA or epoxy, whichever you are most comfortable with. After that is cured, you can trim it on the boom and sand it lightly. Then at the rear edge of the tail boom, on top of the fiberglass you just added, use 2 small pieces of fiberglass cloth to attach the tail skid, pointed down of course. Finally, add one final layer like the first one. Once cured, you may seal the fiberglass with water-based poly.
- Put the Z-bends in place on the servos, and attach them in place using tape or glue or screws or bubble gum or belly button lint or whatever else you use for that. Leave the tail locked into zero deflection as a way to aid in assist in proper servo placement (mostly for mounting with tape of glue) and again, make sure they are centered.
- This is probably obvious to some, but don't try to use the pushrods as an antenna inside an aluminum tube. Run the antenna out to the tip of the tail and fasten with a tiny rubber band over a pin instead.
- The tail cone is attached with scotch tape around the joint between it and the fuselage. You will be able to easily slide it back to inspect the boom/former joint, should it be necessary.
The final thing to remember is that you have lost most of the rear fuselage area, therefore the aircraft's center of pressure has shifted forward some. This means that the CG range is a little bit forward of where it originally was. A good starting point is to put the CG for the test flights at or just a hair in front of the normal forward limit of the CG range, which is now closer to the middle or rear half of the new acceptable CG range. There is also less area behind the CG for lateral stability, but I haven't yet seen anything really awful from this. As I test more, I'll post anything wild and crazy right here J
I would like to hear from anyone who tries this to see how it went. Please email me at firstname.lastname@example.org or catch me at the next IRKS meeting. Or of course you might see me at the flying field or the beach!! May all your launches be high, your thermals be big, and your landings on the spot!
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