DJA Chrysalis - Random Advice|
(Worth what it costs you...)
This sailplane graphic was graciously stolen
from some model sailplane company's website!
I am having a lot of fun and learning a great deal with my latest R/C plane, a DJ Aerotech Chrysalis hand-launched glider (HLG). As I learn more about this ship I will post things I've learned. While the DJA website is one of the best ones ones out there for models, inevitably there are things that aren't always covered that I had to figure out on my own. Of course, these things likely will be applicable to other airplanes and gliders, so here's some stuff that may help you get a head start.
The modification that stands out the most is the fact my Chrysalis has an aluminum tail boom. It is far sturdier than before, and not too hard to do. Whether it helps or hurts you in a competition, I cannot yet say but for slope-soaring and general fun flying, i think it's definitely a good thing. Looks pretty cool too. Read all about it here.
I made the hatch and hollowed out the inside as much as possible ot save some weight and add a little room inside for wiring. There is a 3/32 balsa tab on the front... protrudes about 3/16 inch (this isn't too critical) and it's wide enough to touch the fuselage sides to keep it aligned. The back edge of the hatch has two balsa tabs that hit the side of the fuselage and keeps the rear of the hatch aligned side-to-side. There is a small hook glued near the rear of the hatch in the center, made from a small straight pin and attached with CA and a little piece of fiberglass cloth. There is a matching hook below it on the floor of the fuselage, and a small rubber band between the two holds the hatch on yet stretches enough to allow easy access. The radio switch is mounted on the hatch, towards the left side.
It is very easy to crush the wing top sheeting between the spar and leading edge. I recommend, if possible, adding thin fiberglass cloth to the inside of the sheeting with CA in this area, I didn't and had to cut through the bottom to fix it with a layer of CA after I put my thumb through it just picking up the plane. Another thing is to be sure to install full-span shear webs as described on the DJA site... the wing flutters a little on launches sometimes even with this, and I bet it's downright dangerous without it.
I have had a recurring problem with the wing ribs on my Chrysalis cracking
and breaking during normal use. It's always where they attach to the
trailing edge or anywhere up to an inch or so in front of it, and seems to
affect the wing anywhere from the center to the tip. I have tried putting
CA on the ribs in this area but it hasn't helped much. Don Stackhouse of DJA also gave me his thoughts on this problem.
Be sure to use good foam wing-mounting tape. Gaps between a wing and fuselage, even small ones, add a lot of drag.
I have a bolt-on wing, which is really easy to assemble (just need a small screwdriver, or even your car keys if you want to simplify.) A single 1/6" dowel in the front of the wing goes into a hole in the former at the leading edge. This is attached to the wing so that I can easily replace this dowel, as it is subject to break in a crash. A 3/16 brass tube (1-1/4 inches long now, but I think 3/4 inch would have been more appropriate though) is attached to the bottom of the wing, along the centerline. It is attached using two layers of fiberglass and epoxy. The front edge of the brass tube is even with the leading edge of the wing. A 1/8 dowel (1-5/8 inch long) is covered with heat-shrink tubing, which is just squishy enough to make it fit securely into the tube. A dowel that fits tightly without heat-shrink will be at risk of breaking from stress at the front edge of the brass tubing and will also be darn near impossible to remove if it breaks in a crash. Also seal the entire dowel with CA for a little added strength... while you're at it, make three so you won't have to go home early if you break it somehow.
A single 6/32 nylon bolt going through the center of the wing 3/4" in front of the trailing edge holds down the rear. I did add a couple extra layers of fiberglass cloth about an inch wide at this spot on the top of the wing while I was joining the wing halves, but this is probably not a big deal to omit if you're converting a wing that's already built. I didn't have a nylon washer handy but I used a metal one for a template and made a couple from scrap credit-card type plastic... this spreads the load out a tad across the wing surface. The bolt screws into a blind nut installed to the bottom of a thin 1/8" x 7/8" wide hardwood plate going across the width of the fuselage, about 1/8" below the wing saddle and secured with thin pieces of scrap 1/8" longeron stock above it to help provide a rail to pull against. All this is fairly light, very sturdy, pretty easy to do and really works well.
Definitely add the 1/4 triangle stock on top of the protruding pegs. I still had problems with hooking my fingers over the top edge though, so I carved/sanded out a concave cavity on the back side (a cupped sort of shape) that is very much angled. This way, you really do not get a solid grip on the pegs with your fingertips... this sounds like a bad thing, but in reality, it helps since your fingertips stay on the plane through friction, not by hooking them over the top of the peg (and subsequently pulling down on the plane at the end of the launch and giving the wing a high-G stress test.)
It's damn near impossible to find an L-shaped screw proportioned like a towhook should be. I looked all over but ended up buying an L-shaped wood screw and just bent it myself. Even if you aren't going to do a lot of hi-start launching (more like low-start with this size plane) it makes a good skid to protect the bottom of the fuselage. A word of warning... take a minute or two and ROUND OFF THE EDGE of the hook!! I hit a sharp-edge towhook with my finger that on a friend's HLG once and it drew blood... and worse, it made it painful to keep launching.
Covering / Finishing:
Just two thing here. TRANSPARENT RED MONOKOTE SUCKS. It's brittle, it doesn't shrink well, it wrinkles quickly, it doesn't stick to anything very well, and the color/adhesive layer separates from the plastic very easily. Transparent yellow Monokote works great, regular red is just fine, but transparent red just plain sucks! Never again.
The second thing is that water-based polyurethane is one of the neatest things I've ever learned about for models. It's incredibly easy, safe, cheap, waterproof, nearly weightless, versatile and it cleans up with water. Get a few el-cheapo acrylic paint bottles and make any any color you need. Please try this, it's great! My personal tip here is that clear 35mm film cans make great mixing and airtight storage containers in the right size for small quantities you need for HLGs.
RADIO AND CONTROL SYSTEM INSTALLATION
The transmitter is an FM Flash-4 from Hitec RCD. I ordered it with two S-80 mini servos and the 555 receiver. This system cost about $166 in early 1998 (from Hobby Lobby), so the price was right. I am quite happy with this radio and believe it's a great deal for anything but the most basic of purposes... it's programmable and has a two-model memory, and is simple enough that you don't have to lug the manual to the field with you just to change the basic settings.
I also ordered a tiny Tetra FM 6-channel receiver with two S-80 micro servos from FMA Direct. Unfortunately, I was quite dissatisfied with the service from FMA (they lost my order twice and it took three phone calls over two and a half months to get my stuff... my money order was cashed immediately after I sent the order though) and have heard similar stories from others along these lines. I have had no problems with the equipment and it is very very tiny, in that sense I am happy... but I would recommend if you just have to have something from them, to buy it from a distributor that has them in stock and can confirm that fact before you actually order. FMA seems to have recurring problems getting their ordering and shipping system straightened out and has had for a long time based on other's experiences.
The following equipment was used in my Chrysalis:
The remaining Hitec receiver and servos are being saved for another project. I got the Flash-4 in part because it has two model memory and in part for the mixing function I needed on the V-tail. A good, inexpensive radio that has some basic nice programmable things I could use. Given that I had to add nose weight, I probably could have used the Hitec receiver too but it wouldn't quite fit easily... plus, I had already bought the Tetra and didn't want it to be unused. If the plane was built to be competitive at all costs (ie, one of the 8-oz. Chrysalises that people build for heavy-duty competition) then the few grams would have meant more. For my skills and flying conditions, it's not a big deal on this ship.
- Hitec 250 mAh battery pack slid into the nose, covered in the included "Flight Saver" foam padding and electrical tape to hold it together. Nose weight is taped to the front of this.
- Hitec switch harness mounted towards the left side of the hatch (a homemade toggle assembly worked but just didn't seem safe enough... too easy to switch off or on by mistake)
- FMA Tetra receiver, wrapped in the thin spongy-foam sheet used to pack the Hitec system in and then wrapped in Scotch tape. It was then mounted to the right side of the fuselage behind the battery with stick-on Velcro tabs (this allows room for the switch mounted on the left side of the hatch.)
- Two FMA S-80 servos (see below.)
The S-80 servos are really tiny and are installed in the front of the bay underneath the wing, both fitting ahead of the throwing peg. I covered one side of the servo with foam sticky tape (not from a roll, but the stickier tabs used for mounting pictures.) I covered the other side of the tape with tiny drops of CA glue, and stuck it in place. I glued a 1/32 sheet of balsa to the fuselage (grain going side-to-side), which covers the fuselage floor from the bulkhead to the front edge of the throwing peg. This was because de-mounting the servos kept ripping balsa chunks out of the fuselage floor, and this can be replaced easily if needed without hurting the load-bearing structure.
The servos are arranged so that the servo for the left side of the tail is mounted with its side to the bottom of the fuselage, with the servo arm screw facing the left side of the fuselage and the servo arm pointing up. Both servos are turned such that the arm is more to the front of the aircraft than the rear. The bottom of the servo is against the right side of the fuselage, although I've decided it's not necessary to attach it to the fuselage side (it won't hurt, if you prefer to do this.) The servo for the right side of the tail was mounted behind that on its side as well, but reversed in orientation from left-to-right (its bottom was against the left fuselage side.) I ended up drilling two holes in the fuselage adjacent to each servo arm in order to insert a screwdriver to unlock them for adjusting. It's a lot easier than de-mounting the servos and having to re-trim them each time. A little circle of Monokote and it's not overly noticeable.
The iteration of pushrods used at this writing, to avoid re-making the whole pushrod assembly after adding differential by going one click forward on the servo arms, involves two pieces and is not the lightest possible way to do this. I have the Z-bend on the arms out of one piece of wire, which is soldered to the pushrod wire. I held the ruddervators in place and set the servos to neutral trim, then glued the two together with CA. I then used needle-nose pliers to wrap it loosely with a tiny copper wire strand, and soldered it together for good. not the best way, but I chose the slight weight increase and annoyance factor over going to the store for more wire and re-making both pushrods. With any wire pushrods, you can glue a small piece of coffee-stirrer straw to support places where it would flex excessively. If you glue these supports to a servo, put masking tape on the servo and glue to that so as not to damage the servo.
When I converted to an aluminum tailboom, I couldn't use a pushrod for an antenna and I couldn't run the antenna inside the conductive boom. So, an external antenna it became. I ran the antenna back to exit at the corner of the fuselage side and former at the trailing edge of the wing. I glued a little piece of shrunken heat-shrink tubing as a guide out of the fuselage, and used little pieces of coffee-stirrer straws glued to the side of the fuselage to run the antenna through and keep it out of the way. I put a piece of heat-shrink on the antenna wire to keep it from being pulled farther out of the fuselage than desired. I then took a small piece of thin plastic scrap, and put in two small holes about the diameter of the antenna wire) about 1/8 inch apart. I cut this piece out, leaving enough to hold the piece together.around this, then ran the antenna up through one and down through the other, with a small rubber band in between. Then I pushed a small round-headed sewing pin into the stabilizer, sticking out just enough to let the rubber band go over it (go back and glue this in permanently, but you may want to cut the pin a bit shorter.) Adjust the placement of the rubber band on the wire so the antenna is held loosely but not so loose it falls off easily. This attachment is nice since it absorbs bumps very well and is easy to adjust.
I am using a V-tail on the aluminum boom. Contrary to what DJA recommends as what is normally needed, I have ended up with differential in the tail surfaces set for more up than down travel. Don Stackhouse at DJA took the time to explain this in detail, and it's apparent that my own flying style is basically the problem. Here is Don's explanation. The Chrysalis tends to require you actually fly each phase of a turn differently (just like an airplane is supposed to do) rather than just hold a bank and pull back some. I was used to models that required a constant rudder input to maintain a bank and that weren't too touchy on the elevator.... so I was inducing my own spiral death-dive by holding rudder, then pulling back too much and that of course didn't quite help things like you'd think due to the interaction of the two control functions and my failure to realize I had to allow the plane to stabilize itself in turning flight. Another thing that happened, which was dang near impossible to figure out but that grossly exxaggerated all this, was that I was overcontrolling. I used the programmable Flash-4 transmitter's dual rate function to help with this (on the Flash-4, it really isn't switchable and basically becomes an adjustable travel volume setting, while the Flash-5 does have a dual rate switch.) The elevator is at about 75% of full throw, while the rudder is about 85%. While your numbers will vary, I think the important lesson here is that there needs to be significantly less elevator travel than rudder, and you can't do this without an transmitter capable of dual rates (or are using a mechanical mixer in the plane.) Due to the mixing required, this is not the same as just reducing the throws on two servos if they are mixed together. Of course not all planes will be the same but keep this in mind if you are considering building a V-tail or buying a radio for it.
Other than the mechanical differential I set up (the more fancy programmable radios could do that electronically) and the dual rates, the one other thing that could potential confuse someone is that you do NOT use the "V Tail" mixer!! You use the one for "Elevons". A traditional V-tail mix mixes elevator and rudder, but assumes that the rudder is acting just as the rudder, not being used for roll control like you do with a dihedral/polyhedral aircraft without ailerons. You basically are flying the same stick that you'd otherwise have ailerons on, so you need to tell the transmitter to mix ailerons and elevator.. thus, the Elevon mixing. Other than that,both mixes work exactly the same... up is up, down is down, and left/right makes the surfaces move opposite each other but remain mixed with elevator inputs. So don't panic that you have to pretend you have a flying wing when you're turning on the mixing J
I would like to hear from anyone who finds this information helpful. 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!
Chrysalis Tail Boom Mods
Chrysalis Ruddervator Info
Chrysalis Rib Strengthening
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Hits since Oct. 18, 1998