[Courtesy of Jan Kansky kansky "at" ll.mit.edu, August 1999]
KWRRCG2: The Kansky Water Rocket Radio Controlled Glider Experiment (part 2)
Here is a picture of what I call my "2 liter class" radio controlled water rocket glider! I've christened it the Neptune 2LA. I tried to launch my Chrysalis handlaunch glider last year using a 2 liter bottle, but it couldn't take the stress and broke up due to excessive airspeed on launch number 3. In the meantime, I tried launching my Boomerang EPP slope combat plane but it was too heavy and not very stable in pitch so it would simply do a tight 4 foot diameter loop immediately on launch, quite exciting. I decided that I needed to build a stronger airplane specifically designed for the stresses involved in a water rocket launch. This is what I came up with:
Here are the stats on the Neptune 2LA:
Wingspan: 1 meter
Airfoil:Modified SA7037 at the root, transitioning to a modified SD7038 at the tip with 1 degree washout. The airfoil templates were cut with 1/16" sheeting in mind, and I ended up using 1/32" obechi veneer. I also sanded the obechi at the tips making it even thinner (that's what I mean by modfied, a fancy way of saying really inaccurate, that may explain why so many kits have "modified" airfoils).
Weight: Approx 8 oz. (haven't weighed it yet...)
Rocket: 2-liter soda bottle with modfied garden hose quick disconnect nozzle (8.6mm diameter) I used my Dremel tool to smooth out and taper the interior of the nozzle to maintain smooth water flow. Thanks to info from "Rocket" Bob Kreutzer for this performance enhancing tip.
Construction: The tail boom is a Dave Brown fiberglass pushrod, the pod has 1/8" balsa sides with 1/16"cross-grain top and bottom, the tail is sanded 1/8" thick solid balsa, the wing is white foam with obechi veneer. I used transfer tape to attach the bottom wing skins, and epoxy to attach the top skins mainly because I ran out of tape! I added an 8" section of light fiberglass 1.4 oz at the root, top and bottom, and a 4" layer of 1.4 oz top and bottom on top of that, and finally two 0.5" 1.4oz layers over the center joint.
I was a little concerned about the wind, and because it was the first launch I wasn't exactly sure which direction my plane would decide to vector for the critical first second! I was standing looking at the upper wing surface, so I figured I would dial in some down trim to keep the plane from flying behind me. I filled the bottle up about halfway, pumped with my little bike pump as hard as I could and crossed my fingers. Because of the downtrim the plane executed a nice parabolic arch away from where I was standing. Not exactly the vertical launch I was hoping for, but still pretty good for the first time. The trouble began when the bottle seperated. Instead of falling harmlessly through the v-tail as I had hoped, I heard a bang as the bottle ricocheted off the tail of the plane. Luckily, my excessive superglue bond did the job, and the tails stayed on. Over the course of the next second, I realized that I had no control of the plane! At first I thought it was just slow to respond, but I knew I was in trouble when I pulled full up elevator and the plane continued its nosedive to the ground. It hit hard and the nose fractured at the end of the fiberglass boom. I was disappointed. My main concern was that the bottle would not clear the v-tails when it seperated from the plane. I was worried that I would need to redesign everything. The damage wasn't too bad. I was able to glue the plane together in 5 minutes with some superglue, thanks to the well equipped field box of my fellow club member and photographer, Jose Bruzual. Thanks Jose. I'm still not sure why I lost control, the pull-pull cables probably came loose due to the bottle impacting the tail.
Flight 2: For flight two I decided to leave the trims at the nominal position. Just as I was trying the last few hand tosses to trim out the plane before attaching it to the water bottle, my transmitter started complaining that the batteries were getting low. Good timing... It's hard to ignore an annoying beeping sound coming from your transmitter. Nothing like the good old days when you could squeeze in another flight on fading batteries with an old analog tranny. Anyway, after a short break for charging I was back in business. Same drill, I filled the bottle half way, pumped for all I was worth, stood back and pulled the release cable. This time the plane headed straight up. Jose managed to get some great pictures of the launch! Here they are:
The water comes out with such force that it drills small holes through the grass and about an inch into the mud beneath. Note to self: bring some sort of tarp or towel to prevent a muddy mess. Onlookers from left to right, Helmut, Bob, Les, Anker, Chris.
This setup launches the little Neptune really high! Of course, I want to go higher, but at what cost? Do I hear someone saying "multiple stages"? This picture shows the plane nearing the end of the boost phase, still heading straight up. Computer simulations of this setup predict a 60mph velocity at the end of the boost. The time between these last two images is approximately 1 second.
Of couse, flight three wasn't perfect. For whatever reason, the 2-liter bottle didn't seperate, and the poor Neptune glider struggled to the ground in a severly nose heavy configuration with tons of drag, slightly reminiscent of the SuperGuppy transport plane. Of course, thanks to Murphy's law the parachute promptly deployed with the bottle still attached to the glider, and the plane spun in. Luckily there was no damage.
I couldn't be denied for a third time. I increased the tension in the rubber band that encourages the bottle to seperate after the boost is complete, and launched again. This time things worked out nearly exactly as planned. The plane boosted straight up, the bottle seperated cleanly and the plane continued straight up for a considerable additional altitude gain after the bottle had seperated. Of course, this time the bottle recovery parachute refused to open! The flight was great. I almost caught a thermal and climbed out, but I just missed. Nevertheless, the flight was a minute or two in duration. Next time I'm at the field I'll bring a stopwatch and do some sighting to set a new "world record" for the radio controlled 2-liter class altitude and duration mark. (Note: as far as I know I have no competition). I'll update the flight log with my best time, in the event that anyone wants to make a challenge!