Rocket/Glider Duration
OVERVIEW
This is an event about flight duration: whose rocket can stay up in the air the longest in stable gliding flight after being launched vertically with a rocket motor of a specified total impulse. All parts of the rocket that boost must return together. Therefore, staging and pop-pods cannot be used. The gliding portion may not use flexible materials for its aerodynamic surfaces (if it did, it would belong in the “Flexwing” event) and may not have an attached parachute or streamer
For the full rules for this event, please see the Rocket Glider rules in the Model Rocket Sporting Code.
SCORING
The contestant may make one or two flights. Up to two models may be used. The score is the sum two flights, as long as one is returned.
GENERAL TIPS
article and drawings by George Gassaway, NAR 18723
Design Considerations. It is difficult to make and fly a glider that does not have any parts come off in flight (often with a design that uses some sort of moving parts like a sliding wing or sliding pod). The trade-offs are between high glider performance and visibility with surviving rocket boost, and trying to get at least a reasonable boost altitude.
Rocket Glider designs are more specialized than Boost Gliders, so it is not recommended for newer fliers to try to design their own. Best to go with a kit such as those below, or a plan. With experience, you might later develop a knack for designing your own gliders, but it’s best to learn from something proven first. A list of plans and kits is provided further down on this page.
Many basic design approaches have been used for Rocket Glide. Here are a few.
Slide-Wing. The wing is mounted in a manner that allows it to slide along the fuselage, so the wing is towards the back for launch (moving the CP rearward), then at ejection moves forward for glide. They tend to boost very straight. The wing slide mechanics are somewhat more difficult to build than others, but it is usually worth it.
Slide-Pod. A long engine pod about the length of a normal pop-pop is used. The pod is forward on boost (making the CG forward like a normal pop-pod B/G) then at ejection slides back so that the CG is where it needs to be for glide. It is a reasonable approach which usually boosts OK, and is easier to build than a slide-wing. Well-made slide wings tend to perform better than well-made slide-pods. Slide-pod models use a telescoping external tube glued to the pylon, such as 20mm telescoping tubing, for a BT-20 sliding pod tube to freely slide inside of.
Auto-Elevator. A design which does not move the wing, or move the CG, but changes the angle of the elevator (or canard). The elevator has to be flat (straight) for boost so there is no incidence angle between the wing and tail, so the model will boost straight. For glide, the elevator moves up (or canard trailing edge moves down). This is often one of the simplest and easiest rocket glider designs to use. Although depending on the particular model design, the performance potential might be compromised.
The Xebec-B is an auto-elevator design, which is easy to build as R/G’s go. But it is somewhat finicky as to how straight it boosts, varying from model to model (and sometimes modeler to modeler). Usually it if does not boost straight, it pitches up a bit onto its back, due to unintended slight up elevator angle.
Swing-Wing. Originally a definitive early design for Rocket Gliders, the wings pivot from rearward for boost to forward for glide. They are neat to see fly and deploy, but tend to be too heavy and complex to be competitive other than in the higher engine classes. For B engine class, it’s not very competitive.
Deployment Magic. The above R/G designs use the ejection charge only as a starting point to cause the design to go from rocket boost mode to glide mode. Usually rubber bands are used to make the wings slide, pods slide, wings swing, or elevator to move. What keeps them from moving until ejection? Usually a burn-thread which is rigged to hold the model in boost mode. At ejection, the thread burns, allowing the rubber bands to do their thing.
For Auto-Elevator prep, see the Xebec-3A plan in the files below. Only a light pull is needed to make the elevator pop up: instead of a rubber band, elastic thread tied into a loop of the proper length should be used. A too-strong pull might make it difficult to force the elevator flat for boost.
The burn thread needs to be thin so it will burn easily. The button and carpet type of thread that Estes uses for shroud lines is too thick for anything short of a D12 ejection charge to burn. So use a finer thread. I prefer to use white elastic thread, which will snap free when it burns.
The rubber bands need to be selected carefully. Do not grab just any rubber band and try to force it to stretch really far, as it might cause the model to actually break itself when the band force causes the model’s parts to slam into each other too hard at ejection. Look for rubber bands with 1/16″ cross sections, in various lengths, at Office Supply type stores. Look for the classic pale brown latex bands, colored bands tend to not stretch as much. While the band should not be stretched so much that it causes excessive force, it should not be relaxed at the end of travel so that there is slack, as it might not deploy the wings, pod, etc, 100% as it needs to. This is not hard to get right, just it can cause trouble if it’s overlooked.
Building Gliders. Jeff Vincent’s Nocturne Slide-Wing Rocket Glider plan includes a lot of useful instructions on how to build, trim, and fly a Rocket Glider. It also includes some information on how to rig up the thread and rubber band system, useful to know for any slide-wing. The article with plans is provided below in the files.
Ed LaCroix created some fantastic instructions for the Maxima A Boost Glider. The instructions are so in-depth that they are a must-read for anyone wanting to learn more on how to build, trim, and fly rocket boosted gliders of any kind. They are in the files section of the Boost-Glider page. Please note that the Maxima is a BOOST Glider, not a Rocket Glider, so that design is not legal for this event. But the general “how to build and fly glider” information in those instructions is very useful.
A VERY nice sanding block, useful for gliders, helicopter, and any rocket really, is an all-metal 2-piece clamp-type sanding block made by Red Devil, carried by most Ace Hardware stores (look near where they stock sandpaper). It has a 1/8″ foam rubber backing sheet, which ought to be removed since it allows rounding things too easily in 3-D when you usually want to shape in 2-D at a given time (as with a wing). So, remove the rubberized portion. That sanding block holds a 4.5 x 5.5″ sheet of sandpaper (quarter of a 9 x 11″ sheet), with a 3.5 x 4.5″ sanding surface area on the block. This works far better in most cases than a narrow sanding block. Of course, the wings (or fins) should be shaped and finished before they are glued to the fuselage or model.
When doing very rough shaping for wings, 80 to 120 grit sandpaper is good to grind off a lot of wood in a short time. Don’t over-do it though by sanding off too much. Then go to finer grit like 180 to 220 for finer shaping. Beyond that, 280/320 paper, is sort of a cross between final shaping and setting up for a final finish. A final finish for bare balsa is to use 400 grit paper. Get the “black” type wet or dry sandpaper, it sands better and lasts longer than the reddish types. If you use any clear dope, use 320 to 400 grit paper before and after.
Glider Finish. Never use paint on a contest type glider. For newer fliers, no finish is usually fine. You don ‘t want to weigh the model down too much and maybe having warpage problems. If you want to improve the finish, use some thinned clear dope in one or two light coats, sanding before and after with 240 and 400 grit sandpaper. The idea it not to add weight or cause the wood parts to warp. Some fliers like to use a Japanese tissue finish, but that is more for experts and to an extent is overkill for B R/G’s.
Coloring. OK, so bare balsa (even with clear dope) is not easy to see in the air or on the ground. Use a large black magic marker to color the bottom of the wing and tail surfaces black, as that shows up against the sky pretty well. Use a large red or orange magic marker to color the top surfaces. If you can find true fluorescent markers (not to be confused with wimpy fluorescent highlighters), an orange or red/magenta fluorescent color is highly recommended.
Glide Trimming. It is an understatement to say that it is very important to trim the glider to glide properly. It’s not easy to describe just how to do so.
One mostly universal tip is to have the tail of the glider and the wing to not be parallel to each other (zero incidence). There ought to be a little bit of “up elevator” angle in the tail, relative to the wing, to make the nose pull up a bit. Some plans/kits may be quite specific, and in those cases go with what they say (for example the Xebec-3A auto-elevator design requires absolute zero incidence for a straight boost, then up elevator for glide). Note that free flight model airplane experts sometimes prefer zero incidence, but they are deeply experienced (usually) to have just the right touch and experience to get away with it.
The above being said, one of the simple ways to achieve slight up elevator effect (without adding too much) is to build the model zero-zero (wing and stabilizer parallel to each other), then warp the trailing edge of the stabilizer up. Make sure that the fuselage itself is not warped “down” – if it is warped at all, better to be warped “up” relative to the wing and stabilizer.
Special trimming notes for Rocket Gliders:
- Always trim with a burned-out engine casing in the model, using the same engine type you will fly it with. If you trim with no engine, or an unfired engine, the CG you try to trim for will not be where the real flight’s glide CG will be.
- For Slide-Wing models, do not glue the forward wing stops on before going out to do some basic trimming. Use tape to hold the wing from sliding along the fuselage, and do a few trim throws to see if you have the wing in the right ballpark location. If the model seems tail heavy, move the wing back a bit, and re-tape. If it seems nose heavy, move the wing forward a bit, and re-tape. When it seems to be in the right spot, use a pencil or pen to mark on the fuselage exactly where the wing should stop, then later glue the stops on at that location.
- For Slide-Pod models, do not glue on the glide position stop ring (near the front) to the slide pod until you’ve done a similar series of rough glide trim tosses as described above for slide-wings, in order to determine where the slide pod should stop for glide.
- For Auto-Elevator models, the trimming is unique. The elevator needs to be absolutely flat on boost, so the model will boost straight. Auto-elevator models tend to be designed a bit nose-heavy, so there should be no need to add any noseweight. So the way to trim an auto-elevator model (at least as far as pitch trim), is to adjust the elevator angle until it glides properly. If it stalls, adjust for less elevator. If it goes into a shallow dive or just feels like it is gliding too fast, adjust for more up elevator.
General trimming tips from a number of different sources:
- Kevin McKiou’s article below on Design and Trimming Gliders is a good primer. Items 1 thru 5 are useful only if you are designing your own, or modifying an existing plan/kit. Items 6 thru 9 are useful for trimming gliders for optimum glide performance.
- See also Dave Newill’s article below on trimming boost-gliders.
Launching. Set up the glider on the pad so that it faces into the wind. This means the wing bottom faces upwind, and the wing top faces downwind. Actually the dihedral effect will usually try to make the glider face that way.
Some people prefer to make their own glider launcher that has the launch rod mounted onto a 3/8″ or 1/2″ dowel, so that the whole rod can be used for guidance.
A classic launch problem is for the micro-clips to fall at ignition to grab onto the glider wing or tail, causing damage or even disaster. Some prefer to tape the clips to the rod so they can’t fall, which is effective but a bit messy. Others like to use an umbilical approach. They arrange for a separate launch rod, dowel, or other structure to hold the micro clip wires away from the glider, so when the clips fall they will not fall straight down, but swing away in an arc from the glider. A simple umbilical is to use a piece of 1/4 x 1/4 spruce 18″ long or so, and cut the bottom at a very sharp angle that is about 30 degrees from vertical. Then glue a launch lug to the angled part. That way, the spruce umbilical can be slipped over the launch rod, umbilical angled at 30 degrees from the rod, ready for the micro-clips to be attached to it. Put your name on it, since the next person to use that pad will probably want to remove your umbilical.
MOTOR SELECTION
Rocket gliders are generally draggy and often delicate, with the thin lightweight aerodynamic surfaces tending to “shred” if they reach too high a boost velocity. Because of their drag they do not coast very long, so you usually need a short delay time on the motor so they are not headed downward at ejection — this is hard for a glider to recover from into horizontal gliding flight. You also generally want a motor with a lower average thrust to spread the total impulse out gently over a longer period of time and avoid reaching the “speed of balsa” during boost.
KITS
–Easy-Slide kits (1/4A through D) – Qualified Competition Rockets
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