APPLIED TECHNOLOGY R/C FLIGHT SYSTEMS P.O. BOX 69, FAIRPLAY, CO 80440 BRUCE STENULSON 719-836-2489 Visit our Website at http://shell.rmi.net/~stenulsn/rcflight.htm THE "SNOWSHOE HARE" SKI SYSTEMS COPYRIGHT 1993, 1997 BRUCE K. STENULSON First section Revised 12-4-97 BKS Flying ski equipped R/C aircraft from snow covered areas opens up new possibilities as to where you can take off and land. The snow covered ice of large lakes present especially fine flying These 1/8" birch ply skis and especially the mounting system used were developed and refined over several years of winter flying in the Colorado Rockies, at elevations of 9,000 feet and above. Here, we deal with deep, soft snow on a regular basis, which has led us to using wider skis. With more surface area to support an R/C aircraft, these skis allow flying from and landing in deeper powder snow conditions, while still handling well on hard-packed snow and even bare ice. MOUNTING SYSTEM AXLE MOUNT BLOCKS AND TORQUE WIRES Over the years, many methods and designs were tried in an effort to keep the skis properly aligned on the aircraft, for both ground handling and in flight. We gave up on the spring and cable setups as less reliable, and now favor and recommend the axle mount blocks with torque wires supplied with our ski kits. They do require that your axles can not rotate, allowing the skis to get out of alignment! For bent wire landing gear, this is not a problem, and we recommend custom bending wire gear for your ski equipped plane for three reasons: Non-rotating axle Adequate prop to snow clearance Adequate tracking width for good handling GRIND A FLAT ON THE AXLE FOR THE SET SCREW For the most reliable ski alignment, we highly recommend that the set screw in the torque wire mount block which locks the ski in position on the axle, have a flat area on the axle where it seats. Either a Dremel reinforced cutting wheel or a file can be used to create a flat that is located so that the skis are aligned about 10 to 15 degrees nose up in relation to the wing chord line. It's been our experience that it's not possible to have the skis stay in alignment without this flat, as the set screws simply can not bite into the hardened music wire adequately. TOOLS & SUPPLIES NEEDED Drill and drill bits: 3/32" & 5/32" typically Sandpaper, sanding block Medium and Thin CyA glues ( or other if preferred ) Ruler & pencil for marking Polyurethane Varnish or other fuel-proof paint CONSTRUCTION DETAILS Depending on your aircraft and flying conditions, you may decide to shorten the skis; the 25-40 size skis as supplied are almost 3" wide by 14" long, while the 60-90 size are close to 4" wide by 15" long. If you have a very light weight plane, or never plan to fly from soft powder snow conditions, they can be shortened (or even carefully narrowed if you wish). Electric fliers are particularly conscious of the performance benefits of minimizing weight. Note that the axle is to be located at about 40% to 45% of ski length from the front (to keep it aligned in flight if the set screw should work loose.) Finish sanding of all parts has been left to the kit builder; keep the bottom edges of the skis sharp and square for best snow handling! The mount spines can be finish sanded to round the edges as much as desired, and the front ends of the keel strips should be well tapered as shown in the drawing. (REPEAT STEPS FOR EACH SKI) (1) GLUE the 1/8" ply axle area reinforcing pieces on each side of the wide axle hole area on the 1/4" ply Mount Spines, aligning the bottom edges for the flat mounting surface. Sand away excess wood as desired. (I use medium CyA glue for all assembly.) (2) DRILL a hole the size of your axle (5/32") at the axle location: refer to the drawing. Slip the mount spine on your axle temporarily, or use another piece of 5/32" material through the axle hole, slip the mount block in place, and mark where you want to drill the 3/32" hole for the back end of the torque wire, where it goes through the mount spine. Drill the 3/32" hole there. (3) MARK the MOUNTING SPINE LOCATION on the ski top, measuring to center it from side to side. Sand the glue surface if necessary, and glue it in place. (4) PRE-SAND KEEL STRIP; shorten supplied material as necessary; shape / taper the front end. MARK the center line location on the bottom, and glue it in place. (5) FINISH sand & paint. EDGE GLUE The Plywood before painting if desired with thin CyA glue. Polyurethane varnish is available in aerosol cans, and does an excellent job, as it is fuel-proof, and doesn't soak up much heat from the sun, which can help avoid icing problems on the bottom of the skis. If you paint them, light colors are recommended for the same reason. (6) INSTALL skis on the axles: (Note - it works well to hang the aircraft from the ceiling if possible while setting up the ski alignment. Otherwise, block up the fuselage with the wing about level so that the skiis are not touching for this step.) [NOTE: as mentioned previously, the skis should be set about 10 to 15 degrees nose up when flying, so the tips won't dig in on the landing. When set this way, the torque rods also carry some of the weight of the tail when taxiing on the snow. If you find the tail held too high, so you can't steer the plane with the tail skid or tail wheel, you may need to file the axle flat to readjust where the skis lock on the axles (not quite so nose high) later at your flying site.] Mark where to make the flat for the set screw, matching the skis to each other. (I use this same flat for my outer wheel collar when mounting wheels - you may want to position the flat accordingly.) After you have the flat adjusted the way you want it, so the skis sit at the proper nose-up angle, Add a wheel collar on the inside of the ski mount spine to keep it solodly in place, as well as one on the outside of the mount block on the axle for insurance against losing a ski in flight! Snug down the set screws well before flying. Check & adjust the aircraft's balance with the skis in place; balance will be shifted forward by the additional weight of the skis. For responsive aerobatic models, you may want to keep the balance where it was when flying on wheels. NOW GO FLY!!! If you set a lane out of your warm car directly onto the cold snow, the snow will melt an, then form ice; avoid this! A piece of old carpet, etc., will allow the skis to get cold enough to not ice up when they finally do come in contact with the snow. A liquid product designed for coating no-wax cross country skis, called "Maxi-Glide" could add a non-icing coating in certain conditions. Normally, the polyurethane varnished skis slide very well without any extra treatment.Holding full UP ELEVATOR can help when first starting in soft snow to get your plane up and running across the surface; just let up before too much speed builds so you don't wallow into the air prematurely! **************************************************************************************** THOUGHTS ON OTHER SETUP DETAILS: Written in 1993 by BKS TAIL DRAGGERS VERSUS TRIKE GEAR TAIL DRAGGERS HAVE THE DISTINCT ADVANTAGE FOR SETUP ON SKIS, PRIMARILY BECAUSE OF THE WEIGHT DISTRIBUTION AS WELL AS THE ATTITUDE OF THE FUSELAGE AT THE BEGINNING OF THE TAKEOFF RUN. TRIKE GEAR PLANES CAN ALSO BE SET UP TO FLY WELL FROM SKIS, BUT HAVE TO OVERCOME ONE DISADVANTAGE. A TYPICAL TRIKE GEAR EQUIPPED R/C AIRCRAFT CARRIES AROUND 40% IF THE WEIGHT ON THE NOSE WHEEL; THIS PROVIDES GOOD GROUND HANDLING, AND IS DESIRABLE WHEN USING WHEELS. WHEN YOU SWITCH TO SKIS, HOWEVER, SOME NEW DYNAMICS MUST BE CONSIDERED. WE CAN SPEAK OF THESE AS "FLOATATION" OR MORE PROPERLY "PLANING SURFACE DYNAMICS". CARRYING 40% OF THE WEIGHT WOULD SEEM TO ASK FOR 40% OF THE TOTAL SKI SURFACE AREA; COMMON PRACTICE IN THE PAST WAS TO HAVE MUCH SMALLER NOSE SKIS. IN DEEP SOFT POWDER SNOW, THIS WASN'T VERY FUNCTIONAL. WIDER/LONGER NOSE SKIS OVERCOME THE "SUBMARINE " TENDENCIES ON TRIKE GEAR SETUPS. TWO DETAILS MUST BE DEALT WITH; PROPELLER TO SKI CLEARANCE, AND PROPELLER TO SNOW CLEARANCE. THE NOSE SKI MUST BE MOUNTED SO THAT IT CAN NOT REACH THE PROPELLER IF IT ROTATES OUT OF IT'S DESIRED MOUNTING ATTITUDE. AXLE MOUNTING LOCATION ON THE SKI IS DICTATED BY THE DIMENSIONS OF YOUR PARTICULAR AIRCRAFT. AN AXLE LOCATION ON THE SKI FORWARD OF THE CENTER OF THE SKI CONTRIBUTES TO DYNAMIC IN-FLIGHT STABILITY; IF THERE IS MORE SKI SURFACE BEHIND THE AXLE THAN IN FRONT OF IT, THE AIR PRESSURE IN FLIGHT WILL TEND TO KEEP THE SKI'S TAIL IN LINE BEHIND THE FRONT; THIS IS WHAT WE WANT! IF PROPELLER CLEARANCE ALLOWS, LOCATING THE AXLE BACK FROM THE SKI TIP 35% TO 40% OF THE TOTAL LENGTH SEEM TO WORK WELL IN PRACTICE. THE SAME FORWARD-OF -CENTER AXLE LOCATION APPLIES TO THE MAIN SKIS ALSO FOR THE SAME IN-FLIGHT DYNAMIC REASONS. TALLER NOSE GEAR MAY BE REQUIRED/ DESIRABLE WITH WHEELS OFF AND SKIS INSTALLED, TRIKE GEAR PLANES LOOSE GROUND CLEARANCE. IF THE PLANE SETTLES INTO SOFT SNOW AND THE PROPELLER IS IN THE SNOW, YOU'LL HAVE PROBLEMS. IF THE MOUNT HEIGHT OF YOUR NOSE GEAR IS ADJUSTABLE, TRY LENGTHENING IT TO ACHIEVE PROPELLER TO SNOW CLEARANCE AND A SOMEWHAT NOSE-UP ATTITUDE AT REST. CONSIDER SETTING UP A SEPARATE NOSE GEAR JUST FOR USE WITH YOUR SKI IF NECESSARY. ENGINE DOWN THRUST AND SMALL ELEVATORS MANY TRIKE GEAR R/C AIRCRAFT ARE SET UP WITH A LOT OF ENGINE DOWN THRUST. THIS MAKES THE PLANE HANDLE LIKE IT IS EVEN MORE "NOSE HEAVY" WHEN YOU BEGIN YOUR TAKEOFF RUN. HOLDING IN FULL UP ELEVATOR HELPS TO KEEP THE NOSE FROM DIVING IF YOU HAVE ADEQUATE ELEVATOR SURFACE AREA, THROWS, AND AUTHORITY TO COUNTERACT THESE FORCES, AS THE AIR FLOW/ PROP WASH PUSHES DOWN ON THE TAIL. IF YOU FIND THAT YOUR PARTICULAR AIRCRAFT DOES NOT HAVE ADEQUATE ELEVATOR AUTHORITY, SEVERAL THINGS CAN BE TRIED. (1) INCREASE ELEVATOR THROWS (2) INCREASE ELEVATOR AREA (3) SEAL HINGE LINE AIR GAPS COMPLETELY; (A 1/32" GAP MAY REDUCE ELEVATOR EFFECTIVENESS BY AS MUCH AS 30% ACCORDING TO DON LOWE'S DATA!) ADD CLEAR PACKAGING TAPE TO THE TOP SURFACE, OR ADD "MONOKOTE HINGING" BETWEEN THE CONVENTIONAL HINGES TO ELIMINATE ALL AIR GAPS. (4) THE ADVENTUROUS MAY WISH TO TAKE OUT SOME OF THE ENGINE DOWN THRUST IF THEY FEEL IT IS EXCESSIVE; YOU'RE ON YOUR OWN HERE! (5) CONSIDER CONVERTING YOUR TRIKE GEAR AIRCRAFT TO A TAIL DRAGGER; IT'S BEEN DONE HERE ON A FEW, WITH EXCELLENT RESULTS. TAIL DRAGGER DETAILS TAIL DRAGGERS CAN BE SET UP TO HANDLE WELL ON SKIS. AGAIN, ADEQUATE ELEVATOR AND RUDDER AUTHORITY IS DESIRABLE FOR GOOD GROUND HANDLING AND TAKEOFF RUNS. TAIL WHEELS, TAIL SKIDS, OR TAIL SKIS? SOME LEAVE THE TAIL WHEEL IN PLACE, AND MANAGE OK IN THE SNOW. A TAIL SKI IS ONLY USEFUL ON LARGE, LONG, TAIL HEAVY AIRCRAFT. SEVERAL OF MY AIRCRAFT ARE SET UP WITH A TAIL SKID WIRE, WHICH WORKS VERY WELL ON DIRT RUNWAYS, GRASS, AND SNOW. DON'T WORRY TOO MUCH ABOUT THIS END OF THINGS FOR YOUR FIRST SKI FLIGHTS; EXPERIENCE WILL DICTATE WHAT YOU WANT TO DO AFTER THAT. IF YOU DO INSTALL A TAIL SKI, USE A STEERING KEEL ON THE TAIL END, AND INSURE THAT IT CAN NOT ROTATE OUT OF IT'S PROPER ALIGNMENT.