Introduction to the Hobby of RC Model Sailplanes Copyright (c) 2002 by Bill Kuhl. Right is granted to use this material for non-commercial purposes. All other rights are reserved.
Introduction -The Hobby of RC Sailplanes <ul><li>Is This a Hobby for Me? Some reasons it might be: </li></ul><ul><li>You can start out fairly inexpensively. </li></ul><ul><li>It is clean, quiet, and environmentally friendly. </li></ul><ul><li>You can have success as a beginner, but there are always new challenges as your level of proficiency advances. </li></ul><ul><li>With a dependency on atmospheric conditions there is a good deal of randomness, you never really get it all figured out. </li></ul><ul><li>It can be very educational; you learn about meteorology, aerodynamics, and strength of materials as you advance. </li></ul>
The Origins of Soaring The first creatures to soar upon slope and thermal lift were large winged creatures a couple hundred million years ago. With wingspans well over 20 feet and weighing well in excess of 100 pounds, huge muscles would have been required to maintain flapping flight even with the denser air of that time providing more lift, so it is thought these creatures flew exclusively in lift by jumping off mountain tops. In more recent times large soaring birds such as eagles and hawks, rely on finding lift for much of their flight. Paul Naton Image
The Origins of Soaring Public domain image of one of the Wright Brother’s gliders. The Wright Brothers spent years experimenting with gliders before attempting powered flight. Much of their effort was spent developing more efficient airfoils and perfecting their control system before building a powered plane.
Advances in the Hobby <ul><li>There Has Never Been a Better Time to Take up Soaring </li></ul><ul><li>A Few Good Reasons Why: </li></ul><ul><li>Reliable radio equipment available at low cost. Size and weight of equipment available makes planes practical that were not 10 years ago. </li></ul><ul><li>High quality kits and Almost-ready-to-Fly planes at a reasonable cost. </li></ul><ul><li>Composite construction materials available </li></ul><ul><li>Important technical information made available to everyone. </li></ul>
Radio Equipment Computer radios will handle many functions such as flaps, ailerons, and spoilers, capable of mixing combinations of functions. As you progress through the hobby, most likely this is the type of radio system you will want. Prices around $500 and up. Two or three function radio equipment is available for $50 to $100. Close-up compact radio installation in discus launch sailplane, complete plane weighs under 10 ounces.
ARF - Almost Ready to Fly High-quality sailplanes are available at a reasonable cost. The Primera A.R.F. glider that I am holding, came completely constructed of wood and covered with plastic covering, required only minor assembly of major components. Received third place in two-meter division at 2001 Southwest Classic flying this plane.
Sailplane Design & Construction Design is related to creating a light, strong structure with minimum drag. Because of these goals, many planes use composite materials to get maximum strength from minimum amount and weight of materials. The wing of a sailplane launched by electric winch must be able to withstand a force equal to up to ten times the weight of the aircraft because of the stress placed on the wing during a zoom launch.
Laser Cut Pieces Many of the balsa wood kits come with laser-cut pieces that come out of the wood sheets much easier and cleaner than the die-cut procedure used in the past. Laser cut pieces are very accurate, although the edges of the wood is brown from burning process.
Composite Materials Carbon or carbon fiber is found in various forms such as strips, rods, and woven into cloth. It has a high strength to weight ratio, but the primary strength is in tension. Kevlar is a trade name for this material developed by DuPont. Used in sailplanes for dent resistance, it also used in the making of bullet-proof vests.
Composite Construction The Allegro Lite spar construction consists of end grain balsa sandwiched between carbon fiber strips. This is wrapped with kevlar tow (loose fibers) and then painted with a slow curing epoxy. Note : sheeting, leading edge, and trailing edge have not been added, to show spar assembly of mid panel.
Free Information & Plans Significant advances in model sailplane design were made free for downloading in the form of plans over the Internet by Dr. Mark Drela, professor at MIT. These designs utilize composite construction or balsa wood construction with clever use of hi-tech materials such as carbon fiber and kevlar. Dr Drela has also developed airfoils that offer performance gains for model sailplanes. He has created a computer program “XFOIL” that will analyze airfoils for a variety of flow conditions. XFOIL is free for download for academic use. Dr Drela is also noted for setting the human-powered hydrofoil speed record of 21.3 mph in 1991 and the Daedalus human-powered aircraft that flew 199 kilometers across the Mediterranean Sea. Mark Drela’s RC Sailplane Designs on the Internet Apogee Allegro Bubble Dancer SuperGee
Where Can Model Sailplanes be Flown? It is best to fly at an approved soaring club field as a member. If you must fly from private land, make sure to get permission. Thermal Fields – need big enough area for high start or winch and room to drift downwind over. As there is no glow fuel to be spilled, often sailplane clubs are allowed to fly from sod farms that power fliers can not. Slope Sites - many concerns; access to the site, obstacles, and will there be consistent lift for wind conditions. Safety is a concern, although noise should not be. Do not fly over populated areas.
How Does a Sailplane Stay Aloft? No matter how efficient a sailplane is, the glide path will always be downward unless a source of lift is found. Lift is air that is moving upward faster than the sailplane is descending, (sink rate). Lift is found in two majors ways; slope lift - air moving horizontal is deflected upward, or thermal lift - air rising from the ground because it is warmer than surrounding air. Slope Lift Thermal Lift
Staying Aloft - Thermals Thermals - sailplane enthusiasts have such a fondness for the thermal lift that keep their sailplane’s aloft, many sign their emails and letters with “Thermals” instead of Sincerely. As the ground is warmed by the sun, thermal bubbles are formed and break way from the ground. These bubbles can combine and mature to form thermal columns. Thermals will normally drift with the wind, so the pilot circles the sailplane downwind adjusting to stay within the thermal.
Staying Aloft - Slope Lift Popular California Inland Slope On large slopes such as this one, strong lift is created when the wind blows straight into the slope.
Learning to Fly Learning to fly RC sailplanes is easier than RC power planes because sailplane trainers fly slower but it is best to have an instructor. The new “bounce-able” EPP training sailplanes will survive all but the hardest knocks with no damage. Bob Harold’s TG-3 EPP is a good looking glider. Wooden gliders that are more fragile, will be easier to thermal with if lighter. E. P. P. E xpanded P oly P ropylene foam
Learning to Fly Gentle Lady has been a favorite training glider for many years. Many radio systems include a “buddy box” option, a cable connects two radio systems allowing the instructor to give or take control from the student’s radio.
How Does the Sailplane Get the Initial Altitude? Slope sailplanes normally are thrown directly into the lift off hills or mountains. Thermal sailplanes normally need a fair amount of altitude to begin searching for thermal lift. Methods Used to Reach Thermal Lift are : Hand Launch - plane is launched in an overhand throw. Discus Launch - plane is spun by wingtip and released upward. Hi-Start - stretchable tubing and length of string pulls sailplane upwards like a kite, string detaches from plane. Electric Winch - ground-based electric motor pulls sailplane up. Electric Motor with Propeller - electric motor in sailplane powers to altitude, often prop folds for less drag. Aero Tow - sailplane is towed behind a RC power plane.
Winch Launching Winch on the left and retriever on the right. Pilot holds some tension as the winch line stretches before pilot give the plane a toss. For most larger sailplanes, an electric winch is the preferred method for getting a sailplane to an altitude of a few hundred feet. Electric retrievers are also available that pull back the winch line.
Hi Start Launch Paul Naton launching larger sailplane by hi-start. Hi-starts are available in a variety of sizes. Hi-Start consists of stretchable tubing, stake for anchoring in ground, string, and parachute.
Electric Powered Sailplanes Paul Naton photo of Dieter Mahlein with high-performance electric powered sailplane. Sailplanes powered by electric motors are available in all sizes and performance ranges. The weight of the motor and battery is not as much of a handicap as it once was. Often it is helpful to be able to use electric power to avoid landing off field.
The Need for Efficiency Normally flying in thermal lift and often under slope conditions, there is no excess lift, so a sailplane needs to be as efficient as possible. This means creating adequate lift while creating the least amount of drag, known as the Lift/Drag ratio. Wings of sailplanes are normally long and narrow, the ratio of wing’s length (wingspan) to wing’s width (chord); is known as the “aspect ratio” or AR. Wings with higher aspect ratios have a better glide ratio, but require strong construction.
Efficiency - L/D Glide Ratio Half the drag results in twice the glide ratio. Steeper glide angle result of greater drag.
Airfoils Shape of the wing cross-section (airfoil) is a major factor in the performance of the sailplane. Airfoils are optimized for the conditions that the plane is most likely to be flown under. Factors such as speed, weight, launching factors, wind penetration, and size of aircraft are considered. The following individuals have done considerable research in developing airfoils for model sailplanes and other applications: Dr. Michael S. Selig University of Illinois Dr. Richard Eppler Stuggart Germany Dr. Mark Drela MIT
Airfoil Performance Issue In designing airfoils, desirable to avoid the “laminar separation bubble” on the airfoil upper surface that can lead to high airfoil drag. On the diagram this appears between A & B. The picture below is of oil sprayed on airfoil in wind tunnel observed under black light. Image by permission of Dr. Michael Selig
Competition Sailplane competitions of many types are very popular. Most competitions are low key with entrants more interested in the social aspects of flying with others with the same interests. Tasks are simple enough that pilots with only moderate skills can accomplish. Competition is always a learning experience. You compare your plane and piloting ability with others, and often you find there is much room for improvement. The following is only a small sample of sailplane events.
Thermal Duration Contests Thermal duration contests such as the Southwest Classic held near Phoenix, Arizona every February draw pilots from all over the United States as well as many vendors. Planes are launched off electric winches, pilots try to keep their planes aloft as close as possible to a specified time and then land as close as possible to a small circle on the field.
Discus Launch Flight tasks are performed within a time window, normally ten minutes. Typical tasks might be to get maximum flight time as close to a target time as possible with a limited amount of throws. Penalty for each throw after specified number of throws. The best score for round is given 1000 points and the rest of the scores are normalized, this is “man-on-man” competition. There is a lot of strategy in picking your air and less emphasis on landing. Once known as “Hand Launch” competition, everyone has switched to launching by swinging the plane by a wingtip.
Slope Racing Photo by permission of Gliderking.com Sailplanes race between poles on a course that runs parallel to top of hill. Racing requires very smooth flying.
Hand Launch Golf or DLG Golf Pilots fly hand launch gliders between holes on a golf course using foam noodles that planes must land against. Some pilots have achieved one throw for each hole on the course. The pilot may run or ride in golf cart between holes.
Scale In scale competition, models that are replicas of full-size sailplanes are judged for appearance and flight maneuvers. “ Scale flying points are judged by at least two judges. Each of the ten maneuvers is scored from one to 10. Flying score is added to the static score for the total score.” Bob Harold
The End Special Thanks to the following companies or individuals for allowing me to use their graphics images: Paul Naton www.radioncarbonart.com Robert Bingham GliderKing.com Klaus K Weiss www.h.s.l.rcclubs.com Steve Henke Spring Fling Image Bob Harold Scale Competition Description Dr. Michael Selig Laminar Separation Bubble Image http://www.scienceguy.org - Science Video & Resources