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RADIO CONTROL BY BOB YOUNG Radio-controlled gliders: Pt.2 This month we will look at some of the factors to be taken into account when designing a 2-metre glider and see how these were applied in the Silvertone Stingray, an unconventional 2-metre design. The concept of the MAAA sanctioned 2-metre class was to provide a simple entry level model on which to learn the craft of R/C glider flying. This model was to place few demands on the radio equipment and the model builder’s skills. The main parameters call for rudder and elevator only (no ailerons, camber changing preset flaps or releasable tow hooks) and a span not exceeding two metres. “V” tails are allowed. Wing loading is to be in the range of 12-75g/dm2. For those inter­ested in the complete rules, see the MAAA Official Rules and Instructions Handbook (Chapter 3, Provisional Rules. pp2-41). As a result, the typical 2-metre glider has evolved along rather old fashioned, conventional lines with a polyhedral wing, a simple (lightweight) structure, and rudder and elevator controls. This is typified by the yellow and red glider This is a typical 2-metre glider showing the polyhedral wing and a simple structure. It has just two controls, elevator and rudder. pictured in this article. Sometimes the designs include a butterfly (“V”) tail with mixing on the rudder/elevators. In the thinking of most design­ ers, the rudder-only design dictates that large amounts of dihe­dral (ie, wings sloping upwards) are required in order to induce the model to turn. To my mind, this is wrong as the dihedral can fight the rudder. True, dihedral is required to initiate the turn but it then tends to pull the model out of the turn and the net result is a model that is difficult to hold in a constant rate turn, a most important point in thermal soaring. But 2-metre gliders do not have to look like models out of the 1930s. The design we will be discussing this month does not follow the current trend and had its genesis during the 1970s when I was producing models for the military. While I had often visited glider fields in the past and flown the odd glider, I had never been interested enough to undertake a glider design of my own and fly seriously in competi­ tions. In the good old days, if models did not make a noise and go fast they held no interest for me. Nowadays, if they make a noise I cannot hear them and if they go fast I cannot see them. Much has changed since I was 30 years old. During the early 1980s, Harold Stephenson, a very keen glider flyer, became a regular customer and finally convinced me to design a model for the new 2-metre class just gaining popular­ity at the time. He even offered to help me build it, an offer too good to refuse. I finally relented and drew up the plans on a strip of brown paper from my roll in the shop. Fig.1 shows the finished design, redrawn recently on June 1998  53 In contrast with conventional 2-metre gliders, the Silvertone Stingray has swept-back wings, a “V” rudder and most important, a blended wing/fuselage junction to keep turbulence to a minimum. a computer using a CAD program. Harold built the wing and I built the fuselage and there it sat for the next 15 years or so (in the tradition of all good models). That is until another friend, Barry Ming, incensed that such an interesting model should sit unfinished for so long, offered to take it and finish it. So in 1996 a finished model, painted all over in black, rolled into my workshop. Barry then informed me that the original plan had disintegrated due to age and my only record of the design was gone. It took me another 12 months to apply the colour trim and plug in the radio (one cannot hurry these things) and finally, in late 1997, the model turned out for its test flight. This I might add was on the day of the contest. Why 54  Silicon Chip is it that I sense a lack of surprise at this last statement? Old hands know exactly what I mean. The model still looks quite modern 16 years on and is quite eye-catching in style. Since then the model has flown in four contests, winning several rounds and maxing in several others and has attracted a good deal of interest, largely as result of its excellent flying characteristics and pleasing appearance. The weak link is my piloting, for I simply do not have the finesse necessary to read the subtle signs required for good thermal flying and my spot landings are appalling; with no throt­ tle to adjust the final approach I tend to undershoot all the time. Thus I would be very interested to see this design in the hands of a good flyer, part of my reason for publishing it here. With the addition of ailerons the model would make a great slope soarer and the addition of flaps and ailerons would convert it into an interesting open class sailplane. I am currently working on an F3B version which is scaled up approximately 1.5 times with flaps, ailerons and 2.5° of dihedral and a much better wing section. I must point out that this is not intended as a full con­struction article as it is a difficult model to build and only for experienced modellers. The design lends itself well to fibre­glass but the original is all wood with a built-up wing using 1/2" x 1/8" spruce spars in a “H” girder arrangement enclosed in a 1/16" “D” box leading edge. The plug-in wing dowels for the wings are 1/4" steel rods in brass tubes. The wing stubs are laminated out of 1/2" sheet balsa and hand shaped. Note that the large American influence in modelling tends to favour the Imperi­ al system of measurements in some components. Finished weight is 1.05kg, quite heavy by 2-metre stan­dards, whereas a very simple lightweight can come in at 0.5kg, ready to fly. Even so, the wing loading is still only 33.8g/dm2 (7.9oz/sq ft) due to the large wing. There is some evidence to suggest that this loading is too light for the Eppler 205 section used on this model and the next round of trimming will concen­trate on the effects of ballast and elevator trim on performance. The model certainly likes to fly fast and I feel that I have been flying it too slowly in the last two contests. The original model pictured has several shortcomings. Firstly, the nose is too short and this has been corrected on the drawing presented in Fig.1. The plastic film was also a mistake as it goes slack in the heat. A better approach is a fully sheeted wing covered with silk or Oz Cover and painted all over. Finally, the wing section is over 20 years old and now completely outclassed by the modern thinner sections. However the overall design shows promise and I believe it could be developed into a potent per­former. Design fundamentals There is a fundamental rule in glider design that all glid­ers eventually come down and that little gliders June 1998  55 Fig.1: this 17-year old design has recently been redrawn with a CAD program. The nose has been lengthened slightly to correct an original design shortcoming. The plug-in wing dowels for the wings are 1/4" steel rods in brass tubes. The wing stubs are laminated out of 1/2" sheet balsa and hand shaped. This photo shows the high degree of blending between the wing and fuselage. The fuselage height has been kept to a minimum by laying the servos on their sides. come down more quickly than big ones. Which is just a cute way of saying that one of the key factors in glider design is Reynolds numbers. We examined Reynolds numbers in the recent articles on jet turbines and concluded then that the bigger the chord (width) of the wing, the more efficient it will be. Now there is a fundamental conflict in glider design that arises out of this simple statement. One major source of losses in the wing is the induced drag which arises at the wing tips. Allied to this is the problem of interference drag which arises at the junction of the wing and fuselage. Thus the turbulence from the induced drag extends inwards along the wing panel from the tips and the turbulence from the interference drag 56  Silicon Chip extends outwards from the wing/ fuselage junc­tion. The traditional answer to this problem in sailplanes is to increase the aspect ratio of the wing (ratio of wingspan to wing chord or width), thus increasing the clear span (free of tur­bulence) panel size on each wing half. A good example of this is the 3-metre F3B glider featured in one of the photos in this article. Unfortunately, in doing this we immediately reduce wing chord and thus the Reynolds numbers on the wing and to some extent defeat the purpose of improving the overall efficiency. On full size sailplanes, this is not quite as important as on small models, for there is strong evidence to suggest that a wing section with a chord of less than 200mm falls into the very low Reynolds numbers and ceases to work effectively as an airfoil section at model speeds. A quick glance at the data panel in Fig.1 will show that the mean aerodynamic chord on the Stingray-2M is only 184.5mm, a figure somewhat short of that minimum, so the overall wing effi­ciency is not going to be anywhere near as high as on the F3B version or larger models in general. This applies to all 2-metre gliders and small models. So what to do? I have kept the aspect ratio as low as I could on the Stingray to keep the spar depth and Reynolds numbers as high as possible and yet I have still fallen below the recom­ mended minimum chord. There is little we can do on tip drag (winglets on the tips may offer some help here) but we can do something about wing/ fuselage interference drag. There was a lot of work done during the 1930s and 1940s on wing junction drag and the Vought Corsair F4U was one result. This work showed that wing/fuselage junction angles of 90° or less gave rise to a marked increase in interference drag. The crank­ ed wing of the Corsair was one method of increasing the wing/fuselage junction angle to above 90°. The results were spectacular and the Corsair was one of the fastest piston engine aeroplanes of WWII. The McDonnell XP-67 experimental twin-engined fighter in 1942 was an even more interesting example and the fuselage, nacelles and wing in this design were an almost seamless blend of aerodynamic styling. Thus, by blending the wing/fuselage junction and increasing the junction angles to above 90°, we can substantially minimise the junction turbulence and thereby increase the clear span panel size without reducing the chord. The cross-section at BB on Fig.1 shows just how close the junction angle approaches 180° on the Stingray-2M. One of the photos shows even more detail of the blending. To achieve these angles, the height of the fuselage has been reduced by laying the servos on their side. If we could completely eliminate the wing/fuselage junction we could almost double the effective aspect ratio of the wing without a reduction in chord. Flying wings do just this and the result is a very efficient flying machine indeed. ELECTRONIC COMPONENTS & ACCESSORIES • RESELLER FOR MAJOR KIT RETAILERS • • PROTOTYPING EQUIPMENT • FULL ON-SITE SERVICE AND REPAIR FACILITIES • LARGE RANGE OF ELECTRONIC DISPOSALS (COME IN AND BROWSE) CB RADIO SALES AND ACCESSORIES M W OR A EL D IL C ER O M E Croydon Ph (03) 9723 3860 Fax (03) 9725 9443 Mildura Ph (03) 5023 8138 Fax (03) 5023 8511 Truscott’s ELECTRONIC WORLD Pty Ltd ACN 069 935 397 30 Lacey St Croydon Vic 3136 24 Langtree Ave Mildura Vic 3500 P.C.B. Makers ! • • • • • Bruce Curl with “Calypso” a 3-metre F3B glider. Note the high aspect ratio of the wing, the traditional answer to minimising induced drag and interference drag. So the essence of the Silvertone Stingray-2M is the blended wing/ fuselage. But the design is more complex than this for there are many other factors which can be incorporated into this blend­ed junction. The strakes down the fuselage sides serve a dual purpose. At low or zero angles of attack they serve merely as flow separators, inducing the airflow into a smooth separation at the wing junction. At high angles of attack, when combined with the swept-back wing, they serve as turbulators, inducing the wing to stall at the centre section, well before the tips begin to stall. With the centre of gravity (CG) well back from this point, the nose begins to settle first during a stall, a very handy outcome. The net result is to reduce the need for washout on the wing tips, further increasing the efficiency of the wing overall. An additional minor benefit of the blended fuselage is an im­provement in fuselage lift which can be quite significant in some aircraft. The Grum­man Panther, another blended fuselage aircraft, produced 30% of its SC overall lift from the fuselage. • • • • If you need: P.C.B. High Speed Drill P.C.B. Guillotine P.C.B. 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