RC Soaring Engineering (balsa and film)
Content
In Search of the ‘Missing’ Profile For twenty years now I have asked the same simple question that apparently was just too ‘profound’ for the American aeronautical experts to answer in a simple manner. “What is the best profile form that an entry level sailplane should employ”? The Answer: 10% flat bottom with blunt entry. Why is it that it appears that education breeds stupidity here in the USA?? My dad spent 30 years in college and his professional titles were quite impressive; too bad he had no common sense. When he sent me to Berkley to work for my PhD he did me a great service; because I bowed out of his plan for me to have a PhD before my 21st birthday, and I joined the hippies instead (after acquiring a ‘minor’ engineering degree). Its kinda sad when we have reached the point where a farmer who wants to plow a field has some hotshot ‘Professional’ tell him that this here professor has just cloned the finest race horses in the world, so this new clone should be the best beast for the job of plowing. The inventor becomes deaf to the honest feedback that the farmer generously donates (i.e. “stick it up your nose”) and this attitude simply demonstrates professional ignorance because the professor still thinks (totally in blind faith) his product will fulfill any of his ‘off the wall’ claims. When I got into contest soaring in 1976 I started with a Pierce Paragon (a rather large entry level machine). Flying it against Legionairs, I quickly realized the short comings of a thick flat bottomed airfoil. [[profile #1 My Legionairs that were started in 1978 (including the Shuttle versions) had pronounced Phillips entry. Cecil Haga and John Rimmer spearheaded a drive for thin flat bottom sections that were very effective at the time. With the help of Dr. Eppler, the Legionair’s standard airfoil was completed with it having a degree of Phillups entry to begin with. [[profile #2 My Increasing the amount of Phillips came from the California group of fliers as advice on how to tune up and get more out of the Legionair airfoil. Also their technology expanded upon thickening the wing as the way to handle both the increasing tension of the winch, and the increasing Aspect ratio of the wing. So the answer on improving flatbottom performance, at that time, did not come from the hallowed halls of education, but from the technicians that were searching for improvements. Interestingly John Rimmer’s success story was done with a very thin flat bottom profile, and it was a great approach to low CD flight; however my problems were trying to gain sufficient stiffness with so thin a profile, and the weight of the machines that I was building at the time (because I was already using 12 volt winches to launch).
As I got more into contest flying my Legionairs, they sported spoilers that were reduced in size compared to Cecil’s design; that better finessed the landing, and yielded more strength to the wing. Seeing Mike Bame's success with his MB 253515 I tried a Gemini kit, but was not impressed. I then went to a combination of two profiles that both had shortcomings; Eppler 205 that had a weak low end, and the Bame airfoil that did not seem to tolerate loading. The marriage yielded the ‘Happy Medium’ that was 13% thick, and worked great in two channel high AR machines. When I reached the point that I no longer needed spoilers to land; the Red Bird was born out of simplicity and was the highest reliability machine I owned (that won the most contest hardware for me). Now everyone at this time was seeking a sportsman’s class to stimulate more interest in soaring, but because of my status, I wasn’t allowed to fly my two channel machines in this division. No problem I had 2 ch standard to fall back on.....oops gone also!! Realizing that the profile design for the cruder balsa and film machines was not really advancing or evolving, and that all the ‘experts’ wanted of me and my associates was to support their latest pet project super- whippy-dippy low CD profile....the kind that would be difficult for an expert, let alone a novice, to construct via balsa and film. They had no time to invest in entry level machine design, or they simply felt it was beneath them. [[profile #3 The Beach ball with its very blunt leading edge, slow upper surface ramp, was a natural for many different construction techniques; multi spar, D tube, Plate wing, full depth spar, box spar etc. So far 5 machines all over the country were built with this profile and the findings about the same. Speed range equal to or greater than the Eppler 205! A really great response for so crude a profile. Although the Pierce Arrow company built theirs with a D tube structure, I had originally built mine with a Oly type of wing (only to convert it to a Plate wing at a later date). With the advent of winch power going from high to ridiculous, and the flying sites getting smaller, I had started to investigate toning down my launching equipment and studying effects of ratio types of tasks (using dead air time as a nominal time base). The hope was to attract more newcomers, and keep costs reasonable - and not be a deterrent. At this point in time I am no longer a contest buff because their is no contests available for those of us that just wish to dabble in toy aircraft (unless one wishes to dabble in powered models). At the same time that I was loosing the contest bug I had noticed that many of the individualistic local flyers, that were not in the Dallas League of Silent Flight club, were no longer coming to the Dallas League contests. I believe that they just got tired of blowing their machines up on the club winch (to the glee of the Dallas Club members). Shortly after that time, I initiated the Hot Air Master concepts; only to discover that the machines that performed well in the upper air were a bit too ‘dense’ for the lower launchings. Decreasing weight of a design helped the launch to the point a nominal 10 lb. tensioned (3 to 1 stroked) high start was capable of zoom launching a 12 to 1 AR, 100 inch 36 ounce sailplane in calm air. Once off the launch; however, the machine
was doo-doo; but would climb like mad if you put it right into the eye of lift. Cute, but not good enough for a contest task, ratio evaluated or not. Now needing to speed the machine up to reach out for lift could be done conventionally by current technology, however we get back to the problem of the novice builder unable to cast a molded lightweight wing, or come up with the cost of having someone do it for him. More air speed without weight, along with maintaining the spar depth for reasonable strength, became the engineering problem that I was seeking the simple solution to, and the experts turned their backs on me. The ‘experts’ did not like my first success with this problem; because it is rather crude. The Sugar Trip turbulator at 55% wing chord, 45 mils thick. The bad feature is that it creates premature separation and thus one loses slow speed flight (the float). The good feature is that it works (i.e. statistically proven)! [[profile #4 Those that used this device on lightweight sailplanes liked its reaction, plus the improved handling. The ‘mini spoiler’ trip made sure the sailplane was circulating, and proved that it was the equivalent of adding 20% of the A/C weight in more ballast --- that the launching system of limited force did not see! The Sugar Trip was tried with a myriad of profiles and seemed to work well in speeding sailplanes up that were ‘too light’ (when high tension launching was not available to haul up a ballasted machine). Since I was unable to get cooperation on my project here in America I went to Europe and presented my situation to their talent; and received my most usable information from Denis Oglesby of England. However in the interim, in trying to develop a standard design of a profile that could honestly be labeled as an Entry Level Profile that would be usable by an average soaring individual - with his own available flying sites for practice - and still have a realistic contest format that would be suitable for that individual to grow with; all I got out of the AMA, and its officers was basically resistance. So the ‘ball’ was still in my court. The beginning of this year (1998) I realized (with Denis Oglesby’s information) that my problem was “concern for a MCL”. The Sugar Trip effectively reduced the MCL effect, and thus improved the airspeed capability of a sailplane that was a bit too light, so why not go all the way. Now that the only control for the center airspeed employing a symmetrical profile (streamlined flat plate) would be the mass; lightness thus becomes an important goal for a symmetrical airfoil that operates typically at lower alpha. Because the speed range is actually reduced by this limitation, the controls and thus handling, tend to work more positively. All that is accomplished by doing it this way is that for a given design configuration is that the Symmetrical would be the least CD for the spar height that is necessary. Instead of flying a Sagita type of machine with its Eppler 205 flying at 9 ounces per foot on the currently standard high launch, we are now flying a similar appearing machine at 5 to 7 ounces per foot for a limited launching of 250 feet AGL with a symmetrical
profile of about 10% thickness. [[profile #5 The entry of my symmetrical is similar to the beachball (i.e. blunt) and the upper and lower surface basically straight lines intersecting at the trailing edge for simplicity of structure and construction. The ramp from the leading edge back is so low that it becomes unimportant to shape, and a circular arc would suffice. Although it is hard for the ‘experts’ to realize; the interest here is not for a shape to be employed to enhance F3B machinery, nor is it intended to compete in a high launch high tech state of art world. Its purpose is to give the dabbler (or beginner) a simple and inexpensive to build A/C that has more performance for its weight; then the ancient ‘Nostalgic Dinosaurs’ that are currently available (as ‘crumbs’ from the past), and thus yield larger 2 lb. sailplanes for the ‘buck’ that have more reliability of climb-out from a low launch, and/or have the ability to race to a lift area with some altitude left to successfully climb-out again. Interestingly, this form is not one for soaring only, but apparently is the correct form that is applicable for all model aircraft (fixed or rotary winged); basically it is a unified form. Unified Form: A symmetrical profile (without a MCL curve) that is applicable to all aircraft, powered or not, where low CD is the predominant characteristic, and mass is the only variable to set center gliding airspeed (camber changing symmetricals to be discussed later). The thickness of this unified profile is determined by projected total aircraft weight, aspect ratio, and maneuverability requirements. With soaring applications; if a thin (6% symmetrical for example) form were to be employed, and the resulting weight of the AC would yield too much friskiness so that flaps would be a great benefit to operation, the flap range would be from 0 to +12 degrees (without any considerations for reflex that would be counter productive). For power planes that normally use strip ailerons, the evolution to flaperon control to handle the higher wing loading at low airspeed is much wiser than pursuing some complex aerodynamic form that has fixed programming. Although this theory is rather crude compared to the work of the professionals in high performance profile design, it is very workable for the modelers universe. This attitude mirrors the modern concept that low CD is the primary attribute for an aircraft, and that CL is merely a compromise that compensates for any excessive mass; that may or may not be needed (such as for inertia and/or damping ). In attempts to dissuade me from my HAM adventure and its related profile search, the Experts have assured me that they gave the problem serious attention by instituting a Hand Launch category that “does the same thing”!?? Just zip launch the little ‘chucky’!? Unfortunately for the Newbie; trying to get into soaring with a sailplane that is too small, requiring an experts skill to fly, that uses micro components that are expensive; is not the best introduction to the wonderful world of soaring. Lee Murray on the other hand has spearheaded a gargantuan drive for an RES program (Rudder - Elevator - Servo) that should appeal to the old timers, that have a degree of nostalgia, or for those skilled in building antiques with their
own personal improvements (like slow evolution). I think my ideas would work great in this area because intending to launch off the ‘Club winch’ with less of a zoom (because of lightness), and then dork the landing without the weight, would work well for someone learning the contest experience; providing the wooden sailplane at least had some travel ability for the weight that it was at (preferably w/o exotic hard to fabricate profiles and hardware). Lets look at the entire spectrum of lift succinctly (for sport flier types; it is much simpler than the ‘experts’ intend for us to understand). Starting from a flat plate as the profile to be the reference starting point, we compare it (via peak CL) to the typical flatbottom airfoil that contains a nominal MCL. The ratio of lift between the two is approx. 2 to 1 (or less). Guess what, drag being a direct product of lift also falls into approx. the same ratio so we end up having a circle of logic. The point I am trying to make here (crudely) is that for a 3 degree alpha a sailplane with a flat plate for a profile would be flying at a similar airspeed as a sailplane that weighs twice as much with its flatbottom airfoil also set at approx. 3 degrees alpha. Since we cannot fly on flat plates, we are stuck with some kind of spar thickness to give our wings stiffness. Interestingly, if a 3/4 inch spar depth works for a flatbottom wing; and since lift and weight of the machine to duplicate a similar center flying speed would be cut in half via a symmetrical form, the spar depth for that same area could comfortably be reduced to 5/8 inch; for a little more bonus of the zip! I do believe a spar depth of 9 to10 percent realistic for most applications, because of typical building quality. The best builders of wooden airplanes still fall short of the high tech equipment available that are skillfully built via modern jigs and molds, so setting goals that are very difficult to achieve for a sportsman type of flyer is the wrong way to go for this medium. Current wooden sailplanes end up less than 8 ounces per foot typically; and High Tech machines are peaking out at 16 ounces per foot to handle contest pressure. [I have run into a few 9 ounce per foot high tech 3 meter machine that are quite impressive on the low launch, but they are still no match for a 6 ounce per foot Oly III flying with a tripped Beachball profile (even with enough breeze to utilize launch flap for a really good ‘hi tech zoom’ launch)]. The bottom line for entry level soaring is that I picture a sailplane classification that should be a sportsman’s class where weight (or the limited launching system) being the only limiting factor (along with a total elimination of the ‘what if’ concept). Obviously whether weight is included as a specification or not, is the necessity of the launch having limitations to give the contestants a typical altitude of 250 feet or so to make them work/sweat at climb-out. If controls were kept at 4 channels (the typical RC system - or less) I can see picking up many of those free spirited sportsmen flyers that drifted away from soaring that I mentioned earlier (along with a new flock of hungry to learn newbies that include aging free flighters that can no longer zip their bodies over hill and dale). I have been flying models now for 45 years now, and have dedicated myself to a lightweight RC sailplane that I can go to the local soccer field and zip launch (1/4 size hi start and soar for more than 3 minute flights) and it’s still is a very thrilling adventure for me to the point that I want to share this philosophy with the other earthbound members that are just unaware of the
rewards of this simplistic approach to flight. To be able to read the weeds, grass, birds, bugs, and fluff that surround you all within the same envelope of air, and thus be able to launch timely and chase evasive lift, climbing in it as if a ‘secret’ motor were turned on to increase the models altitude to extend your flight in multiples of what your machine’s dead air time is capable of; is to me what its all about. Once up in the upper air; to scramble seeking lifting areas to play stalled turns, inverted speed runs, slow rolls, loops, snaps and spins while being able to maintain altitude is merely adding more scope to the adventure before whizzing home, slowing down for touch down - and landing softly at my feet.....ready for another adventure. Application of Unified Form Unified Form: A symmetrical profile (without a MCL curve) that is applicable to all model aircraft, powered or not, where low CD is the predominant characteristic, and mass is the recognized variable to set center gliding airspeed @ 6 degrees alpa (power off). The thickness of this unified profile is determined by projected total aircraft weight, aspect ratio, and maneuverability requirements. Soaring aircraft: Use just enough rib height for strength. Acrobatic powered aircraft: Use a 18% thick profile and if drag a bit too much, just use a bigger motor! Slope acrobatics: Use about a 12% thick profile - to fit your flying style. If the resulting mass of an aircraft sets the airspeed too high, instead of searching for a “better profile,” just put in a bit of flap. Interestingly if we take Michael Selig’s data on the E 214, we discover what the application of flaps can do to the performance of this profile. For a given airspeed (alias RE number - when the wing chord is fixed) the charted CD is the value of concern in all cases, so taking the data recorded we find that the E 214 @ RE of 200k (w/o flap) and slow flying it at CL of 1.0 we find we are at Alpha of 6 degrees, and that generates a CD of 0.012 for a theoretical L/D of 83. Feed in 3 degrees of flap and the numbers change for a CL of 1.0 to an Alpha of 2.5 degrees and a CD of 0.012 again for a theoretical L/D of 83 @ RE 200k! As far as the aircraft concerned here, this situation means that at low speed flight we sort of have a tie for performance, however the other attributes of the AC has also changed for the better because of the flap. Adverse pitch increased because of CP moving back, and the handling of AC is better because it is technically more nose heavy. With fuselage flying cleaner at the lower Alpha, the overall L/D of the machine (for the CL reference of 1.0) should be higher at this slow flight situation, then the E 214 w/o flap. Whether or not this is precisely accurate with all profiles is unimportant. The crux of the concept here is that the unified profile is set up for minimum CD without regard to slow flight. The point that I am trying to make here is that to capitalize on slow flight (if desired) the flap at the TE is simple compensation, and in many situations would be superior and less troublesome then the concept of trying to maximize speed range via forming a tight combination of low CD form containing a MCL (to insure that center speed of the AC is within limits the modeling pilot is looking for at the typical loading a model AC would be at). The Unified form theory is such that low CD is paramount for a given beam height that is driven through the
air, and that low Alpha will always be clean flight of which the weight of the model sets up the velocity. If airspeed is needed to be cut back on a symmetrical profile application, and there is no weight to remove, adding a Gurney flap to the TE, or dropping the TE flap about 5 degrees should be enough to compensate for the missing MCL. This should be true whether we are talking indoor-HL, FF, UC, RC thermal or slope Soaring, Precision Acrobatics, Fun fly Acrobatics, etc. Using the unified theory, and if one wishes to build a powered sporter that has fun fly capabilities the 18% thick symmetrical would be a good choice; whereas for a pylon racer we are talking less than a 10% thickness symmetrical using flaperon programming for the Strip Ailerons - for safety in landings, along with getting out of the grass quicker at take off. The Minimax corporation manufactures high quality wooden sailplane kits of extremely light weight (down to 3.5 ounces per square foot). The profile that is employed has a spar depth of 12%; necessary for wing strength. Then to use a flat bottom form for that spar depth makes for a really slow floating type of machine; that is useless for any other locality that has normal turbulent air. Using a 12% symmetrical instead would speed the machine up as if it were loaded to at least 5.5 ounces per foot; and that is a more reasonable situation than adding 10 to 25 ounces of ballast into so light a structure just to be able to fly on nice normal days - other than dead calm. For those that wish to fly a symmetrical ridiculously slow.....Ooops, let me rephrase that! For those seeking the ideal beginners sailplane, the symmetrical can be modified with simple flaps by first cutting the original wing ribs off for the new flap, and using the pieces to make up the flaps (approx. 1 3/4” inches wide). We then place a .062 ply plate as the anchor point on the upper surface glued to the main spar and set between the wing ribs (or in a recess in foam) that is close to the center of the flap, 1/2 inch or so wide - and make the plate fit flush with the upper surface. Put an .062 hole in the center of this ply plate anchor, and then install a Z bent clevis rod into the hole. Install a horn onto the flap, tape the flap in place on the underside and adjust the clevis for about 3/8 inch droop to start. Then lock the clevis, and tape the upper flap surface into place to seal the gap and for better support. Now once tired of the slow, over stable basic trainer, and some stick education has occurred, one merely raises the flap adjustment to about 1/8 inch. To reset flap take the upper tape off, adjust the clevis and re-tape afterward. With a sailplane weighing in at 7 ounces per foot you should be centered in performance to sailplanes carrying a profile with a mild MCL. When you are ready for events like the Hot Air Masters, take the flaps to zero for all the zip you can get. My experience with using servos to control flaps on sailplanes that are very light is a lesson in futility, and my only concern for the programmable flap is for center speed. Now for F3J or F3B applications the symmetrical has some interesting characteristics. The datum line being straight is the least CD state of this profile, which means the flap is always is air loaded. Being unilateral (because flap reflex is undesirable) the flap actuator can be a very light pull only line to set the flap, which then brings the actuator system mass closer to the CG of the AC. Conceptually, I am convinced that a 6% symmetrical with about 4 degrees of flap will perform right
along with the best current F3B profiles Michael Selig has to offer, and still have one major advantage. When the flap of the symmetrical is at zero degrees, the CD is still lower than the CD of the profile compromised with a MCL. Adding reflex to the flap definitely will not help to make the Selig profile catch up! It looks like we have an adversarial situation here that only the “Contest field of Honor” can prove. Personally, I am out of it because of two serious reasons; I’m too old, and I’m just not good enough. Hopefully, some warrior out there may find this concept appealing enough to capitalize upon, and seriously go for the Gold! Reducing speed range by throwing away the low end may appear to be disruptive to many sailplane enthusiasts that usually fly heavy; they use the low end for “parking” i.e. a breather for thinking, maximizing climb in lift (preferable with a bit of flap); and there are some of our successful experts that can build to very close tolerance, and thus have a machine that can come close to doing a typical contest task in dead air. They will cruise about most of the time at a speed close to textbook maximum L/D, and snag a thermal to extend their time aloft to just complete the contest task they are involved with. I have seen great thermallers fly by trims primarily, using the FF stability of their machine to assist in minimizing sink rate. On the other hand, my approach to soaring is quite aggressive. No low-end to me means sharper reaction to controls; thus less throws for the surfaces, more roll rate for the dihedral usually employed for simple 2 chan. machines, and more enjoyment of little adventures that are occurring more rapidly; even though the airspeed of the heavier machine flying at close to maximum L/D, and the airspeed for the lighter machine with a symmetrical is about the same (with the L/D of the heavy sailplane being superior). My attitude of bias for the lighter machine with as much scramble as possible is probably in line with the thinking of the World war I ace, The Red Baron, or the World war II ace Japan’s Yatamoto, whom both capitalized on maneuverability, and rate of climb for their tremendous success. I see the concept of Unified form as the needed relief for model aircraft profile design, so as to return them into the simple toys they were meant to be. With modern technology we can utilize this concept for the design of all flying surfaces; whether rotary (including propellers), flying wings with their reflexed flap, or basic fixed wing aircraft. Once the construction technique is completed for a modelers flying surface that is strong enough to handle his flying air-loads, only the amount of flap needs to be determined to set the most favorable air speed; just that simple. Aircraft that is designed to do work efficiently by carrying some kind of payload in addition to its airframe, are the vehicles that require the addition of a MCL, to compliment the velocity limits of the aircraft’s driving force. Keeping with simple sailplane design; only if one builds too light would there be a need for ballast to make the machine with a symmetrical fly fast enough, and then it shouldn’t take much. Al Sugar PO Box 113315 Carrollton, TX 75011-3315
As I got more into contest flying my Legionairs, they sported spoilers that were reduced in size compared to Cecil’s design; that better finessed the landing, and yielded more strength to the wing. Seeing Mike Bame's success with his MB 253515 I tried a Gemini kit, but was not impressed. I then went to a combination of two profiles that both had shortcomings; Eppler 205 that had a weak low end, and the Bame airfoil that did not seem to tolerate loading. The marriage yielded the ‘Happy Medium’ that was 13% thick, and worked great in two channel high AR machines. When I reached the point that I no longer needed spoilers to land; the Red Bird was born out of simplicity and was the highest reliability machine I owned (that won the most contest hardware for me). Now everyone at this time was seeking a sportsman’s class to stimulate more interest in soaring, but because of my status, I wasn’t allowed to fly my two channel machines in this division. No problem I had 2 ch standard to fall back on.....oops gone also!! Realizing that the profile design for the cruder balsa and film machines was not really advancing or evolving, and that all the ‘experts’ wanted of me and my associates was to support their latest pet project super- whippy-dippy low CD profile....the kind that would be difficult for an expert, let alone a novice, to construct via balsa and film. They had no time to invest in entry level machine design, or they simply felt it was beneath them. [[profile #3 The Beach ball with its very blunt leading edge, slow upper surface ramp, was a natural for many different construction techniques; multi spar, D tube, Plate wing, full depth spar, box spar etc. So far 5 machines all over the country were built with this profile and the findings about the same. Speed range equal to or greater than the Eppler 205! A really great response for so crude a profile. Although the Pierce Arrow company built theirs with a D tube structure, I had originally built mine with a Oly type of wing (only to convert it to a Plate wing at a later date). With the advent of winch power going from high to ridiculous, and the flying sites getting smaller, I had started to investigate toning down my launching equipment and studying effects of ratio types of tasks (using dead air time as a nominal time base). The hope was to attract more newcomers, and keep costs reasonable - and not be a deterrent. At this point in time I am no longer a contest buff because their is no contests available for those of us that just wish to dabble in toy aircraft (unless one wishes to dabble in powered models). At the same time that I was loosing the contest bug I had noticed that many of the individualistic local flyers, that were not in the Dallas League of Silent Flight club, were no longer coming to the Dallas League contests. I believe that they just got tired of blowing their machines up on the club winch (to the glee of the Dallas Club members). Shortly after that time, I initiated the Hot Air Master concepts; only to discover that the machines that performed well in the upper air were a bit too ‘dense’ for the lower launchings. Decreasing weight of a design helped the launch to the point a nominal 10 lb. tensioned (3 to 1 stroked) high start was capable of zoom launching a 12 to 1 AR, 100 inch 36 ounce sailplane in calm air. Once off the launch; however, the machine
was doo-doo; but would climb like mad if you put it right into the eye of lift. Cute, but not good enough for a contest task, ratio evaluated or not. Now needing to speed the machine up to reach out for lift could be done conventionally by current technology, however we get back to the problem of the novice builder unable to cast a molded lightweight wing, or come up with the cost of having someone do it for him. More air speed without weight, along with maintaining the spar depth for reasonable strength, became the engineering problem that I was seeking the simple solution to, and the experts turned their backs on me. The ‘experts’ did not like my first success with this problem; because it is rather crude. The Sugar Trip turbulator at 55% wing chord, 45 mils thick. The bad feature is that it creates premature separation and thus one loses slow speed flight (the float). The good feature is that it works (i.e. statistically proven)! [[profile #4 Those that used this device on lightweight sailplanes liked its reaction, plus the improved handling. The ‘mini spoiler’ trip made sure the sailplane was circulating, and proved that it was the equivalent of adding 20% of the A/C weight in more ballast --- that the launching system of limited force did not see! The Sugar Trip was tried with a myriad of profiles and seemed to work well in speeding sailplanes up that were ‘too light’ (when high tension launching was not available to haul up a ballasted machine). Since I was unable to get cooperation on my project here in America I went to Europe and presented my situation to their talent; and received my most usable information from Denis Oglesby of England. However in the interim, in trying to develop a standard design of a profile that could honestly be labeled as an Entry Level Profile that would be usable by an average soaring individual - with his own available flying sites for practice - and still have a realistic contest format that would be suitable for that individual to grow with; all I got out of the AMA, and its officers was basically resistance. So the ‘ball’ was still in my court. The beginning of this year (1998) I realized (with Denis Oglesby’s information) that my problem was “concern for a MCL”. The Sugar Trip effectively reduced the MCL effect, and thus improved the airspeed capability of a sailplane that was a bit too light, so why not go all the way. Now that the only control for the center airspeed employing a symmetrical profile (streamlined flat plate) would be the mass; lightness thus becomes an important goal for a symmetrical airfoil that operates typically at lower alpha. Because the speed range is actually reduced by this limitation, the controls and thus handling, tend to work more positively. All that is accomplished by doing it this way is that for a given design configuration is that the Symmetrical would be the least CD for the spar height that is necessary. Instead of flying a Sagita type of machine with its Eppler 205 flying at 9 ounces per foot on the currently standard high launch, we are now flying a similar appearing machine at 5 to 7 ounces per foot for a limited launching of 250 feet AGL with a symmetrical
profile of about 10% thickness. [[profile #5 The entry of my symmetrical is similar to the beachball (i.e. blunt) and the upper and lower surface basically straight lines intersecting at the trailing edge for simplicity of structure and construction. The ramp from the leading edge back is so low that it becomes unimportant to shape, and a circular arc would suffice. Although it is hard for the ‘experts’ to realize; the interest here is not for a shape to be employed to enhance F3B machinery, nor is it intended to compete in a high launch high tech state of art world. Its purpose is to give the dabbler (or beginner) a simple and inexpensive to build A/C that has more performance for its weight; then the ancient ‘Nostalgic Dinosaurs’ that are currently available (as ‘crumbs’ from the past), and thus yield larger 2 lb. sailplanes for the ‘buck’ that have more reliability of climb-out from a low launch, and/or have the ability to race to a lift area with some altitude left to successfully climb-out again. Interestingly, this form is not one for soaring only, but apparently is the correct form that is applicable for all model aircraft (fixed or rotary winged); basically it is a unified form. Unified Form: A symmetrical profile (without a MCL curve) that is applicable to all aircraft, powered or not, where low CD is the predominant characteristic, and mass is the only variable to set center gliding airspeed (camber changing symmetricals to be discussed later). The thickness of this unified profile is determined by projected total aircraft weight, aspect ratio, and maneuverability requirements. With soaring applications; if a thin (6% symmetrical for example) form were to be employed, and the resulting weight of the AC would yield too much friskiness so that flaps would be a great benefit to operation, the flap range would be from 0 to +12 degrees (without any considerations for reflex that would be counter productive). For power planes that normally use strip ailerons, the evolution to flaperon control to handle the higher wing loading at low airspeed is much wiser than pursuing some complex aerodynamic form that has fixed programming. Although this theory is rather crude compared to the work of the professionals in high performance profile design, it is very workable for the modelers universe. This attitude mirrors the modern concept that low CD is the primary attribute for an aircraft, and that CL is merely a compromise that compensates for any excessive mass; that may or may not be needed (such as for inertia and/or damping ). In attempts to dissuade me from my HAM adventure and its related profile search, the Experts have assured me that they gave the problem serious attention by instituting a Hand Launch category that “does the same thing”!?? Just zip launch the little ‘chucky’!? Unfortunately for the Newbie; trying to get into soaring with a sailplane that is too small, requiring an experts skill to fly, that uses micro components that are expensive; is not the best introduction to the wonderful world of soaring. Lee Murray on the other hand has spearheaded a gargantuan drive for an RES program (Rudder - Elevator - Servo) that should appeal to the old timers, that have a degree of nostalgia, or for those skilled in building antiques with their
own personal improvements (like slow evolution). I think my ideas would work great in this area because intending to launch off the ‘Club winch’ with less of a zoom (because of lightness), and then dork the landing without the weight, would work well for someone learning the contest experience; providing the wooden sailplane at least had some travel ability for the weight that it was at (preferably w/o exotic hard to fabricate profiles and hardware). Lets look at the entire spectrum of lift succinctly (for sport flier types; it is much simpler than the ‘experts’ intend for us to understand). Starting from a flat plate as the profile to be the reference starting point, we compare it (via peak CL) to the typical flatbottom airfoil that contains a nominal MCL. The ratio of lift between the two is approx. 2 to 1 (or less). Guess what, drag being a direct product of lift also falls into approx. the same ratio so we end up having a circle of logic. The point I am trying to make here (crudely) is that for a 3 degree alpha a sailplane with a flat plate for a profile would be flying at a similar airspeed as a sailplane that weighs twice as much with its flatbottom airfoil also set at approx. 3 degrees alpha. Since we cannot fly on flat plates, we are stuck with some kind of spar thickness to give our wings stiffness. Interestingly, if a 3/4 inch spar depth works for a flatbottom wing; and since lift and weight of the machine to duplicate a similar center flying speed would be cut in half via a symmetrical form, the spar depth for that same area could comfortably be reduced to 5/8 inch; for a little more bonus of the zip! I do believe a spar depth of 9 to10 percent realistic for most applications, because of typical building quality. The best builders of wooden airplanes still fall short of the high tech equipment available that are skillfully built via modern jigs and molds, so setting goals that are very difficult to achieve for a sportsman type of flyer is the wrong way to go for this medium. Current wooden sailplanes end up less than 8 ounces per foot typically; and High Tech machines are peaking out at 16 ounces per foot to handle contest pressure. [I have run into a few 9 ounce per foot high tech 3 meter machine that are quite impressive on the low launch, but they are still no match for a 6 ounce per foot Oly III flying with a tripped Beachball profile (even with enough breeze to utilize launch flap for a really good ‘hi tech zoom’ launch)]. The bottom line for entry level soaring is that I picture a sailplane classification that should be a sportsman’s class where weight (or the limited launching system) being the only limiting factor (along with a total elimination of the ‘what if’ concept). Obviously whether weight is included as a specification or not, is the necessity of the launch having limitations to give the contestants a typical altitude of 250 feet or so to make them work/sweat at climb-out. If controls were kept at 4 channels (the typical RC system - or less) I can see picking up many of those free spirited sportsmen flyers that drifted away from soaring that I mentioned earlier (along with a new flock of hungry to learn newbies that include aging free flighters that can no longer zip their bodies over hill and dale). I have been flying models now for 45 years now, and have dedicated myself to a lightweight RC sailplane that I can go to the local soccer field and zip launch (1/4 size hi start and soar for more than 3 minute flights) and it’s still is a very thrilling adventure for me to the point that I want to share this philosophy with the other earthbound members that are just unaware of the
rewards of this simplistic approach to flight. To be able to read the weeds, grass, birds, bugs, and fluff that surround you all within the same envelope of air, and thus be able to launch timely and chase evasive lift, climbing in it as if a ‘secret’ motor were turned on to increase the models altitude to extend your flight in multiples of what your machine’s dead air time is capable of; is to me what its all about. Once up in the upper air; to scramble seeking lifting areas to play stalled turns, inverted speed runs, slow rolls, loops, snaps and spins while being able to maintain altitude is merely adding more scope to the adventure before whizzing home, slowing down for touch down - and landing softly at my feet.....ready for another adventure. Application of Unified Form Unified Form: A symmetrical profile (without a MCL curve) that is applicable to all model aircraft, powered or not, where low CD is the predominant characteristic, and mass is the recognized variable to set center gliding airspeed @ 6 degrees alpa (power off). The thickness of this unified profile is determined by projected total aircraft weight, aspect ratio, and maneuverability requirements. Soaring aircraft: Use just enough rib height for strength. Acrobatic powered aircraft: Use a 18% thick profile and if drag a bit too much, just use a bigger motor! Slope acrobatics: Use about a 12% thick profile - to fit your flying style. If the resulting mass of an aircraft sets the airspeed too high, instead of searching for a “better profile,” just put in a bit of flap. Interestingly if we take Michael Selig’s data on the E 214, we discover what the application of flaps can do to the performance of this profile. For a given airspeed (alias RE number - when the wing chord is fixed) the charted CD is the value of concern in all cases, so taking the data recorded we find that the E 214 @ RE of 200k (w/o flap) and slow flying it at CL of 1.0 we find we are at Alpha of 6 degrees, and that generates a CD of 0.012 for a theoretical L/D of 83. Feed in 3 degrees of flap and the numbers change for a CL of 1.0 to an Alpha of 2.5 degrees and a CD of 0.012 again for a theoretical L/D of 83 @ RE 200k! As far as the aircraft concerned here, this situation means that at low speed flight we sort of have a tie for performance, however the other attributes of the AC has also changed for the better because of the flap. Adverse pitch increased because of CP moving back, and the handling of AC is better because it is technically more nose heavy. With fuselage flying cleaner at the lower Alpha, the overall L/D of the machine (for the CL reference of 1.0) should be higher at this slow flight situation, then the E 214 w/o flap. Whether or not this is precisely accurate with all profiles is unimportant. The crux of the concept here is that the unified profile is set up for minimum CD without regard to slow flight. The point that I am trying to make here is that to capitalize on slow flight (if desired) the flap at the TE is simple compensation, and in many situations would be superior and less troublesome then the concept of trying to maximize speed range via forming a tight combination of low CD form containing a MCL (to insure that center speed of the AC is within limits the modeling pilot is looking for at the typical loading a model AC would be at). The Unified form theory is such that low CD is paramount for a given beam height that is driven through the
air, and that low Alpha will always be clean flight of which the weight of the model sets up the velocity. If airspeed is needed to be cut back on a symmetrical profile application, and there is no weight to remove, adding a Gurney flap to the TE, or dropping the TE flap about 5 degrees should be enough to compensate for the missing MCL. This should be true whether we are talking indoor-HL, FF, UC, RC thermal or slope Soaring, Precision Acrobatics, Fun fly Acrobatics, etc. Using the unified theory, and if one wishes to build a powered sporter that has fun fly capabilities the 18% thick symmetrical would be a good choice; whereas for a pylon racer we are talking less than a 10% thickness symmetrical using flaperon programming for the Strip Ailerons - for safety in landings, along with getting out of the grass quicker at take off. The Minimax corporation manufactures high quality wooden sailplane kits of extremely light weight (down to 3.5 ounces per square foot). The profile that is employed has a spar depth of 12%; necessary for wing strength. Then to use a flat bottom form for that spar depth makes for a really slow floating type of machine; that is useless for any other locality that has normal turbulent air. Using a 12% symmetrical instead would speed the machine up as if it were loaded to at least 5.5 ounces per foot; and that is a more reasonable situation than adding 10 to 25 ounces of ballast into so light a structure just to be able to fly on nice normal days - other than dead calm. For those that wish to fly a symmetrical ridiculously slow.....Ooops, let me rephrase that! For those seeking the ideal beginners sailplane, the symmetrical can be modified with simple flaps by first cutting the original wing ribs off for the new flap, and using the pieces to make up the flaps (approx. 1 3/4” inches wide). We then place a .062 ply plate as the anchor point on the upper surface glued to the main spar and set between the wing ribs (or in a recess in foam) that is close to the center of the flap, 1/2 inch or so wide - and make the plate fit flush with the upper surface. Put an .062 hole in the center of this ply plate anchor, and then install a Z bent clevis rod into the hole. Install a horn onto the flap, tape the flap in place on the underside and adjust the clevis for about 3/8 inch droop to start. Then lock the clevis, and tape the upper flap surface into place to seal the gap and for better support. Now once tired of the slow, over stable basic trainer, and some stick education has occurred, one merely raises the flap adjustment to about 1/8 inch. To reset flap take the upper tape off, adjust the clevis and re-tape afterward. With a sailplane weighing in at 7 ounces per foot you should be centered in performance to sailplanes carrying a profile with a mild MCL. When you are ready for events like the Hot Air Masters, take the flaps to zero for all the zip you can get. My experience with using servos to control flaps on sailplanes that are very light is a lesson in futility, and my only concern for the programmable flap is for center speed. Now for F3J or F3B applications the symmetrical has some interesting characteristics. The datum line being straight is the least CD state of this profile, which means the flap is always is air loaded. Being unilateral (because flap reflex is undesirable) the flap actuator can be a very light pull only line to set the flap, which then brings the actuator system mass closer to the CG of the AC. Conceptually, I am convinced that a 6% symmetrical with about 4 degrees of flap will perform right
along with the best current F3B profiles Michael Selig has to offer, and still have one major advantage. When the flap of the symmetrical is at zero degrees, the CD is still lower than the CD of the profile compromised with a MCL. Adding reflex to the flap definitely will not help to make the Selig profile catch up! It looks like we have an adversarial situation here that only the “Contest field of Honor” can prove. Personally, I am out of it because of two serious reasons; I’m too old, and I’m just not good enough. Hopefully, some warrior out there may find this concept appealing enough to capitalize upon, and seriously go for the Gold! Reducing speed range by throwing away the low end may appear to be disruptive to many sailplane enthusiasts that usually fly heavy; they use the low end for “parking” i.e. a breather for thinking, maximizing climb in lift (preferable with a bit of flap); and there are some of our successful experts that can build to very close tolerance, and thus have a machine that can come close to doing a typical contest task in dead air. They will cruise about most of the time at a speed close to textbook maximum L/D, and snag a thermal to extend their time aloft to just complete the contest task they are involved with. I have seen great thermallers fly by trims primarily, using the FF stability of their machine to assist in minimizing sink rate. On the other hand, my approach to soaring is quite aggressive. No low-end to me means sharper reaction to controls; thus less throws for the surfaces, more roll rate for the dihedral usually employed for simple 2 chan. machines, and more enjoyment of little adventures that are occurring more rapidly; even though the airspeed of the heavier machine flying at close to maximum L/D, and the airspeed for the lighter machine with a symmetrical is about the same (with the L/D of the heavy sailplane being superior). My attitude of bias for the lighter machine with as much scramble as possible is probably in line with the thinking of the World war I ace, The Red Baron, or the World war II ace Japan’s Yatamoto, whom both capitalized on maneuverability, and rate of climb for their tremendous success. I see the concept of Unified form as the needed relief for model aircraft profile design, so as to return them into the simple toys they were meant to be. With modern technology we can utilize this concept for the design of all flying surfaces; whether rotary (including propellers), flying wings with their reflexed flap, or basic fixed wing aircraft. Once the construction technique is completed for a modelers flying surface that is strong enough to handle his flying air-loads, only the amount of flap needs to be determined to set the most favorable air speed; just that simple. Aircraft that is designed to do work efficiently by carrying some kind of payload in addition to its airframe, are the vehicles that require the addition of a MCL, to compliment the velocity limits of the aircraft’s driving force. Keeping with simple sailplane design; only if one builds too light would there be a need for ballast to make the machine with a symmetrical fly fast enough, and then it shouldn’t take much. Al Sugar PO Box 113315 Carrollton, TX 75011-3315
