ON STABS AND STABILITY: Free Right (FF) is a balancing act. The aptly named horizontal stabilizer (stab) is an important component of that balancing act. However, we often take the stab for granted, shimming it up or down to get the glide right and popping it up for dethermalizer (DT), but otherwise paying it little attention. Basically, the stab's job is to hold the wing at the angle that will give the best glide. That's the easy part. When the model is gliding along comfortably, all the stab has to do is counteract the wing's nose-up tendency. This is usually done by setting the stab at a slight negative incidence relative to the wing. This difference between the wing and stab incidence settings is called decalage. But our models are not just designed to glide. They have to get up in the air first. In most cases, a model in the climb will be going faster than its glide speed. That will result in more lift from the wing. Unless that lift is balanced by increased lift in the stab, the result will be a loop. However, if the stab overbalances the increased wing lift, the result can be a dive. The amount of decalage required to balance the model in the glide depends on a number of factors, including the wing lift, the center of gravity (CG) location, the length of the stab's moment arm, and the stab airfoil. All things being equal, a wing with a higher lift, either from a bigger area or a more undercambered airfoil, will need less decalage than will a smaller wing or one with a lower-lift airfoil. A typical Hand-Launched Glider uses a thin, flat-bottomed airfoil to limit lift in the climb coupled with very little decalage to prevent looping in the launch. Another approach, sometimes used in F1C Power, is to change the wing airfoil from a flat-bottomed, low-lift airfoil in the climb to an undercambered, high-lift airfoil in the glide. This is normally done by flapping the rear third of the airfoil. Another approach, used by Russian flier Leonid Fuzeyev, is to fold the wing during the power phase, reducing the area and converting the undercambered airfoil into a thicker symmetrical shape. In most cases, however, the wing is optimized for maximum glide, and the stabilizer is used to regulate climb and glide. The stab's moment ami affects its ability to balance the lift of the wing, much like the grade-school math problem of the fat kid and the skinny kid on a seesaw. If the wing-to-tail lever is long, a smaller stab can be used. Fuselages were short in modeling's early days, often because of cross-section rules based on the model's overall length. If you wanted to use a long fuselage, you had to increase the fuselage cross-section, increasing drag. (Under the old length squared divided by 100 rule, a 36-inch-long model would require a cross-section of almost 13 square inches. A 48-inch-long model would need a cross-section of just more than 23 square inches.) The answer to the restrictive rules was to use a large stab, often one-third and sometimes more than one-half the wing area. At first glance this would seem like a good thing, since you were increasing total area. But to do its job properly, a stab will be flying at a lower angle of attack than the wing. Each square inch of wing area is producing more lift than each square inch of stab area. That's why models for events with limited total (i.e., wing plus stab) area pack as much area as possible into the wing, using as small a stabilizer as possible with a long moment arm. Typically, stabs are 20% of the wing area or less, with a moment arm ranging from seven to nine times the average wing chord. (Tail moment arm is usually measured from the quarter point of the wing mean chord to the quarter point of the stabilizer mean chord.) For FF. there is a lower limit for stab percentage. Too small a stab will result in poor stall recovery. Often, the model will fly fine in calm conditions but will stall to the ground when upset by wind or thermals. (The same thing can happen when we shim the trailing edge of the stab up too much, trying to wring the best possible glide out of the model.) Besides, the gains in total performance by using small stabs is limited. In his 1976 Sympo paper "Effect of Some Design Parameters on the Glide Performance of an A-2 Nordic Glider," Harry Grogan wrote: "From 164 feel, a 65 sq. in. stab (considered small by present standards) has about a 6 . second advantage over an 88 sq. in. stabilizer (considered large). Further reduction to 52 sq. in. produces an additional 3 seconds in the glide. It would appear that stabilizers below 14% are of little practical help except for very calm conditions." A related factor is the moment of inertia of the tail and aft part of the fuselage. A model with light extremities will react more quickly to slight upsets in climb and glide. I'm convinced that of all the high-tech features developed in the last 20 years, the lightweight carbon-aluminum tailboom may be the most important. In the days of rolled balsa tailbooms, the lightest I was ever able to manage for an Fl B model was roughly 18 grams; a carbon-aluminum tuilboom weighs between seven and eight grams. That coupled with the lighter, plastic-covered stab and rudder and a strict diet on all the fittings has resulted in a total weight for everything aft of the rear peg of 16-18 grams, versus 24-28 grams for the old models. The difference in the air is remarkable. It really does pay to keep the tail light! The stab airfoil is important—perhaps far more important than many of us think. In the late 1970s I was constantly plagued by models that would fly fine in calm air but develop a stall in the heat of the day. The answer, or so I thought, was to increase the stab lift to help hold the nose down and prevent the stall. Wings were cut down, and new, bigger stabs were built, often with thin, highly undercambered airfoils. Things kept getting worse. I even tried some of the slotted, two-part airfoils being touted. Things got even worse. After doing a good bit of reading that brought me to my senses, I switched to a semisymmetrical airfoil with a blunt leading edge. The Wobbeking 2.5-25-8 airfoil I use now has a leading-edge radius of 4% with the high point at 25%. Almost all the stalling problems have gone away, although the model will still stall if pushed too far. In the past I trimmed the model for the best possible glide under calm conditions, jacking up the back of the stab until the model was just off the stall. (A better way is to conduct a glide test, timing the model to the ground and adjusting the stab up or down to give maximum time. That's difficult to do at a small test field.) When the wind or thermal activity increased, the stab's trailing edge could be pulled down slightly to lessen the chance of a flight-spoiling stall. (As an alternative, the glide circle could be tightened up.) But messing with the stab setting presented two problems: it affected the glide and it affected the cruise portion of the climb. With the use of screw adjustments for the stab, there was, at least for me, the problem of keeping track of the setting. Did I screw it back down after the last flight? George Batiuk offered a much simpler alternative which he dubbed "The old Ukrainian trick." To prevent stalling in thermal or wind conditions, he moved the CG forward slightly. Strapping a spare wing wire on the nose of a Wakefield is easy to do, doesn't disturb the climb pattern, and, most importantly, is easy to see. This has become my standard method for dealing with the switch from calm to wind/thermal conditions. The CG location is an important component of stability. Simply stated, the farther back the CG. the less decalage is needed. You can get a model to glide with the CG at 100% (i.e.. at the wing trailing edge) with perhaps 1 ° of decalage. Or you can move the CG forward to 50% (wing midchord) and jack up the stab trailing edge to give approximately 3° of decalage. Which is best? The advantage of the more-reward CG is that less decalage is required. But why would you want less decalage? Not for the glide, but for the climb. A model with low decalage will be less likely to loop under power. In the days before the use of autosurfaces, a typical CG location for a Power model was between 80 and 100% of the wing chord; sometimes it was even behind the trailing edge. Often, the wing and stab were set at zero decalage, with only washin on the right main panel supplying some measure of decalage. It was often a delicate balancing act. Too little decalage would mean an over-the-top dive into the ground. Too much decalage could result in a loop. Spiraling the model up helped turn the loop into a safer spiral climb. But the limited decalage could also cause problems in the glide— most noticeably a spiral dive. With the introduction of reliable timer-operated auto stabs (also called VIT—variable incidence tailplane), the climb and glide phases could be adjusted separately. For the climb, the decalage was reduced to near zero. For the glide, the decalage was increased to roughly 3°. To make the glide work with the increased decalage. the CG was moved forward to approximately 55%. (Note that the CG did not move during the flight.) Because the high-speed climb and the slower glide segments ol' the flight could each be adjusted without affecting the other, trimming the model became much easier. The preceding is a simplified explanation ol the factors affecting longitudinal stability. Other factors can affect stability, such as stabilizer aspect ratio, location of the stab relative to the wing wake, and nose length. There is also a relationship between the tail volume and the optimum CG locution for a non-auto Power model. (Tail volume is determined by dividing the stab area by the wing area then multiplying that by the tail moment expressed in multiples ol the wing chord.) A RamRod 250 has a stab area of 117 square inches, a flat wing area of 263 square inches, a tail moment of 24.7 inches, and a wing mean chord of 6.5 inches. That gives a tail volume of 1.69. By plotting the tail volumes and CG ,, positions of several Power models. Bill ' Bogart and Bud Rhodes developed a graph that was published in the January 1959 Model Airplane News. A diagonal line connecting the plots for the models suggested a stable starting place for the CG location for a given tail volume. If the CG was ahead of the location shown on the plot, the model would tend to loop under power; if the CG was behind the location shown, the model would tend to dive under power. The decalage would need to be adjusted to give a good glide based on the CG location and tail moment arm. If I have learned one thing in the last 20 years, it is to check the CG if you have problems with stalling or a too-fast glide. If the CG is in the right place, then and only then should you start moving the stab trailing edge up or down. If the CG is not in the right place, break out the lead. The flying field is not the place to check the CG. Do it in the shop, out of the wind. Balancing the model on your fingertips is not accurate enough. A simple knife-edge fixture of balsa will allow you to adjust the CG to within 1 % of the chord. Remember too that the CG should be based on the wing's average chord—not its root chord. You also need to allow for tapered or back-swept wings. Once you determine the correct CG location for your model, mark it on the bottom of the wings. Remember to check the CG from time to time. Repairs and patches can cause a shift, usually to the rear. Free Flight Forum 2002: Like our National Free Flight Society (NFFS) Symposium, the British Forum is an annual publication devoted to the art and science of FF. The 2002 edition of the Forum offers wide-ranging articles on a variety of topics. This year the emphasis was more on the practical rather than the theoretical aspects of FF; in other words, there is a lot of useful information and not a lot of formulae. Here's a run-down of the articles: • "Electric Free Flight" by John Godden: Although electric power has become popular for a variety of Radio Control events, its popularity in FF lags far behind. John's article should help rectify the situation by offering specific information on battery, motor, and propeller selection, and wiring. He includes several sketches of timer-operated switches for motor shutoff. • "The 30 Gram Fl B" by Mike Woodhouse: The new, smaller motor for Wakefield will impact performance and, perhaps, model design. Mike explores all the options, from the number of strands used to the possible future implications such as single-blade propellers or variable-diameter propellers. • "Developments in Slow Open Power" by Dave Clarkson: With the new Classic Power event getting started in this country, it would be useful to look at what the British have been doing with their Slow Open Power event. Dave traces his own SLOP models, dating back to the mid-1980s. One item he discusses is the relative merits of various wing dihedral layouts, comparing the typical American scheme with the main panel twice the length of the tip to the British layout with the tip panel two-thirds the length of the main panel. He also includes a plot of tail volume relative to the CG. • "Free Flight Diversity—Strength or Weakness" by Chris Strachan: Britain, like the US, is a land of a thousand events. While the rest of the world concentrates on the Federation Aeronautique International (FAI) events, these two countries offer a seemingly endless variety of classes—too many in Chris's mind. Not only does this result in a dilution of effort, but it can also lead to confusion in contest administration. • "Computers in Free Flight—An Overview" by Mike Evatt: In addition to tracking down contest results and faraway parts suppliers on the Internet, a computer can be a useful tool for a variety of aeromodcling tasks. Mike explores the options in flight simulation, computer-aided design, and 3-D solid modeling. Perhaps most useful is a discussion about using a computer to produce full-size plans. • "Contests in Restricted Space" by John Godden: The subtitle "How to keep them on the airfield" tells it all. This paper explores ways to reduce performance, ranging from shortening the towline length or decreasing the engine run to the more difficult problem of dealing with Rubber models. For Rubber events, two schemes have been tried. One requires ballast equal to the motor weight. The other, adapted from standard indoor testing practice, uses a stick half the length of the motor base combined with a half-length motor. • "Flying the New Rule F IDs" by Bob Bailey: The new rules for the rubber-powered Indoor event require smaller, heavier, more robust models. The idea is to have models that are easier to build, transport, and Oy. In the United Kingdom, this has resulted in unbraced wings covered with Y2K2 plastic film instead of the more-difficult-to-handle microfilm. • "Propeller Pivoting for Flat Folding" by John Barker: What would a Svnipo or a Forum be without a few formulae? John's article offers a technical but easily understood look at the problem of getting a Rubber model's blades to fold flat against the fuselage for minimum drag. Although the narrow blades of a modern FIB or FIG don't require compound-angled hinges, the wide blades of Nostalgia Wakefields or Old-Time Rubber models do. • "Variable Pitch Propellers for Coupe D'Hiver" by John Bailey and Mike Evatt: Widely popular in the larger Fl B Wakefield Rubber event, variable-pitch (VP) propellers are gaining a foothold in the smaller FIG Coupe event. The authors explore a variety of approaches used in FIB and offer several suggestions for simplified, lighter versions for Coupe. Editors Martin Dilly and Mike Evatt have put together a most useful group of papers that should appeal to a variety of FF modelers. You can order a copy of Free Flight Forum 2002 from Martin Dilly. 20 Links Rd., West Wickham. Kent. BR4 OQW. UK. The price, postage-included, is eight pounds sterling (approximately $13). Checks should be payable to "BMFA FF Team Support Fund" and in pounds sterling, only drawn on a bank with a branch in the UK. You can also order by credit card via E-mail to [email protected] or by fax to 44+«))20 8777 5533. The British Free Flight Foruim are also available through NFFS Publications. Contact Bob McLinden, Box 7967. Baltimore MD 2 1221. or E-mail him at [email protected]. You can find ordering information on the NFFS Web site at http://freeflight.org.
Edition: Model Aviation - 2002/10
Page Numbers: 114, 115, 116, 118, 125
ON STABS AND STABILITY: Free Right (FF) is a balancing act. The aptly named horizontal stabilizer (stab) is an important component of that balancing act. However, we often take the stab for granted, shimming it up or down to get the glide right and popping it up for dethermalizer (DT), but otherwise paying it little attention. Basically, the stab's job is to hold the wing at the angle that will give the best glide. That's the easy part. When the model is gliding along comfortably, all the stab has to do is counteract the wing's nose-up tendency. This is usually done by setting the stab at a slight negative incidence relative to the wing. This difference between the wing and stab incidence settings is called decalage. But our models are not just designed to glide. They have to get up in the air first. In most cases, a model in the climb will be going faster than its glide speed. That will result in more lift from the wing. Unless that lift is balanced by increased lift in the stab, the result will be a loop. However, if the stab overbalances the increased wing lift, the result can be a dive. The amount of decalage required to balance the model in the glide depends on a number of factors, including the wing lift, the center of gravity (CG) location, the length of the stab's moment arm, and the stab airfoil. All things being equal, a wing with a higher lift, either from a bigger area or a more undercambered airfoil, will need less decalage than will a smaller wing or one with a lower-lift airfoil. A typical Hand-Launched Glider uses a thin, flat-bottomed airfoil to limit lift in the climb coupled with very little decalage to prevent looping in the launch. Another approach, sometimes used in F1C Power, is to change the wing airfoil from a flat-bottomed, low-lift airfoil in the climb to an undercambered, high-lift airfoil in the glide. This is normally done by flapping the rear third of the airfoil. Another approach, used by Russian flier Leonid Fuzeyev, is to fold the wing during the power phase, reducing the area and converting the undercambered airfoil into a thicker symmetrical shape. In most cases, however, the wing is optimized for maximum glide, and the stabilizer is used to regulate climb and glide. The stab's moment ami affects its ability to balance the lift of the wing, much like the grade-school math problem of the fat kid and the skinny kid on a seesaw. If the wing-to-tail lever is long, a smaller stab can be used. Fuselages were short in modeling's early days, often because of cross-section rules based on the model's overall length. If you wanted to use a long fuselage, you had to increase the fuselage cross-section, increasing drag. (Under the old length squared divided by 100 rule, a 36-inch-long model would require a cross-section of almost 13 square inches. A 48-inch-long model would need a cross-section of just more than 23 square inches.) The answer to the restrictive rules was to use a large stab, often one-third and sometimes more than one-half the wing area. At first glance this would seem like a good thing, since you were increasing total area. But to do its job properly, a stab will be flying at a lower angle of attack than the wing. Each square inch of wing area is producing more lift than each square inch of stab area. That's why models for events with limited total (i.e., wing plus stab) area pack as much area as possible into the wing, using as small a stabilizer as possible with a long moment arm. Typically, stabs are 20% of the wing area or less, with a moment arm ranging from seven to nine times the average wing chord. (Tail moment arm is usually measured from the quarter point of the wing mean chord to the quarter point of the stabilizer mean chord.) For FF. there is a lower limit for stab percentage. Too small a stab will result in poor stall recovery. Often, the model will fly fine in calm conditions but will stall to the ground when upset by wind or thermals. (The same thing can happen when we shim the trailing edge of the stab up too much, trying to wring the best possible glide out of the model.) Besides, the gains in total performance by using small stabs is limited. In his 1976 Sympo paper "Effect of Some Design Parameters on the Glide Performance of an A-2 Nordic Glider," Harry Grogan wrote: "From 164 feel, a 65 sq. in. stab (considered small by present standards) has about a 6 . second advantage over an 88 sq. in. stabilizer (considered large). Further reduction to 52 sq. in. produces an additional 3 seconds in the glide. It would appear that stabilizers below 14% are of little practical help except for very calm conditions." A related factor is the moment of inertia of the tail and aft part of the fuselage. A model with light extremities will react more quickly to slight upsets in climb and glide. I'm convinced that of all the high-tech features developed in the last 20 years, the lightweight carbon-aluminum tailboom may be the most important. In the days of rolled balsa tailbooms, the lightest I was ever able to manage for an Fl B model was roughly 18 grams; a carbon-aluminum tuilboom weighs between seven and eight grams. That coupled with the lighter, plastic-covered stab and rudder and a strict diet on all the fittings has resulted in a total weight for everything aft of the rear peg of 16-18 grams, versus 24-28 grams for the old models. The difference in the air is remarkable. It really does pay to keep the tail light! The stab airfoil is important—perhaps far more important than many of us think. In the late 1970s I was constantly plagued by models that would fly fine in calm air but develop a stall in the heat of the day. The answer, or so I thought, was to increase the stab lift to help hold the nose down and prevent the stall. Wings were cut down, and new, bigger stabs were built, often with thin, highly undercambered airfoils. Things kept getting worse. I even tried some of the slotted, two-part airfoils being touted. Things got even worse. After doing a good bit of reading that brought me to my senses, I switched to a semisymmetrical airfoil with a blunt leading edge. The Wobbeking 2.5-25-8 airfoil I use now has a leading-edge radius of 4% with the high point at 25%. Almost all the stalling problems have gone away, although the model will still stall if pushed too far. In the past I trimmed the model for the best possible glide under calm conditions, jacking up the back of the stab until the model was just off the stall. (A better way is to conduct a glide test, timing the model to the ground and adjusting the stab up or down to give maximum time. That's difficult to do at a small test field.) When the wind or thermal activity increased, the stab's trailing edge could be pulled down slightly to lessen the chance of a flight-spoiling stall. (As an alternative, the glide circle could be tightened up.) But messing with the stab setting presented two problems: it affected the glide and it affected the cruise portion of the climb. With the use of screw adjustments for the stab, there was, at least for me, the problem of keeping track of the setting. Did I screw it back down after the last flight? George Batiuk offered a much simpler alternative which he dubbed "The old Ukrainian trick." To prevent stalling in thermal or wind conditions, he moved the CG forward slightly. Strapping a spare wing wire on the nose of a Wakefield is easy to do, doesn't disturb the climb pattern, and, most importantly, is easy to see. This has become my standard method for dealing with the switch from calm to wind/thermal conditions. The CG location is an important component of stability. Simply stated, the farther back the CG. the less decalage is needed. You can get a model to glide with the CG at 100% (i.e.. at the wing trailing edge) with perhaps 1 ° of decalage. Or you can move the CG forward to 50% (wing midchord) and jack up the stab trailing edge to give approximately 3° of decalage. Which is best? The advantage of the more-reward CG is that less decalage is required. But why would you want less decalage? Not for the glide, but for the climb. A model with low decalage will be less likely to loop under power. In the days before the use of autosurfaces, a typical CG location for a Power model was between 80 and 100% of the wing chord; sometimes it was even behind the trailing edge. Often, the wing and stab were set at zero decalage, with only washin on the right main panel supplying some measure of decalage. It was often a delicate balancing act. Too little decalage would mean an over-the-top dive into the ground. Too much decalage could result in a loop. Spiraling the model up helped turn the loop into a safer spiral climb. But the limited decalage could also cause problems in the glide— most noticeably a spiral dive. With the introduction of reliable timer-operated auto stabs (also called VIT—variable incidence tailplane), the climb and glide phases could be adjusted separately. For the climb, the decalage was reduced to near zero. For the glide, the decalage was increased to roughly 3°. To make the glide work with the increased decalage. the CG was moved forward to approximately 55%. (Note that the CG did not move during the flight.) Because the high-speed climb and the slower glide segments ol' the flight could each be adjusted without affecting the other, trimming the model became much easier. The preceding is a simplified explanation ol the factors affecting longitudinal stability. Other factors can affect stability, such as stabilizer aspect ratio, location of the stab relative to the wing wake, and nose length. There is also a relationship between the tail volume and the optimum CG locution for a non-auto Power model. (Tail volume is determined by dividing the stab area by the wing area then multiplying that by the tail moment expressed in multiples ol the wing chord.) A RamRod 250 has a stab area of 117 square inches, a flat wing area of 263 square inches, a tail moment of 24.7 inches, and a wing mean chord of 6.5 inches. That gives a tail volume of 1.69. By plotting the tail volumes and CG ,, positions of several Power models. Bill ' Bogart and Bud Rhodes developed a graph that was published in the January 1959 Model Airplane News. A diagonal line connecting the plots for the models suggested a stable starting place for the CG location for a given tail volume. If the CG was ahead of the location shown on the plot, the model would tend to loop under power; if the CG was behind the location shown, the model would tend to dive under power. The decalage would need to be adjusted to give a good glide based on the CG location and tail moment arm. If I have learned one thing in the last 20 years, it is to check the CG if you have problems with stalling or a too-fast glide. If the CG is in the right place, then and only then should you start moving the stab trailing edge up or down. If the CG is not in the right place, break out the lead. The flying field is not the place to check the CG. Do it in the shop, out of the wind. Balancing the model on your fingertips is not accurate enough. A simple knife-edge fixture of balsa will allow you to adjust the CG to within 1 % of the chord. Remember too that the CG should be based on the wing's average chord—not its root chord. You also need to allow for tapered or back-swept wings. Once you determine the correct CG location for your model, mark it on the bottom of the wings. Remember to check the CG from time to time. Repairs and patches can cause a shift, usually to the rear. Free Flight Forum 2002: Like our National Free Flight Society (NFFS) Symposium, the British Forum is an annual publication devoted to the art and science of FF. The 2002 edition of the Forum offers wide-ranging articles on a variety of topics. This year the emphasis was more on the practical rather than the theoretical aspects of FF; in other words, there is a lot of useful information and not a lot of formulae. Here's a run-down of the articles: • "Electric Free Flight" by John Godden: Although electric power has become popular for a variety of Radio Control events, its popularity in FF lags far behind. John's article should help rectify the situation by offering specific information on battery, motor, and propeller selection, and wiring. He includes several sketches of timer-operated switches for motor shutoff. • "The 30 Gram Fl B" by Mike Woodhouse: The new, smaller motor for Wakefield will impact performance and, perhaps, model design. Mike explores all the options, from the number of strands used to the possible future implications such as single-blade propellers or variable-diameter propellers. • "Developments in Slow Open Power" by Dave Clarkson: With the new Classic Power event getting started in this country, it would be useful to look at what the British have been doing with their Slow Open Power event. Dave traces his own SLOP models, dating back to the mid-1980s. One item he discusses is the relative merits of various wing dihedral layouts, comparing the typical American scheme with the main panel twice the length of the tip to the British layout with the tip panel two-thirds the length of the main panel. He also includes a plot of tail volume relative to the CG. • "Free Flight Diversity—Strength or Weakness" by Chris Strachan: Britain, like the US, is a land of a thousand events. While the rest of the world concentrates on the Federation Aeronautique International (FAI) events, these two countries offer a seemingly endless variety of classes—too many in Chris's mind. Not only does this result in a dilution of effort, but it can also lead to confusion in contest administration. • "Computers in Free Flight—An Overview" by Mike Evatt: In addition to tracking down contest results and faraway parts suppliers on the Internet, a computer can be a useful tool for a variety of aeromodcling tasks. Mike explores the options in flight simulation, computer-aided design, and 3-D solid modeling. Perhaps most useful is a discussion about using a computer to produce full-size plans. • "Contests in Restricted Space" by John Godden: The subtitle "How to keep them on the airfield" tells it all. This paper explores ways to reduce performance, ranging from shortening the towline length or decreasing the engine run to the more difficult problem of dealing with Rubber models. For Rubber events, two schemes have been tried. One requires ballast equal to the motor weight. The other, adapted from standard indoor testing practice, uses a stick half the length of the motor base combined with a half-length motor. • "Flying the New Rule F IDs" by Bob Bailey: The new rules for the rubber-powered Indoor event require smaller, heavier, more robust models. The idea is to have models that are easier to build, transport, and Oy. In the United Kingdom, this has resulted in unbraced wings covered with Y2K2 plastic film instead of the more-difficult-to-handle microfilm. • "Propeller Pivoting for Flat Folding" by John Barker: What would a Svnipo or a Forum be without a few formulae? John's article offers a technical but easily understood look at the problem of getting a Rubber model's blades to fold flat against the fuselage for minimum drag. Although the narrow blades of a modern FIB or FIG don't require compound-angled hinges, the wide blades of Nostalgia Wakefields or Old-Time Rubber models do. • "Variable Pitch Propellers for Coupe D'Hiver" by John Bailey and Mike Evatt: Widely popular in the larger Fl B Wakefield Rubber event, variable-pitch (VP) propellers are gaining a foothold in the smaller FIG Coupe event. The authors explore a variety of approaches used in FIB and offer several suggestions for simplified, lighter versions for Coupe. Editors Martin Dilly and Mike Evatt have put together a most useful group of papers that should appeal to a variety of FF modelers. You can order a copy of Free Flight Forum 2002 from Martin Dilly. 20 Links Rd., West Wickham. Kent. BR4 OQW. UK. The price, postage-included, is eight pounds sterling (approximately $13). Checks should be payable to "BMFA FF Team Support Fund" and in pounds sterling, only drawn on a bank with a branch in the UK. You can also order by credit card via E-mail to [email protected] or by fax to 44+«))20 8777 5533. The British Free Flight Foruim are also available through NFFS Publications. Contact Bob McLinden, Box 7967. Baltimore MD 2 1221. or E-mail him at [email protected]. You can find ordering information on the NFFS Web site at http://freeflight.org.
Edition: Model Aviation - 2002/10
Page Numbers: 114, 115, 116, 118, 125
ON STABS AND STABILITY: Free Right (FF) is a balancing act. The aptly named horizontal stabilizer (stab) is an important component of that balancing act. However, we often take the stab for granted, shimming it up or down to get the glide right and popping it up for dethermalizer (DT), but otherwise paying it little attention. Basically, the stab's job is to hold the wing at the angle that will give the best glide. That's the easy part. When the model is gliding along comfortably, all the stab has to do is counteract the wing's nose-up tendency. This is usually done by setting the stab at a slight negative incidence relative to the wing. This difference between the wing and stab incidence settings is called decalage. But our models are not just designed to glide. They have to get up in the air first. In most cases, a model in the climb will be going faster than its glide speed. That will result in more lift from the wing. Unless that lift is balanced by increased lift in the stab, the result will be a loop. However, if the stab overbalances the increased wing lift, the result can be a dive. The amount of decalage required to balance the model in the glide depends on a number of factors, including the wing lift, the center of gravity (CG) location, the length of the stab's moment arm, and the stab airfoil. All things being equal, a wing with a higher lift, either from a bigger area or a more undercambered airfoil, will need less decalage than will a smaller wing or one with a lower-lift airfoil. A typical Hand-Launched Glider uses a thin, flat-bottomed airfoil to limit lift in the climb coupled with very little decalage to prevent looping in the launch. Another approach, sometimes used in F1C Power, is to change the wing airfoil from a flat-bottomed, low-lift airfoil in the climb to an undercambered, high-lift airfoil in the glide. This is normally done by flapping the rear third of the airfoil. Another approach, used by Russian flier Leonid Fuzeyev, is to fold the wing during the power phase, reducing the area and converting the undercambered airfoil into a thicker symmetrical shape. In most cases, however, the wing is optimized for maximum glide, and the stabilizer is used to regulate climb and glide. The stab's moment ami affects its ability to balance the lift of the wing, much like the grade-school math problem of the fat kid and the skinny kid on a seesaw. If the wing-to-tail lever is long, a smaller stab can be used. Fuselages were short in modeling's early days, often because of cross-section rules based on the model's overall length. If you wanted to use a long fuselage, you had to increase the fuselage cross-section, increasing drag. (Under the old length squared divided by 100 rule, a 36-inch-long model would require a cross-section of almost 13 square inches. A 48-inch-long model would need a cross-section of just more than 23 square inches.) The answer to the restrictive rules was to use a large stab, often one-third and sometimes more than one-half the wing area. At first glance this would seem like a good thing, since you were increasing total area. But to do its job properly, a stab will be flying at a lower angle of attack than the wing. Each square inch of wing area is producing more lift than each square inch of stab area. That's why models for events with limited total (i.e., wing plus stab) area pack as much area as possible into the wing, using as small a stabilizer as possible with a long moment arm. Typically, stabs are 20% of the wing area or less, with a moment arm ranging from seven to nine times the average wing chord. (Tail moment arm is usually measured from the quarter point of the wing mean chord to the quarter point of the stabilizer mean chord.) For FF. there is a lower limit for stab percentage. Too small a stab will result in poor stall recovery. Often, the model will fly fine in calm conditions but will stall to the ground when upset by wind or thermals. (The same thing can happen when we shim the trailing edge of the stab up too much, trying to wring the best possible glide out of the model.) Besides, the gains in total performance by using small stabs is limited. In his 1976 Sympo paper "Effect of Some Design Parameters on the Glide Performance of an A-2 Nordic Glider," Harry Grogan wrote: "From 164 feel, a 65 sq. in. stab (considered small by present standards) has about a 6 . second advantage over an 88 sq. in. stabilizer (considered large). Further reduction to 52 sq. in. produces an additional 3 seconds in the glide. It would appear that stabilizers below 14% are of little practical help except for very calm conditions." A related factor is the moment of inertia of the tail and aft part of the fuselage. A model with light extremities will react more quickly to slight upsets in climb and glide. I'm convinced that of all the high-tech features developed in the last 20 years, the lightweight carbon-aluminum tailboom may be the most important. In the days of rolled balsa tailbooms, the lightest I was ever able to manage for an Fl B model was roughly 18 grams; a carbon-aluminum tuilboom weighs between seven and eight grams. That coupled with the lighter, plastic-covered stab and rudder and a strict diet on all the fittings has resulted in a total weight for everything aft of the rear peg of 16-18 grams, versus 24-28 grams for the old models. The difference in the air is remarkable. It really does pay to keep the tail light! The stab airfoil is important—perhaps far more important than many of us think. In the late 1970s I was constantly plagued by models that would fly fine in calm air but develop a stall in the heat of the day. The answer, or so I thought, was to increase the stab lift to help hold the nose down and prevent the stall. Wings were cut down, and new, bigger stabs were built, often with thin, highly undercambered airfoils. Things kept getting worse. I even tried some of the slotted, two-part airfoils being touted. Things got even worse. After doing a good bit of reading that brought me to my senses, I switched to a semisymmetrical airfoil with a blunt leading edge. The Wobbeking 2.5-25-8 airfoil I use now has a leading-edge radius of 4% with the high point at 25%. Almost all the stalling problems have gone away, although the model will still stall if pushed too far. In the past I trimmed the model for the best possible glide under calm conditions, jacking up the back of the stab until the model was just off the stall. (A better way is to conduct a glide test, timing the model to the ground and adjusting the stab up or down to give maximum time. That's difficult to do at a small test field.) When the wind or thermal activity increased, the stab's trailing edge could be pulled down slightly to lessen the chance of a flight-spoiling stall. (As an alternative, the glide circle could be tightened up.) But messing with the stab setting presented two problems: it affected the glide and it affected the cruise portion of the climb. With the use of screw adjustments for the stab, there was, at least for me, the problem of keeping track of the setting. Did I screw it back down after the last flight? George Batiuk offered a much simpler alternative which he dubbed "The old Ukrainian trick." To prevent stalling in thermal or wind conditions, he moved the CG forward slightly. Strapping a spare wing wire on the nose of a Wakefield is easy to do, doesn't disturb the climb pattern, and, most importantly, is easy to see. This has become my standard method for dealing with the switch from calm to wind/thermal conditions. The CG location is an important component of stability. Simply stated, the farther back the CG. the less decalage is needed. You can get a model to glide with the CG at 100% (i.e.. at the wing trailing edge) with perhaps 1 ° of decalage. Or you can move the CG forward to 50% (wing midchord) and jack up the stab trailing edge to give approximately 3° of decalage. Which is best? The advantage of the more-reward CG is that less decalage is required. But why would you want less decalage? Not for the glide, but for the climb. A model with low decalage will be less likely to loop under power. In the days before the use of autosurfaces, a typical CG location for a Power model was between 80 and 100% of the wing chord; sometimes it was even behind the trailing edge. Often, the wing and stab were set at zero decalage, with only washin on the right main panel supplying some measure of decalage. It was often a delicate balancing act. Too little decalage would mean an over-the-top dive into the ground. Too much decalage could result in a loop. Spiraling the model up helped turn the loop into a safer spiral climb. But the limited decalage could also cause problems in the glide— most noticeably a spiral dive. With the introduction of reliable timer-operated auto stabs (also called VIT—variable incidence tailplane), the climb and glide phases could be adjusted separately. For the climb, the decalage was reduced to near zero. For the glide, the decalage was increased to roughly 3°. To make the glide work with the increased decalage. the CG was moved forward to approximately 55%. (Note that the CG did not move during the flight.) Because the high-speed climb and the slower glide segments ol' the flight could each be adjusted without affecting the other, trimming the model became much easier. The preceding is a simplified explanation ol the factors affecting longitudinal stability. Other factors can affect stability, such as stabilizer aspect ratio, location of the stab relative to the wing wake, and nose length. There is also a relationship between the tail volume and the optimum CG locution for a non-auto Power model. (Tail volume is determined by dividing the stab area by the wing area then multiplying that by the tail moment expressed in multiples ol the wing chord.) A RamRod 250 has a stab area of 117 square inches, a flat wing area of 263 square inches, a tail moment of 24.7 inches, and a wing mean chord of 6.5 inches. That gives a tail volume of 1.69. By plotting the tail volumes and CG ,, positions of several Power models. Bill ' Bogart and Bud Rhodes developed a graph that was published in the January 1959 Model Airplane News. A diagonal line connecting the plots for the models suggested a stable starting place for the CG location for a given tail volume. If the CG was ahead of the location shown on the plot, the model would tend to loop under power; if the CG was behind the location shown, the model would tend to dive under power. The decalage would need to be adjusted to give a good glide based on the CG location and tail moment arm. If I have learned one thing in the last 20 years, it is to check the CG if you have problems with stalling or a too-fast glide. If the CG is in the right place, then and only then should you start moving the stab trailing edge up or down. If the CG is not in the right place, break out the lead. The flying field is not the place to check the CG. Do it in the shop, out of the wind. Balancing the model on your fingertips is not accurate enough. A simple knife-edge fixture of balsa will allow you to adjust the CG to within 1 % of the chord. Remember too that the CG should be based on the wing's average chord—not its root chord. You also need to allow for tapered or back-swept wings. Once you determine the correct CG location for your model, mark it on the bottom of the wings. Remember to check the CG from time to time. Repairs and patches can cause a shift, usually to the rear. Free Flight Forum 2002: Like our National Free Flight Society (NFFS) Symposium, the British Forum is an annual publication devoted to the art and science of FF. The 2002 edition of the Forum offers wide-ranging articles on a variety of topics. This year the emphasis was more on the practical rather than the theoretical aspects of FF; in other words, there is a lot of useful information and not a lot of formulae. Here's a run-down of the articles: • "Electric Free Flight" by John Godden: Although electric power has become popular for a variety of Radio Control events, its popularity in FF lags far behind. John's article should help rectify the situation by offering specific information on battery, motor, and propeller selection, and wiring. He includes several sketches of timer-operated switches for motor shutoff. • "The 30 Gram Fl B" by Mike Woodhouse: The new, smaller motor for Wakefield will impact performance and, perhaps, model design. Mike explores all the options, from the number of strands used to the possible future implications such as single-blade propellers or variable-diameter propellers. • "Developments in Slow Open Power" by Dave Clarkson: With the new Classic Power event getting started in this country, it would be useful to look at what the British have been doing with their Slow Open Power event. Dave traces his own SLOP models, dating back to the mid-1980s. One item he discusses is the relative merits of various wing dihedral layouts, comparing the typical American scheme with the main panel twice the length of the tip to the British layout with the tip panel two-thirds the length of the main panel. He also includes a plot of tail volume relative to the CG. • "Free Flight Diversity—Strength or Weakness" by Chris Strachan: Britain, like the US, is a land of a thousand events. While the rest of the world concentrates on the Federation Aeronautique International (FAI) events, these two countries offer a seemingly endless variety of classes—too many in Chris's mind. Not only does this result in a dilution of effort, but it can also lead to confusion in contest administration. • "Computers in Free Flight—An Overview" by Mike Evatt: In addition to tracking down contest results and faraway parts suppliers on the Internet, a computer can be a useful tool for a variety of aeromodcling tasks. Mike explores the options in flight simulation, computer-aided design, and 3-D solid modeling. Perhaps most useful is a discussion about using a computer to produce full-size plans. • "Contests in Restricted Space" by John Godden: The subtitle "How to keep them on the airfield" tells it all. This paper explores ways to reduce performance, ranging from shortening the towline length or decreasing the engine run to the more difficult problem of dealing with Rubber models. For Rubber events, two schemes have been tried. One requires ballast equal to the motor weight. The other, adapted from standard indoor testing practice, uses a stick half the length of the motor base combined with a half-length motor. • "Flying the New Rule F IDs" by Bob Bailey: The new rules for the rubber-powered Indoor event require smaller, heavier, more robust models. The idea is to have models that are easier to build, transport, and Oy. In the United Kingdom, this has resulted in unbraced wings covered with Y2K2 plastic film instead of the more-difficult-to-handle microfilm. • "Propeller Pivoting for Flat Folding" by John Barker: What would a Svnipo or a Forum be without a few formulae? John's article offers a technical but easily understood look at the problem of getting a Rubber model's blades to fold flat against the fuselage for minimum drag. Although the narrow blades of a modern FIB or FIG don't require compound-angled hinges, the wide blades of Nostalgia Wakefields or Old-Time Rubber models do. • "Variable Pitch Propellers for Coupe D'Hiver" by John Bailey and Mike Evatt: Widely popular in the larger Fl B Wakefield Rubber event, variable-pitch (VP) propellers are gaining a foothold in the smaller FIG Coupe event. The authors explore a variety of approaches used in FIB and offer several suggestions for simplified, lighter versions for Coupe. Editors Martin Dilly and Mike Evatt have put together a most useful group of papers that should appeal to a variety of FF modelers. You can order a copy of Free Flight Forum 2002 from Martin Dilly. 20 Links Rd., West Wickham. Kent. BR4 OQW. UK. The price, postage-included, is eight pounds sterling (approximately $13). Checks should be payable to "BMFA FF Team Support Fund" and in pounds sterling, only drawn on a bank with a branch in the UK. You can also order by credit card via E-mail to [email protected] or by fax to 44+«))20 8777 5533. The British Free Flight Foruim are also available through NFFS Publications. Contact Bob McLinden, Box 7967. Baltimore MD 2 1221. or E-mail him at [email protected]. You can find ordering information on the NFFS Web site at http://freeflight.org.
Edition: Model Aviation - 2002/10
Page Numbers: 114, 115, 116, 118, 125
ON STABS AND STABILITY: Free Right (FF) is a balancing act. The aptly named horizontal stabilizer (stab) is an important component of that balancing act. However, we often take the stab for granted, shimming it up or down to get the glide right and popping it up for dethermalizer (DT), but otherwise paying it little attention. Basically, the stab's job is to hold the wing at the angle that will give the best glide. That's the easy part. When the model is gliding along comfortably, all the stab has to do is counteract the wing's nose-up tendency. This is usually done by setting the stab at a slight negative incidence relative to the wing. This difference between the wing and stab incidence settings is called decalage. But our models are not just designed to glide. They have to get up in the air first. In most cases, a model in the climb will be going faster than its glide speed. That will result in more lift from the wing. Unless that lift is balanced by increased lift in the stab, the result will be a loop. However, if the stab overbalances the increased wing lift, the result can be a dive. The amount of decalage required to balance the model in the glide depends on a number of factors, including the wing lift, the center of gravity (CG) location, the length of the stab's moment arm, and the stab airfoil. All things being equal, a wing with a higher lift, either from a bigger area or a more undercambered airfoil, will need less decalage than will a smaller wing or one with a lower-lift airfoil. A typical Hand-Launched Glider uses a thin, flat-bottomed airfoil to limit lift in the climb coupled with very little decalage to prevent looping in the launch. Another approach, sometimes used in F1C Power, is to change the wing airfoil from a flat-bottomed, low-lift airfoil in the climb to an undercambered, high-lift airfoil in the glide. This is normally done by flapping the rear third of the airfoil. Another approach, used by Russian flier Leonid Fuzeyev, is to fold the wing during the power phase, reducing the area and converting the undercambered airfoil into a thicker symmetrical shape. In most cases, however, the wing is optimized for maximum glide, and the stabilizer is used to regulate climb and glide. The stab's moment ami affects its ability to balance the lift of the wing, much like the grade-school math problem of the fat kid and the skinny kid on a seesaw. If the wing-to-tail lever is long, a smaller stab can be used. Fuselages were short in modeling's early days, often because of cross-section rules based on the model's overall length. If you wanted to use a long fuselage, you had to increase the fuselage cross-section, increasing drag. (Under the old length squared divided by 100 rule, a 36-inch-long model would require a cross-section of almost 13 square inches. A 48-inch-long model would need a cross-section of just more than 23 square inches.) The answer to the restrictive rules was to use a large stab, often one-third and sometimes more than one-half the wing area. At first glance this would seem like a good thing, since you were increasing total area. But to do its job properly, a stab will be flying at a lower angle of attack than the wing. Each square inch of wing area is producing more lift than each square inch of stab area. That's why models for events with limited total (i.e., wing plus stab) area pack as much area as possible into the wing, using as small a stabilizer as possible with a long moment arm. Typically, stabs are 20% of the wing area or less, with a moment arm ranging from seven to nine times the average wing chord. (Tail moment arm is usually measured from the quarter point of the wing mean chord to the quarter point of the stabilizer mean chord.) For FF. there is a lower limit for stab percentage. Too small a stab will result in poor stall recovery. Often, the model will fly fine in calm conditions but will stall to the ground when upset by wind or thermals. (The same thing can happen when we shim the trailing edge of the stab up too much, trying to wring the best possible glide out of the model.) Besides, the gains in total performance by using small stabs is limited. In his 1976 Sympo paper "Effect of Some Design Parameters on the Glide Performance of an A-2 Nordic Glider," Harry Grogan wrote: "From 164 feel, a 65 sq. in. stab (considered small by present standards) has about a 6 . second advantage over an 88 sq. in. stabilizer (considered large). Further reduction to 52 sq. in. produces an additional 3 seconds in the glide. It would appear that stabilizers below 14% are of little practical help except for very calm conditions." A related factor is the moment of inertia of the tail and aft part of the fuselage. A model with light extremities will react more quickly to slight upsets in climb and glide. I'm convinced that of all the high-tech features developed in the last 20 years, the lightweight carbon-aluminum tailboom may be the most important. In the days of rolled balsa tailbooms, the lightest I was ever able to manage for an Fl B model was roughly 18 grams; a carbon-aluminum tuilboom weighs between seven and eight grams. That coupled with the lighter, plastic-covered stab and rudder and a strict diet on all the fittings has resulted in a total weight for everything aft of the rear peg of 16-18 grams, versus 24-28 grams for the old models. The difference in the air is remarkable. It really does pay to keep the tail light! The stab airfoil is important—perhaps far more important than many of us think. In the late 1970s I was constantly plagued by models that would fly fine in calm air but develop a stall in the heat of the day. The answer, or so I thought, was to increase the stab lift to help hold the nose down and prevent the stall. Wings were cut down, and new, bigger stabs were built, often with thin, highly undercambered airfoils. Things kept getting worse. I even tried some of the slotted, two-part airfoils being touted. Things got even worse. After doing a good bit of reading that brought me to my senses, I switched to a semisymmetrical airfoil with a blunt leading edge. The Wobbeking 2.5-25-8 airfoil I use now has a leading-edge radius of 4% with the high point at 25%. Almost all the stalling problems have gone away, although the model will still stall if pushed too far. In the past I trimmed the model for the best possible glide under calm conditions, jacking up the back of the stab until the model was just off the stall. (A better way is to conduct a glide test, timing the model to the ground and adjusting the stab up or down to give maximum time. That's difficult to do at a small test field.) When the wind or thermal activity increased, the stab's trailing edge could be pulled down slightly to lessen the chance of a flight-spoiling stall. (As an alternative, the glide circle could be tightened up.) But messing with the stab setting presented two problems: it affected the glide and it affected the cruise portion of the climb. With the use of screw adjustments for the stab, there was, at least for me, the problem of keeping track of the setting. Did I screw it back down after the last flight? George Batiuk offered a much simpler alternative which he dubbed "The old Ukrainian trick." To prevent stalling in thermal or wind conditions, he moved the CG forward slightly. Strapping a spare wing wire on the nose of a Wakefield is easy to do, doesn't disturb the climb pattern, and, most importantly, is easy to see. This has become my standard method for dealing with the switch from calm to wind/thermal conditions. The CG location is an important component of stability. Simply stated, the farther back the CG. the less decalage is needed. You can get a model to glide with the CG at 100% (i.e.. at the wing trailing edge) with perhaps 1 ° of decalage. Or you can move the CG forward to 50% (wing midchord) and jack up the stab trailing edge to give approximately 3° of decalage. Which is best? The advantage of the more-reward CG is that less decalage is required. But why would you want less decalage? Not for the glide, but for the climb. A model with low decalage will be less likely to loop under power. In the days before the use of autosurfaces, a typical CG location for a Power model was between 80 and 100% of the wing chord; sometimes it was even behind the trailing edge. Often, the wing and stab were set at zero decalage, with only washin on the right main panel supplying some measure of decalage. It was often a delicate balancing act. Too little decalage would mean an over-the-top dive into the ground. Too much decalage could result in a loop. Spiraling the model up helped turn the loop into a safer spiral climb. But the limited decalage could also cause problems in the glide— most noticeably a spiral dive. With the introduction of reliable timer-operated auto stabs (also called VIT—variable incidence tailplane), the climb and glide phases could be adjusted separately. For the climb, the decalage was reduced to near zero. For the glide, the decalage was increased to roughly 3°. To make the glide work with the increased decalage. the CG was moved forward to approximately 55%. (Note that the CG did not move during the flight.) Because the high-speed climb and the slower glide segments ol' the flight could each be adjusted without affecting the other, trimming the model became much easier. The preceding is a simplified explanation ol the factors affecting longitudinal stability. Other factors can affect stability, such as stabilizer aspect ratio, location of the stab relative to the wing wake, and nose length. There is also a relationship between the tail volume and the optimum CG locution for a non-auto Power model. (Tail volume is determined by dividing the stab area by the wing area then multiplying that by the tail moment expressed in multiples ol the wing chord.) A RamRod 250 has a stab area of 117 square inches, a flat wing area of 263 square inches, a tail moment of 24.7 inches, and a wing mean chord of 6.5 inches. That gives a tail volume of 1.69. By plotting the tail volumes and CG ,, positions of several Power models. Bill ' Bogart and Bud Rhodes developed a graph that was published in the January 1959 Model Airplane News. A diagonal line connecting the plots for the models suggested a stable starting place for the CG location for a given tail volume. If the CG was ahead of the location shown on the plot, the model would tend to loop under power; if the CG was behind the location shown, the model would tend to dive under power. The decalage would need to be adjusted to give a good glide based on the CG location and tail moment arm. If I have learned one thing in the last 20 years, it is to check the CG if you have problems with stalling or a too-fast glide. If the CG is in the right place, then and only then should you start moving the stab trailing edge up or down. If the CG is not in the right place, break out the lead. The flying field is not the place to check the CG. Do it in the shop, out of the wind. Balancing the model on your fingertips is not accurate enough. A simple knife-edge fixture of balsa will allow you to adjust the CG to within 1 % of the chord. Remember too that the CG should be based on the wing's average chord—not its root chord. You also need to allow for tapered or back-swept wings. Once you determine the correct CG location for your model, mark it on the bottom of the wings. Remember to check the CG from time to time. Repairs and patches can cause a shift, usually to the rear. Free Flight Forum 2002: Like our National Free Flight Society (NFFS) Symposium, the British Forum is an annual publication devoted to the art and science of FF. The 2002 edition of the Forum offers wide-ranging articles on a variety of topics. This year the emphasis was more on the practical rather than the theoretical aspects of FF; in other words, there is a lot of useful information and not a lot of formulae. Here's a run-down of the articles: • "Electric Free Flight" by John Godden: Although electric power has become popular for a variety of Radio Control events, its popularity in FF lags far behind. John's article should help rectify the situation by offering specific information on battery, motor, and propeller selection, and wiring. He includes several sketches of timer-operated switches for motor shutoff. • "The 30 Gram Fl B" by Mike Woodhouse: The new, smaller motor for Wakefield will impact performance and, perhaps, model design. Mike explores all the options, from the number of strands used to the possible future implications such as single-blade propellers or variable-diameter propellers. • "Developments in Slow Open Power" by Dave Clarkson: With the new Classic Power event getting started in this country, it would be useful to look at what the British have been doing with their Slow Open Power event. Dave traces his own SLOP models, dating back to the mid-1980s. One item he discusses is the relative merits of various wing dihedral layouts, comparing the typical American scheme with the main panel twice the length of the tip to the British layout with the tip panel two-thirds the length of the main panel. He also includes a plot of tail volume relative to the CG. • "Free Flight Diversity—Strength or Weakness" by Chris Strachan: Britain, like the US, is a land of a thousand events. While the rest of the world concentrates on the Federation Aeronautique International (FAI) events, these two countries offer a seemingly endless variety of classes—too many in Chris's mind. Not only does this result in a dilution of effort, but it can also lead to confusion in contest administration. • "Computers in Free Flight—An Overview" by Mike Evatt: In addition to tracking down contest results and faraway parts suppliers on the Internet, a computer can be a useful tool for a variety of aeromodcling tasks. Mike explores the options in flight simulation, computer-aided design, and 3-D solid modeling. Perhaps most useful is a discussion about using a computer to produce full-size plans. • "Contests in Restricted Space" by John Godden: The subtitle "How to keep them on the airfield" tells it all. This paper explores ways to reduce performance, ranging from shortening the towline length or decreasing the engine run to the more difficult problem of dealing with Rubber models. For Rubber events, two schemes have been tried. One requires ballast equal to the motor weight. The other, adapted from standard indoor testing practice, uses a stick half the length of the motor base combined with a half-length motor. • "Flying the New Rule F IDs" by Bob Bailey: The new rules for the rubber-powered Indoor event require smaller, heavier, more robust models. The idea is to have models that are easier to build, transport, and Oy. In the United Kingdom, this has resulted in unbraced wings covered with Y2K2 plastic film instead of the more-difficult-to-handle microfilm. • "Propeller Pivoting for Flat Folding" by John Barker: What would a Svnipo or a Forum be without a few formulae? John's article offers a technical but easily understood look at the problem of getting a Rubber model's blades to fold flat against the fuselage for minimum drag. Although the narrow blades of a modern FIB or FIG don't require compound-angled hinges, the wide blades of Nostalgia Wakefields or Old-Time Rubber models do. • "Variable Pitch Propellers for Coupe D'Hiver" by John Bailey and Mike Evatt: Widely popular in the larger Fl B Wakefield Rubber event, variable-pitch (VP) propellers are gaining a foothold in the smaller FIG Coupe event. The authors explore a variety of approaches used in FIB and offer several suggestions for simplified, lighter versions for Coupe. Editors Martin Dilly and Mike Evatt have put together a most useful group of papers that should appeal to a variety of FF modelers. You can order a copy of Free Flight Forum 2002 from Martin Dilly. 20 Links Rd., West Wickham. Kent. BR4 OQW. UK. The price, postage-included, is eight pounds sterling (approximately $13). Checks should be payable to "BMFA FF Team Support Fund" and in pounds sterling, only drawn on a bank with a branch in the UK. You can also order by credit card via E-mail to [email protected] or by fax to 44+«))20 8777 5533. The British Free Flight Foruim are also available through NFFS Publications. Contact Bob McLinden, Box 7967. Baltimore MD 2 1221. or E-mail him at [email protected]. You can find ordering information on the NFFS Web site at http://freeflight.org.
Edition: Model Aviation - 2002/10
Page Numbers: 114, 115, 116, 118, 125
ON STABS AND STABILITY: Free Right (FF) is a balancing act. The aptly named horizontal stabilizer (stab) is an important component of that balancing act. However, we often take the stab for granted, shimming it up or down to get the glide right and popping it up for dethermalizer (DT), but otherwise paying it little attention. Basically, the stab's job is to hold the wing at the angle that will give the best glide. That's the easy part. When the model is gliding along comfortably, all the stab has to do is counteract the wing's nose-up tendency. This is usually done by setting the stab at a slight negative incidence relative to the wing. This difference between the wing and stab incidence settings is called decalage. But our models are not just designed to glide. They have to get up in the air first. In most cases, a model in the climb will be going faster than its glide speed. That will result in more lift from the wing. Unless that lift is balanced by increased lift in the stab, the result will be a loop. However, if the stab overbalances the increased wing lift, the result can be a dive. The amount of decalage required to balance the model in the glide depends on a number of factors, including the wing lift, the center of gravity (CG) location, the length of the stab's moment arm, and the stab airfoil. All things being equal, a wing with a higher lift, either from a bigger area or a more undercambered airfoil, will need less decalage than will a smaller wing or one with a lower-lift airfoil. A typical Hand-Launched Glider uses a thin, flat-bottomed airfoil to limit lift in the climb coupled with very little decalage to prevent looping in the launch. Another approach, sometimes used in F1C Power, is to change the wing airfoil from a flat-bottomed, low-lift airfoil in the climb to an undercambered, high-lift airfoil in the glide. This is normally done by flapping the rear third of the airfoil. Another approach, used by Russian flier Leonid Fuzeyev, is to fold the wing during the power phase, reducing the area and converting the undercambered airfoil into a thicker symmetrical shape. In most cases, however, the wing is optimized for maximum glide, and the stabilizer is used to regulate climb and glide. The stab's moment ami affects its ability to balance the lift of the wing, much like the grade-school math problem of the fat kid and the skinny kid on a seesaw. If the wing-to-tail lever is long, a smaller stab can be used. Fuselages were short in modeling's early days, often because of cross-section rules based on the model's overall length. If you wanted to use a long fuselage, you had to increase the fuselage cross-section, increasing drag. (Under the old length squared divided by 100 rule, a 36-inch-long model would require a cross-section of almost 13 square inches. A 48-inch-long model would need a cross-section of just more than 23 square inches.) The answer to the restrictive rules was to use a large stab, often one-third and sometimes more than one-half the wing area. At first glance this would seem like a good thing, since you were increasing total area. But to do its job properly, a stab will be flying at a lower angle of attack than the wing. Each square inch of wing area is producing more lift than each square inch of stab area. That's why models for events with limited total (i.e., wing plus stab) area pack as much area as possible into the wing, using as small a stabilizer as possible with a long moment arm. Typically, stabs are 20% of the wing area or less, with a moment arm ranging from seven to nine times the average wing chord. (Tail moment arm is usually measured from the quarter point of the wing mean chord to the quarter point of the stabilizer mean chord.) For FF. there is a lower limit for stab percentage. Too small a stab will result in poor stall recovery. Often, the model will fly fine in calm conditions but will stall to the ground when upset by wind or thermals. (The same thing can happen when we shim the trailing edge of the stab up too much, trying to wring the best possible glide out of the model.) Besides, the gains in total performance by using small stabs is limited. In his 1976 Sympo paper "Effect of Some Design Parameters on the Glide Performance of an A-2 Nordic Glider," Harry Grogan wrote: "From 164 feel, a 65 sq. in. stab (considered small by present standards) has about a 6 . second advantage over an 88 sq. in. stabilizer (considered large). Further reduction to 52 sq. in. produces an additional 3 seconds in the glide. It would appear that stabilizers below 14% are of little practical help except for very calm conditions." A related factor is the moment of inertia of the tail and aft part of the fuselage. A model with light extremities will react more quickly to slight upsets in climb and glide. I'm convinced that of all the high-tech features developed in the last 20 years, the lightweight carbon-aluminum tailboom may be the most important. In the days of rolled balsa tailbooms, the lightest I was ever able to manage for an Fl B model was roughly 18 grams; a carbon-aluminum tuilboom weighs between seven and eight grams. That coupled with the lighter, plastic-covered stab and rudder and a strict diet on all the fittings has resulted in a total weight for everything aft of the rear peg of 16-18 grams, versus 24-28 grams for the old models. The difference in the air is remarkable. It really does pay to keep the tail light! The stab airfoil is important—perhaps far more important than many of us think. In the late 1970s I was constantly plagued by models that would fly fine in calm air but develop a stall in the heat of the day. The answer, or so I thought, was to increase the stab lift to help hold the nose down and prevent the stall. Wings were cut down, and new, bigger stabs were built, often with thin, highly undercambered airfoils. Things kept getting worse. I even tried some of the slotted, two-part airfoils being touted. Things got even worse. After doing a good bit of reading that brought me to my senses, I switched to a semisymmetrical airfoil with a blunt leading edge. The Wobbeking 2.5-25-8 airfoil I use now has a leading-edge radius of 4% with the high point at 25%. Almost all the stalling problems have gone away, although the model will still stall if pushed too far. In the past I trimmed the model for the best possible glide under calm conditions, jacking up the back of the stab until the model was just off the stall. (A better way is to conduct a glide test, timing the model to the ground and adjusting the stab up or down to give maximum time. That's difficult to do at a small test field.) When the wind or thermal activity increased, the stab's trailing edge could be pulled down slightly to lessen the chance of a flight-spoiling stall. (As an alternative, the glide circle could be tightened up.) But messing with the stab setting presented two problems: it affected the glide and it affected the cruise portion of the climb. With the use of screw adjustments for the stab, there was, at least for me, the problem of keeping track of the setting. Did I screw it back down after the last flight? George Batiuk offered a much simpler alternative which he dubbed "The old Ukrainian trick." To prevent stalling in thermal or wind conditions, he moved the CG forward slightly. Strapping a spare wing wire on the nose of a Wakefield is easy to do, doesn't disturb the climb pattern, and, most importantly, is easy to see. This has become my standard method for dealing with the switch from calm to wind/thermal conditions. The CG location is an important component of stability. Simply stated, the farther back the CG. the less decalage is needed. You can get a model to glide with the CG at 100% (i.e.. at the wing trailing edge) with perhaps 1 ° of decalage. Or you can move the CG forward to 50% (wing midchord) and jack up the stab trailing edge to give approximately 3° of decalage. Which is best? The advantage of the more-reward CG is that less decalage is required. But why would you want less decalage? Not for the glide, but for the climb. A model with low decalage will be less likely to loop under power. In the days before the use of autosurfaces, a typical CG location for a Power model was between 80 and 100% of the wing chord; sometimes it was even behind the trailing edge. Often, the wing and stab were set at zero decalage, with only washin on the right main panel supplying some measure of decalage. It was often a delicate balancing act. Too little decalage would mean an over-the-top dive into the ground. Too much decalage could result in a loop. Spiraling the model up helped turn the loop into a safer spiral climb. But the limited decalage could also cause problems in the glide— most noticeably a spiral dive. With the introduction of reliable timer-operated auto stabs (also called VIT—variable incidence tailplane), the climb and glide phases could be adjusted separately. For the climb, the decalage was reduced to near zero. For the glide, the decalage was increased to roughly 3°. To make the glide work with the increased decalage. the CG was moved forward to approximately 55%. (Note that the CG did not move during the flight.) Because the high-speed climb and the slower glide segments ol' the flight could each be adjusted without affecting the other, trimming the model became much easier. The preceding is a simplified explanation ol the factors affecting longitudinal stability. Other factors can affect stability, such as stabilizer aspect ratio, location of the stab relative to the wing wake, and nose length. There is also a relationship between the tail volume and the optimum CG locution for a non-auto Power model. (Tail volume is determined by dividing the stab area by the wing area then multiplying that by the tail moment expressed in multiples ol the wing chord.) A RamRod 250 has a stab area of 117 square inches, a flat wing area of 263 square inches, a tail moment of 24.7 inches, and a wing mean chord of 6.5 inches. That gives a tail volume of 1.69. By plotting the tail volumes and CG ,, positions of several Power models. Bill ' Bogart and Bud Rhodes developed a graph that was published in the January 1959 Model Airplane News. A diagonal line connecting the plots for the models suggested a stable starting place for the CG location for a given tail volume. If the CG was ahead of the location shown on the plot, the model would tend to loop under power; if the CG was behind the location shown, the model would tend to dive under power. The decalage would need to be adjusted to give a good glide based on the CG location and tail moment arm. If I have learned one thing in the last 20 years, it is to check the CG if you have problems with stalling or a too-fast glide. If the CG is in the right place, then and only then should you start moving the stab trailing edge up or down. If the CG is not in the right place, break out the lead. The flying field is not the place to check the CG. Do it in the shop, out of the wind. Balancing the model on your fingertips is not accurate enough. A simple knife-edge fixture of balsa will allow you to adjust the CG to within 1 % of the chord. Remember too that the CG should be based on the wing's average chord—not its root chord. You also need to allow for tapered or back-swept wings. Once you determine the correct CG location for your model, mark it on the bottom of the wings. Remember to check the CG from time to time. Repairs and patches can cause a shift, usually to the rear. Free Flight Forum 2002: Like our National Free Flight Society (NFFS) Symposium, the British Forum is an annual publication devoted to the art and science of FF. The 2002 edition of the Forum offers wide-ranging articles on a variety of topics. This year the emphasis was more on the practical rather than the theoretical aspects of FF; in other words, there is a lot of useful information and not a lot of formulae. Here's a run-down of the articles: • "Electric Free Flight" by John Godden: Although electric power has become popular for a variety of Radio Control events, its popularity in FF lags far behind. John's article should help rectify the situation by offering specific information on battery, motor, and propeller selection, and wiring. He includes several sketches of timer-operated switches for motor shutoff. • "The 30 Gram Fl B" by Mike Woodhouse: The new, smaller motor for Wakefield will impact performance and, perhaps, model design. Mike explores all the options, from the number of strands used to the possible future implications such as single-blade propellers or variable-diameter propellers. • "Developments in Slow Open Power" by Dave Clarkson: With the new Classic Power event getting started in this country, it would be useful to look at what the British have been doing with their Slow Open Power event. Dave traces his own SLOP models, dating back to the mid-1980s. One item he discusses is the relative merits of various wing dihedral layouts, comparing the typical American scheme with the main panel twice the length of the tip to the British layout with the tip panel two-thirds the length of the main panel. He also includes a plot of tail volume relative to the CG. • "Free Flight Diversity—Strength or Weakness" by Chris Strachan: Britain, like the US, is a land of a thousand events. While the rest of the world concentrates on the Federation Aeronautique International (FAI) events, these two countries offer a seemingly endless variety of classes—too many in Chris's mind. Not only does this result in a dilution of effort, but it can also lead to confusion in contest administration. • "Computers in Free Flight—An Overview" by Mike Evatt: In addition to tracking down contest results and faraway parts suppliers on the Internet, a computer can be a useful tool for a variety of aeromodcling tasks. Mike explores the options in flight simulation, computer-aided design, and 3-D solid modeling. Perhaps most useful is a discussion about using a computer to produce full-size plans. • "Contests in Restricted Space" by John Godden: The subtitle "How to keep them on the airfield" tells it all. This paper explores ways to reduce performance, ranging from shortening the towline length or decreasing the engine run to the more difficult problem of dealing with Rubber models. For Rubber events, two schemes have been tried. One requires ballast equal to the motor weight. The other, adapted from standard indoor testing practice, uses a stick half the length of the motor base combined with a half-length motor. • "Flying the New Rule F IDs" by Bob Bailey: The new rules for the rubber-powered Indoor event require smaller, heavier, more robust models. The idea is to have models that are easier to build, transport, and Oy. In the United Kingdom, this has resulted in unbraced wings covered with Y2K2 plastic film instead of the more-difficult-to-handle microfilm. • "Propeller Pivoting for Flat Folding" by John Barker: What would a Svnipo or a Forum be without a few formulae? John's article offers a technical but easily understood look at the problem of getting a Rubber model's blades to fold flat against the fuselage for minimum drag. Although the narrow blades of a modern FIB or FIG don't require compound-angled hinges, the wide blades of Nostalgia Wakefields or Old-Time Rubber models do. • "Variable Pitch Propellers for Coupe D'Hiver" by John Bailey and Mike Evatt: Widely popular in the larger Fl B Wakefield Rubber event, variable-pitch (VP) propellers are gaining a foothold in the smaller FIG Coupe event. The authors explore a variety of approaches used in FIB and offer several suggestions for simplified, lighter versions for Coupe. Editors Martin Dilly and Mike Evatt have put together a most useful group of papers that should appeal to a variety of FF modelers. You can order a copy of Free Flight Forum 2002 from Martin Dilly. 20 Links Rd., West Wickham. Kent. BR4 OQW. UK. The price, postage-included, is eight pounds sterling (approximately $13). Checks should be payable to "BMFA FF Team Support Fund" and in pounds sterling, only drawn on a bank with a branch in the UK. You can also order by credit card via E-mail to [email protected] or by fax to 44+«))20 8777 5533. The British Free Flight Foruim are also available through NFFS Publications. Contact Bob McLinden, Box 7967. Baltimore MD 2 1221. or E-mail him at [email protected]. You can find ordering information on the NFFS Web site at http://freeflight.org.