YOUR DISTANCE vision not what it used to be? Do you ever have trouble working thermals with an aileron sailplane? Do you bump into a thermal but lose track of it before you want? Is it tough to fly smooth when the model gets far away? Do the polyhedral gliders consistently outclimb your aileron glider? If you answered "yes" to any of these questions, I have a simple solution. It is possible to build aileron gliders that thermal as easily as polyhedral gliders. These models have large dihedral angles and ailerons, which give you the stability of the polyhedral and the maneuverability of ailerons. I got interested in the effect of dihedral in 1997. Blaine Beron-Rawdon wrote an authoritative series of six articles on the effect of dihedral in Model Aviation in 1988 ("Dihedral") and 1990 ("Spiral Stability and the Bowl Effect"). I located the articles and studied them. Frank Baldwin and I built Two-Meter airplanes from scratch in 1998. The models' triple-taper wing cores were identical. They had the SA7035 airfoil at the root, transitioning to the SA7038 at the wingtip. I built my Two-Meter with 16° of dihedral. Frank used the standard 5°. I used a conventional horizontal stabilizer with separate elevator. Frank used an all-moving stabilizer mounted a few inches up the vertical fin. Each airplane weighed approximately 38 ounces and was winchproof. Frank and I standardized our wing-bolt pattern and our wing-wiring-harness connectors so that we could swap wings. Trading out one component at a time is a good way to isolate the effect of changes. We flew our gliders together for a couple years. On a couple of occasions we swapped wings in the middle of a day of flying. Our observations are consistent with Blaine Beron-Rawdon's conclusions. 1) The increased dihedral does not decrease the available roll rate. Frank's model and my model roll equally fast with equal aileron control throws. 2) The increased dihedral did not affect the lift-to-drag or sink rate a perceptible amount. 3) Frank's 5° wing must be watched at all times. On a day with a normal level of thermal turbulence, it would roll into a dive in a short period of time if the pilot did not actively control it. My 16° wing did not have this roll-in problem; it is fully capable of sustained free flight. 4) Thermaling the 5° wing required a lot of concentration and frequent corrections. It was only stable in relatively fast circles. Thermaling the 16° wing was easy; the transmitter stick could be held in one spot. It could fly slow, efficient circles. From "Spiral Stability and the Bowl Effect," the spiral stability factor (SS) = EDA x Lv/span/Ci. Lv is the vertical tail-moment arm to the center of gravity, Ci is the coefficient of lift, and EDA is the equivalent dihedral angle. Spiral stability is thus directly proportional to the dihedral angle and the moment arm of the vertical tail. This Did you ever wish your aileron glider was as easy to thermal as a polyhedral glider? Text tells how this can be done. These Two-Meter gliders were used to test the effect of adding dihedral. The one in the foreground has 16° of dihedral; the one in the back has 5°. This data is from Blaine Beron-Rawdon's "Dihedral" series of four articles in Model Aviation in 1988. explains why a long-boom, flat-wing Hand-Launched Glider such as the Logic could thermal well. According to Blaine, a glider is spirally stable if SS is greater than or equal to 5.7. We can rearrange the equation to solve for what Ci will make an airplane spirally stable: EDA x Lv/span/5.7 = Ci. For a typical full-house thermal duration (TD) airplane, EDA = 5.0 and Lv/span = 0.3. Plugging these numbers in. one can show that the typical full-house glider is spirally stable when Ci is less than 0.22. The ideal Ci for minimizing sink rate thus maximizing climb rate is usually approximately 0.7 for fast multitask airfoils and roughly 1.0 for floaters. So the full-house TD model with 5° EDA is spirally stable as long as you fly fast circles. When you try to slow down to maximize the climb rate, you must fight the roll-in tendency with outward aileron. Many pilots have told me that their low-dihedral airplanes do not have a roll-in tendency. I can say with certainty that they were circling much faster than optimum in the thermals. Why do the manufacturers keep coming out with more low-dihedral airplanes? I speculate that the best pilots don't care if their models are spirally stable at low speeds. They get five minutes of time from a good launch in no lift. Seven- to 10-minute maxes are the norm in contests. Once a pilot finds a thermal, a max is virtually assured. In most cases, the pilot does not have to thermal efficiently to get that additional two to five minutes worth of altitude; loitering in the thermal (or using fast circles) is usually enough. Also, the good pilots are willing to cross control with their ailerons (assuming that the airplane is close enough for them to see what it needs). Another factor in TD contests keeps the market wanting low-dihedral airplanes: there is a widespread belief that models with ample dihedral cannot land accurately in wind. This belief started when people tried to land their polyhedral, nonaileron gliders in turbulence. When a Rudder-Elevator-Spoiler glider is rolled away from level by a gust, it can take a couple seconds of full opposite rudder to get back to level. The heading of the glider may be 90° off by the time the wings are level. After flying several aileron gliders with 16° of dihedral, I can tell you that it is the ailerons that matter. Gliders with ailerons are easy to keep on course, whether they have low or high dihedral. During a three-year period, I don't recall missing any landings from being blown sideways. It was easy to keep the wings close to level, even in turbulence and/or Crosswinds. I had a pleasant surprise with the high-dihedral, full-house airplane: I could raise the ailerons 60° and still have decent roll control. My ailerons and rudder are coupled in all flight modes except reflex. The rudder working with the high dihedral provides roll control in CROW mode. Ninety-degree down flaps and 60° up ailerons allowed the model to come in at a speed that seemed impossible for the wing loading. All the pilots who have flown the design say that it is the easiest-landing airplane they have ever flown. On windy days, my high-dihedral Two-Meter had another unexpected advantage. When the thermals are blowing through quickly, the airplane must drift quickly with them. When I find a thermal, I initiate a turn then hold my control stick steady (off center). This proved to be a huge advantage when drifting with thermals. Frank's airplane with 5° EDA required frequent adjustment of the ailerons and rudder. With a moving frame of reference, it was easy for Frank to lose the thermal because his inputs thus his circle center were changing. One windy day my model hooked a thermal and climbed out in four of four launches. Frank's found the lift but always lost it within a few circles. Then we traded airplanes; Frank's easily climbed out, and the model I had could not stay with the lift. Remember that these airplanes were basically the same except for dihedral angles. The high dihedral was also a great benefit when flying far away. Even when the airplane looked like a flashing piece of Second full-house bent-wing (16°) glider Joe Bishop has built. Jerry Coffin photo. Dihedral and ailerons can work great together on HLGs too. The author's has beaten stiff competition and is a joy to fly. dust on the horizon, I could still fly (and thermal) with confidence. Confidence and low mental workload were great benefits of the high-dihedral aileron design. I could concentrate on placement relative to the core of a thermal and not worry about coordinating turns. I could also look away from my airplane to see what other pilots were doing. There were two minor drawbacks to my airplane. First, the high dihedral looks ugly on the ground. Tom Kiesling dubbed my bent wing "the pigeon." It looks more natural when it is thermaling. Second, the high-dihedral airplane is slightly more sensitive to how it is released for winch launches. As with all polyhedral airplanes, a crooked release will cause it to dart to one side. This can easily be avoided by throwing the model up rather than forward. Mark Drela's Super Gee proved that discus-launch airplanes can have high dihedral. He started with 12° and now recommends 14°. Some pilots have tried ailerons on a polyhedral model and have run into problems. If the airplane is short-coupled, has inadequate vertical tail, and/or has heavy extremities, it can lead to "Dutch roll," which is an unwanted oscillation where the tail swings back and forth fed by a yaw-roll coupling. From my experience I can give parameters that are known to work well on winch-launched airplanes. My vertical tail is 7.4% of the wing area. My vertical tail volumes have been between .023 and .026.1 have not yet had a poor-handling high-dihedral aileron airplane, so I do not know the lower limit of these parameters. I recommend starting with the Super Gee layout for a discus glider. Following is Blaine's recommendation from part four of "Dihedral." "Absolute minimum EDA (equivalent dihedral angle) of 10 degrees (much more like it is 15° EDA). Vertical tail moment arm of 35% to 45% of the wingspan. Vertical tail area of 5% to 6% of the wing area. Keep the wing tips and the tail group light!" Another potential problem with a high-dihedral aileron model is adverse yaw, which is where the drag, because of a down aileron, causes a yaw in the direction opposite the turn. In my experience, adverse yaw is easily avoided with 80- to 100% differential in the ailerons. After testing the high-dihedral full-house glider concept for several years with the help of Frank Baldwin, Joe Bishop, and Jerry Coffin, I strongly believe that all thermal pilots would benefit from at least 10° of dihedral. I used to think that maybe the top 5% of the pilots didn't need high dihedral. Now I am convinced that even the best pilots will fly better with 10 or more degrees of dihedral. With increased dihedral, they can fly out to greater distances. They can thermal more efficiently, especially at long distances. They will also have more brainpower left to think about strategy and/or placement of the glider. It does not harm the glider's landing ability. The high dihedral made flying easier. It alone did not win contests for me. With my high-dihedral Two-Meter and Hand-Launched Gliders, I placed significantly higher than my experience and preparation would suggest. Almost all I could hope for were higher placings than I deserve while having an easy, fun-to-fly model. Now if I could just figure out how to make it prettier ... AM References: "Dihedral" (four-part series) Blaine Beron-Rawdon August 1988-November 1988 Model Aviation "Spiral Stability and the Bowl Effect" Blaine Beron-Rawdon September 1990-October \990ModelAviation Plane Geometry Spreadsheet from Envision Design http://members.cox.net/evdesign/
Edition: Model Aviation - 2002/09
Page Numbers: 110, 111, 114
YOUR DISTANCE vision not what it used to be? Do you ever have trouble working thermals with an aileron sailplane? Do you bump into a thermal but lose track of it before you want? Is it tough to fly smooth when the model gets far away? Do the polyhedral gliders consistently outclimb your aileron glider? If you answered "yes" to any of these questions, I have a simple solution. It is possible to build aileron gliders that thermal as easily as polyhedral gliders. These models have large dihedral angles and ailerons, which give you the stability of the polyhedral and the maneuverability of ailerons. I got interested in the effect of dihedral in 1997. Blaine Beron-Rawdon wrote an authoritative series of six articles on the effect of dihedral in Model Aviation in 1988 ("Dihedral") and 1990 ("Spiral Stability and the Bowl Effect"). I located the articles and studied them. Frank Baldwin and I built Two-Meter airplanes from scratch in 1998. The models' triple-taper wing cores were identical. They had the SA7035 airfoil at the root, transitioning to the SA7038 at the wingtip. I built my Two-Meter with 16° of dihedral. Frank used the standard 5°. I used a conventional horizontal stabilizer with separate elevator. Frank used an all-moving stabilizer mounted a few inches up the vertical fin. Each airplane weighed approximately 38 ounces and was winchproof. Frank and I standardized our wing-bolt pattern and our wing-wiring-harness connectors so that we could swap wings. Trading out one component at a time is a good way to isolate the effect of changes. We flew our gliders together for a couple years. On a couple of occasions we swapped wings in the middle of a day of flying. Our observations are consistent with Blaine Beron-Rawdon's conclusions. 1) The increased dihedral does not decrease the available roll rate. Frank's model and my model roll equally fast with equal aileron control throws. 2) The increased dihedral did not affect the lift-to-drag or sink rate a perceptible amount. 3) Frank's 5° wing must be watched at all times. On a day with a normal level of thermal turbulence, it would roll into a dive in a short period of time if the pilot did not actively control it. My 16° wing did not have this roll-in problem; it is fully capable of sustained free flight. 4) Thermaling the 5° wing required a lot of concentration and frequent corrections. It was only stable in relatively fast circles. Thermaling the 16° wing was easy; the transmitter stick could be held in one spot. It could fly slow, efficient circles. From "Spiral Stability and the Bowl Effect," the spiral stability factor (SS) = EDA x Lv/span/Ci. Lv is the vertical tail-moment arm to the center of gravity, Ci is the coefficient of lift, and EDA is the equivalent dihedral angle. Spiral stability is thus directly proportional to the dihedral angle and the moment arm of the vertical tail. This Did you ever wish your aileron glider was as easy to thermal as a polyhedral glider? Text tells how this can be done. These Two-Meter gliders were used to test the effect of adding dihedral. The one in the foreground has 16° of dihedral; the one in the back has 5°. This data is from Blaine Beron-Rawdon's "Dihedral" series of four articles in Model Aviation in 1988. explains why a long-boom, flat-wing Hand-Launched Glider such as the Logic could thermal well. According to Blaine, a glider is spirally stable if SS is greater than or equal to 5.7. We can rearrange the equation to solve for what Ci will make an airplane spirally stable: EDA x Lv/span/5.7 = Ci. For a typical full-house thermal duration (TD) airplane, EDA = 5.0 and Lv/span = 0.3. Plugging these numbers in. one can show that the typical full-house glider is spirally stable when Ci is less than 0.22. The ideal Ci for minimizing sink rate thus maximizing climb rate is usually approximately 0.7 for fast multitask airfoils and roughly 1.0 for floaters. So the full-house TD model with 5° EDA is spirally stable as long as you fly fast circles. When you try to slow down to maximize the climb rate, you must fight the roll-in tendency with outward aileron. Many pilots have told me that their low-dihedral airplanes do not have a roll-in tendency. I can say with certainty that they were circling much faster than optimum in the thermals. Why do the manufacturers keep coming out with more low-dihedral airplanes? I speculate that the best pilots don't care if their models are spirally stable at low speeds. They get five minutes of time from a good launch in no lift. Seven- to 10-minute maxes are the norm in contests. Once a pilot finds a thermal, a max is virtually assured. In most cases, the pilot does not have to thermal efficiently to get that additional two to five minutes worth of altitude; loitering in the thermal (or using fast circles) is usually enough. Also, the good pilots are willing to cross control with their ailerons (assuming that the airplane is close enough for them to see what it needs). Another factor in TD contests keeps the market wanting low-dihedral airplanes: there is a widespread belief that models with ample dihedral cannot land accurately in wind. This belief started when people tried to land their polyhedral, nonaileron gliders in turbulence. When a Rudder-Elevator-Spoiler glider is rolled away from level by a gust, it can take a couple seconds of full opposite rudder to get back to level. The heading of the glider may be 90° off by the time the wings are level. After flying several aileron gliders with 16° of dihedral, I can tell you that it is the ailerons that matter. Gliders with ailerons are easy to keep on course, whether they have low or high dihedral. During a three-year period, I don't recall missing any landings from being blown sideways. It was easy to keep the wings close to level, even in turbulence and/or Crosswinds. I had a pleasant surprise with the high-dihedral, full-house airplane: I could raise the ailerons 60° and still have decent roll control. My ailerons and rudder are coupled in all flight modes except reflex. The rudder working with the high dihedral provides roll control in CROW mode. Ninety-degree down flaps and 60° up ailerons allowed the model to come in at a speed that seemed impossible for the wing loading. All the pilots who have flown the design say that it is the easiest-landing airplane they have ever flown. On windy days, my high-dihedral Two-Meter had another unexpected advantage. When the thermals are blowing through quickly, the airplane must drift quickly with them. When I find a thermal, I initiate a turn then hold my control stick steady (off center). This proved to be a huge advantage when drifting with thermals. Frank's airplane with 5° EDA required frequent adjustment of the ailerons and rudder. With a moving frame of reference, it was easy for Frank to lose the thermal because his inputs thus his circle center were changing. One windy day my model hooked a thermal and climbed out in four of four launches. Frank's found the lift but always lost it within a few circles. Then we traded airplanes; Frank's easily climbed out, and the model I had could not stay with the lift. Remember that these airplanes were basically the same except for dihedral angles. The high dihedral was also a great benefit when flying far away. Even when the airplane looked like a flashing piece of Second full-house bent-wing (16°) glider Joe Bishop has built. Jerry Coffin photo. Dihedral and ailerons can work great together on HLGs too. The author's has beaten stiff competition and is a joy to fly. dust on the horizon, I could still fly (and thermal) with confidence. Confidence and low mental workload were great benefits of the high-dihedral aileron design. I could concentrate on placement relative to the core of a thermal and not worry about coordinating turns. I could also look away from my airplane to see what other pilots were doing. There were two minor drawbacks to my airplane. First, the high dihedral looks ugly on the ground. Tom Kiesling dubbed my bent wing "the pigeon." It looks more natural when it is thermaling. Second, the high-dihedral airplane is slightly more sensitive to how it is released for winch launches. As with all polyhedral airplanes, a crooked release will cause it to dart to one side. This can easily be avoided by throwing the model up rather than forward. Mark Drela's Super Gee proved that discus-launch airplanes can have high dihedral. He started with 12° and now recommends 14°. Some pilots have tried ailerons on a polyhedral model and have run into problems. If the airplane is short-coupled, has inadequate vertical tail, and/or has heavy extremities, it can lead to "Dutch roll," which is an unwanted oscillation where the tail swings back and forth fed by a yaw-roll coupling. From my experience I can give parameters that are known to work well on winch-launched airplanes. My vertical tail is 7.4% of the wing area. My vertical tail volumes have been between .023 and .026.1 have not yet had a poor-handling high-dihedral aileron airplane, so I do not know the lower limit of these parameters. I recommend starting with the Super Gee layout for a discus glider. Following is Blaine's recommendation from part four of "Dihedral." "Absolute minimum EDA (equivalent dihedral angle) of 10 degrees (much more like it is 15° EDA). Vertical tail moment arm of 35% to 45% of the wingspan. Vertical tail area of 5% to 6% of the wing area. Keep the wing tips and the tail group light!" Another potential problem with a high-dihedral aileron model is adverse yaw, which is where the drag, because of a down aileron, causes a yaw in the direction opposite the turn. In my experience, adverse yaw is easily avoided with 80- to 100% differential in the ailerons. After testing the high-dihedral full-house glider concept for several years with the help of Frank Baldwin, Joe Bishop, and Jerry Coffin, I strongly believe that all thermal pilots would benefit from at least 10° of dihedral. I used to think that maybe the top 5% of the pilots didn't need high dihedral. Now I am convinced that even the best pilots will fly better with 10 or more degrees of dihedral. With increased dihedral, they can fly out to greater distances. They can thermal more efficiently, especially at long distances. They will also have more brainpower left to think about strategy and/or placement of the glider. It does not harm the glider's landing ability. The high dihedral made flying easier. It alone did not win contests for me. With my high-dihedral Two-Meter and Hand-Launched Gliders, I placed significantly higher than my experience and preparation would suggest. Almost all I could hope for were higher placings than I deserve while having an easy, fun-to-fly model. Now if I could just figure out how to make it prettier ... AM References: "Dihedral" (four-part series) Blaine Beron-Rawdon August 1988-November 1988 Model Aviation "Spiral Stability and the Bowl Effect" Blaine Beron-Rawdon September 1990-October \990ModelAviation Plane Geometry Spreadsheet from Envision Design http://members.cox.net/evdesign/
Edition: Model Aviation - 2002/09
Page Numbers: 110, 111, 114
YOUR DISTANCE vision not what it used to be? Do you ever have trouble working thermals with an aileron sailplane? Do you bump into a thermal but lose track of it before you want? Is it tough to fly smooth when the model gets far away? Do the polyhedral gliders consistently outclimb your aileron glider? If you answered "yes" to any of these questions, I have a simple solution. It is possible to build aileron gliders that thermal as easily as polyhedral gliders. These models have large dihedral angles and ailerons, which give you the stability of the polyhedral and the maneuverability of ailerons. I got interested in the effect of dihedral in 1997. Blaine Beron-Rawdon wrote an authoritative series of six articles on the effect of dihedral in Model Aviation in 1988 ("Dihedral") and 1990 ("Spiral Stability and the Bowl Effect"). I located the articles and studied them. Frank Baldwin and I built Two-Meter airplanes from scratch in 1998. The models' triple-taper wing cores were identical. They had the SA7035 airfoil at the root, transitioning to the SA7038 at the wingtip. I built my Two-Meter with 16° of dihedral. Frank used the standard 5°. I used a conventional horizontal stabilizer with separate elevator. Frank used an all-moving stabilizer mounted a few inches up the vertical fin. Each airplane weighed approximately 38 ounces and was winchproof. Frank and I standardized our wing-bolt pattern and our wing-wiring-harness connectors so that we could swap wings. Trading out one component at a time is a good way to isolate the effect of changes. We flew our gliders together for a couple years. On a couple of occasions we swapped wings in the middle of a day of flying. Our observations are consistent with Blaine Beron-Rawdon's conclusions. 1) The increased dihedral does not decrease the available roll rate. Frank's model and my model roll equally fast with equal aileron control throws. 2) The increased dihedral did not affect the lift-to-drag or sink rate a perceptible amount. 3) Frank's 5° wing must be watched at all times. On a day with a normal level of thermal turbulence, it would roll into a dive in a short period of time if the pilot did not actively control it. My 16° wing did not have this roll-in problem; it is fully capable of sustained free flight. 4) Thermaling the 5° wing required a lot of concentration and frequent corrections. It was only stable in relatively fast circles. Thermaling the 16° wing was easy; the transmitter stick could be held in one spot. It could fly slow, efficient circles. From "Spiral Stability and the Bowl Effect," the spiral stability factor (SS) = EDA x Lv/span/Ci. Lv is the vertical tail-moment arm to the center of gravity, Ci is the coefficient of lift, and EDA is the equivalent dihedral angle. Spiral stability is thus directly proportional to the dihedral angle and the moment arm of the vertical tail. This Did you ever wish your aileron glider was as easy to thermal as a polyhedral glider? Text tells how this can be done. These Two-Meter gliders were used to test the effect of adding dihedral. The one in the foreground has 16° of dihedral; the one in the back has 5°. This data is from Blaine Beron-Rawdon's "Dihedral" series of four articles in Model Aviation in 1988. explains why a long-boom, flat-wing Hand-Launched Glider such as the Logic could thermal well. According to Blaine, a glider is spirally stable if SS is greater than or equal to 5.7. We can rearrange the equation to solve for what Ci will make an airplane spirally stable: EDA x Lv/span/5.7 = Ci. For a typical full-house thermal duration (TD) airplane, EDA = 5.0 and Lv/span = 0.3. Plugging these numbers in. one can show that the typical full-house glider is spirally stable when Ci is less than 0.22. The ideal Ci for minimizing sink rate thus maximizing climb rate is usually approximately 0.7 for fast multitask airfoils and roughly 1.0 for floaters. So the full-house TD model with 5° EDA is spirally stable as long as you fly fast circles. When you try to slow down to maximize the climb rate, you must fight the roll-in tendency with outward aileron. Many pilots have told me that their low-dihedral airplanes do not have a roll-in tendency. I can say with certainty that they were circling much faster than optimum in the thermals. Why do the manufacturers keep coming out with more low-dihedral airplanes? I speculate that the best pilots don't care if their models are spirally stable at low speeds. They get five minutes of time from a good launch in no lift. Seven- to 10-minute maxes are the norm in contests. Once a pilot finds a thermal, a max is virtually assured. In most cases, the pilot does not have to thermal efficiently to get that additional two to five minutes worth of altitude; loitering in the thermal (or using fast circles) is usually enough. Also, the good pilots are willing to cross control with their ailerons (assuming that the airplane is close enough for them to see what it needs). Another factor in TD contests keeps the market wanting low-dihedral airplanes: there is a widespread belief that models with ample dihedral cannot land accurately in wind. This belief started when people tried to land their polyhedral, nonaileron gliders in turbulence. When a Rudder-Elevator-Spoiler glider is rolled away from level by a gust, it can take a couple seconds of full opposite rudder to get back to level. The heading of the glider may be 90° off by the time the wings are level. After flying several aileron gliders with 16° of dihedral, I can tell you that it is the ailerons that matter. Gliders with ailerons are easy to keep on course, whether they have low or high dihedral. During a three-year period, I don't recall missing any landings from being blown sideways. It was easy to keep the wings close to level, even in turbulence and/or Crosswinds. I had a pleasant surprise with the high-dihedral, full-house airplane: I could raise the ailerons 60° and still have decent roll control. My ailerons and rudder are coupled in all flight modes except reflex. The rudder working with the high dihedral provides roll control in CROW mode. Ninety-degree down flaps and 60° up ailerons allowed the model to come in at a speed that seemed impossible for the wing loading. All the pilots who have flown the design say that it is the easiest-landing airplane they have ever flown. On windy days, my high-dihedral Two-Meter had another unexpected advantage. When the thermals are blowing through quickly, the airplane must drift quickly with them. When I find a thermal, I initiate a turn then hold my control stick steady (off center). This proved to be a huge advantage when drifting with thermals. Frank's airplane with 5° EDA required frequent adjustment of the ailerons and rudder. With a moving frame of reference, it was easy for Frank to lose the thermal because his inputs thus his circle center were changing. One windy day my model hooked a thermal and climbed out in four of four launches. Frank's found the lift but always lost it within a few circles. Then we traded airplanes; Frank's easily climbed out, and the model I had could not stay with the lift. Remember that these airplanes were basically the same except for dihedral angles. The high dihedral was also a great benefit when flying far away. Even when the airplane looked like a flashing piece of Second full-house bent-wing (16°) glider Joe Bishop has built. Jerry Coffin photo. Dihedral and ailerons can work great together on HLGs too. The author's has beaten stiff competition and is a joy to fly. dust on the horizon, I could still fly (and thermal) with confidence. Confidence and low mental workload were great benefits of the high-dihedral aileron design. I could concentrate on placement relative to the core of a thermal and not worry about coordinating turns. I could also look away from my airplane to see what other pilots were doing. There were two minor drawbacks to my airplane. First, the high dihedral looks ugly on the ground. Tom Kiesling dubbed my bent wing "the pigeon." It looks more natural when it is thermaling. Second, the high-dihedral airplane is slightly more sensitive to how it is released for winch launches. As with all polyhedral airplanes, a crooked release will cause it to dart to one side. This can easily be avoided by throwing the model up rather than forward. Mark Drela's Super Gee proved that discus-launch airplanes can have high dihedral. He started with 12° and now recommends 14°. Some pilots have tried ailerons on a polyhedral model and have run into problems. If the airplane is short-coupled, has inadequate vertical tail, and/or has heavy extremities, it can lead to "Dutch roll," which is an unwanted oscillation where the tail swings back and forth fed by a yaw-roll coupling. From my experience I can give parameters that are known to work well on winch-launched airplanes. My vertical tail is 7.4% of the wing area. My vertical tail volumes have been between .023 and .026.1 have not yet had a poor-handling high-dihedral aileron airplane, so I do not know the lower limit of these parameters. I recommend starting with the Super Gee layout for a discus glider. Following is Blaine's recommendation from part four of "Dihedral." "Absolute minimum EDA (equivalent dihedral angle) of 10 degrees (much more like it is 15° EDA). Vertical tail moment arm of 35% to 45% of the wingspan. Vertical tail area of 5% to 6% of the wing area. Keep the wing tips and the tail group light!" Another potential problem with a high-dihedral aileron model is adverse yaw, which is where the drag, because of a down aileron, causes a yaw in the direction opposite the turn. In my experience, adverse yaw is easily avoided with 80- to 100% differential in the ailerons. After testing the high-dihedral full-house glider concept for several years with the help of Frank Baldwin, Joe Bishop, and Jerry Coffin, I strongly believe that all thermal pilots would benefit from at least 10° of dihedral. I used to think that maybe the top 5% of the pilots didn't need high dihedral. Now I am convinced that even the best pilots will fly better with 10 or more degrees of dihedral. With increased dihedral, they can fly out to greater distances. They can thermal more efficiently, especially at long distances. They will also have more brainpower left to think about strategy and/or placement of the glider. It does not harm the glider's landing ability. The high dihedral made flying easier. It alone did not win contests for me. With my high-dihedral Two-Meter and Hand-Launched Gliders, I placed significantly higher than my experience and preparation would suggest. Almost all I could hope for were higher placings than I deserve while having an easy, fun-to-fly model. Now if I could just figure out how to make it prettier ... AM References: "Dihedral" (four-part series) Blaine Beron-Rawdon August 1988-November 1988 Model Aviation "Spiral Stability and the Bowl Effect" Blaine Beron-Rawdon September 1990-October \990ModelAviation Plane Geometry Spreadsheet from Envision Design http://members.cox.net/evdesign/