TECHNICAL ARTICLE STUNT KITE AERODYNAMICS

TECHNICAL ARTICLE STUNT KITE AERODYNAMICS Written March 16, 1998 Updated January 26, 2010 By DOUGLAS K. STOUT "DESIGNING FOR THE FUTURE" 50 OLD STAGE COACH ROAD ANDOVER, NEW JERSEY 07821 (973) 786-7849 dkstout@ptd.net

1.0 INTRODUCTION This document provides the first technical article, which is entitled Technical Paper, Stunt Kite Aerodynamics. This technical article will allow you to calculate the basic aerodynamic platform of your stunt kite and evaluate your current bridle. 1.1 PURPOSE AND OBJECTIVE The purpose of this stunt kite aerodynamics technical article is to: Teach stunt kite enthusiasts the principals of aerodynamics as they apply to stunt kites; Introduce standard aerodynamic terminology so that stunt kite enthusiasts may discuss the aerodynamic platform of their stunt kite designs; Teach stunt kite enthusiasts how to calculated the aerodynamic platform of their stunt kite designs; and Enlighten stunt kite enthusiasts on how to optimize their stunt kite designs. The objective of this stunt kite aerodynamics technical article is to present a foundation of stunt kite aerodynamics so stunt kite enthusiasts may expand stunt kite designs to the next level of superior performance. 1.2 DISCLAIMER The information presented in this technical article is solely based on my knowledge of aerodynamics and associated experience. I gained this experience while flying and designing aerodynamic platforms for various hobby/sports, over a span of 40 years, as indicated in my biography presented in Section 6.0 of this article. One of my hobby/sports is designing stunt kites for my kite company, Falcon Aero Designs. Since these technical articles are related to stunt kites, some references to design features developed for Falcon Aero Designs are made to clarify aerodynamic principles. Falcon Aero Designs and I admit no liability from the use or misuse of the information presented in these technical articles. 1.3 ARTICLE ORGANIZATION The following provides the sections and associated titles included in this technical article: Introduction Aerodynamic Centers Center of Gravity and Static Margin Rate of Turn and Bridling Conclusions Biography Aerodynamic terms that are relevant to each section of the technical article will be introduced and defined. Each section builds on the principles presented in the previous section. "DESIGNING FOR THE FUTURE" Page 1

2.0 AERODYNAMIC CENTER The aerodynamic center of a stunt kite is the location where all the forces imposed on your stunt kite can be mathematically represented by one point. The aerodynamic center also is the reference point used to measure these forces. In this section, we will discuss the components that are used to calculate the aerodynamic center of a stunt kite. Stunt kites ranging from simple geometric shapes, such a diamonds and rectangles to complex geometric shapes, such as dart style stunt kites with curved leading and trailing edges. Figure 1 provides a labeled diagram for three example stunt kites with different geometric shapes. To evaluate the aerodynamics of a stunt kite, we calculate the aerodynamic center for either the left or right side, defined as the panel aerodynamic center. Since most stunt kites are symmetrical in shape, the left panel aerodynamic center is equal to the right panel aerodynamic center during forward flight. FIGURE 1 "DESIGNING FOR THE FUTURE" Page 2

2.1 PROJECT SAIL AREA The projected sail area of a stunt kite is the effective area used for lift and forward flight. The projected sail area can be represented as the area of the sail you see while flying. The project sail area also can be visualized as the shadow your stunt kite projects on the ground with the sun directly overhead, after you place your stunt kite face down on the ground with the standoffs pointing up. Place your assembled stunt kite face down on a large piece of paper. Carefully trace the outline of your stunt kite. The shape you have transferred to the paper is the projected sail area. The projected sail area is the area that provides lift to the stunt kite. The projected sail area is not the area of the sail when placed your stunt kite flat on the ground without spreaders and standoffs. This area is defined as the construction area or total sail area. The area of the sail that you see when looking down from the nose or top of the stunt kite, or the side of a stunt kite provides stability and maneuverability. In future technical articles, we may discuss how the top and side areas of the sail affect performance. 2.2 SUBPANEL AERODYNAMIC CENTERS Now that you have traced the outline of your stunt kite, we will focus on the right half, from the center spine to the tip, to calculate the right panel aerodynamic center. To calculate the panel aerodynamic center, we have to break the project sail area of the stunt kite into unique geometric shapes, called sub panels. Each sub panel must have vertical sides that are parallel to the center spine, with reasonably straight leading and trailing edges. The leading edge is the part of the sail the air would touch first in flight. An example would be the nose and the edge of the stunt kite sail that contains the wing spars. The trailing edge is the part of the sail the air would touch last in flight. With the rectangular and the triangular stunt kites presented in Figure 1, only one sub panel is required for the right side. The dart style stunt kite presented in Figure 1 is our Falcon. Our Falcon, along with our other stunt kites, were designed using 10 sub panels per side. The following sections will allow you to determine the horizontal and vertical location of the aerodynamic center for a sub panel. Horizontal Sub Panel Aerodynamic Center To calculate the horizontal location of a sub panel aerodynamic center from the center spine, we use the following equation: (RC + 2 X TC) (SPS) SPAC H = X + DS (RC + TC) 3 SPAC H = Horizontal Sub Panel Aerodynamic Center (inches) RC = Root Chord (inches) TC = Tip Chord (inches) SPS = Sub panel Span (inches) DS = Delta Span (inches) "DESIGNING FOR THE FUTURE" Page 3

The root chord (RC) is the vertical length of the sub panel, from the leading to the tailing edge, on the side of the sub panel closest to the center spine. For the rectangular and triangular stunt kites presented in Figure 1, the root chord is the vertical length of the sail material covering the center spine. The tip chord (TC) is the vertical length of the panel, from the leading to the tailing edge, on the side of the panel farthest from the center spine. With the rectangular and triangular stunt kites presented in Figure 1, the tip chord is the tip of the stunt kite. The root and tip chords are equal for the rectangular kite. The tip chord for the triangular stunt kite is zero. The sub panel span (SPS) is the horizontal distance from the root chord to the tip chord, perpendicular to the center spine. For the rectangular and triangular stunt kites presented in Figure 1, the panel span is the horizontal distance from the center spine to the tip of the stunt kite. The delta span (DS) is the distance from the center spine to the root chord of the sub panel. For the rectangular and triangular stunt kites presented in Figure 1, the delta span is zero since the root chord is at the center spine. Vertical Sub Panel Aerodynamic Center To calculate the vertical location of a sub panel aerodynamic center from the leading edge of the nose of the stunt kite, we use the following equation: SPAC V = (RC + 2 X TC) X LS + 0.5 X (RC 2 + RC X TC + TC 2 ) + RS 3 X (RC + TC) SPAC V = Vertical Sub Panel Aerodynamic Center (inches) RC = Root Chord (inches) TC = Tip Chord (inches) LS = Leading Edge Sweep (inches) RS = Root Chord Sweep (inches) The leading edge sweep (LS) is the distance the leading edge at the tip chord is vertically below the leading edge at root chord of the sub panel. The root chord sweep (RS) is the distance the leading edge at the root chord of the sub panel is vertically below the nose of the sail. For the rectangular and triangular stunt kites presented in Figure 1, the root chord sweep is zero since the leading edge of the root chord is the nose of the stunt kite. The distance the sub panel aerodynamic center is below the leading edge corresponds with the quarter chord line of the sub panel. The quarter chord line of the sub panel is calculated by dividing the root and tip chords by four, then measure these distances from the leading edge of the respective chord. As presented in Figure 1, one quarter of the sub panel area will be above this line, while three quarters of the sub panel area will be below. 2.3 CALCULATING SUBPANEL, PANEL AND PROJECTED SAIL AREAS We will now calculate various areas of a stunt kite, which will be used in Section 2.5 to calculate the right panel aerodynamic center. To calculate the area of a sub panel, we use the following equation: "DESIGNING FOR THE FUTURE" Page 4

SPA = (RC + TC) 2 X SPS SPA = Sub Panel Area (square inches) RC = Root Chord (inches) TC = Tip Chord (inches) SPS = Sub panel Span (inches) To calculate the area of the right panel of the sail, we simply add all the sub panel areas together. The following provides the equation used for this calculation: PA = SPA 1 + SPA 2 + SPA 3... + SPA N PA = Panel Area (square inches) SPA 1 = Area of the First Sub Panel (square inches) SPA 2 = Area of the Second Sub Panel (square inches) SPA 3 = Area of the Third Sub Panel (square inches) SPA N = Area of the Last Sub Panel (square inches) To calculate the project sail area of the sail, we multiply the panel area by two. The following provides the equation used for this calculation: SA = PA X 2 SA = Project Sail Area (square inches) PA = Panel Area (square inches) 2.4 MEAN AERODYNAMIC CHORD AND ASPECT RATIO The mean aerodynamic chord is the average width of the projected sail area from the leading edge to the trailing edge. If the stunt kite sail was shaped like a rectangle, then the mean chord is the same as the width of the sail, at any location along the span of the sail. If the stunt kite sail shape was complex, the mean chord is a mathematical representation of the average width of the sail as a rectangle that could not be measured. The mean aerodynamic chord will be used in Section 3.0 to calculate the static margin of the sail. To calculate the mean aerodynamic chord, we must calculate the panel span using the following equation: "DESIGNING FOR THE FUTURE" Page 5

PS = SPS 1 + SPS 2 + SPS 3... + SPS N PS = Panel Span (inches) SPS 1 = Span of the First Sub Panel (inches) SPS 2 = Span of the Second Sub Panel (inches) SPS 3 = Span of the Third Sub Panel (inches) SPS N = Span of the Last Sub Panel (inches) Now that we have the panel span, we can calculate the mean aerodynamic chord using the following equation: MC = PA PS MC = Mean Aerodynamic Chord (inches) PA = Panel Area (square inches) PS = Panel Span (inches) To calculate the wingspan of the stunt kite, we simply multiply the panel span by two. The wingspan is the distance from the extreme tips of the stunt kite when assembled. The following provides the equation used for this calculation: WS = PS X 2 WS = Wing Span (inches) PS = Panel Span (inches) With the above information, we also can calculate an interesting aerodynamic measurement, the aspect ratio. The aspect ratio is the ratio of the wingspan divided by the mean chord. Using a rectangular stunt kite with a wingspan of 10 feet and a mean chord of 2 feet, the aspect ratio is 5 to 1. The aspect ratio of a stunt kite sail is one measure of the sail's efficiency, stability and maneuverability. To calculate the aspect ratio of a stunt kite, we can use one of the following two equations: WS 2 WS AR = or to 1 SA MC AR = Aspect Ratio (unit less) WS = Wing Span (inches) SA = Total Project Sail Area (square inches) MC = Mean Aerodynamic Chord (inches) "DESIGNING FOR THE FUTURE" Page 6

Aspect ratios for stunt kites range from 1 to 1, for some diamond stunt kites, to greater than 10 to 1, for large inflatable traction stunt kites. Most stunt kites have aspect ratios ranging from 6-8 to 1. Our Falcon and Talon were designed with aspect ratios of 7.75 to 1 and 7.0 to 1, respectively. In simplified terms, with a lower the aspect ratio, the stunt kite is more stable, but less efficient. With a higher aspect ratio, the stunt kite is more efficient, but less stable. 2.5 PANEL AERODYNAMIC CENTER We will now calculate the panel aerodynamic center for a stunt kite using the data collected for or calculated from the previous sections. Horizontal Panel Aerodynamic Center To calculate the horizontal location of the panel aerodynamic center from the center spine, we use the following equation: PAC H = SPAC H1 X SPA 1 + SPAC H2 X SPA 2... + SPAC HN X SPA N PA PAC H = Horizontal Aerodynamic Center (inches) SPAC H1 = Horizontal Aerodynamic Center for the First Sub Panel (inches) SPAC H2 = Horizontal Aerodynamic Center for the Second Sub Panel (inches) SPAC HN = Horizontal Aerodynamic Center for the Last Sub Panel (inches) SPA 1 = Area of the First Sub Panel (square inches) SPA 2 = Area of the Second Sub Panel (square inches) SPA N = Area of the Last Sub Panel (square inches) PA = Panel Area (square inches) The horizontal location of the panel aerodynamic center (PAC H ) is the location where the sail area from the center spine to the PAC H is equal to the sail area from the PAC H to the tip. For the rectangular stunt kite presented in Figure 1, the PAC H is in the center of the right panel. For the triangular stunt kite presented in Figure 1, the PAC H is 1/3 of the panel span, measured from the center spine. For the dart style stunt kite presented in Figure 1, you need the above equation to calculate the PAC H. To compare stunt kites with different wing spans, the horizontal location of the panel aerodynamic center is made unit less by using the following equation: PAC H Percent PAC H = X 100% PS Percent PAC H = Horizontal Aerodynamic Center (percent) PAC H = Horizontal Aerodynamic Center (inches) PS = Panel Span (inches) "DESIGNING FOR THE FUTURE" Page 7

For the rectangular, triangular and dart style stunt kites presented in Figure 1, the percent PAC H is 50%, 33% and 26.9%, respectively. The turn of a stunt kite is directly related to the wingspan and percent PAC H. For a specific wingspan, the lower the percent PAC H, the smaller the diameter of the turn. This is why the dart style stunt kites turn so sharply and are preferred for aggressive aerobatics. Vertical Panel Aerodynamic Center The vertical panel aerodynamic center is below the nose of stunt kite at a location where the projected sail area can be split so 25 percent of the sail area is above this line, while 75 percent of the sail area is below this line. Using a rectangular stunt kite with a wingspan of 10 feet, a mean chord of 2 feet, and no sweep in the leading edge, the quarter-chord line is 6 inches below the leading edge of the sail. To calculate the vertical location of the panel aerodynamic center from the nose of the stunt kite, we use the following equation: SPAC V1 X SPA 1 + SPAC V2 X SPA 2... + SPAC VN X SPA N PAC V = PA PAC V = Vertical Aerodynamic Center (inches) SPAC V1 = Vertical Aerodynamic Center for the First Sub Panel (inches) SPAC V2 = Vertical Aerodynamic Center for the Second Sub Panel (inches) SPAC VN = Vertical Aerodynamic Center for the Last Sub Panel (inches) SPA 1 = Area of the First Sub Panel (square inches) SPA 2 = Area of the Second Sub Panel (square inches) SPA N = Area of the Last Sub Panel (square inches) PA = Panel Area (square inches) For the rectangular stunt kite presented in Figure 1, the PAC V is 1/4 of the mean chord below the nose. For the triangular and dart style stunt kites presented in Figure 1, you need the above equation to calculate the PAC V. 2.6 AERODYNAMIC CENTER AND NEUTRAL POINT The location where all the forces of the wind can be represented by one point is called the aerodynamic center. At this location all the forces of the wind are in balance at a specific flying speed. The aerodynamic center is located along the center of the stunt kite where the projected sail area is equal on each side of the center of the stunt kite. For most stunt kites, the aerodynamic center is the same as the neutral point for aerodynamic platforms with only one lifting surface. To calculate the horizontal and vertical locations of the aerodynamic center, we use the following equations: AC H = 0 Since the center spine of the kite is the reference point for horizontal measurements. AC V = PAC V "DESIGNING FOR THE FUTURE" Page 8

AC H = Horizontal Aerodynamic Center (inches) AC V = Vertical Aerodynamic Center (inches) PAC V = Vertical Panel Aerodynamic Center Using a rectangular stunt kite with a wingspan of 10 feet, the aerodynamic AC is at the center of the sail. If you fold your stunt kite in half so the leading edges match, the fold of the sail would represent the horizontal center of the stunt kite. If you cut your stunt kite in half along this fold each half would now have a panel span of 5 feet and be equal in area. The center of pressure moves above and below the aerodynamic center during various stages of flight. The aerodynamic center is used to represent the center of pressure during static conditions. In the next section, we will use the aerodynamic center and measure the center of gravity to determine the static margin of the stunt kite. "DESIGNING FOR THE FUTURE" Page 9

3.0 CENTER OF GRAVITY AND STATIC MARGIN In this section we will determine the center of gravity and calculate the static margin, building on the principles presented in Section 2.0. 3.1 CENTER OF GRAVITY The center of gravity of a stunt kite is the location where the mass of your stunt kite can be represented by one point. The center of gravity is measured from the nose of the stunt kite along the center spine. With your stunt kite fully assembled, position the stunt kite so the center spine is parallel to the ground. The nose and tail of the stunt kite must be the same distance to the ground. The tips of the stunt kite also must be the same distance to the ground. Balance your stunt kite on your thumbnail, along the center spine, so your stunt kite is parallel to the ground. If the tail is lower, move your thumb towards the tail of the stunt kite. If the nose is lower, move your thumb towards the nose of the stunt kite. The distance from the nose to your stunt kite to your thumbnail is the distance we measure to determine the center of gravity location for your stunt kite. Another way to locate the center of gravity is to suspend your stunt kite by the center spine by using a small section of flying line. The flying line would be attached to your ceiling and tied to the center spine. This will allow you to adjust the position of the flying line on the center spine to assure the nose and tail are the same distance from the floor. In traditional non-tethered flying platforms, the center of gravity is along the center, between the leading edge and the neutral point of the platform. The neutral point is the aerodynamic center for aerodynamic platforms with only one lifting surface. We provided the equations to calculate the aerodynamic center in Section 2.0. With tethered flying platforms, such as stunt kites, the center of gravity is below, and sometimes at a significant distance, from the neutral point. This is why stunt kites cannot fly without flying lines and why non-tethered platforms do not fly very well when tethered like stunt kites. This is a very important concept when applying traditional aerodynamic concepts and equations, such as the calculation of the static margin, to tethered flying platforms. With the center of gravity is close, but below the neutral point, the stunt kite is very stable, but the turning performance and the wind window are drastically reduced. With the center of gravity is away and below the neutral point, the stunt kite will have an increase in turning performance and a larger wind window, but the stability will be reduced. There is a point when the center of gravity can be too far below the neutral point. At and below this point the stunt kite will not perform appropriately or even fly. 3.2 STATIC MARGIN The static margin is a unit less measurement of the static stability of flying platforms. The following provides the equations to calculate the static margin for non-tethered and tethered flying platforms. Non-Tethered Flying Platforms To calculate the static margin for non-tethered flying platforms, which usually have the center of gravity in front of the neutral point, we use the following equation: "DESIGNING FOR THE FUTURE" Page 10

NP - CG SM C = X 100% MC SM C = Static Margin of non-tethered flying platforms (percent) NP = Neutral Point (inches) CG = Center of Gravity (inches) MC = Mean Aerodynamic Chord (inches) The neutral point (NP) is the same as the aerodynamic center (AC V ), which is measured in the center, from the leading edge of the flying platform wing, for tailless flying platforms. The center of gravity (CG) is measured in the center, from the leading edge of the flying platform wing. Section 2.0 presented the equations required to determine the mean aerodynamic chord of a flying platform. For non-tethered flying platforms, the larger the static margin, the more stable the flying platform. Conventional flying platforms, such as airplanes, usually have static margins ranging between 5 and 20 percent. Tethered Flying Platforms such as Stunt Kites To calculate the static margin for tethered flying platforms, which have the center of gravity behind or below the neutral point, we use the following equation: CG - NP SM T = X 100% MC SM T = Static Margin of tethered flying platforms (percent) NP = Neutral Point (inches) CG = Center of Gravity (inches) MC = Mean Aerodynamic Chord (inches) The neutral point (NP) is the same as the aerodynamic center (AC V ), which is measured from the nose of the stunt kite, along the center spine. The center of gravity (CG) is measured from the nose of the stunt kite, along the center spine. Section 2.0 presented the equations required to determine the mean aerodynamic chord of your stunt kite. If the static margin is small, the stunt kite is very stable, but the turning performance and the wind window are drastically reduced. If the static margin is large, but below 50%, the stunt kite will have an increase in turning performance and a larger wind window, but the stability will be reduced. If the static margin is at or greater than 50%, the stunt kite will not perform appropriately or even fly. Stunt kites that have been available on the market have static margins ranging from 30 to 49.5 percent. Most of the competition stunt kites have static margins between 40 and 49.5 percent. Good team/precision stunt kites have static margins ranging from 40 to 48.5 percent. Our team kite, the Talon, has a static margin of 48.1 percent. Good ballet stunt kites have higher static margins, ranging "DESIGNING FOR THE FUTURE" Page 11

from 45 to 49.5 percent. Our first stunt kite, the Falcon, has a static margin of 48.5 percent. Good stunt kites used to compete in both precision and ballet have static margins ranging from 45 to 48.5 percent. If you stunt kite has a two-piece center spine, you can move the joiner of the center spine to change the center of gravity. If the joiner was near the tail, when you move the joiner towards the nose, the center of gravity will move about 1/4 inches closer to the nose, which will reduce the static margin by about one to two percent. This will make your stunt kite more stable, but less responsive. If the joiner was near the nose, when you move the joiner towards the tail, the center of gravity will move about 1/4 inches away from the nose, which will increase the static margin by about one to two percent. This will make your stunt kite more responsive, but less stable. Be careful not to increase your static margin above 50 percent. Stunt kites with static margins near 50 percent have a tendency to over steer and do not track out of 90- degree turns. This tendency also can be caused by the sail stalling at the tip during a tight turn. In future technical articles, we may discuss the aspects of the tip stall and how to correct and/or tune for this problem. "DESIGNING FOR THE FUTURE" Page 12

4.0 RATE OF TURN AND BRIDLING In this section will discuss how the panel aerodynamic center affects the rate of turn and how to adjust the bridles of your kite. 4.1 RATE OF TURN For a specific wingspan, the panel aerodynamic center location from the center of the sail determines your turning radius. The further the panel aerodynamic center is from the center spine, the wider the turn will be with the correct bridle lines. As discussed in Section 2.0, the distance the panel aerodynamic center is from the center of the sail can be expressed as a percentage of the panel span. A rectangular sail panel will have a panel aerodynamic center equal to 50 percent of the panel span. A triangular sail panel will have a panel aerodynamic center equal to 33 percent of the panel span. A complex dart style sail will have a panel aerodynamic center ranging from 25 to 30 percent of the panel span. Based on this percentage you can see that for a given wing span, a rectangular sail will have the widest turn while a complex dart style sail will have the smallest turn. The size of the kite in relation to the length of your arms also controls the rate of turn. The length of your arms is fixed, at least the last time I checked, mine were. So let us compare two sails, each with a panel aerodynamic center of 30 percent, but one has a wingspan of 7 feet while the other one has a wingspan of 8 feet. You move your arms 1 foot apart to turn the sail with a wingspan of 8 feet. You will only have to move your arms 7/8 of a foot apart to turn the sail with the wingspan of 7 feet the same relative turn as the sail with the wingspan of 8 feet. Relative turn is defined as the radius of the turn divided by the wingspan. The larger the sail, the greater the distance between the panel aerodynamic centers, the more arm movement you will need to turn the stunt kite. 4.2 BRIDLING Bridle lines are used because we cannot connect our flying lines directly to a point which is at the horizontal panel aerodynamic centers, slightly above the center of gravity. The reference location for the bridle clips from the center of the sail is over each horizontal panel aerodynamic center. This will allow the rate of turn over the range of wind speeds for the stunt kite to be consistent and controllable. The radius of turn is fixed by the panel aerodynamic centers and the center of gravity. A moment arm that the bridle clips can cause is defined as the distance to the bridle clip from the center of the sail minus the distance to the panel aerodynamic center from the center of the sail times the force of the wind. A moment arm is defined as a force applied at a specific distance. If the force or distance is zero, the moment arm is zero. Since the distance to the bridle clip is the same as the distance to the panel aerodynamic center, the moment arm will be zero or no impact on the rate of turn. In future technical articles, we may discuss the impact of the vertical and horizontal center of gravity locations on forward flight, turns and landings. To determine the distance the bridle clips should be from the sail, you measure the angle of two bridle lines. These two bridle lines are the length of bridle line that goes from the center spine to the bridle clip and the length of the bridle line that is goes from the lower wing spar/spreader joiner to the bridle clip. The angle for these two lines should be 90 degrees. If the bridle clips are further away from the stunt kite frame, with an angle of less than 90 degrees, the bridle clips will have a tendency to swing and retard your input for turns. This is due to how the geometry of the bridles divides up the forces imposed by the wind, through the "DESIGNING FOR THE FUTURE" Page 13

tension of the flying lines on each bridle clip. If the bridle clips are closer to the stunt kite frame, with an angle greater than 90 degrees, the bridle clips will be very sensitive to adjusting the angle of attack. The bridle clip should be between 2 to 6 degrees above the center of gravity. The specific angle to achieve the desired flying angle of attack depends the depth of the sail through the use of standoffs. To locate the correct angle of attack, move the bridle clips by performing the following trimming techniques. Fly your stunt kite to a position directly overhead, then dive your stunt kite directly downwind. When you are about 10 feet above the ground, perform a 90-degree push-pull turn to the left or right. If your stunt kite decreases line tension during the 90-degree turn, then your angle of attack is too small because the bridle clips are set too high. Land your stunt kite and lower the bridle clips no more than 1/16 of an inch at a time. If your stunt kite increases line tension during the 90-degree turn, then your angle of attack is too large because the bridle clips are set too low. Land your stunt kite and raise the bridle clips no more than 1/16 of an inch at a time. Be sure that both bridle clips are the same distance from the upper spreader. Once you have located the ideal angle of attack, you will rarely move the bridle clips again. Changes in line drag, such as changing the line length and/or weight, will alter the angle of attack. You will need to conduct the 90-degree pushpull turn to confirm the correct angle of attack when you change line weight and/or length. If the bridle clips are between the center of the sail and the panel aerodynamic center or inside the panel aerodynamic center, the force of the wind will retard the stunt kite rate of turn. Since the distance to the bridle clip is less than the distance to the panel aerodynamic center, the moment arm will be negative causing a slower rate of turn. The stronger the wind, the higher the negative moment arm force, the slower the rate of turn. Some stunt kites turn fair in low winds, but very slow in high winds. If the bridle clip is just inside the panel aerodynamic center, the kite is a little more forgiving, but there will be a noticeable delay going into and out of turns in higher winds. If the bridle clips are between the panel aerodynamic center and the tip of the sail or outside the panel aerodynamic center, the force of the wind will accelerate the rate of turn. Since the distance to the bridle clip is greater than the distance to the panel aerodynamic center, the moment arm will be positive causing an accelerated rate of turn. The stronger the wind, the higher the positive moment arm force, the faster the rate of turn. That is why some stunt kites turn fair in light air, but are uncontrollable in high winds. If the bridle clip is just outside the panel aerodynamic center the kite is a little more aggressive, but the stunt kite may want to jump into and out of turns. If you are not satisfied with the way your stunt kite is turning, make a set of bridles with the bridle clips over the panel aerodynamic center. This will allow you to evaluate the performance capabilities of the sail. If the rate of turn is too slow with the bridle clips over the panel aerodynamic center, then the panel aerodynamic center is too close to the tip of the sail. You should move the bridle clips out past the panel aerodynamic center until you achieve the desired rate of turn. If the rate of turn is too fast, then the panel aerodynamic center is too close to the center of the sail. You should move the bridle clips in from the panel aerodynamic center until you achieve the desired rate of turn. "DESIGNING FOR THE FUTURE" Page 14

5.0 CONCLUSIONS The following silent team kite technical information is provided to illustrate the technical information covered in this article for our Talon : Wing Span - 96 inches Panel Span - 48 inches Leading Edge Sweep - 44.5 inches or 43.75 degrees (Average) Construction or Total Sail Area - 1,524 square inches Project Sail Area - 1,317 square inches Aspect Ratio - 7 to 1 Mean Chord - 13.7 inches Panel Aerodynamic Center from Center - 13.5 inches or 28 percent Panel Aerodynamic Center from Nose - 16.3 inches or quarter chord Center of Gravity from Nose - 22.875 inches Static Margin - 48.1 percent The ideal setting for precision and team flying with the above technical information is with the bridle clips located 0 to 0.5 inches or 0 to 1 percent outside of the panel aerodynamic center. The ideal setting for ballet is with the bridle clips located 1 to 1.5 inches or 2 to 3 percent outside the panel aerodynamic center. In future technical articles, we may discuss how the various materials used to construct stunt kites interact with the horizontal and vertical center of gravities of the stunt kite, and influence forward flight, turning and landings. In future technical articles, we may discuss how the top and side areas of the sail affect performance. "DESIGNING FOR THE FUTURE" Page 15

6.0 BIOGRAPHY Have been flying model aircraft and kites since 1964. Interest in aerodynamics started in 1974, while designing and building the Precision Aerobatics Control Line Stunt Aircraft, the Apparition. Was fortunate to place second in 1974 and first in 1975 in the Senior Class flying the Apparition at the Academy of Model Aeronautics National Championships. Also judged the precision aerobatics event for the Open Class at the 1974, 1975, and 1976 Model Airplane National Championships. Competed in 1979 with Apparition -II for the team to represent the United States in 1980. Was fortunate to place ninth in the United States at the 1979 team selection process. Became interested in using aerodynamic modeling in the design of a canard (tail first) precision acrobatic prototype called ISIS. Interest in aerodynamic modeling continued with the design of various Radio Control Sailplanes. Designed the Super Mirage, Falcon and Falcon -2M Radio Controlled Sailplanes using the aerodynamic computer program called Geosail. Develop a high lift/low drag airfoil call the DS37122. This airfoil was used on the Falcon, Falcon -2M and other North Jersey Soaring Society (NJSS) sailplanes. Geosail was developed for members of the NJSS to evaluate and design complex radio control sailplanes for competition. Started the NJSS in 1982 and also was the representative for Soaring in District II of the Academy of Model Aeronautics from 1986 through 1988. Started flying stunt kites during the summer of 1986 in Stone Harbor, New Jersey with the purchase of a triple pack of Peter Powell Stunt Kites. Started flying and modifying dart style stunt kites in 1989. Adapted Geosail for stunt kites in the summer of 1990 and renamed it Geokite. A Research and Development Program was conducted from September 1990 until May 1991 to refine Geokite and to develop a computer engineered, designed, balanced, and drafted stunt kite. Was the first to incorporate the leach line used in sailing to control trailing edge flutter. With a curved leading edge and a fully floating leach line, was able to make a completely silent stunt kite. Started Falcon Aero Designs in February of 1991. The first stunt kite contest was the 1991 East Coast Stunt Kite Championships, held on the Memorial Day Weekend, in Wildwood, New Jersey. Was fortunate to place first in Intermediate Individual Precision with the Falcon pre-production model. Developed various models of the Falcon using Geokite in 1992 and 1993. Geokite was highly modified by during 1993 to develop the next generation of silent stunt kites for precision and team competition. The Team Silence Talon was developed using the new version of Geokite and introduced to the kiting community at the Norwalk, Connecticut Kite Festival in the fall of 1993. Competed at various Eastern League events in 1991, 1992, 1993, 1994 and 1995. Was fortunate to receive various awards while flying various models of the Falcon and Talon in Experienced Individual Precision. Competed in Master Individual Precision and Ballet events with various models of the Team Silence Talon. Geokite was highly modified by during 1995 to develop the next generation of silent stunt kites for precision and ballet competition. The Total Silence Raptor was developed using the new version of Geokite and introduced to the kiting community at the 1996 East Coast Stunt Kite Championships, held on Memorial Day Weekend, in Wildwood, New Jersey. "DESIGNING FOR THE FUTURE" Page 16

Competed at various Eastern League events in 1996 and 1997. Was fortunate to receive various awards while flying various models of the Raptor in Master Individual Precision and Ballet. Competed in Master Individual Precision and Ballet events with various models of the Total Silence Raptor. Retired from stunt kite competition in 1997, but continued to fly stunt kites on a recreational basis. Based on a request from a close friend who I have flown with since the mid 1990s, began designing the next version of my stunt kites in 1999. Developed a smaller prototype of the Bird of Prey in 2003 to test the concept of the new design. Constructed the full size version of the Bird of Prey in October 2009, which is the currently under evaluation. "DESIGNING FOR THE FUTURE" Page 17