Hang Glider Design and Performance
American Institute of Aeronautics and Astronautics
by Paul Dees
Boeing Commercial Airplanes, Seattle, WA, 98124
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The rigid wings have structural design features more similar to traditional aircraft, offer improved glider performance, and utilize composite materials extensively in their structure, all at increased cost and weight. The flex-wing product line of the Wills Wing company was evaluated in depth with a detailed build-up of drag and glide performance estimates shown.
α = angle of attack, degrees
CD = drag coefficient
CDi = inviscid induced drag coefficient due to wing planform and twist
CDmisc = parasitic drag coefficient due to miscellaneous items
CDprofile = profile drag coefficient
CL = lift coefficient
D = drag, pounds
γ = flight path angle, degrees
L = lift, pounds
L/D = lift/drag
ρ = air density, slugs/ft3
q = dynamic pressure, pounds/ft2
Swing = Wing reference area, ft2
θ = pitch angle, degrees
V = velocity, miles per hour
Vg = velocity over the ground, miles per hour
VG = variable geometry
Vs = sink rate velocity, miles per hour
Hang gliding is an aerial sport whose participant pilots believe comes the closest to humankind mimicking the free flight of birds. What follows is a brief history of the sport, followed by a technical summary of the key design aspects of the most popular style of hang glider, the “flex-wing” which evolved from the original work of Francis Rogallo and John Dickensen during the 1950s and 1960s. Next, the performance of product line of the largest US based manufacturer, Wills Wing, will be analyzed. The drag of four Wills Wing gliders was independently estimated using classical techniques. This drag and the resulting glider performance data were compared to data from Wills Wing.
II. The Rebirth of Hang Gliding
The first heavier than air gliding flight likely took place in 1849 when Sir George Cayley launched his coachman in a flight across a valley in England. The first hang gliders flew during the late 1800s and were built and flown by famous pioneers such as the German engineer, Otto Lilienthal, Percy Pilcher of Scotland, and Octave Chanute and John Montgomery of the United States. Three of the four of these pioneers died piloting their experimental craft, evidence of the dangers faced by the pioneers of flight. Octave Chanute’s biplane glider was the first successful human-carrying glider that set the pattern for biplane aircraft structure for decades to follow. Orville and Wilbur Wright experimented with gliders that improved upon Chanute’s design in 1900-1902. They perfected their control system and basic turns with their 1902 glider before applying power to invent the airplane in 1903. The Wrights also experienced the first well documented, true soaring flight in October, 1911 at Kitty Hawk with a flight that lasted 9 minutes, 45 seconds. References 1 to 10 document many of these pioneering efforts.
After powered flight became a reality, gliding activity became more of interest for recreation. After World War I, Germany was prohibited by the Armistice from flying powered aircraft and gliding thrived as a sport. The first competition was held in Rhoen in 1920 and pioneer Willi Pelzner won in a biplane hang glider. The following years saw the advent of gliders that eventually evolved into the efficient sailplanes of today. Figure 1 illustrates the chronology of these early gliding aircraft.
Hang gliding experienced a rebirth in the 1960s and 1970s when John Dickensen of Australia developed a practical pilot support and weight shift control method coupled with the wing design invented by Francis and Gertrude Rogallo of the USA. Barry Palmer flew a foot-launched Rogallo wing prior to John Dickensen but it was not practical enough to be widely imitated. John Dickensen’s wings were used by Australians Bill Moyes and Bill Bennett to promote hang gliding across the world and it really caught on in the early 1970s. By 1974 there were about 40 manufacturers of “Standard Rogallo” hang gliders in the USA. One could buy a ready-to-fly hang glider for about $400, and it usually came with little or no instruction on proper flying technique. There were many fatalities due to design deficiencies in these early wings as well as poor pilot instruction. Eventually the design technology matured and safety improved greatly with the advent of helmet usage, back up emergency parachutes, hang glider manufacturers agreeing on standardized full scale strength and pitch stability testing, standardized instruction and pilot ratings. The Hang Glider Manufacturers Association (HGMA) oversees the full scale testing of hang gliders in the US and the DHV does this in Europe (References 11 and 12). The sport has been successfully self-regulated in the USA for nearly three decades by the United States Hang Gliding and Paragliding Association., as evidenced by a greatly improved safety record.
Figure 1. Early gliding aircraft through the 1920s.
The gliders evolved into two basic types of aircraft: More sophisticated, advanced Dickensen/Rogallo wings that are now called flex-wings, and gliders with rigid structure more like private aircraft that are known as rigid wings.
In the 1980s and 1990s paragliders also arrived, evolved and have greatly increased in popularity. Figure 2 illustrates the chronology of hang gliders since their rebirth of popularity.
Figure 2. Hang gliders and paragliders since their rebirth in popularity
III. Flex-Wing and Rigid Wing Design Features
The typical flex-wing hang glider has structure and aerodynamic design features that differ significantly from typical lightweight private or ultralight aircraft. Figure 3 shows some of the design nomenclature.
Key design features include:
- A tail-less design that has adequate pitch stability, enabled by a combination of the low pilot position, wing sweep and washout twist.
- Pitch control is by pilot weight shift fore and aft.
- Roll/yaw control is by pilot weight shift side to side, causing differential sail twist.
- Lift loads are carried by the leading edge, side wires, keel and crossbar.
- Battens slide in pockets in the sail, held in tension at the trailing edge using special hardware or ties.
- A tight sail held in place by leading edge tube bending stiffness is used to control spanwise wing twist.
- Passive load relief occurs at high load factors by sail twisting and the outboard LE bending aft.
Figure 3. Flex-wing hang glider parts
There are complex aerodynamic and structural interactions that are known by active pilots and the few glider designers that work for the glider manufacturers but it would be difficult to get quantifiable measurements of them.
Flex-wings vary from easy to fly and inexpensive trainers (Figure 4), to high performance wings capable of very good glide performance and cross country flights of hundreds of miles (Figure 5). The Falcon 3 of Figure 4 has an exposed crossbar that creates more drag, but also enables the glider to be very light weight and have superb handling qualities. The large wing area provides a low sink rate. The Talon 2’s sail (Figure 5) encloses the crossbar and has a tighter wing of smaller area, requiring an advanced pilot rating to safely fly it and fully enjoy its capabilities. It has no kingpost or upper rigging and relies on a heavier, stronger composite crossbar to take negative g loadings in flight. The Talon weighs and costs more than the Falcon. Flex-wings are built in a variety of wing area sizes according to pilot weight. Typical harnesses are very comfortable and enable the pilot’s body except for the head and arms to be enclosed for lower drag and for warmth at higher altitudes. The advanced gliders have a variable geometry (VG) feature where with the pull of a line, the pilot can tighten crossbar tension, reducing sail twist, and effectively trimming at a higher airspeed with lower drag.
Figure 4. Falcon 3 training glider
Figure 5. Talon 2 advanced flex-wing
Rigid wings provide a further improvement in glide performance over topless flex-wings like the Talon, although there is some debate regarding how much or if their cost, weight and greater fragility are worthwhile. A typical rigid wing is shown in Figure 6. Rigid wings typically have greater wing span, higher wing aspect ratio, lower sweep wing planforms and some designs also include winglets for a further boost in effective span.
The ATOS VR design represents state of the art rigid wing design that yields an L/D in the high teens, which is impressive considering the drag of an exposed pilot (Reference 13). Design features of the ATOS VR that differentiate it from the flex-wings include:
- Fully cantilevered wing structure with carbon fiber D-spar leading edge, carbon ribs attached to the LE and covered with sailcloth. It is fragile and must be carefully maintained.
- Less wing sweep so a small tail improves pitch damping and pitch stability.
- Roll/yaw control is by spoilers and is actuated by pilot weight shift side to side. Note that the weight shift by itself provides negligible roll effectiveness.
- Inboard simple trailing edge flaps of triangular planform.
- Ribs fold against D-spar when glider is disassembled. The airfoil is typically constrained to not have undercamber or chordwise curvature reversals.
Figure 6. Air ATOS VR rigid wing (photo via OZ report, vol 10., # 39)
Figure 7. Brightstar SWIFT (photo via http://aero.stanford.edu/Reports/SWIFTArticle1991.html)
The Brightstar SWIFT design, unlike the ATOS VR, integrates the pilot into the wing in a cage-like structure with the pilot in a supine rather than prone position. It has a higher L/D in the mid 20’s if a fairing is used. It has been less popular than the ATOS line of gliders due to its higher weight, but the performance is improved. Design features of the SWIFT that differentiate it from other designs include:
- Fully cantilevered composite wing structure from leading to trailing edge.
- Wing with more sweep, less taper
- Optional fairing for supine pilot
- Pitch, roll control via TE devices actuated by side stick control.
- Inboard trailing edge flaps.
- The disassembled wing is transported in a large box.
IV. Wills Wing Hang Glider Product Line
The USA’s largest manufacturer of hang gliders, Wills Wing, produces four models of flex-wings that are very popular with pilots. Wills Wing has sold thousands of gliders since the early 1970s. Flex-wing hang gliders make up the vast majority of gliders being flown today due to their lighter weight, lower cost and more robust design than rigid wings. There are only a handful of hang glider manufacturers world wide. Others in addition to Wills Wing include Moyes, Icaro, Aeros, Airborne, and Northwing (references 14 to 18).
The Wills Wing product line in order of increasing performance, cost, and weight is shown in Figure 8. The Falcon 3 trainer is a “single-surfaced” wing and is the most produced of the Wills Wing gliders. Many advanced pilots enjoy Falcons due to their convenient set up and superb handling qualities. The Sport 2 is an intermediate pilot’s wing with a double surfaced sail that encloses the crossbar and has a variable geometry feature (as does the U2 and T2). The U2 has a higher wing loading, higher aspect ratio, and higher performance than the Sport 2. The Talon T2 eliminates the upper rigging present on the other gliders to further reduce drag. This results in a weight penalty in order for the structure to still meet strength requirements at negative load factors. All four models of gliders come in two or more wing area sizes to accommodate pilots of various body weights. In this paper a single wing size of glider of each model was analyzed, typically the closest size appropriate for a pilot weighing 160 to 170 pounds.
The generic glide and sink rate polars from the company’s web site (Reference 19) are shown in Figure 9 and are for pilot weights 130% greater than the minimum weight rating for each glider. They assume the pilot is flying with excellent form, with wings level using proper technique to control yaw excursions. These polars are provided in a generic sense so pilots of different weights and sizes of a given model of glider can calculate their glider performance. Data were read from the curves of Figure 9 and by assuming a given glider size and pilot weight, the values of CL and CD could be calculated to generate plots of L/D versus CL (to be shown later in the paper).
Figure 8. Wills Wing hang glider product line
Figure 9. Glide performance of Wills Wing gliders (via Reference 19)
V. Drag and Performance of Four Hang Gliders
- Hang gliders have evolved into safe, efficient, and fun sport aircraft.
- Flex-wing glider performance varies in L/D from 9 to 14 depending on complexity and degree of pilot skill required.
- Rigid wing gliders have achieved L/Ds in the 20s.
- Drag components can be estimated based on factory provided glide data, then adjusted to family better with each other.
- There is still a need for rigorous flight testing to better quantify in flight hang glider geometry and glide performance.
The author is grateful to Steve Pearson at Wills Wing for providing background information on their gliders and their data, to Kay Dees, Darren Darsey, Davis Straub and the Oz Report for the use of photos, and to Ilan Kroo of Stanford University for usage of the SWIFT image. Thanks to Len Baron for an excellent peer review and to Nate Hines for additional help. Thanks also to the United States Hang Gliding and Paragliding Association (www.ushpa.org).
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13 ATOS web site: http://www.a-i-r.de
14 Moyes web site: http://www.moyes.com.au
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17 Airborne Australia web site: http://www.airborne.com.au
18 Northwing web site: http://www.northwing.com
19 Wills Wing hang glider manufacturer’s web site article on glider polars at: http://www.willswing.com/Articles/Article.asp?reqArticleName=PolarData
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