IN THE PAST few years amateur glider pilots in growing numbers have joined sea gulls and pelicans in soaring flight a few feet above the sloping beaches of U.S. coasts. Inland they skim the windward surfaces of sand dunes, ski slopes and other unobstructed hills. The rapidly evolving sport, which is known as sky surfing or hang gliding, makes about equal demands on the enthusiast's skills as a pilot and as an aerodynamicist.

The aircraft on which the sport is based vary widely in details of design. In general they are inexpensive and frequently homemade gliders of ultralight construction. The pilot is suspended by a harness.

Many of the gliders have as a common ancestor a triangular kite that was patented in 1951 by Francis M. Rogallo and his wife Gertrude. The kite has a glide ratio of better than 4:1, meaning that in flight it goes more than four feet horizontally for each foot of drop. Most sky surfers first learn to fly on a Rogallo kite, which is often called simply a "wing." Many of them continue to fly the kite in preference to the hang gliders that were subsequently developed. The hang gliders have a higher level of performance than the kites, but they cost more and are more hazardous.

The Rogallo kite was conceived as part of a multimillion-dollar research project seeking a replacement for the parachutes that lower spacecraft to the earth's surface after they return to the atmosphere. As developed by the Rogallos the kite was a flexible delta wing. It was deployed, and its shape was maintained, by a set of tension lines, much as the shape of a parachute is maintained by its shroud lines.

Although the kite was abandoned by the National Aeronautics and Space Administration as a device for landing space capsules, primarily because it was difficult to stow and to deploy, a number of people interested in aerodynamics quickly conceived of modifications that would convert the craft into a serviceable hang glider. One of them was Michael A. Markowski of Marlboro, Mass., an aerospace engineer who had participated in the design of the Douglas Aircraft Company's DC-10 and subsequently did research for Sikorsky Aircraft. Markowski substituted an airframe of tubular aluminum for some of the tension lines of the Rogallo kite and fitted the structure with a control bar of aluminum tubing in the form of an inverted Y. A harness suspended the pilot in either a sitting or a prone position, from which he could grasp the control bar and exert force against it, thus shifting his weight to control the craft in pitch, roll and yaw [see illustration above]. Markowski describes the evolution of his project.

"My interest in foot-launched gliders began as a spare-time activity, strictly for fun. After designing several rigid-wing biplanes and a monoplane of the flying-wing type I settled on the Rogallo wing as the most practical means of acquiring experience in the art of hang gliding. By taking advantage of several technical reports about the Rogallo wing that had been compiled by NASA I designed a full-scale model that by good fortune flew 'right off the drawing board.' Skimming the ground of a local ski slope turned out to provide great fun and excitement.

"I quickly mastered the art of taking off and gliding to a landing on foot without benefit of an instructor. It is an astonishing sensation to run along the ground for a few steps and rise effortlessly into the air. I scarcely realized that I was attached to a kite, because the craft represents only 20 percent of the gross weight of the pilot-glider system. Suspended prone below the glider, the pilot has a sensation of consummate freedom, perhaps because of his bird's-eye view of the terrain.

"My first hang glider had a sail consisting of .004-inch polyethylene. The machine weighed 40 pounds empty. It served me well for many ground-skimming flights until the sail began stretching, which degraded the glide ratio. As the temperature dropped during the fall of 1971 the plastic started to crack. After consulting a parachute rigger, who was familiar with sail materials, I replaced the polyethylene with rip-stop nylon. This material served for about a year before I retired it. Nylon is too elastic to maintain the curve of a good sail. I now use Dacron sailcloth.

"That prototype glider turned out to be only the first of a series of models. It was followed by both radio-controlled and free-flight models at reduced scale for investigating still other characteristics of the Rogallo wing. Fortunately this series of experiments alerted me to a startling fact. In the jargon of aerodynamics Rogallo kites are 'pitch-down divergent.' When the nose is lowered, the craft speeds up and the sail begins to luff, or flutter, at the trailing edge. The flutter advances toward the apex and the craft dives at an ever increasing angle until it strikes the ground!

"Having observed this disconcerting behavior in the small models, I decided to learn by experiment if the phenomenon would also occur in a full-scale glider. It did. The initial solution that came to mind was the addition of a horizontal stabilizer. I tried one. It worked, but it was awkward and was easily damaged in normal use. The final solution was simply suspending a weight some distance below the keel of the glider. This device worked well and introduced no structural problem.

"Sky surfing is an art. It can be mastered only by diligent practice. Of necessity the hang-glider pilot must be self-taught. The craft do not have dual controls for instruction, as conventional airplanes and gliders do. You start by learning a few additional details about your physical prowess and the reaction of the kite to the wind on level ground before you venture even briefly into the air. Like all young birds, you try your wings many times before leaving the 'nest.'

"The Rogallo kite with a control bar and a safety harness can be flown almost anywhere that suitably sloped terrain faces the prevailing winds. The novice is urged to begin his training at the foot of a small grass-covered hill or a sand dune with a slope of about 25 degrees. There should be a steady, uphill breeze of about 10 miles per hour. Be certain that the breeze is free of gusts that might dump you or abruptly lift you 20 feet or more.

"When you have assembled the kite near the bottom of the slope, put it down with its nose in contact with the ground and pointed directly into the wind. Walk completely around the craft and check the integrity of all its metal components, making sure that they are attached properly and show no bends, breaks or cracks. Simultaneously check the fabric for holes, cracks or rips. Assure yourself that all points of attachment of the sail are sound and that no grommets have pulled away from the fabric. All rigging should be tight enough to twang when it is plucked.

"When you have inspected the glider, grasp the upright members of the control frame, lift the kite above you and run forward with the nose pointed directly into the wind. As you run, tip the nose alternately upward and downward at increasing angles to sense the effect. Raising the nose will cause the sail to inflate, catch more wind and pull upward, thereby reducing the speed at which you can run. Lowering the nose has the reverse effect; it decreases the wind resistance and enables you to run faster. Note, however, that lowering the nose excessively causes the sail to luff, with the result that the wind acts on the top of the fabric and pushes the craft down.

"Continue practicing on level ground until you can unerringly predict and 'feel' exactly how the wing will react to every angle at which you hold the kite. Then strap yourself into the harness. Continue practicing on level ground until the harness feels natural.

"At this stage you can begin to work your way up the slope. At first, however, go up to an elevation of only two or three feet. Always point the nose of the kite directly into the wind, even when you carry the kite uphill. As you gain proficiency, the wind will even help you carry the craft up the slope. When you run downhill, hold the control frame near the bottom, so that the uprights pass close to your shoulders.

"Finally, from an elevation of 10 or 12 feet, begin running downhill with the sail barely inflated. As you pick up speed, push the control frame away from you somewhat, thus pitching the nose upward. If you have reached sufficient speed, you and the kite will rise into the air. If not, your forward motion will simply be retarded. In that case keep trying until you acquire the correct combination of forward speed and pitch angle for flight. Soon you will be skimming the ground.

"Once you have become airborne, maintain your fore-and-aft balance by shifting your weight. Push against the control bar to move your weight backward, which will increase the angle of pitch and thus reduce your speed. Pull the bar to shift your weight forward, thus decreasing the angle of pitch and increasing your airspeed. Shift to the right by exerting lateral force on the control bar to make a right turn, and do the opposite for a left turn.

"Remember, when you are sky surfing only two or three feet above the surface, it is better to err on the side of keeping the nose high and flying too slow than to pull the nose down sharply, which will make the craft dive and expose you to the risk of hitting the ground harder than you would like. Continue practicing near the base of the slope until ground skimming becomes second nature. Then work your way gradually to the summit.

"Concentrate first on learning to maintain fore-and-aft balance, that is, pitch If your craft starts drifting sideways, slow down and land as quickly as possible to avoid being blown into the side of the slope. As you gain skill in maintaining balance in pitch, begin to practice turns. Your first turns should be gentle, smooth and wide. Shift your weight by pressing sideways on the control bar very gently.

"All turns cause the kite to sink at a faster rate than when it flies in a straight line. You can compensate for this tendency somewhat by increasing the angle of pitch slightly during the turn. Avoid pushing the nose up to the angle at which the glider would stall, lose flying speed and drop toward the ground. The optimum angle of pitch can be sensed only with experience.

"As you progress to higher altitudes you will have occasion to make sharper turns and to maneuver in the three dimensions of space, but do not rush your learning process. Word has somehow spread that the Rogallo kite is an exceptionally safe craft. In actuality it is only relatively safe. Aerodynamically the wing is characterized by a very gentle stall, meaning that when the kite begins to lose flying speed, it tends to settle rapidly and to nose down slowly instead of going abruptly into a nose dive. Its performance is governed by the same laws of physics that affect other flying machines.

"The Rogallo kite is basically easier to fly than other aircraft because the pilot controls it by shifting his weight, which is a more or less instinctive action. However, weight shifting as a control technique has its limitations. The forces of the controlling moments remain constant, whereas the disturbing forces are squared with speed. Aerodynamic controls of the kind developed by the Wright Brothers enable one to fly safely in winds that would be unsafe for a hang glider of the Rogallo type. In other words, the Rogallo wing is no toy, and it can be a killer. Most beginners do not know how to land one safely from a stall at a height of, say, 40 feet. This maneuver can and must be learned by patient practice.

"Both wing loading and airspeed are important factors in the performance of all aircraft. With these constraints in mind I designed a series of gliders based on the Rogallo wing for pilots weighing from 100 to 210 pounds. The gliders could be flown in wind speeds ranging from less than seven miles per hour to more than 20 miles per hour. In general the resulting craft have a maximum glide ratio of about 4.5: 1 and a minimum sink rate of 450 feet per minute.

"Although one can have a lot of fun with a glider of this performance, the duration of flights is necessarily limited to minutes rather than hours. Sky surfing with a Rogallo wing is comparable in this respect to riding breakers at the beach with a surfboard. You enjoy a succession of one-way trips.

"I wanted a craft of higher performance that would enable me to glide continuously along windward slopes in both directions by making 180-degree turns at the ends. I also wanted to enjoy this high performance with a minimum of fuss. That meant developing a machine that one person could not only assemble and disassemble but also fold and load for transportation atop an automobile.

"An invention that promised to make possible such a glider came to mind when I recalled a college course in low-speed aerodynamics. During the course a 'sail wing' that had been developed at Princeton University in 1948 was described. The device had been developed as an advanced sail for boats. In 1952 it had been adapted for possible operation as an auxiliary lifting surface on ground-effect machines. Basically the sail wing consists of a tubular spar that supports the leading edge of a fabric envelope and a set of short, rigid booms at the tip and foot of the spar between which a slender cable is stretched to form the trailing edge of the wing [see illustration above right]. The structure can be easily folded and stowed.

"The aerodynamics of the sail wing are both simple and impressive. The performance approaches that of conventional ribbed airfoils in terms of lift and drag. At zero angle of attack, when the plane of the wing lies in the plane of its motion, the sail wing assumes a symmetrical cross section that generates no lift [see illustration above left]. At an increased angle of attack the surfaces of the wing form a cambered airfoil that does develop lift. Moreover, the camber deepens with increasing angles of attack, an aerodynamic effect equivalent to the effect of an automatic wing flap. A graph that depicts the wing's resulting force of lift slopes upward more steeply than that of a conventional 'hard' wing [see illustration, right].

"The overriding factor determining the glide ratio of an aircraft is the relation between the length of the wing and its width. The relation is termed the aspect ratio. It would be incorrect to state that doubling the aspect ratio of a wing would double the glide ratio of the aircraft, but increasing the aspect ratio greatly improves the glide ratio. (The glide ratio, incidentally, is numerically equal to the lift-to-drag ratio. The coefficient of induced drag is equal to in which is the coefficient of drag, ; is the coefficient of lift and AR is the aspect ratio, which is equal to the length of the wing divided by its average chord, or width.)

"After reviewing the tabulated aerodynamic characteristics of several hundred airfoils I came to the conclusion that with a low-speed airfoil the coefficient of lift should increase at the rate of about 7.5 percent of the angle of attack; that the stall should become evident at a lift coefficient of about 1.6; that the larger the diameter of the leading edge the gentler the stall and the higher the coefficient of lift; that the deeper the camber the higher the coefficient of lift, and that the high point of the camber should be more than one third of the distance from the leading edge of the airfoil to the trailing edge. I observed with some surprise that these characteristics described exactly the results of experimental tests made on the sail wing, as reported by Princeton and by NASA. It was clear that the sail wing combined high performance in a simple, foldable structure that should function as the lifting surface of the ultimate hang glider.

"I promptly built and flew a series of scale models, some of which were radio controlled. The first prototype, which I named EAGLE-I, had a wingspan of 40 feet and weighed 70 pounds. The wing lay in a single plane, that is, it had n upswept dihedral angle. The aspect ratio was 8:1, the area was 200 square feet and there was no sweepback. The tail had a conventional rudder, an elevator and a horizontal stabilizer. For lateral control I substituted 'spoilers' for ailerons. The spoilers were small flaps that could be lifted by control cables to create drag near either tip of the wing.

"Initial ground and 'sail inflation' tests were made by the same procedure that applies in learning to fly a Rogallo wing. When the machine was lifted into the slightest breeze, the sail wing assumed exactly the predicted contour. Flight testing was begun on low, shallow sand dunes. The first few ground skims indicated that the elevator gave perfect control of pitch; they also helped me to shift the suspension harness to the proper balance point. The rudder proved to be of some value in controlling yaw, but the spoilers were ineffective. The fabric of the control surfaces became grossly distorted under load, which necessitated redesign.

On the other hand, EAGLE-I was fully stable in flight and had a glide ratio of close to 10:1. Although I enlarged the spoilers 50 percent, the modification was never tested. A critical inspection of the craft indicated that a number of design simplifications could be made in the construction of the airframe. Moreover, I decided to achieve lateral control by the Wright Brothers' system of wing warping. The result was a much cleaner design that weighed 63 pounds.

"Flight tests proved that the machine was a high-performance glider. I took one step in a breeze of 10 miles per hour, pulled back the control stick and was lifted almost straight up. It was a fantastic sensation. The lateral control system, however, was still ineffective. Moreover, it was apparent that the span and area of the wing were much too large to handle in anything more than a flat calm.

"The additional tests and modifications led to the construction of EAGLE-II. This machine weighed 75 pounds and had a wingspan of 34 feet and a wing area of 158 square feet. The removal of a few bolts made it possible to fold the craft easily for transportation on a car top. EAGLE-II had a set of pulleys and control cables and a control stick that enabled the average kite pilot to make an easy transition to the high-performance craft. The wing, with an aspect ratio of 7.25:1, was nonswept. On test it developed a lift coefficient in excess of two, with a gentle stall. Indeed, the 'stall' would be more aptly described as a 'mush,' that is, a parachutelike settling rather than a dive. When the control stick was pulled backward slowly, the angle of attack and the camber of the wing increased simultaneously. The action was followed by a steep but slow descent. Response to the controls remained good up to stall but was sluggish in yaw in the 'mush' condition.

EAGLE-II had a wing taper of 3:1 and a dihedral angle of eight degrees. At first I rigged the craft for a dihedral angle of only two degrees, but it tended to skid too much in yaw. I also rigged two degrees of washout in the tips of the wings to guard against tip stall. (The tips of the wings are twisted to reduce the angle by two degrees.) The structure was stressed by a test load equivalent to six times the force of gravity with a safety factor of 1.5.

"The tail structure consisted of a frame of aluminum tubing that supported fabric of rip-stop nylon. In time it became apparent that this material stretched excessively. The resulting exaggerated camber degraded the glide ratio. As I have mentioned, I now use stabilized Dacron sailcloth.

"Both the rudder and the elevator were balanced aerodynamically, that is, they were hinged slightly forward of the quarter-chord line of the control surface. The horizontal stabilizer and the elevator were removable as a unit from the keel of the airframe. Forces developed by the rudder and the dihedral-angle structure of the wing combined to produce a rollyaw couple that helped make turns easy.

"Hinged structures of covered aluminum tubing that formed the wing tips both warped the wings and served as ailerons. I named them 'warperons.' Coupled directly to the rudder, they deflected the wing tips differentially just enough to produce coordinated turns.

"Exhaustive flight tests have now been completed on EAGLE-III. In theory the maximum glide ratio of the machine alone is substantially better than 10:1. This performance is of course degraded by the presence of the pilot, who creates forces of drag but no lift. The amount of drag introduced by the pilot depends on his position. Computations indicate that when the pilot flies in the prone position, the glide ratio of EAGLE-III approaches 11:1. In the sitting position it is almost 8:1. These figures assume an optimum airspeed of about 24 miles per hour. The sink rate also varies with the amount of drag induced by the pilot; it is about 200 feet per minute. The performance of EAGLE-III is therefore compatible not only with sustained flight on the windward side of sloping beaches and comparable terrain but also with cross-country gliding.

Essentially EAGLE-III is an ultralight, high-performance monoplane with a variable-camber wing. It is possible for the pilot to stall the craft in flight, but the stall is gentle compared to that of a standard airfoil, such as the NACA 23012. Beginners should not attempt to fly EAGLE-III, but pilots who have mastered hang gliding with a Rogallo kite make an easy transition to this high-performance craft.

"Both the Rogallo series of sky-surfing kites and EAGLE-III are available commercially as a kit of raw materials and also as a prefabricated kit that includes machined fittings and certain components that are relatively difficult to make, such as the sails. The kits, together with detailed working drawings and complete instructions for fabrication and assembly, are supplied by Man-Flight Systems, Inc. (P.O. Box 872, Worcester, Mass. 01613)."

Bibliography

HANG GLIDING: THE BASIC HANDBOOK OF SKYSURFING. Dan Poynter. Published by the author, Box 4232, Santa Barbara, Calif., 1974.

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