Otto Lilienthal developed a propulsion system for his gliders that used flapping feathered wingtips, which were actuated by a piston engine. This article presents the known historical and technical facts about this propulsion system and its application in Lilienthal's ornithopters. Although the motor capacity was insufficient for steady level flight, the configuration of the propulsion system is interesting. Wind tunnel experiments with scaled-down wingtips and their results will be presented. Simulations using the unsteady vortex lattice method were performed for comparison. The sensitivity of efficiency to different parameters, as well as the flight capacities of the configuration, will be discussed.

The article describes the historical facts known of Otto Lilienthal's Ornithopters from 1893 and 1896 in some detail. Example results of first numerical simulations of the flapping wing feathers will be discussed at the end of this article.

The article describes the historical facts known of Otto Lilienthal’s Ornithopters from 1893 and 1896 in some detail. Example results of first numerical simulations of the flapping wing feathers will be discussed at the end of this article.

In 1889 Otto Lilienthal published his book ‘Birdflight as the Basis of Aviation’ [1] describing his experiments proving the concept of curved wings and providing the best available instructions of that time on how to design a winged aircraft.

Open AccessHistory of Key TechnologiesFlight Controls of Otto Lilienthal’s Experimental Monoplane from 1895Markus Raffel, Felix Wienke, Clemens Schwarz and Andreas DillmannMarkus Raffel https://orcid.org/0000-0002-3340-9115DLR, German Aerospace Center, 37073 Göttingen, Germany*Head of Department, DLR Helicopter Aerodynamics, Institute of Aerodynamics and Flow Technology; also Professor, Institute of Turbomachinery and Fluid Dynamics (TFD), Leibniz University Hannover, 30823 Garbsen, Germany. Member AIAA.Search for more papers by this author, Felix Wienke https://orcid.org/0000-0003-0081-1084DLR, German Aerospace Center, 37073 Göttingen, Germany†Research Engineer, DLR Helicopter Aerodynamics, Institute of Aerodynamics and Flow Technology. Member AIAA.Search for more papers by this author, Clemens SchwarzDLR, German Aerospace Center, 37073 Göttingen, Germany‡Research Engineer, DLR Helicopter Aerodynamics, Institute of Aerodynamics and Flow Technology.Search for more papers by this author and Andreas DillmannDLR, German Aerospace Center, 37073 Göttingen, Germany§Director, Institute of Aerodynamics and Flow Technology.Search for more papers by this authorPublished Online:25 Oct 2022https://doi.org/10.2514/1.C037047SectionsRead Now ToolsAdd to favoritesDownload citationTrack citations ShareShare onFacebookTwitterLinked InRedditEmail AboutNomenclatureAfflight altitude (Af equal to 0 corresponds to straight legs with feet on the ground), mcDaircraft drag coefficient in aerodynamic coordinate systemcLaircraft lift coefficient in aerodynamic coordinate systemclaircraft rolling moment coefficient in aerodynamic coordinate systemcmaircraft pitching moment coefficient in aerodynamic coordinate systemcnaircraft yawing moment coefficient in aerodynamic coordinate systemEglide ratio CL/CDFPpreload force on levers of leading-edge flaps, NU∞freestream velocity, m/sαaircraft angle of attack, degηgeometric tail plane angle of incidence, degΘrudder deflection angle, degκopening angle of leading-edge flaps, degI. IntroductionIn 1889, Otto Lilienthal published his book Birdflight as the Basis of Aviation, containing the first lift-versus-drag data of cambered wings and other important information required for human flight [1]. In 1895, his experiences were regularly detailed in articles that were published not only in Germany but also in England, France, Russia, and the United States [2,3]. Many people from around the world came to visit him, including Russian Nikolai Zhukovsky, Englishman Percy Pilcher, and Austrian Wilhelm Kress. Zhukovsky wrote that Lilienthal’s flying machine was the most important invention in the field of aviation. Lilienthal corresponded with many members of the Boston Aeronautical Society, of which he was an honorary member. Among them were Octave Chanute, author of Progress in Flying Machines [4]; James Means, who invited Lilienthal to perform flight demonstrations; Samuel Pierpont Langley, who visited Lilienthal in Berlin; and Greely S. Curtis, who even gained first-hand gliding experience during a visit with Lilienthal in 1895. On Wednesday, 29 May 1895, the Verein zur Förderung der Luftschifffahrt did not hold its meeting in an auditorium in Berlin in the evening, as was customary, but at Otto Lilienthal’s training hill Fliegeberg in Lichterfelde in broad daylight. The translated minutes note reads as follows ([5] p. 329): “A large number of members had accepted the invitation of Mr. Otto-Lilienthal yesterday, who demonstrated his widely known and famous flight experiments to those gathered there. Even though the low wind speed at that day didn’t allow the flyer from fully developing his art, the experiments were all the more stimulating and instructive, since the vast majority of those present were only familiar with Mr. Lilienthal’s experiments from descriptions.”Lilienthal demonstrated his 12th aircraft design, a monoplane with a wing area of over 20 m2 to the visitors. Because of its wingspan of almost 9 m and wing chord length of 3 m, Lilienthal’s largest monoplane can only be used in light winds. Lilienthal applied for a patent on the same day. The patent claim was submitted for the front wing (Vorflügel), a leading-edge flap used in a glider for this purpose for the first time. Because of the size of the apparatus, three profile rails were slid onto each wing instead of the usual two.The patent was granted the following December by the German Imperial Patent Office as an addition to Lilienthal’s first airplane patent from 1893. The patent specification states that “the front part of the wing surface is rotatable downward about the leading edge and is pressed downward by rubber bands so that it rotates downward when the air pressure acting from below is released, thereby producing a pitch-up moment on the apparatus” [6].In practice, this translates to rubber bands pushing the moving wing section downward by about 30 deg when at rest. In flight, the wing surface is usually closed. However, when the soaring apparatus starts diving, cutting through the air with a low, or even negative, angle of attack, the wing flaps open due to the pretension of the rubber elements, thereby stabilizing the flight. Lilienthal did not have the vocabulary of modern flight mechanics at his time, yet aptly described that in certain flight attitudes the leading edge of the cambered wings of the patented monoplane can be subject to pressure from above at small angles of attack. Lilienthal noted the resulting danger to stable flight attitudes. He was convinced that the new leading-edge flap provided a nose-up pitching moment in such cases. The new apparatus was demonstrated to the visitors both on the ground and in flight. In the process, Dr. Neuhaus took a series of photographic pictures, in which the folded-down front wing section is clearly visible. It has just under 0.5 m chord length at the wing root and about 0.25 m chord length at the tips. The drawing attached to the patent is schematic, illustrating the patent claim without providing any details. Because the patent was filed as a supplement, the elevator (horizontal stabilizer) was depicted being in front of the fin (vertical stabilizer), as was the case in 1893, even though Lilienthal had positioned both stabilizers in a crosswise configuration at the end of the tail for quite some time at that point [7].Figure 1 shows Lilienthal’s large monoplane from above, which provides a good view of the vertical spoilers and wings’ leading edges formed by flaps. The view from below in Fig. 2 shows the pilot control linkage. These leading-edge flaps gave the apparatus the name Vorflügelapparat(front wing apparatus). In later publications Lilienthal also called it Experimentiergerät (experimental apparatus) as documented in his biography by Schwipps [5]. To follow Lilienthal’s naming and to distinguish this apparatus from his experimental biplanes [8], it will be referred to as Experimental Monoplane for the remainder of this article.From the very beginning, the Experimental Monoplane was designed to serve as a tool for control experiments. The actuating system can already be seen in photos (Fig. 2) [9]. It essentially consisted of the hip cradle, which was formed by two willow withies bent upward toward the rear and joined at the back near the vertical stabilizer, connected by an additional bar in front of the pilot’s body. With wishbonelike levers linking the hip cradle to control rods on each side, a displacement of the hip fork will cause a deflection of the control rods passing through the wing surface near the rear cockpit ring. In pictures, these oblique control rods look like two additional king posts.¶ At first, these rods were only used to deflect the vertical stabilizer laterally as a rudder. The movement of the control rods was transferred to the trailing edge of the tail via strings [7]. These strings ran from the king posts over the control rods to the lower part of the tailplane. It is not clear from the photos whether the flexibility of the tail boom or an articulated pivot provided the lateral deflection of the tail unit’s trailing edge. However, in the main author’s view, an articulated arrangement, which allowed for only moderate rudder deflections, is more likely. To increase the effect of the rudder, Lilienthal extended the vertical stabilizer by adding ribs and fabric to the top. Still, the size of the elevator stayed the same as with his patented monoplane Standard Soaring Apparatus, the world’s first aircraft that was produced in series. The relatively small elevator likely helped to improve the glide ratio, but it also resulted in reduced longitudinal stability of the aircraft with closed leading-edge flaps, making the newly invented automatic pitch control even more important.Fig. 1 Otto Lilienthal in his Experimental Monoplane with flight controls near Berlin in 1895 (photograph: P. W. Preobrashenski, 1895, © Archiv Otto-Lilienthal-Museum).Fig. 2 Paul Beylich in the Experimental Monoplane with automatic balancing.” Hip cradle, control levers, and rods transferring the control input to the strings above the wings can clearly be seen (adapted from the photograph by R. Neuhauss, 1895, © Archiv Otto-Lilienthal-Museum).The photograph depicted in Fig. 1 also shows small vertical control surfaces attached at the outer ends of the wings, which rotate around short, upright posts resembling small sails. A string led from the front edge of such a wing-tip rudder, or roll spoileron, to the control rod, which had been shortened and moved back for this purpose. In normal flight, these control surfaces align themselves in the wind. When the pilot moved his body to the right, for example, the right control surface turned inward via the control rods and strings, while the left one remained unaffected.The Bavarian flight pioneer Alois Wolfmüller had been an important correspondent of Lilienthal since 1893. In 1894, he acquired a copy of Lilienthal’s patented Standard Soaring Apparatus and conducted flight experiments with it [10]. Wolfmüller also began experimenting with his own aircraft designs to improve controllability. In March 1895, Otto Lilienthal introduced his large Experimental Monoplane to Alois Wolfmüller in a personal letter [10]: “I am currently building a larger glider of about 20 m2 wing area, which can only be used at calm winds.” He wrote about the wing tip rudders: “Furthermore, I have attached a surface to each wingtip, which I can straighten up by pulling the string in order to bring back the leading wingtip.” Because Lilienthal said “straighten up the surface,” it is possible that, in addition to the devices visible in the photo, some experiments were performed with simpler spoilerons. These control devices are interesting, as they have the potential to avoid the problem of adverse yaw. If such a control surface is deflected to one side by moving the hip cradle, the drag is simultaneously increased on that wing while the lift is decreased. As an actuated spoiler increases drag, the yaw follows the same direction as the roll [11]. This asymmetric actuation of aircraft spoilers is still used by airliner pilots today, allowing aircraft designers to install smaller ailerons. This technique is prominently used during descending flight, when the drag increase is welcome to reduce altitude.It is also noteworthy that both Wolfmüller and Lilienthal (and later also the Wright brothers) used warping of the wings. Wolfmüller wanted to apply this control method manually, but Lilienthal most likely built a connection to the hip cradle, as did the Wright brothers with their 1902 glider and the 1903 flyer. In August 1895, Lilienthal wrote to Wolfmüller [12]: “You are entirely correct. The shift in center of gravity must be greater than a person can accomplish, when gliding in the wind with large wings. As the simplest method of balancing the lifting capacity of the two wings, I recommend rotating the wings around the longitudinal axis. I have found this to be the safest method compared to any others. It is also the method that is used by birds.” After exchanging assurances of “mutual agreement to the protection of legitimate interests,” a form of a nondisclosure agreement, Wolfmüller presented his thoughts and the results of his experiments on controlling flying machines, which were built using Lilienthal’s design, in a long letter from 29 September 1895. This prompted Lilienthal to write more freely about his own attempts. Wolfmüller designed a wing warping device as well as an installation, that allowed pilots to sit within the flying apparatus. He argued that a sitting position would be advantageous, freeing up the pilot’s hands to operate mechanical control systems such as two levers from his own design for twisting the wings. He proposed that other control elements could be operated using a strap around the upper body.The same year in October, Lilienthal replied [13]: “I tested an arrangement similar to yours for moving and rotating the wings with outer tensioning wires running to different points of a lever mounted at the lower base point that can be pulled to give the wing profile the desired rotation. I also made it so that the tail could rotate to the right or left, making it easier to land. Furthermore, I have attached a surface to each wingtip, which I can straighten up by pulling the string in order to bring back the leading wingtip. These elements were operated by the hips, which press against a hip cradle, when the body is shifted sideways to shift the center of gravity.” Lilienthal concluded by admitting that he had not yet achieved a decisive breakthrough in controllability: “These experiments, which I spent the entire summer investigating, have prompted me to make significant changes that I have not yet fully clarified and for which I regrettably have little time at the moment.”In the course of the investigations described in this paper, a full-scale replica of Otto Lilienthal’s Experimental Monoplane was built, in addition to a 1:5 model. Both featured complete sets of control mechanisms: rubber-band activated leading-edge flaps for automatic pitch control, spoilerons, wing warping, and rudder for yaw and roll control, which were actuated by a hip cradle either individually or in combination. All structural materials relevant to the flying qualities were selected with great care in order to match the characteristics of the original.II. Wind-Tunnel TestsA range of parameter sets was investigated using the 1:5 model of the Experimental Monoplane in two different wind tunnels. The main focus of the investigation was on the aerodynamic effects of the various control elements. The first part of the aerodynamic investigation was carried out in the DLR-SWG (Side-Wind-Tunnel-Göttingen of the German Aerospace center), which is a closed-loop, low-speed wind tunnel. Isolated deflections of elevator and leading-edge flaps were compared to a reference configuration with undeflected control elements. A closed section with a length of 9 m, a width of 2.4 m, and a height of 1.6 m served as the test area. At the maximum power of P=0.5 MW, it is possible to achieve a maximum flow velocity of U∞=65 m/s in the empty test section. The lack of cooling requires an active flow velocity control system, which reduces the variations of the Reynolds number resulting from temperature changes. Each configuration was examined at up to 15 different angles of attack and at three mean flow velocities of U∞=5, 7, and 8.5 m/s. It was not possible to investigate higher velocities in a safe manner due to insufficient structural stability of the model. Wind-tunnel effects on angle of attack, as well as lift and drag coefficients, and pitching moments were corrected using classic linear methods. The measurement system consisted of a six-component RUAG 796-6C strain gauge balance, a Prandtl tube, a Hottinger and Baldwin MGCplus measurement amplifier system, and a computer network.For the investigation of the wing warping, spoileron effects, and hinge moments at the leading-edge flaps, an additional experiment was set up in DLR, German Aerospace Center’s 1 m low-speed wind tunnel (1MG). The right half of the existing 1:5 model was mounted directly on a piezoelectric force and moment balance and exposed to the flow through the wall of the wind tunnel. Yaw and roll moments as well as leading-edge flaps hinge moments were recorded at a mean flow velocity of U∞=8.5 m/s (Table 1).A. PerformanceThe lift over drag polar of the glider is depicted in Fig. 5. The approximately quadratic shape with an offset toward positive lift coefficients is characteristic for a cambered wing. The glider enters the stalled flow regime for lift coefficients above cL=1.1 and achieves a maximum lift coefficient of cL=1.25 at an angle of attack of α=22.3 deg. A minimum drag coefficient of cD=0.078 was recorded. The influence of the freestream velocity is negligible, which indicates a minor Reynolds dependency of the results. It also suggests that the structural deformations are relatively small, because the shape of the wing does not change with the increasing dynamic pressure. Figure 6 shows the lift over drag glide ratio E as a function of the angle of attack. The glide ratio forms a distinct maximum in the range 6.8≤α≤9.2 deg. Because of the limited number of measured angles of attack, the maximum glide ratio and the angle of attack at best glide can only be determined approximately. The maximum glide ratio Emax=5.55 occurs at an angle of attack of α=9.3 deg.To assess the flying qualities in manned flight conditions, the lift coefficients required for pilot masses of mpilot=70, 80, and 90 kg are calculated using a lift curve, which is averaged across the three measured freestream velocities. The resulting trim points for an assumed flight velocity of U∞=11.5 m/s are shown in Fig. 6. The trim angles of attack are located in the range 9≤α≤12 deg, well below the onset of stall and close to the maximum glide conditions. Lilienthal’s reported weight of 80 kg results in a trimmed glide ratio of ETrim=5.3 at an angle of attack of αTrim=10.25 deg, which is only about 5% below the best glide value. A previous investigation by Wienke et al. [14] on Lilienthal’s first patented production aircraft, the Standard Soaring Apparatus, arrived at a trimmed angle of attack of α=16 deg at a significantly lower glide ratio below 4 for the same pilot mass and flight velocity. In comparison, the best glide ratio of the Experimental Monoplane is about 40% higher, which is due to the considerably larger wingspan at identical pilot drag and horizontal stabilizer dimensions. It is questionable whether some experimental imperfections, such as the pilot dummy, which was slightly too small and without clothes, led to a bias toward higher values of the glide ratio. It is understood that the tests performed earlier with full-scale Lilienthal replicas delivered a higher accuracy. However, the main advantage of the Experimental Monoplane is its increased wing surface, which allows an 80 kg pilot to fly very close to the best glide ratio.Fig. 3 Illustration of Lilienthal’s wing warping mechanism (left) and the extreme pilot posture required (right) to counteract a diving flight attitude with weight shift control as sketched and described only in a letter (reprinted in part from O. Lilienthal: Letter to A. Wolfmüller, 1895/10/03, © archive Deutsches Museum München, Acc. 1932-1/11 source [15]: https://lilienthal-museum.museumnet.eu/archiv/objekt/15904).Fig. 4 Reconstruction drawing of the Experimental Monoplane (reprinted from [7], p. 104, © Archiv Otto-Lilienthal-Museum).Fig. 5 Lift vs drag of the Experimental Monoplane with closed leading-edge flaps.Fig. 6 Glide ratios and lift coefficients of the Experimental Monoplane with closed leading-edge flaps (dots indicate lift coefficients required for various pilot weights at 11.5 m/s).B. StabilityBecause many early experimental aircraft designs were not stable with respect to their flight mechanics, the static longitudinal stability characteristics are discussed here based on the measured pitching moment curves. Several conditions have to be met in order to achieve steady, trimmed, and statically stable flight. The total mass of glider and pilot, along with the flight velocity, results in a trim angle of attack on the lift curve, which has to fall within the range of attached flow below maximum lift. At this trim angle of attack, the location of the combined center of gravity and the elevator incidence angle have to be chosen in such a way that the pitching moment around the combined center of gravity becomes zero. Such a flight condition is statically stable when the slope of the pitching moment curve around the combined center of gravity is negative, as it crosses the cm=0 abscissa from positive to negative pitching moments.The Experimental Monoplane is controlled through weight shifts by the pilot and changes in the elevator incidence angle. An increasingly negative elevator incidence angle η shifts the pitching moment curve to higher values. As a result, the elevator can be used as a trim device before takeoff.To give the pitching moment results a better context, they are now compared to data previously published by Wienke [16] for Lilienthal’s preceding aircraft, the patented Standard Soaring Apparatus, trimmed for best glide ratio at U∞=11.5 m/s. Figure 7 compares the pitching moments around the glide center of gravity for the Standard Soaring Apparatus with the results of the Experimental Monoplane at its most negative elevator incidence angle for both open and closed leading-edge flaps. The results of the present study are shown as linear approximations. They were derived from measured data of the attached flow region below α<20 deg in order to suppress measurement noise, which occurred when significant regions of the flow are separated.The pitching moment curves of the two configurations are similar but exhibit different slopes and intersections with ordinate and abscissa. The zero-lift pitching moment coefficient cm0 at the zero-lift angle of attack of α=0 deg will be discussed first. The zero-lift pitching moment of cm0 ≈ 0.05 of the Standard Soaring Apparatus is significantly higher than the one of the Experimental Monoplane at cm0 ≈ 0.03 with closed leading-edge flaps of κ0=0. From this lower zero-lift pitching moment, it follows that the pitching moment coefficients of the Experimental Monoplane are below those of the Standard Soaring Apparatus for the entire angle of attack range up to the trim angle of attack αTrim. It can also be deduced that the slope of the pitching moment coefficient at the trim angle of attack is lower, which indicates less static stability. The zero-lift pitching moment of cm0 ≈ 0.05 of the Experimental Monoplane with open leading-edge flaps of κ1=30 deg is significantly higher and coincides with the one of the Standard Soaring Apparatus. This indicates similar statically stable flight characteristics for both aircraft in this configuration. Because of the more negative pitching moment slope at fully open leading-edge flaps, the trim angle of attack of the Experimental Monoplane for a given center of gravity is also smaller with the open configuration. The main frame position considered for the data analysis shown in Fig. 7 results in the trim angle of attack to be reduced from about αTrim=17 deg down to αTrim=7 deg. To fly the glider with open leading-edge flaps, the pilot would have to shift the center of gravity too far to the front for a sustainable pilot posture. However, the glider was flown with closed flaps that would only open automatically at very low angle of attack. Figure 8 illustrates the working principle of Otto Lilienthal’s automatic pitch control system. The leading-edge flaps were pulled open by rubber bands, whose tension could be adjusted before takeoff. When the angle of attack was reduced, the direction of the net pressure force on the leading-edge flaps eventually changed from lift to downforce, which then opened the leading-edge flaps supported by the tension of the rubber bands. Figure 9 depicts color coded pressure coefficients and streamlines derived from two-dimensional computational fluid dynamics for closed (top) and open (bottom) leading-edge flaps. It can be seen that a closed leading-edge flap leads to a continuously higher pressure on the lower side of the wing and a lower pressure on the upper side. This indicates a relatively constant lift distribution in cordwise direction. For an open leading-edge flap, the flowfield shows a strongly increased lifting pressure difference on the leading-edge flap and a reduced lift force on the main wing due to the reduced pressure values on the main wing’s lower side. The pitching moment is therefore considerably higher with an open leading-edge flap, at the price of a reduced overall lift and an increased drag.Fig. 7 Linearized pitching moment coefficient around the center of gravity of Standard Soaring Apparatus and Experimental Monoplane for two different settings of the leading-edge flap (at maximum elevator inclination) (partly adapted from [14]).The described consequences of the different flowfields can also be found in the measured wind-tunnel data and can be seen in the linear trend of the pitching moment curve as depicted in Fig. 7. This likely extends below the zero-lift angle of attack of α=0 deg, resulting in the open leading-edge flap to produce a higher, nose-up pitching moment than the closed baseline configuration. As a result, the opening of the leading edge adds a returning moment toward positive angles of attack. The beauty of Otto Lilienthal’s approach to gain automatic pitch control by leading-edge flaps lies in their variable deflection. Once the right tension of the rubber bands is set, the pitching moment curve potentially displays both: the relatively high pitching moments at low angles of attack with open flaps and a trim angle of attack around α=10 deg to achieve the required lift with closed flaps. Measurements of the leading-edge flaps lever forces are depicted in Fig. 10. It can be seen that a (closed-flap) pretension of 0.4 N opened the flap at 6.1 deg during the wind-tunnel experiment and led to its automatic closing at 6.9 deg aircraft angle of attack. The applied spring rate of 5 N/m deflected the flaps sufficiently (28 deg with respect to closed position). Considering increased flap areas (25:1) and lever lengths (5:1), as well as the increase air speed (11.5:8.5) during the full-scale flight scenario, this leads to a required pretension of the four rubber bands of approximately 1.8 N each.Fig. 8 Closed (left) and open (right) leading-edge flap positions depending on the angle of attack of the incoming flow.Fig. 9 Color-coded pressure coefficients and streamlines derived from two-dimensional computational fluid dynamics computations for closed (top) and open (bottom) leading-edge flaps.Wind-tunnel and flight-test data of the investigated Standard Soaring Apparatus, the Large Biplane, and the large Experimental Monoplane all show that Otto Lilienthal managed to design flying machines that allowed stable flight within their individual flight envelope. The pitching moment data presented in this article demonstrate the potential of his invention for automatic pitch control and prove how well he understood the necessity to apply control surfaces to a monoplane with a wingspan of nearly 9 m in order to complement his type of weigh-shift control. However, the stability, which was even higher than in some later aircraft designs, was only present for flight attitudes in steady flight, which ensured largely attached flow on the wings. Once the aircraft stalled, the stability vanished, and the pilot had to react rapidly by shifting his weight to the rising front and, in case the stall occurred asymmetrically, to the rising wing’s side. The reason for this limitation lies mainly in the design of the horizontal stabilizer and the tail. It was designed for flights in the vicinity of the ground and for flare landings, which were Lilienthal’s preferred way of landing. In Lilienthal’s American patent description from 1895, which describes the world’s first serial production aircraft, the Standard Soaring Apparatus, he wrote the following [2]: “…on the latter is pivoted the tail in such a manner that it can freely turn upward, but finds downward a point of support on the fixed rudder. This mode of attaching the tail has the advantage that the tail will have no carrying action when the machine is employed like an ordinary parachute, thereby preventing from turning downward.”This ability of the horizontal stabilizer to freely turn upward and thereby to have no carrying action when the airflow acts from below is wonderful when flying in ground vicinity but can become deadly when flying at higher altitudes. In case of a flare landing, which is also commonly used with modern hang gliders, and stall that occurs near the ground, the result is a pancake landing, a vertical fall that sees the wings leveled and acting “like an ordinary parachute” [2]. However, high flight altitudes require the aircraft to start diving in order to accelerate and recover. On 9 August 1896, the day of his fatal crash at age 48, Otto Lilienthal flew his patented monoplane for the first time in several weeks, after concentrating on flying his Large Biplane in the meantime. At an altitude of approximately 15 m above ground, he was stopped by a wind gust and, in spite of his experience, did not manage to lower the rising leading edge of his left wing by weight shifting, as he had frequently done before. His deadly crash on that day confirmed the risk of flying at higher altitudes and outside the flight envelope at high angles of attack. His more than 2000 gliding flights, however, had s

In 1895, Otto Lilienthal patented, built and repeatably flew a research aircraft equipped with a set of control surfaces actuated by the pilot. It had rubber-band activated leading edge flaps for automatic pitch control, and spoilerons, wing warping and rudder for yaw and roll control, which were actuated by a hip cradle. A full-scale replica and a 1:5 model were built for this investigation. All structural materials relevant to the flying qualities, were selected with great care. The 1:5 model was tested in two different wind tunnels at the “Institute of Aerodynamics and Flow Technology” of the German Aerospace Research Center (DLR) in Göttingen, Germany. The full scale replica was used for a set of tethered flight tests on the Outer Banks (NC) in cooperation with Kitty Hawk Kites. The wind tunnel tests gave new insights into the performance, trim state, flight stability and controllability. Based on the commonly used classification of Lilienthal’s gliders the “Experimentiergerät” or in the remainder of this text “Experimental Monoplane” has a wingspan of 8.8m and a wing surface area of 23m2 (appr. 250ft2). Lilienthal wrote that those larger dimensions lead to better performance at low wind speeds compared to his previous designs, but required additional means of control, as his weight shift method provided only very limited control authority for wing spans of more than 7m. Wind tunnel measurements and a limited set of flight tests were performed in order to investigate, whether the patented automatic pitch control and the control surfaces were sufficient to control the glider.

The paper describes full-scale balance tests, which have confirmed the structural integrity and longitudinal static stability of an authentic replica of Otto Lilienthal’s Large Biplane glider. In addition, the replica, which has been built after patent drawings, photographs, and textual descriptions provided by the Otto-Lilienthal-Museum, was towed using a rope winch and, in a next step, finally flown downhill without a rope or guide wires. These tests allow for the assessment of the aircraft’s controllability and stability, not only during steady flight but also during takeoff, landing, and in case of wind gusts. A brief description of the design parameters and applied methods is given, along with force and moment data gained from balance measurements, as well as the evaluation of a limited set of data acquired during flight, including speed, duration, and distances flown.

A full-scale wind-tunnel test of a replica of Otto Lilienthal's Normalsegelapparat from 1893 was conducted in a closed low-speed wind tunnel. The goal was to determine the flight performance and characteristics of this glider for the first time. It is of particular interest to this investigation to resolve the open question about the design's static stability and to clear up whether Lilienthal treated the cloth covering the wings. Therefore, tests were performed both with an impermeable, sealed cloth as well as with untreated, permeable wings. Forces and moments were measured at freestream velocities between 5 and 13 m/s at angles of attack from the linear lift interval up to fully stalled conditions. The recorded data are used to investigate the flight performance and characteristics of the glider with an emphasis on its static stability. The impact of the permeability of the cloth in comparison to the impermeable configuration is discussed.

The paper describes full-scale balance tests, which have confirmed the structural integrity and longitudinal static stability of an authentic replica of Otto Lilienthal’s Large Biplane glider. In addition, the replica, which has been built after patent drawings, photographs, and textual descriptions provided by the Otto-Lilienthal-Museum, was towed using a rope winch and, in a next step, finally flown downhill without a rope or guide wires. These tests allow for the assessment of the aircraft’s controllability and stability, not only during steady flight but also during takeoff, landing, and in case of wind gusts. A brief description of the design parameters and applied methods is given, along with force and moment data gained from balance measurements, as well as the evaluation of a limited set of data acquired during flight, including speed, duration, and distances flown.

Natural wind, roadside obstacles, terrain roughness, and traffic influence the incident flow of a vehicle driven on public roads. These transient on-road conditions differ from the idealized statistical steady-state flow environment utilized in CFD simulations and wind tunnel experiments. To understand these transient on-road conditions better, measurements were performed on German public highways and on a test site. A compact car was equipped with a measurement system that is capable of determining the transient airflow around the vehicle and the vehicle’s actual driving state. This vehicle was driven several times on a predefined 200 km long route to investigate different traffic densities on public highways in southern Germany. During the tests the transient incident flow and pressure distribution on the vehicle surface were measured. With the same test vehicle, individual driving situations were recreated on a test site under weather conditions similar to those of the tests on public roads. This paper presents a comparison of the aerodynamic characteristics measured on public highways and on the test site. Two driving situations were examined at the test site: one is driving the test vehicle without traffic and the other is driving behind a box truck in different distances. This paper compares the realistic properties of the turbulent flow structures and the surface pressure around the vehicle during on-road driving in public traffic with those measured on the testing site. The purpose of this study is to investigate the possibility of reproducing the representative driving on public highways with a simplified approach.

Natural wind, roadside obstacles, terrain roughness, and traffic can influence the incident flow of a vehicle driven on public roads. These on-road conditions differ from the idealized statistical steady-state flow environment utilized in CFD simulations and wind tunnel experiments. For this reason, the transient aerodynamic characterization of the airflow around a vehicle used under such conditions is shown in this paper. A compact car was equipped with a measurement system that is capable of determine the transientairflow around the vehicle and the vehicle’s actual driving state. A fixed driving route on public highways in southern Germany was driven several times, with that vehicle. The tests were conducted under consistent weather conditions and average wind velocities of 2-5 m/s. During the tests the transient incident flow and pressure distribution on the vehicle surface were measured and considered in this paper. Additionally, theground clearance occurring during the tests is discussed. The results of this paper contribute the characterization of the aerodynamic real-world driving conditions on public highways. Furthermore, the results aim to provide a reference case for the integrationof the real-world on-road driving conditions into CFD simulations and wind tunnel experiments.

Open AccessHistory of Key TechnologysFlight-Testing Stability and Controllability of Otto Lilienthal’s Monoplane Design from 1893Markus Raffel, Felix Wienke and Andreas DillmannMarkus RaffelDLR, German Aerospace Center, D-37075 Göttingen, GermanySearch for more papers by this author, Felix WienkeDLR, German Aerospace Center, D-37075 Göttingen, GermanySearch for more papers by this author and Andreas DillmannDLR, German Aerospace Center, D-37075 Göttingen, GermanySearch for more papers by this authorPublished Online:16 Jun 2019https://doi.org/10.2514/1.C035399SectionsRead Now ToolsAdd to favoritesDownload citationTrack citations ShareShare onFacebookTwitterLinked InRedditEmail AboutNomenclatureAfflight altitude; 0≜ to straight legs; feet on grounda0lift slope in lift polar diagramCDdrag coefficient of aircraftCLlift coefficient of aircraftCM,aclongitudinal moment coefficient about aerodynamic centerCM,cglongitudinal moment coefficient about center of gravityDdrag of aircraftLlift of aircraftL′lateral moment about the aircraft’s centerlineMaclongitudinal moment about aerodynamic centerMcglongitudinal moment about center of gravityQpitch rateq∞dynamic pressurettimeV∞freestream velocity relative to aircraftαaircraft’s geometric angle of attack; 0≜ vertical main frameαeaircraft’s trim angle of attackα0aircraft’s absolute angle of attack; 0≜ zero liftθgeometric tail plane angle of attackρair densityI. IntroductionIn 1889, Otto Lilienthal published his book titled Birdflight as the Basis of Aviation [1], describing his experiments proving the concept of curved wings and providing the best available instructions of that time on how to design a winged aircraft. Starting in 1891, Otto Lilienthal made the first, documented, successful flights on a heavier-than-air man-carrying aircraft under the control of a human being. In 1893, he patented the world’s first production aircraft, called the “Normalsegelapparat,” which was a monoplane glider, of which he sold at least nine machines to various places in Europe and America. He performed more than 2000 successful flights, leading to his aerodynamic data and flight reports being circulated around the world. Samuel Langley as well as Octave Chanute and many others corresponded with Lilienthal. Chanute, author of the other most influential book of that time [2]* and later a friend of the Wright brothers, followed Lilienthal’s approach to carefully perform flight tests; with that, he led several younger men to successful flight performances in 1896 and beyond. Wilbur Wright wrote in his article [3]† about Otto Lilienthal: “… he was without question the greatest of the precursors, and the world owes to him a great debt.” According to the patent description, the monoplane glider falls under the category covered by Federal Aviatian Regulations Part 103 [4], which is described in more detail in the Weight-Shift Control Aircraft Flying Handbook (FAA-H-8083-5) of the Federal Aviation Administration [5]. Weight-shift control aircraft are still being flown, safely and successfully, by many thousands of pilots around the world. However, after Otto Lilienthal’s fatal accident in 1896, very little has been known about later attempts to fly this aircraft.II. Glider Dimensions During Wind-Tunnel and Flight TestsWind-tunnel tests gave new insights into the performance, trim state, flight stability, and influence of the permeability of the fabric, which has been woven on an original loom using a formula that was developed based on a careful analysis of fabric taken from an original glider wing. Two parameters that depend on the weight, size, and fitness of the test pilot are the wing area and the pilot’s position in the glider. Their selection was motivated by the analysis of the wind-tunnel data presented in the following. However, in order to focus on the findings gained during the flight tests, only the two diagrams, which helped determining the best possible glider dimensions, will be discussed here.The commonly used classification of Lilienthal’s gliders was introduced by Nitsch ([6] pp. 163–164). According to him the so called “Normalsegelapparat” (“standard soaring apparatus”) has a wingspan of about 6.7 m and a wing area of 13.2 m2. According to Lilienthal’s descriptions, larger dimensions lead to aircraft that are difficult to control in wind gusts. The wind-tunnel model used in the 2017 full-scale wind-tunnel tests at the largest European wind tunnel, the DNW-LLF, had a span width of 6.7 m and a surface area of 13.2 m2. Its main frame location differed slightly from the patent‡ drawing [7] by some centimeters, which has been found to be of great importance to the pilot’s required posture for stable flight. A freestream velocity of 11.5 m/s and an aircraft geometric angle of attack of α=5.82 deg (α=0 deg corresponds to a vertical main frame orientation) leads to sufficient lift to lift the glider (25 kg) plus a person of Otto Lilienthal’s weight (≜80 kg) (see Fig. 1).Fig. 1 Lift coefficient versus geometric angle of attack measured using the wind-tunnel glider at 11.5 m/s.It was discovered during the force and moment measurements that the glider built for the wind-tunnel experiments was difficult to trim. It tended to be tail heavy, even for a small negative angle θ of the horizontal tail plane. Additional tethered flights in the wind tunnel confirmed these findings. During the wind-tunnel run, the test pilot could not continuously hold his body in the most frontal position because of the approximately 1 min time required to accelerate the wind-tunnel flow. In contrast to the glider’s orientation going downhill, in the wind tunnel, the glider and the freestream velocity were inclined upward by approximately 15 deg, which made the pilot’s most forward-leaning posture even more arduous. The tethered flight in the DNW-LLF confirmed that an additional 8.2 kg of trim weight at the front of the cockpit was required to fly the wind-tunnel glider version stably in a horizontal setting. This nosedown force was applied through a ballast cord at the cockpit front pulling downward (see Fig. 2).Fig. 2 Steady tethered flight with an externally applied nosedown moment correction, corresponding to an 8.2 kg trim ballast at the aircraft’s front.The larger weight and body size of the test pilot (see Table 1) led to the following dimensions of the outdoor glider, which later was used for flight testing: the wingspan was chosen to be 7 m and the maximum chord length was 2.5 m, resulting in a wing surface area of 15.6 m2. The fact that the main frame location with respect to the wing area center differed on the wind-tunnel model and the patent drawings (see Fig. 3), and that the wind-tunnel model required a ballast force to reach steady horizontal tethered flight, led to a main frame position of the outdoor glider that was 134 mm further ahead of that of the wind-tunnel glider. The main frame position limits the pilot’s front position, and therefore influences the trim state and static margin and/or the posture required to move the center of gravity in front of the aerodynamic center, which is necessary to obtain a stable and trimmed flight. It is well known that weight-shift control aircraft tend to handle best with small static margins for obvious reasons. However, small static margins also imply that just a few centimeters displacement of the relative center of gravity changes the handling qualities of the aircraft from “easy to control” to “impossible to fly.”Fig. 3 Comparison of wing surface geometry and size at identical main frame positions of three aircraft.Wing surface geometries and sizes at identical main frame positions of the three aircraft versions are depicted in Fig. 3. Instead of the aerodynamic center, the wing’s area center has been chosen as the reference point for the three glider versions because the aerodynamic center was measured only for the wind-tunnel version of glider. However, it is assumed that the wing surface center and aerodynamic center position are displaced at very similar quantities for all three glider versions because of their geometric similarity.Following the wind-tunnel experiments, measurements of the longitudinal center-of-gravity location of the wind-tunnel glider were performed for each of the pilot’s various postures. The evaluation of these data, together with the force and moment data, showed the same deficit of the static margin that was present on the wind-tunnel glider. The necessity of the trim ballast can be explained by the fact that the main frame location, and therefore the pilot’s center of gravity, differed between the wind-tunnel model and the patent drawings depicted in Fig. 3 (top). A detailed analysis of a vertical projection of the glider wings and the location of the glider main frame showed a distance between the wing surface area center and the main frame at the patented glider of 452 mm, as compared to only 348 mm for the wind-tunnel model (see Table 2). A correction to the pilot position within the glider was applied to the measured data, and the results showed that the patented glider could be flown stably at moderate negative tail plane angles without ballast, given a pilot posture as depicted in Fig. 4c (trimmed).Fig. 4 Pilot’s postures during a) start, b) flight middle posture, c) flight trimmed posture, and d) landing.III. Familiarization of the Test Pilot: Tethered Flights and Winch-Supported FlightsThe first thing the test pilot had to learn when attempting to fly the monoplane glider was longitudinal and lateral control by weight shifting. Directional control is only performed indirectly by lateral control. Otto Lilienthal gave the following advice for longitudinal control [2]:The first rule is to keep your legs well extended toward the front, and in landing to throw the upper part of the body backward, so that the front edge raises itself and thus checks the motion, as may be seen whenever a crow alig hts.This leads to the most forward center-of-gravity position during takeoff and the most backward position during landing. They are described by the sketches of postures shown in Fig. 4, which were drawn according to the figures given in the patent drawing by Otto Lilienthal.Lilienthal advised to always use the following procedure to start [2]:The starting and landing must be done exactly against the wind. The fixed vertical rudder will keep the apparatus exactly in the wind when in a state of rest. The horizontal rudder keeps the apparatus from tipping over forward, a thing that arched surfaces are inclined to do.In our case, it was decided to start with tethered flights on a 5×5 m2 platform towed by a passenger car. This allowed for frequent training at reduced risk and costs. The glider as well as the pilot were attached to the platform corners with safety bonds (pilot) and wires (glider), as depicted in Fig. 5. The pilot was wearing a harness connecting him to four safety bonds. The glider had eyebolts anchored to the lower ends of the main frame crossbars through which the steel wires ran from the front corners of the platform to the rear corners on each side. This arrangement limited the glider’s altitude but allowed for directional motion. Two ropes were attached to the car in order to compensate for the aircraft’s drag and to pull the glider forward. Given a glider altitude of less than half of a 1 m above the platform and a lateral position in the center of the platform, the ropes fastened to the platform corners were loose. The pilot could therefore test the influence of weight shifting in longitudinal direction, leading to the postures depicted in Fig. 4.Fig. 5 Practicing at “zero ground speed,” limited height, and limited control authority: platform tests.The two pull ropes were attached to the ends of the glider main frame crossbar by another four ropes, which intersected at a central attachment point 0.7 m in front of the main frame. Therefore, the longitudinal stability was improved and the pilot postures had a reduced influence on the pitching motion.Heading and wind directions did not match during the tethered flight tests because they were conducted on an airfield with just one runway. Therefore, the pilot had to compensate for a crosswind component of up to 5 m/s (see Table 3).The experiences gathered during the tethered flights on the platform were required for the next phase of winch-supported tests. The fact that these flights were performed against the wind (see Table 4) allowed for flights at reduced ground speed (see Fig. 6). The winch tests had to be conducted without the safety wires that, during the platform tests, not only limited the flight altitude but also the roll motion of the glider. The pilot’s lateral control skills became the key issue in cases of moderate gusts on the one hand, as well as in cases of stall that could lead to asymmetric lift on the other hand. (Asymmetric lift during stall occurs especially at low pitch rates Q or initial high attack angles. The influence of the leading-edge vortices inherent to dynamic stall at flare landings will be described later.) Lilienthal described the procedure of lateral control (see Fig. 7) and related difficulties as follows [2]:Fig. 6 Winch speed versus distance traveled.Fig. 7 Practicing lateral control with limited longitudinal control authority: winch tests.The following mistake is to be particularly avoided. The experimenter is soaring in the air and feels himself suddenly raised by the wind, but unequally, as is usually the case--for instance, the left wing more than the right. The inclined position forces him toward the right. The beginner involuntarily stretches his legs to the right, because he foresees that he will strike the earth on the right hand. The result is that the right wing, which is already lower, is loaded still more, and flight tends more and more downward and to the right, until the tips of the right wing strike the earth and are broken. For life and limb there is less danger, as the apparatus forms an efficient guard in every direction, which checks the force of the blow. The correct thing to do is always to extend one’s legs toward the wing that is rising, and thus to press it down again. In the beginning this requires some force of will, but this useful movement soon becomes an unconscious one, after we see how surely the wings can be guided this way and be protected from damage.IV. Trimming of the GliderThe winch tests in Germany were performed with an attachment point using a carabiner clasp placed approximately 0.7 m in front of the main frame center. This additional restoring moment increased the longitudinal stability considerably. Thus, the longitudinal control authority was reduced as long as the pulling rope was under tension. The fact that the outdoor glider was still a bit tail heavy had been overlooked because the pitchup tendency became relevant only at low rope tensions when the glider was in descent and the pitchup was intended and supported by the pilot’s landing posture (Fig. 4d). During the free flight in California, though, this needed to be corrected by either a ballast weight or a reduction of wing area ahead of the pilot. The first measure chosen to increase the static margin for a given stabilizer angle and center of gravity was the reduction of wing area in front of the glider by a v-shaped enlargement of the cockpit “window,” which is the gap in the fabric in front of the pilot. Additionally, after some remaining problems with longitudinal control and stability, the wing area at the glider’s front was further reduced. The front ribs of the wings were moved a few centimeters backward, and the fabric along the leading edge of the wing was cut and reattached accordingly.§ The resulting final shape of the glider is depicted in Fig. 3.Figure 8 depicts CM,cg versus a0 for three different pilot postures for the final version of the outdoor glider. The data have been derived from the wind-tunnel force and moment data measured in 2017, making the following assumptions: The lift increases proportionally with the wing area (15.6/13.24).The weight of the pilot (90 kg) is located further forward than during the wind-tunnel tests because of the altered main frame position of the outdoor glider used for flight tests (134 mm).The dashed line in Fig. 8 corresponds to the middle position (Fig. 4b), the dotted and dashed line corresponds to the trimmed position (Fig. 4c), and the solid line corresponds to the start position (Fig. 4a). It should be noted that the dotted–dashed line intersects with the horizontal axis for CM,cg=0 at a0=17.25 deg. This defines the trim angle of attack; the horizontal tail plane angle; and the pilot posture during trimmed, stable, and controllable downhill flights at 11.5 m/s when the systems weight, drag, and lift forces are in equilibrium. The three necessary criteria for longitudinal balance and static stability (namely, the positive moment coefficient at α0=0, the negative moment curve slope, and the trim angle of attack within the flight range) are fulfilled. The measured data correspond very well to the conditions found during the downhill flights, after the start, but before the landing.Fig. 8 Lift coefficients versus geometric angle of attack derived for the outdoor glider at 11.5 m/s (left). Moment coefficient versus absolute angle of attack derived for the outdoor glider (right).After this modification, the glider could easily be controlled in the longitudinal and lateral directions as well as in free downhill flight, as in cases of flights that were initially accelerated featuring a bungee cord. The latter method was used when the wind and slope of the terrain were too weak for pure foot launching. The bungee cord was a well-suited means to increase the available practicing time and made it possible to get airborne at Dockweiler Beach in the vicinity of Los Angeles and at Tres Pinos in the vicinity of Hollister (in California). Due to the fact that this bungee cord was attached to one central hook directly on the main frame, the trim, stability, and control authority was the same with and without the bungee cord. All final flights were performed freely without any ropes attached at Sand City, north of Monterey (in California).The outdoor glider used in the end for the free flights differed in some other minor aspects from the gliders used by Otto Lilienthal and others at his time:The outdoor glider has been coated with white wood glue. This glue behaves physically just like other types of collodion that was used to coat the Lilienthal gliders. After it is applied to human skin, for example, it acts like a sort of second skin in both cases. The monoplane glider was folded many times, sometimes in very rough conditions, but the coating showed no sign of wear. Ongoing wind-tunnel investigations on the influence of the porosity demonstrate that forces and flowfields are the same for different ways of sealing the fabric. So, this is a chemical difference, but it is aerodynamically of little or no relevance.The skids attached to the lower ends of the main frame were, like the test pilot’s kneepads, only in use after mishaps during landing. So, they were also of little influence during flight. Stainless steel wire ropes of 2 mm in diameter and oval compression sleeves were used instead of steel wires and tension locks (patented by Lilienthal himself).Two parallel short bamboo sticks have been mounted on the outdoor glider on both sides of the cockpit to prevent the pilot from leaning too far back. They substitute the backrests that Lilienthal used and are, like the helmet, a further concession to safety considerations. Luckily, neither the helmet nor the bamboo sticks have ever been needed.The arm rests of our outdoor glider were made more primitively. The pilot could lean on them like Lilienthal on his, but he could not lift the glider in the back in order to get the minimum incidence angle for the start at low-wind conditions. During the winch flights and the final downhill flights, this was not necessary because the head wind lifted the rear part of the glider immediately after the first few meters. At low-wind conditions, the pilot used shoulder straps from time to time to lift the tail before start, and thereby facilitate the acceleration.A structural difference between our outdoor glider and the version that was described by Lilienthal in his patents and publications is described in the following. Willow was used only for parts that had to be bent significantly, like the cockpit frame and the vertical and horizontal stabilizers. However, pine wood sticks, which had been bent into shape while being soaked in water for three days, were used instead of willow for the struts of the wings. This increased the weight to 32 kg and might have shifted the center of gravity of the glider a bit toward the rear. Still, it needed to be done due to the test pilot’s weight being approximately 15% greater than Lilienthal’s, and the required lift forces are approximately another 15% higher when flying horizontally (tethered and at the winch rope) than during downhill flights. As for Lilienthal’s gliders, bamboo was used only when parts of a certain length had to be stiff and straight, like the tail.V. Acquirement of Longitudinal Control Skills: Free FlightsWhen practicing on a training hill at Tes Pinos in the vicinity of Hollister (California), the wind tended to be gusty. During long periods over the year, the winds blow from northwest along the valley; but, on the test day, they were dominated by a strong upper western wind leading to flow separation at the western California coast ranges, which created a few occasional rapid changes in local wind strength and direction. A sudden gust in the early flight phase after takeoff created an upward acceleration, which made keeping the pilot’s posture for trimmed condition impossible. The altitude variation could later be reconstructed by correlation-based image processing from the videos (see Fig. 9). A 10th-order polynomial was used to fit the altitude data estimated for each 0.1 s and to derive the rate of climb (or descent, respectively) and the load factor for a time interval of 3.5 s (see Fig. 10).Fig. 9 Stall and poststall behavior.Fig. 10 Altitude, rate of climb/descent, and load factor during the stall event depicted in Fig. 9.The additional inertial forces made it impossible to keep the legs directed straight and forward. The glider was lifted approximately 4 m within 1 s, and the glider’s incidence angle moved from approximately 15 deg downward to 50 deg noseup. As a consequence, the pilot’s weight moved backward within the cockpit. Lilienthal routinely jumped from a roof of more than 4 m high on a routine basis and managed acrobatically to pitch the nose down and avoid roll, even in those vulnerable stalled conditions without lowering his legs. Lilienthal experimented with an actively steered tail plane, flaps, and wing warping, but he intentionally decided to stay with the weight-shifting control because this allowed him to operate the glider reliably during thousands of flights [8]. The many flights that he started by jumping off the building roof at the “flight station” near Steglitz (a suburb of Berlin, Germany) required the start of the glider reliably at zero ground speed. It is hard to imagine that this can be done better using active control means, and thus weight-shift control is still the method of choice for most unpowered foot-launched aircraft. However, foot-launching gliders in gusty wind conditions require a great deal of practice. As a consequence of the pilot’s inability to counteract stall, in Tres Pinos, the glider stopped and fell approximately 4 m like a parachute: the tail plane flipped upward, although no strong nosedown moment occurred, and the wings stayed nearly level. This safety feature of the glider was described in Lilienthal’s U.S. Patent [7] with the words:On the latter is pivoted the tail q in such a manner that it can freely turn upward, but finds downward a point of support on the fixed rudder r. This mode of attaching the tail has the advantage that the tail will have no carrying action when the machine is employed like an ordinary parachute, thereby preventing from turning downward.It should be noted that the tail could not transfer an upward force because it was rigidly coupled with the upper ropes when the horizontal stabilizer acted downward, but it flipped up when a force acted on it upward. The upward flexibility prevents the tail from breaking when it touches the ground during flare landings. It is well understood that these observations cannot substitute an analysis of dynamic stability of the aircraft. However, they indicate that extreme situations can exceed the pilot’s capacity to maintain a posture required for balanced, controlled flight. Due to the pivoted tail plane, stall seems to be less problematic when a pilot behaves cautiously and does not counteract stall in a later stage. Lilienthal’s annual flight reports not only informed others about successful flights but also about occasional failures [9]. One such flight accident described in detail was a rapid dive that occurred after a small change of wing curvature. This can be interpreted in a way that these modifications had an effect on the static stability of the glider. Additionally, it seems likely from the experiences of the authors’ of this article that the required pilot posture to flare, as depicted in Figs. 4b and 4d, will potentially exceed the pilot’s strength in such a situation. The dimensions of the glider used for the free flights corrected geometric imperfections of the wind-tunnel model and increased the static margin to an amount that matched the static margin of the patented glider. The glider has been safely flown after the described incident, but strong wind gusts were more consequently avoided.The final pitch trim was obtained by a moderate negative angle of attack of the horizontal tail plane. As a result, the glider reacted nicely and sensitively to the pilot’s input and could easily be directed against the wind. During several starts on the sand dune, shear winds required counteracting a descending left wing counterintuitively by shifting the pilot’s weight below the right wing. Here, the training during the winch-supported flights paid off; and when the wind lowered one of the wings, the pilot shifted his legs to the other side in an instinctive manner (see Fig. 11). Basically, the control of the roll angle has to be performed like the one of a modern hang glider, but the legs need to travel greater distance sideward in order to create a similar reaction of the glider, due to the smaller weight that is shifted, when repositioning just the legs. In addition to the fact that a modern glider is laterally controlled by the relocation of the pilot’s whole, it also amplifies the roll moment L′ by shifting the keel, and therefore warp the wings accordingly ([5] pp. 3–9). It has to be noted that turns cannot safely be executed while flying at low altitude in the vicinity of a hill. Therefore, the question of whether steep turns can safely be performed remains unanswered by our tests. It is known from Lilienthal’s flight reports that he, who flew much higher, made turns but (still) tried to avoid them because a safe landing has to be conducted against the wind ([10] p. 267).Fig. 11 Preparation, start, and initial flight phases with lateral and longitudinal control.Lilienthal reported that landing requires a similar counterintuitive move, as is the case with turning the glider ([10] p. 266). He reported that the pilot has to move his legs backward to pitch up and decelerate, even if his instincts advise him to have his feet in the front when approaching the ground at higher speeds. However, this depends on the trim of the glider and, in the case of our test flights with the monoplane glider, it is just enough to lean backward with the upper body, and therefore move the weight of the whole body to the rear (see Fig. 12) in contrast to flights with best gliding performance where the pilot is in most forward position (see Figs. 13 and 14) [11]. The only problems that occurred while coordinating the landings at the beginning were the same problems beginners face during their first hang-glider lessons. When initiating the landing too early and too slowly, stall occurs in a way that the flow on the wings separates slowly but massively. As massively separated flow is never steady or two-dimensional, one wing starts sinking earlier than the other and generates more drag at the same time. This leads to an unintended turn at the end of the flight with both Lilienthal’s gliders as well as with any modern weight-shift control aircraft. The trick to landing the Lilienthal glider well is doing this maneuver a little later and at higher pitching rates Q so that the stall occurs dynamically. The dynamic stall vortices along the leading edges of the wings will then force the flow into a two-dimensional condition [12] while creating a short lift overshoot ([13] p. 1]) and an additional pitchup moment. After these lessons were learned, the glider could be flown reliably and steadily for up to 70+ m, and the coordination of gentle straight flare landings were performed routinely.Fig. 12 Straight, controlled downhill flight and landing.Fig. 13 Otto Lilienthal and Paul Beylich: Fliegeberg near Berlin [11] (top). Markus Raffel and Andrew Beem: Sand City near Monterey (bottom).Fig. 14 Preliminary flight tests with Lilienthal’s large biplane.VI. ConclusionsOtto Lilienthal’s patented monoplane glider (Normalsegelapparat) was flown reliably and steadily downhill over distances of up to 72 m. The static margin of the glider as laid out in the patent drawings of Lilienthal can safely be flown by a person with a weight of not more than 80 kg wh

An analytical approach for the correction of the influence of permeability on aerodynamic lift and drag polars of finite wings is presented. The approach is validated through wind tunnel measurements of a simple wing-body configuration. Polars for both permeable and impermeable wings were measured at varying inflow velocities. Using only measured values for permeability and porosity, the permeable polars can be mapped to the impermeable case and vice versa with good agreement.

Objetivos: Este trabalho se propoe a verificar o consumo maximo de oxigenio de um atleta paraolimpico da modalidade de esqui alpino, apontando a relacao com essa modalidade esportiva, tradicionalmente reconhecida como anaerobica. Os resultados do teste sao apresentados como parâmetro comparativo para a modalidade de esqui alpino.Descricao do caso: O voluntario foi um atleta de esqui alpino de 38 anos de idade, vitima de lesao da medula espinhal desde 1994, classificacao LW10/2 dentro do criterio dessa modalidade esportiva. O atleta foi testado na propria cadeira de rodas, em esteira ergometrica. O teste foi realizado a uma velocidade constante, sendo a carga aumentada 20 W a cada 3 minutos ate a fadiga voluntaria. As respostas cardiorrespiratorias foram aferidas continuamente com eletrocardiograma e analisador de gases. Amostras de sangue foram coletadas antes e depois do teste para medir a concentracao de lactato sanguineo. A maior carga desempenhada pelo atleta foi 100 W (mecânica) e 884,07 W (bruto) quando estava no 5o estagio, com eficiencia de 11,31%, apresentando consumo de oxigenio de 2501 mL/min e lactato de 11,1 mmol/L. A frequencia cardiaca maxima foi de 184 batimentos/min e a pressao arterial de 115/70 mmHg foi medida 5 minutos apos o final do teste.Conclusoes: O esquiador pode responder ao procedimento de diferentes cargas durante o teste. No teste aplicado o atleta tem que aprender a gerenciar sua resistencia, forca e capacidades coordenativas. A avaliacao da capacidade aerobica podera ajudar o desempenho durante o treinamento e competicoes, visto que os atletas devem dividir sua atencao com uma gama de exigencias que juntas sao atendidas em grande parte pelo sistema aerobio.(AU) Aims: This study aims to determine the maximal oxygen uptake of a paralympic alpine ski athlete and relate it to this sport that is traditionally recognized as being anaerobic. The test results are presented as comparative variables for the alpine skiing sport. Case Description: The volunteer was a 38 year old sitting-class alpine ski athlete, who suffered a spinal cord injury in 1994, and is classified as LW10/2 within the criteria of the sport. The test was performed using their own wheelchair on a treadmill at a constant speed. The load was increased 20 W every 3 minutes until volitional fatigue. The cardiorespiratory responses were monitored continuously with an electrocardiogram and gas analyzer. Blood samples were collected before and after testing to measure blood lactate concentrations. The highest load achieved by the athlete was 100 W (mechanical) and 884.07 W (gross) when in the 5ostage, with an efficiency of 11.31% and presenting an oxygen consumption of 2501 mL/min and lactate of 11.1%. Maximum heart rate was 184 bpm and blood pressure, measured 5 minutes after test end, was 115/70 mmHg.Conclusions: The skier was able to perform the procedure with increasing loads. The test administered required the athlete to learn how to manage her stamina, strength and coordinative capacities. The assessment of aerobic capacity may help with performance during training and competition, as the athletes must focus their attention on a range of demands that together, are largely met by the aerobic system.(AU)

An analytical method based on Kummer’s series transformation is presented, which allows for the evaluation of Fourier-Bessel series with poor convergence properties and, in addition, yields the singularities of the series in closed form. This method is applied to the Fourier-Bessel series which arise as solutions of the linearized gas dynamic potential equation for the cylindrical flow field of a supersonic free jet. As an illustrative example, the presented method is applied to D. C. Pack’s classical solution for the axisymmetric free jet with initial homogeneous pressure perturbation, which in its original form cannot be evaluated directly. It is shown that in contrast to the plane jet, the flow field of the axisymmetric jet does not exhibit a strictly periodic behaviour, whereas its singularities are distributed periodically.

By extending the methods of Part 1, the general problem of steady cylindrical supersonic free jet flow is treated in a similar manner to the flow in quasi-cylindrical ducts. It is shown that the presence of a finite pressure jump at the nozzle lip gives rise to a periodic singularity pattern in the flow field. Basic examples of free jet flows are discussed, and for the case of a nearly ideally expanded axisymmetric jet, theoretical Mach-Zehnder interferograms are calculated by analytical integration of the density field. Excellent agreement with experiment proves the validity of linear theory even close to the singularities and far downstream of the nozzle orifice. Furthermore, it is shown that Pack's formula for the wavelength of the shock cell structure is inconsistent; the correct formula is derived and excellent agreement with Emden's empirical fit is found.

Based on linear potential theory, the general three-dimensional problem of steady supersonic flow inside quasi-cylindrical ducts is formulated as an initial-boundary-value problem for the wave equation, whose general solution arises as an infinite double series of the Fourier–Bessel type. For a broad class of solutions including the general axisymmetric case, it is shown that the presence of a discontinuity in wall slope leads to a periodic singularity pattern associated with non-uniform convergence of the corresponding series solutions, which thus are unsuitable for direct numerical computation. This practical difficulty is overcome by extending a classical analytical method, viz. Kummer's series transformation. A variety of elementary flow fields is presented, whose complex cellular structure can be qualitatively explained by asymptotic laws governing the propagation of small perturbations on characteristic surfaces.