Flight Controls of Otto Lilienthal’s Experimental Monoplane from 1895

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