CASE RILE COpy

TECHNICAL 4EMORATDU'S

NATIONAL ADVISORY COMMITTEE POR AEROT.XUTICS

No, 67].

TWELFTH RHN SOARING COITTEST, 1931

By Walter Georgii

Zeitschrift f1r Flugtechnik und. Motorluftschiffahrt Vol. 23, Nos. 4 and. 5, February 9, and. 1arch 14, 1932

Verlag von R. O1d.enbour, Munchen und Berlin

7ashLigton Nay, 1932

https://ntrs.nasa.gov/search.jsp?R=19930094745 2018-05-31T23:54:15+00:00Z

NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS

TECHNICAL MEMORANDUM NO • 671

TWELFTH RHN SOARING CONTEST, 1931*

By Walter Georgii

The Twelfth Rh8n soaring competition formed the fit-ting conclusion of a period of gliding activities never seen before. Throughout the months from spring until mid-summer the gliding enthusiasts were much in' evidence in all parts of Germany, trying for the Hindenburg Glider Prize, Competition was benefited considerably by the general in-troduction of the towing glider in the spring of the year. Towed aloft by airplane, it made every landing field ame-nable for, performance gliders which heretofore had been restricted to certain localities. Groenhoff ! s flight from Munich to Kaaden (Bohemia), an air-line distance of 275 km (171 mi,), in particular, had left a lasting impression of the great advantages of towing gliders. Groenhoff and Riedel!s excursion by towed glider to Munich led, others to undertake similar flights. It was soon followed by the thermic g].iding experiments of Otto Fuchs in Berlin, which culminated in a flight from Berlin to Frankfort on the Ocler, a distance 0±' 80 km (50 mi.), 'by Kronfeld's flights in England, whoso greatest achievement, apart from his spectacular channel flight, consisted of a therraic gliding flight from London to Chatham, by Hentschel's 11*-hour flight at Drnberg near Kassel, and by Dinort's record of 11 hours and 39 minutes at Rossitten.

•Part of the newer research problems embarked upon last year was the glider expedition into the high Alpine region of the Jungfrau in Switzerland. The crew of the Pafnir - Groonhoff, Riedel, and Uarth - was given an ex-cellent opportunity for 'collecting first-hand data as to the usefulness of gliders in the Alps. It is necessary to recunt the remarkable glider achievements ahead of the Rhon Soaring Contest, in order to inderstand the ten-sion and expectation with which the Rhon Contest was an-ticipated.

* IIErgebnisse des 1. Rhn-Segelflug-Wettbewerbo5 l93l•u From Zeitschrift f'ir Flugtechnik und Motorluftschiffahrt, Feb. 29, 1932, pp. 97-102; and Iiarch 14, 1932, pp. 125-132.

2 N.A.C.A. Technical morandum i 'o.' 671

The great many new glid.ers of groups thaking their first appearance on the Rhon, 'and the astonishing number of juvenile flyers with C certificates, prove that the numerous local contests in various paxts of the country constitute an excellent preliminary training for sailplane contests. In spite of the extremely precarious economic conditions, 59 sailplanes were entered, of which 49 ap-•p'cared.. at the field.. '

'The former method of gradjng tho entries into sclool-• ' ing practice and. performance was retained. in principle,' .even though th9 gliding contest was Qliminated.,. For, ac.. cordiig to tho experiences from the 1930 contest, it was found tnt a cortain classification into junior, advanced, and master groups was. absolutely necessary - 'the fist 'two categories being ad.mitt6d. .to.the 11 practico"'cOnt'et only

• and. comprising the .pi1ot having but lately '(a few months before) received. thr. :0 license nd not having made any duration flights of morethan 5 hours; o those' who.' had not flown since 1928. The official,cértificâte with 5 sailplane flights of altogether 30 miñute' duiation, was necessary to qualify the entrant for the. "performance á'ontest. This method,of grading ha 's proved : eminentiy sat-isfactory. , . . ' r

• Another sign.ficant fact revealed, by the Contest was that pr.eci,selytho juvenile flyers.had, in many cases, not eno-agh ezperience to fly performance gliders, and the 'result was .that'rnany a promising looking craft was 'crashed. during the first fev days. ,

The rules governing the "practice" contest were close-ly patterned. after the previous meets; that is, primarily, duration and. 'altitude, The duration was rated., as in for-'mer years, for total flight duration, and. in two catego-ries: Juniors without experience, nd juniors having more than one hour's experience.

The winners of the "practice" contest for total dura-tion, according to this rating, were the Virtt. Luft-. fahrtverband." under the leadership of Kunzer and. Haken-jos, and the glider of the A.rbeitsgemeinschaft Wurzburg" with Schmi. and Endros as pilots, whereas t'he winners of :the alttijdë'total were W. Teichmann, Berlin, th "Akaflag Marcho-Silesia" (pilot Pfeiffer), and again trio 7urtt. Luffii't±band." (pilot Hakenjos)." In 1930 the total"of the I practice U contest for greatest total flight duration was 2? hours and. 28 minutes; this year it totaled 9 hours

NSA.C.A. Technical Memorandum o. 671 3

less, which, however, isnot to be intrpreted as regress-ive. The best proof of th,e never-fading zeal and eager-ness, particularly within the ranks of the jp.venile .roup, is the large iumber of flights made, namely, 300 actual lihts as against l32 in 1930. The main reason

for the smaller total .uration and the considerably less maximum duration of the individual flights is traceable to the peculiar weather conditions. Wost-s,.lope weather with amply strong, continual' west winds, so typical of the 1930 contest, was altogether lacking except for a very few days.

During the entire soconcl.'half of the contoit in par-ticular, the conditions were. altogether abnormal under the influehce of extremely bumpy, easterly winds. They espe-cially prove.. a:drawback for the duration flights:.. , Be-s.des, the . upwind zones, on the slopes facing , south and east, ar'veryiarr .owly .restric .te,d and se-veral gliders in the air at the samo time interfere with, if not actually endanger one anolihor. In spite of this, Schmid, of the °Arbei-tgruppo' Wurzburg,".made an exceptionally boutiful flight. He. attempted, on July 28, to break the Rhon en-durance record with the "Wur.zburger C-eneralanzeiger. t' Al-though wholly an.prepared, he . decided to stay aloft as long as possible', but was finally, forced to land after exactly 9 hours, because of rain'and- . complete abatement ofthe wind. Hemmer's 1930 record of .9 . hours and 36 minutes was not beaten. , ' •. : .

Against this showing of the junior group, that of the advanced flyers uring the "practice contest fell sur-prisingly short. Of the eight entries, not one passed ,the minimum requirements of the prize rules.

Although there was not as much flying as usual be-cause of the weather conditions, the days of the west winds were that much more eagerly taken advantage of, and many beautiful flights were made, notably the squadron flight on July 28. Vith Groenhoff in' the lead, seven ju-venile flyers received their , initiation into the secrets of cloud sailing. It was a beautiful spectacle to watch the youngsters imitate every curve of their leader, and •three planes, in fact - the It Offermann u (Teichmann,.pilot) with. 34.6 km (21.5 iiii.), the ttprofessor tt (Hakenjos. , pilot) with 29.8 km (18.5 mi.), and the H5tadt Stuttgart (Kun-zer, pilot) with 24.8. km '(15.4 niL) were able to . qualify

for the consolation prize. •..

4 N.A.C./k. Technical Memorandum No. 671

The chart (fig,. , .1.) shows four groups of preferred flight directions. The .first and most etensively used is fromqe Wassei1:uppe past the Hohe Rhon. northward. to-ward. the. Ochsen and \Tacha. It is the favorite N.ehring" route of 192.7 and 1928, so popular with the younger groups because of its' favorable upcurrents.

second is eastward from the Vasserkuppe over the Hoho Rhon toward the Geba. The majority of gliders fol-lowed. this route in the storm-front flights of July 25 and August 3, Altogether different from the normal front t)aunderstorms out of the west, this storm of August 3, rose out of the east. It most likely had the character of a more local heat thunderstorm. No effective frontal upcurrent was ascertained. As a result, all the glider.s had to land . agai.n in the neighborhood of the Wasserkuppe, T1esean'dings.form the third intermediate, group in Pig-ur 1, .

The fourh.:.-group i ,s toward the east and the river Pulda. Botsch covered this route of 19 km (11.8 mi.) in 1923, tho last . time.' The altogether abnormal east winds during the second half of this year's contest forced. the flyers into this direction for the first time in 8 years. These east winds, which at first threatened. to become dis-astrous, subsequently proved to be of great value, inas-much as they turned. the contest into entirely new chan-nels and enhanced the possibilities of thermic sailing. The most prominent distance and altitude flights of the performance race are .apended in. Table II.

The rules governing the prize for r,eachln a pre-scribed. destination, run off every year during the per-formance race, were much more exacting this year than heretofore. Whereas last year's rules stipulated. a flight of 15 km (9.3 ml.) to the ' Kreuzberg and return to the Wassrkuppe, this year's rules called. for a flight with the Ochsenberg near Vacha, a distance of 34 km (21 mi.), as destination. The flight with fixed destination thus became more than 68 km (42.3 mi.) with start1from and re-turn to.the Wasserkuppe. Tho flight to the Ochsen was a comparatively easy stretch, whereas the lap, Rhon - Wasser-kuppe was vry difficult.. The last 10 km, es p ecially, from the Hohe Rhon to the plateau of the WasserkuDpe, 'ftrough the 'downwind zone of the WasserkuDpe, presumes 'high-flying altitude, whch can only be at.ned 'by cloud-up wind on the Hohe Rhon. It was foreseen that this last stretch near the goal would call for extraordinary skill,

N.A.C..A. Technical Mernorand.um No. 6?]. 5

and 'o.r• that. reason the rules did...not specify .a.return to theplateau of the Wasserkuppe, but rnerely.a 1aning..any-where'a.t the foot, providing the distance did. not exeed 1500 in (4920 ft,) from the top. The prize was not con-. tested. Hirth and. Groenhoff almost qualified for the en-trance rules by landing loop m (3281 ft.) and. 1500 rn,' (4920 ft.), respectively, outside of the prescribed 1500 m radius. The flight put up by Hirth, Groenhoff and. Kion-feLl to complete this race victoriously, was almost dra-matic.' Again and again Hirth and. Groenhoff tried before they .finally effected a landin within the radius cited above. 1 The efforts of both. flyers to push . on 'from the Roh.e Rhonto the Vasserkuppe, to overorne this last but

..most difficult lap., were an exciting. fight, for every meter of altitude. Groenhoff's attempt, in particular, to obtain

...a last ounce of upwind by skimming the tree tops, ai'id' Kron-feict's .three-time return to the Wass.erkuppe. for 'a ,ne* start was, a spectacular and. gripping sight. These flights of Kronfeld fr the ö.chsenberg prize, one of *hich lasted over seven hours, re new proof of the rem,arkable stamina and. will power of . this pilot..

But the. features which place ti4s year's race at the head of all previoi,,s ones, are: th unique froit sailing flight, by a squadron .of 12, on July 25, in the face of an .approach.ing storm;. Groenhoff. and Hrth r s 220 km (136.7 mi.) and 175 . km (108.7 mi' 0 ) flights to Magdeburg and Halle; further, the first thermic sailing flights of over ,1 100' km. (6'2 mi,), namely, Hirth's 192 km (l193 mi.) to Brohi on the Mosel, Kronf'el&'s 165 km ('1O2.5 mi.) to Arns'berg on •the Westfalen, and G.roènhoff's 107 km (66.5 m1 0 ) to Bad Nauheim. It is necessary, in order to appreciate and clear-ly understand. these flights, to txeat them in connec'ioi with an explanation of 'the scientific principles of ther-mic sailing 'flight. ' . .

• The prevalence of ascending convection currents in the free atmosphere , is contingent upon the local over-heating, of detached air masses above a given, thermically favored territory, or upon .the.liberation f diffused. at-mospheric instability, whicI.is.produce by . regional over-heating of . the nethermost air strat. or cooling off at some height, The escape. of this. instability, manifested by excesive. vertica].. temperatur 'e. gradient, is brought about by the rise of detached quantities of air, whether orographic by. s1ope . up.r.rrent, frontal by frontal upwind, or turbulent by ert 'icl disDlaceme1t of air masses in tur-bulent layers.' The 'ascending bonvection current becomes

6 NOA.C.A. Technical Memorandum No. 671

a'cloud.upcurrent when air mas.se. are elevated beyo.nd their height of condensation by any of these three meth-ods of..escape, and. attain.e buoyancy and. acceleration.. 'by the heat.of condensation set free though cloud forma-t,ion 0 The cloud upcurrent.rhas, .in contrast to the ther-: r!lically rising, dry air masses without. forming clouds, the advantages,, as far.as sailing flight is concerned, of gre..ter vertical extent,...greater.vertical velocity and. 1stly, of being more readily recognised. For that reason it..becomes readily apparent why. thermic sailing flight in cloud 'uDwinds.preceded. the utilization of upcurrents with-out clouds or "invisible!' thermi upwindse For the eval-uation of the latter, the so-called bemagrarnI. (energy-mass diagrams, according to Refsdal*) has proved extremely val-uable., The emagram gives a.direct re:presentation of the, energy mass necessary for an air mass to enable it to exe-cute free vertical movements. To illustrate,, take the state of the atmosphere on'July 28., l931: . .t1'e day of the squadron flight in the cloud. upwind..: If. T is the tern-perature. of air vertically at. rest, T t . , the temperature of the dry adiabatic of an isolated, dry air mass rising from • the ground., T" the temperature of the same air mass on the damp adiabatic after 'inception of condensation, then the difference T' - T .. and.. T." T, the so-called dry, unstable or damp, stable temperature difference is a criterion for the acceleration of the ascending air. Ac-cording to the emagram of JulY 28 (fig. 3), an adiabatic temperature gradient prevailed that day in the lower stra-ta up to 1500 rn (4920 ft.). An ascending air mass on the asse.rkuppe. thus was in thermic equilibrium with the sur-

rounding a.ir . .at any height. The upward. motion of .the air rising on' the slope was n.ot thermically retarded and thus enabled to. reach its condensation level, Between 1500 and 1.6.00 in (4920 and. .5250 ft.) isa minor, negatively d.ry,.un-stable temperature difference. But since this layer is. restricted in height and the negative temperature differ-ence between ascending. and surrounding air .is slight, the air rising on the mountain is able to push through this layer and into the upper, markedly damp, unstable strata. 4fter inception of' condensation the rising ir masses, are •warmer as' the he.ght.increa.ses than the surrounding air which'is vert.ica1iy at rest,. As:. a reultth' cloud air is speeded' up, o,n, i upward pas sage and the cloud upwind, with-in the ol.ou i,t.:el,f grow.s..with increasing height.

'Refs.a',.k"; ' The I'iioist' Unstable Precipitation. Geofysiske P.iblik.a'tioner,."Vôl'. ' IT, No, '12, Oslo, 1930.. '

N.A,C.A. Technical Memo'ran'difln •No. 671 7

Perticularly favorable t l eraiic upcurrent cond.itions prevaired in the scond ' half of the contest, during the long spell of warm east-wind weather, as seen from the ap-pended ernagrams. 'Tile gentle sOutheast wind on A'u.gust 1, was unsuited for: slope ailing, but eminently fitted. for essaying thermic.. sailing flight with towed sai1p1axè... .'The research institute of the R.R.G. had. offered. a special rize for towed. flight on light-breeze days, so as t give

les,s experienced groups a chance' .of becoming fathlliar ..ith towing start methods,. . On the afternoon of Atigist' 1, piLot P. Riedel towed. nine sailpianes into the air 0 Released, at 500 m (1.640 ft.), G'r'oenhof±' in "Pafnir stayed. aloftl hour and. 42 minutes; Hirth in U Wiusterle,, H 1 hour and. 37 min-utes; and Kronfeld. in "Wien, 1 hour and 10 minutes. The emagram for this day .augured extremely propitious condi-tions for the formation of convection ' currents. By a d.ry-? unstable state up to 2000 rn (6560 ft.), air masses, risen orogr 'aphically'on. the Wasserkuppe, were free to push on nd upward,. (Fig'. .4,) From 2000 m on, the air was damp-

unstable, so that hair m'asses risen to this height could continue to ri'se as cloud-uflwind. as soon as condensation began. The ascent continues up to 2400 rn (7874 ft..), where a strong tem-oerature inversion forms an effective inter-cepting layer. And. Groenhoff's, as well as Hirth t s flight are in perfect accord. with these'theories. After unhook-ing from the airplane, G'roenhoff soared along the east slope of the Wasserkuppo till the released. slope current had. changed into a free,' ascending warm ai'r current topped. by a cumulus cloud, and in this convectiçn current he gradually ci'rcl.ed. 'to greater height. It 'is a characteris-'tic of the ascending conve'ctio.n currents formed from slope upwinds that their upwind zone from the windwar:d slope shifts more ' and. more with the general wind. toward the lee-ward. side, whereby the'free upciirrent very often does not attain its greatest strength' until it reaches the leeward. side, with the result that in the lee above. downward-slope currents the ascending convection current'..is particularly powerful0 Thi is well illustrated by Hirtb's and. G'roen-hoff t s flights. (See figs. 5 to 8,) In.excellont agree-

ment with the theory of vertical expanse of ascending con-vection currants, G'ioehoff reached a maximum absolute ceiling of 2300m (75,45 ft.), and Hirth t s route was very much like it. (See figs. 5 and. 6.) Hirth's flight was also mainly mado on the west slope of the Wasserkuppe de-spite the east winds, and. this flight likewise revealed' the previously .rnentioned phenomenon that thq unstable en-ergy: sot free. on the windward, slope of the mountain as a result of forced rise, of the air, shif's the strongest ascending convection currents far to leeward.

8 N.A.C.A. Technical Memorand.um No. 671

On August 2., the thermic conditions on the Was:ser-kuppe were practically the same as the day before.I The atmosphex'ic instability was even higher. Up to 1900 in (:6234 ft..), the air was markei1y . dry-unstable, beyond it and, up to 2200 in (7218 ft.) damp-unst;able where, as on the previous day,. a strong temp.er.ature' inversion formed an of-. fective barrier. The entirely successful flights of Pfeiffer, in his excellent "Schiesienin Notu were a rev-. alation to the spectators. Taking advantage of the brisk east wind, he flew the little explored east slope of the Eube and, to the astonishment of all, reached a height of 800 (2625 ft.) in very little time,

His example was immediately followed, by the other en-thusiasts The heights attained in the. practice" and. "performance" contests as compiled in Tables I and. II, were practically all made that day, Groenhoff and. Hirth util-ized the favorable thermic conditions prevailing that date for the first distance sailing flights into West Germany'. G'roenhoff landed after 107km (66.5 ml.) at Lauba,chnéar Ba Nauhei.m; Hirth crossed tho river Rhine, and landed at Brohl on the Mosel, after covering a distance of 193 kin (120 ml,).

This rapid development of ascending convection cur-rêiit sailing is due in a large measure to the towing of gliders. It makes it possible to develop a systematic method of thermic sailing flight, which stipulates that the pilot must utilize as long as possible every indicated. free upwind zone (whether by feel, or better, indicated. by sensitive variometer). by continuous circling in the zoe. During this circling the pilot must attempt to attain the height which enables him to continue until he finds a iow upwind zone, When theascending convection currents rise beyond. the height of condensation, the cumuli serve as guideposts to new upcurrents. In the absence of clouds the pilot must rely on his °feel" or his variometor, in order to orientate himself as to location, expanse and. strength of the "invisible" ascending air masses. This method of thermic sailing flight was explored. by the re-markable French glider enthusiast, P, Idrac, in several expeditions to North Africa in l9l9_1923,* Pursuing dif-ferent methods the research institute of the R.R.G. uses sounding balloons, A example of such records is given in Figure 11.

The xtilization of ascending .co:nvec.tion currents was heretoforehandicappo.d by the quest.ion.of frequency. Long-

Idrac, P.: Exprimenta1 Studies on Sailing Flight. Li-brairie des sciences aronautiques, Paris, 1931.

NCA:.:. Te.èhnicaii1ernorandum iTo, 671 9

di'át 'ance f1gbtbI the sole usèof thermicupwind. zones ithpüt cloud. frthät ion are possible only when thermic up-iiret arèfeqiiént enough so that the pilct can feel cnfidét een accidentally finding., the mos.t .usial1y ivisI'ble upind zones which are far from being local con-ditions. In this respect, Kronfe1ds flight oh the 'after-noon of August 5, from the asserkuppe to Arnsborg (Viest-ha1'ia) a diitace &f 165 . km (DM2.5 mi,) is worthy of

note. .: . . ."'

: " .

On this r'ticu.ar' day the atmospheric conditions' wer O: so'thewhat different than on:.,th'e other days, According to the 'e'rnagra.m the airwas dry-unsta'b].e tp to 1500 m (4920 f't.)' from th'èie to 2000 m (6560 ft.)' the stratum was. neu-trál - in p'ar.t, stable. Beg.inning:.at' 2000. rn, tlie air be-. came increasingly damp-unstable. The sky was', .],'most cloud-less, 11 1n the east, far from the iVasserkuppe, pe'r'h-aps on the Thuringer d, a few •umuli could be seen. The escape of the instability of •the atmosphre in the ambit of the Wasserkupe' was therefore restricted, to the lower dry-. unstable strata. I'tev'i . d'ent1y'lacked the impulse to re-lease the d'am"initabi1ity of the upper layer.' As 'a ro-suit the convetibi ' Otrreiit wasconfined. to the strata below 1500 m, 'The cloud' upwind of the layers above 2000 m failed to release that day. Since the hori g ontal wind. velocity did not 'exceed. 5 m/s, it was extreme,y difficult to connect with the ascending convection currents. 0nl Kronfeid. made 'as'erious attempt, and even:thçgh the Itwienui was superior o'à'thérs á g 'far as sinking speed is concerned, he had. great difficuitie in keeping in the slope upcirent. He flew for over an hour, at times very low, n the s1pe of the Eube trying to gain altitude. . At timeø it seemd hopeless, but after one hour he was able to get hig. enough above his starting height, to rid himself: of the s1e arid. connect with the free ascending convection currento. His subsequent method of flight was again a maste?ly ehibi-tion of the art of sailing flight and a classical example of thermic sailing, a fitting sequence of.his earlier flights - his Himmeld.ankberg flight in 1928, and his 0 km (87 mi.) flight to Bad Hermsd.orf in 1929. . A ,l.ance at Figure 13 shows how Kronfeld., after leaving the slope of the,Was, sekuppe, gained altitude 'after finding a convo.-io' zone.....' Abut 10 km (6.2 mi) away from the slope, he

wäs'600m'(i69ft.) above'.his'a't.arting point. A sec,t.on the baró'gram' of this flight' (fig. 14) is the best proof

of his 'skill ' ànd 'resourcefu1noss:.': Ascent and,deuceit fol-low one another withy almost wavelike regularity. . 'Rising, he circled in 'the found upwind zne;. descending hee.nd.eav-

10 , N.A.C.A. Technical •Mem.oranthm No.. .671

ored,t,o locate a new u4pwin&,.zone, always mi'nt:a'ining.his course in the direction of the wind, so as to- cover d.is-tance at the san1e time. He stayed. aLoft over 'six hours. Toward pie'ningt,he intensity o.the cnvec't•ion.&ur±'eits a"ated. 'and'he'. ,anded 1:65 km (1.02.5 ml.), away,t"Arnsberg (Ve stphalia) .' ' .

aside from.Kronfeld)s feat, this flight offered the first important scientific information about the frequency of ascending convection currents. The flight proved that finci.ing convection currents in sai1iig'flight ca actually be left to chance, because theyapparentl:are 'so plenti-ful if the weather is favorable and the sailplane has

• enough altitude that a short gliding flight againleads to,an effective..upwind. zone. FQr the rest, KronfeIds flight indicates that the depondenco of . the ascending

• óoñ r otion currents on the surface conditions of the ground. • (fields, woods, ineadows, wator etc.) , is not .o'noral, and 'my even 'be of a •sacon.dary nature. The systemdtic se-quencé of the upcurrents, manifested in the 'b.ar.ograrn, is n'ot indicative of any marked dependenco. of the convection current on the surface conditions. It even appear's pos-sible that the certai1y remarkable wavalie 'sequence 'of the upwind zones is not accidental, but ra,ter'.ciontin.gent upon a certain systematic disposition of the,.verti'cal :-arragmetit of the unstable air masses.: . ..'

•.....'. Themic, sailing, flight finds its greatest .evelop-''ent in. Sbtropia1 and' tropical countries. •'Lt:'; i.s entire-

ly ' feasible that '. 'n uch, countries, gliding may even at-tain real practic:al. significanc.e. It is for this reason that' the nternationa1 Study Committee for engineless flight, laid particular stress upon exploration of thermic sailing flight l the tropics, during its October session in L 0nd.on, '•

'The main . .feature from the sports standpoint, of the whole contest, was, without a doubt, the unique storm' flight of July 25. The sound of the siren nOtified. the contestants of the approach of a storm front. Before its arrival the wind was. from the south. Everybody awaited. eagerly th shift. of the wind, to the west. At the in-"stant the win changed. the first machine started, eleven others fo1iow,iig..inuick. succession within seven minutes. It wás ' a j1it 'never to be forgott ,en. The singleness of purpá -and. the cheerful enthusm with which even young pilots who had. not learned. as yette,secrets of storm-front sâiiixig 4 ,took off for th hyy clouds amid thunder

N.LC.A. Technical Memorandum No. 671 1].

.nd flash of lightning and the subsequent thick sheet of rain, were astonishing. Those not cognizant of the famil-iarity of the si1l±ig-,f1ight pilot with clouds or wind, may have looked upn ti start as foolhardy, daring, but the end of the f1ightroved that the pilot has ample pro sence of mind to ntust :himseif to. the stoirn oiily ó far and so long as iti.sof advaxtage for him to do so, A great number of the younger fliers were unable to acc91 in-pany the storth front for more than 30 km (18.6 mi.). ohm in the "Stadt Stuttgart 11 followed for 38 km (23.6 ml.); Hurttig in the "blihister Leuschner' for 40km (24,9 mi.).; wle Groenhoff andHirth stayed with the front from the Rhon to the Elbe river. The path of the storm of July 25, 1931, s shown in Figure 15. At 5 o'clock the storm neared the Rhon, and 15 minutes later commenced the start at the asserkuppe, At 6 o'clock the storm front crossed the val-

ley of the Werra, passed rapidly over the northern Thurin-gian Forest, but clung for a long time to the southern Thuringian Forest,

After 9 o'clock at night the storm gradually disap-. peared. Groenhoff and Hirth kept together in the van of the front as far as Erfurt, where they became separated, Hirth flew over Weimar-Apolda toward the valley of the Saab near Halle, where he landed after 8 o'clock at night at Schloss Friodeburg, having covered.a distance of 175 km. (108.7 ml.), Groenhoff left Erfurt for the northeast and. landed at 9 o'clock at Meitzend.orf, a suburb of Magdeburg, after covering a distance of 220 (136.7 ml.).

• The altitude-time curves of this flight are extreme-ly illuminating. After his start from the WasserkuDpe, Groenhofi' quickly reached an altitude of 500 m (1640 ft.) but just as quickly lost it again in the lee of the Vasser-kuDpe. The same thing happened. on the Hohe Rh8n. A gain on the windward side of this mountain massif is followed. by an etraordinarl1y pronounced drop in the lee of the Hohe Rhon. This downwind zone, which apparently weakened. the frànt considerably, also was the cause of the prema-ture landing of several flyers. Hirth's altitude-time curve is very similar and likewise shows the effect of the terrain on the flight path. The two flyers did not reach the actual front until at the Geba, where both attained an enormousrate of climb. Groenhoff was carried. from 900 rn (2950 ft.) to 2500 m (8200 ft.) in incr.e.dibly short time, his rate of climb reaching 79 rn/s (25.9 ft./sec.) at times. Hirth's altitude curve is identically the same, after hav-ing found the front a little later. For the next two hours

-12 N,A.C.A. Technic]. Memo.randura No, 7l

the a1titude-tie curve of both .shovëd.t.h;same typical, uniform course of other front s.ai1izi.fl.i.gits. Groen-hoff t s subsequantlap over .Querfrt-tdwrd the eastern foothills of the. Harz was :perfec.tly calm- and. devoid. of any disturbances. ;.-.Hirth, on the otiex hand, evidently encoun-tered less propitious upwi. i4s. A± . t.s:r p assing Arnstadt neer Erfur, he gradually but consistently 1st altitude till, he: lauded. at Halo, ;Groenho'ff"s1st .hour was beset with uue'peted. difficulties' After 'lo;sing'lOOO m (3280 ft.) in fl.eignt toward the eid of the t.iird nour, his course ,aDpeared extreino .iy: :agd.tated. :•. .'viden't'Iy he encoun-tered -more turbulent::aiir ma..se.s int:esp:ersé with strong aounca.rrents, wich et up critical stresses in parts of his. -oLano... Moreover, since it was quite dark after 9 oc1ock 'anA: he did not feel..justified in taking the risks involvea ..n 'a storm flight in, the dark, he offectd laid-ing. : : '. ' -; .

Groenhoff made two such flights in the same ye, 'but rather than being easy victories, they were the results of a..euccossful 'battl of' human skill, endurance, and unshak-able compsr.o matched agaiat:; -the unrelenting p owers of the elements, in a hailstorm over. Liuiiich and in storm soa11s ov,or Magd.eburg, at: times bordering. on the limits of; human e,nduanc&... '.-

• ' 'T:e :enthusiasm.wit'hwhich •the. 12 flyers started. for the storm flight is yet another' forcible argument for the great valie of eigineloss flight as part :Of .a general flight-training schedule. And. we are particularly justi-fied. in ropea't.ing 'a statement 'voiced bvor twb years ago. Prcis;el' r . the. : s.trmfJght.o Jily. 25 proved. our conten-tion that "the present-day methods of 'sailin.g: flight at-. tracts a young generation: which, instead of being : appr:e_ hensive of an apro.aching storm, actually welcomes the ar-rival of the storm.front and knows bow to utilize its pow-erful energy. This generation of flyers accumulates a larger amount 'of s p ecial knowledge and experience than ma-ny an aIrplane p ilot after years of flying practice.0

Progress in sailing flight has been commendable,: a fact, however; which in. itself, isno l9nger a surprise. It has been the same at . evry: . other Rhon contest. But it does prove the clear-sighted:, development 0±' German sailing 'light in the right direction. Sortsmanship of. the pilot, scientific resOarch, methodical training, large-scale ex-

. p ansion of flying activities toward one single purpose have ',':aided'in the re.tention.'o±' the.uniqueDosition which German

gliding activities command in the world of aeronautics.

N.A.C.A. Technical Memorandum NOG 671 13

And in this respect we wish to emphasize the perform-nces of the three masters of sailing flight: Groenhoff,

Kronfeld. and Hirth, who have contributed much toward this world position.

Translation by J. Vanier, National Advisory Committee for Aeronautics.

N.A.C.A. Technical Memorandum No. 671

14

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N.A.O.A. Technical Memorandum No. 671

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N.A.C.A. Technical Memorandm No.671

Pigs. 34, 5

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Pig.4 Ilmagram for August 1,1931, i40 PM ascent Waseerkuppe

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Fig.b Route of Hirth in Ngter1eI Aug. 1,1931

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N.L.C.A. Technica1 Memorandum No.671

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N.A.O.A. Technical emorandum No.671 Figs.7,8

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27

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,' 0 I U9 1931 o4 7'

Fig.8 Groenhoff in Fafnir,afternoon of August 1, 1931, towed tart,re1eased at 1460 m(4C00 ft.),

scale 1:50000 (altitude 10 times).

N.A.C.L. Technical Memorann No.671

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N.A.C.A. Technical Memorandum No.671

Fig. 11

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N.A.C.A. Technical Lemorandum No.671

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'tWien", August 5, 1931

Fkigzl4,, Flyu +ifl1e,rnnU+es Fig.14 Barogram of Kronfeld.

•j•_ flight ,August 5,1931.

N.L.C.A. Technical Mecnorandum No.671 ø'ig.lS

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II Hirth in Muter1e Fig. 16 Utitude-tirie curve of front flight by Groeniioff and Hirth.

Ju1T 25, 1932. Start.- WasserkllDpe

x -c A1titud by barogrRm