HOCHMUTH, G.

MAHNKE, R.

English

ISBS (International Society of Biomechanics in Sports): Conference Proceedings Archive

RECENT FINDINGS CONCERNING AERODYNAMIC EFFECTS IN SKI-JUMPING WAHNKE, R.; HOCH13VTH G. Institut of Physical Culture and Sport Leipzig GDR In the last  few years tbere has been a rapid bprovernt  of p c r f o ~ n c a  M ski-jumping. Plight postures bave visibly w e d ,  as regards both body amfiguration and ski ~t t i tndos .  Tb mt noteworthy developments are the latest  variations in sltf attitude, the spctafular V-fon (Boklw-position), tbe oblique ski position to the flight path plane, and in general substantially bmder  ski a t t i M e s  tban before. N 1  these nev ski att i tuies variations coatravere the current mStandards for Position and WDverentW in force on ohich the scoring system of the jmp is b a d :  with the result tbat pints an deduced. Ibwever, tbe gain in jumping distance for sow jumpers pore than ouebalanw the loss of points. As SO often in sport, tbe practice of sport has led to the discovery of new and Pore effective technical solutions vhich contradict current opinions and as a result the co~equently new criteria on uhich the scoring system i s  based are necessary. Biolecbanics rhould be applied in order to find out the causal relations involved in tbe mre effective use ot windforce; new rcccrrondations should a h  be loud for favourable flight postures and performance of the conplete flight by neans of wind tunnel e r p e r h t s  and trajectory simulations. The ptqnse of the wind t m l  c rpc r iw t s  carried out a r e  based on th following observations: a )  I t  lurt bc ass& that by vans of the l a tu t  ski attitudes and an increased extension of the body, the jutperfski unit attains a wider l i f t  and drag area and changes in the aerodynamic conditions. Wind force i s  increased as a result and greater junpiq distancc can be achieved. b) I: i s  to be expected that the iwportant dwge i n  bcdyfon in Ure right angled projection vil. alter significantly the relationship, discovered until now, between clearly defined flight-posture features in Be flight path ( i.e. the a q l e  of attack of tbe ski and the uppr and lower parts of the body ) and wind force. In other words, the effective angles of attack of the ski and body are well to be found in other areas tnan halve bean discovered up to now. c ;  Finally, i t  i s  also wrL. carryirq out tests to  ascertain whether the effect of physical differnnies in the janprs has significantly inc:eased thouqh iaportanc chanqes in body-fom and increased diversity c l  wdy-:3n. Consequently i t  right be nuch mre  necessary than previously to determine the mst  favourahle fliqht posture of a given jumper by leans of individual wind tlnnel erperiments in conjuction with computer s imlat ims (i.e. individual, 'made-to-measure solutions ). The results of the f i r s t  wind tunnel tests concerning tbe latest ski attitude variations carried out on t.40 jumpers in 1988,  confirmed the abve wntioned observatiars. Aiter this,  further tests involving top-class j q e r s  were staged. The aim of these tests was to d e t e h  tbe individual effective flight posture for the fornation of the middle flight segment. The starting point in these tests was the desire to find more effective, hdividual ski attitude variations in conj3wtion irith gccd body postures vhich could be assessed as realizable according to the level of development and achievable capabilities of a given jumper. 2. ESXGN CF HK'JREENTS MLl PRCCFClURES IN THE h'I!Kl MI2 CCHPTTE2 SIRULATICNS 4 With the help of a special support the jumper i s  positioned in  the wind tunnel's a i r  current in s~;ch a xiy as to be able to forn various flight postures uhich are approximtely reproduceable. By adjusting axle 1 and 2 (Fig.1 and 2) it i s  possible to  vary the ski and body positions to  the a i r  current i t  i s  on. Consequently, a complete series of measurements can be taken during the air  current process. A control monitor i s  positicned in the jumper's field of vision belov the jet on uhich he can check the side-view and top view of his fliqht p s i t i on  and correct the errors as they appear on the screen ( Pig. 2 ) .  Sketcbes are made of the pictures taken by video-cameras and other data characterisirq the flight pasture i s  deternined, using the respective taken previously. Ibe measurement series vere set i n  such a way that f i r s t  of a l l  the typical flight postures vhicb had been detenined earlier froa the analysis of the j q e r s  actuaI movement sequences vere shokn and then the VIII Symposium ISBS - 99 - Prague 1990 body position, the sk i  position and the body configuration were varied. Figure 1: Figure 2: In the presentation of the r e su l t s  as  ell as for  the f l igh t  path calculat ions (wnputer  s h u l a t e d ) ,  those l i f t -and drag-ccefficients are  used which are based on the pressure, be& recorded a t  the merit of the measurirq process. The wird tunnel measurements w s t  be supplemented witS. f l i g h t  pat! calculations (Computer s inulat icns)  k c a u e  the qual i ty  of a flig'nt generally i s  not calculable from the wind tunnel f igures.  The reason for  th i s  l i e s  on the m e  hand in  the fac t  that  various wPbinations of l i f t  and drag can lead t o  equal j q i n g  distances, and on the other hand, a given !light position in tbe different  f l iqh t  segaents can be judqed as k i n g  good or  h d  t o  varyinq deqrees. C o n s q ~ e n t l y ,  the f l ign t  pai t ioff i  must be assesed in  t h i s  unity as a course of m v e m t s  (khnke  1983). Accordirq t o  these f ac t s ,  there is larger  scope :or equal variants in the conps i t ion  of f l igh t  with .ia:yinq conbinat~ons of l i f t  and drag levels  in  conjunction with varylrq changes in  the course of the f l igh t .  For the f l l g h t  path calculations a s i q l i f i e d  &el was used with vhich the jumper-ski system i s  taken as aji point for  application (Pig.]). The c o q u t e r  simulation vas carr ied out  by the Siarsyst Simulationssystem f rcz  KMufjSchnabel (1981). (bhnke  1989). Figure 3:  Kcdel fo r  the f l igh t  phase VIII Symposium ISBS - 100 - Prague 1990 According to the respective puestions involved, different variants in the f o n  of varying degrees of l i f t  and drag in flight time vere set up in the mnputer siulation. Ihiq tbe established jump distances, the effects of tbe changes i n  flight path course carried out vere detenined against a set variant used for reference. 3 .  BeSUtR 3.1. THE INKOECB OF SKI WIlUDE ON YMDWRCE AND JUHPIWC DISTAUCE In comparison to the conventional, narrow ard parallel ski attitude vith a ski's-break distdnce betwen the skis (Pig. 4 with 10 a), a l l  otber ski-attitude-variant, researched show in some cases quite clearly even increased wind force. As a rule, vinl force rises vith h incredse in distance beteen tbe ski. Obliqw ski attitude variants produce greater ~ i n d  force tban the parallel (to tbe flight path plane) ski attitude: Boklev-forn generally brings about tbe gratest increase in wind force howver. INCLUEWCE OF THE SKI ATTITUDE, (JUMPER A) , Figure 4 :  Here, the rise in l i f t  and drag occur almost to the same extent. lhese tendencies are clearly visib:e In P:q. 4 ,  as they are in the other figures of the wind tunnel results. The results of the computer sinu!ati:n (representation of the jumping distances achieved in Fig. 4 )  verify that these increases in l i f t  and drag, vbile t5e lift-to-drag ratio rvnains the same or even increases, prcduces a clear increase in jumping distance. The increase i n  distance as a result of the V-fon i n  comparison to the conventional, narrcv and pr?!le! ski attitude would, according to the computer s idat ion on the normal jumping bill,  amunt to a good 13 wtr?s. !kvertbe!ess, the increase i n  distance is considerable at 7 metres with the oblique positioned skis. 1: is understandable that, winq to this siqnificant influence, it i s  rare to find ski a t t i t l e s  i n  s;.i-jqinq today where f ie  distance bebeen t!!e skis is snaller than 25 centimetres. VIII S\mposium ISBS - 101 - Prague 1990 In Figurer 5 anl 6 thc results of cxarirratim of tvo ju+n (Juqn B and C )  are colpared. The coqirison shows thdt for tbe MY flight posture (sam mqle of attack of body anl ski position, as well as similar ski attitude) i n  a l l  stages of -t, different l i f t  and drag levels occurred (see Pig. 5). Tbe figure for j q r  B exceeded those of w r  C right thmugh, in  l i f t  as nll as in drag. A t  the same t h  bowever the w e e  to vhich vind force W e d  to be dependent on body anl ski position vas the sdm in both jumpers. A reason for this fact is obwimcrly to k found in th different body fon. ? b a result of t!h jPqa I achieves larger juaping distances vith tbe sana fliqht postures than jumper C. ?be differem can k as hi@ as 3 mtres on b e  m m l  jwbg hill (in average 13 1). Jlnper D achieves his o p t h  vith quite an uRMdrd (stretched) flight position ad an angle of attack for the louer half of the body of approxhak 50'). In both caw an increase in distance of 5 to even 7 metres is achieved by a s  of the V-form ski attitude. The coqxrison conducted here draws our attention to the fact that a quantitative assessement of the effectiveness of flight postures can only be lade vith the help of wind t m l  experiments for the given jumpers. Osirq- jmpr D (top class) as the exiuple, the results e r e  as follow: h i s  junpers' typical flight posture i s  cbaracterised by a lorq extension of tBe bcdy and ldrge forward lean, during which h p i t i o n s  his skis oblipw to tbc flight pth. With this flight posture be achieves in the dddle flight segwt verf good vird force figures and consequently large juapinq distances. However, this junper could achieve even better results if be puts the skis in the V-form (Fig. 7 ) .  The mrease in d~stance on W lkml Jumping Hill is ,  on the basis of computer simulation, to be estisated as approx. 5 letres. If we compare the wind force figures he achieved vith the V-form ski attitude with those o! jus~ers B and D (e.g, with 35 '  of ski arqle position) then tbe overall higher efficiency of his flight position is clearly sbm.  During the search for reaainiq reserves to achieve further perfection of jumper D's flight posture, interesting observations were lade. With an oblique ski attitrde, both windforces figures - l i f t  and drag - rise by a praximte 10; with an i increase in the angle position (anqle of attack) of the bcdy (lower body froli 45' to 52 and upper bcdy from 30' to 35'). Figure 5: IWDIVIWU D m  betmen Jumper B and C Coefficients fA anl N VIII symposium ISBS - 102 - Prague 1990 Pipre 5:  INDIVIDUAL D I ~ e S  betveen Jllnper 1 and C INFLZXCE on JW(PIWC DISTW Figure 7: WER D ( t op  class) oblique and V-Para VIII Symposium ISBS - 103 - Prague 1990 lhis causes an imease in distance of 5 mtsar. A# a result the assmptioa could k confined that j q e r  D has to suffer a loss of distance achieved kcalrrc be often tuds to mduce the arqle of attack (angle position in the course of th flight. I n a  r M l a r w y , t h e o f f e c t r  o f m b v r p t w n t t u p i c a l r ~  i n t h e b o o f  tberovuc~tufthis jlqar a u l d  be quanitifiad; signs rdrich m t i l  nar bad twn d at bating mptir, affect. t Above a l l ,  thh wa~ nuavidrat in tfY rductio~l of the I]ti LPgh [ p i t i m )  h the recond balf of the flight. As vi tb  the obliqw ski Ittihdc, Juqu D alao adricvlr vitb th V-ton flight pwture with approxiate 50' aagle of at- of tln lorr prt of tbe bDdy the 1-t figures in l i f t  ad drag jrnpiq distaws. Should be have to ulcrad tberr lrwL bovlw, ttn f i g u m  uollld ba nh rrrr w f f r t f w  (w Fig. I). e The drag figures in am^ h t i c a l l y  and thb c a w  aaridcrabh losm ia distance. %ese ohservationa lead to a s ta tusnt  of gemral Validity to tbe effect tbat espccially with the V-torn ski attitude a given optiml angle of attack of bodg poritim sbwld (mder n, ckmtance~) be exceeded. !his fact i s  confirmd in practice in tbe executioa of aijroping in lo tar a, thorr j q r s  ubo prefer the V-mn ski attitude, do s o m t i l a  rfat obtain th large distances &at t h y  might otbervise achieve, because their a q l e  of attack of the body was to large during the fliqbt. Figure 8: JDPW D (top clatD)/ - Pon MLDUCE of the *lo of attack of the lover body VIII Symposium XSBS - 104 - Prague 1990 4 .  FINAL OBSPRVATICNS The result of the wind tunnel eyperiments carried out and tbe computer simulations clearly show that vith the new ski attitude variants, tbe aercdynanic effects can be very efficiently used for the achievement of greater jmphq distances. This represents a pmgress in tbe development of ski jumping technique vhich cannot be stalled any !oqer . and .hich the jumpers are naturally exploiting. The current mles and regulatims on posture and mvement i n  the judging of jup - execution (Micle 431.2.1. of the Interrntio~l Ski Competition rules of the PIS) conflict with this developcent. Tbest reylations o q h t  to be changed and adapted to the development further, for tbese develop@?nts r i l l  continue to 8 disregard the mles anyway. WAiM[E,R.: Investigations into the Criteria for the effective Take off-Transitonal - and Fiight phase - techniques in ski-jumpinq - 1983-Leipzig, PICj, Diss.A IIAANKE,R.: The use of Computer Siaulation i n  Skijumping in the deterni~tion of effective flight pcstxe 1993, Leiprig, Triining unl. ilettkaqf, 0 213 ,  149/165. VIII Symposium ISBS Prague 1990 

RECENT FINDINGS CONCERNING AERODYNAMIC EFFECTS IN SKI-JUMPING

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RECENT FINDINGS CONCERNING AERODYNAMIC EFFECTS IN SKI-JUMPING

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