﻿SOAEliq^G 
  FLIGHT 
  KLEMPERER 
  229 
  

  

  On 
  a 
  sailing 
  boat, 
  the 
  angle 
  of 
  attack 
  of 
  the 
  sail 
  by 
  the 
  wind 
  and 
  

   that 
  of 
  the 
  keel 
  by 
  the 
  water 
  lie 
  toward 
  opposite 
  sides. 
  The 
  equiva- 
  

   lent 
  condition 
  has 
  to 
  be 
  brought 
  about 
  in 
  dynamic 
  soaring 
  by 
  facing 
  

   the 
  two 
  portions 
  of 
  the 
  wind 
  in 
  opposite 
  directions. 
  Tliis 
  involves 
  

   certain 
  maneuvers 
  which 
  deserve 
  some 
  particular 
  investigation. 
  

  

  Let 
  us 
  consider 
  an 
  idealized 
  form 
  of 
  gustiness. 
  Imagine 
  the 
  wind 
  

   to 
  fluctuate 
  between 
  the 
  extremes 
  of 
  20 
  and 
  60 
  miles 
  per 
  hour. 
  Let 
  

   the 
  acceleration 
  during 
  the 
  freshening 
  period 
  be 
  constant, 
  say 
  4 
  feet 
  

   per 
  second 
  -, 
  or 
  one-eighth 
  of 
  that 
  of 
  a 
  dropped 
  body. 
  It 
  would 
  take 
  

   10 
  seconds 
  between 
  two 
  extremes. 
  Suppose 
  we 
  had 
  been 
  flying 
  facing 
  

   the 
  slow 
  20 
  miles 
  per 
  hour 
  wind. 
  Now 
  the 
  wind 
  freshens 
  up. 
  If 
  we 
  

   should 
  not 
  react, 
  we 
  would 
  gain 
  excess 
  speed 
  against 
  the 
  air 
  due 
  to 
  

   our 
  own 
  inertia. 
  However, 
  we 
  do 
  not 
  want 
  more 
  speed, 
  since 
  our 
  

   lift 
  just 
  balances 
  our 
  weight. 
  Thus 
  we 
  decide 
  to 
  throttle 
  down 
  our 
  

   motor, 
  and 
  assuming 
  the 
  drag 
  of 
  our 
  machine 
  to 
  be 
  one-eighth 
  of 
  its 
  

   weight, 
  we 
  could 
  just 
  shut 
  off 
  that 
  engine 
  completely. 
  Our 
  drag- 
  

   would 
  just 
  retard 
  us 
  so 
  much 
  that 
  our 
  speed 
  agaiiist 
  the 
  swelling 
  gust 
  

   will 
  be 
  constant, 
  and 
  we 
  continue 
  our 
  level 
  flight, 
  without 
  any 
  pro- 
  

   pulsive 
  power 
  otiier 
  tlian 
  the 
  reaction 
  of 
  inertia. 
  liovvever, 
  this 
  con- 
  

   dition 
  will 
  last 
  only 
  10 
  seconds. 
  Upon 
  reacliing 
  the 
  climax 
  of 
  the 
  

   gust, 
  the 
  negative 
  acceleration 
  of 
  the 
  calming-down 
  period 
  would 
  

   have 
  a 
  retarding 
  eii'ect 
  on 
  us 
  and 
  we 
  would 
  need 
  twice 
  our 
  normal 
  

   engine 
  horsepower 
  in 
  order 
  to 
  catch 
  up 
  with 
  it. 
  After 
  completing 
  

   the 
  cycle 
  we 
  would 
  have 
  gained 
  notliing 
  at 
  all. 
  Hou'ever, 
  the 
  idea 
  

   of 
  this 
  sort 
  of 
  dynamic 
  soaring 
  is 
  not 
  to 
  wait 
  until 
  the 
  lull 
  spoils 
  the 
  

   temporary 
  gain, 
  but 
  to 
  turn 
  away 
  in 
  the 
  meantime. 
  Then 
  when 
  

   the 
  calming-down 
  stroke 
  arrives, 
  one 
  is 
  licaded 
  already 
  the 
  other 
  

   way 
  and 
  gets 
  another 
  free 
  lift 
  equal 
  to 
  the 
  preceding 
  one, 
  since 
  now 
  

   we 
  transport 
  the 
  impulse 
  from 
  the 
  fast 
  wind 
  portion 
  into 
  the 
  slow 
  

   one. 
  

  

  This 
  manipulation 
  is 
  by 
  no 
  means 
  a 
  m3^sterious 
  case 
  of 
  perpetual 
  

   motion. 
  In 
  fact, 
  the 
  airplane 
  or 
  bird 
  so 
  circling 
  picks 
  energy 
  from 
  

   that 
  stored 
  in 
  those 
  mcteorojogical 
  irregularities. 
  Its 
  action 
  some- 
  

   what 
  resembles 
  that 
  of 
  a 
  traflic 
  cop 
  on 
  the 
  road, 
  slowing 
  down 
  fast 
  

   elements 
  of 
  (wind) 
  trafiic 
  and 
  speeding 
  up 
  slow 
  ones, 
  with 
  the 
  result 
  

   of 
  smoothing 
  the 
  entire 
  motion. 
  If 
  he 
  does 
  a 
  good 
  job 
  there 
  is 
  

   nothing 
  left 
  for 
  his 
  fellow 
  farther 
  downstream 
  to 
  do. 
  

  

  It 
  is 
  interesting 
  that 
  not 
  the 
  actual 
  accelerative 
  structure 
  of 
  a 
  gust 
  

   is 
  what 
  counts, 
  but 
  the 
  average 
  acceleration 
  only, 
  between 
  the 
  times 
  

   of 
  change 
  of 
  course 
  of 
  the 
  craft. 
  If 
  this 
  is 
  the 
  same 
  or 
  more 
  in 
  pro- 
  

   portion 
  to 
  gravity 
  than 
  the 
  ratio 
  of 
  drag 
  to 
  lift 
  of 
  the 
  airplane 
  or 
  

   bird, 
  the 
  soaring 
  effect 
  is 
  100 
  per 
  cent. 
  The 
  conditions 
  assumed 
  in 
  

   our 
  numerical 
  example 
  happen 
  frequently 
  in 
  the 
  atmosphere, 
  and, 
  

   since 
  we 
  can 
  refijie 
  glider 
  design 
  so 
  as 
  to 
  reduce 
  drag 
  to 
  one-twentieth 
  

   of 
  lift, 
  chances 
  of 
  riding 
  the 
  gusts 
  would 
  not 
  look 
  sad 
  at 
  all. 
  How- 
  

   74906—28 
  16 
  

  

  