﻿170 
  

  

  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1922. 
  

  

  Following 
  this 
  process, 
  Prandtl 
  and 
  his 
  associates 
  have 
  shown 
  how 
  

   one 
  could 
  calculate 
  the 
  influence 
  of 
  one 
  wing 
  of 
  a 
  biplane 
  upon 
  the 
  

   other, 
  so 
  that 
  if 
  the 
  behavior 
  of 
  one 
  wing 
  is 
  known 
  that 
  of 
  two 
  

   may 
  be 
  deduced; 
  and 
  they 
  have 
  proceeded 
  much 
  further 
  and 
  made 
  

   aeronautics 
  into 
  a 
  beautiful 
  theoretical 
  science. 
  

  

  But 
  in 
  the 
  end 
  the 
  function 
  of 
  aeronautic 
  research 
  is 
  obviously 
  

   to 
  learn 
  all 
  there 
  is 
  to 
  be 
  known 
  about 
  the 
  forces 
  acting 
  on 
  an 
  aero- 
  

   foil 
  or 
  wing. 
  How 
  can 
  this 
  be 
  done? 
  It 
  is 
  not 
  possible 
  to 
  make 
  

   actual 
  airplanes 
  of 
  various 
  types 
  and 
  test 
  them, 
  nor 
  would 
  this 
  help 
  

   us 
  much 
  if 
  it 
  were 
  practicable. 
  We 
  must 
  actually 
  measure 
  the 
  

   forces 
  involved 
  in 
  any 
  one 
  case, 
  and 
  must 
  vary 
  our 
  conditions 
  in 
  

   every 
  conceivable 
  way, 
  but 
  in 
  a 
  S3^stematic 
  manner. 
  There 
  are 
  

   several 
  methods 
  open 
  to 
  us. 
  One 
  is 
  to 
  make 
  a 
  model 
  of 
  a 
  wing, 
  

   say, 
  as 
  nearly 
  full 
  size 
  as 
  possible, 
  suspend 
  it 
  by 
  wires 
  30 
  feet 
  or 
  

  

  Fig. 
  3. 
  — 
  Diagram 
  of 
  flow 
  past 
  an 
  aerofoil 
  showing 
  an 
  increased 
  pressure 
  below 
  and 
  a 
  

  

  decreased 
  one 
  above. 
  

  

  more 
  below 
  an 
  airplane 
  which 
  can 
  carry 
  it 
  in 
  flight, 
  and 
  then, 
  by 
  

   inserting 
  measuring 
  instruments 
  in 
  the 
  wires, 
  study 
  the 
  forces 
  

   under 
  varying 
  conditions. 
  This 
  method 
  is 
  being 
  used 
  with 
  marked 
  

   success 
  at 
  Langley 
  Field, 
  near 
  Old 
  Point 
  Comfort, 
  Va., 
  by 
  the 
  staff 
  

   of 
  the 
  National 
  Advisory 
  Committee 
  for 
  Aeronautics. 
  It 
  is 
  called 
  

   the 
  " 
  free-flight 
  method." 
  

  

  A 
  second 
  method 
  is 
  to 
  make 
  a 
  small 
  model 
  of 
  a 
  wing 
  or 
  a 
  

   fuselage 
  or 
  an 
  entire 
  machine, 
  say, 
  one 
  twenty-fifth 
  the 
  size 
  of 
  the 
  

   actual 
  part, 
  and 
  then 
  to 
  investigate 
  the 
  forces 
  acting 
  on 
  it 
  when 
  

   it 
  is 
  placed 
  in 
  a 
  rapidly 
  moving 
  stream 
  of 
  air. 
  This 
  is 
  known 
  as 
  

   the 
  wind-tunnel 
  method, 
  and 
  is 
  now 
  in 
  general 
  use 
  in 
  all 
  countries. 
  

   England 
  has 
  10 
  or 
  more 
  such 
  tunnels, 
  France 
  has 
  several, 
  Ger- 
  

   many 
  has 
  a 
  large 
  number, 
  etc. 
  In 
  this 
  country 
  there 
  are 
  12, 
  and 
  

   more 
  are 
  being 
  made. 
  

  

  Other 
  methods 
  have 
  been 
  used 
  in 
  the 
  past 
  but 
  are 
  no 
  longer. 
  

   Langley 
  attached 
  his 
  models 
  to 
  one 
  end 
  of 
  a 
  long 
  arm 
  which 
  could 
  

  

  