﻿172 
  

  

  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  1922. 
  

  

  up 
  of 
  a 
  number 
  of 
  separate 
  surfaces, 
  and 
  when 
  the 
  shaft 
  is 
  rotating 
  

   each 
  of 
  these 
  is 
  thought 
  of 
  as 
  an 
  aerofoil 
  moving 
  rapidly 
  through 
  

   the 
  air. 
  In 
  this 
  way 
  the 
  action 
  of 
  a 
  propeller 
  may 
  be 
  calculated, 
  

   and 
  the 
  theory 
  may 
  be 
  tested 
  by 
  trying 
  the 
  actual 
  propeller. 
  But 
  

   there 
  is 
  one 
  all-important 
  difficulty 
  in 
  applying 
  the 
  knowledge 
  thus 
  

   available 
  to 
  the 
  design 
  of 
  actual 
  aircraft. 
  Is 
  one 
  justified 
  in 
  drawing 
  

  

  Lift 
  balance 
  

  

  Seg- 
  

   - 
  rnent 
  

   for 
  

   chang- 
  

   ing 
  the 
  

   angle 
  

   of 
  attack 
  

  

  ^Lifi 
  counter 
  v/eiyhtt 
  

  

  Fig. 
  6.- 
  

  

  Scheme 
  of 
  the 
  balance 
  used 
  to 
  measure 
  the 
  forces 
  

   on 
  a 
  model. 
  

  

  conclusions 
  as 
  to 
  the 
  properties 
  of 
  large 
  bodies 
  from 
  knowledge 
  of 
  

   the 
  properties 
  of 
  smaller 
  ones? 
  Suppose, 
  for 
  instance, 
  that 
  on 
  com- 
  

   paring 
  two 
  models 
  on 
  a 
  scale 
  of 
  one 
  twenty-fifth 
  of 
  full 
  size 
  in 
  a 
  

   wind 
  tunnel 
  through 
  which 
  the 
  velocity 
  of 
  the 
  air 
  stream 
  is 
  40 
  miles 
  

   per 
  hour, 
  it 
  is 
  found 
  that 
  one 
  model 
  has 
  a 
  greater 
  lift 
  or 
  a 
  less 
  drag 
  

   than 
  the 
  other, 
  what 
  conclusion 
  could 
  one 
  draw 
  as 
  to 
  the 
  comparative 
  

   properties 
  of 
  two 
  actual 
  airplane 
  wings 
  in 
  flight 
  at 
  120 
  miles 
  per 
  

  

  