﻿AERONAUTIC 
  RESEARCH 
  AMES. 
  

  

  173 
  

  

  hour 
  ? 
  The 
  answer 
  is, 
  " 
  Possibly 
  none." 
  The 
  exact 
  condition 
  under 
  

   which 
  results 
  from 
  tests 
  on 
  models 
  may 
  be 
  applied 
  to 
  actual 
  con- 
  

   struction 
  is 
  known. 
  This 
  can 
  best 
  be 
  described 
  by 
  discussing 
  what 
  

   is 
  called 
  in 
  aerodynamics 
  the 
  " 
  Reynolds 
  number." 
  When 
  a 
  body 
  is 
  

   moving 
  through 
  the 
  air 
  there 
  are 
  two 
  types 
  of 
  forces 
  acting 
  on 
  it 
  — 
  

   the 
  air 
  exerts 
  a 
  pressure 
  upon 
  it 
  and 
  there 
  is 
  friction 
  between 
  the 
  

   moving 
  air 
  and 
  the 
  layer 
  of 
  air 
  sticking 
  tight 
  to 
  the 
  solid. 
  It 
  is 
  not 
  

   difficult 
  to 
  see 
  that 
  the 
  four 
  physical 
  quantities 
  involved 
  in 
  the 
  aero- 
  

   dynamical 
  action 
  are 
  : 
  The 
  velocity 
  of 
  the 
  air, 
  its 
  density, 
  its 
  viscosity 
  

   (i. 
  e., 
  measure 
  of 
  the 
  frictional 
  property), 
  and 
  the 
  size 
  of 
  the 
  solid 
  

   body 
  as 
  given 
  by 
  its 
  length 
  or 
  its 
  thickness. 
  Lord 
  Rayleigh 
  showed 
  

   many 
  years 
  ago 
  that 
  if 
  we 
  formed 
  the 
  quantity 
  — 
  

  

  density 
  X 
  velocity 
  X 
  length 
  

   viscosity 
  

  

  Window 
  

  

  -26'0"- 
  

  

  Floorline 
  Manhole 
  5 
  X 
  3' 
  Electric 
  wires 
  Floor 
  line 
  

  

  Fig. 
  7. 
  — 
  Compressed 
  air 
  wind-tunnel. 
  

  

  which 
  is 
  now 
  called 
  the 
  " 
  Reynolds 
  number," 
  we 
  would 
  be 
  justified 
  in 
  

   saying 
  that 
  the 
  properties 
  observed 
  in 
  any 
  experiment 
  would 
  also 
  

   be 
  found 
  to 
  be 
  the 
  same 
  for 
  any 
  other 
  experiment 
  having 
  the 
  same 
  

   Reynolds 
  number. 
  It 
  is 
  seen 
  by 
  looking 
  at 
  the 
  definition 
  of 
  this 
  

   number 
  that 
  we 
  can 
  have 
  the 
  same 
  Reynolds 
  number 
  for 
  a 
  large 
  

   number 
  of 
  different 
  experiments. 
  Thus, 
  compare 
  an 
  actual 
  airplane 
  

   in 
  flight 
  at 
  90 
  miles 
  per 
  hour, 
  say, 
  with 
  a 
  wind-tunnel 
  experiment 
  on 
  

   a 
  model 
  of 
  one-twentieth 
  scale 
  but 
  having 
  the 
  same 
  velocity 
  of 
  air 
  

   flow. 
  If 
  the 
  Reynolds 
  number 
  is 
  to 
  be 
  the 
  same 
  for 
  the 
  two 
  cases, 
  

   it 
  is 
  necessary 
  to 
  increase 
  the 
  density 
  of 
  the 
  air 
  in 
  the 
  wind 
  tunnel 
  

   twentyfold. 
  This 
  shows 
  that 
  if 
  one 
  were 
  to 
  make 
  a 
  wind 
  tunnel 
  

   in 
  which 
  the 
  air 
  is 
  compressed 
  to 
  20 
  or 
  more 
  atmospheres, 
  experi- 
  

   ments 
  on 
  models 
  placed 
  in 
  it 
  would 
  give 
  results 
  immediately 
  appli- 
  

   cable 
  to 
  full-size 
  airplanes. 
  It 
  is 
  perfectly 
  possible, 
  of 
  course, 
  that 
  

  

  