title_body: Do aerodynamics scale for model aircraft? If I wanted to make a scale model plane that looked like a real aircraft, could I just scale down all of the measurements of the actual aircraft to make a model aircraft with similar flight characteristics? If not, what is the reason?
upvoted_answer: Aircraft do not scale linearly, for a number of reasons.
  
  Most aerodynamic equations include a speed squared term. If you halve the speed, you only get a quarter of the lift and drag, so models tend to be lighter and less powerful than you'd expect.
  
  If you make a plane with half the wing span, it only has a quarter of the wing area. So now a half size model flying at half speed only has one sixteenth of the lift.
  
  Reynolds number - air flowing over a surface will start with laminar flow but turn turbulent after a while, depending on the speed (and density and viscosity but we can ignore those). A smaller, slower model will see more laminar flow than the full size aircraft. In practice, this is a relatively small effect compared to the previous factors but it's an important consideration for high performance models.
  
  
  Example
  A full size Spitfire had a wing span of 36 ft 10 in (11.23 m), a weight of 5-6000lbs (2-3,000kg) and between 1000hp and 2000hp depending on the variant, with a maximum speed of around 400mph.
  If you scaled it linearly, a 1/10th scale model would have a 1.1m span, weigh 200kg, have 100hp and fly at 40mph!
  If (as Phil suggests) you scale weight and power by 1000 (10 cubed) it would weigh 5-6lbs and have 1-2hp, which is close but still heavier and more powerful than real models:
  A 72&quot; model (1/6th scale) might weigh 15lbs and have 2.5-3KW (4hp) and fly at 100mph.
  A 44&quot; model (1/10th scale) might weigh 2-3lb and have 2-300W (1/2hp) and fly at 60mph.
  A 36&quot; model (1/12th scale) might weigh about 1lb and have 100W (1/7th hp) and fly at 50mph