Reynolds number (engineering notes)

[David Lewis]

woundedbear wrote: What is a Reynolds number ?

David Lewis wrote: As you shrink an airplane down, air molecules do NOT get smaller, therefore aerodynamic behavior between different scales is not similar. Generally drag goes up as Reynolds number (Re) decreases. Most often Re is used to compare the performance of a given airfoil at different scales or speeds. In that case it's called the "chord Reynolds number", and the characteristic length plugged into the formula is the chord of the airfoil or wing.

Full scale engineers rely on test flights or wind tunnel data collected at (or corrected for) the Re in the case at hand. With models, however, Re effects are more important, and data is rarely available and less useful. Even if you have two wings with identical Re, they may not fly the same because small differences, such as building tolerances and surface roughness, have a significant effect on aerodynamic behavior.

[dirk gently]

Basically, there are two major forces that govern the dynamics of air molecules - viscosity and inertia.
As the airflow gets faster and the distances under consideration (like, the distance from the leading edge to the trailing edge of the wing) are getting larger, viscosity has lower and lower impact on the overall airflow, while the impact of inertia increases. Hence, different laws of physics tend to apply when describing the aerodynamics of small and slow aircraft (like rubber-powered balsa models) and large and fast ones (like full scale jets).

Reynolds number catches this, by simply multiplying the size by the speed of the flow (times some constants). Airflows tend to behave similiarily provided that their Reynolds numbers are similar.

This has an interesting consequence - a 1:X scale aerodynamic tunnel model needs to be examined at X times the airspeed of the full scale aircraft (*not* 1/X speed!) in order to reliably mimic it's airflow.

[woundedbear]

sorry guys but i still don't have a clue
ALSO WILL SOMEBODY PUT SOME MORE OPINIONS ON THE OTHER QUESTION I HAVE ON THIS FORUM
DOES ANYBODY KNOW ANYTHING..... ANYTHING AT ALL ABOUT FLIGHT STABILIZERS AND GIROS

[dirk gently]

It's also explained quite well on wikipedia: https://en.wikipedia.org/wiki/Reynolds_number

[woundedbear]

well when you mix numbers with letters I am hopelessly lost... However if you put a $ in front and a .00 with only two numbers to the right I can usually figure it out

[woundedbear]

OH one more thing Mr. Lewis you are one smart dude

[David Lewis]

Reynolds number = inertial forces/viscous forces = air density x speed x length/viscosity

Standard units:
[length] = metre (m)
[time] = second (s)
[mass] = kilogram (kg)
[pressure] = pascal (Pa)

Air density (standard atmosphere at sea level) = 1.225 kg/m^3
Air viscosity = 17.3 uPa-s (micropascal-seconds)

Re = 70810 x speed x length