Thread: reynolds number
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Old June 22nd 09, 07:04 PM posted to rec.aviation.homebuilt
jan olieslagers[_2_]
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Default reynolds number

cavelamb schreef:

Detailed explanation at:
http://www.aerodrag.com/Articles/ReynoldsNumber.htm


Thank you Richard, you really did your best but I only feel more stupid,
this text still leaves me confused. At one time the Reynolds factor
seems a property of the plane, and one Questair plane can even have two
values for it, another time it seems a property of the wing and yet
another time the Reynolds factor is different for various places of the
wing. Excuse my being confused!

Here's a simple example for a wing with a 10 feet chord at 100 mph
flying speed, at Sea Level and "Standard Day" conditions.

Re = 9346 x 100 x 10 = 9346 x 1000 = 9,346,000.


This sounds like "it is a property of a given wing at a certain speed"
OK, I can digest that. So just like drag, Re will go up by speed
squared. And it might vary with atmospheric conditions, I'm still with
you, great!

Reynold's Magic Number basically shows the ratio between inertial
forces and viscous forces in a fluid.

Think of it as (how fast it's moving) / (how sticky it is).


OK, how sticky it is depends on the wing (airfoil and "smoothness" I
should think, and speed is speed. OK, got that.

At low R, viscous forces predominate. (and generally laminar flow)
At high R, is dominated by inertial forces. (resulting in higher sheer
forces and turbulence)


This much I gathered from the Wiki page, but I still don't get the
point. Given the mission (design a plane with so much max gross, with
xxx HP engine power, and make it go as fast as you can) is it possible
to determine an optimal Reynolds number for the wing or for the damned
plane or its f....g mother in law?

Or why is this Reynolds factor important, and where does one apply it?

Thanks for bearing with me,
KA