reynolds number

Gerry van Dyk schreef:
Don't feel the least bit stupid over this, Reynolds number is a
horribly misunderstood thing.

Thanks for reassuring me Gerry, you mailed this while I was replying to
Cavelamb.

(snipped useful explanation)

Reynolds number basically puts a value on the quantity of air working
on a wing for a given unit of time. If you reduce speed or reduce
size, then less air works on it. Increasing speed or increasing size
increases the amount of air working on it.

This is a hard nut to crack, but it looks like it might be the key to my
understanding. Will sleep over it now, and let the information soak this
poor old brain...

At the risk of making things more murky, let me add another point.

Given that bigger wing = higher R = more lift, or higher speed =
higher R = more lift, it also works higher air density = higher R =
more lift. Both lift and drag drop off with altitude, and increase as
you decsend to sea level, IE Renolds number decreases with altitude
and increases going down.

As a thought exercise, think about how fast you'd need to make a wing
travel in water to lift a given weight. It might take only 10 mph to
lift a C-150 in water vs 60 mph in air.. Water is so much more dense
than air, the low speed makes just as many molecules of water contact
the wing during 1 second of time, as air molecules work on it for a
second at 60 mph in air. Therefore, you get the same Reynolds number
at 10 mph in water as 60 mph in air. When moving at an equal Reynolds
number a given wing will give the same lift and drag. In this thought
exercise slow speed thought water gives the same lift and drag as high
speed in air. We've now added fluid density to the equation, all
three factors are part of Reynolds number.

The bottom line is, regardless of the fluid density, physcal size of
the wing, or speed, a given Reynolds number will produce a specific
lift and drag. Change the density, change the size and work out a
speed that will give the same R number, then you will get exactly the
same lift and drag.

Getting back to the question in the other thread, asking about
Reynolds number is engineer-speak for "what wing chord and what speed
will you be running. We'll assume 'standard air' for density, then
we'll select an airfoil that will give you enough lift for the weight
you'll need to pick up."

Hope I didn't just make it worse.
Gerry

On Jun 22, 12:08*pm, jan olieslagers
wrote:
Gerry van Dyk schreef:

Don't feel the least bit stupid over this, Reynolds number is a
horribly misunderstood thing.

Thanks for reassuring me Gerry, you mailed this while I was replying to
Cavelamb.

(snipped useful explanation)

Reynolds number basically puts a value on the quantity of air working
on a wing for a given unit of time. *If you reduce speed or reduce
size, then less air works on it. *Increasing speed or increasing size
increases the amount of air working on it.

This is a hard nut to crack, but it looks like it might be the key to my
understanding. Will sleep over it now, and let the information soak this
poor old brain...