The short version (for dummies like me) is that as scale gets smaller, the
air molecules don't...therefore they become "large" relative to the airfoil.
When they're large, they're harder to move out of the way of the wing in the
same way that it's easier to dig through fine sand than through course sand
at the beach.....the fine sand "flows" better - it has less friction. This
same effect takes place at the leading edge of your wing - if the molecules
are "smaller" (as in a large scale wing) less of them are hit dead on by
your leading edge (causing drag) than hit above or below the exact edge and
flow AROUND the airfoil (creating lift).
Matt
----- Original Message -----
From: "Don Stackhouse @ DJ Aerotech" <[EMAIL PROTECTED]>
To: <[EMAIL PROTECTED]>
Cc: "Clifford Schwinger" <[EMAIL PROTECTED]>
Sent: Sunday, February 04, 2001 10:49 PM
Subject: [RCSE] Re: Who is this Reynolds dude?
> Date: Sun, 04 Feb 2001 19:25:43 -0500
> From: Clifford Schwinger <[EMAIL PROTECTED]>
> To: RCSE <[EMAIL PROTECTED]>
> Clifford Schwinger asks:
>
> >I have been seeing a lot of references being made to the "Reynolds
> >Number" recently. Could someone please explain to me - in simple terms -
> >what this number is and what it means when a sailplane has a small "Rn"
> >vs. a large "Rn".
> >
> >>From what my feeble mind has figured out so far, the "Rn" has something
> >to do with the increasing difficulty of an airplane to fly as it gets
> >smaller? 2-meter sailplanes have a higher Rn than an rchlg? An rchlg has
> >a higher Rn than a ffhlg?
> >
> >Does the Rn explain why I can't build a glider with a 1" wingspan?
> >
> >How does one compute the Rn for an airplane? If I have a sailplane with
> >a 1.5 meter wingspan is the Rn for that plane "set in stone" or can you
> >do something with the geometry or weight of the plane to vary the Rn and
> >make it fly better?
> >
> >Does anyone know of any websites that have a clear explanation of the
> >Rn?
>
> There are quite a few articles in the "Ask Joe and Don" section of our
> website that discuss the nuances and implications of Reynolds numbers. It
> is such a significant factor that there are quite a few articles where it
> makes at least a cameo appearance. Just type "Reynolds" into the Ask Joe &
> Don search engine and you'll get a big list of articles to browse on the
> subject.
>
> One article that discusses the number itself is at:
>
> http://www.djaerotech.com/dj_askjd/dj_questions/hlgairspeed.html
>
> BTW, I have built FF gliders with wingspans as small as 5/8", and kites
> (scale models of Monarch butterflies, complete with all the black lines
and
> white spots) with 3/8" spans, both of which flew successfully. However,
the
> max L'd's are definitely lower than what is possible at larger sizes.
>
> Reynolds number (or "Re" in engineering shorthand) is air density times
> length (usually wing chord) times speed, divided by air viscosity. It's a
> numerical measure of what modellers call "scale effect". It gets smaller
as
> you go up in altitude, slower in airspeed, or smaller in chord. At sea
> level standard day conditions it's:
>
> Re = 778 * Chord (in inches) * Speed (in MPH)
>
> In general, decreasing Re means that your drag will increase, and your max
> lift coefficient will decrease. Airfoils designed for one Re will
generally
> not work well at a substantially different Re. However, it is possible to
> design airfoils with reasonably good efficiencies at very low Re's (as we
> did for our Spectre series), although it becomes increasingly difficult,
> especially at Re's below about 100,000. Below that number, things really
> get squirrely, and most of the traditional ways at analysing airflow tend
> to quickly get into serious trouble. Most of the commonly available
airfoil
> codes generate mostly garbage below that number, and most wind tunnels
> aren't significantly better. In my experience, at very low Re's, what is
> more important than the software you're using is your experience and skill
> in properly interpreting its outputs.
>
>
> Don Stackhouse @ DJ Aerotech
> [EMAIL PROTECTED]
> http://www.djaerotech.com/
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