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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