reynolds number

Thanks for your post Oliver, you are of course absolutely correct in
everthing you've said. I was regretting my second post with the
'thought exercise' as I hit the button, the intent was to demonstrate
the relationship of Re to density, and I think I missed the mark.

My intent for posting was not to provide an accurate engineering
description of Reynolds number, but to help a non-engineer understand
the basic concept. As you've provided, Re = v * L / nu. If we
consider air viscocity / density / stickyness to be constant, then it
holds that an airfoil operating at half the speed and twice the length
of another will calculate out to the same Reynolds number, and both
will essentially behave the same.

I will submit for your consideration that giving a fully accurate and
complete engineering description of the effects of Re that is gold for
we engineers will do little or nothing to help the layman understand
the underlying concept. (I'm a mechanical engineer, but not an
aerodynamicist) I will contend that the terms kinematic and dynamic
viscosity, drag polar, laminar and turbulent flow, laminar bucket,
lift coefficient, while joyful for us engineers to bat around in
aircraft design, they are abolutely meaningless to the non-engineer.
(You did mean kinematic and not 'cinematic' didn't you? Sorry, just a
friendly dig there. ;^)

I'll try to restate: Reynolds number is essentially a count of how
much air is acting on a wing in a unit of time. If a given length of
airfoil traveling at a given speed calculates to a certain Re, then
the same airfoil shape in a smaller size will have to go faster to
have the same quantity of air working on it.

Would that statement pass the accuracy test for you?

Jan, is the concept starting to come together for you?

Best regards all
Gerry

To All:

I've always appreciated my father's explanation, which some of you may
find suitable if you have a nine-year old who is just getting into
free-flight.

Model competitions were quite popular up until WWII but never regained
their past glory after. My dad and several of his friends were
discussing a new wing for an existing fuselage and Reynold's number
was mentioned several times. When I asked what it was his friends
started to grin; a couple even laughed but he told me there were
somethings that we couldn't scale, such as the size of a molecule of
air. so a fellow named Reynold came up with a way of calculating a
number that could be used as a kind of artifical scaling factor.

-R.S.Hoover

(Do they still use Banana Oil?)

First, thanks to Oliver for a much better explanation.


It's not about the size of a molecule, but how they interact
with each other and surfaces at different energy levels.



These links are a few animations and film clips that depict air flow
and increasingly higher Reynolds numbers.



This first one is at VERY LOW Reynolds and displays a nearly Newtonian
reaction. I suspect that if people think about air flow, this is how
they would expect it to behave.
http://www.youtube.com/watch?v=rbMx2...eature=related



But kick the velocity up to flying speeds and see what actually happens...
http://www.youtube.com/watch?v=A4taH...eature=related
http://www.youtube.com/watch?v=vQHXI...eature=related



Transonic range...
http://www.youtube.com/watch?v=ZMEQJhiebu4
And a standing shock wave on a sub sonic airliner wing!
http://www.youtube.com/watch?v=duCHF...eature=related



Actual photographs of supersonic shock waves (an interesting film
about supersonic flight too)
http://www.youtube.com/watch?v=atItR...eature=related



So, what about an actual airfoil?
Department of Engineering, University of Cambridge, multimedia video from
Physics Education, 2003, by Holger Babinsky.
The smoke tunnel really lays it out clearly.
And the pulsed smoke flow can start a lot of arguments!
http://www.youtube.com/watch?v=6UlsA...eature=related



This one I like because it shows a "long bubble" developing on the top
surface of the wing.
It's not what they were after, but it clearly shows the bubble development.
http://www.youtube.com/watch?v=J-xxC...eature=related

Oliver Arend schreef:

The Reynolds number is determined as

Re = v * L / nu

v is the speed of the airflow,
L is the "characteristic length" (I'll get into that)

OK OK thanks but that wasn't really my question.
Excuse me if I wasn't clear enough, what I really meant to ask is
"when designing a plane, need I be concerned about the Reynolds number
and if so, in what way?" My first understanding was that it is a
property of the airfoil, that seems wrong now.

( ... )

This means you can get an infinite number of Re on an airplane, just
as has been written. It cannot (really) be chosen, but is determined
by size and operating conditions of the airplane.

But NOT by the airfoil, then? Does one first determine the (max?) Re the
plane will be operating at, and choose an airfoil accordingly?

In short, you need it if you (seriously) want to design an airplane
and estimate its performance.

That is a very useful answer to me.

But the difference between wind tunnel testing and reality is much
greater than the difference between Re = 1 * 10^6 and 2 * 10^6, so it
doesn't really matter for homebuilders. It can become interesting for
builders of high-performance model airplanes and of course
aerodynamically challenging tasks such as designing sailplanes.

So if I'm not wanting the ultimate bit of performance from my DreamBird,
I needn' t bother too much? How then do I go about selecting an airfoil?


PS1 I am very happy with the tone of this discussion: only positive
reactions, and people willing to accept correction. It's one of the
reasons I like to lurk a bit here, and even dare to launch some
questions about a silly dream I am not likely to ever realise.

PS2 Oliver, verstehe ich gut du bist Deutsch? Das koennte mal Spass
machen, anderes als Englisch zu schreiben...

jan,

Perhaps it would be easier to think of it as a property of the fluid
(in this case - air) - not the wing...



The air is moving past the wing(!) and will stratify (or separate into
layers) moving at different speeds.



Read these arrows as a vectors - direction and length = velocity

----------------------------------------- Free stream velocity
------------------------------------
------------------------------ boundary layers move faster
--------------------------
---------------------
-----------------
------------- boundary layers move slower
--------- closer to surface
---
========================================= surface





In special cases, air in the the boundary layer can even move
in the OPPOSITE direction!

----------------------------------------- Free stream velocity
------------------------------------
------------------------------ boundary layers move faster
-------------------------- farther from surface
---------------------
-----------------
------------- boundary layers move slower
--------- closer to surface
-----
- here, the flow has reversed
---
========================================= surface



Note: the Re of the layers could be different due to differences in
velocity.

"jo" == jan olieslagers writes:

jo OK OK thanks but that wasn't really my question. Excuse me if
jo I wasn't clear enough, what I really meant to ask is "when
jo designing a plane, need I be concerned about the Reynolds
jo number

jo PS1 I am very happy with the tone of this discussion: only
jo positive reactions,

Hmmm. This may be the first negative one. If someone doesn't
understand Re, should they be designing an airplane? I don't think
one can pick up sufficient fluid and structural mechanics on Usenet to
be a reliable aircraft designer.
--
Suppose you were an idiot, and suppose you were a member of congress;
but I repeat myself.
~ Mark Twain

OK OK thanks but that wasn't really my question.
Excuse me if I wasn't clear enough, what I really meant to ask is "when designing a plane, need I be concerned about the Reynolds number and if so, in what way?"

Yes, you should. But not much (see below, answer to Bob's post).

My first understanding was that it is a property of the airfoil, that seems wrong now.

True. As Richard said, it's more of a property of the air flowing
around the airfoil.

But NOT by the airfoil, then? Does one first determine the (max?) Re the plane will be operating at, and choose an airfoil accordingly?

You should determine the range of Re the plane/airfoil is operating
at. For low speeds/low Re, evaluate cLmax for stall. For higher speeds/
higher Re, evaluate drag in the cL range you're going to encounter.
For example, I'm currently trying to fit winglets to an existing wing.
The wing itself operates (roughly) at Re=1M...5M, the winglets at
300k...5M. Esp. the latter poses a serious challenge for the airfoil
designer/airfoil choice.

So if I'm not wanting the ultimate bit of performance from my DreamBird,
I needn' t bother too much? How then do I go about selecting an airfoil?

Well... cLmax predictions from calculations and wind tunnels don't
always relate well to what you get or seem to get in flight. The so-
called "laminar bucket" (range of cL with low drag) does so much
better. So what you do is choose an airfoil that gets you low drag in
the your cruise speed range, and if you're lucky it has a decent
cLmax. Size the wing accordingly (e.g. to satisfy stall speed
requirements). It all boils down to a trade-off, which is easier with
flaps.
If you don't want to use flaps, the NACA airfoils that have been
proposed (4- and 5-digit-series) do the job pretty well, but don't
expect a high-performance aircraft to come out of it. There's also a
lot of other areas you can work on to reduce drag. The plane I'm
working on has a no-lift-cD of around 0.035, out of which only maybe
0.006 come from the wing (the rest is fuselage, turbulence from the
prop, struts, empennage, landing gear etc.). RVs for example use
countersunk rivets much more than other metal homebuilts it seems (and
maybe larger engines), so they achieve higher performance.

Hmmm. This may be the first negative one. If someone doesn't
understand Re, should they be designing an airplane? I don't think
one can pick up sufficient fluid and structural mechanics on Usenet to
be a reliable aircraft designer.

You don't really have to understand Re. You only have to apply it. I
think that's a difference between somebody who specialized in
aerodynamics (me ;-) opposed to someone who specialized in aircraft
design (me too ;-). And I don't think anyone would design a plane with
knowledge from Usenet. Rather pick up a book and get questions that
remain unanswered in the book answered on Usenet (neither Raymer nor
Roskam _explain_ Re, but they probably mention it somewhere). And
structural design on a (homebuilt) aircraft can be simplified to the
point of only having beams under tension/compression, bending and
torsional loads with maybe some buckling thrown in. Gets you three or
four different equations. The rest is structural testing.
Wasn't there a post about "Designing your homebuilt in 10 equations"
mentioned a while back?

PS2 Oliver, verstehe ich gut du bist Deutsch? Das koennte mal Spass machen, anderes als Englisch zu schreiben...

Absolut richtig, Jan, und so weit ich das verstehe bist Du
Niederländer? Oder Flämisch-Belgier? Ik kann een wenig Platt verstahn,
aber dat is nich dat selbe wie Nedderlannsch ;-)

Oliver

Bob Fry wrote:
"jo" == jan olieslagers writes:

jo OK OK thanks but that wasn't really my question. Excuse me if
jo I wasn't clear enough, what I really meant to ask is "when
jo designing a plane, need I be concerned about the Reynolds
jo number

jo PS1 I am very happy with the tone of this discussion: only
jo positive reactions,

Hmmm. This may be the first negative one. If someone doesn't
understand Re, should they be designing an airplane? I don't think
one can pick up sufficient fluid and structural mechanics on Usenet to
be a reliable aircraft designer.

There are two kinds of design (and science, and lots of other stuff..)
incremental, and original.
As it happens, the great majority of design (and lots of other stuff) is
incremental. That's not such a cop out as you might think.

In terms of airplanes: you examine the mission. (EVERY airplane has a
mission, but some folks don't necessarily realise that)
Then, you gather data on every airplane with a similar mission.
Then (as best you can) you evaluate the positive user feedback, and the
negative stuff, and associate design features with the desirable features.
Then you try to package all the desirable features and none of the
undesirable features in one package. At NO point have I mentioned Re
have I?
I could even go a little further: if you get yourself in a situation
when you have to deploy your considerable engineering skills in
evaluating Re, it is because you forgot to use your even more
considerable judgment is selecting well-liked, useful, relevent airfoils.

:-)

Brian Whatcott Altus OK

Bob Fry schreef:

jo PS1 I am very happy with the tone of this discussion: only
jo positive reactions,

Hmmm. This may be the first negative one. If someone doesn't
understand Re, should they be designing an airplane? I don't think
one can pick up sufficient fluid and structural mechanics on Usenet to
be a reliable aircraft designer.

Bob, I do not consider your reply negative. Concerns about excessive
risks are very positive indeed. Thanks very much!

"Brian Whatcott" wrote

I could even go a little further: if you get yourself in a situation when
you have to deploy your considerable engineering skills in evaluating Re,
it is because you forgot to use your even more considerable judgment is
selecting well-liked, useful, relevent airfoils.

:-)
Amen!

Ya define the mission, and how fast you think you will go and look at the
list of airfoils used on airplanes of similar speed and mission. That makes
airfoil choice a real choice.
--
Jim in NC