reynolds number

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

"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

Morgans schreef:


"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.

And that's the lines along which I was thinking, until this Reynolds
thing crossed my way. My gratitude to all who responded, I have received
a powerful lot to put under my thinking cap for the next couple of
weeks/months/years

KA

On Sat, 27 Jun 2009 17:43:27 -0400, "Morgans"
wrote:



"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.

....and then you look at mark langford's web site and pick the aerofoil
according to thickness. ...from the list of excellent aerofoils
developed for the KR2S on his web site. :-)

you dont have to be correct or competent to design an aeroplane.
you stand the chance of designing a damn site better one though if you
are. having experience and attitude sometimes gets you there.
competence can mire you in decisions and see you achieve nothing.

Why is the Wittman W8 tailwind still a standout in the efficiency
figures?

Stealth Pilot

"Stealth Pilot" wrote

Why is the Wittman W8 tailwind still a standout in the efficiency
figures?

Seems to me that it has a few things that keep it on top.

See if you think I am on the right track.

The shapes used in the fuselage and anything that is sticking out in the wind
are all good aerodynamic tradeoffs of slippery and light.

The basic shape of the fuselage is good for contributing to the lift of the
aircraft, more than most other designs. Probably the most important feature of
the design, in my eyes.

Attention is always on making structures easy to build light and no extra weight
is there that does not contribute to lightness.

The airfoil and fuselage are light enough and slippery enough to be powered by a
small engine, so extra engine weight and fuel weight does not have to be carried
around, which allows the structures to be built more lightly. It is sort of a
good circle that keeps weight down, versus the other circle that keeps growing
the weight of the aircraft.

How did I do? g
--
Jim in NC

On Tue, 21 Jul 2009 00:40:21 -0400, "Morgans"
wrote:


"Stealth Pilot" wrote

Why is the Wittman W8 tailwind still a standout in the efficiency
figures?

Seems to me that it has a few things that keep it on top.

See if you think I am on the right track.

The shapes used in the fuselage and anything that is sticking out in the wind
are all good aerodynamic tradeoffs of slippery and light.

The basic shape of the fuselage is good for contributing to the lift of the
aircraft, more than most other designs. Probably the most important feature of
the design, in my eyes.

Attention is always on making structures easy to build light and no extra weight
is there that does not contribute to lightness.

The airfoil and fuselage are light enough and slippery enough to be powered by a
small engine, so extra engine weight and fuel weight does not have to be carried
around, which allows the structures to be built more lightly. It is sort of a
good circle that keeps weight down, versus the other circle that keeps growing
the weight of the aircraft.

How did I do? g

pretty damn good.