reynolds number

[jan olieslagers[_2_]]

It's been puzzling me for a long while and there it is again popping up
in the "WIG airfoils" thread: what is this sacred Reynolds number?
I tried our alther friend en.wikipedia but its theory was quite beyond
my level of education* and its examples of oil in a pipe were not really
illuminating - not to mention the spermatozoa and the Major League Baseball.

Is it a property of the wing, or of the whole plane, or do the fuselage
and wing and empennage &C each have their own Reynolds number?
I seem to understand this figure is a measure of aerodymanic quality?
Given a plane's weight and engine power, will it be faster (or slower)
for a higher Reynolds number?

Excuse my stupidity,
KA


*I am only a modest Solaris sysadmin, never went to university...

[cavelamb[_2_]]

jan olieslagers wrote:
> It's been puzzling me for a long while and there it is again popping up
> in the "WIG airfoils" thread: what is this sacred Reynolds number?
> I tried our alther friend en.wikipedia but its theory was quite beyond
> my level of education* and its examples of oil in a pipe were not really
> illuminating - not to mention the spermatozoa and the Major League
> Baseball.
>
> Is it a property of the wing, or of the whole plane, or do the fuselage
> and wing and empennage &C each have their own Reynolds number?
> I seem to understand this figure is a measure of aerodymanic quality?
> Given a plane's weight and engine power, will it be faster (or slower)
> for a higher Reynolds number?
>
> Excuse my stupidity,
> KA
>
>
> *I am only a modest Solaris sysadmin, never went to university...

Detailed explanation at:
http://www.aerodrag.com/Articles/ReynoldsNumber.htm

Here's a simple example for a wing with a 10 feet chord at 100 mph
flying speed, at Sea Level and "Standard Day" conditions.

Re = 9346 x 100 x 10 = 9346 x 1000 = 9,346,000.


Reynold's Magic Number basically shows the ratio between inertial
forces and viscous forces in a fluid.

Think of it as (how fast it's moving) / (how sticky it is).

At low R, viscous forces predominate. (and generally laminar flow)
At high R, is dominated by inertial forces. (resulting in higher sheer
forces and turbulence)


Straight from wiki...

If an airplane wing needs testing, one can make a scaled down model of the wing
and test it in a wind tunnel using the same Reynolds number that the actual
airplane is subjected to. If for example the scale model has linear dimensions
one quarter of full size, the flow velocity would have to be increased four
times to obtain similar flow behaviour.

Alternatively, tests could be conducted in a water tank instead of in air
(provided the compressibility effects of air are not significant). As the
kinematic viscosity of water is around 13 times less than that of air at 15 °C,
in this case the scale model would need to be about one thirteenth the size in
all dimensions to maintain the same Reynolds number, assuming the full-scale
flow velocity was used.

The results of the laboratory model will be similar to those of the actual plane
wing results. Thus there is no need to bring a full scale plane into the lab and
actually test it. This is an example of "dynamic similarity".

Reynolds number is important in the calculation of a body's drag
characteristics. A notable example is that of the flow around a cylinder. Above
roughly 3×106 Re the drag coefficient drops considerably. This is important when
calculating the optimal cruise speeds for low drag (and therefore long range)
profiles for airplanes.

[Gerry van Dyk]

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

In broad terms, it puts a value on the physical length of an airfoil
and the speed it's traveling. Say you have a 4 foot wing chord, and
the wing is travelling 100 mph will produce a R number around 3.3
million. That same airfoil scaled up to 8 feet will give an R at 6.6
million, or the 4 foot wing at 200 mph also gives 6.6 million, and the
8 foot chord going 200 mph gives an R of 13 million. An airfoil built
half the size, traveling at the same speed as another will give much
less lifting force. (not necessarily half) but that half size airfoil
travelling at twice the speed will act exactly the same as the larger
airfoil at the lower speed.

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.

Hope this helps your understanding
Gerry

On Jun 22, 10:33*am, jan olieslagers >
wrote:
> It's been puzzling me for a long while and there it is again popping up
> in the "WIG airfoils" thread: what is this sacred Reynolds number?
> I tried our alther friend en.wikipedia but its theory was quite beyond
> my level of education* and its examples of oil in a pipe were not really
> illuminating - not to mention the spermatozoa and the Major League Baseball.
>
> Is it a property of the wing, or of the whole plane, or do the fuselage
> and wing and empennage &C each have their own Reynolds number?
> I seem to understand this figure is a measure of aerodymanic quality?
> Given a plane's weight and engine power, will it be faster (or slower)
> for a higher Reynolds number?
>
> Excuse my stupidity,
> KA
>
> *I am only a modest Solaris sysadmin, never went to university...

[jan olieslagers[_2_]]

cavelamb schreef:

> Detailed explanation at:
> http://www.aerodrag.com/Articles/ReynoldsNumber.htm

Thank you Richard, you really did your best but I only feel more stupid,
this text still leaves me confused. At one time the Reynolds factor
seems a property of the plane, and one Questair plane can even have two
values for it, another time it seems a property of the wing and yet
another time the Reynolds factor is different for various places of the
wing. Excuse my being confused!

> Here's a simple example for a wing with a 10 feet chord at 100 mph
> flying speed, at Sea Level and "Standard Day" conditions.
>
> Re = 9346 x 100 x 10 = 9346 x 1000 = 9,346,000.

This sounds like "it is a property of a given wing at a certain speed"
OK, I can digest that. So just like drag, Re will go up by speed
squared. And it might vary with atmospheric conditions, I'm still with
you, great!

> Reynold's Magic Number basically shows the ratio between inertial
> forces and viscous forces in a fluid.
>
> Think of it as (how fast it's moving) / (how sticky it is).

OK, how sticky it is depends on the wing (airfoil and "smoothness" I
should think, and speed is speed. OK, got that.

> At low R, viscous forces predominate. (and generally laminar flow)
> At high R, is dominated by inertial forces. (resulting in higher sheer
> forces and turbulence)

This much I gathered from the Wiki page, but I still don't get the
point. Given the mission (design a plane with so much max gross, with
xxx HP engine power, and make it go as fast as you can) is it possible
to determine an optimal Reynolds number for the wing or for the damned
plane or its f....g mother in law?

Or why is this Reynolds factor important, and where does one apply it?

Thanks for bearing with me,
KA

[jan olieslagers[_2_]]

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...

[Jim Logajan]

jan olieslagers > wrote:
> Or why is this Reynolds factor important, and where does one apply it?

My rough understanding:

It is useful to aircraft designers who want to first build small models.
The flow over small models will generally remain laminar over a larger
portion of the small model than the full-sized aircraft. Reynolds number
can be used as an _estimate_ of the amount that turbulent flow contributes
to aspects like drag. (The equation, Re = V*L/nu, doesn't even include
surface roughness; hence the approximation aspect of its nature.)

One could use the Reynolds number equation to estimate when the flow goes
from laminar to turbulent. In fact since length L is in the equation, there
are an infinite number of Reynolds numbers for a body! For example, at the
leading edge of a wing, the length L starts at 0, so Re = 0. That indicats
laminar flow at that point (in theory!) Then Re gets larger as the flow
moves along the wing because the L in the equation gets larger. If one
could factor in wing surface roughness and how much the fluid is already
edging toward turbulence before it even reached the leading edge of the
wing, then one could presumably estimate the value of L when the flow
transitions to turbulent flow (or laminar separation from the surface) for
a given V.

So when you see some publication saying that Re is, for example, 1,000,000
for a wing of length L in a fluid moving at speed V, they mean that is the
value Re reaches at the trailing edge of the wing. Roughly halfway along
that wing Re would be about 500,000 for the same V.

I can't think of anyone other than aircraft designers and testers needing
an understanding of the number.

[Gerry van Dyk]

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...

[cavelamb[_2_]]

A different example...

A golf ball operates at a very low Re (small length and low speed),
so it's form drag would be very high - without the dimples.

The dimples trip the boundary layer forcing a premature turbulent transition
in the boundary layer. This turbulent layer provides a high energy boundary
layer that reduces form drag by reducing the boundary layer thickness.
The dimples, in effect, increase the Re.

Back to wings:

In choosing airfoils (and thus wing areas), Re is important because all airfoils
do not work equally well
at all Re.

Laminar airfoils generally don't work very well at exceptionally low Re.

The 5 digit NACA series, as an example, don't do well below Re of 2 million.

On that little low wing single seater I was sketching on a while back, the
wing tips are only 24" chord. At landing and take off speeds ( 40 mph?)
the tips aren't making very much lift!

That leads to higher take off speeds (or a larger wing?), lousy aileron response
at low speed, and makes it very easy to encounter a tip stall (roll on take off
any one?).

But the rest of the wing, due to longer chord looked to be fine.

So my "solution" was to change from a 23015 airfoil at the root to a
2412 (a 4 digit - or turbulent flow airfoil) at the tips.
(That would be a lofting nightmare - without CAD)

This airplane was also supposed to operate at higher than "usual" altitudes
(12,ooo+ feet for cruise) the same thing could have happened in cruise.

The thinner (lower density) air lowers Re to the point that we could possibly
encounter high speed tip stalls...


Now, does that mean that small chord wings can not operate at low speeds?
Of course not.
But we left wing size, efficiency and altitude out of the question.


Richard

[cavelamb[_2_]]

jan,

I had a nice graphic that explained the range of effects on Reynolds Number from
the "fall of dew" to Mach 3. Unfortunately I haven't found it - yet.

But getting away from airplane wings for a moment,

How fast does dew fall?
How big is a droplet of water in the morning dew?

At that scale, viscosity (stickiness of the air - and yes, air is "sticky")
effects are the major forces involved.

Accumulate more water into a raindrop.
Now there is enough mass for much higher velocities.
Now the inertial forces predominate.
The wake left behind is fairly clean, but noticeable.

Freeze a bunch of that into hail stones!
Lots of mass - much higher speeds.
Now we start seeing funny things in the shape of the wake - swirling
vortices.

http://en.wikipedia.org/wiki/Von_K%C3%A1rm%C3%A1n_vortex_street
Notice how the animation shows the wake whipping back and forth.


Get enough mass and size, and altitude to fall (ie much higher speeds),
and we see the air ahead of the object starting to pile up - can't get
out of the way fast enough - resulting in super sonic shock waves.

It's all the same air.

The differences are how long (chord length) and fast something is
going through it.


Or, as another pointed out, increase the density of the fluid.

[Oliver Arend]

Guys, please, if you really only do have a marginal understanding,
don't post or at least mark it as such. Explaining in entirety what
the Reynolds numbers means will fill one or two chapters in a good
aerodynamics textbook.

What Cavelamb wrote is good, what Jim wrote is, as he said, marginal
understanding, but not wrong, but what Gerry wrote is between
borderline and wrong.

I will try and summarize a bit (but after 7 years of university, it's
probably pretty theoretical):

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)
nu is the "cinematic viscosity" (dynamic viscosity divided by density,
which yields roughly 1.5 * 10^-5 m^2/s at ISA MSL conditions - no
fudge factors such as "9346" when using SI. The density effects Gerry
wrote about are 99% due to the impact of density on the dynamic
pressure, and not on Re - and the dynamic viscosity of air and water
are much different to begin with)

Physically speaking, it gives you the relation between viscous and
inertial forces in a fluid (as has been said). Which won't really tell
you anything in the beginning. Very roughly it means whether the
airflow will be laminar or turbulent (see last post by Cavelamb).

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.

On an airfoil, the characteristic length L is the chord. Wind tunnel
measurements on airfoils are done at certain Re. So if you know your
airflow speed and chord, you can get the right polar to figure out how
your airfoil will behave. Re mostly has an effect on the length of
laminar airflow (transition usually occurs at a certain Re, which is
then not calculated with L, but rather with x, meaning the length of
surface the air has travelled on the airfoil up to this point) and
hence drag, and since laminar and turbulent flow behave differently
with regard to flow separation (check for pictures on Google what this
means), it also has an effect on the maximum lift coefficient the
airfoil will achieve.
In general, higher Re lead to lower drag coefficients and higher max
lift coefficients, but a smaller "laminar bucket", which means the
range of lift coefficients the airfoil can operate in to achieve low
drag is smaller (especially interesting for sailplane and other low-
drag applications).

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

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.

Oliver

[Gerry van Dyk]

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

[Bob]

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?)

[cavelamb[_2_]]

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=rbMx2NMqyuI&feature=related



But kick the velocity up to flying speeds and see what actually happens...
http://www.youtube.com/watch?v=A4taHkU7lKQ&feature=related
http://www.youtube.com/watch?v=vQHXIHpvcvU&feature=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=duCHFaWm-ms&feature=related



Actual photographs of supersonic shock waves (an interesting film
about supersonic flight too)
http://www.youtube.com/watch?v=atItRcfFwgw&feature=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=6UlsArvbTeo&feature=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-xxCkebdZs&feature=related

[jan olieslagers[_2_]]

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...

[cavelamb[_2_]]

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.

[Bob Fry]

>>>>> "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

[Oliver Arend]

> 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

[Brian Whatcott]

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

[jan olieslagers[_2_]]

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!

[Morgans[_7_]]

"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

[jan olieslagers[_2_]]

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

[Anders]

First, I'd like to say that I am no professional aerodynamicist, and the following text may contain errors. I'd be happy if someone points them out!

About Reynolds numbers:

Let's say you're a pioneer of flight, and you want to build an airplane that can fly. How big a wing do you need? To answer this, you could simply build a wing, put it in your pioneer-wind tunnel, and measure the forces on it at the speed you think your airplane will be able to attain with the pioneer-engine you have available. Let's say the wing wasn't big enough. Now, what do you do? You have to build another one! And another one... etc, until you get it right. You don't wan't a too big wing (weight!), and not a too small (can't fly!).

Anyway, this quickly grows old. Someone said: Why can't we predict how much lift a given wing will have without building the darned thing every time?

So someone set out to build lots of different wings, and tried to see if there was some kind of pattern to the amount of lift different wings gave. Let's see, this one over here is 1m wide, and gives 1N at this speed, but that one over there gave 2N at the same speed. Hmm, why.. oh, it's twice as wide, but identical otherwise! Okay, lift seems to increase linearly with wingspan! And people built lots of wings, and thought hard, and came up with a formula:

Lift = chord * span * CL * Velocity * Velocity * airdensity / 2

Where:
chord = Width of wing, from leading edge to trailing edge
span = Length of wing, from tip to tip
velocity = The relative forward speed of the wing in relation to the air
air density = The weight of air, per volume (1.2kg / cubic meter)
CL = Lift Coefficient, (varies with angle of attack, and reynolds number!)

Anyway, this worked pretty well.

The was just one problem! When you build a 30 cm (1') wing, and it gives 10N of lifting force, you'd expect the same kind of wing, but 3m (10'), to give 100N (if it has the same chord), at the same speed! But, this did not occur! Rather, it turned out, that when you take a small wing, and make it many times bigger, or run it many times faster through the air, the above formula doesn't work anymore.

When does it work? Does speed have to be the same? Does chord have to be the same? No! It works when the Reynolds number is the same!

So, if you build a small wing, and run it in a wind tunnel with a different medium (perhaps water), and this makes the reynolds number the same as it will be when your real aircraft flies its real mission, you can expect the lift equation above to 'work'.

So, the Lift Equations allows the lifting force from a wing to be predicted (very handy when constructing aircraft). But the lift equation only works when supplied with a CL measured at the same Reynolds number as you'll be flying at! So you need the Reynolds number in order to be able to use the lift equation!

[Brian Whatcott]

Anders wrote:
> First, I'd like to say that I am no professional aerodynamicist, and the
> following text may contain errors. I'd be happy if someone points them
> out!
>
> About Reynolds numbers:
>
> Let's say you're a pioneer of flight, and you want to build an airplane
> that can fly. How big a wing do you need? To answer this, you could
> simply build a wing, put it in your pioneer-wind tunnel, and measure
> the forces on it at the speed you think your airplane will be able to
> attain with the pioneer-engine you have available. Let's say the wing
> wasn't big enough. Now, what do you do? You have to build another one!
> And another one... etc, until you get it right. You don't wan't a too
> big wing (weight!), and not a too small (can't fly!).
>
> Anyway, this quickly grows old. Someone said: Why can't we predict how
> much lift a given wing will have without building the darned thing
> every time?
>
> So someone set out to build lots of different wings, and tried to see
> if there was some kind of pattern to the amount of lift different wings
> gave. Let's see, this one over here is 1m wide, and gives 1N at this
> speed, but that one over there gave 2N at the same speed. Hmm, why..
> oh, it's twice as wide, but identical otherwise! Okay, lift seems to
> increase linearly with wingspan! And people built lots of wings, and
> thought hard, and came up with a formula:
>
> Lift = chord * span * CL * Velocity * Velocity * airdensity / 2
>
> Where:
> chord = Width of wing, from leading edge to trailing edge
> span = Length of wing, from tip to tip
> velocity = The relative forward speed of the wing in relation to the
> air
> air density = The weight of air, per volume (1.2kg / cubic meter)
> CL = Lift Coefficient, (varies with angle of attack, and reynolds
> number!)
>
> Anyway, this worked pretty well.
>
> The was just one problem! When you build a 30 cm (1') wing, and it
> gives 10N of lifting force, you'd expect the same kind of wing, but 3m
> (10'), to give 100N (if it has the same chord), at the same speed! But,
> this did not occur! Rather, it turned out, that when you take a small
> wing, and make it many times bigger, or run it many times faster
> through the air, the above formula doesn't work anymore.
>
> When does it work? Does speed have to be the same? Does chord have to
> be the same? No! It works when the Reynolds number is the same!
>
> So, if you build a small wing, and run it in a wind tunnel with a
> different medium (perhaps water), and this makes the reynolds number
> the same as it will be when your real aircraft flies its real mission,
> you can expect the lift equation above to 'work'.
>
> So, the Lift Equations allows the lifting force from a wing to be
> predicted (very handy when constructing aircraft). But the lift
> equation only works when supplied with a CL measured at the same
> Reynolds number as you'll be flying at! So you need the Reynolds number
> in order to be able to use the lift equation!
>
>
>
>
Building on your response, I should mention that lift coefficients are
sometimes based on cross section area perpendicular to the wind, rather
than on chord times span. The former gives bigger numbers than the
latter method. So a flat board perpendicular to the wind was sometimes
given as Cl = 1 to 1.2, whereas a flat board at zero incidence might be
given as span times chord and make the contrast between the angle of
attack of a flat board for AoA = 0 versus AoA = 90 look even look bigger
than it already is.

Brian W

[Stealth Pilot[_2_]]

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

[Morgans[_2_]]

"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

[Stealth Pilot[_2_]]

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.