Thrusting or Sucking (where's Howard Stern when we need him.)

[Ken Kochanski (KK)]

OK, what is the better explanation to give fledgling students. Should
you say the wing deflects/pushes/thrusts the air down to hold the
aircraft up ... or should you say the wing/airflow creates a low
pressure area that sucks the wing/aircraft upwards.

Like many people, Bernoulli was the initial and only explanation I was
aware of ... but I now think it is easier and more accurate to explain
that a wing/airfolil pushes the air downward. Yes, you do have
pressure differences, but that is just an artifact of the process.

A Bernoulli based explanation seems to create some inconsistencies.
For example, boat and airplane propellers are basically identical
devices and differ in engineering specifics primarily because they
operate in different fluid mediums. Both employ "fluidfoils" ... and
both produce "thrust" ... but wouldn't a Bernoulli view argue that the
airplane prop is actually sucking ... and how would you use Bernoulli
to explain the thrust produced by the boat prop.

What about a Bernoulli view of SR-71 flight ops as it climbs from
sea-level to 85,000 feet.

thanks,

[J. N.]

You`re pushing air, water or whatever downwards. The result is a
pressure and shear distribution on a plane which creates both lift and
drag.
In reality you can only apply bernoulli on a slow (<200 kts), airfoil
shaped device because of compressibility and possible detatchment of
the boundary layer. When you start adding slats, fowler flaps or start
flying fast or very slowly (model airplane`s) Bernoulli is far beside
the thruth.

A propellor or rotor is simply a rotating wing, and works exactly the
same. It both sucks and pushes the air afterwards.

The Blackbird would probally not fly at all if you use Bernoulli.

That`s it...

Jarno Nieuwenhuize,
The Netherlands.

[Marty]

Stick the cambered side of a standard spoon under a running tap! Ease
the cambered side of the spoon gradually into the running water until
the flow ‘sucks’ the spoon into the flow. The faster the flow of water
from the tap, the more "lift"… and the slower the flow the less "lift".
You can demo the stall by reducing the flow of water until the spoon
fails to sustain flight!!

If you want to look like more of an idiot, blow on the back of the
spoon whilst it's in the flow of water. It effectively demonstrates a
few principles of fluid dynamics… even if it isn't as scientifically
correct as it should be. You students won’t forget the demo.

> OK, what is the better explanation to give fledgling students.

What's wrong with explaining both? There’s a lot of aerodynamics that
requires a fairly comprehensive knowledge of both principles?

> Yes, you do have pressure differences, but that is just an artifact of
> the process.

I'm not sure if I understand that sentence. Explaining 'deflection of
air' alone without reference to pressure differential essentially means
that the many reasons for a cambered wing are essentially ignored.
Explain reasons why some wings are heavily cambered and why some are
not and it qualifies your explanation.

> What about a Bernoulli view of SR-71 flight ops as it climbs from
> sea-level to 85,000 feet.

My point exactly. What about a SFTOL aircraft as an example of the
opposite end of the performance spectrum? Compare wings (L/D, span,
chord, etc) and compare power/thrust/weight ratios. An example of an
all-in-one wing might be an airliner with slats, slots, spoilers, and
flaps etc that effectively create a new wing for a different purpose in
different phases of flight.


--
Marty
Posted at www.flight.org

[Nyal Williams]

Newton explains it better than Bernoulli. The FAA
written for PSEL no longer requires the Bernoulli answer
about lift.

Go to Amazon.com or Bookfinder.com and look for a book
by Gale Craig, titled, 'Stop Abusing Bernoulli! - How
Airplanes Really Fly '. (Search under Craig, Gale.)
Gale is a physicist who enquired into this subject.
Used copies are listed there, but the book is self-published
and you can order fresh copies from him. He lives
in Anderson, Indiana. Anyone who is interested in
that address should write me and I will get it for
you.

At 17:00 09 January 2006, Marty wrote:
>
>Stick the cambered side of a standard spoon under a
>running tap! Ease
>the cambered side of the spoon gradually into the running
>water until
>the flow �sucks� the spoon into the flow. The faster
>the flow of water
>from the tap, the more 'lift'� and the slower the
>flow the less 'lift'.
>You can demo the stall by reducing the flow of water
>until the spoon
>fails to sustain flight!!
>
>If you want to look like more of an idiot, blow on
>the back of the
>spoon whilst it's in the flow of water. It effectively
>demonstrates a
>few principles of fluid dynamics� even if it isn't
>as scientifically
>correct as it should be. You students won�t forget
>the demo.
>
>> OK, what is the better explanation to give fledgling
>>students.
>
>What's wrong with explaining both? There�s a lot
>of aerodynamics that
>requires a fairly comprehensive knowledge of both principles?
>
>
>> Yes, you do have pressure differences, but that is
>>just an artifact of
>> the process.
>
>I'm not sure if I understand that sentence. Explaining
>'deflection of
>air' alone without reference to pressure differential
>essentially means
>that the many reasons for a cambered wing are essentially
>ignored.
>Explain reasons why some wings are heavily cambered
>and why some are
>not and it qualifies your explanation.
>
>> What about a Bernoulli view of SR-71 flight ops as
>>it climbs from
>> sea-level to 85,000 feet.
>
>My point exactly. What about a SFTOL aircraft as an
>example of the
>opposite end of the performance spectrum? Compare wings
>(L/D, span,
>chord, etc) and compare power/thrust/weight ratios.
>An example of an
>all-in-one wing might be an airliner with slats, slots,
>spoilers, and
>flaps etc that effectively create a new wing for a
>different purpose in
>different phases of flight.
>
>
>--
>Marty
>Posted at www.flight.org
>
>

[Don Johnstone]

I think you can also obtain books that prove the earth
is flat and the Holocaust never happened. Oh and I
forgot there are several that prove global warming
:-)


At 18:24 09 January 2006, Nyal Williams wrote:
>Newton explains it better than Bernoulli. The FAA
>written for PSEL no longer requires the Bernoulli answer
>about lift.
>
>Go to Amazon.com or Bookfinder.com and look for a book
>by Gale Craig, titled, 'Stop Abusing Bernoulli! - How
>Airplanes Really Fly '. (Search under Craig, Gale.)
> Gale is a physicist who enquired into this subject.
> Used copies are listed there, but the book is self-published
>and you can order fresh copies from him. He lives
>in Anderson, Indiana. Anyone who is interested in
>that address should write me and I will get it for
>you.
>
>At 17:00 09 January 2006, Marty wrote:
>>
>>Stick the cambered side of a standard spoon under a
>>running tap! Ease
>>the cambered side of the spoon gradually into the running
>>water until
>>the flow �sucks� the spoon into the flow. The faster
>>the flow of water
>>from the tap, the more 'lift'� and the slower the
>>flow the less 'lift'.
>>You can demo the stall by reducing the flow of water
>>until the spoon
>>fails to sustain flight!!
>>
>>If you want to look like more of an idiot, blow on
>>the back of the
>>spoon whilst it's in the flow of water. It effectively
>>demonstrates a
>>few principles of fluid dynamics� even if it isn't
>>as scientifically
>>correct as it should be. You students won�t forget
>>the demo.
>>
>>> OK, what is the better explanation to give fledgling
>>>students.
>>
>>What's wrong with explaining both? There�s a lot
>>of aerodynamics that
>>requires a fairly comprehensive knowledge of both principles?
>>
>>
>>> Yes, you do have pressure differences, but that is
>>>just an artifact of
>>> the process.
>>
>>I'm not sure if I understand that sentence. Explaining
>>'deflection of
>>air' alone without reference to pressure differential
>>essentially means
>>that the many reasons for a cambered wing are essentially
>>ignored.
>>Explain reasons why some wings are heavily cambered
>>and why some are
>>not and it qualifies your explanation.
>>
>>> What about a Bernoulli view of SR-71 flight ops as
>>>it climbs from
>>> sea-level to 85,000 feet.
>>
>>My point exactly. What about a SFTOL aircraft as an
>>example of the
>>opposite end of the performance spectrum? Compare wings
>>(L/D, span,
>>chord, etc) and compare power/thrust/weight ratios.
>>An example of an
>>all-in-one wing might be an airliner with slats, slots,
>>spoilers, and
>>flaps etc that effectively create a new wing for a
>>different purpose in
>>different phases of flight.
>>
>>
>>--
>>Marty
>>Posted at www.flight.org
>>
>>
>
>
>
>

[Bruce]

Here is a stab at a simple explanation -

The crux of the matter is that aircraft fly by the transfer of energy. Fluid
dynamics and physics provide us with a variety of tools to model how this happens.

A famous quote from IT applies - "All models are false, some models are useful"

Depending on the situation one or another model may be more useful and/or
accurate, but they remain models. We are generally concerned with low Reynolds
numbers and laminar flow over low lift+low drag wings. In this regime the
contribution of upper surface lift is apparently no less than 1/3 of the total,
and at low angle of attack it may be greater than 50%.

The net result of Bernoulli, Prantl et al is that a moving wing transmits
kinetic energy to the air. As long as the air flows around the wing in a
relatively orderly fashion, the reaction is lift (varying amounts of suck and
blow), if the flow becomes chaotic (stalled) we get primarily drag.(Very simple.)

Conversely - calculating how much will be created, and optimising efficiency
over a range of performance requires serious computational power and a head for
calculus. (very complex)

Nyal Williams wrote:
> Newton explains it better than Bernoulli. The FAA
> written for PSEL no longer requires the Bernoulli answer
> about lift.
>
> Go to Amazon.com or Bookfinder.com and look for a book
> by Gale Craig, titled, 'Stop Abusing Bernoulli! - How
> Airplanes Really Fly '. (Search under Craig, Gale.)
> Gale is a physicist who enquired into this subject.
> Used copies are listed there, but the book is self-published
> and you can order fresh copies from him. He lives
> in Anderson, Indiana. Anyone who is interested in
> that address should write me and I will get it for
> you.
>
> At 17:00 09 January 2006, Marty wrote:
>
>>Stick the cambered side of a standard spoon under a
>>running tap! Ease
>>the cambered side of the spoon gradually into the running
<SNIP>


--
Bruce Greeff
Std Cirrus #57
I'm no-T at the address above.

[Shawn]

Don Johnstone wrote:
> I think you can also obtain books that prove the earth
> is flat and the Holocaust never happened. Oh and I
> forgot there are several that prove global warming
> :-)

"Climate Change" C'mon don, get with the times. "Global Warming" is so
1997.
;-)
BTW, was the Earth flat before Magellan sailed around it?

Shawn

[Alan Baker]

In article >,
Bruce > wrote:

> Here is a stab at a simple explanation -
>
> The crux of the matter is that aircraft fly by the transfer of energy. Fluid

Point of order: transfer of *momentum*. In order to fly, a
heavier-than-air aircraft must apply a certain force to the air,
transferring momentum to it. The amount of momentum transferred is
constant across the aircraft's performance envelope, whereas the energy
transfer varies.

> dynamics and physics provide us with a variety of tools to model how this
> happens.
>
> A famous quote from IT applies - "All models are false, some models are
> useful"
>
> Depending on the situation one or another model may be more useful and/or
> accurate, but they remain models. We are generally concerned with low
> Reynolds
> numbers and laminar flow over low lift+low drag wings. In this regime the
> contribution of upper surface lift is apparently no less than 1/3 of the
> total,
> and at low angle of attack it may be greater than 50%.
>
> The net result of Bernoulli, Prantl et al is that a moving wing transmits
> kinetic energy to the air. As long as the air flows around the wing in a
> relatively orderly fashion, the reaction is lift (varying amounts of suck and
> blow), if the flow becomes chaotic (stalled) we get primarily drag.(Very
> simple.)
>
> Conversely - calculating how much will be created, and optimising efficiency
> over a range of performance requires serious computational power and a head
> for
> calculus. (very complex)
>
> Nyal Williams wrote:
> > Newton explains it better than Bernoulli. The FAA
> > written for PSEL no longer requires the Bernoulli answer
> > about lift.
> >
> > Go to Amazon.com or Bookfinder.com and look for a book
> > by Gale Craig, titled, 'Stop Abusing Bernoulli! - How
> > Airplanes Really Fly '. (Search under Craig, Gale.)
> > Gale is a physicist who enquired into this subject.
> > Used copies are listed there, but the book is self-published
> > and you can order fresh copies from him. He lives
> > in Anderson, Indiana. Anyone who is interested in
> > that address should write me and I will get it for
> > you.
> >
> > At 17:00 09 January 2006, Marty wrote:
> >
> >>Stick the cambered side of a standard spoon under a
> >>running tap! Ease
> >>the cambered side of the spoon gradually into the running
> <SNIP>

--
Alan Baker
Vancouver, British Columbia
"If you raise the ceiling 4 feet, move the fireplace from that wall
to that wall, you'll still only get the full stereophonic effect
if you sit in the bottom of that cupboard."

[Tim Hanke]

Ken,
Howard Stern is on Sirius Satellite radio... Channel 100... first day
today! Channel 101 replay for West Coasters ..... LOL

[Eric Greenwell]

Ken Kochanski (KK) wrote:

> OK, what is the better explanation to give fledgling students. Should
> you say the wing deflects/pushes/thrusts the air down to hold the
> aircraft up ... or should you say the wing/airflow creates a low
> pressure area that sucks the wing/aircraft upwards.

I suggest you tell them to hold their hand out a car window like a wing,
and experiment with the angle of attack. Anything more complicated than
that isn't going to help them fly a glider better. You don't have to
understand the physics to fly well, as ras demonstrates repeatedly, and
I don't see how explaining it with Bernoulli's theorem, or f=ma, or
pressure differentials is an aid to flying. It's hard enough to get
across the idea of angle of attack for stalling, much less Bernoulli or
Newton.

> Like many people, Bernoulli was the initial and only explanation I was
> aware of ... but I now think it is easier and more accurate to explain
> that a wing/airfolil pushes the air downward. Yes, you do have
> pressure differences, but that is just an artifact of the process.

And this illustrates part of the problem. Ken, whom I believe to be a
good instructor, wants to explain it to the student, but he doesn't
understand it either (I'm not suggesting I do, either). Nonetheless, his
students can fly well, because you have to know what to do at the right
time, and (fortunately) you don't have to figure it out from an
explanation of the physics involved.

Pragmatically, telling the student whatever explanation makes them happy
is probably good enough, but maybe referring the really interested to
good book like "Fundamentals of Sailplane Design" would be a good idea.

--
Change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

[Steve Hill]

Eric Wrote
"You don't have to understand the physics to fly well, as ras demonstrates
repeatedly"

In terms of agreeing with Eric's two cents worth on the phsyics and
aerodynamics of it all...I tend to agree...I still find myself clueless as
to how the toilet actually flushes...and yet it's use, I accomplish rather
handily.

Please...I beg....don't explain the whole physics issues...I'll sleep
soundly, knowing that...
" It just swirls around...and then disappears down that hole there..."




Happily clueless.


Steve.

[Capt. Geoffrey Thorpe]

Spell checking is left as an excercise for the reader.
"Ken Kochanski (KK)" > wrote in message
oups.com...
>
> OK, what is the better explanation to give fledgling students. Should
> you say the wing deflects/pushes/thrusts the air down to hold the
> aircraft up ... or should you say the wing/airflow creates a low
> pressure area that sucks the wing/aircraft upwards.
>
> Like many people, Bernoulli was the initial and only explanation I was
> aware of ... but I now think it is easier and more accurate to explain
> that a wing/airfolil pushes the air downward. Yes, you do have
> pressure differences, but that is just an artifact of the process.
>
> A Bernoulli based explanation seems to create some inconsistencies.


Berhoulli's equation (pressure + (1/2 *density* velocity squared)
+(densty*g*elevation) = constant) is simply a special case of Newtons law
applied to ideal gasses under specific circunstances.

So, no inconsistancies, no problems.

Air is deflected. Delta pressures are created.


--
Geoff
the sea hawk at wow way d0t com
remove spaces and make the obvious substitutions to reply by mail

[1MoClimb]

Bernoulli is not explaining 70% of the lift an ordinary airfoil
generates. Bernoulli completely fails when using a flat plate which
creates plenty of lift at normal angles of attack. Newton, bah humbug!
The key to understanding lift is circulation, nothing else gives
compliance with wind tunnel and test data.
Problem is that it takes about 20 minutes to read through the whole
story and many people give up before they understand. Here is by far
the best explanation I have found:
http://www.av8n.com/how/htm/airfoils.html
This guy put an entire book about flight training on the web, God bless
him!
He disposes with many myths that simple minded and uneducated wannabe
physicists promote, you gotta love it!
Ken, I know you as a reasonably smart guy, you're gonna figure it out!

Herb, J7

Capt. Geoffrey Thorpe wrote:
> Spell checking is left as an excercise for the reader.
> "Ken Kochanski (KK)" > wrote in message
> oups.com...
> >
> > OK, what is the better explanation to give fledgling students. Should
> > you say the wing deflects/pushes/thrusts the air down to hold the
> > aircraft up ... or should you say the wing/airflow creates a low
> > pressure area that sucks the wing/aircraft upwards.
> >
> > Like many people, Bernoulli was the initial and only explanation I was
> > aware of ... but I now think it is easier and more accurate to explain
> > that a wing/airfolil pushes the air downward. Yes, you do have
> > pressure differences, but that is just an artifact of the process.
> >
> > A Bernoulli based explanation seems to create some inconsistencies.
>
>
> Berhoulli's equation (pressure + (1/2 *density* velocity squared)
> +(densty*g*elevation) = constant) is simply a special case of Newtons law
> applied to ideal gasses under specific circunstances.
>
> So, no inconsistancies, no problems.
>
> Air is deflected. Delta pressures are created.
>
>
> --
> Geoff
> the sea hawk at wow way d0t com
> remove spaces and make the obvious substitutions to reply by mail

[Bob C]

Nasa has some great tutorials on aerodynamics, rockets,
etc. Check this page, then hit the 'next' arrow at
the bottom. Also, bounce around the text links. Good
stuff. Their foilsim site is fun, too.

http://www.grc.nasa.gov/WWW/K-12/airplane/wrong1.html



At 23:30 09 January 2006, 1moclimb wrote:
>Bernoulli is not explaining 70% of the lift an ordinary
>airfoil
>generates. Bernoulli completely fails when using a
>flat plate which
>creates plenty of lift at normal angles of attack.
> Newton, bah humbug!
>The key to understanding lift is circulation, nothing
>else gives
>compliance with wind tunnel and test data.
>Problem is that it takes about 20 minutes to read through
>the whole
>story and many people give up before they understand.
> Here is by far
>the best explanation I have found:
>http://www.av8n.com/how/htm/airfoils.html
>This guy put an entire book about flight training on
>the web, God bless
>him!
>He disposes with many myths that simple minded and
>uneducated wannabe
>physicists promote, you gotta love it!
>Ken, I know you as a reasonably smart guy, you're gonna
>figure it out!
>
>Herb, J7
>
>Capt. Geoffrey Thorpe wrote:
>> Spell checking is left as an excercise for the reader.
>> 'Ken Kochanski (KK)' wrote in message
>> oups.com...
>> >
>> > OK, what is the better explanation to give fledgling
>>>students. Should
>> > you say the wing deflects/pushes/thrusts the air
>>>down to hold the
>> > aircraft up ... or should you say the wing/airflow
>>>creates a low
>> > pressure area that sucks the wing/aircraft upwards.
>> >
>> > Like many people, Bernoulli was the initial and only
>>>explanation I was
>> > aware of ... but I now think it is easier and more
>>>accurate to explain
>> > that a wing/airfolil pushes the air downward. Yes,
>>>you do have
>> > pressure differences, but that is just an artifact
>>>of the process.
>> >
>> > A Bernoulli based explanation seems to create some
>>>inconsistencies.
>>
>>
>> Berhoulli's equation (pressure + (1/2 *density* velocity
>>squared)
>> +(densty*g*elevation) = constant) is simply a special
>>case of Newtons law
>> applied to ideal gasses under specific circunstances.
>>
>> So, no inconsistancies, no problems.
>>
>> Air is deflected. Delta pressures are created.
>>
>>
>> --
>> Geoff
>> the sea hawk at wow way d0t com
>> remove spaces and make the obvious substitutions to
>>reply by mail
>
>

[Bruce]

T o d d P a t t i s t wrote:
> Bruce > wrote:
>
>
>>In this regime the
>>contribution of upper surface lift is apparently no less than 1/3 of the total,
>>and at low angle of attack it may be greater than 50%.
>
>
> I'm interested in where you got these figures. The numbers
> I've seen put the contribution from the upper surface at
> normally somewhat greater than 50%, although these were
> numbers for general aviation, not laminar flow sailplane
> airfoils/wings.
>
> My personal opinion on the original question is that the
> Bernoulli and Newtonian explanations are complementary.
> They both explain 100% of the lift, but some people
> understand one better than the other. I'm also of the
> opinion that the Newtonian "air gets pushed down"
> explanation is easier to misunderstand and/or misapply to
> aviation than the Bernoulli form, so I prefer starting with
> Bernoulli.
>
> It's hard to explain why the contour of the upper surface is
> so critical to flight if you focus on Newtonian "pushing air
> down," which is usually understood as implicating only the
> lower surface of the wing.
>
OK - I consider myself chastised on the momentum thing. I know it is transfer of
momentum, but then you have to explain momentum. "Energy" is easier for the
luddites.

The range of coeficient of lift contribution came from my (mis-)understanding of
Dr Fred Thomas's "Fundamentals of Sailplane Design" + John D Anderson's
"Fundamentals of Aerodynamics" Both of which get past my mathematical fundament
quite easily...

--
Bruce Greeff
Std Cirrus #57
I'm no-T at the address above.

[Ken Kochanski (KK)]

Thanks to all for the explanations and links. I now (for the first
time) understand the conservation of energy and momentum components
involved in lift generation. Not bad for a Psych major ... :-)

KK

[toad]

What ??? Bernoulii works great for flat plate airfoils ! Or are you
confusing the real Bernoulli with the bogus equal transit time BS that
some "dumb the truth down for the stupid pilots" wiseass created ?

The site has a good and complete explanation.

Todd Smith, 3S

[Alexander Georgas]

Ok, here is another way this can be explained (if just
a bit inaccurate):

The critical point to consider is angle of attack.
You just need to think of the airflow meeting the wing
at a specific angle. If you now imagine the airflow
separating to move above and below the wing section
as two particles, you have the following explanation:

-The particles are going to separate at the leading
edge and meet up together approximatelly at the wing's
trailing edge.

-Because of the angle of attack (and of the wing's
thinkness), the air particle that will travel above
the wing section will have a greater discance to cover,
so it will have to run faster if it is to meet up with
the other particle which is traveling below the wing
section.

-Particles traveling at greater speeds (compared to
particles in neighboring areas) create areas of low
pressure -- just think: because the particles are running
faster over the wing compared to under the wing, there
are fewer of them in a specific area. Alternativelly,
you can take Bernoulli's word on this one.

-Now let's examine the situation from the perspective
of the wing. There is a low pressure above and a high
pressure bellow. The resulting force is thus upwards
(in relation to the angle of attack).

The only presumption which remains to be explained
is why the two particles have to meet at the back of
the wing section. The easiest way to gloss over this
is to consider that if this is not approximatelly the
case, an imbalance will be created whereby there will
be more air crossing below the wing section and a low
pressure area buildup at the top back end of the wing,
created by the lack of air (teh air has just not had
the time to reach there). This would probably invite
air from below the wing to bleed back up towards the
back top. In this case, the airflow above the wing
would separate from the wing section before the trailing
edge (does this sound a bit like a stalled wing?)

I don't know how much science there is behind this
explanation. I just find it a bit easier to swallow
than the Newtonian explanation. In my view the action-reaction
way of putting it (air is deflected downwards) is true,
but unable to shed any light on the mechanism. It is
just an energy checks-and-balances way of explaining
lift, which we already know occurs.

Alexander Georgas

[Michel Talon]

Alexander Georgas > wrote:
> The only presumption which remains to be explained
> is why the two particles have to meet at the back of
> the wing section.

The only problem is that this is blatantly false as amply demonstrated
by all the papers that have been quoted in this thread. In particular
you can see images clearly showing that particles don't meet at the trailing
edge by far. Moreover this is necessary so that circulation is created and
there is a theorem in fluid mechanics relating lift and circulation.
Other figures show that the distribution of speed around the wing may be as
unintuitive as possible. So that, even if Bernouilli theorem is perfectly
true, you can get zilch intuition using it to understand lift.

>
> Alexander Georgas
>
>

--

Michel TALON

[Alan Baker]

In article >,
Alexander Georgas > wrote:

> Ok, here is another way this can be explained (if just
> a bit inaccurate):
>
> The critical point to consider is angle of attack.
> You just need to think of the airflow meeting the wing
> at a specific angle. If you now imagine the airflow
> separating to move above and below the wing section
> as two particles, you have the following explanation:
>
> -The particles are going to separate at the leading
> edge and meet up together approximatelly at the wing's
> trailing edge.
>
> -Because of the angle of attack (and of the wing's
> thinkness), the air particle that will travel above
> the wing section will have a greater discance to cover,
> so it will have to run faster if it is to meet up with
> the other particle which is traveling below the wing
> section.
>
> -Particles traveling at greater speeds (compared to
> particles in neighboring areas) create areas of low
> pressure -- just think: because the particles are running
> faster over the wing compared to under the wing, there
> are fewer of them in a specific area. Alternativelly,
> you can take Bernoulli's word on this one.

These first two points are precisely the ones that have been debunked
already...

>
> -Now let's examine the situation from the perspective
> of the wing. There is a low pressure above and a high
> pressure bellow. The resulting force is thus upwards
> (in relation to the angle of attack).
>
> The only presumption which remains to be explained
> is why the two particles have to meet at the back of
> the wing section. The easiest way to gloss over this
> is to consider that if this is not approximatelly the
> case, an imbalance will be created whereby there will
> be more air crossing below the wing section and a low
> pressure area buildup at the top back end of the wing,
> created by the lack of air (teh air has just not had
> the time to reach there). This would probably invite
> air from below the wing to bleed back up towards the
> back top. In this case, the airflow above the wing
> would separate from the wing section before the trailing
> edge (does this sound a bit like a stalled wing?)

They don't *have* to meet up, and it real life, they don't *actually*
meet up.

>
> I don't know how much science there is behind this
> explanation. I just find it a bit easier to swallow
> than the Newtonian explanation. In my view the action-reaction
> way of putting it (air is deflected downwards) is true,
> but unable to shed any light on the mechanism. It is
> just an energy checks-and-balances way of explaining
> lift, which we already know occurs.
>
> Alexander Georgas

--
Alan Baker
Vancouver, British Columbia
"If you raise the ceiling 4 feet, move the fireplace from that wall
to that wall, you'll still only get the full stereophonic effect
if you sit in the bottom of that cupboard."

[Maule Driver]

Since the FAA still tests on Bernoulli, you have to know that. I too am
much more comfortable with the Newtonian approach. As others have
pointed out in extreme detail, both are imperfect but they can both
provide useful understandings and numbers.

Such a well behaved community.... you know an item like this should
never be allowed to stand unabused. :-)

Ken Kochanski (KK) wrote:
> OK, what is the better explanation to give fledgling students. Should
> you say the wing deflects/pushes/thrusts the air down to hold the
> aircraft up ... or should you say the wing/airflow creates a low
> pressure area that sucks the wing/aircraft upwards.
>
> Like many people, Bernoulli was the initial and only explanation I was
> aware of ... but I now think it is easier and more accurate to explain
> that a wing/airfolil pushes the air downward. Yes, you do have
> pressure differences, but that is just an artifact of the process.
>
> A Bernoulli based explanation seems to create some inconsistencies.
> For example, boat and airplane propellers are basically identical
> devices and differ in engineering specifics primarily because they
> operate in different fluid mediums. Both employ "fluidfoils" ... and
> both produce "thrust" ... but wouldn't a Bernoulli view argue that the
> airplane prop is actually sucking ... and how would you use Bernoulli
> to explain the thrust produced by the boat prop.
>
> What about a Bernoulli view of SR-71 flight ops as it climbs from
> sea-level to 85,000 feet.
>
> thanks,
>

[Capt. Geoffrey Thorpe]

"toad" > wrote in message
oups.com...
> What ??? Bernoulii works great for flat plate airfoils ! Or are you
> confusing the real Bernoulli with the bogus equal transit time BS that
> some "dumb the truth down for the stupid pilots" wiseass created ?

Does anyone know who the aforementioned wiseass was? Or when this myth was
invented?

--
Geoff
the sea hawk at wow way d0t com
remove spaces and make the obvious substitutions to reply by mail
Spell checking is left as an excercise for the reader.

[Ian Johnston]

On Mon, 9 Jan 2006 15:52:44 UTC, "Ken Kochanski (KK)"
> wrote:

: OK, what is the better explanation to give fledgling students. Should
: you say the wing deflects/pushes/thrusts the air down to hold the
: aircraft up ... or should you say the wing/airflow creates a low
: pressure area that sucks the wing/aircraft upwards.

It does both, obviously. Equal and opposite reactions and all that.
Plane gets pushed up, air gets pushed down.

: Like many people, Bernoulli was the initial and only explanation I was
: aware of ... but I now think it is easier and more accurate to explain
: that a wing/airfolil pushes the air downward. Yes, you do have
: pressure differences, but that is just an artifact of the process.
:
: A Bernoulli based explanation seems to create some inconsistencies.

If you think there is a "Bernouilli based explanation" you have been
misinformed.

Ian

[Ian Johnston]

On Mon, 9 Jan 2006 19:05:38 UTC, Don Johnstone
> wrote:

: I think you can also obtain books that prove the earth
: is flat and the Holocaust never happened. Oh and I
: forgot there are several that prove global warming
: :-)

Indeed. Any book which explains why Bernouilli doesn;t work was
written by someone who doesn;t understand what Bernouilli's theorem
say in the first place, or when and how it is applied.

Ian


--

[Ian Johnston]

On Mon, 9 Jan 2006 16:49:18 UTC, Marty
> wrote:

:
: Stick the cambered side of a standard spoon under a running tap! Ease
: the cambered side of the spoon gradually into the running water until
: the flow ‘sucks’ the spoon into the flow.

Bad example. That's (almost) nothing like a wing - the force is a
result of the momentum transfer of the water as it follows the curve
of the spoon, thanks to the Coanda effect.

Ian

--

[Ian Johnston]

On Mon, 9 Jan 2006 23:27:10 UTC, "1MoClimb" >
wrote:

: Bernoulli is not explaining 70% of the lift an ordinary airfoil
: generates. Bernoulli completely fails when using a flat plate which
: creates plenty of lift at normal angles of attack.

Rubbish. Bernouilli explains it perfectly well, as long as you have
the right velocity distribution. What Bernouilli's theore, does /not/
do is predict velocity distributions, and certainly not thos ebased on
fatupus ideas like "it's the different lengths of the top and bottom
surfaces that matter."

As you go on to say, rightly, it;s circulation that matters, and one
you know how big the bound vortex is a simply momentum equation on a
cylindrical control volume around the vortex/wing produces the
Kutta-Joukowski Theorem: lift = free stream velocity x circulation x
density. But do do that, no need the pressure distribution on the
control surface. Guess how you do that?

Yup. Bernouilli.

Ian

--

[Ian Johnston]

On Tue, 10 Jan 2006 14:55:28 UTC, Alexander Georgas
> wrote:

: -The particles are going to separate at the leading
: edge and meet up together approximatelly at the wing's
: trailing edge.

Utter rubbish. Why the hell should they?

Ian


--

[Ian Johnston]

On Tue, 10 Jan 2006 14:20:38 UTC, T o d d P a t t i s t
> wrote:

: Circulation theory is 100% consistent with Bernoulli.

What the unititaited don't realise is that forces can be transmitted
by pressure or by momentum flux. At the surface of a wing, all the
force comes from pressure. A long way away from the wing, all the
force comes from momemntum transfer (there is effectively no pressure
difference). As you go from one to the other the balance changes. Most
people who "prove" theories of lift wrong simply forget to take
account of momentum flux.

Ian

--

[Alexander Georgas]

OK OK,

I said it was inaccurate to begin with!

Good to see this has stirred up some debate, thought.

While I agree this is more urban myth than fact, I
do feel that it helps more in visualization than statements
such as 'pressure builds below the wing' or 'it is
the Bernulli effect above the wing' (some even refer
to the Venturi effect).

I feel that the equal transit time proposition is a
good first step in helping people visualize what happens
when lift is generated, i.e. people start to think
about the flow fields over the entite wing section
in a more visual manner.

It is quite intuitive and easy to visualize. Once one
has actually understood this, he can now accept that
it is not true and move to the next level of understanding.

So if you drop the wing geometry idea (which I do not
aspise to begin with) and replace it with the concept
of angle of attack and also do not get too stuck with
the 'particles exit at exactly the same time' proposition,
you are starting to visualize things more properly.

Once you understand this, and the fact that the flow
over the wing is much faster than what the equal transit
time proposition must suggest and that the angle of
attack has created a low pressure area which has accelerated
the top flow field quite considerably, you can start
visualizing things like the vortices that are generated
once the two flows meet up behind the trailing edge
etc.

Now you are getting somewhere!

Otherwise, jumping from the 'there must be a Bernulli
effect somewhere' to the Euler equations is just too
much of a leap for the imagination (at least my imagination).

Now, concerning the Newtonian explanation, I consider
this too much of a cop out. Ok we all know it is correct,
and it is plain simple to understand. It just doesn't
explain anything about the mechanics of flight. It
is as saying (it is, in fact, saying) that the laws
of energy conservation will be upheld and that the
laws of thermodynamics will not be broken and that
the universe will keep on working the same way as it
was before. We sort knew this already. So where is
the understanding that comes from this explanation?

So I feel that while the equal transit time paradigm,
while false, is a good starting point for introducing
people to the aerodynamics of wing sections. They just
need to be told, once they understand the concept,
why it is false.

It is as useful as the Bohr model of the Atom: extremelly
inaccurate, but easy to visualize as a starting point
to understanding a very complex phenonenon. Would you
ever introduce particle physics to a student by writing
down the Schroedinger equation and solving a few Hamiltonians?

Alexander Georgas

At 16:24 10 January 2006, T O D D P A T T I S T wrote:
>Alexander Georgas
>wrote:
>
>>Ok, here is another way this can be explained (if just
>>a bit inaccurate):
>
>Sorry, but this is a lot inaccurate (a.k.a 'wrong')
>
>>The critical point to consider is angle of attack.
>>You just need to think of the airflow meeting the wing
>>at a specific angle. If you now imagine the airflow
>>separating to move above and below the wing section
>>as two particles, you have the following explanation:
>>
>>-The particles are going to separate at the leading
>>edge and meet up together approximatelly at the wing's
>>trailing edge.
>
>The particles do *not* meet up at the trailing edge.
> This
>is often referred to as the 'equal transit times'
>explanation of Bernoulli, which is thoroughly discredited.
>
>>-Because of the angle of attack (and of the wing's
>>thinkness), the air particle that will travel above
>>the wing section will have a greater discance to cover,
>>so it will have to run faster if it is to meet up with
>>the other particle which is traveling below the wing
>>section
>
>The particles do not have to meet up.
>
>>-Particles traveling at greater speeds (compared to
>>particles in neighboring areas) create areas of low
>>pressure -- just think: because the particles are running
>>faster over the wing compared to under the wing, there
>>are fewer of them in a specific area. Alternativelly,
>>you can take Bernoulli's word on this one.
>
>Bernoulli does not explain the flow pattern. Bernoulli
>explains the lift (pressure differentials given a specific
>flow pattern. You have to call in some other physics,
>namely that the air flows smoothly off the trailing
>edge, to
>establish the flow pattern. Once you have that, you
>apply
>Bernoulli.
>
>>-Now let's examine the situation from the perspective
>>of the wing. There is a low pressure above and a high
>>pressure bellow. The resulting force is thus upwards
>>(in relation to the angle of attack).
>>
>>The only presumption which remains to be explained
>>is why the two particles have to meet at the back of
>>the wing section. The easiest way to gloss over this
>
>Take a look at John Denker's start page:
>
>http://www.av8n.com/how/
>
>See the graphic and the bands of red, orange green
>blue?
>The blue band air over the top hits the trailing edge
>before
>the blue band air at the bottom, even though they started
>out together.
>
>>is to consider that if this is not approximatelly the
>>case, an imbalance will be created whereby there will
>>be more air crossing below the wing section and a low
>>pressure area buildup at the top back end of the wing,
>>created by the lack of air (teh air has just not had
>>the time to reach there). This would probably invite
>>air from below the wing to bleed back up towards the
>>back top.
>
>Guess what, that 'low pressure buildup' causes the
>air on
>top to accelerate, and it does tend to cause the high
>pressure to flow around to the top (resisted by air's
>finite
>viscosity and the sharpness of the trailing edge).
>
>>In this case, the airflow above the wing
>>would separate from the wing section before the trailing
>>edge (does this sound a bit like a stalled wing?)
>
>In a stalled wing, this does happen, but we're talking
>about
>a non-stalled wing and in that case the air flows faster
>over the top.
>
>>I don't know how much science there is behind this
>>explanation. I just find it a bit easier to swallow
>>than the Newtonian explanation. In my view the action-reaction
>>way of putting it (air is deflected downwards) is true,
>>but unable to shed any light on the mechanism.
>
>I agree that the Newtonian explanation is hard, but
>so is
>the Bernoulli. That's the nature of lift.
>
>I think of it like this: When the wing starts forward
>at a
>positive angle of attack, the space behind the wing's
>upper
>surface is being swept clear of air molecules - at
>least
>that's what the wing is trying to do. However, in
>a fluid
>(air), sweeping the molecules clear would produce a
>vacuum
>behind the wing, so the instant that the pressure begins
>to
>fall behind the wing, air from on top and ahead of
>the wing
>and air above the wing begins to rush towards this
>region.
>
>The air from ahead and on top of the wing that rushes
>back
>towards this low pressure region is being accelerated
>-
>exactly what the Bernoulli explanation needs for its
>lower
>pressure due to faster flow. The air above the wing
>moving
>down, coupled with higher pressure air below the wing
>(also
>moving down to escape that higher pressure) produces
>downwash - exactly as the Newtonian explanation needs.
>
>
>In reality, these are just two faces of the same coin.
>There's no need to use the false 'equal transit times'
>explanation of Bernoulli.
>
>>It is
>>just an energy checks-and-balances way of explaining
>>lift, which we already know occurs.
>>
>>Alexander Georgas
>>
>
>--
>T o d d P a t t i s t - 'WH' Ventus C
>(Remove DONTSPAMME from address to email reply.)
>

[Bert Willing]

Well, trying to explain something by starting with a statement which is
completely wrong is a very strange educational twist...
The Bohr model is not accurate enough to explain all details, but it is not
wrong.

Bert
ASW20 "TW"

"Alexander Georgas" > wrote in message
...
> OK OK,
>
> I said it was inaccurate to begin with!
>
> Good to see this has stirred up some debate, thought.
>
> While I agree this is more urban myth than fact, I
> do feel that it helps more in visualization than statements
> such as 'pressure builds below the wing' or 'it is
> the Bernulli effect above the wing' (some even refer
> to the Venturi effect).
>
> I feel that the equal transit time proposition is a
> good first step in helping people visualize what happens
> when lift is generated, i.e. people start to think
> about the flow fields over the entite wing section
> in a more visual manner.
>
> It is quite intuitive and easy to visualize. Once one
> has actually understood this, he can now accept that
> it is not true and move to the next level of understanding.
>
> So if you drop the wing geometry idea (which I do not
> aspise to begin with) and replace it with the concept
> of angle of attack and also do not get too stuck with
> the 'particles exit at exactly the same time' proposition,
> you are starting to visualize things more properly.
>
> Once you understand this, and the fact that the flow
> over the wing is much faster than what the equal transit
> time proposition must suggest and that the angle of
> attack has created a low pressure area which has accelerated
> the top flow field quite considerably, you can start
> visualizing things like the vortices that are generated
> once the two flows meet up behind the trailing edge
> etc.
>
> Now you are getting somewhere!
>
> Otherwise, jumping from the 'there must be a Bernulli
> effect somewhere' to the Euler equations is just too
> much of a leap for the imagination (at least my imagination).
>
> Now, concerning the Newtonian explanation, I consider
> this too much of a cop out. Ok we all know it is correct,
> and it is plain simple to understand. It just doesn't
> explain anything about the mechanics of flight. It
> is as saying (it is, in fact, saying) that the laws
> of energy conservation will be upheld and that the
> laws of thermodynamics will not be broken and that
> the universe will keep on working the same way as it
> was before. We sort knew this already. So where is
> the understanding that comes from this explanation?
>
> So I feel that while the equal transit time paradigm,
> while false, is a good starting point for introducing
> people to the aerodynamics of wing sections. They just
> need to be told, once they understand the concept,
> why it is false.
>
> It is as useful as the Bohr model of the Atom: extremelly
> inaccurate, but easy to visualize as a starting point
> to understanding a very complex phenonenon. Would you
> ever introduce particle physics to a student by writing
> down the Schroedinger equation and solving a few Hamiltonians?
>
> Alexander Georgas
>
> At 16:24 10 January 2006, T O D D P A T T I S T wrote:
>>Alexander Georgas
>>wrote:
>>
>>>Ok, here is another way this can be explained (if just
>>>a bit inaccurate):
>>
>>Sorry, but this is a lot inaccurate (a.k.a 'wrong')
>>
>>>The critical point to consider is angle of attack.
>>>You just need to think of the airflow meeting the wing
>>>at a specific angle. If you now imagine the airflow
>>>separating to move above and below the wing section
>>>as two particles, you have the following explanation:
>>>
>>>-The particles are going to separate at the leading
>>>edge and meet up together approximatelly at the wing's
>>>trailing edge.
>>
>>The particles do *not* meet up at the trailing edge.
>> This
>>is often referred to as the 'equal transit times'
>>explanation of Bernoulli, which is thoroughly discredited.
>>
>>>-Because of the angle of attack (and of the wing's
>>>thinkness), the air particle that will travel above
>>>the wing section will have a greater discance to cover,
>>>so it will have to run faster if it is to meet up with
>>>the other particle which is traveling below the wing
>>>section
>>
>>The particles do not have to meet up.
>>
>>>-Particles traveling at greater speeds (compared to
>>>particles in neighboring areas) create areas of low
>>>pressure -- just think: because the particles are running
>>>faster over the wing compared to under the wing, there
>>>are fewer of them in a specific area. Alternativelly,
>>>you can take Bernoulli's word on this one.
>>
>>Bernoulli does not explain the flow pattern. Bernoulli
>>explains the lift (pressure differentials given a specific
>>flow pattern. You have to call in some other physics,
>>namely that the air flows smoothly off the trailing
>>edge, to
>>establish the flow pattern. Once you have that, you
>>apply
>>Bernoulli.
>>
>>>-Now let's examine the situation from the perspective
>>>of the wing. There is a low pressure above and a high
>>>pressure bellow. The resulting force is thus upwards
>>>(in relation to the angle of attack).
>>>
>>>The only presumption which remains to be explained
>>>is why the two particles have to meet at the back of
>>>the wing section. The easiest way to gloss over this
>>
>>Take a look at John Denker's start page:
>>
>>http://www.av8n.com/how/
>>
>>See the graphic and the bands of red, orange green
>>blue?
>>The blue band air over the top hits the trailing edge
>>before
>>the blue band air at the bottom, even though they started
>>out together.
>>
>>>is to consider that if this is not approximatelly the
>>>case, an imbalance will be created whereby there will
>>>be more air crossing below the wing section and a low
>>>pressure area buildup at the top back end of the wing,
>>>created by the lack of air (teh air has just not had
>>>the time to reach there). This would probably invite
>>>air from below the wing to bleed back up towards the
>>>back top.
>>
>>Guess what, that 'low pressure buildup' causes the
>>air on
>>top to accelerate, and it does tend to cause the high
>>pressure to flow around to the top (resisted by air's
>>finite
>>viscosity and the sharpness of the trailing edge).
>>
>>>In this case, the airflow above the wing
>>>would separate from the wing section before the trailing
>>>edge (does this sound a bit like a stalled wing?)
>>
>>In a stalled wing, this does happen, but we're talking
>>about
>>a non-stalled wing and in that case the air flows faster
>>over the top.
>>
>>>I don't know how much science there is behind this
>>>explanation. I just find it a bit easier to swallow
>>>than the Newtonian explanation. In my view the action-reaction
>>>way of putting it (air is deflected downwards) is true,
>>>but unable to shed any light on the mechanism.
>>
>>I agree that the Newtonian explanation is hard, but
>>so is
>>the Bernoulli. That's the nature of lift.
>>
>>I think of it like this: When the wing starts forward
>>at a
>>positive angle of attack, the space behind the wing's
>>upper
>>surface is being swept clear of air molecules - at
>>least
>>that's what the wing is trying to do. However, in
>>a fluid
>>(air), sweeping the molecules clear would produce a
>>vacuum
>>behind the wing, so the instant that the pressure begins
>>to
>>fall behind the wing, air from on top and ahead of
>>the wing
>>and air above the wing begins to rush towards this
>>region.
>>
>>The air from ahead and on top of the wing that rushes
>>back
>>towards this low pressure region is being accelerated
>>-
>>exactly what the Bernoulli explanation needs for its
>>lower
>>pressure due to faster flow. The air above the wing
>>moving
>>down, coupled with higher pressure air below the wing
>>(also
>>moving down to escape that higher pressure) produces
>>downwash - exactly as the Newtonian explanation needs.
>>
>>
>>In reality, these are just two faces of the same coin.
>>There's no need to use the false 'equal transit times'
>>explanation of Bernoulli.
>>
>>>It is
>>>just an energy checks-and-balances way of explaining
>>>lift, which we already know occurs.
>>>
>>>Alexander Georgas
>>>
>>
>>--
>>T o d d P a t t i s t - 'WH' Ventus C
>>(Remove DONTSPAMME from address to email reply.)
>>
>
>
>

[Alexander Georgas]

Maybe someone more acquainted than me in nuclear physics
can shed some light on this, but my understanding is
that the Bohr model is false: The electrons revolving
around the atom would spin into the core, for one.
It is thus a model that could not sustain an atom in
life. After all, this is one of the main paradoxes
that lead to the creation of quantum mechanics.

But I'm digressing. The point I want to make is that,
just as the Bohr model is taught in school almost a
century after it has been found to be false (and not
just a bit inaccurate), there is an instructional role
for models that are false, but useful as a stepping
stone to the understanding of very complex phenomena.


One has to state the model, make people understand
it AND THEN exlpain why it is not correct.

Otherwise, it is most often the case that complex phenomena
tend to remain within the purview of the very few that
will come to understand the very complex mathematics
that describe the phenomenon in its full details.

There is plenty of good understanding that can be gained
by explanations that contain false premices. Should
we be as purist as this to reject even a cursory mention
of things that are not 100% accurate in the educational
process?

So, when trying to explain the world of physical matter,
do we bring out the Bohr model, or should we skip this
and directly introduce the fuzzy world of QED.

In the same principle, when trying to explain the very
complex phenomenon of wing lift, do we go from stating
the Bernulli effect and Newton's 2nd law to diferential
equations of fluid dynamics -- or possibly stop there,
explaining that the actual phenomenon is too complex
for 'common people' to understand?

Or can we bring up a series of models, each more complicated,
to ease in an understanding of the phenomenon by helping
people visualize what is taking place. Of course this
can only happen up to a point and then we are faced
with the pure mathematics.

I would like to know what this forum thinks in terms
of this? In my experience, quite a few gliding instructors
are either very much missinformed about the aerodynamics
of lift (and keep on missinforming students), or offer
explanations that while accurate fail to answer a student's
question by advancing the understanding of what take
places. This is very discouraging for most people and
it is only the few who will then go back into the literature
and try to sort things out.

The funny thing about this is that, while it is not
very important to know about aerodynamics to fly a
glider, all sorts of misinformation eventually trickles
into discussions and advice about such things as the
best bank to turn in, or when a stall occurs etc.

So what should we do about this?



At 10:30 11 January 2006, Bert Willing wrote:
>Well, trying to explain something by starting with
>a statement which is
>completely wrong is a very strange educational twist...
>The Bohr model is not accurate enough to explain all
>details, but it is not
>wrong.
>
>Bert
>ASW20 'TW'
>
>'Alexander Georgas' wrote in message
...
>> OK OK,
>>
>> I said it was inaccurate to begin with!
>>
>> Good to see this has stirred up some debate, thought.
>>
>> While I agree this is more urban myth than fact, I
>> do feel that it helps more in visualization than statements
>> such as 'pressure builds below the wing' or 'it is
>> the Bernulli effect above the wing' (some even refer
>> to the Venturi effect).
>>
>> I feel that the equal transit time proposition is
>>a
>> good first step in helping people visualize what
>>happens
>> when lift is generated, i.e. people start to think
>> about the flow fields over the entite wing section
>> in a more visual manner.
>>
>> It is quite intuitive and easy to visualize. Once
>>one
>> has actually understood this, he can now accept that
>> it is not true and move to the next level of understanding.
>>
>> So if you drop the wing geometry idea (which I do
>>not
>> aspise to begin with) and replace it with the concept
>> of angle of attack and also do not get too stuck with
>> the 'particles exit at exactly the same time' proposition,
>> you are starting to visualize things more properly.
>>
>> Once you understand this, and the fact that the flow
>> over the wing is much faster than what the equal transit
>> time proposition must suggest and that the angle
>>of
>> attack has created a low pressure area which has accelerated
>> the top flow field quite considerably, you can start
>> visualizing things like the vortices that are generated
>> once the two flows meet up behind the trailing edge
>> etc.
>>
>> Now you are getting somewhere!
>>
>> Otherwise, jumping from the 'there must be a Bernulli
>> effect somewhere' to the Euler equations is just too
>> much of a leap for the imagination (at least my imagination).
>>
>> Now, concerning the Newtonian explanation, I consider
>> this too much of a cop out. Ok we all know it is correct,
>> and it is plain simple to understand. It just doesn't
>> explain anything about the mechanics of flight. It
>> is as saying (it is, in fact, saying) that the laws
>> of energy conservation will be upheld and that the
>> laws of thermodynamics will not be broken and that
>> the universe will keep on working the same way as
>>it
>> was before. We sort knew this already. So where is
>> the understanding that comes from this explanation?
>>
>> So I feel that while the equal transit time paradigm,
>> while false, is a good starting point for introducing
>> people to the aerodynamics of wing sections. They
>>just
>> need to be told, once they understand the concept,
>> why it is false.
>>
>> It is as useful as the Bohr model of the Atom: extremelly
>> inaccurate, but easy to visualize as a starting point
>> to understanding a very complex phenonenon. Would
>>you
>> ever introduce particle physics to a student by writing
>> down the Schroedinger equation and solving a few Hamiltonians?
>>
>> Alexander Georgas
>>
>> At 16:24 10 January 2006, T O D D P A T T I S T wrote:
>>>Alexander Georgas
>>>wrote:
>>>
>>>>Ok, here is another way this can be explained (if just
>>>>a bit inaccurate):
>>>
>>>Sorry, but this is a lot inaccurate (a.k.a 'wrong')
>>>
>>>>The critical point to consider is angle of attack.
>>>>You just need to think of the airflow meeting the wing
>>>>at a specific angle. If you now imagine the airflow
>>>>separating to move above and below the wing section
>>>>as two particles, you have the following explanation:
>>>>
>>>>-The particles are going to separate at the leading
>>>>edge and meet up together approximatelly at the wing's
>>>>trailing edge.
>>>
>>>The particles do *not* meet up at the trailing edge.
>>> This
>>>is often referred to as the 'equal transit times'
>>>explanation of Bernoulli, which is thoroughly discredited.
>>>
>>>>-Because of the angle of attack (and of the wing's
>>>>thinkness), the air particle that will travel above
>>>>the wing section will have a greater discance to cover,
>>>>so it will have to run faster if it is to meet up with
>>>>the other particle which is traveling below the wing
>>>>section
>>>
>>>The particles do not have to meet up.
>>>
>>>>-Particles traveling at greater speeds (compared to
>>>>particles in neighboring areas) create areas of low
>>>>pressure -- just think: because the particles are running
>>>>faster over the wing compared to under the wing, there
>>>>are fewer of them in a specific area. Alternativelly,
>>>>you can take Bernoulli's word on this one.
>>>
>>>Bernoulli does not explain the flow pattern. Bernoulli
>>>explains the lift (pressure differentials given a specific
>>>flow pattern. You have to call in some other physics,
>>>namely that the air flows smoothly off the trailing
>>>edge, to
>>>establish the flow pattern. Once you have that, you
>>>apply
>>>Bernoulli.
>>>
>>>>-Now let's examine the situation from the perspective
>>>>of the wing. There is a low pressure above and a high
>>>>pressure bellow. The resulting force is thus upwards
>>>>(in relation to the angle of attack).
>>>>
>>>>The only presumption which remains to be explained
>>>>is why the two particles have to meet at the back of
>>>>the wing section. The easiest way to gloss over this
>>>
>>>Take a look at John Denker's start page:
>>>
>>>http://www.av8n.com/how/
>>>
>>>See the graphic and the bands of red, orange green
>>>blue?
>>>The blue band air over the top hits the trailing edge
>>>before
>>>the blue band air at the bottom, even though they started
>>>out together.
>>>
>>>>is to consider that if this is not approximatelly the
>>>>case, an imbalance will be created whereby there will
>>>>be more air crossing below the wing section and a low
>>>>pressure area buildup at the top back end of the wing,
>>>>created by the lack of air (teh air has just not had
>>>>the time to reach there). This would probably invite
>>>>air from below the wing to bleed back up towards the
>>>>back top.
>>>
>>>Guess what, that 'low pressure buildup' causes the
>>>air on
>>>top to accelerate, and it does tend to cause the high
>>>pressure to flow around to the top (resisted by air's
>>>finite
>>>viscosity and the sharpness of the trailing edge).
>>>
>>>>In this case, the airflow above the wing
>>>>would separate from the wing section before the trailing
>>>>edge (does this sound a bit like a stalled wing?)
>>>
>>>In a stalled wing, this does happen, but we're talking
>>>about
>>>a non-stalled wing and in that case the air flows faster
>>>over the top.
>>>
>>>>I don't know how much science there is behind this
>>>>explanation. I just find it a bit easier to swallow
>>>>than the Newtonian explanation. In my view the action-reaction
>>>>way of putting it (air is deflected downwards) is true,
>>>>but unable to shed any light on the mechanism.
>>>
>>>I agree that the Newtonian explanation is hard, but
>>>so is
>>>the Bernoulli. That's the nature of lift.
>>>
>>>I think of it like this: When the wing starts forward
>>>at a
>>>positive angle of attack, the space behind the wing's
>>>upper
>>>surface is being swept clear of air molecules - at
>>>least
>>>that's what the wing is trying to do. However, in
>>>a fluid
>>>(air), sweeping the molecules clear would produce a
>>>vacuum
>>>behind the wing, so the instant that the pressure begins
>>>to
>>>fall behind the wing, air from on top and ahead of
>>>the wing
>>>and air above the wing begins to rush towards this
>>>region.
>>>
>>>The air from ahead and on top of the wing that rushes
>>>back
>>>towards this low pressure region is being accelerated
>>>-
>>>exactly what the Bernoulli explanation needs for its
>>>lower
>>>pressure due to faster flow. The air above the wing
>>>moving
>>>down, coupled with higher pressure air below the wing
>>>(also
>>>moving down to escape that higher pressure) produces
>>>downwash - exactly as the Newtonian explanation needs.
>>>
>>>
>>>In reality, these are just two faces of the same coin.
>>>There's no need to use the false 'equal transit times'
>>>explanation of Bernoulli.
>>>
>>>>It is
>>>>just an energy checks-and-balances way of explaining
>>>>lift, which we already know occurs.
>>>>
>>>>Alexander Georgas
>>>>
>>>
>>>--
>>>T o d d P a t t i s t - 'WH' Ventus C
>>>(Remove DONTSPAMME from address to email reply.)
>>>
>>
>>
>>
>
>
>

[J. N.]

Alexander Georgas schreef:
> There is plenty of good understanding that can be gained
> by explanations that contain false premices. Should
> we be as purist as this to reject even a cursory mention
> of things that are not 100% accurate in the educational
> process?
Being an aerospace student myself I`m probally a bit of a purist as
well. Nevertheless I think that the final goal of teaching aerodynamics
(anything actually) is understanding, not perfectionism.


> In the same principle, when trying to explain the very
> complex phenomenon of wing lift, do we go from stating
> the Bernulli effect and Newton's 2nd law to diferential
> equations of fluid dynamics -- or possibly stop there,
> explaining that the actual phenomenon is too complex
> for 'common people' to understand?
What`s wrong about only telling the short version of the theory,
without complicated stuff:
"An airfoil creates an flow which accelerates air downwards. This
results in a pressure (and shear) field on you`re wing which causes
both lift and drag. Nothing really complicated about that I guess, and
no problem with applying it to reality at all.


> I would like to know what this forum thinks in terms
> of this? In my experience, quite a few gliding instructors
> are either very much missinformed about the aerodynamics
> of lift (and keep on missinforming students), or offer
> explanations that while accurate fail to answer a student's
> question by advancing the understanding of what take
> places. This is very discouraging for most people and
> it is only the few who will then go back into the literature
> and try to sort things out.
About a year ago I was having a discussion with a couple of gliding
instructors. None of them understood "anything" about aerodynamics and
I heard the weirdest theories. Nevertheless they seem to teach flying
quite well without any problems for their student. Not understanding
flow completely doesn`t seems to be such a problem in flying a glider.


> The funny thing about this is that, while it is not
> very important to know about aerodynamics to fly a
> glider, all sorts of misinformation eventually trickles
> into discussions and advice about such things as the
> best bank to turn in, or when a stall occurs etc.
>
> So what should we do about this?

Just skip the complicated theory?
Real world aerodynamics is extremely complicated with a lot of issues
(especially vortices) still beyond oure understanding. You can`t expect
every glider pilot to understand complex differential equations or the
navier-stokes equations.
Instead of putting a lot of effort in explaining lift we maybe should
concentrate a bit more in the relationship of weight, surface and so
on, which is very usefull in normal flight operations. The square and
squareroot equations seems to be complicated enough for most pilots.

Jarno Nieuwenhuize,
The Netherlands.

[Udo Rumpf]

> I would like to know what this forum thinks in terms
> of this? In my experience, quite a few gliding instructors
> are either very much missinformed about the aerodynamics
> of lift (and keep on missinforming students), or offer
> explanations that while accurate fail to answer a student's
> question by advancing the understanding of what take
> places. This is very discouraging for most people and
> it is only the few who will then go back into the literature
> and try to sort things out.
>
> The funny thing about this is that, while it is not
> very important to know about aerodynamics to fly a
> glider, all sorts of misinformation eventually trickles
> into discussions and advice about such things as the
> best bank to turn in, or when a stall occurs etc.
>
> So what should we do about this?

I agree with your statements.
When do your instructions begin?
I regularly abandon wrong assumption. Through this process
I am learning. For example the lift equation
CL * V^2 * area / density , how does it fit into the circulation
equation? I assume it was a stripped down version that I have
seen in an earlier post. I view the circulation as a 3D flow
about the wing and glider structure. As this flow must effect
every thing in one continues interaction, to express this in
a formula and still have a visual understanding of the process
must be nearly impossible. As a person interested in
aerodynamics I find the incremental approach much more to
my liking and I get results that are still within what I experience.
Regards
Udo

[For Example John Smith]

Do I have to go to the back of the class? I thought a propeller was a
screw.

"J. N." > wrote in message
oups.com...
> You`re pushing air, water or whatever downwards. The result is a
> pressure and shear distribution on a plane which creates both lift and
> drag.
> In reality you can only apply bernoulli on a slow (<200 kts), airfoil
> shaped device because of compressibility and possible detatchment of
> the boundary layer. When you start adding slats, fowler flaps or start
> flying fast or very slowly (model airplane`s) Bernoulli is far beside
> the thruth.
>
> A propellor or rotor is simply a rotating wing, and works exactly the
> same. It both sucks and pushes the air afterwards.
>
> The Blackbird would probally not fly at all if you use Bernoulli.
>
> That`s it...
>
> Jarno Nieuwenhuize,
> The Netherlands.
>

[For Example John Smith]

Schroedinger says yes! and no!
"Shawn" <sdotherecurry@bresnannextdotnet> wrote in message
...
> Don Johnstone wrote:
>> I think you can also obtain books that prove the earth
>> is flat and the Holocaust never happened. Oh and I
>> forgot there are several that prove global warming
>> :-)
>
> "Climate Change" C'mon don, get with the times. "Global Warming" is so
> 1997.
> ;-)
> BTW, was the Earth flat before Magellan sailed around it?
>
> Shawn

[Denis]

Ian Johnston a écrit :

> What the unititaited don't realise is that forces can be transmitted
> by pressure or by momentum flux. At the surface of a wing, all the
> force comes from pressure. A long way away from the wing, all the
> force comes from momemntum transfer (there is effectively no pressure
> difference). As you go from one to the other the balance changes. Most
> people who "prove" theories of lift wrong simply forget to take
> account of momentum flux.

Please explain us how momentum flux can be transmitted in a gas without pressure change. I think you will get at least a Nobel Price for such a revolutionary theory ;-)

[Ian Johnston]

On Fri, 13 Jan 2006 16:11:29 UTC, Denis
> wrote:

: Ian Johnston a écrit :
:
: > What the unititaited don't realise is that forces can be transmitted
: > by pressure or by momentum flux.

: Please explain us how momentum flux can be transmitted in a gas without pressure change. I think you will get at least a Nobel Price for such a revolutionary theory ;-)

Eh? Any fluid crossing the boundary of a control volume takes momentum
with it. That's a momentum flux. Elementary fluid mechanics.

Ian

[Ian Johnston]

On Fri, 13 Jan 2006 17:14:46 UTC, T o d d P a t t i s t
> wrote:

: The point is that the sum (pressure plus MF) is always the
: same, equal to the lift on the airfoil. The point of the
: original post was that you have to account for *both*
: pressure and momentum flux to figure out the force.

Thanks - that was exactly my point. Peraps I was not terribly clear.

Ian