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

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.

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

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

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

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.

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

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

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

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

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."