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

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,

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.

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

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

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

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.

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

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."

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

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