Gliding lecture slides

Hi,

I've put some lecture slides on the following topics:

* Principles of glider flight
* Glider instruments

onto the web site:

http://www.carrotworks.com/

Regards,

Richard

On Apr 14, 9:18*am, (Richard Lancaster)
wrote:
Hi,

I've put some lecture slides on the following topics:

* Principles of glider flight
* Glider instruments

onto the web site:

http://www.carrotworks.com/

Regards,

Richard

You might want to check on the accuracy of the oft-repeated myth that
wings develop lift because of the pressure difference between the
upper and lower surfaces. My understanding of the physics is that
this component is negligible compared to the second mechanism you
mention of the downward deflection of the airflow.

If you think I am mistaken, then please explain how symmetrical
airfoils develop lift!

Mike

If you think I am mistaken, then please explain how symmetrical
airfoils develop lift!

Mike

angle of attack.

Richard and Mike, you're both a little bit wrong.

I like the slide showing the airflow around the airfoil section, few
illustrations show how the streamlines change on the lower surface. A
lot just keep them flat. However, if I'm to interpret the length of
the vectors as the local velocity in the flow field, then the vectors
on the lower surface look in error. They also typically accelerate to
a velocity greater than freestream. Which brings me to the problems I
have with the chart on page 8. The pressure forces on the lower
surface are not acting in the direction as shown. Because the air also
has to accelerate around the lower surface, it too is a lower pressure
than ambient and is therefore acting positive to the surface normal.
Or in the diagram, downward. However, the net difference between the
upper and lower surface yields a positive lifting force. This site has
very good diagrams that show the direction and magnitude of the
pressure forces on an airfoil. It also shows how the pressures react
for symmetric airfoils.

http://www.dynamicflight.com/aerodyn...pres_patterns/

Mike, I think the common mis-understanding to which you are referring
to is the oft stated quote that because the surface is longer on the
top than the bottom, the air has to travel faster to meet at the end.
Geometrically, this difference is near 1%, and clearly any velocity
delta driven by the distance would not be significant. However, no
such rule requires that they meet at the end, and in reality they
don't.

There are two ways to look at how an airfoil creates lift. And both
are correct. The Pressure theorists look at the difference between the
upper and lower surface and calculate lift. If you integrate the
pressures on the airfoil you'll end up with the same calculation that
you get from momentum theory which looks at the imparted change in
momentum of the airmass which is deflected downward. Both are correct
and both happen. The difference is how you look at the problem. You
can examine it near field in which case you look at the pressures on
the surface, or you can examine it far field and examine the change in
airmass movement imparted by the airfoil as it moves through the air.

As for symmetric airfoils, they won't produce lift at an angle of
attack = 0. But as Tony says, change the angle of attack and it
produces lift. And now you know why.

-Kevin

Mike the Strike wrote:

You might want to check on the accuracy of the oft-repeated /myth/ that
wings develop lift because of the pressure difference between the
upper and lower surfaces. My understanding of the physics is that
this component is negligible compared to the second mechanism you
mention of the downward deflection of the airflow.

If you think I am mistaken, then please explain how symmetrical
airfoils develop lift!

Mike

Amusing how partisans grow zealous for their chosen explanation! :-)

Brian W

On Apr 15, 5:14*pm, brian whatcott wrote:
Mike the Strike wrote:
You might want to check on the accuracy of the oft-repeated /myth/ that
wings develop lift because of the pressure difference between the
upper and lower surfaces. *My understanding of the physics is that
this component is negligible compared to the second mechanism you
mention of the downward deflection of the airflow.

If you think I am mistaken, then please explain how symmetrical
airfoils develop lift!

Mike

Amusing how partisans grow zealous for their chosen explanation! :-)

Brian W

Actually, we physicists get annoyed when people get the science wrong.

Mike

Mike the Strike wrote:
On Apr 15, 5:14 pm, brian whatcott wrote:
Mike the Strike wrote:
You might want to check on the accuracy of the oft-repeated /myth/ that
wings develop lift because of the pressure difference between the
upper and lower surfaces. My understanding of the physics is that
this component is negligible compared to the second mechanism you
mention of the downward deflection of the airflow.
If you think I am mistaken, then please explain how symmetrical
airfoils develop lift!
Mike
Amusing how partisans grow zealous for their chosen explanation! :-)

Brian W

Actually, we physicists get annoyed when people get the science wrong.

Mike


Hehe....good one, Mike! :-)

Brian W

On Apr 15, 9:20*pm, Mike the Strike wrote:
On Apr 15, 5:14*pm, brian whatcott wrote:

Mike the Strike wrote:
You might want to check on the accuracy of the oft-repeated /myth/ that
wings develop lift because of the pressure difference between the
upper and lower surfaces. *My understanding of the physics is that
this component is negligible compared to the second mechanism you
mention of the downward deflection of the airflow.

If you think I am mistaken, then please explain how symmetrical
airfoils develop lift!

Mike

Amusing how partisans grow zealous for their chosen explanation! :-)

Brian W

Actually, we physicists get annoyed when people get the science wrong.

Mike

Mike,

You might want to check your own physics, before making comments like
this.
Because us aerodynamicists get annoyed when people get the science
wrong :-)

Symetrical airfoils work the same way asymetric foils work. The air
is pushing up on the bottom, stronger than the air is pushing down on
the top. If the pressure was the same on the top and bottom surfaces,
there would be no lift.

Please, don't confuse pressure differential with the incorrect "equal
transit time" theory.

I have lost my favorite link to a well presented and complete theory
of lift, so I can't post it here. But the short version is:

angle of attack makes the air flow change direction
that change of direction makes
air pressure rise on the bottom ( but not by much )
air pressure drop on the bottom ( most of the change)

And it all happens because of the Kutta Condtion.
http://en.wikipedia.org/wiki/Kutta_condition

If you are going to berate people for their own incorrect knowledge,
you should be able to improve on that knowledge.

The original posters slides look like a pretty good explanantion. I
think that it is more information than a pilot really needs, but some
people won't relax and actually learn until they think they understand
"why" it works.

Todd Smith
Masters of Science Aerospace Engineering 1990

On Apr 16, 1:18*pm, toad wrote:

If you are going to berate people for their own incorrect knowledge,
you should be able to improve on that knowledge.



The only flight theory that most pilots need to know is that sustained
flight requires a continuous supply of money.

It is easy to prove that when the supply of money is exhausted flight
is no longer possible.

Andy

toad wrote:

The original posters slides look like a pretty good explanation. I
think that it is more information than a pilot really needs, but
some people won't relax and actually learn until they think they
understand "why" it works.

That's probably the best reason I've heard for teaching pilots about
aerodynamics.

Despite my deep interest in aerodynamics, my observation and experience
is knowing the aerodynamics doesn't seem to be an asset to flying
correctly, and that it is really all about keeping the airspeed up and
horizon in the correct place on the canopy. When I fly, thoughts of AOA
and pressure distributions are not flitting through my mind, and things
happen too fast to be deriving your next action from first principles.

You definitely want the designer of your glider to understand
aerodynamics, but the pilot - completely optional!

--

Eric Greenwell - Washington State, USA (netto to net to email me)

- "Transponders in Sailplanes - Feb/2010" also ADS-B, PCAS, Flarm
http://tinyurl.com/yb3xywl

- "A Guide to Self-launching Sailplane Operation Mar/2004" Much of what
you need to know tinyurl.com/yfs7tnz