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

On Apr 14, 10:18*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

Generally excellent for an introductory presentation. There are a few
very minor issues that only an engineer or aerodynamisist would
catch. One is the use of the Greek letter Alpha to label the AoA of
the fin in a skid - Beta is correct.

Another is the description of the secondary effect of roll with the
application of yaw inputs (Yaw to roll coupling). You are correct
that this is momentarily due to a spanwise difference in airspeed but
you failed to mention that the continued, and much larger, effect is
due to dihedral. Your description of 'adverse' yaw is correct but I
cringe at the negative connotation of the word. Adverse yaw is not
"bad" yaw - it's just what ailerons do. In fact, 'adverse' yaw can be
quite useful in soaring turns as well as in crosswind landings.

BTW, your description of how lift is generated is just fine for this
sort of presentation. Some will argue endlessly about it without
reaching a consensus but yours is more than good enough.

On Apr 14, 1:24*pm, bildan wrote:
On Apr 14, 10:18*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

Generally excellent for an introductory presentation. *There are a few
very minor issues that only an engineer or aerodynamisist would
catch. *One is the use of the Greek letter Alpha to label the AoA of
the fin in a skid - Beta is correct.

Another is the description of the secondary effect of roll with the
application of yaw inputs (Yaw to roll coupling). *You are correct
that this is momentarily due to a spanwise difference in airspeed but
you failed to mention that the continued, and much larger, effect is
due to dihedral. *Your description of 'adverse' yaw is correct but I
cringe at the negative connotation of the word. *Adverse yaw is not
"bad" yaw - it's just what ailerons do. *In fact, 'adverse' yaw can be
quite useful in soaring turns as well as in crosswind landings.

BTW, your description of how lift is generated is just fine for this
sort of presentation. *Some will argue endlessly about it without
reaching a consensus but yours is more than good enough.

I was maybe a little imprecise in my niggle, but Kevin has described
it very well. My main criticism would be that glider pilots don't
really need to know this. The rest of the stuff is fine.

Mike

Mike the Strike wrote:
... There are a few
very minor issues that only an engineer or aerodynamisist would
catch. One is the use of the Greek letter Alpha to label the AoA of
the fin in a skid - Beta is correct....
Mike

Hmmm...beta is yaw angle, and alpha is AoA.
In referring to side forces generated by a vertical fin,
beta is not quite the controlling factor, is it, when the rudder effect
is included?

Brian W

On Apr 15, 8:24*am, bildan wrote:
One is the use of the Greek letter Alpha to label the AoA of
the fin in a skid - Beta is correct.

In mathematics variable names are without meaning and therefore
interchangeable, so I don't see how you can say that a particular
labeling is correct or incorrect, but at most that it follow or does
not follow some convention.

It is true that when considering an entire aircraft it is conventional
to use Beta for yaw, but if you are considering the fin as an airfoil
in isolation then Alpha is perfectly reasonable in order to use the
same equations as you would for other airfoils.

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