Curve fitting to airfoil coordinates

Since the good Doctors are done explaining how it should be done in
the electronic world, here is a bit of advice for you to ponder,
Larry.

It was good enough for NACA to have the leading edge be above and
ahead of the origin of the coordinates system (Remember, Aero
specialists. Leading edge radius and slope on the old sections, hmm?)
Spline it that way, from upper surface to lower surface, right on
around the old leading edge. Use a point at 0,0. See how far above
and ahead the "actual" leading edge is in percent. Then, look at how
much this is on the 36 inch or so root chord of your wing. And then
look at it on the 14 or so inch tip chord.

You could mislocate a template by twice that much if they only go as
far back as you showed in your drawing!

Get it close and make it as smooth as possible. If only it was as
easy to smooth a wing (and maintain the desired contour) as it is to
smooth a curve to give a desired pressure distribution in the world of
electronics!

BTW, I spent hours smoothing the 67K150 section coordinates. Making
changes at the thrid and fourth decimal point. Made HUGE changes in
the plot of spline curvature. When looked at on the wing sized
templates, less than .001 inches.

Steve Leonard

Steve....
Excellent points...
Thats why I said to grab compufoil, the selig website data
and plot it on their pc printer. Game over and you
don't have to worry about all the corrective actions.....

Scott Correa.


"Steve" wrote in message
om...
Since the good Doctors are done explaining how it should be done in
the electronic world, here is a bit of advice for you to ponder,
Larry.

It was good enough for NACA to have the leading edge be above and
ahead of the origin of the coordinates system (Remember, Aero
specialists. Leading edge radius and slope on the old sections, hmm?)
Spline it that way, from upper surface to lower surface, right on
around the old leading edge. Use a point at 0,0. See how far above
and ahead the "actual" leading edge is in percent. Then, look at how
much this is on the 36 inch or so root chord of your wing. And then
look at it on the 14 or so inch tip chord.

You could mislocate a template by twice that much if they only go as
far back as you showed in your drawing!

Get it close and make it as smooth as possible. If only it was as
easy to smooth a wing (and maintain the desired contour) as it is to
smooth a curve to give a desired pressure distribution in the world of
electronics!

BTW, I spent hours smoothing the 67K150 section coordinates. Making
changes at the thrid and fourth decimal point. Made HUGE changes in
the plot of spline curvature. When looked at on the wing sized
templates, less than .001 inches.

Steve Leonard

Many thanks to everyone who has (will?) responded!!!! The technical info
is great. I now have a copy of Xfoil and am trying to figure out how to
use it. The "realistic" advice is also well taken.


Steve wrote:

Since the good Doctors are done explaining how it should be done in
the electronic world, here is a bit of advice for you to ponder,
Larry.

It was good enough for NACA to have the leading edge be above and
ahead of the origin of the coordinates system (Remember, Aero
specialists. Leading edge radius and slope on the old sections, hmm?)


Steve, et. al., did NACA _actually_ intend that the leading edge be above
and ahead of coordinate origin?? Or is that simply how we choose to
conveniently spline it currently? Seems like that messes up the whole
"chord" reference mechanism...

Thanks again to all... this is really interesting!

Larry Goddard
"01" USA

Hello Larry:

It's hard to add anything to the expert advice given by Dr.'s Drela &
Pfeiffer, and the practical value of an experienced
pilot/builder/engineer like Steve Leonard.

However, you might glance at :
http://www.appledor.net/tsillas/soaring/pikt7/pikt7.htm

As is so often the case, Dick Johnson has "been there, done that". A
couple of key points from this work with the related FX 17% section:

1) The practical eyeball approach yielded two serious failures when
Dan Somers found pressure waves in the NASA Langely tunnel
2) The final results give you some idea of what you *might* gain for
the work involved.

I enjoyed meeting and flying with you at Hobbs in '98. One of these
days, I might enter another contest.

Best Regards,
Gary Osoba


Larry Goddard wrote in message ...
Many thanks to everyone who has (will?) responded!!!! The technical info
is great. I now have a copy of Xfoil and am trying to figure out how to
use it. The "realistic" advice is also well taken.


Steve wrote:

Since the good Doctors are done explaining how it should be done in
the electronic world, here is a bit of advice for you to ponder,
Larry.

It was good enough for NACA to have the leading edge be above and
ahead of the origin of the coordinates system (Remember, Aero
specialists. Leading edge radius and slope on the old sections, hmm?)


Steve, et. al., did NACA _actually_ intend that the leading edge be above
and ahead of coordinate origin?? Or is that simply how we choose to
conveniently spline it currently? Seems like that messes up the whole
"chord" reference mechanism...

Thanks again to all... this is really interesting!

Larry Goddard
"01" USA