Airfoils

Fred Thirty-two gave me a call this past weekend. Could I help him
plot an airfoil?

Sure, I sez, and commence doing so. After a minute or so I realize
there is a pregnant pause bulging out of the telephone. "Hello?"

Ummm, Fred Thirty-two says. Also, ah, er, uh and eh. Finally he says,
could I mebbe SHOW him how to do it?

Turns out, Fred Thirty-two has a problem with verbal instructions.
Written ones, too, but I didn't find that out until he was sitting
across from me in the patio. He's a trim looking fellow in his
fifties, a working man judging from his hands and clothes. Not a lot
to say at first, which I took to be nervousness. Got him some coffee,
showed him pictures of the grandkids. Once he warmed up I was
delighted to find he had a keen sense of humor. A bigger surprise is
that he's probably smarter than me. Based on his emails, I'd
expected the opposite since his spelling is uniquely adventurous and
his grammar like a literal translation of German. Talking to him
one-on-one, it doesn't take long to figure out he probably suffers
from dyslexia. Which isn't necessarily a problem; being dyslexic
doesn't mean a person is dumb nor insensitive, although it can be
something of a challenge in a classroom since guys like Fred usually
learn best by actually doing the work. Which can be horrifyingly
difficult if the subject is history, government, geography and so on -
- the core subjects our educational system requires for graduation.

I was an instructor in the Navy and ran into cases similar to Fred's
in that their dyslexia wasn't recognized before they'd already
flunked out of school and were ****-canned by society. (Fred is in
'Facilities Engineering,' by the way. Meaning he pushes a broom.)

Fred has recently acquired a used computer and is learning how to use
it; he thinks the OS is Windows 95. He's also got a copy of DeltaCAD
and has used it to download most of the HVX files. He's a good
mechanic and a fair machinist, having used a mild steel steam-pipe
flange to make the drive-hub for the coaxial dynamo, which was what
told me he's smarter than the average bear.

Fred showed me some neatly done sketches and several sheets of figures
derived from the 1960 edition of the 'Amateur Aircraft Builder's
Manual.' From what I saw, his Dream Machine didn't break any of
the rules, although it was pretty big, designed around a liquid cooled
industrial engine. Right now he needed a hand turning the NACA
coordinates of an airfoil into ribs for a tapered wing, 64 inches at
the root, 48 at the tip.

The 230-series isn't an airfoil I'm familiar with. I go into the
house, dig around for Abbott, grab the lap-top and before you can say
'Theory of Wing Sections' we're sitting there, sipping coffee and
having a high old time keying the coordinates of the 23018 into
DeltaCAD.

NACA cites 16 x-y coordinates for each surface of the airfoil, listed
as a percentage of the chord; 18 if you count the 0% and 100%
x-location, which nobody does. Fer instance, the upper surface 5%
coordinate for the 23018 is 6.92% of whatever chord you're using. In
DeltaCAD you simply draw a line equal in length to the chord, select
the POINT function, click on the start of the line as your initial
location (ie, 0,0) then enter .05*64, .0692*64 and hit enter. That
will give you a dot precisely 3.2" back from the leading edge (ie, the
x-location) and 4.4288" above the chord-line (ie, the y-location). If
you were doing the lower surface you'd add a minus to the y factor
(ie, .05*64, - .038*64).

See how it works? You're letting DeltaCAD do the calculating.
Telling it .05*64 means you want it to use the VALUE equal to 5% of 64
inches (for the x-location), whereas telling it .0692*64 tells it to
calculate the value for the y-location. Since the POINT function
expects the x and y to be separated by a comma, you put one in: .05*64
, .0692*64. Hit the big red button and there's your point. Do that
thirty-two times, connect the dots and there's your airfoil. There's
some fiddly bits having to do with the angle of the slope on which you
draw the radius of the nose but but Fred picked right up on it once I'd
shown him how it works.

Not very elegant but it gets the job done. So long as the rib (and
wing) is being hand-fabricated, using just sixteen coordinates for
each surface works just fine. Modern coordinate systems plotting
points as close as every thousandth of an inch of chord (!) were
developed to satisfy the input of CNC machines and super-computers.
Building a rib out of quarter-inch sticks, the accuracy and precision
of the resulting surfaces depends largely upon how willing the wood is
to follow the curve. Ditto for banging ribs out of aluminum or sanding
a foam wing-surface to match a few templates.

Unfortunately, Fred Thirty-two never learned to type by touch. I watch
him hunt & peck for a couple of coordinates, determined look on his
face - - but a happy one - - until I scream in frustration, snatch the
keyboard away and show-off at a hundred words per minute. (Entering
the coordinates is the EASY part of the job. Takes mebbe ten minutes.
The tricky bit is turning the airfoil into patterns and jigs.)

Once we get the coordinates entered I select the SPLINE function and
connect the dots. DeltaCAD turns it into a smooth curve and there's
the 23018 airfoil flying on the computer's screen, a fat, jolly
looking teardrop that makes you think of Ford Tri-motors.

Pure joy on the face of Fred Thirty-two. Just having the basic airfoil
in front of him offered bags of information he didn't have before,
such as spar depth (nearly 12") and approximate fuel tank capacity
(huge!). The Dream had come a bit closer.

To get the airfoil out of the lap-top I took it into the house, laid a
two inch grid of faint blue lines over the drawing, transferred the
file into another computer and printed it out. It took fourteen sheets
of normal sized typing paper. I showed Fred how to use a thread on a
sliding glass door to align the sheets. After taping them together I
kicked around under the workbench, found a scrap of eighth-inch
doorskin ply, spritzed it with spray glue then eased the drawing onto
it. Sawn near the line and then sanded to it, the plywood can serve as
a pattern. But so will the original drawing, now that it exists as a
file Fred can load into his computer and manipulate with DeltaCAD.

DeltaCAD's simplistic POINT function and spending ten minutes
hand-entering coordinates is the ultimate in Lo-Tek but this is an
inexpensive tool anyone can master. The fat 23018 may not be the ideal
airfoil for Fred's Dream Machine but if it doesn't work out he now
knows how to generate ANY airfoil for which he can find x-y
coordinates, an option he didn't have just a few days ago.

-R.S.Hoover

PS - His name isn't Fred, of course. But after posting messages
about Flying on the Cheap, inexpensive methods of converting VW engines
for flight and using locally available lumber, I've been contacted by
a surprising number of 'Freds' here and abroad who are doing
exactly that.

PPS - I've never been asked to autograph an airfoil before. Thank
you, Fred. I consider it quite an honor.

wrote in message
oups.com...


Fred Thirty-two gave me a call this past weekend. Could I help him
plot an airfoil?

Sure, I sez, and commence doing so. After a minute or so I realize
there is a pregnant pause bulging out of the telephone. "Hello?"

(...)

Once again pearls of wisdom,
witty humour,
and warm commiseration with us
less bright members of society.
My heartfelt gratitude for this text
even if it has no practical value for me.

Ya done good, RS...

denny

On 28 Sep 2006 12:58:57 -0700, "
wrote:




PS - His name isn't Fred, of course. But after posting messages
about Flying on the Cheap, inexpensive methods of converting VW engines
for flight and using locally available lumber, I've been contacted by
a surprising number of 'Freds' here and abroad who are doing
exactly that.

PPS - I've never been asked to autograph an airfoil before. Thank
you, Fred. I consider it quite an honor.

Way to go Bob. I wish more people took that time and had the patience
to help out people like that.

Many years ago (probably 30 or so) I knew a guy like that who had an
eye disease, the name of which I've forgotten, (Retinitis Pigmentosa
or something like that..I think) but they thought he was retarded,
kicked him out of school and put him on welfare. By the time they
actually discovered the disease he was blind and well into his 20s..
Without the basic school back ground he was smart enough to learn the
regulations, code (13 WPM), and theory to earn his general class
(Conditional back then) Amateur radio license.

BTW my MS spell checker thinks Retinitis should be spelled
Creationists? That's not even in the ball park.


Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

wrote:
an excellent post...thanks,

For any given application of an airfoil there is an appropriate level
of accuracy needed. A biplane strut will not need much accuracy as a
canard that may be sensitive to disruptions in the laminar flow. At
some point the accuracy of the points used to plot an airfoil will
affect the desired aerodynamic performance. While working for a
contractor at NASA Ames once I needed to plot some airfoils for a
shrouded air probe to fly on a science mission on the NASA DC-8. In the
process I learned about the sensitivity of the surface of an airfoil to
the accuracy of the plotted points. I also developed a simple and crude
method of improving the accuracy of plotted points which may be useful
in some cases.

As I was about to put a few hours into some drawings which would then
direct the shop to cut metal and the mission was to fly in a few months
I did not want to find out later that the final product ended up with
noticable and unwanted waviness in the surface so the first thing I did
was take a careful look at the airfoil points provided by the
aerodynamicist based on his CFD design.

I was using Autocad to plot the points and the various spline functions
to generate the curve. The data was provided to three decimal places
and I typed in the points and used the spline to generate the curve.
Since the points came from the aerodynamicist's program I fully
expected the curve to form the airfoil, no problem. As I took a close
look I was suprised to find bumps as the spline function tried to do
what I asked and apply the polynomials to the points. But there were
the bumps. Neither spline for curve fit would yield a smooth shape. I
called him and asked 'are your sure'? He checked and verified yes the
points are correct.

I played around with the points and curves and came up with a crude
method to try to interpolate more digits of accuracy. Part of the mehod
involved keeping the percent cord exact. Then I used two adjacent
points, skip the third point, and the fourth point to draw a circle.
Then I intersected the circle with the percent cord to estimate the
third point. Using this mehtod I worked down the 30 or so points a few
times until the curves became more accurate. When the spline finally
smoothed out I regenerated a new set of data points and sent the set
back to the aero to be checked. It was good. The resulting parts were
smooth.

Two decimal places is usually good enough for low speed applications.
Abbot and Von Doenhoff TOWS gives standard NACA data to three decimal
places; the industry standard. To get a smoother airfoil a few hours of
Autocad can help achieve a little better performance for applications
like laminar flow. Of course the composite layup and the sanding of the
gel coat will smooth out the curve but why not start with better data?
Airflow likes smooth shapes and any deviation from smooth will only
detract from the optimum recovery of pressure and result in earlier
separation. All I am saying is as long as you are going through the
trouble to lay out an airfoil, take a close look at the curve until you
are satisfied with the shape for your applicaton. Three decimal places
on a 100 inch cord is 0.1 inches, not too bad. But what concerned me
was the possibility that the Autocad spline function might bulge out
the curve between points in unknown ways and start to affect
performance.

Here is a website with an image of the shroud probe.

http://www.espo.nasa.gov/sonex/pages/photos.html

About halfway down the page is a photo of the shroud which is still
flying missions once in a while. It is the barrel hanging below the
viewport which looks like a mini jet engine. I still can't believe that
they let us do stuff like this to the aircraft. We had to be very
careful to make sure the structure was strong enough. To be very
conservative we would over design it for loads such as full speed flow
from the side times a Cd of 1.0, and even then multiplied by a factor
of three because we had to achieve airworthiness by analysis and a test
flight.

James