Best airfoil?

If one were to design their own sailplane, which airfoil would be a good
choice for low drag and fairly low speed (ie, an ultralight part 103
sailplane for exampl)? In other words, is a thicker airfoil (more lift)
better at slow speeds or a thinner airfoil (less drag)? Which type would
give the best glide ratio with all other things being equal?

--
Scott
http://corbenflyer.tripod.com/
Building RV-4
Gotta Fly or Gonna Die

Scott wrote:
If one were to design their own sailplane, which airfoil would be a good
choice for low drag and fairly low speed (ie, an ultralight part 103
sailplane for exampl)? In other words, is a thicker airfoil (more lift)
better at slow speeds or a thinner airfoil (less drag)? Which type would
give the best glide ratio with all other things being equal?

Have you read Fundamentals of Sailplane Design? That'd be a good place
to start if you haven't. Join the Sailplane Homebuilder's Associaton at
the same time (buy the book from them, too).

Thicker and thinner don't mean much; instead, you need to be talking
about the characteristics you want, such stall speeds, desired L/D at
max and cruise speeds, wing loadings, construction (wood and fabric,
carbon, or ?) and so on. Theennn you can start asking about the airfoil!

--
-----
change "netto" to "net" to email me directly

Eric Greenwell
Washington State
USA

Hello Scott:

Along with Eric Greenwell's very good suggestion, here are a few other
considerations. If you are designing for FAR part 103, then your
weight constraints and therefore structural considerations will drive
your design more than subtle aero concerns. From a practical
standpoint, the thicker sections will give you greater freedom in the
design process.

Here are the opposite ends of the spectrum in the current state of the
art for sub 155 lb. empty structures:

Carbon Dragon @ 145 lbs. Root section nominally 18% at root but
actually increased to 21% over a short section of the span for
structural reasons. 13% tips, highly tailored aerodynamically due to
large q-differential across span at very slow circling speeds.

SparrowHawk at circa 153 lbs. With pre-preg carbon construction and
excellent engineering, able to use relatively thin and very slippery
sections for the R-numbers throughout. Not practical for most
homebuilders to take this route. If you like this design approach, it
is very well executed and you might consider buying one as they are
reasonably priced.

Best Regards,
Gary Osoba

Eric Greenwell wrote in message ...
Scott wrote:
If one were to design their own sailplane, which airfoil would be a good
choice for low drag and fairly low speed (ie, an ultralight part 103
sailplane for exampl)? In other words, is a thicker airfoil (more lift)
better at slow speeds or a thinner airfoil (less drag)? Which type would
give the best glide ratio with all other things being equal?

Have you read Fundamentals of Sailplane Design? That'd be a good place
to start if you haven't. Join the Sailplane Homebuilder's Associaton at
the same time (buy the book from them, too).

Thicker and thinner don't mean much; instead, you need to be talking
about the characteristics you want, such stall speeds, desired L/D at
max and cruise speeds, wing loadings, construction (wood and fabric,
carbon, or ?) and so on. Theennn you can start asking about the airfoil!

Earlier, "Scott" wrote:

If one were to design their own
sailplane, which airfoil would be
a good choice for low drag and
fairly low speed...

In addition to what some others have written on this topic:

These days it is getting more and more rare to see a sailplane
developer pull an airfoil straight out of a catalog and use it. Now
that 2D and even 3D airflow analysis software has gotten more common
and more affordable, most developers and designers are using custom
airfoils specifically created for with their objectives in mind.

What we are doing with the HP-24 kit glider wings is pretty common: we
started with a catalog airfoil used on some modern racing ships and
used a 2D analysis package (PANDA, I think) to tweak it slightly.

However, it's not unknown for folks start with a pair of straight
lines from 0,0 to 1,0 and nudge them around until they get the
pressure distribution that they're looking for.

As Gary Osoba else wrote, the wing thickness is generally more of a
structural thing than an aerodynamic concern. More thickness gives you
better stiffness/weight, since the stiffness of a beam is relative to
the cube of its depth - or something thereabouts.

In addition to Gary's good suggestion about the Sailplane
Homebuilder's Association: there's a couple of YahooGroups forums that
are good for topics like the ones you raise. There's glidertech for
design/development issues, and HomeSail for construction issues. Of
course, there's a lot of overlap between the two. You can contact me
via email for subscription info for both.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com/hp-24

On 26 Jan 2004 18:09:18 -0800, (Bob Kuykendall)
wrote:

As Gary Osoba else wrote, the wing thickness is generally more of a
structural thing than an aerodynamic concern. More thickness gives you
better stiffness/weight, since the stiffness of a beam is relative to
the cube of its depth - or something thereabouts.

In addition to Gary's good suggestion about the Sailplane
Homebuilder's Association: there's a couple of YahooGroups forums that
are good for topics like the ones you raise. There's glidertech for
design/development issues, and HomeSail for construction issues. Of
course, there's a lot of overlap between the two. You can contact me
via email for subscription info for both.

The Falco web site, http://www.seqair.com/, has a PDF version of a
book its designer wrote on glider design. Look under 'The Glider' Its
a pretty old book, but looks to be advocating the design approach Bob
talked about.

Is that book of interest still?

BTW the site's worth a visit just to drool over the Falco, a very
pretty design.


--
martin@ : Martin Gregorie
gregorie : Harlow, UK
demon :
co : Zappa fan & glider pilot
uk :

Earlier, Martin Gregorie wrote:

Is that book of interest still?

It's a good and rigorous survey of the glider design process. Of
course, much of it is dated. However, the general approach is still
valid.

The thing I like about it is that it gives insight into the thinking
of Stelio Frati, who designed some of the prettiest small airplanes
ever, and did it about fifty years ago. When you look at the diagrams
of how to use the method of tangents to develop fuselage
cross-sections, there's an embryonic slice of Falco staring out at
you.

BTW the site's worth a visit just to drool
over the Falco, a very pretty design.

Alfred Scott is my hero. Resurrecting the Falco was one of the finest
and most elegant things ever done for sport aviation. Not in terms of
quantity, but in quality, in Robert Pirsig's sense of the event in
which subject invents and envelops object.

Thanks, and best regards to all

Bob K.
http://www.hpaircraft.com