From: David Lednicer <dave@amiwest.com>
Newsgroups: rec.aviation.homebuilt
Subject: Re: GA Airfoils
Date: Thu, 02 Dec 1999 09:42:55 +0000

My experience with Riblett and his airfoils is this:

1) He does his airfoil design work with the Eppler code, a program with
some serious shortcomings.  I am not aware of his airfoils ever being
wind tunnel tested.  This is necessary to properly quantify their
behavior.

2) He has claimed that the proper choice of airfoils can reduce induced
drag.  This is incorrect and shows a lack of knowledge of the subject.

3) I think he probably has improved the NACA 60-series airfoils a bit.
His modifications probably do produce better stall characteristics.

4) The claim that his airfoils "lift better" is meaningless.

5) His claims about the harsh stall characteristics of the NACA 5-digit
series are correct, but nothing new.  His extrapolation of these claims,
calling for the grounding of the ATR-42, show his ignorance of 3-D
aerodynamics.  There are a lot of aircraft with NACA 5-digit airfoils
that stall just fine, because the stall behavior has been properly
tailored.  For more information, see the Incomplete Guide to Airfoil
Usage at: http://amber.aae.uiuc.edu/~m-selig/ads/aircraft.html

6) His knowledge of US copyright and patent law is lacking.  I wanted to
include his airfoils in the UIUC airfoil database, but he claimed they
were copyrighted, which is impossible.  Airfoils can be patented (one of
mine is) but not copyrighted.  Because of his agitation, his airfoils
are not in the database, but I have the coordinates in my own personal
database.  On the other hand, his "book" contains unapproved reprints of
copyrighted material, which is illegal.

7) I am not aware of any rotorcraft airfoils he has designed.  However,
the UIUC database contains rotorcraft airfoils designed at Boeing and
Sikorsky.

-------------------------------------------------------------------
David Lednicer             | "Applied Computational Fluid Dynamics"
Analytical Methods, Inc.   |   email:   dave@amiwest.com
2133 152nd Ave NE          |   tel:     (206) 643-9090
Redmond, WA  98052  USA    |   fax:     (206) 746-1299

From: highflyer <highflyer@alt.net>
Newsgroups: rec.aviation.homebuilt
Subject: Re: GA Airfoils
Date: Mon, 06 Dec 1999 10:46:54 -0600

David Lednicer wrote:

> My experience with Riblett and his airfoils is this:
>
> 1) He does his airfoil design work with the Eppler code, a program with
> some serious shortcomings.  I am not aware of his airfoils ever being
> wind tunnel tested.  This is necessary to properly quantify their
> behavior.
>
> 2) He has claimed that the proper choice of airfoils can reduce induced
> drag.  This is incorrect and shows a lack of knowledge of the subject.
>
> 3) I think he probably has improved the NACA 60-series airfoils a bit.
> His modifications probably do produce better stall characteristics.
>
> 4) The claim that his airfoils "lift better" is meaningless.
>
> 5) His claims about the harsh stall characteristics of the NACA 5-digit
> series are correct, but nothing new.  His extrapolation of these claims,
> calling for the grounding of the ATR-42, show his ignorance of 3-D
> aerodynamics.  There are a lot of aircraft with NACA 5-digit airfoils
> that stall just fine, because the stall behavior has been properly
> tailored.  For more information, see the Incomplete Guide to Airfoil
> Usage at: http://amber.aae.uiuc.edu/~m-selig/ads/aircraft.html
>
> 6) His knowledge of US copyright and patent law is lacking.  I wanted to
> include his airfoils in the UIUC airfoil database, but he claimed they
> were copyrighted, which is impossible.  Airfoils can be patented (one of
> mine is) but not copyrighted.  Because of his agitation, his airfoils
> are not in the database, but I have the coordinates in my own personal
> database.  On the other hand, his "book" contains unapproved reprints of
> copyrighted material, which is illegal.
>
> 7) I am not aware of any rotorcraft airfoils he has designed.  However,
> the UIUC database contains rotorcraft airfoils designed at Boeing and
> Sikorsky.
>

I have argued with David Lednicer in the past about aerodynamics and some
aerodynamic details.  David is one of the leading people in the country
in this field.  He earns my respect far above Mr. Riblett.  Mr. Riblett
does do an excellent job of marketing his book and his airfoil designs.

The fact is, that any of a number of airfoil designs will do an excellent
job for a GA aircraft.  Each airfoil design represents a compromise
between features, and that compromise should be selected in the light of
ALL of the many compromises that are balanced to design an airplane.

Wing area is normally determined by the low speed characteristics
required to make the airplane safe and easy to land.  Blunting the nose
and increasing the camber and thickening the airfoil will generally
increase the maximum lift coefficient for an airfoil of a given general
shape.  Other factors are the Reynold's Number where the airfoil will
operate.  This varies widely in GA aircraft.  That is why the airfoils of
sailplanes tend to be thicker and more "tadpole" shaped than those for
powered planes.

Washout is generally used to tailor the lift distribution along the wing
to optimise it.  Before they figured that out, there was a tendency for
designers to design beautiful elliptical wing planforms to ensure the
desired elliptical lift distribution without washout!  Unfortunately,
cutting into the trailing edge to insert things like ailerons and flaps
destroyed the lift distribution anyway and required compensation.  When
designers found that they could take a simple straight tapered wing and
twist it just a little and still get the optimal elliptical lift
distribution and have a wing that was MUCH easier and cheaper to build,
those lovely "Spitfire" wing planforms went south.

A "soft and gentle" stall depends on many factors, of which the airfoil
lift characteristic near the stall is only one relatively minor one.
Even the 23012, which has a discontinuous lift curve at stall, can give a
gentle and progressive stall when the wing is designed for that purpose.
Any rectangular planform, with NO twist, will tend to stall progressively
and give a relatively gentle stall characteristic with good burble for
warning over the tail surfaces.  This can be degraded if the wing is
allowed to deform when it picks up a load, and render the stall less
predictable, as in the Tomahawk.

Actually, a "symetrical" airfoil does not often have a zero moment
coefficient.  In fact, the symetrical airfoil may have a moment that is
opposite that of the cambered airfoil.  The cambered airfoil tends to
have a moment coefficient that is "destabilizing" because the actual
center of lift of the airfoil tends to move forward as the angle of
attack increases.  With most symetrical airfoils, the center of lift
tends to move aft with increasing angle of attack.  This movement is
"stabilizing" since it tends to reduce the angle of attack.

A cambered airfoil can have the moment coefficient reduced to approximate
zero by merely reflexing the trailing edge.  This is how the 23012
airfoil obtains its near zero moment coefficient.  I have heard it said
that this moment coefficient of the 230xx series airfoils is what made
the monospar wing of the DC-3 possible.  I cannot verify the truth of
that, however.

In any event, the moment coefficient can be caused to vary clear off the
scale by the simple expedient of lowering a flap or moving an aileron!
That generally becomes the design constraint in wing torsional rigidity.
Dynamic behavior of the wing/control surface pair also becomes extremely
important.  Bad dynamic behavior is a common cause of flutter that will
cause rapid disassembly in flight.  :-)

Laminar flow airfoils can give a "drag bucket" for the angle of attack
range where significant laminar flow can be achieved.  No airfoil can
sustain laminar flow after about the thickest part of the airfoil.  That
is why most laminar flow airfoil designs have their maximum thickness
quite far back from the leading edge.  Typically, a "laminar" airfoil
will have MORE drag than a more conventional airfoil outside of the
drag bucket, or with a "standard roughness" surface.  Any spanwise
waviness in the wing will totally destroy laminar flow, as well as any
protuberances from the surface.

In summary ...  airfoil selection is only one of the many compromises
that make an aircraft design. It is not one of the more important items,
and there are many others that will have more effect on handling
qualities and flight performance, although airfoil design is important.

--
HighFlyer
Highflight Aviation Services

From: "Walter Lounsbery" <Walt@Lounsbery.com>
Newsgroups: rec.aviation.homebuilt
Subject: Re: GA Airfoils
Date: Thu, 02 Dec 1999 02:00:37 GMT

I would strongly encourage you to look at a wider range of airfoil
information, and it seems that you are already on that path by posting this
message.  I highly recommend looking at some of the more accepted works by
Eppler, several NASA GA airfoil investigators, some of the older Wichita
State University stuff (thanks Gene!), and even Barnaby Wainfan's material.
I'm afraid I only have dim recollections of some of Riblett's articles from
years ago, and I haven't read his book.

There is a big difference between your question "Have I missed something?",
and my question, "What did Mr. Riblett leave out of his book?"  Airplanes
seem to be flying quite OK.

But maybe I can add my two cent productively in some other areas than
disputing Riblett (which is surely an easy debate).  I designed a few
airfoils for some Boeing projects about 14 years ago, using cutting-edge CFD
analysis and empirical data.  Some airfoils were based on rotor airfoil
design technology obtained from Boeing Helicopters.  Since these were
exercises in tailoring, proof-of-concept, and building expertise on
underfunded military studies, the airfoils never made it to actual testing.

The non-symetric airfoils are intended to either obtain a higher maximum
lift coefficient or to shift the "drag bucket" to higher lift coefficients.
I would not discourage use of a non-symmetric airfoil for a rotor as long as
it is already proven in use at a similar size (chord) and speed (actual
speed at the rotor, if the chord is similar it is likely the aircraft speed
will also be similar).  In any case you have to be careful of shock
formation on the airfoil.  Some airfoils will perform nasty tricks in the
transonic regime.  This is why it is best to stick to proven airfoils in
this case.  If the Riblett profile has been tested in a transonic wind
tunnel at the proper Reynold's number, as well as on a vehicle similar to
what you are thinking about, have at it.  Considering that you will not
likely see significant performance differences from tweaking the rotor
profile on an experimental, GA aircraft, you might want to employ safety as
your guiding factor.

Good luck with your gyrocopter!

Walt Lounsbery

Terrben <terrben@aol.com> wrote in message
news:19991201113935.19422.00000315@ng-co1.aol.com...
> I purchased Harry Riblett's "GA Airfoils" booklet at Osh Kosh this year
> and have been digesting it since.  He makes a compelling argument for
> avoiding many of the popular NACA and NASA airfoils and using his
> "evolutionary" airfoils, instead. His airfoils, he claims, stall softer,
> lift better, and are within in a drag count or two of the slickest NASA
> airfoils without the detrimental characteristics they apparently
> exhibit.  His reasoning and evidence seem solid.
> 
> My question is:  If what he says is correct, why is there not a strong
> move by designers and homebuilders to incorporate his airfoils into
> their projects?  Am I missing something?
> 
> On another tack but still in reference to Mr. Riblett's work, rotor
> blades usually, if not excusively, use very low pitching moment
> airfoils; not necessarily symmetrical but with Cm's of essentially zero.
> Mr. Riblett's airfoils are derived from actual or modified NACA
> thickness distributions which are, in fact, symmetrical and are,
> apparently, quite good airfoils in their own right for uses requiring
> low pitching moment (tail surfaces, aerobatic wings, and, I would think,
> rotors).  Would anyone be willing to comment on the use of Mr. Riblett's
> symmetrical thickness distributions as the planform of choice for a
> gyrocopter rotor blade?
>
> Regards,   Terry Bendickson

From: David Lednicer <dave@amiwest.com>
Subject: Re: GA Airfoils
Newsgroups: rec.aviation.homebuilt
Date: Fri, 10 Dec 1999 08:38:35 -0800

In article <msieweke-0612991231400001@prt-or29-234.ix.netcom.com>,
msieweke@nospam.ix.netcom.com (Mike Sieweke) wrote:

> I'm not sure what you are saying here.  It is pretty obvious that an
> airfoil with a high moment coefficient will cause a relatively large
> tail down load at cruise.  This tail load increases the lift required of
> the wing, and thus increases the induced drag.  Choice of an airfoil
> with low moment coefficient will reduce induced drag over an airfoil
> with high moment coefficient, all else being equal. There is a common
> misconception that a tail down load can be compensated by "thrust" from
> the horizontal tail operating in the wing's downwash. NASA tests
> indicate that this is false, and that a negative tail load always
> increases drag.

  Mr. Riblett's arguments were presented in a forum at Oshkosh.  I
think the forum was in 1988.  He claimed that the pressure distribution
of a 2D airfoil could in itself reduce induce drag.  At the time I was
working for John Roncz, and I remember spending some time discussing
Mr. Riblett's assertions.


> Where did you hear that Riblett called for the grounding of the
> ATR-42?

  You obviously don't read the letters column in Aviation Week and
Space Technology!  He had a letter published there after the Roselawn
ATR-72 crash.

> Unfortunately, Lednicer shows a limited knowledge of Riblett's
> work.

   No, I think you do.  I have read his book, heard his forums, read
his letters to me and argued with him on the phone.

> I have never heard of this from anywhere else.  My readings indicate
> that washout is used to tailor stall characteristics.  When NACA tested
> tapered wings, they discovered that several tapered wings with zero
> washout performed as efficiently as the elliptical wing (i.e. high e
> factor).  Some tapered wings had higher e factors than the elliptical
> wing NACA tested.

   Washout is usually used to tailor stall characteristics, not span
loading.

> Also witness the BD-5A, which had unusually high stall speed due to bad
> airfoil choice,

  Most BD-5s that have nasty stalls, have the characteristic due to the
method of wing construction.  They make the leading edge bend in such a
way that a very tight radius develops in the metal, giving a full span
stall strip.  BD-5 wings constructed by creating the leading edge bend
in a better fashion have better stall characteristics.  This info comes
from a BD-5 builder I know.

> the Questair Venture, which had to graft on several mods to tame the
> dangerous stall characteristics of the 230xx airfoil series.

   The 230XX airfoil series has a sharp, not dangerous stall.  There
are many airplanes flying with the 230XX airfoils that stall just fine.

                         -David Lednicer

From: David Lednicer <dave@amiwest.com>
Newsgroups: rec.aviation.homebuilt
Subject: Re: GA Airfoils
Date: Wed, 08 Dec 1999 08:59:47 +0000

Gee, thanks for the compliments Highflyer!  By and large I agree with
your post.  Just a few comments:

> Washout is generally used to tailor the lift distribution along the
> wing to optimise it.  Before they figured that out, there was a
> tendency for designers to design beautiful elliptical wing planforms
> to ensure the desired elliptical lift distribution without washout!

I am not certain which aircraft you are referring to here.  If you are
referring to the Spitfire, it did have washout.  My analysis shows that
it didn't have an ellptical loading due to this.  R.J. Mitchell used the
elliptical planform to get greater airfoil depth outboard, without
increasing airfoil t/c.  He needed the depth for the outward retracting
landing gear and the ammunition trays.

> This can be degraded if the wing is allowed to deform when it picks up
> a load, and render the stall less predictable, as in the Tomahawk.

An even better example of this is the Fw 190.  Under load, as much as
50-60% of the wing stalls all at once.

> Actually, a "symetrical" airfoil does not often have a zero moment
> coefficient.  In fact, the symetrical airfoil may have a moment that
> is opposite that of the cambered airfoil.

Here I disagree.  Most symmetrical airfoils have near zero moments until
separation begins to develop on the airfoil.  This is why they were long
used on helicopter rotor blades.  Modern helicopter rotor blades now
have cambered airfoils, but this is because the means have been found to
design low moment cambered airfoils.

> I have heard it said that this moment coefficient of the 230xx series
> airfoils is what made the monospar wing of the DC-3 possible.

Interesting idea, but not true.  The root airfoil on the DC-3 is the
NACA 2215 and the tip is either the 2209 or 4412, depending on who you
believe (I am still working on tracking this one down).

-------------------------------------------------------------------
David Lednicer             | "Applied Computational Fluid Dynamics"
Analytical Methods, Inc.   |   email:   dave@amiwest.com
2133 152nd Ave NE          |   tel:     (206) 643-9090
Redmond, WA  98052  USA    |   fax:     (206) 746-1299

From: David Lednicer <dave@amiwest.com>
Subject: Re: GA Airfoils
Newsgroups: rec.aviation.homebuilt
Date: Mon, 13 Dec 1999 09:18:10 -0800

> Interesting.  What is induced drag anyways?

  Induced drag is a 3D effect that comes from the creation of lift.
Lift is created by deflecting a volume of air downwards.  This means
the deflected air has a small vertical velocity, called downwash.  This
downwash has an influence back at the wing, raising the angle of attack
slightly.  This then tilts the force vector, which is normal to the
oncoming flow, back slightly, resulting in small component of force in
the drag direction.  This small component is called induced drag.
There is no way to escape induced drag, but there is a way to minimize
it, by tailoring the 3D lift distribution.

> Do you know what issue this was in, or perhaps the year?  I would
> like to look it up.  So far I have only your assertion that Riblett's
> stance was not reasonable.

  I'll have to dig for this.  It appeared quite a while after the
Roselawn crash.  We all got a chuckle, but I didn't save a copy.

> The factory demonstrator BD-5 also had unusually high stall speed.

  From what I have been told, the factory demonstrator had a wing
constructed the "wrong" way.  I have been told that BD-5s constructed
the "good" way have much better stall characteristics.  However, I
think you have a point - some of this sensitivity is probably due to
the low Reynolds number at stall.

  Yes, I can picture where yawing can change a OK straight-ahead stall
into a bad stall.  With one exception, I have only stalled Cessnas
which strike me as quite benign.  My exception is the Tomahawk.  There,
I was more aware of the tail buffet than the sharpness of the stall.
Come to think of it, I have also "stalled" a 777 fixed base simulator,
but there the stick pusher was really what I was experiencing.

  However, look at the list of aircraft that have NACA 5-digit
airfoils.  (http://amber.aae.uiuc.edu/~m-selig/ads/aircraft.html).
There are a lot of them.  Do they all have nasty stalls characteristics?

                        -Dave Lednicer