Conventional RES Airfoil Choice - AG why?

[atjurhs]

I am wondering if the AG family of airfoils would also be the best choice for a "Conventional RES Sailplane"?

What's become of all the other (Epplers, Selig, Hepperle, HN, and Rolf Girsberger) airfoils? Do they just not compete against the AG family when it comes to a RES sailplane?

Or are we all just one of many following the same trend, dare I say fad? I don't intend to defame or belittle the AG series, I'm really just curious why so many folks are going to this airfoil choice, and what's wrong with the others, or what the AG has over them?

I suppose I should define what I mean by "Conventional RES Sailplane", one that is NOT constructed like the AVAs and Bubble Danncers, etc. etc. - a sailplane that has a traditional fuse and tail that we all knew and loved 10 years ago, back before boom and pod and V mount construction.

If this were the case, would the AG series still be your first choice? Is whatever there is that is so magical about the AG series really only applicable to the AVA type planes?

Thanks for your insight.

[Ollie]

A better question: Which designs for RES?
I quote:

"Design Philosophy by Don Stackhouse

"There have been some rather heated discussions lately on the R/C Soaring Exchange about the relative merits of computer analysis versus old cut-and-try methods in the aerodynamic design of wings. Similar threads have argued over the shape of the "ideal" planform, the "best" airfoil, the "optimum" aspect ratio, and the validity of modified airfoils or of blending from one root airfoil into a different tip airfoil. All this controversy reminds me of one of my favorite stories:

"The Four Blind Men and the Elephant (an old Hindu parable)

"One day four blind men encountered an elephant for the first time. They approached it cautiously, but with great curiosity. The first one grabbed hold of the trunk and declared "Aha! An elephant is just like a snake!" The second found an ear and replied "No, an elephant is exactly like a tent." The third bumped into a leg and decided the elephant was just like a tree, and the fourth caught the tail and maintained that the elephant was just like a rope. They all went home arguing, each steadfastly insisting that he was right and the other three were wrong.

"The flight of a model sailplane is a complex phenomenon, each portion of the model seeing its own unique set of conditions at any given time, yet still having an influence on all of the other parts of the model at the same time. In addition, we expect our models to perform well at a wide variety of operating points within the overall flight envelope. To help us achieve this aim we have available a large database of experimental data and theoretical analysis tools, plus the insight garnered from all of our own experiences and the experiences of others, and the results of actual tests of the model. The results depend on how well we use all of this information together to reach the final design.

"If we get hung-up on one parameter, or one design technique, or one phase of the design process, we automatically give ourselves a case of "tunnel vision". There is no single airfoil, aspect ratio, planform, tail size or type, etc., that is optimum at all flight conditions for even a single model, much less a variety of models.

"While it is true that the section at the mid-span of a wing of two wildly different tip and root airfoils may have or may not have any of the characteristics of its parents, it is also just as risky to believe that the airfoil that is optimum for the conditions at the root will be equally appropriate at the tip. Ideally you should study the sections at a variety of points along the wing, as well as local chord, twist, flow characteristics, et cetera.

"The effects of what is happening at the tail, along the fuselage, along the span of the wing, all influence each other in different ways at different flight conditions, with corresponding effects on the overall control, stability and performance of the model.

"None of our design tools is perfect. None of our data is completely reliable at all conditions, and some of it isn't very reliable at any condition. In my experience the best approach is to use all of the available tools and data to the fullest extent possible, then look for the consensus forming between the different approaches. This way the strong points of the different approaches can compensate for their individual shortcomings. The first '93 Monarch hlg went through 150 hours of computer work, PLUS six fuselages, seven tails and eight wings before we froze the design.

"If you try to build an elephant with only a tree, or only a rope, a snake, or a tent, your result will almost certainly fall short of your expectations. Likewise, the model that was designed with only theoretical methods, or only past experience, will probably not be the best design possible. Only by using all of the available tools as cooperative members of a team effort can you achieve a design that is more than just the "sum of its parts"."

My opinion is the "best" airfoil depends on the lift spanwise distribution, mass spanwise distribition, airfoil thickness (spar: strength, stiffness), wing planform (AR, twist, tapers, chord length range, etc) wing loading, range of Cl, range of Cdo, Cm, Re range, air speed range, etc.

See:
http://www.charlesriverrc.org/articles/drela-airfoilshop/markdrela-ag-ht-airfoils.htm
And
http://www.mh-aerotools.de/airfoils/index.htm
Index: Airfoil, sailplanes; Other topics, Aerodynamics, Designing an Airfoil

[atjurhs]

Yes Ollie, exactly! My intetnion here is NOT to kick the shins of Dr. Drela and all the work that he has done in derriving the AG series of airfoils (and all the benefits that modelers have gained from his work, for that I thank him), but simply not to be one of "The Four Blind Men".

Finding information about the design choices (input parameters) that Drela Eppler, Selig, or Hepperle used in the development of ANY of the thousands of airfoils availble to choose from seems nearly impossible!

To quote from above, "There is no single airfoil, aspect ratio, planform, tail size or type, etc., that is optimum at all flight conditions".

I fully agree and understand, so perhaps the better question I should have phrased goes something like, "How well will the AG series of airfoils perform at twice the wingloading (10-12 oz/ft²) and 50% faster airspeeds?" If Dr. Drela had used these design parameters in XFOIL, would he still have come up with the AG35-38 that we seem to gravitate twoards?

[spatial]

I don't think they would work particularly well to be honest. If you check the Drela airfoil shop (http://www.charlesriverrc.org/articles/drela-airfoilshop/markdrela-ag-ht-airfoils.htm) on the Charles River web site you'll see the AG35-38 series is designed for light built-up 2m and 3m poly ships, and AG34-36 is listed as appropriate for heavier 3M poly gliders. The notes on the AG34.dat read "thicker version of AG35, for large/heavy gliders".

[markdrela]

Some of the airfoils are not as different as you think.
Overlay the AG34 and S3021 --- they are pretty darn close.
Same for the AG40d and MH32.

But the new thing I've tried to do is make airfoils in groups, to be used across the span to account for varying Reynolds number. So for example, the AG40d and MH32 both work well at the center of a 3m TD glider wing, but the MH32 gets really bad near the tips. In contrast, the AG41,42,43d work properly all the way to the tip. One common technique to alleviate this has been to simply thin the airfoil near the tips, but this doesn't work well -- the thinned airfoil loses a lot of speed range and maximum lift capability. For example, the AG43d is very different from a MH32 thinned to the same 6.5% thickness.

Spanwise tailoring has gotten more and more important as TD glider weights have come down. The MH32 works best on ~100 oz gliders (it's a slope section), and also here the spanwise tailoring is less important since the tip Reynolds numbers aren't all that small. On a 50-60 oz glider, the MH32 really starts to suffer, and completely falls apart at the tips.

[Ollie]

atjurths,
What do you mean one "Conventional RES Sailplane"?
Wing span?
Wing area?
Weight?
Lift distribution?
Chord distribution?
Maximum G's?
Stalling air speed?
Mass spanwise distribution?
Spar design? ($5 wood spar or $50 caps carbon +balsa shear web + wrap)
Etc, etc, etc.

Read about Dr. Drela's designs:
Group Information
Members: 2422
Founded: Oct 16, 2000
Language: English
http://groups.yahoo.com/group/Allegro-Lite/?yguid=108420033

Quote from Dr. Drela:
"Mini-quiz: Consider the following range of weight versus span, from a modern 1.5m DLG, up to a modern light 3m TD ship:

span weight
1.5m 10oz
2.0m ??
3.0m 65oz

Q: For these to fall along a straight line on a log plot, what should the 2m weigh?
A: ...drumroll.... About 22 oz!

"It's no wonder that the typical 30-40 oz full-house 2m glider flies like a lead sled compared to the DLG or 3m ship. Bleah. I think this is one reason why 2m ships have dwindled in numbers -- they're just plain unpleasant to fly in weak lift, and for good reason.

"This observation is what prompted me to build the 2m Aegea, which at 21 oz is quite fun to fly in the weakest lift. It handles much like an aileron DLG, and can be comfortably flown as a javelin HLG by anyone with a half-decent arm. I'm convinced that if 2m glider weights had stayed in the 20-25 oz range rather than spiralling out of control, there would be a lot more of them today.

"One intriguing possibility is discus-launching such a glider, which would allow easy operation of a good-sized thermal glider without the hassle of a winch or histart. I considered putting a tip peg on the Aegea, but I didn't have the time needed to figure out a good way to make the plug-in joints take the spanwise tension load. It's certainly an attractive prospect."

More:
"The objective for an RC glider is not only to fly slow, but to fly slow OR fast as needed. Being able to fly fast is important if you fly in any significant wind. I would switch to a lower-camber airfoil, and reduce the wing loading if you want the same slow flight capability as before. You can just increase the chord by maybe 10% to do this. This new wing will be able to fly faster than the old wing, despite its larger wing area."

"Compared to spruce, plain carbon has 15x more stiffness,
and 20x-40x more strength. Which strength factor to use
depends on how well the carbon is restrained against
delamination and buckling.

So you want 0.014" carbon to match the stiffness,
and 0.014" or 0.007" carbon to match the strength.

- Mark"

"From Joe's description he sent me some time ago, the Icon was designed considering the sink rate, L/D, and launch height in some combination. Joe can probably fill in here.

" The problem with talking about "optimum" Aspect Ratio is that it's almost impossible to define in a realistic design problem. You can optimize AR for min sink rate or max L/D in straight and level flight, but RC gliders do not spend their life in straight and level flight at the optimum airspeed. There's also launch, zoom, circling slowly, running back, landing, etc, etc. The end result strongly depends in how these factors are weighted against each other.

"The main contributions to total drag are
1) Profile drag, varies as 1 / Re^0.5 or so
2) Induced drag, varies as 1 / AR
Most people are probably not aware of this, but there is a third source during maneuvers
3) Unsteady-lift drag, varies as 1 / AR
This becomes important when "pulling G's" during slow flight, such as when working a spotty thermal. Since this 3rd component also varies 1/AR, its presence favors a larger AR than if it were ignored.

"So the bottom line is that the Supra's large AR is better geared towards working small spotty thermals. You can crank it around harder than usual and it won't lose energy as much as a lower-AR glider. The combination of narrow tips and slight washout also helps here."


Small samples from my files about Dr. Drela writing.

[Trisquire]

So apparently some airfoils are efficient at the root of the wing, but not at the tip. What are the draw backs of a constant chord wing? Remember the Thermal Queen?

Tom

[atjurhs]

Ollie, yes I agree the question is rather complicated.

For sake of discussion, let's define generally a "Conventional RES Sailplane" to be:

120inch WS (30 inch main panels, 25inch outer panels, 5 inch tip panels)
1000 sq.inch Waing Area
9.5inch MAC
CF used for wing construction to attain stiffness
60oz
2inch wide FG fuse cross section
Whatever your favorite tail is T, V, X

A RES that in days gone by probably would have spec'd the E205 or S3021

Hopefully these numbers give us room for discussion

[Mike K]

What are the draw backs of a constant chord wing?

Vortex drag increases. Martin Simons covers these points relatively well in his Model Aircraft Aerodymanics book.

Regards.
Mike K.

[atjurhs]

Mark,

Thank you for chimming in the discussion!

I too landed upon the AG34-36 series based on the Charles River Website information. I find it interesting that you mention the AG40d, 41, 42, and 43d. Could you please amplify your suggestion/statement relative to the "Conventional RES Sailplane" that we've defined?

[Mike K]

I understood that the 40 series was designed for composite construction, whereas the 34-38 series was equivalent, but for built-up construction.
So am guessing that the discussion would hinge upon the construction method used.

Regards.
Mike K.

[atjurhs]

I understood that the 40 series was designed for composite construction, whereas the 34-38 series was equivalent, but for built-up construction.
So am guessing that the discussion would hinge upon the construction method used.

Regards.
Mike K.


Ok, let's say it is balsa build-up with CF reinforcements wherever needed, so we have a very stiff wing for no flexing during winch launching.

[Ollie]

Just use the BD plans! You couldn't design it better, in my opinion.

[BMatthews]

Just use the BD plans! You couldn't design it better, in my opinion.

Maybe not "better" but "different" has it's own intrest as well... :D I'm at the point where I would prefer to give up a little bit of absolute performance in order to have a design with some charisma that makes me smile while it flies overhead. For me that means fatter fuselages with at least the suggestion of the full scale types. I know that they are efficient and that efficiency has its own beauty but the current crop of fishing poles with wings just leaves me cold. That's probably why I've been enjoying my free flight old timer models so much over the past two decades.

Trisquire, the reason the airfoils work better if they change has to do with the local airflow and lift coefficients. Wings have varying degrees of spanwise flow for a number of reasons as you move along the wing and altering the shape of the airfoils can better tune the airfoil to the local needs.

Constant chord wings will experience this as well. But with the constant chord the optimum airfoil choices will change as well.

PS: OK then, BD planform and flying surfaces with a new fuselage and "proper" hinges for the tail surfaces... :D

[Mike K]

Ok, let's say it is balsa build-up with CF reinforcements wherever needed, so we have a very stiff wing for no flexing during winch launching.

Point is, no matter how good you are at building and covering the design, there will be differences in built-up construction that will not be present (to the same degree, at least) with composite construction.

Correct me if I am wrong, Dr. Drela, but everything I read points to the 30 series being optimized for built-up models, with accuracy of the airfoil being within a few hundredths of an inch, whereas current composite molded ships could get within a couple of thousandths. And the 40 series was directed to these molded or bagged ships with their higher, more exacting tolerances.

Regards.
Mike K.

[Ollie]

From my files:

Written by Vern Bailey:

History and Technology - The Bubble Dancer Backstory
Revealing their kinship in the photos, Allegro-Lite and Bubble Dancer
were actually designed at about he same time in Fall, 2000. In the design,
Drela consulted extensively with Californians Bob Parks, Blaine Rawdon,
and Bill Watson via email. Part of the glider's heritage comes from Blaine's
112" Mirage poly glider, designed in the mid-70's. The Paragons and Sailaires
of that era featured thick, flat bottom airfoils which suffered when running
back upwind. In contrast the Mirage introduced a penetration-type airfoil
and yet retained excellent thermalling qualities via a very low wing loading
of 5.5 oz per sq. ft. . This proved a most potent combination for soaring,
especially in tight, low thermals. Blaine used his Mirage to win the first-ever
hand-launch contest in 1976! The Bubble Dancer design extends the Mirage's
features a bit further, with more modern custom-designed airfoils, and precise
use of carbon-capped balsa in lieu of spruce to achieve winch-stalling strength.
A bulletproof Kevlar pod, a carbon boom fuselage and lightweight tail feathers
completes the design.
Bubble Dancer challenges the wide-spread perception in the RC soaring community
that built-up construction automatically implies an impaired ranging ability.
With Xfoil simulations, Drela identified the real likely culprit to be covering sag.
Even D-tube sheeted wings still have some sag over the rear upper surface which is
critical at higher speeds. Drela then designed the AG35,36,37,38 airfoils specifically
to sidestep this problem in the wing structure of Allegro-Lite and Bubble Dancer.
On these airfoils, the upper surface behind 40% chord is intentionally composed of
three flat facets, which can be spanned perfectly by the film covering with no sag.
The absence of sag makes the airfoils work exactly as intended, and gives penetration
performance which appears to rival that of composite wings of similar camber and
wing loading. The AG3x airfoils also have a flat bottom surface behind the 30%
chord location for easy built-up assembly on a flat surface.
The carbon/endgrain balsa spars on the Allegro-Lite and Bubble Dancer
give very low weight for the required strength, especially at the wingtips.
The carbon boom and carbon mount for the all-moving stab likewise enable
a very light empennage -- everything behind the trailing edge on the
Allegro-Lite weighs less than 1.5 oz! Such light extremities produce a low
yaw inertia, crucial for good handling qualities on a rudder/elevator glider.
Adding generous dihedral and a powerful rudder gives a nimbleness invaluable
in working small thermals near the ground, especially for the smaller Allegro-Lite.
In windy conditions, adding the desired ballast at the CG necessarily diminishes
small-thermal capabilities, but considerably improves the damping and control qualities.
An oversize central spoiler allows good glidepath control for contest spot landing tasks.
Admittedly, the landing precision of a flap/aileron system cannot be fully reached
by RES, but in the hands of a skilled pilot the low yaw inertia and good control
characteristics can make Allegro or Bubble Dancer competitive.

Bubble Dancer
Drela's own prototype Allegro-Lite proved a huge success. It has won
every RES contest it was entered in, and has placed near the top in an
Open TD contest, flying against mostly 3-meter composite ships. Competition
has proven repeatedly, however, that, all else being equal, two-meter models
can't win against the top-notch models of the inherently more efficient open
or three-meter category. Aerodynamic scaling laws can be pushed but not cheated.
The answer is the 3-meter Bubble Dancer.
Although Drela delayed building his own Bubble Dancer while the Allegro-Lite
internet effort was gathering steam, a number of the group members pressed ahead
with foam-wing versions of Bubble Dancer. Ollie Wilson of Florida and Mike Glass
of Texas were among the first to build composite Bubble Dancers, and Rick Hallett
in Maine has now built six, striving to reduce weight each time. Although these
gliders perform very well, with composite construction it is virtually impossible
to reach the low weight and low yaw inertia that can be obtained with built-up
construction.
Encouraged by the very favorable flight reports of the composite prototypes,
Drela finished the first built-up Bubble Dancer in February 2002, with an
all-up weight of only 31 oz, and a wing loading of 4.5 oz per sq. ft.
(see photos) Initial tests of the glider revealed a remarkably
small minimum-turn radius and almost no tendency to tip stall.
On the third hand-toss during initial glide testing, Drela stumbled
into a small bubble and easily climbed out (danced?) for a long flight.
The small-thermal capabilities of Rawdon's Mirage have clearly been
matched and exceeded. Full-pedal winch launches have demonstrated
the glider's strength and zoom capability. When ballasted for wind,
it shows its legs -- a ranging ability which rivals that of the SchpotDorkers.
And in light air conditions, it will be able to hook weak or tiny thermals
which a SchpotDorker can only nibble at.

[atjurhs]

Maybe not "better" but "different" has it's own intrest as well... :D I'm at the point where I would prefer to give up a little bit of absolute performance in order to have a design with some charisma that makes me smile while it flies overhead. For me that means fatter fuselages with at least the suggestion of the full scale types. I know that they are efficient and that efficiency has its own beauty but the current crop of fishing poles with wings just leaves me cold.

Bruce,

BINGO!!!!!!

I couldn't agree with you more!

Ollie,

You remind us of the "Four Blind Men" Hindu proverb, and then you say just go grab the BD plans, that I can't do it better. You are right. I can't improve upon them, nor is that my question, nor is that my goal. In my opinion, the BubbleDancers and AVAs (and like ships) are the very best at what they do - TD.

My goal and question is not to further examine the merits (or shortcommings) of those designs, but to understand what airfoils make for an excellent all-around RES ship. If I wanted to just grab something off the shelf, I'd grab somthing like the Houston Hawk or a Scooter 3M, and I may will do that. I just don't want to be a "Houston Hawk Blind Man" either. I'm looking to understand the design criteria behind the sailplane, and more specifically the airfoil, and I think a lot of folks can benefit from this discussion.

[Ollie]

"I'm looking to understand the design criteria behind the sailplane, and more specifically the airfoil, and I think a lot of folks can benefit from this discussion."

Compare designs. Try up your designs. Use PC Soar:
http://my.athenet.net/~atkron95/pcsoar.htm

And
http://www.profili2.com/eng/default.htm

[spatial]

AVL is great tool for exploring planform design. It makes very clear the overall lift distribution of a wing design...

http://web.mit.edu/drela/Public/web/avl/

[schrederman]

Maybe not "better" but "different" has it's own intrest as well... :D I'm at the point where I would prefer to give up a little bit of absolute performance in order to have a design with some charisma that makes me smile while it flies overhead.

Amen to that. My Houston Hawk is in my opinion a "typical RES sailplane." In the right hands it is a formidable performer. It uses the S3021 airfoil. The models have weighed in at between 54 and 60 oz. I'll be the first to admit that they would possibly perform better with Mark Drela's AG series airfoils. The VERY few that are flying and the number of wins they have racked up is testiment that they are good performers. Their main drawback is that they are not available as an ARF. Another drawback is the guys that claim to be kitting them. No one can seem to get in contact with them so the project has basically died on the vine, while the AVA and other Bubble Dancer derivatives have flourished due to their availability as ARFs. Then again, the model proved it's worth as a teaching tool.

Happy thermalling...

Jack Womack

[histarter]

Amen to that. My Houston Hawk is in my opinion a "typical RES sailplane." In the right hands it is a formidable performer. It uses the S3021 airfoil. The models have weighed in at between 54 and 60 oz. I'll be the first to admit that they would possibly perform better with Mark Drela's AG series airfoils. The VERY few that are flying and the number of wins they have racked up is testiment that they are good performers. Their main drawback is that they are not available as an ARF. Another drawback is the guys that claim to be kitting them. No one can seem to get in contact with them so the project has basically died on the vine, while the AVA and other Bubble Dancer derivatives have flourished due to their availability as ARFs. Then again, the model proved it's worth as a teaching tool.

Happy thermalling...

Jack Womack
Jack you are underrating your Hawk. It is far better than "typical"!

The magic of flight is conducted by the pilot, not the profile. The pilot's skill is to utilize all the attributes dealt him, and those pilots that shine can really milk duration out of an exposure by knowing how long to hang onto lift if flying a bit slow because of profile, or when to bail out and go for better, when flying a bit fast. Sink is a cross to bear, however when launching a model to 700 ft and manipulating controls for positioning, trimming, steering, and setting up for a spot landing, one will find he is sinking at 2'/sec as an average. A Full size sailplane zapping along at 50 mph with an L/D of 40 is also sinking at 2'/sec. And the model has the ability to really core a thermal with 1/6 the spiral diameter when working the same air!
The bottom line: When the pilot takes the time to get intimate with his existing profile, he will do a lot better than the pilot that flys profile d'jour, or the pilot that seeks the magic profile that will enhance, or overlook his shortcomings! :D
Check out histarter's latest blog.