Thermal Wing Design, Input Appreciated - RC Groups
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Mar 04, 2010, 07:22 PM
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wardo78's Avatar

Thermal Wing Design, Input Appreciated

Hello All,

I recently finished building my MWC 84 ( and have been enjoying it quite a bit. I always had in mind that I wanted to build a 3+m thermal duration wing to play around with this summer. I've read every thread I could find on the subject, and understood some of what I read. Originally I thought to find an existing design and replicate it with some mods, I may still do that, but after playing around with Curtis' (CloudyIFR) wing program (which can be downloaded here: ) I felt encouraged to try my hand. The program does an excellent job of doing much of the number crunching, although GIGO is applicable as always. In addition to reading many threads, I've PM'd with NMasters, CloudyIFR and Minphase. I want to thank them for getting me going. I've looked at many euro websites and Vern Hunt's work. I am not sure how much I've understood of all I've seen. Minphase encouraged me to just give it a go and solicit input.I know there are a lot of other very knowledgeable community members, so I thought I'd stop reading and put something together.

I am in this for fun, so this doesn't need to be perfect. I want a TD oriented powered wing that is of my own design. The wing doesn't need to break down, because I fly out my back door. It doesn't need to be fast, I've got that covered elsewhere. It should look good, thermal well, climb quickly and have no nasty surprises. The output from Curtis' program is below. Going into it, I knew I wanted a tapered high aspect ratio wing in the 3m range. I also wanted a Yehudi which NMasters had recommended in this thread: . I figured it looks good, gives me a thicker center section for electronics and allows me to mount a longer, more centralized spar. I'll use winglets and maybe a center fin underneath. For construction, I'll have the cores cut and I'll cover them with glass, CF and epoxy (no bagging, low tech). I'll put in some solid spars from Goodwinds ( and/or a D-Cap. Obviously, I am only in the very preliminary stages, so there is a lot I don't know.

Total Wing Results:

Total Span 120.00 in.
Total Area 1200.00 in2
Wing Loading 9.60 oz/ft2
Effective Wing Loading 12.48 oz/ft2
Mean Chord (area/span) 10.00 in.
Mean Aerodynamic Chord (length) 10.80 in.
Wing Aspect Ratio 12.00
Wing Taper Ratio 0.11
Location of 0% point 13.28 in.
Location of 25% point 15.78 in.
MAC distance from root 25.33 in.
Average .25 chord sweep angle 24.00 degrees

Pankin Wing Twist Comps:

Reynolds Number
Speed 35 mph
Result 273000

Airfoil Data
Root Zero Lift Angle -3.650
Root Cmo -0.097
Tip Zero Lift Angle 1.730
Tip Cmo 0.025
CL Manually Calculated 0.60 Note: Enter the number "zero" to have Cl automatically calculated
CL Automatic Value 0.600
Static Margin 5.00 %
Aerodynamic Twist -7.91 degrees

Geometric Twist -2.53 degrees
Geometric Twist 0.26 inches

Speed Calculator
Estimated Speed 35 mph Cl = 0.192
CL = 0.6 Speed = 20 mph

There is a lot above that I don't completely understand. I know the individual definitions, but the interrelationships are a bit fuzzy in certain instances. There are a few concerns and questions that come up right away, here are a few in no particular order:

-I haven't selected an airfoil yet, there are tons to choose from and I am poorly equipped to differentiate. I do know I would like a thin airfoil, but not so thin as to be weak out at the tips.

-Is the taper too severe? I can build it pretty stiff, but if the design is flawed, that might not be enough.

-The Pankin formula shows two different twist numbers, aero twist at -7.91 degrees and geo twist at -2.53 degrees. Is the difference attributable to the sweep?

-What is a realistic speed to use for calculations?

-I know from my limited experience that a powered wing can be tricky to balance. With the Yehudi the motor needs to be even further back. Is there a proven way to mount the motor forward and have a long, possibly supported prop shaft to assist in balancing the wing?

-Is there any history of using spoilers versus flaps on wings? I am thinking of using long, narrow ailerons and would rather avoid flaps if possible.

-What haven't I asked that I should have?

There is little chance I'll learn everything I need to know anytime soon. Hell, I don't need to know everything about wing design, or even this wing design. I just need help identifying problems early and making relevant adjustments. This is my first pass at this, so it may be way off. I am looking forward your input.

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Mar 04, 2010, 08:07 PM
Red Merle ALES VI
Curtis Suter's Avatar

Very nice, I can't wait to see the flight results! Oh yea, the building has to start first.

>>-The Pankin formula shows two different twist numbers, aero twist at -7.91 degrees and geo twist at -2.53 degrees. Is the difference attributable to the sweep?<<<

On the Glossary tab of the spreadsheet I wrote:
Aerodynamic Twist
The angle between the zero-lift angle of an airfoil and the zero-lift angle of the root airfoil." In essence, this means that the airfoil of the wing would actually change shape as it moved farther away from the fuselage
Geometric Twist
An actual change in the airfoil angle of incidence, usually measured with respect to the root airfoil.

Perhaps this will help better:

>>>>>-Is there any history of using spoilers versus flaps on wings? I am thinking of using long, narrow ailerons and would rather avoid flaps if possible.<<<<

Are you planning on full span ailerons? The aerodynamics are quite out of my expertise but a flying wing is like any other plane, it has an elevator and ailerons, normally they are the same surface and called elevons. But you know that. However, I think efficiently speaking the wing will perform better with using outer elevons and leaving the inner section of the airfoil intact. This is where the lifting occurs in a flying wing anyway. Please don't confuse reflexed airfoils and what you're trying to accomplish using the Pankin method. You're method is superior, IMHO.

Hope this helps some, I'm sure Herk will chime in, it's his original design scaled up.

Mar 04, 2010, 08:28 PM
Registered User
wardo78's Avatar

Thanks for all your help. I ended up starting from scratch, although I did use the Tinamou as one of my major data points given it's proven design and excellent documentation. For the moment, I am using your data from the SD7037 airfoil for the Pankin calcs. Obviously, if I select something else, the airfoil data will change. I am not sure how you divined the zero lift angle for the tips, I do see the root. I know you used different airfoils, maybe this explains it. In my case, it is possible that I will use a different airfoil for the inboard portion of the wing and start the twist past 10". I'll need some help sorting through those calcs, but it is too early to worry about as I need to settle a few other basic decisions first.

I am not sure what I was thinking about 'rons, but I think outboard 'rons with a solid inner trailing edge would be nice. I imagined spoilers would be viable, but I'd love to hear more from others who have tried them.

Mar 04, 2010, 11:31 PM
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pval3's Avatar


off to a good start
i am subscribed!
Mar 05, 2010, 01:00 AM
internet gadfly
nmasters's Avatar
Originally Posted by wardo78

-Is there any history of using spoilers versus flaps on wings? I am thinking of using long, narrow ailerons and would rather avoid flaps if possible.
At this time of night I can barely read, much less wright, so I'm not commenting but I just found this video of a glider with spoilers for yaw yesterday:
Nurflügel horten rc (1 min 28 sec)

Not saying that it's perfect but it's interesting. Good night all. I'll be away for the weekend so I'll check in on this thread Monday
Mar 05, 2010, 04:21 AM
the answer 42 is
Hey Wardo,

we are looking for the same, I will probably go fro soemthing smaller to star, also i will be using an electric motor (laying somewhere on my shop) Looking forward for your design.

I was wondering what happed if we use an autostable profile (e.g. PW51) on a sweep flying wing?

Mar 05, 2010, 05:13 AM
Registered User
wardo78's Avatar
Phil, nice to have you aboard.

Norm,That is a very cool video. I can envision how they are using the spoilers, but it is hard to see. It did appear to have a nice coordinated turn, even at slower speeds.

I envision putting the spoilers inboard more, and using them strictly for lift degradation. My original concern was that the moment between the pitch control surface (elevon) is much smaller than a conventional glider, thus leading to difficulty compensating for the pitch change when deployed. I realize that spoiler placement is key, are they usually placed over the neutral center of lift?

EZ, thanks for chiming in. In my research, it appeared that you had a ton of great resources available online in Deutsch. I can see the pretty pictures, read the numbers and understand every fifth word. I haven't really looked at profiles yet, and am REALLY not qualified to weigh in on your suggestion. Maybe on of the more learned readers can add something. My wing will also be powered by an inrunner. I imagine the set-up will be very similar to my MWC 84, although I'd love to come up with a way to move the weight of the motor forward, and the folding prop further aft so it is in cleaner air.

Mar 05, 2010, 07:19 AM
HerkS's Avatar
Hi Wardo,

Looks like this will be an attractive project. Very nice layout.

Regarding spoilers - yes, they will work on a wing like this. There are two issues that need to be understood. First, if they are top surface spoilers, they should be located out near the MAC. Too far from the MAC and they will have a strong pitch effect when they are deployed. If they are on the bottom, then they will work OK if they are more inboard. Second is lift. Spoilers on the top reduce the lifting capability of the wing. Since flying wings have reduced lift capability to begin with, top spoilers reduce that further. Thus they can actually increase the landing speed of the model.

Inboard flaps on the other hand, increase the wing's slow flight capability and introduce a favorable lift distribution at slow speed.

Regarding motor placement, you will probably find that a model like this needs quite a bit of nose weight. Placing the motor on the front will help with this, and it will also make the model much easier to hand launch.

Regarding design trim speed, 35 mph is a bit high for a thermal design. You would probably find a plane like this soaring at something more like 25 mph. Though you will find that if you use that as your design point, the plane will require quite a bit more total twist (aerodynamic plus geometric).
Mar 05, 2010, 07:31 AM
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miniphase's Avatar
looks interesting Wardo

To reiterate what others have already said- you won't need full span elevons, I'd go for half semi-span at most (based on the TLAR method)

If the red triangle on the plan is the cg, why not put a flap along the straight portion of the trailing edge (with a split for motor mount) rather than spoilers.

This is going to be a great thread!!
Mar 05, 2010, 07:48 AM
Red Merle ALES VI
Curtis Suter's Avatar

Airfoils / Stability 101

Well I'm only half way through my first cup of coffee but let's see if I can shed some light on the different techniques for flying wings.

1) Planks that use a reflexed airfoil for stability
2) Swept wings that use an airfoil in the center section that has good lifting capabilities and an airfoil at the outer section that is fully symmetrical or is reflexed slightly which will counteract the negative pitching moment of the very nice lifting airfoil in the center section.

So let's back up and explain some of this.

Airfoils have a chord line, this is a line drawn from the center of the leading edge to the center of the trailing edge. If the airfoil shape is the same top and bottom then the airfoil is fully symmetrical.

So let's take this airfoil and blow air across it.
If the air is blown parallel to the chord line the airfoil will neither pitch up or down. This angle, which is zero, is called, you guessed it, the zero lift angle.

If the airfoil is tilted up or down slightly, increased/decreased Angle of Attack (AoA) the airfoil will be pitched up or down.

Now lets take add some camber to an airfoil. Now if you blow air parallel to the chord line the airfoil will pitch nose down. How much is determined by the airfoil chosen i.e. shape or amount of camber. This is called the pitching moment and expressed as Cmo. This will also rotate around the quarter chord location for all practical purposes. i.e. 25% chord line.

So by wind tunnel testing different airfoils we can get the zero lift angle and the pitching moment which is what's required for Dr. Panknin's formula.

Back to the Plank airfoil such as the PW51, they are reflexed in the aft section of the airfoil and optimized as such so that when air is blown parallel to the chord line the pitching moment is near zero. However, they don't lift as well as a cambered airfoil. Are you getting the drift here yet as to the advantages of one over the other? Read on.

All airplanes need have some positive static and dynamic stability.
So if we take a nice lifting airfoil that has a negative pitching moment, i.e. the leading edge pitches down, then we need something to counteract that. That's where a fully symmetrical airfoil at the outer portion of the wing that's twisted nose down slightly i.e. washout, an amount that's proportionate to counteract this negative pitching moment of the center airfoil will give a stable flying platform.
So you'll have a good lifting airfoil in the center section of the wing and the outer portion will give the airplane the stability required for flight.

Summing up:
If you chose to use a reflexed airfoil across the entire span, it would not thermal as well as using a good cambered airfoil. However, reflexed airfoils for powered flying wings or slope models are widely used and successful. But in this author's opinion, for a thermal model they can't perform as well as a cambered airfoil.

One last comparison. In a conventional airplane the horizontal tail has a tail down force, and in many airplanes you'll see that the tails airfoil is actually upside down as it's more efficient this way. This is exactly what the outer portion of a flying wing is accomplishing. So the outer portion of a swept wing is the tail!

There is a book called the Tailless Tail, you see a flying swept wing has a tail too. Just a very short one compared to the overall aerodynamic center.

There are two books at
one called The Tailless Tail and the other called Understanding Polars without Math.

The link isn't working right now but I'd recommend them both.

Mar 05, 2010, 08:02 AM
the answer 42 is
Hey Curtis thanks a lot for the lesson and the evercomming questio?? why so few flying wings for thermal flying?? is there a real impedment on their flying characteristics or they are just too difficult to see and read on the air?? what do you think??

Mar 05, 2010, 08:03 AM
Registered User
wardo78's Avatar
Herks, thanks for the input, I was hoping you would chime in. I'll rethink the spoiler idea, I could put them on the bottom, but flaps might be the ticket. Also, miniphase's suggestion of using the center portions trailing edge is interesting. I like the simplicity and likely need for only one servo. The major rub I see is conflict with the motor and prop. On that subject, I was thinking of trying to find a longer prop shaft which would allow the motor to be mounted forward, I would then support the shaft at the rear with a bearing arrangement of some sort. I don't have ready access to a machinist, but I am sure I could turn something up.

Mar 05, 2010, 08:05 AM
Registered User
wardo78's Avatar
Curtis, you posted while I was writing. I have an appointment in a moment, but I'll try to digest things and respond with questions later.

Mar 05, 2010, 08:11 AM
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miniphase's Avatar

I think they go up quicker in lift, but have an inferior 'hang time' in still air, and are harder to land accurately than a conventional layout.

As most thermal flying is competition orientated, these qualities mean there's no benefit to flying a wing in the competition environment. As such the development of thermal ships has only be persued with tailed designs.

I still think there may be more potential to be unlocked with the help of gyros, but I'm not sure these would be allowed within F3B/J rules.
Mar 05, 2010, 08:19 AM
Registered User
miniphase's Avatar
maybe split the flaps either side of the motor, and just accept you'll have to use 2 servos?

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