Design HELP on a Box Wing Config

by MESA RC | March 23, 2016 | (12) Posted in Projects

AN FT STEM Exemplar School

To the Flite Test Community;

Susannah, 8th grade top design student at MESArc, came across a cool plane design that gave her the design itch to try to create... the box wing style aircraft. See here the video she got some insight from;

Using the FTSTEM Design brief to guide her thought process, she conducted research, put up some designs, and created a prototype that you see in the featured video.

After going through a series of designs on CAD, she dry fit one to figure out final touches. The one you see completed in the video was a good enough prototype to actually test out.  The initial glide test proved to be just fine, there was no issues with the exception of where to put everything, including control surfaces.  As you saw in the video, we have no idea and are completly stumped on what could possibly be going wrong.  I told Susannah that I know exactly who to reach out for, the Flite Test Article Community.  Is there anyone out there that could make suggestions so that Susannah could redesign, rebuild, and get this cool idea in the air?

Questions about design include:

  1. Placement of Box wing configuration?
  2. Placement of Motor?
  3. Size and placement of Ailerons, Elevator, and Rudder?
  4. Additional design elements that we are leaving out?

Thank you guys for helping out with this project, with your input we hope to get her idea in the air before she moves on to High School.

For more information about our program:

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CMS_1961 on March 23, 2016
I'm not an expert, but just a suggestion based on the visual flight of the aircraft. I would try using elevons and do away with the upper elevator. It appears that the CG may be off (too far forward???) or the engine thrust line may need to be up slightly or neutral --looks like the engine was pulling the plane down and elevators were not effective at all. Nice design!!! I am sure you guys will get it going.
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Techno16 on March 23, 2016
I partially agree with CMS, it is an issue with the thrust-angle. What's going on is that the thrust-line is above the CG (center of mass), this is creating a torque on the entire plane, pulling the nose down, but only when the motor is on. This is why the motors on pusher planes (Bixlers) are at a funny angle. The thrust line MUST be on the CG. It may look funny, but it should work. For this plane, the motor should be angled DOWN, not up (which would increase the torque) I suggest mounting the motor on robust armature wire, to make it easy to tweak the angle, once you've got the angle, you can make a more permanent mount. I also found some picture for a visual representation of the problem:
The article I found the pictures, giving a better explanation:

The same principles apply even though you're using a tractor motor on the back of the plane
Also, the elevators do look a little small for a plane of that size; you'll need to keep the motor on to get enough air moving over them, you might want to make them bigger (longer) and then dial it in from there

Good Luck!
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LordVader on March 23, 2016
Great design and build, I have to agree with CMS_1961. The CG is too far forward and the thrust angle isn't right. Every time you gave it throttle, it wanted to pitch down while it was in your hand. So that is where I would start. Also, just use elevons, like on a delta, with the wing area basically coving from leading edge to the rear. Anyway, just a thought. Good luck, I know you guys will figure it out.
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jgupton on March 23, 2016
Hey Jake and Crew,

Nice work as always - and Susannah, I dig how you are going after a new/cutting edge idea. One of the things I ask my STEAM kids to do when trying (to figure out) something new is to eliminate variables. I notice that you are not only attempting a box-wing airplane, but the location of the motor (as others have mentioned) is also unconventional. I wonder if eliminating the variable of the motor placement in order to focus more on control surfaces and identifying a flyable CG would help move the project forward more. Once you get those two issue figured out, you might be able to reattempt the rear/high mounted motor.

Observationally speaking, it did seem like the thrust angle of the motor was pushing the nose down and that, once you were able to achieve some form of flight, the aircraft appeared tail heavy.

Best of luck to you all and keep up the good work!

Jack Gupton
Walker Jr. High STEAM
La Palma, CA
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kknapp on March 23, 2016
From watching the video my recommendations are:
1. Your motor angle is wrong. The article linked above by techno16 will give a good explanation to fix that problem. It should be a quick fix after you read that.
2. CG is way to far back. If you check Youtube for videos of tail heavy wings you will see them crash almost exactly like yours. From my understanding of the aerodynamics of a box wing they will act functionally like a wing.

As for the control surfaces elevons would be my suggestion.

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Willsonman on March 24, 2016
In my design experience, you are very close. The number one concern I have is the forward-swept wing. Nearly all technical papers on this wing design expresses concern about wing torque (or twist) being induced during flight. To help alleviate this I would highly suggest making sure that your wing incidence is EXACTLY the same for your top and bottom wings.

Relative to your wings, you also have a very short-coupled design. Perhaps decreasing the sweep angle on both wings and moving the wings more aft will enlarge your neutral point's static margin.

To determine your proper CG you could go with deriving a very complex formula. You have a couple of ideas her: 1) flying wing 2) biplane 3) forward-swept wing There are known formulas for these wing configurations but not having a flying tail to determine your neutral point can be the hard part. I would start with using basic flying wing formulas. Determine the CG for the top and bottom wings separately. Then use the Biplane formula and see where all three coincide.

Use this flying wing calculator: (note you can use a negative number for sweep)

Also give this a read:

Hope this helps... It looks to be a great project!
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Willsonman on March 24, 2016
Another note: Aft-swept wings have span-wise flow going away from the fuselage, Forward-swept wings are the opposite. So in effect, your span-wise flow is exaggerated on your elevators due to how the air currents run. Not to mention they are right in the prop wash too so they are already sensitive.
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RAM on March 24, 2016
I'm a rookie but looking at the video I would suggest cutting your elevators in half. Simple to try, just slice each one at the middle and tape the remainder in line with the wing. If that doesn't help just tape them back together. (if your control horn is in the middle you will have to slice each end instead to get to half the control surface)

The only other thought I have is adding canards but that would change the original design.
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MikeRobey on March 24, 2016
As we know normally the wing provides lift and the horizontal stabilizer helps keep the plane balanced in pitch. Question is when does that rearward sweeping wing become a horizontal stabilizer? One design I would check is the Mignet Pou-du-Ciel from the 30's its sort of the reverse of your idea in that its a canard where the canard plane is the same size as the main wing but the same issues must have come up. I think your basic idea is sound but you need to look into the basic physics of the centre of lift for both the wings and the where the cg should be first.

If it was me I would start with a box biplane with a normal tail. Then I would progressively move the top wing backwards in increments to see how the cg has to shift in relation to the change. From all of that I would then return to your original design and make changes from what I have learned. This test prototype would be a simple chuck glider so its quick and easy to knock up. I would do the tests indoors to eliminate the wind as a factor. You could build in control surfaces that could be fixed in various positions to see the effect of applying up and down elevator in varying degrees.

Once you had all that sorted I would stick the motor up front as it eliminates all the issues with high mounted pushers. Most of the time a tractor with a couple of degrees of down and right thrust will do fine. As others have suggested the elevators need to be larger. I would make them at least twice as long or, as others have suggested, eliminate them in favor of elevons on the "main" wings.

Lastly, I know it can be very frustrating but keep in mind all the frustration will only make the thrill of finally succeeding all the more exhilarating.

I wish you the very best of luck and look forward to the successful flight video that I know will result from your efforts.
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Thorsclaw on March 24, 2016
If you need more design inspiration check out for an rc model with the same wing configuration
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Mid7night on March 24, 2016
First off, let me lead a virtual standing ovation for this young lady's tenacity in attempting a Box-Wing scratchbuild - AWESOME!! Do not, DO NOT give up Susannah! The box wing is a fickle beast, and "normal aircraft" intuition may lead you astray if you're not careful. As a fellow RC-enthusiast of over 18 years, and an aerospace engineer; I'm actually really glad this first attempt crashed and pitched because it's going to make you scratch your head and you will end up learning MORE than if it had just sailed out of your hands into the air. Embrace the trails and learn from them!

The elevators and ailerons are in fine locations - don't move them. You just have balance issues with....

....Thrust Line and CG!

Thrust Line:
Aircraft CG is not just a fore/aft placement problem, it also has a vertical component. This vertical element usually doesn't come into play because the motor/prop are usually placed near the main body centerline, which is also usually close to the vertical CG position. HOWEVER; once you put the motor/prop higher above the aircraft, you will encounter some serious "Powered Effects". Throttle becomes a coupled-pitch-force, instead of just a forward-push.

To counteract powered-effects, you need to point the thrust-line AT the CG, which in this case means angling the motor/prop down.

Center of Gravity:
You may be almost fooled into thinking that this plane was nose heavy after viewing all of its nose-dive launches, but that is entirely due to the powered-effects stated above. The one ground-takeoff observed showed a RAPID nose-up and stall once sufficient airspeed was reached for lift, and then the later nose-in-loop launches. This actually means your plane was quite tail-heavy, or at BEST, something aero-engineers call "Neutrally Stable" - This basically means that the Aerodynamic Center (AC) and Center of Gravity are at the same longitudinal spot. A Neutral-Stable aircraft will be almost IMPOSSIBLE to balance/fly manually - state of the art jets and fighters use massively powerful computers and thousands of hours of simulation to program control laws to enable near-neutral stability and maintain control. Near-neutral stability is good for commercial cruise efficiency, but that's not something you need in a foamboard parkflyer. :) You want POSITIVE stability, which means the CG needs to be well in front of the AC.

Moving the CG forward will improve static stability, and it will also reduce the amount by which you have to angle the thrust; the further forward the CG, the less you need to point the motor down. Start by ignoring the upper/aft 'wing' and calculate the desired CG based purely on the main wing "Mean Aerodynamic Chord" (MAC).

The MAC is an imaginary chord-length that is used to represent the whole wing and estimate where the center of lift might be. A good rule of thumb is to balance your plane at the "Quarter Chord" of the MAC - This means your CG should be 1/4 of the way down the MAC.

For a constant-chord wing, the MAC is just the wing chord. But for a trapezoidal wing, it's a geometric 'average' of sorts between the root and tip chord.
This link is to a simple wing-MAC calculator, that ignores other surfaces (it's intended for flying wings), but it's a perfect tool for calculating the MAC of the wing-alone:
PRO TIP: Click the "Show MAC Lines" button when using the above link, and it will show you graphically how it uses geometry to calculate the MAC location!

I know that's quite a wall of text - my apologies. :) Please don't hesitate to ask me any followup questions if you need. :D

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Stingray Aviation on March 24, 2016
I agree with the above posters on the thrust angle. That seems to be your main problem since you say it was gliding okay. I would suggest angling the motor down a few degrees or, better yet, converting it to a pusher or tractor.

Your CG is also probably too far back. I once built a flying wing that behaved about the same way, gliding fine and then becoming uncontrollable under power. The issue was that the CG was far behind the aerodynamic center of the wing instead of in front of it, causing the aircraft to rapidly pitch backwards with up elevator input. I would suggest reading for a better explanation of how this works.

Another thing you might want to try is playing with the ratio between the area of the forward wing and the area of the aft wing. First, I would try enlarging the forward wing and making the aft wing slightly smaller. This will allow the planform to work more like a conventional airplane; the forward "main" wing is producing most of the lift while the aft wing is mostly there for stability. (Keep in mind that the whole point of a closed wing is to eliminate the drag caused by wingtip vortices, not necessarily to have two wings producing equal lift.) Another thing that may help is increasing the distance between the aerodynamic centers of the two wings. This combined with the right wing areas should give you a lot more stability.

This is an interesting project! Engineers have been throwing around full scale closed-wing concepts for awhile, so maybe we'll see one get built in the near future. Good luck!
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HilldaFlyer on March 24, 2016
Wow, that is a great project. Can you believe the toughness? What was that, 5 or 7 hard crashes and it was still in potentially flying shape. Great build!

I can't add to the above advice for solving your issues. Most apparent is the thrust line. Aim the motor towards the leading edge of the front wing. Next advice, keep going - I want to see it fly! I wish more people would publish their learning experience because we all can learn from it. Good luck!
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hotwax on March 25, 2016
In the 80's I read about trials to get such a boxwing model flying and it did not work. This config has nonlinear stability. The only successful application of a boxwing I know is an ultralight called Sunny Boxwing with the upper Wing in front and the lower wing aft. I googled a bit and found a long article telling what difficulties are hidden.

My suggestion: CG forward, Motor to the front. make ailerons to elevons, put GYROS on all axis, don't expect too much, don't trust computergraphics. And yes, you will need a big rudder because an aileron input will induce a lot of adverse yaw.
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Krümelmonster on March 27, 2016
I wouldn´t put gyros in, because that would only hide weaknesses in the design and the plane would never fly as good as it would when it would be figured out properly.
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hotwax on March 28, 2016
Well, we have seen the design doesn't work without gyros yet. With gyros it might work. Why spending precious time when cheap technology can help?

On the other hand, if it doesn't work with gyros, it's time to try another design.
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Berge on March 25, 2016
Awesome plane, I too think its tailheavy, with the rates for elevators set too high. Throw a cc3d on it , its possible that the forward swept top wing along with the incorrect thrust line of the motor and too far to the rear cg position are making it topply, and pitchy.
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oneiwily on March 27, 2016
1st thing 1st, well done on taking this challenge.
I want is to endorse you on taking this challenge and to hope that you will keep it up.

I don’t want to repeat previous comment regarding to CG and thrust angle.
I want to suggest steps that will help you succeed and help you build the confidence.
1. Place the engine in the font so that the trust will be on the centerline.
2. Now do glide test, No power, just glide the airplane and see how it handles (still control it with the elevens).
If it stalls whenever you turn it ... It's tail heavy.
If it drops like led... It's nose heavy.
3. Fix the CG according to the glide test, and do some more until you are happy with it.
4. Power up and fly away .
If you finished the steps above you should have airplane flying properly.
Now it's time to modify and experiment.
Do your thing, move the engine to the back, up, down as you see fit. You know where the CG is and what to expect. Change the wings, change the control surfaces, do whatever you want, this is your design.

Don’t forget to enjoy the process this is a hobby after all

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Air-headed Aviator on March 27, 2016
I love this! It's great to see other you g people flying and building, and seeing then tackle unconventional designs. Keep doing what y'all are doing! As of advice on your airplane Sussannah I recommend moving your motor forward, to be over or past the center of gravity. As high as it is, and as far back as the motor is, it pulls the tail end of the plane forward and over. Moving it up will make it act more like a nose mounted motor. You could extend the firewall of the motor forward with a big, long foam nose! Good luck to you and your airplane.
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VolksRocket on March 28, 2016

Always build small profile gliders first to find the "flying" CG, then you will have a flyable aircraft to start. Your design is fine, it is the thrust angle of the engine, some down thrust will fix it.

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psuaero on March 30, 2016
Plenty of great suggestions above with thrust angles and elevons. Great job Susannah! Don't get discouraged and you'll have your box wing flying in no time. That's an ambitious design choice and I'd love to see a followup video once you get it flying.

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Ron DuBray on March 30, 2016
try about 5 deg down angle on the motor. Its pulling the tail up as is. take a look at the sea planes with the motor high on the stabilizer like this. Their motors almost angle down so they are pointing to a point just in front of the nose of the plane.
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Mogeley on April 5, 2016
That's a very cool plane! It's a bit more complex than a normal aircraft. Keep at it! You'll get it!!!

SHORT ANSWER: I think the main issue is the motor placement position is blowing air the wings unequally. This causes more lift on the back and causes the plant to go nose down in slow flight. Moving CG back will make it worse. Pointing the motor down may help though your still producing more lift on the rear wing than the front wing.


Take a look at the images for the Box wing patent:

High AOA image:

Note the inverted airfoil (the top wing is not a lifting wing in their design) and the all flying stabilizer.

I their design the entire top wing moves to act as a stabilizer.

In aircraft stability you need to consider all the forces acting on the craft.

First, find Aerodynamic Center. This is pretty easy for one wing but because you have 2 wings it becomes a little more complex. To do this find the aerodynamic center of each wing separately. It's roughly 25% from the front of the Mean Chord. See Diagram: (more info:
Now you'll want to plot each wings AC in relation to each other. To get the final AC, getting the middle point may work if the wings are the same surface area. If not you'll need to find a weighted position based on surface area of each wing. Something like this should work.


Front Wing: 100 sq in
Back Wing: 85 sq in
Total: 185 sq in

Front Wing Lift Percentage: 100/185 * 100 = 54%
Rear Wing Lift Percentage: 85/185 * 100 = 46%

Distance between front and rear AC = 4 inches
Combined AC from front AC = 4* 46/100 = 1.84 inches

In this example the combined AC would be 1.84 inches behind the front wings AC.

This allows you to determine where the CG should be.

Second, consider any conditions that may cause the the AC to change mid flight in a bad way. AC in reality is very dynamic. One such condition is creating more lift on the back wing than the front wing. A blown wing which you have will do this causing more lift on the back and causing it to nose down when more power is applied. You may want to reconsider your motor placement location. In line with the fuselage is best. Perhaps a twin engine would be better for your plane since the plane is large for the hardware.

Third, make sure your control surfaces can produce enough force to rotate and hold the aircraft in the need ed AOA. In my estimation the ailerons looked very large and the elevators too small. Others noted the location of the elevators that they would be enough for when the propeller is blowing air over them. But they should also work well when simply gliding in. Note, control surface size can be reduced as the control surface is moved farther from the CG. Control surface size must be increased when closer to the CG. This is simple leverage relationship.

Fourth, there may be some under camber issues. The back wing is negative cambered. Also swept wings act as camber as well. The forward swept rear wing may enhance the negative camber. The high back wing adds roll stability so it may work itself out. It's something to keep an eye on.
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Mogeley on April 5, 2016
I looked at the two wings. They are very different. One has an airfoil. The other is flat. This effectively makes the top wing more a stabilizer. Finding the combined AC is more difficult as it's really a balance of lift between the two wings. Which changes in different conditions. Easiest thing is to treat it as a single wing only. Start with CG far forward and slowly move it back.
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oldjoe1931 on April 5, 2016
I am not an aeronautical engineer; however, I have built many odd shaped aircraft. This one is extremely tail heavy! I believe this will fly with just this one change. My suggestion is ignore the top wing & calculate/set CG using bottom wing only. This can easily be accomplished with moving the battery or adding weight to the nose. Good luck!

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Quasi-Flyer on April 7, 2016
That's an awesome design and I'm really trilled that there are great kids like you that could be competing with my daughter for the top job soon... It is an inspiring new idea and build. Specially if you are trying a design that is new.
Seeing the video you you have four or 5 things you need to work out.
1. There needs to be balance of thrust of and placement of your rotor of that size. Perhaps if you add a pair of slightly smaller rotors pushing underside of the fuselage and wings. at the same time you could program the ESC's on each of the rotors of the sides so that they change speeds independently to adjust some pitch and yaw. And the added weight of the smaller motors on the front need to match and balance the center of gravity. The bigger the difference of rotor thrust of the back rotor with the front would keep balance. (kind of like drag cars have a set of wheels behind. .. Or like Mater flips the tractors on that Disney movie. ,,,, Anyway you get the point.
2.I would suggest a swept opposing boomerang wing design where you use more wing surface and just have elevators and ailerons that are two opposite surrounding to one on each side of the planes. So it would be a set of three on one side and a set on the other. There is where you need to proggram servos one pushing down and two pushing up. I suggest you work on esc and flight controller programing to help with the flying.
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JasonEricAnderson on April 26, 2016
Super big kudos to Susannah for tackling a box wing design head on. I've also seen that video on Synergy Aircraft but never had the confidence to tackle designing a box wing. You've got lots of great feedback here, and I'm not an aerospace engineer but I would echo the idea of putting some control surfaces on a glider version to help work out the control surfaces and general CG. I'd also recommend the first powered version after that to be a tractor configuration and use a folding prop if you can to save on lost props. My gut tells me this would help the wings do their work by having more consistent air flow.

Another thing you'll notice about the Synergy plane (since it seems they at least have a good sized RC prototype flying) is that top 'wing' surface area is much smaller than the bottom. This helps lower the issue of forward swept wing twist. Based on what I've seen with your layout, If I were to build one, I'd try a design that had a slightly swept lower wing and the top wing having a straight leading edge. That would at least eliminate the forward swept wing as one of the points of failure.

Cheers and great work!

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Neskair on May 11, 2016
If this is still going, Id like to add my 0.02$
I think it needs cg tuning and new motor placement. I think it looks a little tail-heavy. Maybe the motor could go on the back of the fuselage. That would require the vertical stab to be modified a bit. Maybe add more vertical fins for tracking.
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COLAB on September 4, 2017
Hello everyone,

This wing principle was abandoned by NASA working on this geometry, because too unstable.

We have been working on the same kind but reverse and have filed patents before NASA and will soon produce an ultra-light airplane within the next 2 years (2018 or 2019).
Visit our website:

With this triangulation you will have extraordinary results right away, and can even use a fuselage anywhere and install the COLAB wings.

Have fun
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Design HELP on a Box Wing Config