The Tern: 60" Flying Wing

A part of me has a problem with building the same thing twice. There no such thing as a perfect airplane, but I enjoy finding the niche filled by every creation of mine that manages to sutain flight! That's why I would like to share with you the airframe that has been my favorite so far this summer, the Tern.

I build every airframe with the hope that it will live to see some form of moisture, and live through it. That is why i completely delaminate all paper from my dollar tree foam board, before relaminating one side with packaging tape.

The plans I make are desgined specifically to be cut out of a 20x30" sheet of dollar tree foam board, you can assemble this whole plane from scratch in under two hours if you are comfortable because there are really not that many cuts, and the designs don't need to be printed out you just make a few measurements with a yard stick.

All lines here are cut lines except the dotted line. You can see here the upper and lower surfaces of the wing. The lower surface has an inch and a half subtracted from it for later use in an elevon. Because I will be using a flat bottomed airfoil for this wing, the lower surface doesn't need to be as large as the top, so I can save some of it by saving it for the control surfaces. The right trangles are cut from the edges to give the plane its sweep. A fun idea would be to try this without any sweep, and leave the right triangle out. Your bet is as goot as mine if it would be stable or not (removing sweep decreases stability). But if you keep reading I will share some of my home remidies for pitch stability.

Here is a simple cuttout following the plans. The packaging tape is on the other side, what you see is exposed foam without any paper, this is crucial because of the curves that will follow. Because the tape has already been layed out on the other sides there is no need to laminate each peace seperatly, most of the taping is already done by this point. The only detail left out of my dimensions the position of the spar. I will be using flite test's go-to double layer spar, and although I love removing paper, it will be left on because of the great strength it adds, and its protection from moisture inside the wing. Once I have cut out the lower surface of the wing I label the spar location as starting 4.75 inches from the leading edge of the root chord, to 1.75 behind the leading edge of the tip chord. This leaves the maximum chord thickness at 40% of the root chord, and 30% of the tip chord, a good setup for any flying wing. I then draw a line connecting theses two points for the spar to be laid on later. I have not been laying keys in my spar because I have found it is not needed for strength and it is easy enough to glue the spar in the right place if there is a penline guiding you. It is almost easier than trying to fit keying joints together while hot glue is drying.

If you think these spars are simple, then you are right, but these determine a lot about how the airplane will fly. These are cut out of the smaller side of a sheet of foam board, so they are 20 inches long. The thicker side of the spar is 1.5 inches and the small is 1 inch.

This wing has a smaller root chord than the versa wing (13 inches to 11.75 here) and yet it has 22 inches more span, so how could it be very strong? It is all in the simple shape of the spar. In trying to mimic the 2415 airfoil, I relized that the spar would have to be 0.15 x 11.75 = 1.7625 inches (look up NACA four didget airfoils). If I subtract the thickness of the top and bottom surfaces, I get about 1.5 inches. As stated above that spar already startes at 40% of the root chord, so I should have a very rough 2415 if I can fold the foam right. If the spar is at 30% chord on the tip airfoil than I would need a 2315 at the tip. With approx. 6 inches chord length at the tip the maximum chord thickness should be 0.9, and the spar should be about 0.75 inches tall once the skins material thickness is subtracted. But my spar is only 20 inches long (for ease of assembly) so to reach 0.75 inches at the tip I will end the twenty inch long spar at one inch ( decreasing 1/4 inch per 10 inches) to form the tip airfoil. I cannot pretend to be able to accuratly create the 2415 and 2315 because it will be too hard to control the camber without any chord profiles along the spar, but I don't need them for strength, and I am happy with the glide slope I have with this method, so I see no point in complicating the build. I have already secured two important variables of an airfoil ( maximum thickness, and location of maximum thickness along the chord.

But in aiming to build a 2415/2315 variable air foil wing, I have acomplished something else; strength. This spar is double the height (not thickness, height!) found in other builds and it gives this airframe more than enough rigidity for its span. I will attach a video showing some acrobatics and some good crashes. I have done loops and rolls with this plane with plenty of gear on board. I have also had collistion with silos, barbed wire, and swingsets, and been able to pick up the plane and just toss it again without any repair.

Here you can see the result after the first fold. Glue is only needed along the spar for the fold; none on the leading edge. It is good to keep the top and bottom surfaces parallel while folding and gluing. If your surfaces are parallel then your leading edge at the root chord shold be off the surface of the table, and you can hold the leading edge at the tip chord against the table while the glue is drying the put some wash out into you airfoil for some stability. The extra material on the wing further away from the camera is left so that the wings can be perfectly fitted after being finished. Because of the variable wing geometry the top wing surfaces tends to move outward toward the tips after the fold. The bottowm surfaces, and the spar will still be perfectly aligned, but there would be a gap between the top surfaces... not good... that is what the extra material is for. Your two wings should combined seemlessly before you glue them and fit together like a jigsaw puzzle. That way, when you glue them together, the upper and lower wings surfaces, along with the spar are all contacting. This will create an amazinly strong wing that you could not achieve with blue/pink foam and carbon fiber while being so light at the same time.

Here you can see the second fold in action. Do whatever is natural for you. I keep the top surface on the table, and roll the spar until the bottom surface has the "flatish" shape I want. Mark the position of the bottom surface on the upper surface so that you can reference it, apply glue, refold, and press a long object along the trailing edge while the wing is resting on a flat surface. Trim off the excess material from the upper surface, and seal the deal by closing the trailing edge with packing tape.

Here are some 3/4 inch mini spars I add in the wing tips, they do a good job of making sure the wing tips have the shape I want. I add them right along the line I drew for the main spar. They also provide another surface for the winglets to adhere to when it comes time to add those.

 Here is an expample of fitting the two wings together. You can see the two bottom surfaces match perfectly like they should. The closer wing has the top surface pulled out by the folding of the variable wing, and the further wing would have the same problem if it where not for the extra material I left to compensate for the missing material on the closer wing. If you have OCD you can leave extra material on both wings and trim them both so that the seem is perfectly straight, but letting one side err, and compensating with the other side is less work, and the bottom seem should always be perfectly centered if you need to make measurements for fitting electronics. It should look pretty spiffy when it comes together.

Under 170 Grams as is without elevons and vertical stabs, a conservative 200 grams with those added. I would love to see what the FT community does with this. This is a perfect blank slate for, well anything except raw speed. Room for plenty of elctronics and rigid enough to support whatever you add. Well... anything lighter than a toaster. If you want to make a toaster fly just build the kraken.

Thick wing bonus: 9g servos fit completely in wing... smooth

Always looking for new ways to mount electronics. Skewers don't go through the spar, just out the other side of the blue foam enough so that the skewers catch the upper wing surface to prevent the foam falling out.

There is enough room for the 2200 under the blue foam because of the thick spar, here is my go to battery mount, rubber bands and bambo skewers

Warning: If you are adding drag below aircraft and therefore creating downward pitching moments, do not experiment at the same time with how small you can make you elevons. :D ...... D: seriously though. They are the only thing that provide the reflex to keep your plane pitched up!

650 gram non FPV model, unlimited vertical or 30 minute flite times! w00t!

Here is picture just to show the wash out you can achieve by following the building methods described above. A few degrees is all you need.

Wash out is needed for two reasons. First of all, air naturally aproaches the wingtips at a higher angle of attack than it does at the root chord. So, if you wanted the whole wing to interact with the air the same you would need wash out. But we don't want the whole length of the wing to behave the same, we want to avoid tip stalls and spins, by designing the wing to stall in the center before it stalls at the tips. Things brings us to  our second reason for wash out. If we increase the wash out past what we need to counter the natral increase in angle of attack from root chord to tip we can make sure that the wing tips are always at a lower angle of attack with respect to the air that is flowing over them promoting the center or root of the wing to stall first. When the center of the wing stalls first the pitch of the airplane is decreased, the angle of attack decreases and speed increases so that a recovery can be made from the stall.

I don't know it it actually works, but i usually leave off a bit of the elevons towards the wing tips. Elevons are important for controling pitch, but they destroy some lift. I see this as another way to fight tips stall. It serves the the same purpose of wash out at the tips, without actuallt decreasing the tip chords angle of attack any further.

There are other methods of making sure to avoid tips stalls. One great one is changing the airfoil at the tip, so that it is different from the root chord and provides more lift. A great example of this is the undercamber you find in so many of the FT designs. In these plans a shift from a 2415 to a 2315 serves the same purpose. I am currently trying out some differnt combinations like a 2515 root chord to a 2415 tip chord.

Someone with money please take a 500-1000mah 6s lipo and a nice 400kv motor and 14" prop and invest in some solar panels. I would love to see what some quad motors and a little foam could do.

I would also love to see someone make a blunt nose version of this. No center section, just move the root chord back to 50% chord and cut the nost blunt, should work!

ACTION SHOTS:

Anyway here is a flight video, just make sure to tilt your head to the right before you start watching because I suck at editing video!

Shameless Plug: If you want to see some more simple designs you can measure and cut I have some more and would like to get some money around to build a website and host it. If you want to support me or even just want an airframe shipped to you I am selling these on ebay, shipped in two peices for shipping reasons. Just search "Tern Flying Wing"

Further designs: The wing on the pusher plane is similar to a simple soarers wing. I hope to finish up the biplane flying wing I am working on also.

Don't forget to send me a message @ holbrook77@gmail.com if you make a model I would love to see if I have helped anyone have some fun with this design.