The Tern was designed to deliver a quality FPV flying wing, that is as easy to build as it is to fly. The dimensions are shown below, and can be drawn up much more quickly than any other design that is meant to be printed, trimmed, and taped together from dozens of pages of tiled plans.
There are a few simple design features used in this plane that I wanted to share in this article, and they are:
- The simple chord profiles help give the wing the washout needed to keep the wing tips from stalling, and create a strong surface to hold onto when luanching the wing.
- The wingtip airfoil's maximum thickness is a much larger percent of the local chord line than the airfoil found at the root chord. This construction allows a larger portion of the lift and drag to be created toward the rear of the aircraft, where it creates stablity, and negates the famous problem of adverse yaw common in flying wings.
- All paper is removed from the foam that becomes the skin of the aircraft, while the spars and chords remain covered in paper. One side is then laminated. The removal of the paper from the inside of the bending foam allows for smooth curved airfoils that will match the profiles given below. Removing the paper from the outside of the wing creates waterproof layer that will not delaminated from the foam beneath it, because there is no paper below the tape that can get wet.
Although I have never tried it, it would be possible to leave the paper on what will become the outside of the wing if you did not want to spend the time to laminate with tape. But in any case, the paper must be removed from what will become the inside of the wing, or else you will not be able to create the curves defined by the chord profiles below
The rectangle you see here is exactly the the size of a piece of dollar tree foam board. The prototypes I had were so close to fitting, so I simplified the cuttout to fit perfectly onto one piece of dollar tree foam board. All that is needed to draw the design is two or three marks from each corner,and a yard stick to connect the dots.
Instead of measuring the driven dimension (seen in parennthesis) of 3.077 inches, I hold my yard stick in the same place immediatley after drawing the root chord, and use the opposite side of the yard stick to draw the parallel tip chord line.
Since the curvature of the airfoil causes the top wing surface to be longer than the bottom surface once the wing is completed, I subtracted some material from one of the wing surfaces to shorten it so it may be used as the lower wing surface. The 1.5 inch of material that was subtracted can later be made into elevons.
It is important to notice that two of these need to be cut out for each plane, but the second must be a mirror image, or optical isomer of the original. If one side is laminated before cutting, and is intended to face outward, the lower wing surface from the drawing above, and the upper wing surface from the mirror image cuttout will complete the left wing and vice versa (the upper wing surface from the drawing above and the lower wing surface from the mirrored drawing, will combine to complete the right wing).
One last picky detail. If you want to create a perfect fit between the two wings, leave one of the isoceles triangles on the root chord of just one of the upper wing surfaces. Without this, each wings upper surface will shift towards the wingtip during folding because of the variable spar thickness, and create a dihedral in the wing when combined. You could follow flite test suggestions from the Versa build and sand until both surfaces are flush, but i do not like subtracting from the lower wing surface because doing so would change the sweep, and every build would have slightly different sweep and CG location. EEK that's gross!!! Just draw several lines on the extra material you left on the side of the root chord. Start each line at the same point on the leading edge, and end each line 1/4" further out from the original line that made the root chord. Once both wings are completed, one of the lines will match the material missing perfectly. Make one cut on that line and both sides will fit together like legos, and every wing will have very similar dimensions because the unmoddified bottom wing surface will not have been sanded.
Unlike the wing surfaces where all the paper is removed, the board that is to become the spars will have all the paper left on it becuase of its great strengthening traits.
The length of these spars is 20 inches and will fit perfectly along the short side of dollar tree foam board.
Before cutting the spars out i recommend marking the right angles made by the spars, because the angles are so small, it is hard to differentiate the right angle from the acute one at the root of the spar.
Two spars are used for each wing, just like the versa, but all cuts go 100% perecent through the foam, so there is nothing left to hinge two spars perfectly together. Instead face two bases toward each other and make two simple tape hinges before glueing together.
Spar and Chord Profile Locations:
This only needs to be drawn on the inside of the lower wing surface.
The wing spars shown further below will span the first twenty inches of the wing from the root chord, and end before they reach the tip chord.
The four perpendicular lines indicated where the chord profiles shown further below will go.
The chord profiles are not paralell to the root chord and do no represent the airfoil that the wing will have. It is much easier to draw right angles, and use the profiles below to produce the wing's final shape.
If you feel the tip airfoil is too undefined, or slightly non uniform because of the shortness of the spar, just cut out four 5/8th or 11/16th inch squares, glue the pairs togeher, and wedge them in between the upper and lower wing surfaces where the spar line ends on the lower wingtip.
*Center of gravity for 20% MAC is at 6.5" from root chord leading edge, with 2 22"x1.5" control surfaces*
When printed profile 1 should be 6" from leading edge to trailing edge.
The image is intended to fill a full piece of paper 8.5x11"
feel free to email me at email@example.com if you would rather have a .pdf to print out.
LINE OF SIGHT FLIGHT
This design is the product of my lazyness, and dred of the downtime In between large crashes. It is designed to deliver a quality airframe in a small amount of build time, to get you flying again, and in style. Please take this from where I have left off and add your own spice to it. Maybe someone will be daring enought to try a forward swept design, or add some more vertical surface for added yaw stability. I have been tempted to leave the camber on the trailing edge of the wing (seen at 3:25 in the video) and make a partial cut instead to use it as a control surface that is already built into the plane the second it is made. My conservative half (warning conservative estimate; more like 3/4conservative) says that the control surface is to week because it is only laminated on one side, the control surface is too short after having taken the longer route along the top wing surface, and making it longer would only subract from the MAC of the wing making a more accurate balancing needed before flight. Maybe you could use the lack of stiffness of the unlaminated control surrface to your advantage if the servo held it more towards the wingtip; that way even more of the drag was moved rearwards! Anyway, I hope this inspires you to enjoy the design aspect of the hobby. Flite test always has great plans, but everyone here has something that is worth sharing with community!