Making Fully Symmetrical Curved Wings from DTFB

Hello everyone,

This article shows how to make fully symmetrical wings with curved surfaces using a slight modification of the basic FT building methods.

The development for this method started with my desire to reproduce one of my favorite older designs from my nitro days (more than 20 years ago) in a foam / electric version.  The original unpublished design which I called "Blue Streak" was a low wing aerobatic flyer patterned after the late WWII fighters and/or some of the pre-war racers.  The closest comparison is probably the Mustang.  Here are a couple of shots of the original lite ply, balsa and Monokote model:

Here it is again with the first three mock-ups of the symmetrical wing sections in the foreground.  The first complete wing panel is in the middle of the picture to the left of the canopy.

The nitro version was pretty well powered with an OS 40 and performed to the limit of my skill level at the time.  I also built an OS 60 size which met its demise when an aileron came loose on its first flight in 20 years last summer. I probably had 50 or 60 hours in designing and building it so I am still upset at myself for not checking it over more carefully before putting it in the air again.

I have done a 1 to 1 conversion of the smaller design to Foam Board and Electric.  The foam board version looks like this:

And this:

The fuselage was pretty straight forward and similar to many FT designs.

I needed a fully symmetrical wing equivalent to the original balsa and Monokote one to complete the design.  Everything I have done in foam board up to this point has been following the FT methods featuring flat bottom airfoils and segmented top surfaces.  I have seen the nerdnick symmetrical designs with segmented top and bottom panels.  Somehow that would just not do for this design.  I have seen some other methods for curved symmetrical airfoils but wanted to do something simpler and closer to the basic FT wing building methods.  The wing in the foreground above is what I came up with.

The basic concept is to provide a series of score cuts on the inside surface of the wing panels to allow them to bend.  The uniformity of the cut widths and compressibility of the foam assures that the curvature will be even.  The assembly method assures that the top and bottom curves are equal.  Squiggles of glue across the score cuts help to lock the curved surfaces into place in the assembled wing.

On this tapered wing it is possible to build wash-out into the wing as the wing is being assembled.  In fact it is actually easier to include it than avoid it.  This design also creates a higher percentage of wing thickness at the tip than at the root. The combination of higher percentage of thickness and the wash-out act together to avoid tip stalls.  With this design under camber is not needed.

Here is a look at the completed wing section:

You can see that the top and bottom surfaces have equal curvature, as close as the eye can tell.

You will also note that there are two spars.  The one toward the front defines the thickest part of the wing.  The one toward the back keeps the unscored portion of the wing at the proper spread.  Together they produce adequate stiffness to assure the structural integrity of the wing.  This was my first build in which I used only one spar at the front.  In the interest of additional strength I have doubled it on later builds.

The shape and size of my wing was determined by the original "Blue Streak".  I wanted to duplicate the original as closely as possible.  That dictated the tapered design as well as the basic dimensions.  It also determined the size and shape of the ailerons.  Fortunately this fit almost exactly on a sheet of foam board with very little waste.  Here is the design:  

This is a jpg file.  You should be able to right click and print it so you can refer to it on your workbench.

This is the left hand wing layout transferred onto the first sheet of foam board:

For uniformity transfer the layout from the first sheet to the second one rather than doing separate measured layouts.  Lay the two sheets end to end with the 20" sides butted and transfer the marks to the second sheet. Like so:

Note: The marks above are for the 3/8" score cuts at the wing tip end.  They do not appear to line up because of the camera angle.

Do the same on both ends.  

Once you have both sheets laid out make all of the through cuts (marked "C") and score cuts (marked "S").  The score cuts should be about 2/3 of the thickness of the foam so they will "pop" when you flex them. It should look like this:

Bend the wing back on itself to break the score line at the double bevel and then cut the two bevels:

While it is folded, mark the positions of the edge of the bottom panel on the inside face of the top panel on both ends.  These marks will be used for alignment later:

Lay it back flat and cut the score cuts for bending the wing.  These should only go about 1/3 of the thickness of the foam.

I use cheap snap-off razor knives from Harbor Freight set with one section extended and let the handle run along the straight edge to control the depth of cut.

Changing the angle of the handle changes the depth of the cut.  These cuts should only go about 1/3 of the way through the foam.  There will be 8 score cuts on each side of the "Double Bevel" cut.  These cuts will not be broken.  They will just flex and compress a little to allow the board to bend to the airfoil shape.  If you cut more than half way you will see lines on the face of the wing.

Here is a picture showing cutting the shallow bending score cuts using the straight edge as a depth gage:

This method can also be used for the 2/3 depth cuts by holding the knife at a higher angle.

Next we will cut and install the spars.  I used the scraps from the wing to make the spars.  First cut a thin strip off to square up the factory edge.  Then cut them into 3/4" strips.  Like so.

Then cut off the tapered parts in combinations of lengths to make two 30" pieces like this:

The 3/8" spar shown above can be cut from scraps as well.  It is just a spacer and does not have to be all one piece.

Glue the four 3/4" pieces together to make a double 3/4" X 30" spar staggering the joints the way they are arranged in the picture above.  Hold flat against the building board for straightness as the glue sets.  Here are the completed spars ready to install:

Run a pencil in the score cuts on either side of the double spar line so you can see them for positioning the spar:

Run a moderate bead of glue on the wing panel along the double spar layout being careful to miss the score cuts.  Carefully place the spar in place between the score cuts and press it into place.  Use the same procedure to install the 3/8" spar being careful to keep it on the right side of the line.  The installed spars should look like this:

The next step will pre-form the curved surface on the lower wing panel.  Run generous squiggles of glue on the lower panel touching the spar on each side and crossing all of the score cuts.  Be very careful not to get any glue on the spars or in the double bevel cut.  Remove it immediately if you do. It should look like this:

Then quickly fold the wing together into its final shape.  Align the trailing edges of the bottom panel to the pencil marks made earlier.  This will assure the proper shape.  Make sure the spars are in contact with the top panel. Hold about a minute till the glue sets firmly.  

Unfold it and the lower wing panel should hold the airfoil shape like so:

Lay the wing flat again.  Spread glue squiggles quickly across the score cuts on the top wing panel.  Then spread glue quickly in the double bevel and along each spar.

Quickly fold the wing together and press firmly against each spar along their full length of the wing while aligning the bottom panel with the pencil marks again.  The top wing surface trailing edge and aileron should be flat on the table and the edge of the bottom surface should be on both marks made earlier. This all has to be done very quickly so the glue stays hot enough for the small adjustments required to hit the marks.  You only get one shot.  If done properly both spars should be in tight contact with the wing panels for their full lengths and the trailing edge of the bottom panel should be exactly on the pencil marks.  The unscored portion of the upper wing panel and the aileron should be flat on the building board. The trailing edge of the bottom panel should be tight against the upper panel.  The airfoils on both ends should be symmetrical.

Here are the resulting airfoils.  The wing root is on the bottom and the tip is on the top.

The bottom panel must be aligned with the pencil mark so the top and bottom panel lengths are the same.

Hold both ends till set.

Next glue the trailing edge of the lower wing panel to the top wing.  First use the tip of the glue gun to separate the two panels slightly and spread a light bead in the gap end to end.  Press the joint down to the table till it sets.  Then run another small bead along the joint using the tip to melt away any bumps and spread the bead evenly. Like this:

Next fold the Aileron back and cut a 45 degree bevel on the leading edge:

Cut about 1/8" off the tip end of the aileron so it will have clearance when the wing tip is installed:

Now we will make the wing tip. Here is the template:

This is a jpg file.  You should be able to right click and print it full size.

Make two matching pieces of foam board from the template. It would be best to make one from the template and trace the second from the first.  Place the two together and trim till they are identical.

Bevel the edges to the dashed line on the template leaving only the thickness of the paper on the other side. Make one of each hand, mirror images.  Remove the paper on the bevel side:

Sand the edges to remove any lumps or extra thickness left after beveling the foam:

Starting at the front tip, use short pieces of Scotch tape to join the paper along the tapered edges. You are making a pocket with the paper on the outside and the foam on the inside:  

As you follow the curve you will place short pieces of tape on the top side, split the free part into short tabs and then fold the tabs over the back side.  In the process you will be pulling the two edges of the paper together to a tight edge. It should roughly match the wing tip and look like this:

After the edge is all sealed we will make several foam board wedges to spread the open side to match the wing tip.  Start at the thickest part of the wing and make the wing tip match the thickness of the wing:

Add as many additional wedges as needed to control the rest of the length.  You will have to spread some areas and squeeze some others.  Once you have the shape right fill the voids at the tip that was taped with glue to make the tip solid and more durable.  It should look like this:

Check it against the wing tip.  Adjust it till it matches as closely as possible. Trim the matching edges till it fits tightly to the end of the wing.  Then apply a light even bead of glue to the end of the wing, press the tip into place and hold it till it dries. Adjust it carefully till all of the edges match as well as possible. After the glue dries run a small bead on the surface and squeegee it to fill any gaps in the joint.   Remove any excess with the squeegee.  Use a heated pie lifter or similar object to remove any excess glue and "iron" the joint smooth: 

The completed wing tip should look like this:

The finished wing panel should look like this:

Repeat the process to make a second wing panel opposite hand.

The final step is to join the wing halves.

This should be done by the standard FT method: Trim the panels to length and for dihedral. Align and tape the top panels. Spread glue on all joining edges. Swing the panels into place pushing and pulling to match all surfaces top and bottom as close as possible.  Add glue to fill any gaps.  Squeegee and /or iron the glue to a smooth surface.  Tape the bottom joint.  I recommend doubling the tape top and bottom for additional strength.     

I am showing this wing for a particular application.  The cut lines shown on the plans are for Blue Streak, the wing shown at the beginning of the article.  This requires cutting to a shorter length, cutting to an angle (shown on plans) to produce the front and back tapers and trimming to produce the dihedral for the low wing design. It also has mounts for the application so rubber bands are not needed.  Here it is again:

Here are two wings that can be made from this wing panel:

The upper wing is the one for "Blue Streak".  It was trimmed to 54" long before the tips were added.  It was also trimmed at an angle so the wing has both leading edge and trailing edge taper similar to a Mustang wing. The joining cuts were made at an angle vertically to produce the dihedral angle.

The lower wing has the two wing panels joined without any trimming. It is 60' long with no added tips.  I plan to use it on a high powered two sheet version of my "No Waste Twin Tail Boom" series.  It will be a shoulder mount wing with no dihedral.  The leading edges are straight and the trailing edges are tapered.  It has 2 degrees of wash out for tip stall control but no under camber. This version will have two tractor motors with differential thrust rather than one pusher. It should be pretty fast and very aerobatic.

I am planning to do a separate article showing construction of a 60" constant cord fully symmetrical wing to be used on my "Stout Trainer" to complete the advanced trainer potential of that design. I also plan to do an article on "Blue Streak" at some point in the future but I need to be able to produce printable plans before that becomes practical.  Unfortunately some of my skills are still stuck in the Stone Age.

This wing panel can be modified to suit other applications. The aileron size and shape can be modified to suit the application.  The amount of taper can also be adjusted as needed to fit a particular design.  Using these principals you could modify almost any flat bottomed wing to a fully symmetrical one and change the flight characteristics of your aircraft considerably.  The possibilities are endless.

Symmetrical wings are less efficient for normal flight but they open a lot of doors in aerobatics.  The forces of lift are equal whether inverted or right side up. They are therefore better at inverted flight as well as rolls, loops and many other maneuvers. Advanced aerobatic aircraft generally use symmetrical airfoils.

Hopefully this will inspire you to try something similar to match your application.

Update 1-25-16: Blue Streak Wing Mount Details:

Based on a reader comment / request (jdfilkins) here are a couple of photos showing the Blue Streak's wing mount details.

This is the wing portion of the mount assembly:

The front of the dowels is supported by a piece of 1/8" lite ply that is slotted into the wing top panel and glued to the bottom panel.  It also acts as a positioning block between the sides of the fuselage.  The back ends are glued to two tongue depressors with the round ends trimmed off. There is an additional positioning block at the rear. The other parts are just glued on top of the wing. 

The nylon bolt goes through a piece of 1/8 lite ply the width of the fuselage glued to the bottom of the wing.  It screws into a threaded hole in a hard wood block mounted inside the fuselage.  There is a donut of hot glue on the top side to keep the bolt from falling out.

There is a small 1/8" lite ply bulkhead with holes matching the dowels at the front of the wing opening.  The holes are carefully positioned so the wing is snugly wedged up to the fuselage.

Stick the dowels into the holes and snug the nylon screw up.  That is it! 

Thanks to jdfilkins for the comment / question.

(End of update)