How to build a double-tapered balsa wing (50" span, .45 size)

[Frank v B]

Next project is a plane.

Problem
Mark Satin ordered a short kit of a '90's design that is quirky.  It should be a blast to fly.
The short kit has foam wing cores but was very short, especially on the instruction side.

Solution
We will team-build two of them.  Mine will have a built-up wing so I can start on that right now.  Will post it as this "How to build a double-tapered balsa wing."
Now that Rob Dickinson has decided to build one too, this serves as building instructions for his built-up wing.

Mark will reveal the project soon.  Experienced builders will recognize the planes*.

Frank

* clue is "Bill Evans".

[Frank v B]

Step 1- cut hardwood root and tip ribs.  The root rib was drawn on tracing paper.  The tip rib was traced onto regular bond paper.
Step 2- cut slots for the spars.  I picked two top spars and one bottom one.
Step 3- mark the datum lines on the ribs (straight lines front to back visible on both ribs)
step 4- measure the sweep of the wing half- the leading edge of the tip rib was 1 1/8" behind the root rib.  The photo of the two ribs on top of each other are the proper 1 1/8" stagger.

Note the photo of the two ribs on top of each other with the datum lines lined up.  It clearly shows* that the ribs have a progressive wash-out built in.  Wash-out is when the trailing edge at the tip is raised when compared to the root rib.  It helps delay stalls at low speed.

Ready to make the ribs.

Frank

* the spacing of the bottom of the tip rib increases as it goes back to the trailing edge from the bottom spar.

[Frank v B]

A few main prep items:

1) Rib spacing- looked at the rib spacing on my 48" Taylorcraft- 2".  Decision made. Need 24 ribs plus a center rib.  Cut the blanks- see photo.
2) Wing style.  Decision: "D"-tube, 3 main spars- two on top, one on the bottom, cap strips top and bottom.  Note: "D" tube is when the leading edge is sheeted from the top main spar to the bottom main spar.  In cross-section it looks like the letter "D".
3) Dihedral- the plans show dihedral is small.  The wing is joined upside down on a flat surface.  That is equal to about 1" of dihedral.  I may add a bit more.  A low wing airplane with a flat-ish wing looks odd, as if it has anhedral*.  Note: the upside down wing joining trick is the same as we used on our Nooner pylon racer. It looked fine because it was a shoulder wing.
4) Dihedral joint and landing gear mount.  I use my standard pine continuous piece.  The plans show mounts between ribs.  I never use this because it will rip out on my trademark hard landings.  Will explain later.

Frank

* anhedral is negative dihedral or "droop"

[Frank v B]

Step 1- Doctor up the wing plan.  Draw in the ribs (red ink).
I cut the wing plan (foam) off and drew in the rib spacing.  Outside the last rib will be a shaped horizontal wing-tip.

Frank

ps: the red squiggle line is a mistake.  I'm only human.

[Frank v B]

Make the ribs.
1) stacked rib blanks between the root and tip ribs
2) with a drill press, drilled 2 holes all the way through the stack
3) slipped brass tubes into the holes and clamped the stack tight with wheel collars
4) sand the ribs so they meet the root and tip rib templates.

[Frank v B]

The stack of ribs sanded.  Used a 12" piece of pine 1x6 and stapled 100 grit sandpaper to it.  If I were to do it again I would use 80- grit.  It is a lot of sanding. (45 minutes).
Cut the spar slots with a razor saw.

Frank

[Frank v B]

Match the ribs.

The stack of ribs means every rib is smaller than the one beside it.  What I do is:
- number all the ribs from #1 (largest) to #24 (smallest) just in case you drop them or mix them up accidentally.
- put rib #2 (smaller) on top of rib #1 (larger).  Cut the larger rib to the size of the smaller one.  They now match.  Re-number them both as #1
- do all 24 this way and re-number them from 1-12. 3 on top of 4, 5 on top of 6, etc. the first photo shows 12 piles of two matching ribs.
- now create two piles of new numbered ribs 1-12.  These become the left and right wing panel ribs. The second photo.

Frank

[msatin]

Who needs laser cutting?! 

[Frank v B]

Finishing the ribs:

Since the ribs were twinned with larger ones trimmed to the smaller one, it means the centre rib was missing.
Made 2 centre ribs out of 3/16" balsa scraps and labelled them with a red "1".  Now the stack of ribs is complete.

Mark: re: your comment "Who needs laser cutting?!  "  No way it could handle this assignment.  The ribs were all made out of scrap wood, 4 different sizes (1/16, 3/32, 1/6, 3/16) and one centre rib was glued together out of 4 pieces of balsa. That's why the cap strips to strengthen them.

Not a single piece of balsa was the size as sold. 
I call it "pre-cycling".  Crashing becomes "re-cycling"

Frank

[Frank v B]

Building the wing.

step 1- line up and pin the centre rib on top of the bottom spar.  I am using a hardwood bottom spar because I found two pieces.  Measure from the datum line on the rib to the building board (red mark at the leading edge (LE) and trailing edge (TE) of the rib).  Make sure it is the same distance from the table top.  This datum line is marked on the plans and on my two rib templates.
Step 2- line up and pin the tip rib.  This time, I measured the red mark on the leading edge and trailing edge so that the trailing edge is 1/4" higher off the building board.  This automatically adds wash-out to the wing (trailing edge higher than the leading edge to reduce stalls)
Step 3- cut the leading edge and trailing edge so it sits on the table top and at least 3/32 above the centre and tip ribs.  My leading edge was cut from 3/16" sheet and is 1 1/2", the trailing edge is 1".
Step 4- mark the top of the centre and tip rib on the leading and trailing edge.  Join these points with a metal yardstick.  That is the red ink line along the LE and TE
Step 5- glue the centre rib, tip rib to the leading edge and trailing edge.  Let it cure.
Step 6- line up the LE and TE so it is straight between the two ribs (no wow in it).  I do this by placing the ruler against the two pinned ribs.  Then push the middle of it up to the ruler so it is absolutely straight and pin it in the middle.  This is a must because as you add the other ribs it wants to curve the LE and TE.

Let everything cure so it is solid before you install the other ribs.
Note: I never cut spars, leading and trailing edges to size.  They stick beyond the wing so it can be pinned down outside of the wing so sheeting can be added while it is pinned down.  It allows adjustments (mistakes, poor measurements, etc.) while it is being built.

When each rib is added it is glued and pinned so the top of the rib touches the red line on the LE and TE.  A quick adjustment is when you push down on the metal ruler as in photo 70.  It automatically lines up all the ribs.

Note- the LE and TE stick above and below the ribs so the sheeting can be glued to it.

Tomorrow- install the rest of the ribs.

Frank

[Frank v B]

Install the ribs.

Step 1- secure the center- install, glue and pin a rib in the middle.  Its purpose is to lock the leading edge and trailing edge in place.
Step 2- size each rib.  Each rib was a bit too big.  To make each one fit I used the same order so that all the upper spar gaps stay in line:
i) trim the forward end of the bottom spar gap.
ii) trim the front edge of the rib if needed
iii) trim the trailing edge
The size is correct when the front end stayed under the steel ruler and the top of the rib hit the red line on the trailing edge.  Sometimes I had to sand the bottom of the rib to make this happen.
step 3- once the glue holding the rib in the middle has set, glue and pin the other ribs in place. Make sure you do not push the LE or TE outward.

First photo show the rib glued into the middle with 5 minute epoxy.  I then sized all the other ribs while the glue cured.
The second photo shows all the ribs glued and pinned in place.  At the leading edge all the ribs were held in place by tucking it under the steel ruler taped in place with masking tape.
The 3rd photo shows the two top spars in place.

Rob, no need for 13 ribs in each half.  11 is enough.  It looks a little "ribby".  Plenty strong.

Frank

ps: in case you wondered why his wing is built in the air without pinning the trailing edge to the board.  This is a special wing section (reflex wing section) that can best be described as an "S" curve.  The bottom does not have a flat portion all the way back.  This plane does not have a stabilizer so both ailerons are cranked upward 1/8" for level flight.

[Frank v B]

finishing the top side:

Photo 14- Check for high ribs at the trailing edge- I used a metal foot long ruler and moved it over the ribs.  It immediately showed high spots which I cut off with an #11 blade.  The ruler would rock back and forth over a high spot.

Photo 15- after cutting off the high spots I used a sanding block to smooth everything out.  Use the sander from front to back, never span wise (left to right).  You will break the ribs if you go left to right.

Photo 16- curve the leading edge sheeting.  This is what one pass of a wet paper towel does to the wood.  It curled the wood.  By the time I finished applying the glue, the curve was 50% greater.  It takes the tension out of the wood.  I used 15 minute epoxy to glue the sheeting in place. I never use wood glue for this.  The wood absorbs the moisture from the glue and can warp the wing.  Don't ask me how I know.

Photo 19- ready to dry overnight.  Note- make sure it is pinned down so it cures straight.
Note- no cap strips yet.  Will do that after the wing halves are joined.  A finishing job.

Frank

[Frank v B]

Second wing half.

Main message: no matter how accurately or inaccurately the first wing was built, make the second wing half with the same mistakes.  It will fly fine as long as all mistakes are symmetrical.  You can quote me on this.... that's why transmitters have trim levers.

i) Took the wing off the board and trimmed all the spars and balsa at the centre rib.
ii) Calculated the dihedral associated with the dihedral angle.  The wing is joined upside down flat on a board.  That means the dihedral is equal to 1.3 cm off the board for one wing tip when right side up.  I doubled this and rounded up to 3 cm one tip.  A low winger with small dihedral looks like the wing droops.  This should avoid it.
iii) pinned the finished wing to the board at the centre rib, raised the finished wing half off the board 3 cm using the dihedral block I cut (photo 21 at the tip of the knife)
Built the second half directly off the first half.  Used the same rib spacing.  Made all dimensions on the leading edge, trailing edge and tip rib the same as the first wing half.  Glued these 4 pieces in place at the same dimensions and same angles.
iv) installed a rib halfway to keep everything straight.  Let it cure.
v) installed all the other ribs and am letting the glue cure.

Next steps:
- when dry, install the two top spars and top sheeting.
- make and drop in the landing gear mount/dihedral brace (one piece of wood).  Glue and secure.
- add the bottom sheeting at the leading and trailing edges.
- add centre sheeting top and bottom
- add to the leading edge to make it the proper thickness.
- sand the wing to the proper shape, trim leading and trailing edge, shape the leading edge to the proper airfoil shape.
- add the cap strips top and bottom
- make and hinge the ailerons
- install the servos and feed the wires.
- fit the wing to the fuse.

Frank

[Frank v B]

Calling all porcupines.

Sheeting the top section of the wing.  Used 15 minute epoxy.
Almost ran out of pins!

Am only sheeting the top to strengthen it.  Will not sheet the bottom until the dihedral race/Landing gear mount is installed.
The dihedral brace will be installed from the open bottom.
Stay tuned.

Frank

ps: yes, the little triangle is missing on the left side.  Will wait until I get a scrap from sheeting the bottom.

[Frank v B]

Installing the dihedral brace/landing gear mount.

Used a scrap piece of 1x2 pine from Home Depot. Yep, found it in my car!  

- determine the length of the brace and cut it to length.  Done by looking at the end of the landing gear mount on the plans and extending it to the next rib (5 ribs on each side). Between the green tape marks on the wing.
- place the brace on the bottom of the wing and mark the 10 ribs affected.  Relieve a channel through all the ribs.  Make the depth conservative. Photo 36.
- test slip the brace into the channel and determine how deep it should be at each rib (see red pen marks). Photo 36.
- cut the depth to the approximate amount.  It will probably be crooked.  I liked the depth on one side and measured it.  Marked the same depth on the other side.  Cut the channel to the approximate depth, making sure it was equal on both halves.
- fine tune the channel.  There will always be one rib not deep enough....like an annoying little kid. Photo 37.
- Mark the brace. I followed the bottom spar with a pen.  It shows the dihedral angle. Photo 38. The "X"s are the part to be cut away.
- glue in place and clamp. Photos 39,40.

Frank