34" span Chipmunk- House of Balsa- build.

[Frank v B]

Stabilizer and elevator halves.

Problem: these are made of 3/32 sheet balsa.  The elevators are separate, then joined with a hardwood dowel.  When the glue dried the halves would/could twist.
Real issue- this plane has about 3 times the power for which it was designed.  This elevator would split or deflect because only one half has a control horn/pushrod.

Solution:
- add a piece of balsa between the elevators to reduce twist. It adds beef.
- fibreglass* the entire joiner area. Used 1/2 ounce cloth.  The waxed paper (again!) allows me to squeeze the epoxy flat and eliminates the annoying glass fibres sticking up.  When cured it will be absolutely smooth. 
The 4 clamps are to make sure the radius of the wax paper at the leading edge stays tight to the wood.  Otherwise an air bubble will form there.  Then the only solution will be to cut the bubble out or sand it out.

Have done it all my planes with a dowel or balsa joiner for 35 years.  Works well.

Frank

* for 4 summers in University I worked at CS Yachts in Brampton so have been around fibreglass for 45 years.  My strengthener of choice.

[Frank v B]

Wing hold-down bolts.

On a small plane, the blind nuts/T nuts require a fairly large hole in a small plywood plate.  Not a good combination.

Trick- drill a bolt sized hole through the plywood plate and install the blind nuts backwards.  Hold them in place with a piece of balsa.  See photo.  In the photo, the epoxy is on top of the balsa and it is slipped under the plywood hold-down plate.

benefits:
- the hole through the plate is smaller, leaving the plywood stronger.
- the back of a blind nut is tapered at the back side.  It acts as a funnel to guide the nylon bolt into the threads.  Works really nicely.
- the balsa acts as a brace for the plywood.  Doubles the gluing surface.

Note the missed centre hole.  That's when I went to the double bolts and the reversed blind nuts solution.

Frank

[Frank v B]

It looks like an airplane.  First time assembled.


Trick: there were some dents and scratches in the balsa sides.  See photo 71- One pass of a wet paper towel and the dents swelled and disappeared within 5 minutes.  Prefer this over filler because covering does not like to stick to light filler.

[Frank v B]

Wing hold-down bolts

I run the wing bolts through a pencil sharpener to make a point.  This auto-feeds the bolt into the blind nut.  Works like magic.

When cutting bolts (metal or plastic) put a regular nut or a blind nut on the bolt so when you shorten or sharpen it, the screw thread automatically removes the burrs and opens up the threads at the cut.
Photo 83 is of the actual bolts for the Chipmunk
Photo 84 is a sample larger nylon bolt so it is easier to see.

Frank

[msatin]

I had a lot of trouble beveling the joiner between the 2 halves of the elevator because it was ply.
In trying to bevel the joiner, I would sand too much on the balsa sections
Does using a dowel prevent the need for beveling?
Looking great btw!

[Frank v B]

Mark,

Thanks.  Your post contains 2 questions.  The easiest one first:

1) "Does using a dowel prevent the need for bevelling?"
No, it does not but it gives you a heck of a head-start.


2) "I had a lot of trouble beveling the joiner between the 2 halves of the elevator because it was ply."
It is only difficult if you use the wrong sanding tool.

Below is the photo of the 4 sanding tools I use:  (left to right)
i)  loose sandpaper- best for contour hand-sanding of any surface with multiple curves like fuselage bottoms, turtle decks, wing leading edges, curvy transitions like wing fillets.  See next post how to do the official "Sandpaper Fold"
ii) Dura-Grit flat file- It has a coarse and a fine side.  Great for sanding small straight surfaces.  benefit: it allows club member Roger Mason to keep buying model airplanes.
iii) sanding handle- it has foam under the sandpaper so it does not leave hard edges on curved surfaces.
iv) Sandpaper block  The best item and best for sanding the leading edges of elevators, rudders and ailerons.  A piece of hardwood (1" x 2" x a length) with sandpaper glued on each side- coarse on one side, fine on the other side.  This one is 15 years old and still sands like new. 
The sandpaper needs to be glued onto the wood, not nailed, not stapled and not held in place.  It cannot move or it will self destruct.  Use either contact cement or epoxy.  Do not use wood glue, it will wrinkle the sandpaper.  When glued, place sandpaper side down with weights on top so it dries absolutely flat.  Next day, glue the other side.
Important: do not wrap the sandpaper around the block. If you do this the sandpaper will radius at the corner and will cut grooves as you sand.  Best way:  glue the sandpaper to one surface, let it dry, then trim it around the edges with a knife.


This tool would have fixed your plywood joiner bevel issue in 5 passes and be absolutely straight.

I use other stuff a lot like files, razor planes, X-Acto blades, etc. but these are the major sanding tools.

Frank

[Frank v B]

The sandpaper fold.

I was taught this in 1972 when sanding the bottom of a racing sailboat*.  Sailboat bottoms could only be hand-sanded wet because no surface was straight or flat.

Take a full sheet of sandpaper. Fold it in half one way, unfold it and fold it in half again the other way.  It leaves a cross on the paper.  Tear any one of the folds from the edge to the middle.  Fold it so no two sandpaper surfaces touch each other.  This way it does not slip.  Sandy surface to sandy surface would destroy itself.
When the piece of sandpaper has lost its edge, use the other side until it is gone, then unfold it and re-fold it so the two remaining surfaces are exposed.

See the photos.

Frank

* For Rob and BJ... it was a Redwing 30 C&C design called "Herself" out of Hamilton.  The reason for the name: the owner was Hugh Drake and raced with his wife and 3 daughters.
The person who taught me the sandpaper fold was Dr. Doug Hood.  The Susan Hood race in June every year out of Port Credit is named after his daughter who was born while he raced in this overnight race (85 nautical miles) and won the first one.

[Frank v B]

Covering!!

Prepping for covering:
- sanding all corners smooth, filling dents and imperfections with light weight spackling.
- make sure that all moving surfaces work properly and have clearances.  reason: all clearances need to be big enough to allow 4 thicknesses of covering (esp. edge of ailerons)
- all hinging is in place on the ailerons but not glued yet.  Hinges will be glued after covering.
- the edges of the rudder and elevator have been angle-sanded.  Will be using the covering as a hinging material.  The 3/32" balsa is too thin for CA hinges.
- wheels off, no pushrods yet, no control horns.  I do those after covering.
- stabilizer and rudder/fin are covered separately and installed after all the covering is done.

Order of things:
fuse- cover the bottom first, two sides next, then top.  This is so edges of the covering are least visible.
wing- cover the bottom of both wings first, then the top the wings.  Reason- so  overlaps happen on the bottom of the wing and not visible.

Colours
Rule- do the lightest colours first so the darker colours will not show through.
try to match the temperature range of different covering.  Lower temperature covering (Solarfilm) can be installed over top of higher temperature covering (Monokote, OraCover).  It is very frustrating to try the reverse.

Cover the biggest pieces first- wings, stab, fuse, fin/rudder.  This is so you can use cut-offs of the larger items on the smaller ones.

The filler is drying overnight so the covering will start tomorrow.

Frank

[Frank v B]

Covering

Gentlemen, start your heating irons.

prep work:
i) make sure you have enough covering for the model.  See photo 98 of half of my collection.  Found it. (red, white, black, grey)

ii) always use a brand new blade.  The new blade is in the red knife for covering. The next to new blade is in the blue knife for wood trimming. Photo 95.
  note: the green masking tape...to stop the knife from rolling off my desk.  5 Years ago one rolled off and stuck straight into my thigh and hit a vein.  The blood circle on my pants was 6" in diameter.  That's not what hurt.  Trying to explain it to my wife did!

iii) this plane uses two colours on the wing.  Grey inside half, red outside half.  To give it a flat surface for the transition I sistered the ribs in that area.  Attached 3/16" balsa to the ribs and held in place with clamps.  Photo 96.  When dry, will trim it to the shape of the rib.  A lot simpler than making templates and pre-cutting the wood.

iv) did some final filling.  Here is a photo of the filler I bought at Home Depot.  There are about 3 brands.  Pick the one that feels empty.  It is the good light-weight one. See photo 97. I apply it with my fingers.  Easy to shape.

Did I tell you to use a new blade to cut covering?    Otherwise it tears and hooks.

Frank

[Frank v B]

starting to cover:

Wing

- cover the inside corner of the ailerons first...always.  Doing it later will be ugly! Photos 99 and 01
- lay the covering over the wing and tack down one corner (size of a quarter).  Stretch the long side (leading edge) and tack that down.
- lightly stretch the covering and do the opposite two corners so 4 corners are stuck to the wing.
- attach each of the 4 sides between the corners carefully.  Goal: to make it as flat as possible...without wrinkles... without shrinking it. Photo 03
- clean up each corner, do overlap edges at the leading edge or trailing edge by about 1/4". Photo 28

Note: do not shrink the covering with a heat gun until all the covering is in place, properly sealed and trimmed.  reason:  Both sides of a surface should be shrunk at the same time or the wing will warp.  Not too risky on this model but guaranteed to ruin your day if you are covering a 2 meter glider wing.

Frank

[Frank v B]

Covering the wing continued

- cover servo protrusion first.  This way it will be covered when the main sheet is added.  Note- it is off-centre.  See photo 29
- tack down the 4 corners.  Do not go past centreline of the wing.  This needs to be slit because of the dihedral.  See photo 48
- do the edges completely around the perimeter.  Note: none of the center was tacked down onto the ribs.  This will be shrunk in place when heat gun is used.
- Photo 50 shows the top and bottom done.  Note: nothing has been shrunk yet with the heat gun.

Doing the red tips
- cut the overlap with a metal ruler. Photo 52.  Note the ruler is upside down.  It is more accurate that way because of the cork glued onto the bottom.  That space allows the knife to wobble.
- start at the colour overlap so it is absolutely parallel to the rib line.  Then tack the 2 overlap corners, then seal the colour overlap completely....If you forget, the seam will pull apart when you go to shrink the covering.  Don't ask me how I know.

Wingtip
- pull it as hard as you dare and tack it down on the high point of the tip. Photo 80.  Instead of tacking down the corner, only tack the high point.
- OOPS! Pushed my thumb through the trailing edge.....just so Mark will feel better about doing it several times when covering his latest project. Moved it back together again, and glued a doubler onto the bottom between the 2 ribs.  Clamped it in place. Photos 83, 84

Time to quit while I'm ahead.  Let it cure overnight.

Frank

[Frank v B]

The wingtip


Always tough because of the curves involved.
Do the bottom first.  Hold the wing between your knees with the wingtip up.  Pull the covering as hard as you dare, touch the covering iron to the film as you stretch it around the curves. Photo 93.  Try to bring it halfway up the tip.  Only the front of the wingtip needed the covering slit to get it around. Photo 94.
Do the same for the top covering.  This encounters greater curves.

This covering has not been shrunk yet.  Only sealed at the perimeter.

Trick
Finding the hinge slots- when the wing is done, hold the aileron beside it and cut the hinge slots in the wing. Photo 98.
When you cover the aileron, do the reverse and transfer the wing slots onto the aileron.

One wing half done both sides.

Frank

[Frank v B]

Oops.

Two mistakes:
1) the red tip looked looked angled on one wing.  I made the joint 90 degrees to the trailing edge, not the leading edge.
Fix- cut a small strip of red with a ruler , make the angle and overlap it to correct the mistake.  See shots 00 and 03.

2) The red on one aileron did not match the wing. Cut a piece of red, one edge with a ruler, line up the line and attach it with heat.  Photos 06, 07, 08

The wing is now done.  Now it can be treated with the heat gun.

Frank

[Frank v B]

Trick- how to remove the backing from a piece of covering.

One method- poke your X-Acto #11 blade into a corner of the backing (photo 09), pull up and to the right with the back edge (not the sharp edge; photo 10).  It will peel it.

Frank

[Frank v B]

Heat shrinking the covering:

When doing the heat shrinking, I do one wing half at a time both top and bottom.
Notice my left hand (photo 12) is adding twist so that the trailing edge is higher than the leading edge. (wash-out).  I hold this twist while shrinking both top and bottom.
Wash-out adds stability to the wing.  When nearing a stall, it will force the middle of the wing to stall first. The tips keep flying and stops the plane from flipping onto its back.

Reason: In general- a wing stalls when the angle of attack is about 15 degrees*.  By adding twist to the wing of about 2 degrees, it will have an angle of attack of 17 degrees before it stalls.  When the inner wing stalls, the outer halves are still flying by 2 degrees.

Wing is finished- ready for servo installation and hinge glueing.

Frank

ps: the Northrop flying wing YB-49 has a wash-out of 7 degrees to make it fly.

ps2: corrections made to stall angles courtesy of "Full Size Fly Guy"