Covering a model with heat shrink covering (e.g. Monokote)- Graupner Kadett

[Frank v B]

Covering a Model

This post will show the steps in covering a model.  There are about as many ways to do it as there are modellers. 
Here is the method I use.  Feel free to post tricks and tips and the normal insults. 

The model is the Graupner Kadett which is a 3 channel vintage model (no ailerons).  The link to the build post  https://temac.ca/smf/index.php?topic=8107.0

Overall approach:

Fuselage- bottom first, then the sides, then the top.  This way the bottom edge of the sides overlaps the bottom and is not visible.

Wing and stab- bottom first then the top.  This way the overlap edge is on the bottom and not visible.  Any wing with dihedral should be covered in 4 separate pieces.  The stab in this project can be done as a single top and single bottom.

Shrinking the covering
:  Fuse- do it as you go along.  It is best to start at the nose and work your way back.  Always leave an air escape route clear.  Trapped air will create a bubble.  Use a pin prick at one end of a bubble and work your way towards it with the covering iron so air can keep escaping.
: wing/stab- Take a pin and prick a hole in each wing rib (anywhere) and make sure there is an air escape hole at the centre of the wing.  If you use independent aileron servos, the holes for the wires will do a great job.  Just make a small hole in each rib outboard of the aileron servo bay.  The reason- when you go to shrink the covering with high heat, it will balloon each rib bay and the covering will pop off the rib.  In many cases the colour backing will stay on the rib and you will now have a clear plastic section in your film.

Hatches: I do these last because it is likely that they can be covered with cut-offs from the wing and fuse.

Hinges: I will show you how to do Solarfilm hinges-.  It uses the fused covering to make a sealed hinge on the rudder and elevator.

Frank

[Frank v B]

Covering the wing.

Remember:
- Bottom first.
- half a wing at a time.
- attach the covering at the edges only, not the middle of the wing half.
Do not shrink the covering until the wing has been fully covered.

Step 1- cut the piece of covering about 3/4" larger around the perimeter.  Remove the clear plastic backing that protects the adhesive/colour.
Step 2- lay the piece over the wing with proper overlaps on all 4 sides.  Tack down one corner with the iron.  Just a dot the size of a dime.  I start at the center of the wing.
Step 3- tack down the second corner after stretching it a bit while gripping it between thumb and forefinger. (Photo 08) I tack down a long edge first (leading or trailing edge) and not a short edge (chord-wise).  When I have tacked down the short edge first, it has gone crooked and the covering misses the wing on the long edge.
Step 4- tack down each side with heat around the perimeter only.  Curl the leading edge all the way around to the top of the wing's leading edge (not touching the ribs).  This adds strength. The trailing edge is curled around to the top of the wing and adhered to the wood with heat.  Again, stop at the ribs. (photo 811)
Step 5- the wingtip.   Cut the covering about 1/4" long and make cuts about every quarter inch (photo 09).  Curl it around the wingtip starting at the back (photo 810).  This is so that the overlaps are downwind.

Do the second half the same way.  Bottom of the wing covering is done in photo 812.  Shrinking is done at the end*.

Frank

* shrinking is done at the end for two reasons
- if you shrink each panel as you install it, the wing will twist.
- you need to shrink it while managing the wash-out twist correctly.  To lock this twist in, you have to heat both sides alternately.  I will explain "wash-out" when we get there.

[Frank v B]

Covering the stabilizer- How to do a "Solarfilm hinge"*
This is safe to use on small planes up to about 450 size.  I used it on my Nooner pylon racers and worked fine.

Prep work.   
- The trailing edge on the stabilizer should be square with sharp corners (no rounding).
- the leading edge of the elevator should be undercut (sanded) to a bevel of at least 30 degrees.  On many models I glued tri-stock balsa to the leading edge so there was an instant and accurate 45 degree bevel.

Covering the bottom of the stab
- cut the film so it is big enough to do the entire stabilizer and elevator in place
- start covering the bottom of the stab.  Seal all 4 edges.  Seal the training edge while folding the remaining covering back so it does not see any heat.  Seal the flat (bottom) portion only. Do not go up the trailing edge. 
- with the stab flat on the table with the covering on the bottom, lay the elevator in place.  I use three pieces of cello tape on the top to hold it in place. photo 24
- fold the elevator back onto itself and wrap the covering around the bottom of the elevator. See photo 25- Notice I put a small clamp on the assembly so it will not shift while lining things up.
- attach the covering to the bottom of the elevator first (photo 26), then seal the hinge line at the stab's trailing edge and the elevator's leading edge.
- finish the bottom covering around all edges.  Do not heat shrink the middle of the stab.  Done last.

Covering the top of the stab.
- attach the top covering onto the stab (not the elevator) and seal all 4 edges.  Keep the heat away from the hinge line.
- grip the covering and the stab with one hand in such a way that the covering is tight and the elevator is in the full "down" position (Photo 29).  Sounds tougher than it is.
- attach the covering to the entire elevator while holding full "down". Photo 31
- seal the hinge line- while holding full down elevator seal the entire hinge line everywhere.
- finish all the edges.

Now heat shrink covering on the entire stab and elevator combination with the elevator run "neutral" position.  This sticks the top covering to the exposed hinge line from the bottom.  Your mission is to stick the covering together at the hinge line. Finished tail feathers photo 32

Stand back and admire. 

Added the graphic of the covering process.  The corporate artist was out so I tried....   Photo 34

F.


* I saw this in a British magazine about a quarter century ago.  Britain's main covering material has the brand name Solarfilm.  In North America it is MonoKote.  Pick your poison.

[GeoffM]

Going to copy and paste this post into a PDF.  Refer to it later.  Have a fuse to cover and been getting tips from Greg and this is also helpful.

[msatin]

Frank walked me through this process on my Stevens "Shaft" and it came out perfect.

[Frank v B]

Covered the top of the wing but did not have my camera.  It was in the cold car a cold 20 feet from our front door.

Covering the fuselage
- Photo 35 shows that the sides were made of two pieces because I cut the covering width-wise (less waste).  Started at the tail and worked my way forward so the last seam will be down wind (front piece overlaps the back piece).  Did it in two colours top and bottom.  The straight lines are cut via a metal yard stick and a model knife.  Notice that I messed up the front two inches of the red piece.  It droops down.  Not a problem as long as I make the front piece overlap it.
- Photo 36- the opposite fuselage side finished.  I will add a racing stripe and cockpit windows.
- Photo 37 the finished side.

F.

[Frank v B]

What the heck is "wash-out"?

Wash-out is used in a wing to soften/avoid stalls.  When a plane stalls it essentially loses lift and falls out of the sky.  Not a problem at height when you can dive down, pick up speed and fly out of it.  When you are slowing down for a landing, especially when turning on final and the plane stalls you have no alternatives to picking up an airplane..in a garbage bag.

In a wing, there should be about 2-3 degrees of "wash-out" where the trailing edge is higher than the leading edge at the wingtips*.  When one wing half is flat on a building board, the trailing edge at the wingtip should be about 1/4- 3/8" higher than the leading edge.  This way the wing at the center loses lift first, allowing the wing to keep flying at the wingtips to keep lift going.

A wing generally stalls when the wing has an angle of attack of about 12 degrees.  This means the wingtips do not stall until the angle of attack of 14 degrees.

When the reverse happens it is called "wash-in".  The trailing edge is lower than the leading edge.  This way if the plane is at an angle of attack of 12 degrees (at stall), the wing tips will be at 10 degrees and losing lift.  Whichever wingtip has the most wash-in will lose lift first and the plane will want to flip on its back.

How to add wash-out in a wing
i) in an open structure wing (like this project)- it is best to build it flat and add wash-out when the wing is covered.
ii) in a "D" tube wing (sheeting on the leading edge on the top and the bottom) I would build the wing half flat, then lift the trailing edge by about 3/8" when doing the top sheeting, let it dry and then sheet the bottom.  Final adjustments can be done at the covering stage.
iii) in a fully sheeted wing (balsa or balsa covered foam) you must build it in.  No way to significantly twist it once the sheeting is applied.

How to add "wash-out" (twist) : cover the entire wing but do not shrink it until you grip the center section between you knees and add the twist with one hand and shrink it while holding the twist in place.  See photo 840

Photo 838 is what the wing twist looks like.  Sight the tip along the center rib while holding the wing upside down.  It will clearly show the twist.  The photo shows the bottom of the wing (the leading edge is on the left).

F.

* interesting trivia.  Dr. Keith Shaw told us in an EMFSO presentation that the Northrop flying wing (YB-49) had a wash-out of 7 degrees.  A flying wing without wash-out is unflyable.  An F-18 has 4 degrees of wash-out.  See article https://www.boldmethod.com/learn-to-fly/aircraft-systems/how-wing-washout-makes-your-airplane-and-wings-more-stable-in-flight/

[Frank v B]

Covering is finished.  Final assembly is next.

F.