Chapter 25: Wings and Strakes


 

After the winglets, I decided to officially start Chapter 25 by contouring the wings and the strakes.  Why the wings and strakes?  My reasoning is most builders start on the bottom of the fuselage to develop their technique so that they are experts by the time they get to the wings.  But by the time they get to the wings, they are so burned out and just want to finish so badly that they end up botching the finish work on the wings.  The surface finish of the wings get perhaps the most scrutiny from fellow canardians.  I decided to do whatever it took to get the wings and strakes right.  If I burned out later down the road, then people would have to get under the fuselage to find the mistakes.

 

I filled and sanded the wings and strakes at the same time to ensure I got a good transition (blend) at the wing/strake junctions.  I started with the bottoms first so I could perfect my technique some more before contouring the tops for everyone's enduring scrutiny.  I did one wing/strake at a time.

 

Inspections and Repairs

I repaired two air bubbles that formed on the wings during cure.  One bubble formed between the 1st and 2nd BID plies that cover the 9 plies for the wing-to-winglet attachment.  I sanded away the bubbled glass, feathered back the surrounding glass, then applied a 1-BID patch over that area.  The other bubble formed over that sliver of foam used to get the wing root core to the correct thickness prior to hot-wiring.  I repaired it the same way.

 

Surface Preparation

The wings were removed and sandblasted.  The strakes were sanded by hand because they had been peel plied.  Note the West epoxy containers in the corner of the second picture! :-)

 

 

 

With the wings removed, I rolled the plane outside, installed the clamshells, and rolled the plane inverted.  I built a firewall jig and jack stands to hold the plane up while it sits inverted for a few months.  The firewall jig is bolted to the firewall using the holes for the engine mount.  I have wheels mounted onto the jig so I could roll the plane around if I need to.  My initial plan was to roll the plane out of the hangar every day and do all the sanding outside (to keep from burying the hangar with a foot of dust).  As it turns out, this was not a very good idea.  It's a hassle to do this and there's always the risk of dropping the plane or banging the wings on the ground or into the hangar doors.  So, the plane stayed in the hangar and I cleaned up the dust later.  The jack stands are built from 3/4-inch plywood.  They are placed at the ends of the center section spar to help stabilize the plane.  The firewall jig and the jack stands are sized so that the winglets are about 3 inches off the ground.  I lifted the wing up and rested the top of the winglet onto a piece of blue wing foam.  The weight of the wings on the blue foam helps to keep the wing from moving back and forth as you sand. 

 

 

 

With the winglets so close to the floor, I decided to put something around them to keep them from being whacked by feet, shop-vacs, extension cords, etc.

 

 

 

Pre-Filling

My hangarmate is always saying, "You can tell a pilot, you just can't tell him much."  At this point, I was still too stubborn to prefill the low areas on the wings.  Not doing the pre-fill caught up to me once again.  I ended up filling the wings three times (!!!) before getting rid of most of the low spots.  It was clear I had not perfected my spreading technique and I still SUCKED at gauging how thick to apply the micro.  Try as I might, I never filled the low spots adequately the first time.  Thus, after a few hours of sanding, I had to reapply the micro and try again.

 

I learned my lesson after requiring the three fill-n-sand cycles on the wings.  I dedicated myself to trying the pre-fill on the strakes.  I found the low spots and filled them.  I was VERY surprised at how many low spots there were on a surface the LOOKED flat.  I immediately became sold on the need for prefilling.  In the long run, it made the big fill on the strakes so much easier to do.

 

The low spots will vary from wing to wing.  But in general, the low spots will typically be forward and aft of the 9-ply layups at the wing tips, at the outboard end of the aileron spar, and either over the spar caps if they're filled low, or forward and aft of the spar caps if the spar caps were overfilled.

 

Big Filling

 

Before frosting the wing/strake, I slipped some plastic film into the joints between the wing and the strake.  I'm using transparency film.  This is an old trick I learned from another builder who achieved absolutely perfect seams.  I'm not sure if you can decipher this from the pictures or not, but you end piling some dry micro onto each side of the plastic film.  Once it cures, you trim off as much of the plastic film as possible, then just start sanding.  The sanding takes away the excess micro and the excess plastic film.  The real purpose of the film is to keep the micro from gluing the wing to the strake!!!  But after it's all said and done, you will end up with fantastic seams that are so well matched as to be nearly invisible.  Please note that at this point the seams are really too well matched.  You will need to widen the seams a bit later to account for the thickness of the paint.  But at this point, it is easier to widen the seams later than to close off the seams later.

 

 

 

I perfected my spreading technique while doing the bottoms of the wings.  I got rid of the cement trowel I was using and switched to the 6-inch wide dry wall paddle with a spring steel blade.  The cement trowel wasn't working for me.  It was too bulky and was way too wide (12 inches).  Because the blade was rigid, I felt like I had no control with it.  I kept digging the blade into the micro and causing gouges (which cure into low spots).  The dry wall paddle is about the right size for me.  Because its spring steel blade flexes under load, I had more control of how much micro got squeezed between the blade and the surface.  I held it very flat to the surface, which allowed me to spread the micro more evenly with the same thickness across the wing.

 

I also got better at spreading the micro more evenly without all the peaks and ridges.  What I started doing was gently heating my paddle with my heat gun.  The heat of the paddle warms the dry micro, making it spread easier and thinner without balling up behind the paddle.  As I was spreading the micro, I alternated between heating the back side of the paddle with gently warming the micro in front of the paddle.  After the initial spreading I used the same technique to knock down and smooth out the ridges and peaks.  Ridges and peaks form off the edges of the paddle as you pull the it across the surface.  If allowed to cure this way, you end up wasting time having to first sand down all these ridges and peaks.  So the last step I did was to warm the trowel and _very_lightly_ drag_it_ over the peaks and ridges to knock them down and spread them out a bit.  The trick (and the art) is to do this without interrupting the surrounding micro.  Here's a picture of me using this technique while adding micro to fill a low spot on the strake.  When I say to apply heat, it must be very gentle heat.  You'll know it when you get the paddle too hot.  The micro will start smoking big time and it will exotherm (cure) onto the paddle!  Don't do that!

   

 

Contouring

It was while sanding the wings that I switched from the home-brewed sanding boards to the professional fairing boards and Durablocks.  These made a world of difference.  Unfortunately, I had not yet heard of the raw epoxy finishing technique.  So I was sanding all the way through to 120 grit.  I now only contour to 80 grit.

 

Here are what the wings and strakes look like after being contoured.  Note the obvious dark patches (high spots) that have developed on the strakes.  The bottom of the strakes are NOT flat.  They are gently curved almost everywhere!  So you will end up with a lot of high spots (and that's a good thing).  Hopefully you can also see how translucent the micro has become on the wing area ahead of the where the ailerons are supposed to be.  If you were to see the wing in person, you'd see that the micro is much more translucent than shown in the pictures.  It's sanded to within a few passes of bare fiberglass. Each wing is different and each wing will have its own unique distribution of high spots.  But in general you can expect the high spots to be along the wing/strake junctions, at the leading and trailing edges, over the 9-ply layups at the wing tips, and either on the spar cap itself (if the spar cap is overfilled) or immediately ahead of and behind the spar cap (if it is too low). The pictures here are of the starboard wing and strake.  Note the bright white micro over the spar cap, which means the spar cap ended up a little lower than the surrounding surfaces.  My port wing was just the opposite because most of the micro was sanded off above the spar caps.  The moral of this story is NOT to use one wing as a benchmark for the other.  Just keep sanding, shaping, until you are about to hit the high spots.  Then STOP and WALK AWAY!  I wish I had better pictures of the contouring, but I've found it extremely hard to take good pictures of a white wing.  Now I understand why so many other builders don't have pictures on their sites either. 

 

   

 

Skim-Coating

The skim coating worked extremely well on the wings.  Just as advertised!

 

Priming

I removed the wings from the strakes and primed them separately.  Again, I had about 20-30 pinholes to fill in each wing, all in areas where I purposely sanded through the skim coats and into the micro.  This is a picture of the sacrificial primer.  Unfortunately I never took a picture of the wings in final primer.  Again, the wings will stay in their final primer for a while.  I'm going to wait to paint the wings until all the other parts of the plane are in primer.  I'll wet sand the primer to 320 grit just before I'm ready to paint the plane.  I've moved my wings and stored them in another hangar to prevent them from getting banged up (hangar rash).

 


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