Chapter 25: Canard


Shape is everything on the canard!  For the canard to perform well the shape needs to be as close as possible to the nice, rounded airfoil that John Roncz specified.  The problem is, most canards end up with a flatted section across the top of the spar caps.  Why?  Well, the canard starts out with a flat spot in it because the spar caps are laid up flat.  Then, we don't realize how much micro is required to build up the profile high enough to match the Roncz profile.  We might sand to a "pleasing shape", but it's to a much flatter profile than what Roncz specified.  Finally, the 45-degree sanding pattern has the tendency to flat-spot the top of the canard.  Walk down the EZ flight line and rub your hand over the canards.  Are the canards nicely rounded, or do you feel a flat spot over the spar caps?    

 

 

 

My "solution" is to use a milling machine!  Yes!  A milling machine, a canard sanding block that is run side to side over a level table.  I set up my canard on the very same lay-up table I used years ago.  Remember those "E" jigs we used in Chapter 10?  I screwed the "E" jigs to the table and set the canard on top of them.  With the table leveled, I used the "G" jig and set the canard at zero degree incidence to the table.  I bondoed the canard to the jigs so it wouldn't move during contouring.  Next, I made a 4-inch wide sanding block shaped to the Roncz airfoil templates from the plans.  You use it by placing a strip of sticky-backed sanding paper onto the block, then sliding it side to side along the span of the canard.  Lag bolts at each corner of the sanding block slide along the top of the table.  The lag bolts set the height of the sanding block above the table.  They also set the incidence of the sanding block!  As more and more of the micro is sanded off, you screw the lag bolts in by an eighth of a turn to lower the sanding block.  You keep screwing in the lag bolts and lowering the sanding block until you hit the high spots!  In this manner, you can precisely shape the micro to the Roncz airfoil.  This method also ensures that both sides get sanded the same with no twist!  This approach may sound anal, but I can tell you it's quicker and more accurate than spline sanding and having to stop every few strokes to measure the canard shape with little templates.  My buddy George used a sanding block on his GU canard for his Long-EZ.  He had one of the lowest stall speeds of any canard in the fleet.  It's what sold me on the approach.  It's quick, accurate, and fool proof!

   

 

Here are the details of the various steps in the process.

 

Inspections and Repairs

None required.

 

Surface Prep

I prep-sanded the surfaces by hand. I didn't bother dragging out the sandblasting equipment.  UNI sands easily enough even if it's not peel-plied.  (In retrospect, I wished I would have sandblasted the canard.  It took me longer than expected to sand it by hand.)  I sanded the top of the canard first, then contoured it.  I then flipped the canard over and sanded the bottom.  Then I contoured the bottom.

 

Before doing ANY filling and ANY sanding, I wrapped the lift tabs and elevator hinges with lots of tape to prevent them from being scratched.  A scratch on a lift tab can cause a stressor, which may induce a crack to form.  That could ruin your flying career and your life in one quick crash!  Now, I realize the lift tabs are in the center area of the canard and that there's really no need to fill-n-sand what's inside the fuselage.  But the lift tabs are really close to the surface that does get filled and sanded.  So they can be inadvertently nicked.  Wrap those puppies up! 

 

Canard Top -- Filling & Contouring

I knew the low spots would be over the spar caps, at the joggles in the fish tails, and over the joggles where the wingtips are glassed on.  The canard has so little surface area fore and aft that as long as you're piling the micro into the prefill areas, you might as well fill the whole thing.  Try as I might I didn't put on enough micro over the spar caps.  It took me three fill cycles to build up the spar cap areas high enough!  (See why most canards end up too flat???)  I was amazed at how thick the micro ended up being. (See why most canards end up too flat???)  I mean, it's not incredibly thick (because that would require it to be covered by fiberglass).  It's just thicker than I thought it was going to be. :-)    The sanding block made quick work of the micro.  I was impressed.  Sure beat trying to wield a 4-foot spline sander and guessing at the shape.  From the photos, you can see that the high spots are forward of the spar caps and between the spar caps and fish tails.  Again, there's alot of micro over the spar caps (as expected), and it is very tempting to keep sanding that area.  But don't do it!  You want that micro built up to the correct shape.  It's what you're after.   Shape is more important here than a few ounces of extra weight. 

 

The port side of the canard sanded out fine.  I wish I could say the same for the starboard side.  When I built the canard in Chapter 10, the outboard core cured a little low of the adjacent core.  That junction proved to be a very high spot!  In fact, I had to bury the starboard side of the canard under alot more micro to make up for that one high spot.  No matter though.  The shape is more important to me than the few ounces of extra weight.  By the way, the last picture of me is staged.  You can tell I posed for the picture because I'm not wearing my respirator.  I always wear a respirator when sanding micro.

     

 

Canard Bottom -- Filling & Contouring

This "sanding block" approach won't work on the bottom of the canard because the elevator hinges get in the way.  Obviously, a canard sanding block cannot be run back and forth full span because of the elevator hinges.  So I just contoured the bottom by hand.  The bottom of the canard is quite flat, so it lends itself well to being sanded with the usual 45-degree pattern.  Sanding the trough and fishtails proved to be the toughest parts to contour (again, because of having to work around the elevator hinges).  I was down to using small, flat boards and sticks to get the job done.  Many people dismiss the importance of contouring the trough and fishtail correctly.  Our canards are Fowler flaps.  The trough geometry determines the amount or air that flows over the elevators.  Therefore, try your best to accurately shape the trough and fishtail.  

 

Skim Coating

By now, skim coating is old hat for me.  What's not old hat is skim coating in 35-degree weather!  The skim coats don't get warm enough to tack up.  So I had to apply the skim coats differently.  First, I put cabosil in the epoxy.  I did this to thicken up the skim coats and to keep the skim coats from running.  (The skim coats still ran a bit, especially over the leading edge, wing tips, and the edge of the trough.)  Second, I squeegeed the first three coats to ensure the pinholes were mostly filled.  Then I floated on two final coats.  "Floating" means to intentionally cover with a very thin coat, making no attempt to squeegee it off.  I used a foam brush to float the coats on.  I cured the skim coats under a heat tent. 

 

Here are pictures of the canard bottom after the skim coats were applied.  Note that I applied masking tape around all the edges of the canard.  Even with cabosil, the skim coats do have a tendency to run over edges.  The masking tape redirects the drips onto floor.  I'd rather the drips being on the floor than rolling under the canard and hardening on the surface.  Also, note how I treated the holes in the wingtips for the elevator hinge pin.  I was a bit sloppy when I made that hole way back in Chapter 11. So, what I did was put a section of the same size rod through the hole and waited for the left-over epoxy/cabosil mixture to gel (kick off).  I scooped some out of the mixing cup and packed it into the hole.  Once cured, I heated the rod slightly so that it would release from the epoxy.  Once the wingtip is sanded, you're rewarded with a very tough, cosmetically appealing, fully-functioning hole.  Cool. huh? 

 

        

 

Priming

I waited until late Spring '07 arrived before priming the canard.  Again, by now this is old hat stuff.  I sprayed on the sacrificial primer and sanded most of it off.  Take a close look at the picture on the left and you can see that most of the primer was sanded off.  In fact, you can see a lot of white micro showing.  This is proof that I was still finessing the shape of the canard even at this stage of finishing!  I filled one or two pinholes and fixed a few rough edges before spraying the final coat of primer.  I'm very happy with how the canard came out.  It is now ready for top coat paint.  I'll try to paint the plane later this Fall (2007).

 

   

 

 


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