Repairing a Composite HLG Wing
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Home > Articles & Tips Index > Repair Tips > Repairing a Composite HLG Wing

[Courtesy of David Walter, who heard it from Jim Tyrie, who got it from???, modified a bit by Joel Foner, July 1998]

If you fly a competition HLG with composite wings and throwing peg, you will likely eventually do "the no-no".  Throwing pegs can give great launch height, but there is one thing you should never do - you should never pull down at the end of the release (as your reflexes tell you that you should from throwing a baseball).  If you pull down, you'll hear a funny (well, not so funny) crunch/crack sound and at the landing you'll find a nasty looking crease along the chord of the wing - on top - typically around the wing servo bay area. 

Often, the bottom skin will be unharmed, even though the foam and the upper fiberglass layer got "hurt".  The damage may look like a crease from front to back on the wing, or it may actually cause a fairly straight front-to-back fracture of the upper skin fiberglass.

DON'T THROW THE WING AWAY!  THIS CAN BE REPAIRED PRETTY EASILY WITH LESS THAN 3 GRAMS ADDITIONAL WEIGHT IN THE WING!

Here's a technique that will bring your wing back to life (detailed steps, but will likely only take you an hour or two - far less after the first time).  The basic idea is that we'll be inserting two 1/32" thick spars, spanwise, one in front of the servo bay and one between the servo bay and trailing edge.  These spars will be the full height of the wing at the installation point, minus the lower skin thickness.

  1. Gently (and this does mean gently) warm the upper injured surface of the wing with a heat gun from "a ways away".  Repeat a few times after cooling - don't let the surface get too hot to touch, since you don't want to melt the foam below.  This will cause the creased area to shrink a bit, smoothing out the crease as much as possible before we start the repair.
  2. We'll be making two "spars" of 1/32" ply, so if you don't have any 1/32" aircraft ply then now is the time to buy or borrow a sheet.  (David used 1/16" for the front spar and 1/32" for the rear - I decided that 1/32" would do for both, and after many weeks of use it has held just fine - choose whatever you're comfortable with after reading these instructions).
  3. Measure the width of your servo bays (spanwise), and then mark a line parallel to the span, about 1/2" ahead of the servo bay (which will usually be slightly in front of the wing's high point).  Make the line about an inch longer than the spanwise width of the servo bay - 1/2" extra on the inboard and outboard side.
  4. Put one of those thin cutoff wheels in your Dremel, and carefully cut a straight line through the wing upper surface, spanwise, the full length of the line.  You do want to go down as far in the foam as you can, but you don't want to go through the bottom fiberglass skin!
  5. Measure the foam height at the front of the servo bay - or the height of the slot you just cut.
  6. Cut a 1/32" spar that is the length of the cut, and the height of the wing foam.
  7. Cut a 45 degree angle at each end of the 1/32" spar, so that the stiffness of the wing will gradually increase, not spike at the start of the spar.  The bottom of the spar will be shorter than the top of the spar when you're done, by typically 1/2" or so in total.
  8. Mix a small amount of 5 minute epoxy, then "push" some into the spanwise cut that you put in the top skin (I used a toothpick), and put a thin layer on both sides of the 1/32" spar.   Push it gently into the slot until the top of the spar is at or very slightly below the skin surface, wipe off any excess at the top surface, and let the epoxy cure
  9. Do 2-8 again for the rear spar - eyeball the rear spar to be partway between the rear edge of the servo bay and the trailing edge.

Once this is done, you've got a wing that is stronger than it was in the break area, with very little additional weight.

What you do from here depends on your esthetics and how much effort you want to put in.   Some just fly it this way.  You can also sand the affected area with very light sandpaper for better adhesion and then fill the low spots with filler to get as close as possible to the correct airfoil with as few chordwise ripples as possible.   Rather than paint over the filler (since many fillers are water-based and will wash away...) you can put some clear packing tape over the repair chordwise - from front to back.

Back to flying!


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