(This modification has not yet been implemented. It has been posted exactly as Jim sent it to me in an email. If you decide to undertake it, please document your entire work with images and let us know how it works. I will open a new section for the web site just for this, to showcase the work of the first person to submit their work. -- Juan)
The BD-5 Wing was designed for a specific gross weight. The tubular spar was designed for this gross weight. We collected a lot of data on extensive flight tests. Les Berven, our test pilot, logged well over a 1000 hours of BD-5 time. The results of flight test verified these design parameters. Structurally we knew the spar would take a greater load. What we intended was for the design deflection there would be no skin buckling at normal gross. When the aircraft is operated at a higher gross weight, the deflection obviously increases. This is the primary source of wrinkles in the upper wing skin. There are a variety of solutions but most add a significant amount of weight. This may get rid of the wrinkles but the weight increase surely does add to the stall speed and take off distance. There is also the high cost to some of these fixes. The simplest, cheapest fix, of course is to build it light to its original design empty weight. There are a number of BD-5's already flying that are heavy. There are some BD 5's under construction that are trying to solve the wing skin-buckling problem.
What I am proposing now is a possible simple solution to BD-5's already flying and for those under construction. What I am proposing is a carbon composite tubular spar added to the original metal spar. Composite carbon fiber structure is very rigid and would add a great deal of stiffness to the wing spar. What I am recommending is that you take carbon fiber material and roll it up into a long narrow tube. Roll it up so you have about two to three layers of material. What carbon material you use such as type of cloth or Uni directional tape is not real critical. It is stiffness not strength we want.
Since we have not done this ourselves, it is difficult to say which is the optimum material to use. The object here is to roll up the cloth so as to make a tube that you can slide inside the metal wing spar. What you will need to do is to take this cloth tube material that I will call a "sock". You make the length of this sock about half the length of the wing spar. You will have to make something that you can insert in the center of this sock that you can inflate. Possibly taking a bicycle inner tube and cutting and sealing one end and coming up with a method of inflating the other end. You may be able to use plastic film and make a tube to inflate. The objective here is to take this sock and impregnate it with epoxy resin the same as if you were going to make a hand lay-up fiberglass part. Insert this sock with the inflating tube down through the center of the wing spar. When you get it to the proper location, you then inflate the rubber bladder to form the sock to the inside of the wing spar. You do not have to use a lot pressure just enough to push the sock up against the metal spar. You then leave it there until the epoxy is cured. By using this method you will not have to be concerned about the ends of the rivets from the ribs protruding into the spar. The composite graphite tube will conform around these rivets. Obviously before you insert the sock, you will need to clean the inside of the spar the best you can with MEK. It would be best if the sock bonds to the inside of the metal spar but even if it does not it will add to the stiffness.
We definitely do not recommend making the sock go the full length of the spar. The thermal expansion difference between the metal spar and the graphite can cause high stresses on the bond line over that length. The shorter the distance the less stress. Obviously you do not want the carbon fiber sock to be in the region where the metal wing spar slides over the center section. Obviously when you put the inflated tube down through the center of the sock, you need the inflation tube to be coated with some form of release agent material so the rubber bladder doesn't bond itself to the carbon/epoxy material.
Here is what you can expect from this modification. You will have stiffened up the spar significantly with the carbon fiber. For modest over gross weights this should totally eliminate wrinkles with even the original skins. The strength of the wing will go up but never be less than the original spar. In other words if you do every thing wrong the wing will still be as strong as it was originally. Based upon how well it adheres to the inside of the metal tube, as to how much additional strength you will gain. It really isn't the added strength that you are looking for but instead, added stiffness. One of the beauties of the metal tubular spar is what Bobby Bishop discussed and that is that it will bend but not snap. An all graphite spar would not do that. We believe that this modification will have the minimum weight increase and can be done for no more that the cost of the materials.