A word about adhesives. One of the great leaps in technology that occurred during WWII was the development of Epoxy Adhesives. T-88 adhesive was developed by Howard Hughes for the HK-1 or Spruce Goose. Prior to the use of synthetic adhesives all wood bonding agents were organically based, either they were derived from curdled milk (Casein Glue) or from the fat of rendered animals. (Hide Glue) these products can all be attacked by bacteria and will eventually break down. All of these early adhesives required good fit up between the wood joints and high clamping pressure.
But along came Epoxy. A good wood joint occurs when the wood fails under test away from the joint. The adhesives need only be slightly stronger than the materials they are splicing. Epoxy adhesives are different from epoxy laminating resins. The main difference is viscosity and the addition of polymers (rubber) into the resin matrix. These polymers are what make adhesives different from resins. In a bond joint, the stresses tend to concentrate at the ends of the joint. The addition of the rubber molecule allows the peaking stresses to be slightly relived. This dramatically increases the most important property of an adhesive, the peel strength. In the construction of the Robin there are joints between wood and wood, wood and foam and wood and fiberglass. The foam I am using for the ribs in the case of the tail feathers and wing is extruded 3 lb density Styrofoam sheet. Testing has shown that this foam will fail at approximately 90 psi in shear. The design allowable by the way is only 15 psi. Therefore there is no need to use a high strength adhesive like T-88 in any joint between Styrofoam. All Styrofoam joints in the Robin use 5 minute epoxy. This dramatically speeds up the build process. The remainder of the joints will all use T-88. light clamp pressure is all that is required, the only requirement for the epoxy joint is contact pressure. The shear strength of the T-88 is around 5000 psi, where as the shear strength of the Spruce is around 900 to 1200 psi depending on the grain direction. 5 minute epoxy is in the region of 2700 psi.
|Top sheet of the Vertical fin drawings|
|Completed front and rear spars|
Birch plywood is used for the shear webs of both spars. There are intermediate blocks of wood separating the spar caps called intercostals. They serve two purposes, they react the compression component of the shear load and serve as a panel breaker reducing the size of the shear bays. The maximum allowable shear stress before buckling is a function of the shear bay size.
|Another view of the completed spars and skins|
By following the print exactly these foam ribs are cut out. The edge bevels are carefully laid out and also cut out. The loft of the vertical fin is a constant section between the front and rear spar. This simplifies the construction.
|The ribs are used to jig the spars together|
|Rib caps being added|
this is an idea that belongs to Steve Wood first used on his Sky Pup. By bonding spruce wood rib caps to the foam core. this ensures that all axial bending loads will be transmitted through the wood and all shear loads will be transmitted through the foam. Because of the thick low density section, the foam is an efficient shear web. analysis has shown that a built up wood truss rib would be slightly lighter, i decided the simplicity of the construction outweighed the ounces I could save. where this design differs from the sky Pup,is in the use of the rear gusset plate and fwd skin. These pieces overlap the rib caps and create a nice clean aerodynamic joint. the bond between the foam and wood is again 5 minute epoxy. the later skin bond will be T-88 adhesive.
|Nose ribs being added|
This view shows the nose ribs being bonded with 5 minute epoxy. The technique for cutting out these ribs is to first make a metal template from either Aluminum or Sheet tin. I use a 24 pitch metal cutting band-saw blade. Rather than use the front of the blade, i guide of the back of the blade where there is no kerf or teeth. The templates all have at least 3 #30 holes to accept some round toothpicks that are used to hold the foam to the template.
|Leading edge being soaked in Ammonia|
This is where more magic happens, this forming technique was developed during WWII by the US forest wood laboratories in Madison Wisconsin. Again, this work supported the non strategic materials aircraft program. the Ammonia soaks through the total thickness of the plywood. This usually takes about 30 minutes to be effective. The wood is not damaged and neither is the adhesive. The wood fibers are softened and will undergo dislocation when being formed. After the ammonia drys, the wood is good as new and the strength is unaffected. The wood will not have any spring back either. This is the beginning of the wrapping process for the leading edge. This is the oversize sheet that I previously mentioned that had to be remade.
|This next step is best done outdoors!!!|
a series of Velcro belts and clamps are used to wrap the skin around the leading edge foam ribs. try to start from the center and gently fold the skin downward. There are some space blocks under the belts that allow them to stand off from the from spar web. Let the set up dry over night. he next day the bond is made between the wood and the foam. Use T-88 adhesive here for all joints, the working time need to be the same at all of the joints, so 5 minute cannot be mixed into the joints. Before the skin is bonded to the ribs, a coating of epoxy resin is rolled onto the inside surface as a moisture barrier.