Vertical Fin Construction

One of the reasons I like to build with wood is the ease of construction. The construction of the vertical fin required only 3 days to complete. The main reason it took this long is because of the cure cycle time of the T-88 epoxy. 

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



This is the top drawing of the beginning of the Vertical fin section of the plans. The fin airfoil is a symmetrical section . There are two spars, a leading edge and a trailing edge. The leading edge is skinned with 1/32nd plywood and is supported by a series of perpendicular nose ribs. 

Completed front and rear spars

All spars in the empennage are built off of a 10 foot x 8” flat jig board.  The main spars are assembled on the jig board first, and then the jig board is used to locate the rear spar.  Construction begins with the assembly of the two 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

 This picture shows the pre-trimmed leading edge skin and rear spar gusset plate. this turned out to be a mistake, the final contour came our slightly larger than the design and these parts ended up being too short. The way to build these parts are to cut them slightly oversize, form them fit them and then trim them.

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

 This is why its important to cut the ribs accurately and with the correct bevel angles.The ribs are being used to locate and jig the two spars together. Again, these bonds are all with 5 minute epoxy allowing the builder instant gratification!!!!!
 

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.

Final Assembly

3 days and 3.1 lbs!!!!!!