Fuselage Construction

This is an area where I have spent the largest portion of my time in both design and construction. I have redesigned the fuselage 3 times. My original concept was similar to my original ultralight the "Wren" , a fuselage built from 4 sheets of 1/2" Styrofoam. The foam would have been covered by a light weight glass. I couldn't make the weight on this design, I had tried two iterations before I threw it out and discovered Natures composite, Spruce wood!!!  The following is a picture of the original fuselage truss I designed.

  Now this may seem minor, but I could not show this design "Good" or structurally viable. The main loading for the aft portion of this fuselage is a downward acting tail load. It is always a percentage of the main wing loading. Usually around 10%. An airplane actually carries 110% of its design gross weight in the max pull up design condition. This is the balancing tail load that reacts the wing pitching moment. So in this design, for positive loading all of the diagonal members of this truss will react this load as a shear component. The angle of the member and the direction it faces will determine the actual load (vector sum of the load) and whether this load is in tension or compression. To simplify this design I chose to build the truss with 1/2" x 1/2" spruce sections. The diagonal members are longer than the vertical members, this means that they become a much longer column when placed in compression, I could not show most of these members "good" based on the compression loading. The solution was to reverse the members such that they are all in tension for the positive loading. Remember, negative loading which WOULD place these members in compression  is 1/2 that of the positive load. Its a simple trick, but it saves almost a pound!! All of the longerons are doubled up 1/2" x 1/2" sections. The pitch of the vertical members was determined by the critical length of the longerons in compression. The design of the gussets by the way is very straight forward and conservative. The assumption I made on all gussets is that they should be able to transmit the net tension load of the attaching member. Since all attaching members are 1/2" x 1/2" , I assumed a tension load equal to the max allowable of the spruce and added a safety factor. This is conservative, because actual loading is less than this design stress. Most members are actually sized for compression. Here is an Isometric view of the final fuselage design. 




Fuselage Isometric everything in this picture weighs 26 lbs


Construction of the fuselage started by building a flat level work table 17 feet long x 4 feet wide.

   





Fuselage jig table

  








I used particle board and painted the surface white. The base had sufficient cross members to keep the surface from sagging. I used the old wing assembly jig as a table base. The next step in the construction is to lay out the full size jig from the fuselage plans.






marking jig



I made a pretty useful jig tool here. This is used to locate the bevel or end trim of a member. The untrimmed spruce member is set on top of the existing trimmed pieces. The jig is set on top of this untrimmed piece with the two blocks aligned to the trimmed member. The bevel angle is then drawn on the untrimmed member. I usually cut to within a 1/16th of an inch and then carefully sand the rest. I will do this numerous times until the fit is exactly line to line. Not too tight to where the adhesive is forced out, or too loose where it could actually flow out.




jig being used

 

Wood can only be ordered economically in 109" lengths. Because of this the longerons need to be spliced. The type of splice I used is a 15:1 scarf splice. I used a scarfing jig on my table saw to cut the exact same angle in both pieces of the longeron. The longerons are then splice "insitu" or during the build up of the side truss. The 12:1 to 15:1 taper is a standard aircraft wood splice angle. The idea here is to have more bond strength than the ultimate tension of the piece being spliced. A proper splice will fail in the wood before the glue joint. Its important not to sand these edges, sanding overturns the surface wood fibers and does not allow the epoxy to flow into the exposed wood cells. All cutting was done on the table saw using a hollow ground planer blade. The long locating blocks on the assembly jig board are screwed into place and are used as clamping blocks for the splice. Excessive clamp force must be avoided to keep from forcing out all of the adhesive. Remember, epoxies only require contact, not pressure for a successful bond. The jig board was coated with 3 coats of carnuba paste wax  to ensure everything would release.


Scarfing jig



The fuselage sides are identical for the L/H and R/H sides. The joint between all members is made with 1/16th" plywood gussets. Gussets are used on both the inner and outer surface. So when a pattern is made for one gusset, it is duplicated 3 more times. There is no real drawing of a gusset, just minimum areas that need to be maintained. The idea here is that the gusset will be sufficient in tension area to develop the full ultimate tensile load of the member. This simplified my analysis because I only had to worry about a single master gusset. In the design of a truss, the idea is to have all of the members intersect at the centroid of their sections. This eliminates any induced out of plane loading. from a practical stand point, its pretty tough to do, so in the plans I approximate it. Attach the gussets with T-88 adhesive and 3/8” staples. Shoot the staples through a piece of heavy Dacron fabric or Peel ply. This make removal a snap after the bond cures.

The following picture is the completed fuselage side and my dog Boo (Aussie Sheppard). Both sides are built up in the truss jig and gussets are attached to both surfaces.

 


side truss and "Boo" my Australian Shepard

 

Once the side truss is completed, the builder has the option to either make a new top view template or clean up the side truss template and lay out the top view template. Since the Robin was fully modeled in a 3D CAD system, it was possible to lay out an exact developed flat pattern of the side truss. In reality the flat pattern is slightly longer than the actual side view of the truss. This becomes apparent when the sides are brought together on the plan view template.



side truss jig


The procedure for building the completed truss is similar to building the side trusses. The fuselage is constructed up side down because the upper longeron is flat. When cutting both the horizontal members and the gussets a duplicate is also made for the lower longeron. When fitting the horizontal members, use the jig earlier described and cut an accurate bevel. I usually cut a piece long and then sand the parts on the belt sander to a perfect fit. When attaching the lower longeron horizontal members, be sure that the pieces are parallel to the lower longeron surface where the gussets are to sit. Make some large 90 degree triangles out of particle board to act at squares to keep both sides square and parallel.



Truss flat pattern



After the side trusses are attached, let the fuselage sit in the jig. At this time you will build up the wing box carry through and the landing gear attach frames. These can be built up ahead of time and installed when the two sides are jigged together. Both frames are built up separately and then jigged into position before the additional box members are added. The main wing attach pins are used to locate the wing frames parallel to each other. The landing gear frames have two large bushed holes; these holes are used in conjunction with a maple dowel pin to align the frames. After the frames are bonded into position, the dowel is removed and the bushings are replaced. 



completed basic fuselage truss. 21 lbs


Here are some more pictures of the completed truss.



after the truss is finished, build up starts for the  Roll over cage and turtle deck. The Vert fin is atached and the  landing gears can be assembled  and Whoopee!!! A milestone is reached!!  "Weigh on Wheels" Normally in the business, everyone gets to go to the main assembly area and they give you an ice cream bar. Because I'm a little smaller than than the "big boys" I just cracked a Bottle of Merlot and a Neighbor and I killed it!!!

Weight on Wheels