Sig 1/3 Scale Spacewalker Review
Fuselage
7/6/2002 - A lot of work and time has elapsed since the last update. My job, family matters, and travel have made the building process very slow.
The fuselage is constructed in two pieces. The aft section is constructed from 1/4" x 1/4" spruce stringer with 1/4" balsa cross pieces. This is a fairly simple process, but I made one mistake during construction. The last foot or so of the aft section is angled slightly to allow a +1 degree positive incidence for the tail feathers. You are supposed to splice the top stringers to allow this angle. Unfortunately this is not specified in the instructions - only on the plans. Well, I stared at those plans for two days and still didn't see the notation. I simply laid out the stringers along the plan and then glued in the balsa cross pieces. I ended up with the proper angle, but the stringers are under "stress". I don't think this will be an issue since the cross pieces ensure the proper angle, but it would have been better to splice them as indicated. Look for this on the plan sheet!
The front section of the fuselage in constructed from 1/4" spruce top stringers and 1/4" balsa. Each side is then glued to a 1/8" light ply side former. To glue the top and bottom cross pieces in place, I screwed several carpenter squares to my work surface then clamp the fuselage sides to them to ensure they were parallel and square. After the front section of the fuselage was constructed I then glued the two aft section sides to the front section of the fuselage. I used 1/16" ply over the joint for additional strength. The ends of the aft sides are sanded on the inside to allow the ends to be joined together. The top and bottom cross pieces are then glued into place.
Since I had to remove a significant amount of the firewall to accommodate the mufflers I decided to build an engine box to mount the engine. I have used engine boxes on several large planes and like the additional strength the provide. I glued two 3/16" pieces of ply together to make the 3/8" engine box firewall. I used 1/8" ply for the sides of the box, and reinforced them with 3/8" triangle stock. I pinned the engine box firewall to the sides using 1/8" bamboo skewers (you can get these where most grills are sold). I had to cut the F1 and F2 formers to accommodate the firewall box. This setup added a little weight up front, but since I plan to install two elevator servos in the tail I think it will be a good trade off in terms of balance.
I used ny-rod for the throttle linkage. I plan to mount the throttle servo inside the wing saddle area. The ignition will be strapped to the bottom of the engine box, and the battery will be placed in the area between the engine box and the side of the fuselage. The spark plug wire and spark advance wire will be run through the bottom of the firewall. Both the receiver and ignition switches will be mounted inside the cockpit.
I decided to change the construction sequence slightly and go ahead and complete the wing center section and mount it to the fuselage. I did this because a lot of work is required, and I didn't want to install the turtle deck stringers and sheeting and then have to turn the fuselage over to work on the wing center section. When I began working on the wing hold-downs I noticed that I had glued to balsa cross piece to which you glue the hold downs in slightly crooked. I simply added a lite ply spacer to solve this problem. The wing dowels are installed through the leading edge of the wing center section. A piece of of 3/4" balsa is glued behind the leading edge with the grain running vertically for strength. While this is probably sufficient, I decided to glue a piece of 1/8" ply between the leading edge and the 3/4" balsa for additional strength. The holes were drilled in the hold down plates and then the ends of the ends of the dowels are sharpened and pushed through from the front side to mark the drill points. Holes are drilled using successively larger drills until the holes are 5/16". The dowels are then glue into the leading edge of the wing with about 1/2" sticking out.
Note on the wing center section. When I test fit the center section to the wing saddle I was most dismayed. The fit was not what I expected - especially since I took great pains to build the wing section and fuselage sides as accurately as possible. Shaping of the wing saddle was required (which the instructions say should not be required.....). I don't know if my construction was not accurate, or the wing saddle contours on the light ply sides was inaccurate.... Anyway, I used an incidence meter to obtain the +2 degrees positive incidence. NOTE: You will not find this in the instructions. The +2 degrees incidences is only noted on the plan sheets! To obtain the proper contour of the wing I made a template from heavy cardboard. I started by tracing the contour of the ends ribs of the center section and then carefully shaping them until I got a good fit where the wing meets the wing saddle. I then transferred this contour to the wing saddle and shaped the saddle accordingly - checking the incidence numerous times during this process! All-in-all, the fit turned out pretty good.
The rear hold-down blocks are made from hard maple blocks. The installation of these blocks is fairly straight forward if you have ever built a kit that uses this method. 3/4" triangle stock was then glued around the hold-down blocks for strength. The wing center section was placed the saddle in the proper position and holes drilled through the mounting plates in the rear of the wing and through the hold-down blocks. The hold-down blocks were then tapped using a 1/4x20 tap and the threads strengthened with CA.
Next steps will be to install the turtle deck stringers and sheeting, the side balsa stringer caps, and then on to the tailfeathers.
8/31/2002 - Most of the fuselage work is now complete. The tailfeathers have been mounted and hinged. I used a Great Planes Slot Machine to do the initial cutting of the hinge slots, the finished them with a hinge tool. The decking has been completed, but I have not yet cut out the cockpit. I will save this for last to prevent damage to the edges. I didn't follow the instructions for sheeting since I thought it would be hard to join the sheeting in separate pieces. Instead, I glued all the sheets together before sheeting. I then made a poster board paper template of the area to be sheeted and cut the sheeting to fit. I then moistened the sheeting with a mixture of water and ammonia and bent it around the formers and taped it in place. When it dried it was a very good fit - this made gluing much easier. I then sanded off the excess in front and then end result turned out pretty good.
Next I added the stringers to the top of the fuselage, gluing the formers in place first. I then used CA to tack the stringers in place, and then came back later and put a fillet of epoxy around each joint where the stringer attached to the former. I then cut and installed the apron pieces that fit directly behind the cockpit. This was a fairly simple task, but took a while to get each piece to fit.
Next I installed the top fuselage formers and stringers. This went fairly easily, except once again I didn't like the way the stringers ran along the fin. I thought this would be very hard to cover, so I made some fairings that continued the curve of the turtle deck all the way back to the rear of the fin. See pictures below for a description of how I made the fairings.
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