Finishing the Bubble Doors

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 I used a black spray to finish the inside face of the Plexiglas called “Lacryl” which is a specially formulated Lacquer used in the sign business.  It has a very nice eggshell appearance on the sprayed side and a very glossy black look on the outside.  I used a metal reinforced black plastic edge to finish the plexiglas flange that extended beyond the door frame and I attached the plexiglas to the steel door frame using special black sheet metal screws with a waxed black plastic washer.  A Plexiglas drill bit was used to drill slightly oversized holes in the plexiglas to eliminate cracking or splitting when the sheet metal screws were attached and tightened.

The Forming Secret

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 The Fabricator basically uses a big pizza oven to heat up an oversized sheet of Plexiglas. When it arrives at the correct forming temprature it is removed from the oven and draped over the lower part of the mold.  The upper plywood frame is then clamped over the Plexiglas sheet and air is immediately pumped in from the center of the lower mold.  There is a “Secret” as to how the air is introduced to the lower mold. If you just attach an air hose nozzle the result of the form will look very localized around the air nozzle location. The preferred shape is a balanced uniform inflation of the bubble around the entire frame shape.  The way this is done is amazingly simple and crude.  A piece of cardboard approximately 12 inches square is stapled on the corners over the air inlet and that’s it. That is enough air diffusion to uniformly lift the heated sheet of acrylic to its lofty formed shape.

The Photos above show the finished parts trimmed and reclamped in the mold. For Insurance, I had the Fabricator make two sets just in case I broke one later and that’s exactly what happened a few years later when I ground looped the airplane and shattered the left door with a safety cable flinging off the landing gear!

Completing the Tooling

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 The last component of the tooling was a continuous plywood frame approximetely 3 inches wide that covered the shape of the steel door frame and adjacent flange.  The object of this plywood frame was for clamping over the heated Plexiglas sheet while forming the bubble.

The Fabricator also instructed me to add a silicone bead around all joints and edges to make the Tooling as air tight as possible.  With both Left and Right Tooling Fixtures completed it was time to bring them over to the Fabricator for the forming operation. The goal post shown in the photo above was for measuring the maximum height of the bubble during forming.

Fabricating the Bubble Doors

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 The Bubble Doors utilize the same fabrication method as Plexiglas Skylights – the same skylights as typically seen in school gymnasiums and factory spaces.  Luckily I had a local company who made these Skylights.  They had the equipment and know how to make my Bubble Doors and instructed me on how to make the tooling.

The first thing was to weld together a steel door frame that followed the contour of the airframes door opening.  I used a 1″ square 4130 steel tube and carefully tack welded the door frame inside the airframes frame.  With one of the wings attached I could determine if the future Gull Wing type Bubble Doors would interfere with the bottom side of the wing when it was raised.  I also  designed the latching method and gas lift spring locations needed to open and close the doors.   After I finish welded both left and right steel door frames I then began to build the wooden molds around the front and back sides of each door frame.

Fabricating the Belly Panels

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I decided to experiment with a material used in the display business called Alucabond.  It is a 6mm panel consisting of two .020″ aluminum skins thermo bonded to a solid polyethylene core with an overall thickness of 6mm or 1/4″.  It is a very durable material and easy to form. I chose the 6mm thickness to fit the standard 1/4″ set back of the “Z” channels however in retrospect I should have used the 3mm thickness and remade custom 1/8″ “Z” channels instead. The thinner panel would have been easier to form and be less weight. 

My shop was not equipped with a metal forming machine so I designed my own contraption to form the bends.   I used a cardboard tube from my local Home Depot that is used for making footings for decks.  These come in various diameters and I used a slightly smaller diameter to compensate for springback. I filled the tube with concrete to make it as ridgid and hard as possible. A 1.5″ thick plywood panel was hinged at the edge of the work bench and was used for bending the Alucabond around the concrete filled tube.  As crude as it sounds, this fixture worked amazingly well.  There were of course a few R & D bends that did not fit right but I eventually developed a method that worked.  Fitting the panels to the airframe was also a trial and error process.  My only regret was that I did not make an extra set of panels. Ground looping the airplane during flight testing damaged two of the panels that I would later replace using an altogether different material and forming method that I will describe in a later post.

Removable Belly Panels

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The plans called for a fabric encased belly which means that once the fabric is attached, I would have to cut it open to get inside the belly or tear out the complete interior and remove the floorboards to get at what I need to, which is an equally bad idea . . . and there’s no way I’m going to use inspection rings in the fabric because I’ll never place them exactly where they should be and they are just too small to ever get any work done.

Because it’s the belly, it’s an area that is likely to get a lot of abuse including water, dirt, stones, rocks and worse. I decided to divide the belly up into four removable panels with fixed sections between them. At each fuse tube crossover point I welded a pair of channels approximetely 10 inches apart.  These channels would be used to rivet fixed aluminum skins to.  In-between these fixed sections I would fabricate removable panels shaped with the same belly contour as the fixed sections. Using the rotisserie, I welded the channels and pre fitted the fixed skins.  The channels were actually “Z” channels that had an edge that would provide the removable belly panels a lip to fit up against.  The next question was, what material should I use to make the belly panels from. They have to be sturdy, light weight, follow the contour shape of the belly and be easily fastened.  On my next post I’ll describe what I used.

Floorboards and Re-Welded Seats

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While my main effort was to complete all the welding on the fuselage I had to re-focus my attention to the wood floorboards and seating attachment issues.  That’s because metal tabs and seat mounts had to be welded on the fuse to hold them down.  I chose 5/16″ exterior grade plywood and carefully cut it out to fit around tube joints and around the control sticks.  This was not to be the final finished floorboard.  In fact I ended up fabricating three different floorboard patterns until I ended up with a suitable design that evolved with other changes I made along the way.

During this early phase of construction I was reminded of the labor intensive method used for annual inspections for my Cessna Skyhawk.  My A&P (Airframe & Powerplant) Mechanic at the time had the complete cockpit interior removed including the seats and carpeting and all the floorboard inspection panels were also off.  This close up inspection is to check for airframe corrosion, control cable integrity, pass through wiring integrity, fuel line integrity, etc. But my mechanic also found a large mouse nest which certainly did not belong there!

This led me to think how easy or difficult it would be to inspect my homebuilt when it was finished. The plans specified that all interior sidewall sheet metal panels have a lip or 90 degree edge to be used to attach to the wood floorboards.  This would require me to completely remove all the metal sidewalls just to get to the floorboard removal and that would be incredibly difficult and time consuming.  The plans also specified a fabric belly and Yes, I could have added several removable inspection rings but they are very small and difficult to get into. Thus I decided in the months ahead to design a system of easily removable full width belly panels and omit the fabric belly altogether.

During this phase of building was the time I also purchased and modified a pair of Cessna 172 seats and removed the upholstery and removed 3 inches of metal frame width and re-welded them back together.

2 + 2 Seating

2 + 2 Seating

The Wag Aero “2 + 2” model designation refers to the airplanes seating arrangement which means there are two seats in front and two seats in back. However the seats in back are really only suited for children of 12 years old or less or one adult passenger. The fuselage measures only 5 inches wider compared to a Piper Super Cub which makes one wonder why the Sportsman designer thought the extra five inches would allow side by side seating versus the tandem seating arrangement for the Super Cub,

I soon realized this cozy seating arrangement when I jury rigged a couple of front seats using cement blocks and 2 X 6 board seat backs and also placed a fabric sling in position for the back seat. I also made a cardboard template of the instrument panel and for the first time I could sit in the cockpit and feel for myself the interior layout of the aircraft.

I was struck with just how narrow the cockpit was. I immediately rechecked the plans to see if there was any error, but it checked out right. I then measured the interior width of my Cessna 172’s cockpit and was quite surprised to find it was only one inch wider than the Sportman. These measurements were taken at the width of the instrument panel. The major width difference between these two airframes is because the Cessna’s cockpit width remains mostly constant all the way down to the floorboard where as the Sportsmans cockpit width tapers down from 38 inches to 30 inches nominally, thus affecting the seat width. It was now no surprise why Wag Aero specified to remove 3 inches of seat/back width from a Cessna 172 seat and reweld them back together. Using standard Skyhawk seats would never have fit.

My concern for this ergonomic issue was not over and many months later I would develop a solution for this narrow width problem by designing special plexiglas bubble doors that would add 8 inches of interior elbow room to the cockpit without modifying the airframe. In later posts I will describe the tooling and fabrication methods used.

Flight Test Video of Elevator & Horizontal Stabilizers

Let’s look a few years ahead at a three minute flight video showing the Aircraft Tail behavior during Cruise, Landing and Side slip maneuvers. A noticeable misalignment of the Elevator and Horizontal Stabilizer occurs in these various flight control movements. An adjustment to the Forward Spar will be needed to realign these surfaces.

Welding the Elevators, Trim Tab, Stabilizers, and Rudder

Welding Tail partsMaking the tail parts require accurate welding fixtures.  Plywood panels were purchased and painted flat white.  Then a 2″ pencil grid was drawn on the entire face of the panels.  I then transposed the full size shape of the stabilizer, elevator and rudder on the plywood panels and located  each rib location, hinge knuckle, cross brace etc. per the plan. The tubing was bent using a spring and soft rubber hammer and plenty of hand persuasion.  Gradually I got the metal to agree with my curvy drawing.  The horizontal stabilizer had a tricky leading edge taper that required spliting the last outboard length a few inches from the end and then removing sufficient material and then squeezing the ends together and re-welding the seam back together.  The seam was then ground smooth.

The metal tubes and ribs were cut to size and fitted tightly together on top of the plywood pattern. Wood blocks were used to keep everything in their place during welding.  Only tack welds were used to temporarily hold parts together.  The assembly was then removed and finish welded on a welding table.

Welding the thick hinge knuckles and bushings was the most difficult due to the differental thickness of the knuckle and the parent tube.  It was important to keep the heat directed to the heavier wall tubing and avoid burning through the adjoining thinner parent tube.  Also keeping the hinge knuckles aligned  was done with sacrificial bolts that sometimes became unknowingly welded to the finished assembly.

The plans also called for small 1/8″ rods in certain end locations.  This was used as a anti bending brace to avoid end deformation during the later fabric cover and shrinking process.