Re-Fitting, Sandblasting, Priming and Storing the Tail Parts

Preparing Tail Parts for Covering
With the tail parts re-attached to the fuse it was time to re-check control movement of the elevators, trim tab and rudder.  When this checked out, the elevators and horizontal stabilizers were removed for sandblasting and priming.  A strobe light bracket from Univair was welded on the top of the rudder and later wired with a quick link harness prior to fabric cover.

The vertical stabilizer remained attached to the fuse and all the remaining tail parts were then hung from the rafters until the rest of the fuse was ready to cover.

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Mating, Squaring, Leveling, and Aligning the Tail

Mating and Alignment of the TailAfter the tail parts were finish welded per the drawings it was time for a trial fit on the fuselage.  The rear spars of the horizontal stabilizers were joined together to a rotating sleeve while the front spars were joined together into a moveable and adjustable forward stabilizer mount.  This arrangement is different than the traditional jack screw found on the Super Cub.  That is because the Sportsman utilizes a cockpit adjustable trim tab on the left elevator very similair to the Cessna 172.  The adjustable range on the front spar provides a way to adjust the aircrafts pitch to keep the elevators centered during normal cruise flight and then use the cockpit adjustable trim tab for reducing control pressures.  During test flights the pitch angle of the stabilizer was changed several times until an optimal setting was found.  Future video posts will show the before and after effects of making these pitch adjustments.

The next trial fit was to attach the elevators to the rear spar hinge points and to my shock and displeasure there was a huge 1″ gap between the left and right elevator horns.  These two horns should have had only 1/8″ between them to receive the upper and lower cable attach lugs.  What went wrong?  Did I make a mistake or were the plans incorrect?  It didn’t matter – it had to be corrected and this became my first welding repair.  The tubes were cut near the end by the horns.  An insert tube was rosette welded between the two and a larger tube doubler was joined over the joint and finger welded over the existing tube.

Flying wires or tail brace wires were attached and a level was used to check tail alignment across the hinge lines of the left and right elevators.

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.

Finishing the Turtledeck

Fabricating the Turledeck 3 of 3After the metal fabrication and system testing of the lift and lock mechanism was complete it was time to finish and protect the metal.  A backyard sandblasting set up was used to clean and prepare the metal for paint.  This set up was used for all smaller parts like tail pieces, rudder, landing gear legs, etc.  I found out that sandblasting is very messy and its impossible to recover used blasting material when you do it outside. All future sandblasting was done by Southwest Sandblasting in Grand Rapids who do an outstanding job.

After the turtledeck was sandblasted it was cleaned with MEK and hand brushed with a 2 part epoxy primer from Randolph Coatings quickly followed by a 2 part epoxy J-3 yellow finish color.  The 2 part epoxy paint protects the metal from corrosion and is impervious to MEK and all other Poly Fiber chemicals used for fabric attachment and finishing.

Because the inside fabric of the turtledeck can be seen when it is raised I used untinted Poly Brush to avoid having pink brush marks visible.

Designing a Lift & Latch Mechanism

Fabricating the Turtledeck 2 of 3The plans do not show how to lower and lock the turtledeck down or how to lift it and keep it  up.  As an Interior Designer (my regular occupation) I have designed a lot of specialty store fixtures, concierge desks, etc., so I decided to use a lift mechanism typically used to raise and lower custom tables made by Suspa in Grand Rapids. It is a manually operated self contained (non-electric) single acting cylinder system and hydraulic pump.  I located the pump under the sheet metal deck of the turtledeck compartment with a detachable crank handle facing out on the pilot side.  You crank it clockwise to raise the turtledeck or counterclockwise to lower it and it will stay put wherever you stop cranking.  I bought a complete second system just in case it ever failed but to my surprize it has been remarkably dependable after being raised and lowered hundreds of times during several hot and cold seasons.

Another concern was keeping the turtledeck locked down for flight. You obviously don’t want something 8 feet long flopping around back there.  An architectural panel latch available from Southco was used to latch and lock the front and rear ends of the turtledeck. This mechanism uses two detachable hex wrench operated  thru-bolts that when rotated hook to brackets and pull and lock down both ends of the deck.

At this point in time I had my first visit from my EAA Tech Advisor, Bud Potts who offered a lot of technical know how and made many other visits throughout the building process.  All aircraft builders should take advantage of this useful EAA resource.

Deciding on a Hinged Turtledeck

Fabricating the Turtledeck 1 of 3The Sportsman 2+2 plans provide the builder 2 Turtledeck options – A fixed in place Turtledeck or a hinged Turtledeck.  I chose the hinged option and I have never regretted it.  The hinged turtledeck has its origin from the U.S. Navy Piper HE-1 designed to carry a patient litter.  From a practical weight and balance stand point there is only so much you can load behind the passenger compartment, but also from a practical stand point having a hinged deck allows for very easy preflight inspection of all things below and makes sense for easy fabric repair, for access to replace aging rubber fuel lines and for checking elevator cable tension all without ever needing to remove or cut fabric.

Fabricating the thin wall steel channel puts your welding skills to a real test.  Most all of the light weight structure is made from .035″ X 3/8″ x 3/8″ Piper mild steel channel, including the stringers and frame channels.  Too much heat and you burn right through it!

My styling side got the best of me when I could not resist to change the shape of the window from trapezoids to nautical portholes similar to ones found on one of my favorite STOL aircraft, namely the Helio  295 Super Courier!  Granted my Super Sportsman is no Super Courier but why can’t it look like one!   I welded  small steel rings to the channel structure so the fabric would have something to attach to and used clear plastic lenses from wall clocks for the round bubble windows.  Luckily they still make the clock so I have a replacement source for the windows. My next post shows these porthole windows installed.

Determination Video

Let’s look ahead a few years and view a short 2 minute video taken from a belly camera during a 2012 test flight. This video was made after the aforementioned landing incident and was created to instill a sense of confidence for the test pilot…Me!

A bounced landing can feel especially severe but after reviewing the video I was quite surprised that the bounce was not nearly as high as it felt.

There are 4 cameras used in various locations on N728DC and in future posts I will describe their purpose and demonstrate their capabilities for both aircraft performance review and Pilot flight test analysis.

Learning to Weld

Welding ProjectsWith the fuselage securely attached to the rotisserie it was time to learn to weld the 4130 steel tubing.  Using a oxygen/acetylene gas rig and jewelers torch I test welded several scrap pieces together but it was’nt until I got expert instruction from Chuck & Craig Garret from my local EAA Chapter 145 that I finally gained some confidence.

I started welding on the fuselage lift handles and then the wing spar brackets, elevator bell crank assembly, floorboard mounts, rudder pedal mounts, engine mounts and landing gear and wing strut brackets.  They say the best way to test a weld is to try to tear or break it apart.  Unfortunately that destroys your weld.  Sadly, a few years later I put some of my welding to a real world test during a bad landing/ground loop event.  The good news is, the welds survived – the bad news is, the landing gear did not.

I will post more about that event in a future post including photos of the damage and the repairs made to the aircraft.

Preparing the Fuse

Fuselage on RotisserieThe newly purchased fuselage was attached to a homemade rotisserie fixture on wheels.  The fixture supported the fuse and allowed it to be turned and held in place for welding parts on, attaching floorboards, installing components, and for fabric covering and painting.

The fuselage stayed on the rotisserie for 4 years until it was removed for sand blasting and painting.  It was then re-attached and the rotisserie was motorized for easier turning. It stayed attached for another two years for fabric covering and painting until it was finally removed and let to stand on its own wheels.

The Starting Point

Trip To Orillia, OntarioEvery Project has a starting point and N728DC started with a trip to  Orillia, Ontario to inspect a Sportsman 2+2 fuselage that was for sale.  In November of 1995 we packed up the family, rented a trailer and headed to Canada to inspect and possibly buy a former builders uncompleted project.  A set of color coded plans were used to inspect tube diameters and a thorough set of measurements were also taken to confirm if the fuselage fabrication was done correctly.

It turned out that everything was spot on.  The frame was square and true, the welds looked good, and there was no corrosion.  In fact it also turned out that the fuselage was actually fabricated by Wag-Aero and sold as a kit in their catalog for a much higher price.

There was still a lot of welding to complete but this fuselage would provide a huge head start to a long and complicated project.