Flap Mechanism

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The Wag Aero Sportsman 2+2 is designed as a four place aircraft, however the “+2” refers to two children or one adult for the back seat. When determining the routing of the flap cables, consideration had to be given on the location of these cables to keep clear of the back seat passengers, thus a side mounted cable system was needed.  The flaps are engaged by pulling a Johnson Bar Lever between the front seats that pulls the cable that run down the centerline under the floorboards until they reach behind the baggage panel and are then directed upward and over to the right side of the fuse where they terminated to a pivoting bell crank.  The bell crank cable was attached to a “Y” connection from cables routed from each wing.  As my previous post described, I found that this cable geometry limited the flap travel and it was later greatly simplified by removing the bell crank and using only one cable from the Johnson Bar.  The return springs in the wings provided the means of retraction and are assisted greatly by the thrust of the airstream.

The Northstar Wing instructions advise to set the maximum deployment angle to 52 degrees down. While that seemed extreme, the actual angle was between 40 to 45 degrees when the air load was applied.  The Northstar flaps are massive and one difficulty is being able to pull on that much flap. Your airspeed is very important and the slower the aircraft, the easier it will be to get that last notch of flaps engaged. The larger wing area and the Fowler like flaps help the aircraft fly much slower, so it takes some getting use to  as when you can pull on that last notch of flaps.

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Deploying the Flaps

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The Wag Aero wing plans use spoilers instead of flaps.  The spoilers have almost no similarity to flaps. They are very small panels above and below the wing that you deploy upward and downward by pulling a “T” handle just before you flare to land.  They are designed to kill the lift of the wing. The Northstar wing kit on the other hand has massive Fowler-like flaps.  I decided to use a lever to deploy the flaps manually using a standard Johnson bar flap handle located between the seats. The challenge was how to rout the cables and pulleys from the underside of the belly and back up to the ceiling of the cabin. I bought a used Johnson bar from an old parted out Piper and designed a mount and ratcheting system for the bar to attach.  The first set of 3 inch pulleys were welded offset to the centerline of the fuse to avoid competing with the push-pull tube that operates the elevator.  I made the cables turn at a right angle behind the baggage panel to keep the turtle deck opening clear and useable. Bringing the cables up and along the side of the fuse allowed for a solid and secure location for a welded pivot point for the bell crank to attach. The Johnson bar had a welded horn centered on the lower bar and the horn had two holes for cable shackles which led me to design a twin cable system all the way to both ends of the bell crank. The theory in my mind was that one of the two cables would always be pulling when raising or lowering the flaps.  It was not until I was attaching the wings during the aircraft’s final assembly that I discovered that the travel movement of the bell crank was not sufficient to operate the flap operating range of the wings. To correct this I used only one cable that attaches directly to the cables from both wings and removed the fuse bell crank location entirely.  The flaps have a very strong return spring on the rear spar that assure flap retraction as soon as the flap handle is lowered.

Wing Rotator Fixtures

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While saw horses were used during the early stages of wing construction, it would now be neccesary to have some other method to support the wing for the fabric covering process.  I would need an easy way to rotate the wing from right side up to upside down without having someone to help me.  The wing would need to rest on something flat when it was right side up and rest on something contoured to the airfoil shape when it was upside down.  I purchased two very inexpensive engine stands for each wing from the Harbor Freight store and mounted a 2 X 10 board that attaches to the rotating hub of the engine stand and connected to the front and rear wing spar connections.  When the wing was upside right I would use a simple saw horse on the opposite end.  When I needed to flip the wing upside down I would use a special plywood box that has a carpet tile covered contour that matched the airfoil contour of the Northstar wing.  I also wanted a method to hold the wing up vertically for certain fabrication reasons and also for efficient storage before final assembly to the fuse.  I used the same plywood box and created a vertical airfoil profile of the first third of the Northstar wing by kerfing a piece of plywood to create the shape and then covering it with the backside of carpet tile to protect the wings surface.  When I needed to flip the wing I would lift the wing tip and turn the wing over and use a wing box or a saw horse to support it as needed.

Wing Root Design Inside the Cockpit

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Looking up toward the skylight shows how the Northstar wing mates up with the Wag Aero Fuselage.  The forward fuel line leads down the fuselage “A” pillar and into the fuel selectors right port and the rearward fuel line leads down the “B” pillar and back down behind the baggage panel to a belly sump.  From there it goes forward and also ties into the right fuel selector port.  This arrangement provides positive fuel head pressure whether the aircraft’s attitude is nose high or nose low without the need for an electric fuel pump.  The photo also shows how the ailerons leading edge cable travels up from the control stick and enters the internal leading edge area of the wing.

Wing Test Fit

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With the right wing structure completed, it was time to fit it to the fuselage and check for spar and fuse fitting alignment and see how the bird cage of the fuse lined up with the wing root profile.  This was also the first time I could actually see how the all the plumbing terminations (fuel ports, fuel vent line, and fuel site gauge ports) fit between the bird cage structure.  With the wing in position I could also finish weld the upper pulley entry mount for the wing and evaluate where the aileron cross over balance line fairleads run through the upper head area of the cockpit. Because I was adapting a Northstar wing to a Wag Aero 2+2 Fuselage I knew there would be issues for locations of these aforementioned items but luckily I really had only one problem and that was the aileron balance line fairleads on the fuse that did not line up with the fairleads on the the Northstar wing.  I got the torch out and burned the paint off the proposed affected weld area and welded new 4130 fairlead barrels to line up with the wing. Instead of removing the old fairleads I adapted them as a crossover visor bracket.

It was also at this time I needed to make a small temporary addition on my pole barn so I could attach both wings at the same time. The photo above is when I had only enough space to attach one wing at a time. I had the wings on and off the fuselage a few different times as it was necessary to design how the flaps would function and how to trim out the wing root with the side of the fuselage.  The Northstar wing has flaps and the Wag Aero plans only used spoilers, thus I was on my own to design the pulley and cable locations between a Johnson flap bar between the seats to transit the underside of the fuselage and activate the raising and lowering of the flap system. I left the wing struts off during this process and used a temporoary brace to hold up the wings in order to expedite the design of the flap system.

 

Miscellaneous Wing Projects

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A number of various plumbing, electrical, and access issues needed to be completed before covering the wing with fabric.  Fuel vent lines were installed on both wings, fuel ports, sump drains, and site guage fittings were terminated through the wing root for future connections.  The wing tip nav/strobe light wiring was fished through the trailing edge void and operationally tested.  Any electrical problem occuring inside the wing will become a lot more troublesome to fix after the fabric is attached. Inspection covers were also fabricated for aileron and flap operations along with wing strut connection locations.  I elected to use 8/32 machine screws and anchor nuts for all covers which took a little longer than the conventional sheet metal screws. Northstar provides the templates for these covers and also explains on their video a very clever method to bump out the aluminum as needed to go around a protruding aileron pulley.

Finishing the Wing Tip

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The Northstar wing has a very elegant wing tip shape.  Instead of the popsicle stick end found on the Piper wing,  the Northstar wing tapers down and back into a graceful rake. The wing tip extends the leading edge and maximizes the aspect ratio by  increasing the effective wing span, this according to it’s designer. The kit includes a custom shaped fiberglass leading edge element, a tip rib, a wing bow, and a preformed trailing edge sheet metal piece. The fiberglass part is firmly attached with Avex rivets to the end of the leading edge sheet metal and along the top and bottom sides of the wing bow.  The trailing edge metal is preformed and temporarily held in place with strapping tape to keep it positioned for drilling rivet holes.  The holes are then cleco’d down and a stiffener channel that runs diagonally is placed on top for locating rivet holes.  The stiffener channel is then placed inside the trailing edge piece and attached using  flush Avex rivets. Northstar also welded the mount on the wing bow for the attaching the standard Whelen nav/strobes light fixture.  The design of the wing tip is especially appreciated when fabric is attached making the finished tip an extraordinarily beautiful shape.

Fabricating the Flaps

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 Fabricating the flaps and ailerons required a special flat table fixture. I had my local lumber yard rip some flat MDF panels into very square pieces and then assembled them around  4 X 4 posts with adjustable leveling glides.  The Northstar Kit included several angle iron brackets that attached to the table top and would be used as squaring stops to keep the skins aligned with the spar and ribs during assembly. The preformed ribs and spar were attached off the table using plastic milk crates to support the irregular shaped parts during riveting.  The rib and spar assembly was then placed against the angle iron stops on the table for attachment of the preformed leading edge sheet metal.  This procedure was basically the same as was done for the leading edge metal on the wings.  The top edge is riveted first and the assembly is then turned upside down and ratcheting straps are used to pull the leading edge metal tight against the edges of the ribs and spar. When everything is tight and square the holes are drilled through the leading edge metal and structure beneath and cleco’d together.  The clecos and skins are then removed and holes are deburred and dimpled for flush riveting.  Almost all riveting was done with a pneumatic riveter gun using Avex brand rivets.

 The corrugaated sheet metal skins are attached to the assembly in preformed sections that overlap the ribs. Reenforced strapping tape is used to keep the trailing edge metal flat and tight against the table surface and stops. The corrugated sheets are then placed over the assembly and riveting lines are aligned with rib edges beneath. The convulusions of the corrugated skins need to be carefully placed to avoid locating them over a riveting line. The strapping tape was very useful to keep the skins down and flat.  Clecos were attached after each hole was drilled and then removed along with the corrugated skin. Each hole was deburred and skins were reattached using Avex dome rivets.  When completed, the Flaps and Ailerons were extremely straight, square and very strong.

Fuel Tank Installation

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 The Northstar wing kit includes two custom TIG welded fuel tanks.  The tanks hold 52 gallons in total or 312 pounds of fuel.  That quantity of fuel will give the aircraft approximately a 6 hour/ 600 mile range.  The fuel tanks were crafted to form the airfoil shape of the wing and were held tightly in place with three stainless steel tensioning straps.  Inside the tanks were three baffles to reduce sloshing of fuel.  After the tanks were installed in the wing a set of “X” braced drag wires were inserted through the fluid tight internal tubes in the tank.  

Using the Northstar wing made it unnecessary to use a header tank.  Piper aircraft and many Piper replicas require a header tank which is a small fuel tank mounted in the cockpit just above the Pilots feet. The header tank purpose is to ensure consistent fuel flow during climbs, dives and maneuvering. I did not like the idea of highly combustible 100 octane racing/aviation fuel just above my feet. The Northstar wing plumbing and fuel line routing provides positive fuel flow during steep climbs or dives making the header tank unnecessary. The fuel system is gravity fed from a forward port and a rear port that feed together to a central fuel selector. The head pressure is sufficent to also eliminate the need of an electric fuel pump.

One last feature of the Northstar fuel tanks are the fuel sight gauges in the root of each wing. These sight tubes provide the pilot an honest indication of remaining fuel on the ground and in the air.

 

Attaching the Leading & Trailing Edges

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 With the drag wires squared up and firmly tightened, it was time to lock the wing into place. The Trailing Edge sheet metal came in preformed “S” shaped sheets approximately four feet wide.  The first sheet was attached at the root end and then each subsequent sheet was added with a slight over lap.  The preformed “S” shaped profile fit nicely against the shape of the trailing edge of the ribs and the predrilled holes allowed an easy way to back drill through the ribs and into the sheet metal.  As holes were drilled, Clecos were attached to keep the sheet in position.   Before  final riveting, the sheets were removed, holes were deburred and dimpled to receive countersunk Avex rivets. 

The Northstar Wing uses .020 for the Leading Edge and is designed to extend further down  creating a firmer and truer airfoil shape.  The Leading Edge sheet metal also came in preformed sheets and were also attached starting at the wing root however they had to be stretched over the wing in tension to assure that the sheet metal came in contact with the edges of the ribs.  This was done by first attaching the sheet metal on the underside of the ribs with a set of Clecos and then using tension straps and racheting the sheet down in place.  When the sheet metal was in place holes were back drilled and Clecos held the sheet in place. The sheet was later removed, holes were deburred and dimpled and countersunk Avex rivets were installed for final assembly  

The tip of the wing Leading Edge  ends with a raked wing tip bow and tip rib covered with a wrap around fiberglass termination cover that is also attached with Avex rivets. With the Leading and Trailing edges firmly attached the wing no longer twists and is permanently locked into shape.