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.

 

Fine Tuning the Wing

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 There are four sets of criss-crossing drag wires in each wing. Each wire is a 3/16 inch heavy duty stainless steel wire threaded on each end.  The wires are used to literally tune and square the wing.  A trammel point tool is used for this process which is basically a measuring device that measures the “X” distance where the wires attach to the spars.  These trammel points are drawn on the top of the spar connection points and the object is to measure all the wire connection points until they measure the same. This is done by adjusting the threaded drag wires back and forth until the trammel points are equal by turning a nut against a block. When everything is squared a couple of jam nuts are used to fix the drag wires into final position. Where the two wires cross in the middle a nylon cable tie is used to keep the wires from rattling against each other. The drag wires help tune and square the wing but their most important role is to keep the wing spars parallel and prevent the spars from sweeping or parallelograming.  

At this point in the construction the wings need to be squared and finished because the next step will be to attach the leading and trailing edge sheet metal across the length of the wing. Once this sheet metal is attached it will fix the shape and squareness and prevent the wing from twisting. Before the leading edge sheet metal is attached the concealed aileron pulley and cable transit had to be completed because soon it would be mostly covered over by the sheet metal.  Fairlead brackets also had to be attached along the spar to keep the aileron cable inline and away from interfering with the ribs.  It will be important to remember that when the wing is later covered in fabric that inspection holes be located below these fairleads so they can later be replaced due to wear.

Assembly Details

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 The wing spar brackets on the fuselage must be 30″ on center to agree with the spar attachment points on the wings.  This has to be a precise fit.  Unfortunately my welding experience or lack thereof caused me to not compensate for all the distortion that can happen during a weld and thus I ended up with a fractional difference between the spar brackets.  Fortunately Morgan Williams had an easy solution using a series of thin precision made aluminum plates stacked between the aluminum spars and the steel attach brackets accommodated the measurement difference to thus make a precise fit.  

The Northstar wing like the standard Super Cub wing uses the forward wing strut to carry the aileron cable up to the wing. I wanted the aileron cable concealed and run inside the leading edge zone of the wing.  The lower right photo shows a preliminary rigging set-up of the proposed forward pulley location and it’s necessary alignment of the pulley behind the spar. The carry through hole in the spar had to be kept  small and avoid interfering with the adjacent hard riveted reinforcement plate.

The lower left photo shows the aileron hinge bracket located at the rear wing strut attach point and it’s alignment with the aluminum wing compression strut. Also note that there is an additional steel tube strut providing additional triangulation strength where the wing struts will be attached. The hinge bracket, compression struts and the rear wing strut bracket all come together in a collective assembly of precision made aluminum and steel plates all held firmly together with AN stop nut hardware. 

 

Wing Assembly Begins

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 The Northstar Wing uses the same airfoil shape as the Piper USA 35B as used on a Super Cub but the similarity ends there. For the design of the Northstar wing, Morgan Williams used the heavy duty features of the Piper Pawnee PA-25 Agplane with a higher gross weight. From there he made several more improvements including the alignment of all wing compression struts with the aileron and flap hinges thus directing the airloads in a straight line from the aft spar instead of zig-zagging through the spars.  There is also a new raked wing tip to maximize the wing aspect ratio and increasing the effective wing span that allows side-slips even with full flaps. Four sets of stainless steel drag wires including one set that runs through the gas tank extend through the spars and are tightened using a simple nut and block attachment system. Morgan also designed the control cables to stay inside the wing including the upper aileron cable thus maintaining a more aerodynamic and water resistant enclosure.

What was most impressive is the high quality of the parts. The spars came predrilled with reinforcements hard riveted at the factory. The aluminum ribs were preformed and predrilled and when any drilling was required Morgan provided a very detailed video explanation of how to make the set-up, what clamps to use, measurements needed and what type of rivets or AN hardware to complete the assembly with.  Also if I ever had a question he was always available to help me work through it.

The ribs were also predrilled for attaching the fabric with fabric rivets which I found to really speed up the fabric covering process.  The old method of sewing and tying knots to attach the fabric to the ribs is not needed on the Northstar wings.