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.
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.
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.
This was a little over ten years ago (2003) when we took the family truckster up to Georgian bay in Ontario Canada to pick up our newly made wing kit. I was assured by Morgan Williams of Custom Flight Limited that it would all fit in our van except for the four 16 foot wing spars that had to ride in a huge tube on the roof. At the time there was a currency exchange rate that worked in our favor and for some unknown reason there was no duty or tariffs for aircraft parts. While going through Customs, they were more concerned whether Tony sitting in the back seat was really our son. Another advantage on the trip was visiting the “Beer Store” where Tony had his picture taken! He was 15 at the time – too young to be a customer at the Beer Store!
I could hardly wait to get home and start the assembly which required viewing over 12 hours of video tape instructions which I was a bit skeptical of. I later found out that the video tapes were a very efficient way to learn the proper methods and practices for assembling such a project. Morgan estimated it would take 50 hours to assemble both wings (not including fabric). It ended up taking me twice as long but I had no complaints. I wanted to take my time!
Unlike other Experimental Aircraft Plans I had a choice on what wings to build. I could build the Wag Aero 2+2 Sportsman Wings per the plans or select other wing options. The Wag Aero Wings are wood construction with square tips and wing spoilers instead of flaps. At the time I did not have a high confidence level for fabricating wood parts and I really wanted flaps so my attention turned to a wing kit made in Canada and available for use on the Sportsman 2+2.
The Northstar Wings are made by Custom Flight Limited of Perkinsfield Ontario. The Northstars airfoil is the same as the Proven Piper USA 35B and is totally adaptable to fitting my Wag Aero Fuse. Morgan Williams is the owner and Chief Engineer for Custom Flight Limited and is the brightest and most knowledgable person in Aviation that I have ever met. His company (http://www.customflightltd.com) makes the Northstar Bush Plane Kit which is a Super Cub type airframe and they sell the same wings for Sportsman builders like me. In the next several posts I will describe a lot more about this fantastic wing kit and what a joy it was to build such an extraordinary piece of engineering.
However before proceeding I always like to point out misguided ideas I had as a novice homebuilder. Early in the process I read somewhere the need to build an absolutely flat wing table needed to fabricate the wings. That seemed so logical to me that I did not hesitate to build a huge 4 foot wide by 16 foot long wing table. Later on, after purchasing the Northstar wing kit I discovered that they are assembled on a simple pair of saw horses. The Northstar instructions provide all the necessary methods for keeping things square and true. The wing table I built would actually have been a hindrance but it was a nice work bench for other things including the installation of Micro Vortex Generators on the wings during the time of flight testing.
The project made a dramatic change when it came off the rotisserie and sat for the first time on it's own gear. It took six years to get it this far and it looked about half done. The wings would be the next goal along with the engine, instrument panel, cowl, windshield, wiring, rigging and hundreds of smaller projects that took another seven years to complete!
I assembled everything to the fuselage that was completed including the tail, bubble doors, and the turtle deck and it started to actually look like an airplane. It was now time to start the wings!
The most important finish of a fabric covered airplane is the sprayed on silver finish. These multiple coats of silver are basically aluminum particles suspended in the vinyl chemistry to protect the fabric from the harmful effects of the Sun. There is usually a minimum of three coats of the Poly Spray material needed to bury the fabric in metal. A work light is placed behind the fabric to inspect if enough silver has been applied to totally eliminate any light penetration through the fabric. The silver/fabric material is carefully sanded with 320 / 400 grit sandpaper between coats.
The next spray coats will be a White Poly Tone Color that is used as a base finish prior to spraying the final classic 146 J3 Yellow. The White is needed to cancel the gray tone of the silver otherwise the Yellow color would be a greenish and hideous color.
Attaching large sheets of sticky medium weight Polyester fabric to the sticky Poly Tak edges of the fuselage frame became a two person job. The objective is to attach the fabric before the Poly Tak glue starts to set up and dry while also leaving enough time to get the right amount of slack in the fabric for shrinking allowance. Too little slack could deform the metal frame during shrinking and too much slack could result in a saggy skin.
Side walls of the fuse were covered first, then shrunk and trimmed. The Belly panel was attached next and overlapped the side walls. The roof panel was attached last. Fabric tapes were attached on the edges and over the stringers using Poly Brush as the glue. Poly brush coats were then brushed on next over the entire fabric surface. Poly Brush coats were then sprayed on as seen in the photos above.
The Turtle deck had its own spray booth and is shown with tapes attached. There is no “Red Tint” on the fabric finish since the “Untinted Poly Brush was used.
The next step will be the Silver Coats using Poly Spray for UV Protection.
The Poly Fiber Instruction manual warns about over tensioning the fabric during the three stage Ironing process as it can deform metal parts. I was deeply concerned that this may occur on the Turtle Deck since it is an open ended frame so a Test Fit was in order to check for proper fit on the Fuselage. Luckily there was no deformity and as I gained expierence I found that it was next to impossible to deform parts, even light weight aluminum parts. The only caveat is, if you glue the fabric too tightly around the part to begin with then it is entirely possible to deform it as the fabric pulls itself together. The best practice is to leave a little slack in the fabric when it is first glued to the frame, then as it shrinks it will not deform the part it is covering.
The sides of the Turtle Deck were fabric covered first and heat shrunk and then the top fabric was attached and shrunk down. The above photo shows the fabric before tapes were attached. Because the inside of the Turtle Deck can be seen when it is raised, I used the “Untinted” Poly Brush which is basically a clear finish product. The regular Poly Brush has a red tint and as you brush it on before, during and after the taping stage the brush marks and runs all begin to show. The red tint is there so you can see where you have brushed and where you haven’t. Theses brush marks will all be coated over with the sprayed on silver and color finishes but they will always be seen from the back side of the fabric. That’s not a problem when it’s inside the fuselage and can’t be seen but any fabric that can be seen from the back side such as fabric that faces the cockpit or in my case is seen when the turtle Deck is raised is an area where you would want to use the “Untinted” Poly Brush product