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Old Oct 31, 2005, 09:08 PM
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SoarScale
United States, WI, Wind Lake
Joined Nov 2004
814 Posts
P61C Black Widow series (#2)

Wing Center section.

I have built many sailplanes in my modeling history, both small (10ft span) to large (38ft span) and have used foam exclusively for the core material. I have gained a significant amount of experience with foam cutting and wanted to use that experience for the P61. I did choose however, not to build the flying surfaces with foam as a core material and sheet them. What I did choose to do is use the foam “beds” from pre-cut foam wings as the “building boards/jigs” for each wing section. You’ll see this in the pictures. Essentially, the wing structure was built ON the foam beds providing a stable, consistent surface for each wing. There were some other benefits from this approach also.

1). The foam beds AND cores were used to pre-form the airfoil contours into the sheeting PRIOR to rib application therefore removing any wood bending stresses (read warping) from the skins
2). the foam beds became very useful when applying the final sheeting panels to lock the wing in shape.
3). The dihedral was already built into the foam beds making the complete center section a breeze to build
4). The washout on the wing tips was pre-cut into the foam cores using templates with washout built in. This created a wing building jig again, with all the necessary top and bottom surface requirements and provided an excellent building surface for the relatively complex spoilerons present on the P61. In the case of the tip panels, these were cut upside-down so that the ribs and spars were applied to the top surface sheeting first.

So, to construction:

Two cores were cut from white foam. It was important to create the center section foam beds to the exact height on both top and bottom surfaces for center section building AND for later alignment and building of the tip panels – specifically the joiners (see later). Due to the dihedral of the center section and then anhedral (with respect to the center section) of the tip section, careful calculations were made to get the template heights perfect for the resultant foam beds. The center section of the wing was to be built “top-surface up” and the tips were to be built “top-surface down” due to the spoileron assembly requirements. If you choose to use a method like this don’t forget to keep the cores themselves. They become a wonderful form in association with the beds to pre-form the skin sheeting prior to their assembly using alcohol/water.

Bottom line, when all the foam beds for the three wing sections were laid out, upside down, on my building bench, the resultant “building” surface was an exact form for the top surface of the wing from tip to tip – almost like making a mold of the top surface of the wing in one piece and building the structure within it. Great for setup, alignment of joiners etc – see later

The first piture shows a “dry run” of the first half of the wing center section – checking to see if the parts are accurately cut and fit well. Note the two separately cut foam cores as the building base. One other requirement of this method is that the spar locations and rib spacing must be marked on the sheeting prior to construction – maybe a drawback, but I didn’t find it so!

If you have a real keen eye, you may notice a “black” layer laminated between wood in the spar caps. That’s carbon – or, specifically, Graphlite. Graphlite is a pultruded carbon that can be purchased in differing rectangular cross sections in very long spools. I typically special order my spools in 500ft+ lengths and have used it extensively in my sailplanes for spar caps. Don Bailey, the guy who decided to post my original P61C pictures in this forum, mentioned the 35ft ETA I built a while back. This is a sailplane that weighs approx. 45lbs, has a 10 ½” wing cord at the root and a 35 foot wingspan. Graphlite and end-grain balsa were used exclusively for this wing spar structure. This aircraft looped under high speeds with wing flexing at the tip in excess of 3 feet. The graphlite spar caps were the primary reason for both the flexibility and strength of this spar system and consequently the sailplanes’ wing. Anyway, I digress!

The second and third picture shows both sides of the center section internal structure – Ribs and bottom spars are glued in place, the remaining items are dry fit – this includes top spars, the wing joiners, servo wire routing tubes, spar braces and all servo hatch mounting rails. Note that the servo hatches and the general shape of the bottom surface sheeting is already precut prior to gluing the ribs in place. As long as you are accurate and mark the sheeting well, this assembly method works very well.

The forth picture shows the main structure gluing complete, servo mounts added for the small flaps, flap structure in place, wing mounts in place (note lightening holes to reduce weight)
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