SOGWING Build Log

[Andy M]

Wing Spar Design:

I considered the following:

a) The Kevlar skin provides significant structural strength and durability so the need for a wing spar at all is somewhat debatable. However I hate to build a model and later find it is structurally weak, so decided to include a minimal spar(s).
b) The foam alone seems stiff enough for flight loads even without the Kevlar, so a spar will primarily benefit crash resistance. If a tow hook is added, I would not be confident of sustaining launch loads without a spar.
c) I'll use balsa/CF/epoxy composite spars, and as the CF tensile strength is significantly higher than the balsa, the balsa can be minimal.
d) A flat spar fitted tightly in the foam is well supported from deflection, buckling and twisting in the fore/aft direction, so it doesn't matter too much if it is weak in that direction before insertion in the foam. Stiffness against bending in the roll direction is most important.
e) To be stiffness/weight efficient I'd like to have a larger vertical spar dimension at wing root than tip. But it is difficult to cut a varying depth groove in the foam, so decided to have a constant depth spar.
f) Foam removed to accommodate the spars needs to be minimal.
g) Because this is a swept wing, a spar parallel to a % of chord line will have a kink at the root, which will be weak at the root where bending moment is highest. So I decided to have a spar at 33% of chord along each wing, and 2 short length spars perpendicular to the chord line at the wing root, spanning across the wing joint. These short spars are effectively also wing joiners. One of the short spars is at the CG so loads via a tow hook can be transferred directly to that spar.

I eventually decided on spars that are a sandwich of CF tow in-between 2 strips of 1/32” hard balsa glued with epoxy. So only 1/16” wide slots are needed for each spar. The spar depth is dictated by the minimum wing section along each spar line. This is 10 mm for the long spars and 15 mm and 20 mm for the short spars spanning the wing root line. The spars are fitted flush to the wing lower surface as that is where the maximum tensile bending stress is (when not flying inverted).

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[Andy M]

Wing LE

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[Andy M]

Elevon hinges:

The elevon hinges will be the Kevlar lower skin. However Kevlar skin at the hinges can de-laminate over time with flexing of the hinge. This is a pain to repair so I'm adding some ripstop nylon secondary hinges. These secondary hinges are trapped between a strip of light 1/32" balsa and both the TE and LE of the wing and elevon respectively. So if the Kevlar de-laminates, the elevon will not sag below the wing and alter the wing lower surface geometry.

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Wing Leading Edge:

The LE is reinforced with 2 strips of hard 1/16” balsa. The foam wing as cut is slightly flat at the LE, so I cut off 1/16”, glued on the 1/16” strips and planed/sanded a LE.

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[Andy M]

Early build pics cont.

[Andy M]

Early build pics

[Andy M]

The SOGWING (the name may change later) is a Southern Ontario Glider Group Inc. 1.2m (47") span foam core slope soaring wing that several members are building as a Club project. Airfoil and planform I believe were decided by Ed Smith and the foam cores were cut by Ray Munro. Ed built his last winter (2010/2011) and I've just got around to starting my build.

Wing area is ~31 dm2 (3.37 sq ft)
The weight of each unfinished wing foam core is 68 and 69 gm.

Construction details may change as I progress and with input from club members, but my current intent is as follows:-

Balsa/CF composite wing spars
1.7 oz/sq yd Kevlar skin on main wing and elevons, sealed with Minwax Polycrylic
Elevons cut from main wing (as opposed to separately fabricated)
Vertical stabilizers on wing tips, 1/32" glassed heavy balsa
Reinforced leading edge and nose to provide durability in nose landings
Small ventral fin at CG to aid launching, provide a balancing point and possibly incorporate a tow hook
Compartment at CG for ballast or a RAM3 altimeter
Spektrum AR6255 Rx. I'd like to use a cheaper Rx, but need full range type with end pins, not side pins, hence the 6255
Futaba S3114 servos
CR2, CR123A or Duracell 223 non-rechargeable lithium/manganese dioxide batteries (more on that in a separate topic). Battery choice will eventually depend on how much ballast is needed. Looking at Ed's build I suspect it will need a lot.

I have a 24" Halfbad foam wing which is almost identical in planform, so I'm going to use experience with that for guidance.

This build log will include mistakes and changes in construction intent as I progress and learn from any problems.

Some notes on adhesives.

The foam cores are common blue insulating foam. I'm not sure of the density that Ray used. I've never used this foam before so I did some up front testing with adhesives.
Foam safe CA (at least the Great Planes PRO brand) DOES NOT work on this foam and results in some minor melting.
Titebond III wood glue works fine foam to foam and wood to foam.
Weldbond works as well.
Goop and PU (Gorilla glue) work, but not as well as the Titebond or Weldbond and I'm not as confident with these.
3M Super 77 spray adhesive works well to adhere the Kevlar to the foam.
Polycrylic fills and seals the Kevlar well as an alternative to laminating epoxy (I also suspect it is a lot lighter). Polycrylic does not seem attack the foam though I've heard it may.
A low iron setting will attach Coverite Microlite to the Kevlar skin without melting the foam for cosmetics if desired. I've tested this.

Note on cutting Kevlar:

Kevlar is notoriously difficult to cut. However, I've found this trick works well.
1. Mark the cut outline on the Kevlar with a fine point permanent marker.
2. Paint a thin line of Polycrylic, ~2-3 mm wide along the marked outline and allow to dry.
3. The Kevlar will now cut much easier with a pair of high quality scissors. But you still have to cut slowly near the fulcrum of the scissors.

Initial construction steps:

1. Ensure wing root edges are smooth for good surface adherability to one another. If necessary cut smooth and sand with 400 grit emery cloth. Only one of my wings needed smoothing.
2. Smooth out any surface bumps from the foam cutting with 400 grit cloth.

Note it is much easier to perform work on each half of the wing, as opposed to on the joined wing. So I will do as much as I can before joining the wings.

Elevons:

I considered sheeting the elevons with 1/32” balsa, but calculated this would add 20 gm at the trailing edge relative to Kevlar which would translate to about 40 gm total. So decided to stay with Kevlar and a CF reinforced TE.

1. Mark out the elevons on both wing upper/lower surfaces, including rounding the root and tip corners. I made the elevon chord length 40 mm at root and 50 mm at tip. This was scaled from my Halfbad.
2. Chamfer and smooth the elevon corners.
3. Trim the TE to be straight and a thin edge.
4. Note, attaching the Kevlar top surface while the elevons are still attached to the wing I hope will reduce any bowing of the elevons resulting from the Kevlar attachment.
5. Glue (with 10% water diluted Weldbond) CF tow on the top of the trailing edge.
6. Cut out a Kevlar strip for the top surface of each elevon. Make the strip an accurate edge for an exact fit on the leading edge of the elevon but allow overlap for the trailing edges.
7. Mask the wings top surface except for the elevons and spray the elevons only with 3M Super 77. Allow ~10 mins to become tacky.
8. Apply the Kevlar to the elevon upper surfaces and allow to dry.
9. Paint the Kevlar generously with Polycrylic and then soak up as much as possible with a paper towel. Use the towel to firm down the Kevlar onto the foam. Allow 24 hrs to dry.
10. Cut off each elevon ensuring the blade is perpendicular to the wing lower surface.