SOGWING Build Log

Tell us about your new construction project. Seek advice if you have flight trimming problems.
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Andy M
Posts: 205
Joined: Wed May 13, 2009 9:17 pm

SOGWING Build Log: Hatch Cover

Post by Andy M » Tue Mar 27, 2012 9:46 pm

Hatch Cover for Battery and Rx:

The forward position of the battery is such that I could not recess it completely within the wing thickness without compromising the strength of the nose. So the hatch needs to accommodate the battery proud of the surface. The hatch is built up layers of 1/16” and 1/32” soft balsa. It is built up on the wing with the battery in place so it smoothly conforms to the required shape of the wing and proud battery. This is more accurate and provides a neater fit than carving from solid block. The hatch is held in place by a 1.3 mm short CF rod at the front end and a Velcro type fastener at the rear.

The nose and tail pieces at front and rear of the hatch to provide streamlining are also fabricated in the same fashion as the hatch. The rear of the nose piece has a 1/32” ply bulkhead in the front of the battery recess and also includes a female tube (insulation from 14g house wire) to house the hatch CF pin.

I fibreglassed the hatch with 1.5 oz glass as it will be handled a lot and it is a bit thin at the edges to blend nicely with the wing.
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Andy M
Posts: 205
Joined: Wed May 13, 2009 9:17 pm

SOGWING Build Log: Servo Pushrods

Post by Andy M » Wed Mar 21, 2012 6:45 pm

Servo Pushrods:

The pushrods are 0.032” wire with a Dubro #917 micro clevis at the servo end and an out of plane Z bend at the control horn. The advantage of an out of plane Z bend is that the pushrod can be inserted/removed from the horn by just twisting the wire.

The servos are quite far forward, so the distance between the servos and the horn is quite long for an unsupported rod. So the rods run through an 1/8” dia. Teflon tube glued to the wing top surface. I wrapped some thread at the ends of each Teflon tube to improve adherability to the wing.

The control horns are cut from milk bag ties and glued into a slot in the elevons.

I tried just holding the servos in place with only double sided tape, but it would not hold the servo securely under the reaction torque. So I made the top of the servos flush with the wing surface with balsa glued to the top of the servo, and fastened them down with a 1/64” ply cover plate, slightly overlapping the servo opening, glued to both wing and servos. The servos are wrapped in masking tape before glueing.
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Andy M
Posts: 205
Joined: Wed May 13, 2009 9:17 pm

SOGWING Build Log: Vertical Stabilisers

Post by Andy M » Wed Mar 21, 2012 5:30 pm

Vertical Stabilisers:

The V. Stabs are 1/16” balsa skinned with ¾ oz/sq yd fibreglass each side and sealed with Polycrylic. I would like to have used 1 mm Coroplast but couldn't readily find any and it was not worth buying a sheet just for this. The shape and area were just scaled from my Halfbad.
I've found the following works well for lightly fibreglassing tail feathers.

1. Cut a generous overlap on the fibreglass (2+ cm).
2. Lay the glass on the balsa and coat generously with Polycrylic.
3. Smooth over with a paper towel and then with another paper towel, soak up as much Polycrylic as you can.
4. Repeat for the other side
5. Sandwich between plastic sheet, Mylar is best if you have it. Do not use food wrap as it will stick. I use page protectors or photocopy transparent plastic.
6. Weigh down heavily between 2 flat surfaces. I use 3/4” oak with a 4l paint can on top.
7. Leave a minimum of 24 hrs, preferably 48 hrs. Polycrylic needs air exposure to dry, so dries slowly when compressed between the plastic sheets. But it does dry eventually.
8. When removed from the sheets, trim the glass and round off the edges.

The finished stabilisers weigh 5gm each, before any colouring/covering. I'll decide how to finish them when I know what the balance looks like.
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Last edited by Andy M on Wed Mar 21, 2012 9:43 pm, edited 1 time in total.

Andy M
Posts: 205
Joined: Wed May 13, 2009 9:17 pm

SOGWING Build Log: Kevlar skin lower surface

Post by Andy M » Sat Feb 11, 2012 5:21 pm

Kevlar skin lower surface:

I skinned the lower surface after adding the ventral fin and tow hook. Trimming the Kevlar after the Polycrylic was dry ended up a much neater job than the upper surface. All part of the technique learning process.
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Andy M
Posts: 205
Joined: Wed May 13, 2009 9:17 pm

SOGWING Build Log: Ventral Fin, Tow Hook and CG Position

Post by Andy M » Sat Feb 11, 2012 4:56 pm

Ventral Fin, Tow Hook and CG Position:

CG position
We need to know where the CG will be before we design and position the ventral fin and tow hook. The only purpose of the ventral fin is to hold the model and help hand launching. The tow hook is not essential but if it works out might just make this model flyable at thermal fields too.

The rule of thumb for Tailless Wing CG position seems to be between 20% and 25% of Mean Aerodynamic Chord. My Halfbad is recommended at 21% and it flies well there. Here are the Sogwing parameters for calculating CG position.

Span: 118.7 cm
Root chord: 31.4 cm
Tip chord: 17.1 cm
Sweep: 35.9 cm

Note the chord dimensions include the elevons because of course these are also part of the lifting surface.

The resulting CG positions as a function of %MAC are as follows:

%MAC CG Position from nose (mm)
20 212
21 214
22.5 218
25 224

I'm lazy so did not calculate the above from first principles. I have 2 calculators for this which I've previously checked. This is the calculator I used;
http://fwcg.3dzone.dk/

It seems to be well vetted on RC Groups over a 3 yr period;
http://www.rcgroups.com/forums/showthre ... ghlight=cg

I assumed the CG at 21% for starters and put the hook position at 16mm (5/8”) forward of the CG. I figure it is best to err on the forward side as although this hook will not be adjustable, the hook can effectively be moved back by adding material in the knee of the hook. This position also places the hook immediately behind the rear spar, so launch loads are transmitted directly to it.

Ideally the ventral fin needs to be at the CG, but with a tow hook there is not room. So it has to be forward of the hook.

The tow hook is 1/16” rod through the complete depth of the wing. It is anchored in 2 plywood laminations of 1/32” ply at the lower surface and 2 laminations of 1/64” plywood on the upper surface. A brass grommet in each of the upper and lower plywood anchors transfers the load to the plywood. The launch loads are carried as follows:-

Hook pull out load is taken by brass tube CA'd to the hook rod at the upper surface so the hook cannot pull through the grommet. Epoxy between the hook and the wing also adds pull out resistance.
Bending moment on the hook is reacted by shear between the upper and lower plywood anchors and the wing.
Shear on the lower surface is also reacted by the lower anchor. The anchors are epoxied to the wing.
At least that is the theory, but I won't be too surprised if it needs improvement in practice.

The ventral fin is just big enough to grip between thumb and forefinger. It is 1/8” balsa skinned with 1/64” ply. The fin fits in a slot in the lower anchor ply.
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Balancing sling point: Where possible I balance my models by including a sling point at the CG. This allows accurate balancing in both pitch and roll. In this case it a piece of ply with 3 balancing holes at 21%, 23% and 25% of MAC.
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Last edited by Andy M on Sat Feb 11, 2012 5:30 pm, edited 1 time in total.

Andy M
Posts: 205
Joined: Wed May 13, 2009 9:17 pm

SOGWING Build Log: Joining Wing Halves

Post by Andy M » Sat Feb 11, 2012 4:47 pm

Joining Wing Halves:

I glued the 2 cross joint spars into one wing half before joining the wings. Align the spars by putting both wing halves together with the spars, but only glue the spars on one side.
On final wing assembly glue the 2 halves and the other half of the cross joint spars. I used 1 hr epoxy for the wing halves because they were not exactly flat with each other and I wanted to fill the gaps. I used Titebond III for the spars.

The long spars which do not span the wing joint can be added independently of joining the wing. I added them beforehand. I first used PU glue for these spars, cleaning up the foam as it dried. But it still ended up a not very neat job with removal of excess glue necessary afterwards. That is why I used Titebond for the other spars.
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Nose reinforcement:

One gets some hard landings on the slope so I added some reinforcement. This consists of a triangle of 2 laminations of 1/32” ply epoxied into a slot in the nose. The triangle is ~15 mm x 45 mm spanwise. I also added small pieces of epoxied Kevlar at the nose on both wing surfaces.
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Last edited by Andy M on Sat Feb 11, 2012 5:05 pm, edited 1 time in total.

Andy M
Posts: 205
Joined: Wed May 13, 2009 9:17 pm

SOGWING Build Log: Adding elevons with the secondary hinges

Post by Andy M » Fri Feb 03, 2012 7:19 pm

Adding elevons with the secondary hinges:

1. Make 2 x 1/32” thick light balsa strips for the leading edge of the elevons at the hinge line. Pin these strips to the trailing edge of the wing at the hinge line with the secondary hinges free to wrap at the trailing side of the strip. Remember a similar balsa strip has already been added to the wing TE with the secondary hinges. Be sure to align the lower edge of the elevon strip with lower edge of the wing TE strip.
2. Glue the secondary hinges to the elevon strip. I used Weldbond.
3. When the hinge to strip glue is dry, attach the elevons to the strips. Do this on a flat surface ensuring there is compression between the elevon and wing. I pinned down the elevon, pushed the wing against the elevon and used tape to hold it together. A few pins to hold the 1/32” strip against the elevon also helps. I used Titebond III to glue the elevon to the rear strip.
4. When adhered, the top of the strips may need minor planing to be flush with the elevon/wing.
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Last edited by Andy M on Sat Feb 11, 2012 5:06 pm, edited 1 time in total.

Andy M
Posts: 205
Joined: Wed May 13, 2009 9:17 pm

SOGWING Build Log: Kevlar skin upper surface

Post by Andy M » Fri Feb 03, 2012 7:05 pm

Kevlar skin upper surface:

OK I've made my first mistake, being too skimpy with the excess when cutting out the Kevlar. Kevlar is expensive so I thought if I did an accurate job I could just allow ~1 cm of excess at the edges of the covering. I made a paper template for the half wing, transferred the shape to the Kevlar using a fine point Sharpie, and cut out the fabric per my note in the first post of this blog. I've learnt the following;

1. After cutting, because of the weave flexibility, the cut shape is not exactly the same as the template and,
2. A straight line marked on the fabric does not necessarily end up as a straight line after cutting. This all may be slightly affected by use of Polycrylic to help cut the fabric, as the cut line is stiffer than than rest of the fabric because of the Polycrylic.
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I thought Kevlar would be a lot easier to work with than fibreglass in this respect, and it is, but the weave still allows quite a bit of distortion. So the end result of all this is that there are some minor uncovered spots at the edges of the foam. It doesn't look great, but is no big deal and is not worth a rework.

The Kevlar to foam application process is exactly the same as described for the elevons earlier.

Note though, the Polycrylic causes the 3M Super 77 to lose adherence when first applied, but is OK as long as you smooth down well. It ends up sufficiently adhered.

Note also, don't expect this method to be as neat and as smooth a job as vacuum bagging, it isn't, but it works and is adequate.

The good news is that after the Polycrylic is dry, the Kevlar edges cut very easily with a #11 blade. I keep my used blades, which are usually only blunt at and near the tip. The edge further back remains quite sharp and is still good for cutting Kevlar coated with Polycrylic.
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I skinned the upper surfaces before joining the wing halves as it is easier in 2 halves. The lower surface though has to be skinned after joining because the skin covers the spars. And of course the elevons also have to be included when skinning the lower half.
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Last edited by Andy M on Sat Feb 11, 2012 5:06 pm, edited 1 time in total.

Andy M
Posts: 205
Joined: Wed May 13, 2009 9:17 pm

SOGWING Build Log: Equipment recesses

Post by Andy M » Tue Jan 24, 2012 8:23 pm

Equipment recesses:

On a flying wing, to decide on equipment positions to minimize additional balancing ballast, it is best to assemble the entire model as much as possible, then experiment with equipment positions to balance. Then cut the recesses at the ideal positions. This is how I did it with my Halfbad, but it is quite difficult and crude, as all the equipment has to be temporarily taped in place before cutting the recesses. The end result with the Halfbad was that everything needed to be as far forward as possible and there is evidence that it will be just the same with the SOGWING. So I took a gamble and cut the recesses without performing this temporary balance step. Moreover, I want to cut the recesses before skinning with Kevlar and that will alter the balance anyway.

I decided in the end not to provide a recess for an altimeter or ballast for windy conditions, as the total amount cut out was getting too large for my liking and the recess would interfere with the spar at the CG.
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It is much easier to cut the central recesses while the wing is in 2 halves. I marked the recesses, cut and removed the bulk of material with a #11 blade and then finished off with a Dremel.
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Last edited by Andy M on Sat Feb 11, 2012 5:07 pm, edited 1 time in total.

Andy M
Posts: 205
Joined: Wed May 13, 2009 9:17 pm

SOGWING Build Log: Slots for Wing Spars

Post by Andy M » Tue Jan 24, 2012 8:02 pm

Slots for Wing Spars:

1. Scribe 2 lines 1/16” apart along each spar position on the wings lower surface.
2. With a piece of tape as a depth gauge on a modelling knife, depth set for the specific spar, make a cut along one line perpendicular to the wing surface.
3. Make a 2nd cut on the other line 1/16” away at an angle to cut out a wedge of foam. Remove the wedge with the point of the blade.
4. Use a 1/16” dia. grinding spindle in a Dremel to turn the triangular slot into a rectangular slot. It helps to place or tape a straight edge along the slot line for both the knife and Dremel cuts.
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Last edited by Andy M on Sat Feb 11, 2012 5:08 pm, edited 1 time in total.

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