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Author Topic: Sikorsky S-39 "Spirit of Africa"  (Read 9112 times)
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g_kandylakis
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« Reply #25 on: March 01, 2018, 03:56:03 AM »

Yes, it is an enjoyable plane to fly, especially in calm uncrowded conditions (not the case with this video, with 3D planes flying around).

Slow progress, here is a small update.

Work concentrated on the ailerons, their hinging in particular. After drawing the necessary outlines, a plywood template was cut to shape and then used for cutting the 6 fixed hinges. The hinges are cut of 0,3mm polystyrene, a good compromise between weight and stiffness. A drop of CA on one edge is enough for fixing the polystyrene to the template for accurate cutting and shaping.
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g_kandylakis
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« Reply #26 on: March 01, 2018, 04:03:48 AM »

The position of the hinges on the wings was defined and using various balsa pieces, corresponding slots were created.

The hinges will be glued after covering and doping, but prior to painting.

Next in turn are the aileron halfs of the hinges, which will be similar in concept as the wing side...

George
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« Reply #27 on: March 01, 2018, 04:11:14 AM »

Clean, and elegant  George. The hinge brackets look enough to do the job and no more.

Enjoying the detail.

John
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Jürgen Be
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« Reply #28 on: March 06, 2018, 06:50:46 AM »

Hello George,

a great work on the wing can be seen here. Makes me want to grab some balsa...

It was nice tomeet you in Crailsheim and see your smaller Sikorsky flying.

Jürgen
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g_kandylakis
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« Reply #29 on: March 10, 2018, 03:33:32 PM »

Thanks John,

I do have a minor concern with using plastic for structural members, but it seems the forces are minimal, so it should handle it well. Up to now it was always plywood, but plastic is so much better to work with for such fittings.

Hi Jürgen, yes, it was nice meeting you too, a very nice event and flying hall.

Some more work done, actually the hinges are finished for now...

Same process as already shown, cut a template out of plywood (oversized on one end for easier handling), use extra positioning holes and cut multiple copies out of plastic. For the aileron hinge half 3 parts are needed, two outer and a middle one, glued together in the end. A carbon rod serves as axis.

6 hinges were made.

Suitable slots created on the wing structure for future positioning after covering
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g_kandylakis
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« Reply #30 on: March 10, 2018, 03:38:16 PM »

and to finish with these miniature fittings, the aileron control rod fitting was also made...

No chance without a magnifying glass  Grin

Also visible are small balsa pieces glued to the outer ribs, to prevent future wrinkles due to tissue pulling. I prefer this to gussets, as it is ivisible after covering...

Next are the strut attachment points and the leading edge sheeting. Then the wing can be glued together as one piece.

George
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g_kandylakis
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« Reply #31 on: March 16, 2018, 10:33:28 AM »

Wing sheeting completed. Some segments needed to be done twice. The choice of very thin and light balsa (0,3mm) requires extra care and does have me worried for what might happen in the future. It can easily get punctured.

My initial wing design had a major flaw: there was no support where the sheeting ends, about one centimeter from the leading edge.
So, even the slightest extra pressure on the balsa between two ribs would crack it. A small spar was needed, but too late to add it later.
Not to mention that my brilliant idea of rib lightening holes prevented me from even sanding a groove to the ribs of the finished wing in order to insert one-piece spars.
So, individual striips had to be glued between every rib, one for top, one for bottom. A parallel task to watching football, so not much time lost, only a couple of evenings  Angry...

Wings segments were glued together, using a simple jig for scale dihedral. Small carbon rod pins were used to align the root ribs as perfect as possible. After everything was set up and preglued, small carbon strips were CA-ed to the spars, ensuring a very strong continuous structure.

Total weight is just short of 6 grams. Not bad but I had hoped for less...

Time to start thinking about the tail booms and tail. The hull will have to wait for a very long time  Grin

George
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« Reply #32 on: March 16, 2018, 07:22:48 PM »

Inspiring work George. I like the delicate structure and the engineered to be just strong enough approach.

John
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« Reply #33 on: March 17, 2018, 04:23:18 AM »

That wing is what dreams are made out of Smiley. Beautiful!
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Tim Horne
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« Reply #34 on: March 17, 2018, 09:06:14 PM »

Hi George, That is really beautiful and inspiring work! I love the way you have made the hinges rather than (as I would have done Embarrassed) using commercially available parts which wouldn't have been at all scale.

My initial wing design had a major flaw: there was no support where the sheeting ends, about one centimeter from the leading edge.
So, even the slightest extra pressure on the balsa between two ribs would crack it. A small spar was needed, but too late to add it later.
Not to mention that my brilliant idea of rib lightening holes prevented me from even sanding a groove to the ribs of the finished wing in order to insert one-piece spars.
So, individual strips had to be glued between every rib, one for top, one for bottom. A parallel task to watching football, so not much time lost, only a couple of evenings  Angry...

This makes me feel so much better Cheesy. I thought I was the only one who didn't think through my construction beforehand and ended up making a "modification" to make up for it. If you can do that then I am in good company!
However, if we compared the weights of our respective current build wings then you would again be way ahead of me. That seems very light- superb Smiley.

Thanks as well for the video you posted of the original model flying. Great to see and lets hope this one does even better.

Tim
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g_kandylakis
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« Reply #35 on: March 28, 2018, 09:31:02 AM »

Thanks for the positive comments, I am glad you like what you see...

Tim,

you might have it all wrong if you think that not everybody who builds from scratch has to come up with "modifications" (what a nice way to put it btw...)
That does not mean we have to advertise our bloopers, of course, but they certainly are there...

While glueing the right wing to the center section, I had to do it three times. Luckily I was using thinnable glue, so I could soften the glue joints with thinner. First, still unresolved, the root rib was not perpendicular to the leading edge, so once glued I realized the two leading edges were at a significant angle as viewed from above. This led to the jig shown, for attempt two.
Attempt two was perfect in execution, but not in design. I miscalculated the tip height for the correct dihedral by 50%, so, too much dihedral. A nice way to lose static points. So, attempt 3, in order to arrive at the reslut of the pictures... Embarrassed

Anyway, that is part of the fun of scratch building, isn't it?

Back to my progress, last days were devoted to the tail booms. A very simple item really, you just cut a piece of sheet wood, form the ends and that is done. I cut 6 pieces, in order to choose the lightest pair in the end and was almost done. A look at the weight, a look at the drawing and the suggestion of Bernard above for the spars came to haunt me... How about built up, as in the original? So, I gave it a  try.

Cross section is 9,5mm x 2,4 mm, so I built up in 3mm thickness, in order to sand to final thickness. much more accurate and a super smooth surface for covering in the end.

Various jigs were used, for cutting of the ~30 diagonal pieces and for shaping the front and rear ends. End result looks good, seems strong enough and is certainly lighter, at 0,42gr for both booms, compared to about ,6 for the solid ones. Wether the time investment was worth it is another question. Yes, in terms of personal satisfaction, no in terms of rational building.
Luckily I have not set a deadline in my head, so no pressure at the moment.


PS. in picture #2 one can see the first version of the tailboom, finished within minutes...
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« Reply #36 on: March 28, 2018, 09:33:50 AM »

a quick jig for sanding to thickness, made of basswood. By inserting strips of paper under the part, you can adjust the actual thickness. The top surface is covered with scotch tape, to make sure you do not oversand the jig. The black marks are a reminder if you sand the tape away...

The jigs for the front and rear ends have positioning guides on both sides, so they can be used for either side of the boom assembly. Thiin polystyrene was used again to simulate the scale fittings. It is not going to be a load-bearing connection there, though. The booms will be glued to the wing where they mate (hence the extra hole for a locating and strengthening pin).

I have printed the tailplane drawings and will start work on them later in the day.

George
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Pete Fardell
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« Reply #37 on: March 28, 2018, 10:39:47 AM »

I'm really enjoying this build, George. You always choose such interesting subjects and your approach and expertise never fail to inspire. This model will be magnificent!
Pete
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« Reply #38 on: April 07, 2018, 03:10:48 AM »

Hi George,
What glue did you use for the plasticard(?) to balsa ends of the fuselage booms?
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g_kandylakis
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« Reply #39 on: April 07, 2018, 03:39:39 AM »

Thin CA.

I positioned the (oversize) plastic pieces and applied CA, which got everywhere through capilary action. One side at a time. Then everything was trimmed to shape.

I imagine any glue will work though, as long as it attacks polystyrene, like thick nitrate dope, which I plan to use for the hull.
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« Reply #40 on: April 09, 2018, 01:59:58 AM »

Thanks, George. I used to use Poly-Zap CA on larger RC models. Not often seen in the model shops, and works out expensive as it does not store well.
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« Reply #41 on: April 11, 2018, 01:20:22 PM »

Ian, I use "simple" pink Zap. it seems enough for this. I do not know of the poly-zap, never seen it I guess.

Back to building progress, two weeks since the last update.

Next up were the tail surfaces. As already seen in previous builds of mine, I have a preference for a specific building way for tail surfaces, which involves a thin sheet core and sticks glued on either side. Advantages I see in this are very accurate outline shapes, very thin edges and a better torsional rigidity. Drawbacks are perhaps more weight (not a problem for this model) and certainly more time consuming. Anyway, whatever makes one feel more comfortable...

Here are some photos of the progress, maybe too many, but since I took them...

Contour drawings glued to the balsa core with spray mount, then the inner contours cut and the drawings peeled off.

Core aligned on actual building drawing, all sticks glued on one side, then removed from building board and sticks on other side added, aligned to the first side (hence the small holes seen on the rudder)

Excess sticks dut off and sanded to outline
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« Reply #42 on: April 11, 2018, 01:23:20 PM »

Curved outlines were laminated, straight outlines from straight sticks, everything glued together
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« Reply #43 on: April 11, 2018, 01:24:45 PM »

Horizontal tail, same process more or less
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« Reply #44 on: April 11, 2018, 01:25:42 PM »

elevators...
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« Reply #45 on: April 11, 2018, 01:34:51 PM »

A lot of sanding required after that, first a "leveling" to bring everything to the correct thickness, then the outlines on each side sanded to the required thickness. Ribs were sanded with a suitable template and finished by hand.

Final parts await minor reinforcements for hinges, strut attachment points etc...

And as you can see in the pictures, there are two stabilisers, where the plane only needs one. As already discussed in Tim's Doulas thread, just another design error  Grin
My first stab and elevators were build to a 4mm thickness, they should have been 5mm thick. So I made a second stab and capped the elevators with 0,5mm on either side, so these were rescued...Not a real problem, I will use the ecxtra stab for covering and painting tests...

Next, the hull...
A lot of drafting to be done first, so not much should be expected soon.

George

PS. component weights as shown:
stab 0,8gr
elevators o,4 gr
fin + rudder 0,4 gr
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« Reply #46 on: April 11, 2018, 07:42:45 PM »

Fascinating - thanks George!

John
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« Reply #47 on: April 11, 2018, 09:56:01 PM »

Γεια σου    g_kandylakis!     I really like your wood work.   Shocked    You are very neat and meticulous.    Cool    I am enjoying your build of a very intriguing subject - your single engine light amphibious flying boat aircraft - the Sikorsky S-39.    Smiley

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OH, I HAVE SLIPPED THE SURLY BONDS OF EARTH ... UP, UP THE LONG DELIRIOUS BURNING BLUE ... SUNWARD I'VE CLIMBED AND JOINED THE TUMBLING MIRTH OF SUN-SPLIT CLOUDS ...
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« Reply #48 on: April 12, 2018, 12:09:52 AM »

Hi George,

One of the most intriguing builds of all time!!  Beautiful craftsmanship of a unique design.  Thank you for sharing!!!

Don
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« Reply #49 on: April 22, 2018, 06:02:02 PM »

Thanks John, γεια σου to you too Richard ! Don, I have no reply to your comment, a bit too much, but thanks  Embarrassed.

Things are not progressing very quickly but they do seem to be moving along at a steady pace.

Do not expect to see a finished hull yet, but I did start work on it.

I managed to draw some initial outline plans based on the excellent Paul Matt drawings. He shows extreme detail of the full size aircraft, often tempting for model use as well... There are minor accurace errors, which can be traced to the original being hand drawn, compared to the 0.000001 accuracy of today's CAD systems. Nothing great though, plus he has almost everything dimensined as well.

Here is a screen output of part of the hull drawing. Mainly outlines and cross sections, to understand the target shapes. Contrary to my previous S-39, I hope to have interior detailing as well, so my previous construction method of a keel and formers cannot be used 100%. Again the top front part of the hull will be removable, to provide access to the inside of the hull bottom where all the hardware will sit. I also want a rathere large opening for that, so the sides of that section will be rather weak in case of a head-on collision. The cabin too has to be clean inside.

So I decided to make a composite contstruction of the main hull part, laminating balsa and carbon strips. A laminating form was made of 2mm balsa, same as the lamination width, cut and sanded at the inside contour The contact area was covered with scotch tape, to prevent the strips from firmly sticking to the form when applying CA to the laminations.

The sheet balsa under the form is also covered with scotch tape, for the same reason.

The laminating went in steps of small lengths at a time, applying enough pressure to the strips and then some glue, making sure no glue goes to the pressure piece.

Here you can see the one side done. The front area has 3 carbon strips, the rest 2.
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