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Author Topic: Sikorsky S-39 "Spirit of Africa"  (Read 9120 times)
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g_kandylakis
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« on: February 16, 2018, 08:25:17 AM »

Hi everybody,

here is a thread for my latest "long term" project, the Sikorsky S-39 amphibion, for indoor electric scale.

This is a model I have already built in the past, as a micro RC model. this was at 1/32 scale, now I am opting for 1/24 scale. I also considered 1/20 but got freightened from the resulting size.

As planed, the span will be 660mm and I am aiming for a weight of 45 grams.

As usual with most of my recent projects, this will again be a dual "purpose" model, both for free flight and for radio control (please do not throw me out of this forum  Grin).
This does cause an extra weight penalty for the RC equipment, but on the other hand provides me with the possibility of more frequent flying under rc, instead of only 1-2 times per year as free flight in large halls.

It is going to be another long term project, not to be expected within 2018. I do not even dare to make a prediction at the moment, I 'll take it slowly and enjoy the building process. Not to mention the masking and painting one  Grin

George
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Sikorsky S-39 "Spirit of Africa"
Sikorsky S-39 "Spirit of Africa"
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« Reply #1 on: February 16, 2018, 08:38:38 AM »

I chose to start with the wing.

I have a personal rather time consuming method for wing rib creation, rough cutting each rib in a sandwich between ply templates, then shaping it with fine cutting and sanding to the templates.

Normally it is not as bad as far as productivity is concerned. For the Sikorsky I want go as light as possible, so I added a lot of lightening holes  Angry.

Result was, about 4 ribs per hour... Plenty of hours in total... You only realise the consequences of some decisions when it is too late to start over...

A lot if fine needle filing and sanding, but at least in the end you get a large stack of uniform nicely shape ribs...

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Re: Sikorsky S-39 "Spirit of Africa"
Re: Sikorsky S-39 "Spirit of Africa"
Re: Sikorsky S-39 "Spirit of Africa"
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g_kandylakis
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« Reply #2 on: February 16, 2018, 08:49:00 AM »

Wing is in three sections, with a nice scale dihedral.

Center section was first.
One can already see that the wing chord of 90mm drove my favorite building board (a 25mm thick balsa sheet) to its limit...
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Re: Sikorsky S-39 "Spirit of Africa"
Re: Sikorsky S-39 "Spirit of Africa"
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Re: Sikorsky S-39 "Spirit of Africa"
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g_kandylakis
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« Reply #3 on: February 16, 2018, 08:55:20 AM »

Next was the left wing, then the right one...

This is as far as I have taken pictures of. More will follow later.

not that there is much progress, just fine work on the wings. Currently working on the ailerons.

I always do separate control surfaces on larger models, never had a problem with it.
Until I read a comment by Richard Crossley in his Piper Tripacer thread, where he compared the effort for separate ailerons to building a second set of wings. How true...

George
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Re: Sikorsky S-39 "Spirit of Africa"
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Re: Sikorsky S-39 "Spirit of Africa"
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« Reply #4 on: February 16, 2018, 10:15:39 AM »

I agree about the workload burden of separate control surfaces but it's very satisfying when you have everything done. I love the choice of subject. Didn't this design feature in the Howard Hughes bio-pic a few years ago, "The Aviator"?
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« Reply #5 on: February 16, 2018, 12:23:14 PM »

Partly true. It was the same design concept, but a twin motored version, the S-38

http://www.everystockphoto.com/photo.php?imageId=8232537

Continuing with the ailerons, here is the current status.

The ailerons are hinged at their underside and have slanted leading edges (?), as the drawing in the first picture better shows. A template was made of thin ply with grooves that allow exact positioning on every rib.

Next, very carefully, the ribs were scored just enough, to allow later removal of the unwanted rib section. Deep enough to allow following the scored line, but not too much to separate the rib (90% success...)

In order to add some stability for when the ribs are cut, external temporary strips were added. These will later be removed with thinner, but only after everything else is glued in place.

Next, with a slow steady hand  :'(, the scored balsa is cut through and you get a structure ready for glueing. A light pass with a thin sandpaper block will ensure all ribs are exactly the same. To follow...

Just my personal way of ensuring undistorted and exactly matching surfaces...
Attached files Thumbnail(s):
Re: Sikorsky S-39 "Spirit of Africa"
Re: Sikorsky S-39 "Spirit of Africa"
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Re: Sikorsky S-39 "Spirit of Africa"
« Last Edit: February 16, 2018, 12:56:19 PM by g_kandylakis » Logged

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« Reply #6 on: February 16, 2018, 12:33:54 PM »

And here is an extra, which you do not see in free flight...

The aileron bellcrank assembly, so far on the left wing only.

A simple balsa base and a 0,4mm ply bellcrank. All holes are lined with thin brass tubing, to ensure minimum friction and minimum play.

Especially the later is most important for free flight. The control surfaces need to be completely "locked" in order to get consistent flights. In RC this is not such a major problem, of course it is always better to have zero play.

Driving rod is 0,3mm carbon, fed through holes in the ribs. Weight penalty is small at such size, as is the distortion through the continuous support provided by all ribs.
Attached files Thumbnail(s):
Re: Sikorsky S-39 "Spirit of Africa"
Re: Sikorsky S-39 "Spirit of Africa"
Re: Sikorsky S-39 "Spirit of Africa"
Re: Sikorsky S-39 "Spirit of Africa"
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« Reply #7 on: February 16, 2018, 12:52:30 PM »

Sikorsky had some great amphibians in the late 20's - 30's era.  Another great one is the S-38.  This one, originally owned by the S.C. Johnson company, was used to fly around South America scouting for sources of Carnuba wax.  Both of these planes would be fun builds if you love rigging!



Attached files Thumbnail(s):
Re: Sikorsky S-39 "Spirit of Africa"
Re: Sikorsky S-39 "Spirit of Africa"
Re: Sikorsky S-39 "Spirit of Africa"
« Last Edit: March 16, 2019, 05:18:19 PM by Ratz » Logged
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« Reply #8 on: February 16, 2018, 01:57:55 PM »

Very nice George,
I wonder how much weight is really saved with hollowed ribs? I would guess that you could have gone with solid ribs and then used top and bottom I beam spars constructed with 1/32 to get a lighter stronger wing (maybe??). Either way the effect is nice.

BG
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« Reply #9 on: February 16, 2018, 02:40:37 PM »

Hi Bernard,

interesting question, woke up the engineer in me (my profession, actually  Grin)

I did some measurments and calculations:

weight of rib without lightening holes = 100%
weight of ribs with lightening holes = 66%

(not taking into acount the necessary holes for spars...)

Ribs are 0,8 mm thick, I happened to have weighed them during making, weight was 0,03 grams. So, a rib with no holes would weigh 0,03/66% = 0,454 gr.
The extra weight would be 0,0154 gr per rib.

There are roughly 61 ribs (double count for the thicker joining ribs), so the total additional weight would be 0,93 grams, roughly 1 gram.

Expected wing structure is 5,5-6 grams, (4,5 grams  so far without some LE sheeting missing)

Model weight hoped at 45 grams.

So, we are talking about 2% more.

A gram here, a gram there, yes I think it makes sense to save wherever I can... Plus, it looks better  Grin

George
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« Reply #10 on: February 16, 2018, 03:54:17 PM »

Thanks for crunching the numbers George. You omitted the weight saving from using 1/32 I beam spars though .... looks like you are using solid 3/32 spars. My intuition is that it would all come out in the wash in the end, with the addition that solid rib and I beam spar wings are easier and faster to make.

 Wink
BG
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« Reply #11 on: February 16, 2018, 04:11:11 PM »

Yes, you are correct, I only thought of the ribs.

Spars are 2mm thick, the lightest C-grain I had. Total spar weight was about 1,5 grams.

So, I guess the correct answer is: built up spars and ribs with lightening holes...

Too late now, but I will keep it in mind.


I have never done a spar like that.
How do you ensure the straightness of the built up spars?

George
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« Reply #12 on: February 16, 2018, 05:37:00 PM »

continued work on the left wing.

Oversize wood strips were glued to the wing and aileron. The temporary external strips were removed with thinner and patience.

Next the oversize strips were trimmed and sanded to follow the contour of the ribs. I use low tack scotch tape as a protection for the nearby structure, which gives a very good guid for the sandpaper to follow.

Final picture, the completed center section. Completed is a relativ word, of course, plenty of fittings and reenforcements still to be done...
Attached files Thumbnail(s):
Re: Sikorsky S-39 "Spirit of Africa"
Re: Sikorsky S-39 "Spirit of Africa"
Re: Sikorsky S-39 "Spirit of Africa"
Re: Sikorsky S-39 "Spirit of Africa"
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John Webster
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« Reply #13 on: February 17, 2018, 04:13:41 AM »

It is a pleasure to watch and learn from a master.

When you were cutting out all those ribs, how often did you wish you had a laser cutter?
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« Reply #14 on: February 17, 2018, 04:41:38 AM »

Hi John, thanks for calling me that  Embarrassed

I did not wish I had a cutter. It would be enough if I only had access to one  Wink. Or someone who had access to one... Wink Wink

I do now, thanks to an offer from a friend (thanks...)

The main thought I had was, I hoped I had chosen the correct scale. I hated the idea of having to redo everything.


But, funny thing, I enjoyed cutting them in a sense (no, not a masochist...). A bit like, going back to crafstmanship only, and a proof that similar good results can be achieved manually.

Plus, the added bonus of clean outlines with no sanding necessary.


My real test of man vs machine will be the dummy engine. 3D printing vs completely hand made? But way back in the future for that...

George
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« Reply #15 on: February 18, 2018, 07:27:05 PM »

Exquiste detail wormanship.
 There is also the Dave Rees style of Rib construction. Should one want an alternate technique.
Curious re the bottom hinging of the Ailerons.   
You did this because you are an adherent of dr Drela.. or?
Drela suggests a bottom hinged fitment.. whenever possible.
However his area is in High Perf  hand launched glider airfoil/research.
 His claim is that uninterrupted flows on the undersurface of an airfoil are critical... whereas on the topside .. not so much, certainly not 'back there' where ailerons usually live,
 as flow separations are already well established by that point.
His view is that upper surface  aileron V grooves are merely less attractive, rather than problematic.
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« Reply #16 on: February 18, 2018, 08:05:40 PM »

Beautiful workmanship as always George, I will really enjoy watching this come together however long it takes.

I'm sure you will get to this but I am very interested to know how and with what you will hinge the ailerons, and how you will attach the hinges.

How well did the smaller one fly? I remember seeing pics of it but I don't think I have seen it flying; on film or in real life.

Tim
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« Reply #17 on: February 19, 2018, 07:30:48 AM »

Hi Fred,

The Dave Rees style is very good and very light. It would not fit my aim, though, which is thin ribs at 1/32". Also, the bottom of the ribs is not straight but mixed with some undercamber as well.

Regarding the hinging of the ailerons, there are two very simple answers. First. it is much better, aerodynamically speaking (reduces adverse yaw compared to traditional centered hinges)
The second is much simpler and much more important, this is how it was done on the original, so a cale model should follow suit....

When I built my first S-39, it was started as a free flight model (no worries then...). And when I decided to convert it to micro RC, I was faced with many problems, especially how to control the tail surfaces with a Rx brick located in the hull. After some thought and head scratching, it proved to be much easier, to simply copy the original pull-pull with cables, which was quite effective. The challenge then was not how to do it, but how to do it in that size, with fine thread and guides for routimg the thread from the servos to the control surfaces.

So, if someone took some time to sort it in the big plane, why reinvent the wheel...Same now...
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« Reply #18 on: February 19, 2018, 08:08:43 AM »

Hi Tim,

I got jealous from your Douglas build thread, or rather, inspired... Some nice projects currently underway.

Regarding the aileron hinging, I have it sorted out in my head at ~90 %. Which menas there is still 50% left to do...

Hinges in the original were made of sheet metal, see images attached. I intend to copy that with either thin ply, thick paper or thin polysterene sheet. To be tested and decided later. The sheet hinge will be glued into a slot created in the rib structure. Correct position is to be defined through a simple jig, both for the wing and the aileron. Hinge axis will be a short piece of carbon rod, glued to the hinge base and sanded down as close as possible. Actually, I think if you look at the pictures and think about it, you will see there is not much to it...

The smaller one flew and still flies fine. It had some initial problems with the Rx browning out, which would cause the power to drop and rise, which then developed an oscillation along the longitudal axis. A newer Rx with more Amp solved this problem. Now it is a rather cosistent flyer, only a little bit too fast for my liking. Therefore a new, bigger one...

I had it with me and flew it in Nijmegen in 2016, but you were not there.. Here is a video from way back 2009, when I still had some problems. Surprisingly, I have nothing newer... I will try to make one next Sunday. https://www.youtube.com/watch?v=R2umolEqAaU (you can hear some of the brownouts...) Not the best quality, but you get an idea, I hope.

George

PS, I notice that using a laptop instead of my traditional computer with keyboard, I make many more typing mistakes. I correct most of them, but some slip through... My apologies...
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Re: Sikorsky S-39 "Spirit of Africa"
Re: Sikorsky S-39 "Spirit of Africa"
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« Reply #19 on: February 22, 2018, 03:08:45 PM »

Beautifull work so far George, definitely an ambitious project! labour intensive configuration and scope for detailing is infinite... I am sure it will be a beauty when finished!
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« Reply #20 on: February 22, 2018, 06:21:55 PM »

Thanks for all the interesting build detail George. I was fascinated by your ribs and the aileron marking jigs in particular. Are the little nibs on top of the ribs near the LE just to locate the LE sheeting?

John
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« Reply #21 on: February 23, 2018, 12:18:14 AM »

Exquisite workmanship George!!   Truly a work of art, and maybe too nice to cover.  I get the part about saving weight, but for me it is also a lot about just looking neat.  Following with great interest!

Don
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« Reply #22 on: February 26, 2018, 11:53:18 AM »

Thanks for the coments guys...

John, yes. I knew I would have difficulty geting a uniform outline if I shaped the ribs prior to glueing the sheeting, so I thought of leaving extra material on the ribs at the step and sanding it to shape, to follow the sheet precise thickness. 1-2 mm in height, 4-5mm in length were enough.

Not much more progress at the moment  Embarrassed

George
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« Reply #23 on: February 26, 2018, 04:45:54 PM »


How well did the smaller one fly? I remember seeing pics of it but I don't think I have seen it flying; on film or in real life.

Tim

Hi Tim,

I went to an indoor event yesterday and managed to make a flight and have it filmed. That should give an approximate idea about how it flies. Of course, do not forget this is radio controlled. You can correct anything... I would have liked the flight speed to be a bit lower... Hope the bigger one flies slower  Cool.

https://youtu.be/gX9gHzB8YnQ

George
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« Reply #24 on: February 27, 2018, 09:13:14 AM »

I think that's a fine achievement. The bigger one will probably be a little slower, and of course will appear to be slower still, because of its larger size.
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