Thread Tools
Sep 28, 2003, 10:27 AM
Registered User
Thread OP

Building a SZD-9 Bocian


I am starting the build of a 1/6 scale SZD Bocian from a Traplet plan by Mike Trew.

The SZD-9 or Bocian is a 1950's Polish twin seater that has been used succesfully in competitions:



This will be my first scale build, and I'll document it here looking for tips & feedback. Ah - as to scale: I'll try to capture the scale looks, but I'm no purist with regards to things like the profile, the exact shape of spoilers or hinges etc, and I won't even bother with pilot(s) or things like instruments, so may be scale-like or semi-scale is better termed.

Anyway - all feedback is welcome, and I'll certainly post questions along the way.

Arthur
Last edited by ArthurWerner; Sep 29, 2003 at 01:56 AM.
Sign up now
to remove ads between posts
Sep 28, 2003, 11:01 AM
Registered User
Thread OP

Getting the ribs sorted


No airfoil is mentioned on the plan. However, even if it was, it would probably be outdated, as the construction details in the plan hint that it is not very recent. (E.g. there are no servo's in the wing.)

The plan shows the root rib, as wel as the tip and suggests using the sandwich method to create 22 ribs. The root rib is fairly thick, fairly symetric and has a slight camber, the tip looks mostly.. small - the wing has strong taper.

After taking the plan to the club, I got recommended to look at the HQ profiles, which are being used succesfully by German scale manufacturers. I finally settled on the HQ 2.5-12 profile.

To print the ribs, I used Profili 2, an inexpensive Italian program which I can recommend to everyone, see www.profili2.com

I printed the ribs in 2 sections, 1-15 and 16-22, to be able to include the washout (from 0 to -3degrees) automatically in the latter third of the wing, but not in the first part. I told Profili to include building tabs, then printed everything on sticky back paper and cut out.




Arthur
Sep 29, 2003, 09:50 AM
Where's the lift?
dgliderguy's Avatar
Keep it coming, Arthur! I'm a big fan of this bird, too, and I have considered doing a quarter-scale version from the Martin Simons three-view, as a tempting project for Someday. I've found lots of good pics of the Bocian on the 'net for documentation. It is a regal-looking 'plywood skybarge', and the wing and tail planform and that long multi-panel canopy really get me off.

Incidentally, as long as your lines are correct, it's a scale model. "Semi" scale is the term used for models that deviate from the three-view. Scale detailing, such as cockpit details and panel markings, are for extra points. Everyone else agree here, or did I make this up?

Anybody know how to pronounce 'Bocian'? ...Boshun? ...Boceeyahn? I don't speak Polish!

Don Bailey
Sep 29, 2003, 10:42 AM
MTT
MTT
I care about rising air !
MTT's Avatar
Hi, Arthur & Don, I have also just embarked on my first plan-build, it will be a 1/4 scale Sperber jr., based on plans I got from Traplet in England.
When I say embarked, it means that I just made copies of the plans, and have cut out the fuselage former paper templates and glued them to the plywood, that is as far as I got for now....

I was also thinking about the airfoil :
Just as on Arthur's Bocian plans, no airfoil is specified, the one shown is approx. 12-13% thickness, with very slight undercamber, could it be the venerable Clark Y ?
Anyway, if I want to change the airfoil, now is the time, any recommendations ?
Mr. Quabeck has on his HP also some airfoils specifically for oldtimers, might any of these be better ?

Michael
Sep 29, 2003, 12:10 PM
Registered User
Thread OP
Hi guys,

Thanks for your feedback, I'm going to need all the encouragement I can get along the way!

@ don: Traplet do a 2 part canopy kit for the Bocian which I ordered. The canopy will be an interesting piece of work, with the back part sliding and the front one hinging. The parts look huge!

@ michael: I initially wanted to stick to the original plan airfoil and went about it this way: Copied the airfoil from the plan, then scanned it in and cleaned it up. Imported as a BMP into Profili 2which will do an analysis and recommend the most similar airfoil. It will even allow you to save your scan as a new airfoil, and allows you to compare airfoil inc. reynolds numbers... This program is extensive. (I'll gladly analyse your airfoil for you if you scan it in).

I didn't use one of the oldtimer airfoils, as they did not seem to match with the original aircrafts high-performance.

@all
This doesn't mean Profili2 doesn't have any shortcomings, but I could work around them.

1) The wing has a false leading edge. As you have to choose either a slotted LE or a conventional LE, you can't create this directly. I choose to add the width of the LE to the spruce - which means I'll have to chop off the first 5 mm of each rib manually after printing.



2) You can set Profili to include building tabs which take washout into account. However you can only do this over the full range of ribs. Solution: measure the width of the rib where the washout starts. Then generate a set from root to that rib. Then a second set from that rib to tip, which included the washout. Discard the double rib.

3) Spars go all the way through. You can't specify a spar slot to only apply to the first X ribs. As a result, be careful when cutting out the ribs - some of the slots should not extend all the way.

That done, I printed the sets of ribs and stuck them to balsa.
The plan wants to use 1.6mm (1/16 inch) which I think is too flimsy. I am using 3 mm, 2.5 mm, 2mm towards the tips.

The ribs go from sizable (30 cm) to quite small (9cm).



Arthur
Sep 30, 2003, 03:18 PM
Registered User
Thread OP

Ribs cut out


All ribs have been cut out with a sharp knife and than notched. The good thing is that you can do this in the living room - no dust whatsoever.



The white you see is the sticky label. I leave it on as it adds some extra strength. It also keeps the parts nicely together for glueing if you happen to snap a rib. You will also notice the building tabs, that should give me a straight wing, with progressive washout in the last 1/3.

The difference in size from biggest rib to smallest rib is impressive:



Arthur
Oct 05, 2003, 07:09 AM
Registered User
Thread OP

Wing joiners


The method of joining the wing halves have given me some headaches. This is as the wing platform is swept forward... so the spars are not in line when viewed from above, but start to point forward from the fuselage to the tips.

In the original drawing the wing halves are joined by aluminium joiners made of L stock, whreby both the dihedral and the 'swept-forward' angle are taken up in the fuselage. However this depends on very exact angles on the aluminium, two aluminium joiner pieces to be bolted on the ends that stick out of the wing inside the fuse at every assembly, exactly drilled holes, etc. Too much margin for error! I'm not a metal worker.

Therefore, I've devided to take up all angles in the wing, with a metal wing joiner going straight through the fuselage. The dihedral is not a problem, but because the wing is swept forward, the wing jhoiner no longer runs paraller to the spars. I will have to make sure it transmits its forces to the wing spars though.

BR
Arthur
Last edited by ArthurWerner; Oct 05, 2003 at 07:12 AM.
Oct 06, 2003, 12:26 PM
Registered User
Thread OP

Setting up the wing


I don't build on the plan, but transfer the outlines and key points to paper instead.

I would say the ribs line up pretty wel though:



Viewed from the other side, you can see the washout:



I plan to leave the building tabs on until the topside is fully covered. That will hopefully give enough rigidity to cover the bottom without introducing warps.

Arthur
Oct 06, 2003, 12:29 PM
Registered User
Thread OP

Time to add the spars


Spars are two pieces - so a tapered joint goes in:



Howeverm before the spars can be glued into place, the first 4 ribs need to be cut to accept the wingjoiner (testfit, not glued yet)
:



Arthur
Oct 06, 2003, 12:35 PM
Registered User
Thread OP

Mounting the joiner box


With ribs cut, the joiner box can be put in.



The joinerbox meets the ribs at right angles. In the 4th bay, it is completely encapsulated by the spars.



Next will be the bottom spars and the shear webs.

Arthur
Oct 09, 2003, 06:23 AM
Registered User
Thread OP

Thinking of the aillerons


Ok, spars & shearwebs are in. Spoilers are in, and am currently adding extra strength to stress areas. (Photos coming)

The main wing structures for both wing halves have now been finalised, and I am starting to look at the ailerons. I will need some advice here.

The ailerons aren't really 'there' until the wing is sheeted. This is as they are not very deep (especially towards the tip) and the wings TE is formed by the sheeting. Towards the tip the ribs hardly extend to the aillerons...

So I plan on sheeting the wing fully, then cutting out the ailerons with 5mm margin each side, then glue 5 mm hard balsa strip to both wing and aileron.

Does that make sense? Any better ideas?

Of course I will still have to hinge them. Somehow tape hinges don't seem right. Should I use a "Hohlkehle"? Hidden hinges? Or just hinge in the middle? How will the washout effect the hinging?

Any advice appreciated!


BR
Arthur
Oct 09, 2003, 07:37 AM
MTT
MTT
I care about rising air !
MTT's Avatar
Your method of makeing the ailerons sounds good to me.

I'd go with the "Hohlkehle". What the heck do you call that in english.....?
You can easily make it by gluing square balsa stock to the LE of the aileron, and triangle balsa stock to the wing, in the area of the aileron.
I have done this one my Flair Ka8 ( the instructions call for tape hinge ), and used Robart hinge points for hinging the aileron.

Michael
Oct 09, 2003, 12:10 PM
Registered User
Thread OP
Michael,

I'm not sure I understand exactly what you describe.
If you say "...gluing square balsa stock to the LE of the aileron, and triangle balsa stock to the wing, in the area of the aileron", are you meaning this?



OR are you meaning using 2 pieces of triangular stock to help create the hollow bit, which is further formed by sanding?


(This would be my definition of a "hohlkehle")

Arthur
Oct 09, 2003, 12:55 PM
MTT
MTT
I care about rising air !
MTT's Avatar
Arthur, your second option is exactly what I meant.
This is how Flair tells you to do in the instructions of their Ka8 kit, on the levator and rudder.
On the ailerons, however, they tell you to use your first option.
I did not like that, so I modified my Ka8's ailerons to option 2.

Of course, I forgot to mention that the square balsa stock you glue to the aileron LE has to be planed and sanded to the circular form.

Michael
Oct 09, 2003, 03:45 PM
Registered User
Thread OP
Adding carbon.

It took me some effort to find a place where I could buy it, but eventually managed to get some 240 gms unidirectional carbon band. This has 10 individual strands and is 2.5 cm wide. The single strands can easily be pulled out as it is only one single glas fibre that loosely weaves them together.

Though I am building according to plan specs I feel I should reinforce the wing as it will have to go up on a winch.

I am adding the carbon strands to the top and bottom spars. Starting with 3 strands next to each other at the root and reducing to 1 towards the tip. The strands can be laid out nicely flat and neat. At the moment I have attached them only with a tiny drop of thick cyano every 20 cm or so. I will epoxy them all in one go - both halves, top/bottom.

Also in the picture - the box that will hold the (Graupner) spoiler.



Next is to wrap the joiner box / main spar junction with carbon, which has been beefed up with ply shearwebs and 'joiner plates', effectively creating a box-like structure.


Arthur


Quick Reply
Message:

Thread Tools