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Bistormer 60" (A Barnstormer with more ribs)

Barnstormer with more ribs

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john stones 125/01/2014 00:03:53
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where you supposed to cut the 3 inner ribs

and put a full strip in ?

seems an odd method

Danny Fenton25/01/2014 00:10:58
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Hi John according to the attached notes those blocks get added above the spar and in each bay. I haven't made it very clear and the joint is good so perhaps you cannot see it but the rear spar is intact all the way from root to tip, I have simply added blocks to bring it up to the top of the rib
Cheers
Danny
john stones 125/01/2014 00:27:08
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no you were clear enough Danny

I didn't engage brain before typing wink

kc25/01/2014 17:13:20
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Danny you questioned whether the webs need to be a tight fit against the ribs. This is not conclusive, but it's interesting to note that the Avicraft Mini Frantic has only small bits of webbing. Each rib has a only a bit of web material glued either side of the rib , with a large gap until the next bit. Presumably to save weight but even these small bits of web stiffened the wing a lot.
My instinct is it doesnt matter if the web touches the rib but it's important to have it firmly glued to the spars all the way along. Doubtless the engineers amongst us will quote the apropriate theory! (It's quite common to see steel girders with large cutouts in the central rib and this is much the same)
Anyway as always the definitive answer could come from a practical test to destruction on small test pieces. Remember Alistair Sutherland's RCMW article on balsa Warren girders compared to T truss girders etc tested until they failed? Very informative.
Danny Fenton25/01/2014 22:15:25
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Hi Chaps, we all do it John, me more than most crook

Thanks KC I do not recall the article in question, can you enlighten us on Alastair's findings?

Managed to get both wing panels to the same point, in between trips to stagecoach and swimming for my little 'un thumbs up

bs 45.jpg

The rib cap strips were actually done after the leading edge sheeting, its just the leading edge was fitted using Titebond and the capping with Zap, hence the pins wink 2

Cheers

Danny

kc26/01/2014 12:26:05
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Alasdair Sutherland's practical tests revealed that the T triangle (similar to the Barnstormer etc rear fuselage) was about 50 percent stiffer than a Warren girder and only 7percent heavier. He proved that gusseted square structure was nothing like as stiff as the T triangle or Warren. He also tested how the strength improved with covering by film or by tissue. All this was in RCMW Aug 2002.
I would like to put a copy of this here on Modelflying but RCMW are the competitors so I wont to avoid problems!
My point in mentioning this is that Alasdair did simple tests with balsa sample structures to prove what was best for balsa models. It's almost the same as the webbing spars discussion but not exactly. So the field is open for someone to do a test on spar webs -touching the ribs and not touching etc-- in a practical but scientific way. Maybe a project for engineering students to investigate?
Anyone who is interested in this sort of balsa structures to test engineering principles might also want to read the book Basic Structural Behaviour via Models by Barry Hilson pulished by Crosby Lockwood in 1972. This shows other balsa structures being tested to destruction.

Someone built a model ( almost certain it was the great David Boddington) and used tape or ribbon in a double diagonal manner (rather like the herringbone struts used in roofing)instead of webbing the spars. He said it increased stiffness remarkably.
So to sum up I believe that its not essential for the webs to actually touch the ribs but of course it would be better.

Edited By kc on 26/01/2014 12:28:43

Danny Fenton27/01/2014 17:28:25
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Thanks KC, It is an interesting area for sure. I have responded via a PM in more detail as KC very kindly sent me a scanned copy of the article by Alisdair.

Not much to show, the lower wings have followed the same sort of construction as the upper ones. I promised to show how I do the belcranks, probably much the same as everybody else. We will see how they stand up the test of time.

bs 46.jpg

These are the component parts, I very carefully remove any bur from the plastic parts

bs 47.jpg

The hole in the plywood is made with a quality 2.5mm drill bit. The screw is then threaded through the ply wood until it is snug. The Nyloc nut is then fitted over the top until every thing is done up but still free.

bs 48.jpg

a simple Z-Bend is all that is needed, but the hole must be a good fit!

Next up is the small centre section which I will lash together next.

Cheers

Danny

Nigel Day27/01/2014 17:43:23
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'Lash together'

One man's lash is another man's really neat piece of work. smiley

Danny Fenton27/01/2014 18:03:29
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Thanks Nigel wink 2

Cheers

Danny

Biggles' Elder Brother - Moderator27/01/2014 19:20:55
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The webbing acquires no significant strength from touching the ribs as such, webbing adds strength via two geometric routes;

Firstly it couples the upper and lower spars with a member that is extremely stiff in bending about the chordal axis - ie a sheet of balsa "end on".

Second because the grain is vertical this stiffness is even further anhenced by a resistance to buckling under load which it might display were the grain spanwise.

The complicating factor here is that what maximises the effect of webbing is maximising its area - the bigger the area of webbing we can create the stronger the wing will be and the lower the stress levels will be in the wing for any given deflection. Now of course the absolute maximum area of webbing would be achieved by completely spanning each bay with a webbing piece - then the webs would indeed touch the ribs. But this would be maximally strong not because the webbing is in contact with the ribs, but because the area of the webbing was maximised! In fact there would be no detectable difference in strength between a wing where the webs did contact the ribs and one where the webs were 99.9% of the bay width, but just fell short of actual contact.

If fact it could be the case that contact is sub-optimum because it would introduce another deformation mechanism that is not necessarily desirable - ie the bending wing would start to "compress" the webs between the ribs. While balsa (like most woods) is very strong in compression, its even stronger in "edge-on" (or in-plane) pure bending. To prove that to yourself take a small piece of balsa sheet - you can crush it comparatively easily, but you will not bend it "in-plane"! In well installed webbing the joint with the spar would probably fail before the wood in bending "in plane".

BEB

Danny Fenton27/01/2014 19:35:20
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Thanks BEB well that clears that up! However BEB makes no mention of the power of OCD this makes the webs fit perfectly. I CANT make gaps
Cheers
Danny
Tony Bennett27/01/2014 19:42:51
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OCD has its good points then, cos you produce some excellent models.

Danny Fenton27/01/2014 23:44:29
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Thanks Tony, you don't do half bad yourself thumbs up

"lashed together the centre section. Used a different approach again, this time I pinned the spars and ribs straight onto the board, then the 1/4 leading edge, webbing then sheeting. Finally lifting the assembly from the board and sheeting the leading edge underside section.

bs 49.jpg

bs 50.jpg

bs 51.jpg

It suddenly dawned on me that there was no provision for wing attachment. So a quick scratch of the noggin and look at the plan, a couple of dowels set into the leading edge, and some blocks in the trailing edge should sort that. Good job I didn't sheet the undersides yet crook

Next up is to notch the ribs adjacent to the webbing and epoxy the dihedral braces into place.

Cheers

Danny

Nigel Day28/01/2014 08:49:58
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This is a bit like watching golf on the telly. On the rare occasion a pro shanks it, it comes as a shock. smiley

Danny Fenton28/01/2014 09:31:17
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Thanks Nigel, in my enthusiasm to get the wings joined I have overlooked the servo! doh! Another affect of modelling at the end of a day crook Anyway I wanted to have the top sheeting on to keep some rigidty in the centre section during wing joining, so its not really wrong. Just don't be surprised to see a hole appear in the centre section upper sheeting through which to mount the aileron servo. Hopefully more later. teeth 2

Cheers

Danny

Tony Bennett28/01/2014 09:43:25
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5079 forum posts
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Ha. that is always my mistake, the blooming aileron servo's, i am forever forgetting the wiring runs for them.

glad to find out that you are human Danny. wink

kc28/01/2014 11:14:06
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169 photos
Danny, how did you ensure the centre section fits both panels?
The last model I built with a separate centre section ( a Vic Smeed design) I built to the plan but found a slight angular error which meant the joiner didnt touch the spars in both the panel by a little. I scrapped the centre section and built a new one on from the wing panel. I didnt glue it on at that stage but just dry fitted the joiners.

Has anyone else noticed that this Boddington style of construction and even wing sections follows the Vic Smeed designs of a little earlier?
Danny Fenton28/01/2014 11:29:04
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Hi KC when i say I was in a hurry to join the wings that doesn't mean they are joined wink 2 Just trying to get in a position to do the deed. Wont be until I get back to the bench, two club nights will stop that happening yet.

bs 52.jpg

Here you can see that everything lines up well at the moment. Still have to cut the slots and see if the dihedral braces line up both in angle and touching the spars as you say.

Cheers

Danny

Bob Cotsford28/01/2014 11:46:36
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Posted by Danny Fenton on 27/01/2014 19:35:20:
Thanks BEB well that clears that up! However BEB makes no mention of the power of OCD this makes the webs fit perfectly. I CANT make gaps
Cheers
Danny

I'm far from ocd on most things but regardless of the theory I can't help feeling that webs SHOULD fit snugly, something to do with weakest links and stress points I think.

Was the original bolt on wings or rubber bands? I've converted the Mustfire to bolts, now I can't help thinking I should have beefed up the wing area doublers to suit a rigid wing mount.

 

ps - on the string triangulated bracing, isn't that the way WW1 era fuselages were built, with straight wooden cross pieces and diagonal wire bracing?

Edited By Bob Cotsford on 28/01/2014 11:48:27

Danny Fenton28/01/2014 12:18:53
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Hi Bob, I am with you despite BEB's very clear explanation wink 2 On the original Bistormer wing it is retained by bands, as is the undercarriage.

You are right about the older aircraft and not just WWI, the Hurricane has a tubular frame fus with diagonal cables for strength. The thing that really leapt out of Alisdair's article was how strong any frame was when it was covered in tissue and dope (acting like the tension wires I guess?) Film added some strength but not a lot in comparison.

I am still in two minds re building it as a tribute and keep everything as it was intended, ie banded wings and U/C or alter things slightly.

Any thoughts?

Cheers

Danny

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