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Martyn's Ballerina @ 115%

It's called Ballerina 115 - sounds more impressive than Ballerina 70

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Martyn K12/01/2016 20:46:01
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5001 forum posts
3658 photos

Been a few days since the last post. Unfortunately I have been struck down by the lurgy - so bad I didn't even get to the shed on Sunday.

Anyway - a bit of progress.

I was working on the tailplane at the last post. Brain not being fully engaged, I missed a couple of important photos in the next sequence - read the fin posting for the missing bit.. blush

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The junction between the fuselage and tail plane is delimited by thick strips of 3/16" soft balsa - allowing plenty of meat to secure the covering when the time is right.

At this point - there are a couple of missing images - the separation and hinging of the elevators.

Basically, each half of the tail plane is sanded to section and then joined as a pair with pins locating the join line.Before the elevators separated, I added a piano wire brace linking the two halves and glued with slow setting epoxy. Joining the elevators before they are separated like this ensures that they remain in perfect relationship with each other. The wire brace has been added to one half only.

When dry, separate the elevators from the tail plane -

Sand a bevel - for some time now, I have kept the tail plane side square and only bevelled the elevator. If you only need 20 degrees of elevator movement, then you only need to sand a 20 degree bevel..

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Now - apply the hinge material (see later) and glue together

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When dry separate and then complete the sanding of the tail plane - using finer sandpaper - on a block

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I have decided to laminate the fin from 2 sheets of 1/8" balsa and have a built up rudder from 2 x 1/16" sheet laminated core with 3/8 x 1/16 strip ribs. The rudder was scanned and scaled, the fin measured from the plan and scaled manually. Quite easy with straight edges.

Again a couple of photos missing (sorry). Space the 'ribs to suit but use a 'rib' along the bottom of the rudder to add a little toughness. It is important that there are sufficient ribs to ensure that the rudder remains warp resistant, I used 50mm spacing.

Sand the bevel - in this case 35 degrees each way.

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Now that the rudder halves are completed and the fin halves have been cut to size, I will outline my method of hinging. I have to admit that it has certain advantages and one big disadvantage - the flying surfaces are assembled before the model is covered. Not to everyone's taste.

For a few years, I have been using Kevlar cloth as the hinge material. It its very strong, very light, provides a full air seal and also adds strength to the structure. The cloth is actually sail cloth - as seen on ocean racing yachts.

With all 4 components completed - but not final sanded, the cloth is cut to about (ideally) 15-20mm wide - I have used this successfully at only 10mm wide strips.

I use Cyano to attach the cloth then PVA to glue the wood parts together. The cloth glues very quickly with Cyano and can be reinforced using capillary flow by simply running glue down the seam if required.

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Join the fin halves first and then using a couple of map pins as spacers, attach the rudder halves

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The extra piece of cloth at the bottom will be used to hinge the rudder into the fuselage.

Weigh it all down while the glue dries

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And then sand it all to shape

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and the hinge - free moving with the correct amount of travel

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Completed fin and tail plane - fuselage next

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Hopefully my next posting may be a bit more coherent..

More to come

koff koff sniff sniff

Martyn

john stones 112/01/2016 21:31:02
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10759 forum posts
1481 photos

Look good with those ribs, under the covering yes

John

AndyD12/01/2016 22:51:13
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712 forum posts
503 photos

might try your hingeing on my next model,and must remember to to add elevator joiner before seperating them,nice collection of batterys,ideal weights.

AVC12/01/2016 23:00:04
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539 forum posts
147 photos
Martyn, I like your hinging process, very neat 👍
Also those tail surfaces look very good
Biggles' Elder Brother - Moderator12/01/2016 23:46:27
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15748 forum posts
1460 photos

Interesing hinge material, I bet it is very strong!

BEB

Martyn K13/01/2016 08:32:16
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5001 forum posts
3658 photos

Hi BEB

Its very strong - you will never tear it. Ever. The other big plus is that the contact area for gluing is huge, providing the parts are held together properly while the glue dries it will never come adrift either. I have only ever had one partial failure - I tried to use a cheap poundland cyano on the hinge once - it appeared to dry OK but did fail - slightly.

The Cyano seems to attack the "varnish" on the cloth - I have had a play with this to see just why it works so well, There is definitely a change in structure.

The problem is - of course - covering the components after they have been assembled - that is more tricky.

The batteries came from an old computer UPS (Uninterruptable Power Supply). Wheelchair batteries are the next size up. I have just found 2 of those. They are very handy in the workshop. Make a good doorstop in Summer as well

Martyn

Martyn K13/01/2016 08:38:01
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5001 forum posts
3658 photos

Just spotted one thing I forget to mention. You can see the spar doublers on the tail plane. I am very nervous about tail planes folding after a couple of incidents in the past few years so I have added spar doublers. The difference in length top and bottom is different, the longer side extending 1/2 bay out further each side. The end of each doubler has a taper cut with a razor saw

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It doesn't have to be a long taper, just long enough to remove an obvious stress discontinuity - i.e. the load should flow smoothly down the structure.

M

Peter Miller13/01/2016 08:45:08
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10321 forum posts
1231 photos
10 articles

AH! Someone else who knows about the danger of sudden changes in section

Martyn K13/01/2016 09:15:48
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5001 forum posts
3658 photos

I learnt from bitter experience flying contest A/2 free flight gliders. They always break at the end of the wing joiner.

M

McG 696913/01/2016 18:37:49
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2714 forum posts
1017 photos

Hi Martyn,

Even if the load gets 'split' by different lengths top & bottom, may I ask you how 'long' you are making your taper?

Cheers

Chris

BRU - BE / CTR Spar Control

Peter Miller13/01/2016 18:45:32
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10321 forum posts
1231 photos
10 articles

Speaking personally I like to taper 1/4" sq as near as possible one inch

McG 696913/01/2016 19:23:21
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2714 forum posts
1017 photos

Thanks, Peter

By the way, did you visit the Bella Ballerina build recently? wink

Cheers

Chris

BRU - BE / CTR Taper Control

Martyn K13/01/2016 20:28:50
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5001 forum posts
3658 photos

Hi Chris

I normally taper a 30 degree angle - a 1:2 taper - irrespective of the width of the strip (or sheet for that matter) - its easy to use a school 30:60 rectangle as a guide. If the angle is much sharper then the benefit of the taper is lost - much shallower and it becomes more difficult. Peter uses a 1:4 taper - about 15 degrees - which is probably better but I find more difficult to get a neat or consistent joint

Martyn

AVC13/01/2016 20:43:30
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539 forum posts
147 photos
I normally use 30 degrees using the same tool that you, but sometimes I've used 45 degrees for spars junctions. Is this not enough acute in your opini?n?
Martyn K13/01/2016 21:08:12
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5001 forum posts
3658 photos

Do you mean splice joints? A lot depends on where the taper is. Sometimes, if the junction is at the unloaded end of a spar or longeron, its not really that critical. The splice angle - really the transition from one section of material to another becomes more significant and important as the load bearing demands increase. Back to my Free Flight days, I used to use the 30 degree angle regularly at the centre spar doublers - reducing an i-beam spar from 1" wide to 1/2" about 6" out from the very heavy load bearing centre section to fuselage joint. Never had a failure there - but of course, it could have been well over engineered and inappropriate for the actual load demanded,

I think we used the TLAR acronym earlier - I hate to say it - but assuming a parallel chord wing, I think that the load down a spar diminishes like a cosine curve (if you can visualise a half span in 90 segments). I know there is linear function that can describe this more accurately - BEB is far better to qualified to advise. Don't forget that these are only the flying loads - landing and (err) incident loads can turn the demand totally upside down.

For info, there are also 2 design schools (or there used to be when I worked on aeroplanes). One thought was that structures should be rigid and will have very little flex - we use to call this the British way - and was typified by aircraft such as HS Tridents and BAC1-11 which demonstrated very little flex either on the ground or in the air (we used to test this by springing up and down on the wing tips - but done tell anyone - apprentices eh). The other school of thought was that structures can be flexible and (if designed correctly) the loads can be transmitted across a flexible airframe. We used to watch aircraft from Boeing and DC take off and we could see the dihedral increase as the airspeed increased as the wings gradually took on increased flying loads before they took off. I have used both techniques on my models - the most flexible model I have is my Aquila glider - that wing really bends whereas my F3A models are very rigid. One thing you need to be careful of though is that torsional rigidity is paramount whether building a flexible load bearing surface or a rigid load bearing surface

Now That Paragraph May Cause Some Controversy..

 

Martyn

Edited By Martyn K on 13/01/2016 21:10:09

AVC13/01/2016 21:18:05
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539 forum posts
147 photos
Thanks Martyn. Yes I asked about junction (sorry, I did't clarify).
And thanks for the lesson on aeronautical trends, very intetesting 👍
AndyD13/01/2016 23:05:27
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712 forum posts
503 photos

I learnt most of my building from Gordon Whiteheads book rc scale aircraft so i use a Taper of about 3:1 which is about what G W uses from memory,a must book in my mind,my 1st book was David Boddingtons radio control primer what got me started,Hes got a lot to answer to.someone i would of loved to meet.

Martyn K15/01/2016 21:50:45
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5001 forum posts
3658 photos

Still koffing and spluttering a bit so progress still a bit slow, but there has been some.

Now working on the fuselage. I have already cut out the formers while doing the prep, I now had to mark and cut out the fuselage sides.

I haven't enlarged the plan for this, there are only a couple of curves to worry about, but first I had to scale the plan.

I cut the rear part of the fuselage on the plan and joined it to the longer front section and then started measuring.

My scale factor is 1.15 and so:

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Using a calculator and measuring the position of the front of each former from the very front of F1 then multiply by a factor of 1.15 then write the revised amount on the plan. Always measure from F1 rather then the difference between subsequent formers - F2 to F3, F3 to F4 etc as any error will be propagated down the fuselage.

I have used 4mm balsa sheet for the fuselage sides. Conveniently (and expensively), the most suitable wood came in 4ft lengths so I didnt have to do any splice joints to increase the length, however, I did have to increase the width just in front of F2 by about 10mm

True up a corner and then measuring from the nominated front edge mark the front edge of each former down the fuselage sides - both sides - and then check they are both correct - or at least look correct!

Use a square against one edge to mark the positions of the formers.

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When happy, measure the width from the plan, again multiply by 1.15 and you can complete the basic outline.

There is a curve linking F1 and F2 under the fuel tank. I have made mine a few mm deeper and will cut it back when the engine is mounted (inverted). I may have to alter the cowl line slightly.

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The wing cut outs were drawn by using the wing template - carefully positioned

For the first time I have used Evo Stick Makes You High Contact Adhesive - note the 1/32 ply doublers lurking and waiting to be joined up. Learning from other peoples blogs here.. Also note that each half and doublers are clearly marked left and right on the inner edge.. Its so easy to get this bit wrong.

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Another little tip is that I will be covering up the positions of the formers when I put the doublers in. Just extend the lines onto the edge of the balsa - you can then draw them back in again (using a square) after the doublers are in place.

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The rear ply doublers have also been added - I used PVA here (I had just cleaned up after using the contact adhesive..

One thing I had forgotten to do was to cut the hole in F1 for the fuel pipes. The hole only needs to be big enough for 3 tubes - any bigger and fuel and crud will find its way in.

So, I had to cut a hole. Ply is difficult to cut nice holes - the wood tears as the drill exit the other side.

But, if you partially drill through from one side

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Then flip the former over

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and drill through from the other side - using the pilot hole you have just made

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You end up with a lovely clean hole and no tears..

Now the formers are complete, we can glue them into place. The fuselage is parallel back to F7 (I think) so we can assemble the front formers on the board using a square to keep the em vertical

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The second use for UPS batteries. They make wonderful former steadies. Each former is glued then clamped to a battery and placed into position there is enough mass in the battery to hold the former very steady while the glue dries - in fact I also added the triangle section while the glue was drying.

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Finally tonight, the 1/4" square (mine is cut slightly deeper and will be sanded back) spline has been added along with F3 and all allowed to dry.

More to come

Martyn

PS - Just spotted - I haven't drilled the holes for the engine mount in F1 - doh...

AndyD15/01/2016 22:07:43
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712 forum posts
503 photos

love the battery former steadies great idea.

Colin Leighfield15/01/2016 22:18:19
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5965 forum posts
2494 photos

Great build Martyn and thanks for going to the trouble to describe it so well. I liked your comments about the different approaches to airframe stiffness. As I've read previously, Boeing used the idea of a flexible wing with dynamic loads offset by the underslung engines originally on the B47 and it's always been obvious on the B52. When you see a 777 or 787 take off the upward curve on the wing is remarkable! I remember reading Winkle Brown's description about the extreme flexibility of the Windsor bomber geodetic airframe, it sailed on serenely in rough air with the wings flapping up and down about 6 feet and the whole structure moving in sympathy. Automatic gust response?! Mitchell designed some of these principles into the 317, with a very light wing structure carrying the bomb load, which provided a downward force to offset the upward aerodynamic loads. You must have had an unforgettable experience during your early days, I assume that you worked in Hatfield? What a disaster that it isn't there any more.

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