Martyn's Ballerina @ 115%

[Peter Miller]

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

[McG 6969]

Thanks, Peter

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

Cheers

Chris

BRU - BE / CTR Taper Control

[Martyn K]

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

[AVC]

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 K]

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

[AVC]

Thanks Martyn. Yes I asked about junction (sorry, I did't clarify).
And thanks for the lesson on aeronautical trends, very intetesting 👍

[AndyD]

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 K]

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:

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.

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.

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.

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.

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

Then flip the former over

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

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

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.

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...

[AndyD]

love the battery former steadies great idea.

[Colin Leighfield]

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.

[john stones 1 - Moderator]

Using Evostick contact, will make you forget your own name Martyn

John

[Martyn K]

Posted by Colin Leighfield on 15/01/2016 22:18:19:

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.

Thanks Colin

I had a fantastic apprenticeship - courtesy of BEA > British Airways,  Based initially at the Engineering Training School at Hatton Cross then over into the engineering workshops and Hangers at the end of 28L - Heathrow. Watched the first take off of Concorde from the car park. That was another unforgettable moment as the exhaust blast tried to knock us off our feet.

 

Show me an apprenticeship that (in one course) cover machining, welding brazing and heat treatment, servicing gyroscopes and radar and electrical wiring and avionics in theory and practice, aerodynamics, stress analysis, principles and practice and maintenance of gas turbine engines, hydraulics, airframe maintenance and repair. Eventually finished on airborne computer systems.

I worked on Viscount, Vanguard, BAC1-11 (std and stretched), Tridents 1,2,3 and Tristar, (And Jackie O's Lear Jet).

It was an amazing apprenticeship - famous for being the worlds best.

 

Martyn (the fumes have worn off)

 

Edited By Martyn K on 15/01/2016 22:57:47

[Biggles' Elder Brother - Moderator]

Nice detailled fuselage build Martyn, well done for taking the trouble. Its important that (if they feel they can) the more experienced buiders do go into some detail - its helps those less exerienced and frankly we can all pick up the odd tip even if you've been doing this for years! And its true that things that seem obvious to one person can come as a complete revelation to another!

BEB

[Andrew Price 2]

I don't do envy Martyn, but, if I did........

I well remember going to Hatfield when they had an open day. A fantastic way for a young aviation mad boy to spend a day. Even bits of Comet looked really good. Later I worked at Oldings Corner which was right under the final approach. Deep joy.

[Chris Barlow]

4' lengths are so expensive compared to 3'

115% looks a handy size though. I was quite surprised just how chunky the fuselage is once built up with quite a deep section after the cockpit. Should knife edge well!

Have you still used 1/32 ply for the doublers or did you increase it?

[Peter Miller]

Posted by Andrew Price 2 on 16/01/2016 13:13:25:

I don't do envy Martyn, but, if I did........

I well remember going to Hatfield when they had an open day. A fantastic way for a young aviation mad boy to spend a day. Even bits of Comet looked really good. Later I worked at Oldings Corner which was right under the final approach. Deep joy.

When I left the RAF I worked at Hatfield on permanent nights for about 6 weeks, four nights a week for ten hours. Gave that up. to wrk in local factories. Well, I couldn't build models!!!

[Andrew Price 2]

My Dad was at Pages, Radlet. He put his hand up when they were desperate for people to work nights "for about 4 wks". He had to get quite strident to go back to days - after about 10yrs!!

[Martyn K]

Part 1

Nice to reminisce occasionally, one day, I may explain why I left. Those circumstances were not so good.

Back onto the build.

Chris - yes - I have used 1/32" ply doublers.

I now have a fuselage side with a subset (parallel section of fuselage) - with all the formers vertical. Time to flip it over and glue the other side.

One thing that I maybe didn't make clear was the importance of getting the fuselage halves identical. This means that the wood should be of a similar density for both sides so that it bends evenly. You need straight grained medium density wood which has a certain springiness when you whip it. Hard to describe this, you need to avoid the very 'anaemic' white short grained stuff that crumbles when you break it. You also need to be sure that both halves are identical in shape - including any err induced errors. This will be obvious in a moment.

When the fuselage halves are joined, we need to make sure that the halves aren't twisted in alignment.

You need to use an engineers square and move around the fuselage checking that both halves are parallel to each other. A barely noticeable 1mm error at the front will magnify to a huge 4mm error at the rear.

Simply place your square on your flat (glue blob free) board and check that the the sheets line up like this.

Move all the way round and check and double check then allow the glue to dry thoroughly.Note I am using my batteries as weights now ensuring good compression on the joints. Add the triangle section reinforcement.

When happy, we can move the fuselage into the jog, confident that when we pull the the tail in, we wont be inducing a twist into the fuselage..

Like most here, I use the SLEC jig. It works well although the rights angles could be more rigid.

We need to align the fuselage down the centre. Remember when I cut the parts, I suggested that you should also draw the centre lines.

This is why.You can see clearly that the fuselage needs a nudge to the right.

Use the square to extend the lines up to formers that are too short to reach the mat..

The fuselage end is pulled in and held using a bulldog clip. Use the square on the inside (between the fuselage halves) to make sure it is centred correctly.

As I mentioned, the SLEC squares could be stiffer. It can help to put a lightly load elastic band to help hold the top sides in.

 

Edited By Martyn K on 17/01/2016 21:46:35

[Martyn K]

Part  2

With all the formers in place, leave it alone until the glue dries. I haven't glued the rear of the fuselage together yet, I need to keep it separate to add the lower rudder hinge.

You can now start adding the stringers. Its very tempting to try and do both sides without turning the fuselage round, but I can guarantee that unless you can clearly see what you are glueing you are likely to end up with some funny shapes. So, working from the top - that centre line comes in handy again.. Elastic bands are used to add a little tension.

The spacers on my enlarged version turn out to be 12mm wide - I cut a dozen or so pieces 40mm long

They need a slight bevel sanding on each edge reducing in angle as you move down towards the fuselage side.

Because i have used thicker fuselage sides than shown on the plan - but the same sized stringers, my lowest stringer is cut to 4mm square hard balsa. It needed a slightly smaller filler piece. It looks OK.

While the stringers were drying, I sheeted the front upper cowling above the fuel tank. Again, 4mm medium balsa (same as the fuselage sides)- which didn't want t bend fitted in 2 pieces - one each half.Spray water on the outside edge and lots of clamps to hold it in place while the glue dries.

And while this was happening, the model and jig was turned around and the other side stringers were fitted.

When the glue on the stringers dried, the scallops were sanded back using a piece of sand paper around a suitable sized piece of tube. Quite easy and also quite pleased with that.

Back on the front cowling. With the first half dry, the second half can be added. There is no space for clamps now so a mixture of well tensioned elastic bands and pins to hold the dampened second piece into place.

No magic, just careful marking, cutting slightly oversize and sanding back to fit.

With wood construction of the front end mainly completed, the fuselage was fuel proofed and the inverted engine mount fitted. I wanted to add a couple of degrees of right thrust (I don't think any is shown on the plan), so the mount if offset to the left by 3mm and then packed with a piece of 16g aluminium (under the left side of the mount to deflect the thrust line to the right). If I have got this right, the prop driver centre line will be down the centre line of the model. Its not massively critical if this isn't spot on, but it looks better.

Captive star nuts on the inside of F1

With the front decking and stringers done, the lower rear fuselage sheeting can be done. The grain runs across the fuselage and the wood is cut oversize, the glue allowed to dry and then trimmed back later. Leave the last part (at the end) unsheeted until the snakes or push-rods have been fitted.

The weights make sure we have a good strong joint.

 

Edited By Martyn K on 17/01/2016 21:46:56

[Martyn K]

Part 3

​We can now get the tail plane mounted. The fuselage datum is along the upper edge of the fuselage and the tail plane needs to be mounted with 0 degrees incidence relative to this. Those using flat plate tail planes don't need to worry about this as it is very easy to see that the tail plane is level with the top of the fus. Those who have built up symmetrical tail planes need to check that the tail plane is indeed 0 degrees before it is glued.

I had already guessimated the shape of the half section for the lower edge and cut/sanded this to approximate shape

I need to check its accurate. The only reference that I can use is the cockpit floor that is inlaid between the fuselage halves at the rest of the fuselage has now been hidden by structure.

So using my mobile phone with the free 'Bubble Level' app..

Zero on the cockpit floor

Then repeat the measurement across the high point - my spar - and sand the tail plane seat until the error is acceptable (or better - no error)

At the moment, I have 0.5 degrees tilt and -0.2 degrees incidence. Pretty close, but not perfect

Finally, before the tail plane is glued, we need to triangulate it. Following BEBs method (described wonderfully a couple of years ago in the Tucano mass build if I remember correctly), place a pin in the centre line at the nose:

Loop a piece of line around the pin body

and with pins pushed into the tail plane at the same point at each half, check that the length from centre to each tail plane tip is identical.

Tack glue the tail plane down and let it dry. Double check your measurements and the end liberally glue the tail plane into place.

Not bad for about 12 hours work...

 

More to come

 

Martyn

Edited By Martyn K on 17/01/2016 21:47:15

[john stones 1 - Moderator]

Very nice and well described, i'll tune in for part 4

John

[Martyn K]

Thanks John

Its only in 3 parts because I overloaded the MF data input field - be a factor of 3! Hopefully only 1 part entries from now on

M

[AndyD]

yea lots of detail and description,the newer builders will love this,older ones as well,keep up the good work.

[Devcon1]

Nice work Martyn

Great method for doing those internal scallops and I like the mobile tip.

Looking forward to the end product.

John

[Peter Miller]

Impressive build there. Decidedly better than my "Throw it together and a quick sight along it!!"