Tiger 72.

[Jim Newberry - Nuviation Ltd.]

With SWMBO and daughter away visiting the outlaws, I have been able to spend pretty much the entire evening in the workshop, pausing only to reheat the remnants of a chicken vindaloo and take the dog out for his evening constitutional. Consequently, a pleasing amount of progress has been made on the wing, with ribs, spars, upper sheeting and capping strips for the left hand panel all now drying on the building board. Once again, I piled up the extraneous bits of ply that fell out of the lightening holes and popped them on the kitchen scales. The total weight saved from all ribs now stands at 80g. Not exactly a tonne of weight saved (to put it into perspective, it's about the same as 3 AA cells) but I'd say it was worth doing.

I did have a minor revelation as regards the plan though - as it stands, the printed plan shows a left wing and a right wing but of course the model has a completely symmetrical aerofoil section so it makes no difference whether the wing is built upside down or the right way up; consequently there is no need to print both panels as building one panel upside down and the other right side up over the same drawing has the desired effect. Since this frees up three square feet of paper (or thereabouts) it should allow me to reduce the page count from three to two which, considering the "pages" are A0 size, represents a significant reduction in printing costs. Obviously, a certain degree of care needs to be taken to ensure that the servo trays are built into the correct side of each panel, and that the inner rib is set at the correct dihedral angle but these are minor considerations.

[onetenor]

So how does Cyparis compare to Obeche ?.The original balsa substitute :

[onetenor]

BTW those stringers seem enormous. Do they need to be that big :

[Jim Newberry - Nuviation Ltd.]

Obeche and cyparis are very similar - I'm sure obeche would work just as well where I have used cyparis. I suppose the stringers could be a bit slimmer, maybe 3/16" instead of the 1/4" I have used, or even 1/4" x 1/8" set edge on.

[Masher]

The stringers are 3/16 on the standard size plan so 1/4 seems more appropriate for the enlarged.

Coming along nicely Jim, I'm just completing the standard size model and hoping to build the bigger one next. It would be interesting to put the Tigre and Ballerina side by side, lots of similarities.

[Former Member]

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[Jim Newberry - Nuviation Ltd.]

I have a few days off work so hope to make some more progress this week, today I have been continuing work on the wing and here is the result.

I changed the design somewhat here - the original version had the outline made up from a number of laminations of 1/16" balsa shaped around a balsa core. It worked well but I felt that I could simplify the design a bit - especially as I was having the parts laser cut. Had I been cutting them by hand I wouldn't have bothered with the lightening holes. The space at the front will be filled with solid balsa before being shaped to match the curve of the wing skins.

If I build the wing again I will change the arrangement of the lightening holes in the outermost rib so that they provide more surface area for the wing tip to glue to - in other words, where you can see in the picture that the ply ribs of the wingtip pass over the lightening holes in the wing rib, there will be wood in contact with the inner faces of the wingtip. It will be quite strong enough as it is but it would be tidier and neater with the modified rib.

Edited By Jim Newberry on 25/01/2016 17:49:57

[Former Member]

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[Jim Newberry - Nuviation Ltd.]

Back in the workshop last night and again this morning, and the fin support blocks have been added and shaped.

First of all, I made this spacer piece from two scraps of 3/8" balsa to hold the blocks in the correct alignment while the glue dried.

The fin blocks have now been shaped, leaving the spacer piece in position to lend them some support as at this stage the blocks are only fixed to the model by gluing the forward edges to the rear face of the last former. The spacer can now be slid out of the way to make way for the fin and tailplane.

Meanwhile, work has continued on the right wing.

This is being built upside down over the same drawing as the left wing. The rear of the ribs is being supported off the building board by a length of thick dowel. The charring around the edges of the ribs, caused by the laser cutting process, has led to them taking on a grubby appearance but this will be invisible beneath the covering.

[Jim Newberry - Nuviation Ltd.]

Another couple of hours spent in the workshop this afternoon and I now have all ribs, three out of four spars and all the shear webs added.

Here you can see the 1/4" aileron bay facing. I used a piece of wood thick enough to extend down to the building board - obviously, this is going to mean a fair amount of wasted wood as approximately 3/4" will need to be planed off to bring it level with the tops of the ribs when the wing is removed from the board and turned over, but it does help to keep everything square and true while work progresses.

For the same reason, the false leading edge is also made from a piece of 2" x 1/8" balsa which extends down to the building board. Note the grain direction on the shear webs which should be vertical to take full advantage of the natural strength of the balsa.

Annoyingly, I found that one of my 1/2" x 1/4" cyparis spars was warped along its length and, as it was my last length of cyparis, I had to cut out the bent section and create a scarf joint with a new piece to make a straight length. The joint appears well out towards the tip on the upper surface, where stresses are smallest, and is secured with a pair of small dowel pins (barely visible in the picture above) so I am pretty confident that there is no loss of strength.

[Former Member]

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[Jim Newberry - Nuviation Ltd.]

Thanks Percy, glad you noticed the thickness of the ribs as it raises a point about which I have given a great deal of thought so I am glad of the opportunity to discuss it. I look at an awful lot of plans and think that the designers tend to over specify the rib thickness in the belief that they are adding strength to the wing. Actually, I think that thickening the ribs adds weight but not strength - or at least, no strength in an area where it is required. The spars and the shear webs, between them, resist the urge of the wing to fold vertically under flight loads (i.e. "clap hands" above the model) and also to snap downwards after a heavy landing. Torsional loads are resisted by the balsa skin on the D box and the covering itself. Fore and aft loads are resisted by the relationship between the main and sub spars which are locked in position by the ribs, reinforced by the D box skin. Since there are 12 ribs on each side, each individual rib assumes only a very small proportion of this load; so the primary job of each rib is to maintain the aerofoil section at its own particular position. Above the wing there is suction, below the wing there is pressure; so the total force acting on a rib - and more importantly, trying to distort the rib - is minimal. Of course, thin ribs tend to flex more than thicker ribs and so construction requires a certain amount of care to avoid a curved rib, but when I build models I work on the basis that I build them once but fly them repeatedly so a bit of care in the build stage is paid off many times over at the flying field.

The ribs are all 2mm lite ply.

[Jim Newberry - Nuviation Ltd.]

Close of play for the day and this is where we are.

D box skin and servo cover fitted, I need to fit the capping strips tomorrow before turning the wing over and adding the upper sub spar, D box skin, leading edge and wing tip.

Time to think about colour schemes!

[Former Member]

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[Jim Newberry - Nuviation Ltd.]

I may yet be wrong! It all looks very convincing on paper but this thing has yet to fly!

[Former Member]

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[Jim Newberry - Nuviation Ltd.]

Every now and then, at work, I have what I call a "niff naff and trivia day" whereby lots of little jobs get done without any significant progress being made. Today has been a niff naff and trivia day in the shed with lots of sanding and shaping taking place, hinge blocks being glued into place and general fettling going on. It doesn't really feel as though I've made much progress towards having a completed model, but all these things need doing.

First up, here are a pair of wingtips, one sanded and shaped, one not quite there.

Next, reinforcement blocks have been added to the aileron bay and the aileron itself, ready to accept the Robart hinges I'll be using. I had planned on using fluffy mylar hinges - and I think they would have worked - but the sundries drawer yielded a dearth of mylar and a surfeit of Robarts, so Robarts it shall be.

[Former Member]

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[Jim Newberry - Nuviation Ltd.]

Thanks Percy, it's nice to be at the stage where all major components are complete and ready to be assembled to make a model! All being well, I hope to have a complete model ready for a bare bones picture by the weekend. I've done some more bits of shaping today and tomorrow is "Wing Joining Day" which I always view as a major milestone in a model build.

[Jim Newberry - Nuviation Ltd.]

Wing Joining Day. For me, this is the day when the model starts to come together. Up until now it has been a collection of sub assemblies which I know will eventually form a whole model but which require an effort of imagination to picture them actually coalescing into a flying machine. Wing Joining Day is the tipping point where, suddenly, I have a model aeroplane that just needs assembling, not building.

I was very pleased to find that the dihedral braces slid into position without any need to cut slots in the wing skins. A light coating of aliphatic resin and a bit of jigging had everything set in position while I took the dog for his morning walk. One thing I have found since building the new workshop is that the combination of heating and dehumidifying leads to dramatically improved setting times for aliphatic and similar glues - in my old draughty single garage, a wood joint made with aliphatic would take many hours to harden but not any more! Also, I found that my questionable building skills led to some work being required to achieve a good fit between the three wing sections. In some areas I had to trim a bit from the joint, in others I had to add a sliver of balsa to fill a gap but we got there in the end! If you look closely at the pictures below you will be able to see where I have added material.

It is worth mentioning that I have reduced the dihedral compared to the original design. Time may yet prove me wrong but I felt the original had a little too much. Anyway, here are the pictures of today's progress...

It is one of those jobs where you need to get all the bits together before you start. Then, you need to make sure you won't be disturbed halfway through! Once the wings have been joined with the dihedral braces in place, cut two lengths of fibreglass bandage to size, leaving approximately 1" overhang at the trailing edges. Lay the bandage in place, with approximately half the width on either side of the joint line (as you can see, I nudged the bandages outboard a little, to clear the holes in the upper skin where the aileron servo wires and wing bolts will pass), and clip the tails with clothes pegs. Use a soft pencil to mark the outer edges of the bandage on the balsa wing skin and then move the bandages out of the way. Mix up some slow setting epoxy (at least 30 minute epoxy but slower if you have any doubt about your ability to complete the next stage in time) and paint or smear it between the pencil lines. Now position the wing so that it is pointing vertically upwards - in other words, as if it were fitted to a model in a vertical climb. Lay the bandage over the two stripes of epoxy, smoothing it out as you work downwards, and refit the clothes pegs. The picture above shows us at this stage. You will see that, while some portions of the bandage have clearly soaked up the underlying epoxy, some have not.

The picture above shows us halfway through the next step. Once the first layer of epoxy has set, mix up another batch and paint it over the bandage, making sure that it is all thoroughly wetted through. It is easy to see when a particular section is properly covered as it will change in appearance from what you can see on the right to that shown on the left.

Both sides done and the wing has been returned to its vertical orientation to set overnight.

Once I cut the excess free at the trailing edge I will probably find the clothes pegs are permanently glued shut and will be thrown away, however (to take a tip from SWMBO's cooking), you can't make an omelette without burning down the kitchen.

[john stones 1 - Moderator]

Chuckle

No supper for you Jim lad.

John

[Jim Newberry - Nuviation Ltd.]

I'm safe - she doesn't read this!

[Former Member]

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[Jim Newberry - Nuviation Ltd.]

You're not wrong Percy - 80% done, 80% still to go! I didn't get around to putting it all together for a picture so that'll have to wait. Now I am at work for a few days and then it's half term next week (when I'm off work but won't be able to get into the workshop very much) so there will only be limited progress for the next ten days or so. Still, it gives me a chance to think about colour schemes.

[Jim Newberry - Nuviation Ltd.]

I got home from work this afternoon and quickly nipped out to the workshop to check on things now that the epoxy on the wing joint has cured. I have trimmed off the excess bandage and couldn't resist putting all the bits together just to check the fit of the wing and the fuselage. Happily, they slid together with a satisfyingly snug fit and the wing bolts passed straight through the corresponding cutouts in the centre section and into the blind nuts in the fuselage (the drawings all lined up in TurboCad but of course there's no substitute for actually putting it all together as there's always that nagging suspicion that a millimetre or so of inaccuracy makes all the difference!)

Anyway, here it all is.