PSS Zlin 526A

[Steve Houghton]

Steve - It would be nice to have it done for April, but I'm not renowned for being a fast builder. However, I am trying to move quickly to advance the build before the Skyhawk pack arrives. I just saw on the Skyhawk thread that delivery time is running at 10-14 days, so I may have a couple of week's grace yet.

Andy - I'll try to arrange a dry fit and photo soon.

[Steve Houghton]

Well, what do you know? No sooner had the ink dried on the previous post than the postie arrived with my A4 Skyhawk pack. Now, shall I resist opening it until 1st March??

So many dilemmas to resolve!!

P.S. I'm at home today with a bout of Lurgy which I don't want to pass on to my work colleagues. I might just squeeze in a bit of building to assist my recuperation.

[Phil Cooke]

The wing looks great Steve, nice to see a built up structure utilising foam and balsa composite. Some good skills there.

Make yourself a brew, or some lemon and honey if you are feeling rough, and spend an hour looking over the A-4 plan and the woodpack - no harm in that, is there?

[Steve Houghton]

Well Phil, I gave in and opened the Skyhawk pack while supping Lemsip (other Flu remedies are available!).

F2 is missing, as discussed on the Skyhawk thread, but the plan modifications have been made for the 5/16" TE size and the wing skin overlaps.

Meanwhile, on with the Zlin - I will finish before starting the Skyhawk, I will!!!!

Boxed in

The servo box has been fabricated from 3/32" and 1/8" balsa from the scrap box, with 1/4" ply servo bearers.

Centre section sheeting finished and a trusty HS225MG trial fitted. I shall be replacing the arm with a circular output disc, so that I can offset the pushrods to introduce some aileron differential. I'm a little concerned about the length of the wire pushrods and the scope for whipping, but they will be shorter than Jack Edwards' prototype that had the servo in front of the spar. I'll suck it and see.

Trailing Behind

I then spent the first part of this evening cutting and shaping the triangular TE stock from raw 3/8" sheet; and spent the second half hoovering the shavings off the carpet! Now need some daylight to finish sanding it to profile outside.

[Steve Houghton]

Torque Rods

We all have jobs we hate doing, and mine is aileron torque rods, with so much that can potentially go wrong. Hence, it's taken me a few days of careful planing, cutting and bending to get them to fit into the fixed TE section. I used 12g rods made by Flair, supplied by Blackburn Models. The plan calls for 3mm wire, but these are 2.4mm and I think they'll be perfectly adequate. I had to cut the vertical part down to 20mm to avoid fouling the wing bolt plate - I see this is also a likely problem with the Skyhawk.

Washed Out - A Question

The plan says to build the wing flat and then add 1/4" washout by planing the ailerons to shape, as per the plan below:

However, I have built 3/16" of the specified 1/4" washout into the wing by packing it up during the build. So, the big question is - Do I still need to plane some washout into the ailerons (which is sure to end in disaster), or can I get away with straight ailerons, which I have already prepared? Do any of you experienced builders have a view on this, please?

[Phil Cooke]

Do the ailerons run right out to the tip or is there a static section of tip outermost?? I think this dictates the answer...

[Steve Houghton]

Hi Phil,

There is a small static section as part of the 1/2" wingtip, as below:

However, the note on the plan in my previous post says "wingtip modified to new aileron shape".

My instinct is to shape and fit the wingtip first, but without any 'reshaping', to match the true tip airfoil section (already tilted up to include washout), and then trial fit a straight aileron and see if it fits. I can't get my head around what shape it really needs to be!

If that fails, I can keep the straight ailerons for use on the Skyhawk, by planing them down to 5/16". I can't see any reference on the Skyhawk plan to planing the aileron with a 'twist', even though that also has washout.

 

 

 

Edited By Steve Houghton on 22/02/2016 22:59:14

[Phil Cooke]

Yes, we didn't do anything clever with the ailerons on the A-4 - they are just straight, this will result in a little mismatch at the tip as they should have a little twist in them really. But not as much as you are requested to put in here.

That's another alternative - wet your straight strip ailerons out thoroughly in hot water or ammonia and pin them down to dry over a form so that you can build the twist in. Then dry fit them to the wing in neutral position and sand the tip to suit.

[Steve Houghton]

Thanks, Phil. I did wonder about trying the twist with ammonia, but it feels a little too imprecise. I shall probably re-carve some new ones with a built in twist.

[Steve Houghton]

Carve the Twist

To achieve the washout in the ailerons I decided to carve the twist into the aileron blanks. To do this I needed a reference point at the tips, so I fitted the wing tips first. The tips were laminated from 1/32" ply sandwiched between balsa blocks. I drew the tip airfoil onto the blanks and aligned them with the tip ribs, which already had the washout built in. Thus, the TE should be at exactly the right point with the washout.

 

The aileron blanks were cut to outline from 3/8" sheet and pinned into place with the bottom aligned with the bottom of the root TE. This is the left wing:

This is the tip of the left wing, clearly showing that I have managed to build in the washout, and the aileron needs a twist of approx 1/8" carving into the tip:

 

Thankfully, the right wing was almost identical in the amout of twist needed. Here's the root:

Here's the right tip, again needing 1/8" twist:

I was very pleased to finally 'see' the twist needed, and rapidly carved, sanded and hinged the alierons. It's taken three weeks, but for me that is rapid!

Stay tuned for full dry assembly photos soon!

 

 

Edited By Steve Houghton on 21/03/2016 21:15:06

[Steve Houghton]

Dry Bones!

I'm finally able to dry fit everything, complete with carved and hinged ailerons. Apologies for the wrinkled bed, as it's too dark to photgraph outside:

Starting to look like a Zlin:

Yes, definitely a Zlin - long nosed version.

Later Zlin 526 variants had the nose shortened so that the cowl was flush with the wing LE, but this one will be a 526ASM, special edition, which kept the long nose. Should be great for achieving the right CG.

Spinning Ahead

I haven't decided how to make the spinner yet. The plan calls for a cut down plastic spinner, but:

a) It's not the right shape - it needs a parallel section at the rear, behind the conical front.

b) It will have screw holes. I can't find one that doesn't.

I'm deliberating on the best solution. I may try a lost foam moulding, as a trial run for producing a full fuselage for my F4U Corsair plan in due course. Or, I could go the whole hog with a full male mould, which would take longer, but I don't want to delay the start of my A4 for too long or I'll never catch up!

 

 

Edited By Steve Houghton on 21/03/2016 21:39:43

[Andy Meade]

If you get stuck, you can send me a side profile, a diameter for the back, and an overall length, I will 3d print you one

Looking great so far though Steve!

[Steve Houghton]

Take a Seat

The immediately next job is to shape the wing seats to fit better. This is another job I dislike! The plan shows raw foam left for the wing seat, but I have an idea to sheet them with 1/32" balsa to add a little durability and to give the film covering something to stick to. Here's the left side:

.. and the right side:

I usually just get to work with a knife and sanding block to make the wing fit, but the exposed foam will require a more subtle approach. I'll explain when I've figured it out!

Mind the Gap!

Due to a combination of plan rib template inaccuracies, over zealous sanding of the rib ends, and poor workmanship, the wing has ended up slightly undersize. I chose to leave a small gap at both sides when I drilled for the wing bolt, otherwise the wing incidence would have been affected, or more seat cutting would have been needed.....

The front gap will need a small filler behind the ply facing that I want to fit (not shown on plan):

Not sure what to do with the rear yet, but I want an inclined face so that the wing can slide back when I make a less than perfect (i.e. 'normal' landing:

[Steve Houghton]

Andy, Thanks for the kind offer. That printer of yours is seeing some service!

For now, I'll persevere with one of the old fashioned methods. I could even carve it from balsa, but the ingenious designer fitted his noseweight (such as it is) inside the spinner, into the hollow, so it needs some internal expansion capability.

[Peter Garsden]

Steve, nice to see a lot of progress - looking forward to seeing it finished in April?

I would counsel against a lost foam nose because it is very difficult to take fibreglass round tight corners without bagging it under vacuum. I don't have that equipment and have tried it but it is difficult to stop the fibre springing away round the front of the nose. You would have to make a female mould which as you say would take ages.

I would go for balsa like you say out of 1/2 inch sheet with a hollow the same shape as the outside diameter. If you need lead you can then dip the spinner in sand and pour lead into the hollow which should fit neatly inside your hollow.

Or you could make a square shape inside the nose and put square pieces of lead in there glued down with epoxy.

You could make it out of blue foam but it won't take much hammer before breaking open and you losing the lead. That is unless you coated it with 25gram per sq metre fibreglass cloth and water based resin on top - that does go round tight bends because it is so light

Or 3D print it of course but that is beyond my experience.

There's a few ideas to put in your pipe and smoke?

[Phil Cooke]

Looks lovely that Steve, very nice job! Will make a great addition to the slope on those calmer days!

[Steve Houghton]

Peter,

Many thanks for your tips from the 'Master of Lost Foam'. You might have just saved me from some wasted efforts.

I am now wondering if I can use the inside of a commercial spinner as part of a female mould, with the screw holes plugged. Just thoughts at the moment.

Yes, the final solution may be plain and simple balsa, but I was hoping to continue my foray into modern building techniques by trying something different. I'll report in due course.

Phil,

Yes, those balmy Great Orme days when we wait for the wind to rise - I just hope this will turn out light enough to float on a whisper. The prototype loading was 10oz/sq ft. If I can bring it even lower with lighter modern radio, I shall be delighted. Must remember to fit a ballast tray for stronger blows!

[Steve Houghton]

Have a Seat

I decided that the bare foam wing seat would be too vulnerable to damage, so I Iined it with 1/32" balsa. I fastened the wing in place to hold it to shape while it dried. End result is satisfactory.

Now on to the wing fairings..

Edited By Steve Houghton on 08/04/2016 21:20:50

[Steve Houghton]

Fair Enough

To fill the previously mentioned gap at the front of the wing I added a 1/8" balsa plate to the LE of the wing root, faced with 1/32" ply:

The TE needed a reinforcing plate around the wing bolt, so I made it faily large to overlap the TE to cover the other gap:

This left a nice ledge on the top that just needed filling:

A small piece of hard balsa sheet to finish it off, angled so that the wing can slide backwards if knocked:

The end result, with some scrap block fairing added around the LE, was quite a good fit:

[Steve Houghton]

Bubble Trouble

Now for the job I've been fearing - my first serious attempt to make a heat moulded bubble canopy. I've read other threads about it (thanks Steve McLaren, Matt Jones, Phil Cooke), so I reckon I know all I need to know!!!!

Here we go, warts and all. There may be cries for help during this process!

I have started to make the male plug with a 1/16" ply egg crate frame to provide the basic profile and several cross sections:

The three-view I have only shows one cross section of the canopy, which is the rather square section at the rear. I am guessing that the front needs to merge into a more rounded shape as it approaches the fuselage top. Hence my three estimated sections shown below, looking from the front. It appears crooked because it's loose and slipped during the photo session:

Next job will be to fill the frame with balsa block, using a job lot of offcuts I picked up from Balsa Cabin during a recent show.

[Steve Houghton]

Blocking In

The ply frame was filled with various pieces of block balsa (from that Balsa Cabin bargain bundle). The middle was easy, as it sat on a flat base. The ends had to be laminated with sheets of thicknesses from 1/2" to 1/8", so that the bottom could be rounded to fit the fuselage curvature.

It has only been roughly shaped at this stage - the ends still need more curvature.

Hollow to Follow

Inside the plug, the ends are hollow to follow for the fuselage curvature - will be filled in later.

After final shaping of the canopy ends, the end hollows were filled in with more layers of sheet, and a 1/2" base was added, to allow some 'run-in' for the moulded plastic.

Some areas are rough, but will shortly be filled with filler:

Overall, I'm pleased with the shape so far:

 

Edited By Steve Houghton on 10/06/2016 01:01:49

[Steve Houghton]

Fill Her Up!

After careful shaping and honing, all that lovely work has now been covered in Isopon P38 car body filler (easy sand variety).

I have to confess to a major error. On the first attempt I didn't add enough hardener to the filler, and after 24 hours it was still soft. I had to scrape it all off and start again. The instructions call for a 2% ratio of hardener to filler, or a pea-size lump of hardener to a golf ball size lump of filler. I covered it in several mixes, each less than a golf ball, and obviously didn't use a large enough pea!

Next time I was generous with the hardener, but it started going off after just a couple of minutes, causing a few more peaks than I was hoping for. However, the final three mixes were just right, and I had 10 minutes of working time to smooth it a little better.

Now, I just need to find time to:

• Sand it down
• Pluck up the courage to mould the canopy
• Fit the radio
• Cover the model
• Start my A-4 build, as the deadline is looming, especially for my pace of building!

To achieve the latter, I'll have to put this to one side for a while, until the A-4 has progressed a bit.

[Steve McLaren]

Ah, good to see that you're still making progress Steve. This one could have been perfect for this weekend!. But i'm sure there will be plenty more light wind days.

Are you planning to do the canopy moulding yourself, or do you have a LMS who will do it for you?

[Steve Houghton]

Hi Steve,

I was hoping to have it finished for this weekend, but we've landed a large project at work (installing the signalling infrastructure for the new Mersey Gateway Bridge), and my spare time has evaporated this year. It was precisely for these light wind weekends that I decided to build the Zlin.

I'm planning to mould the canopy myself, but am a little nervous of failures and don't have a lot of spare acetate if it needs several attempts.

I'll probably not make it to the Orme this weekend, as family events require my attention as well as modelling. Having missed my planned deadline for completing the Zlin, I'm determined not to miss the A-4 deadline, so I'll try to make a start today. I'll be starting a thread soon to reveal my intentions.

 

 

Edited By Steve Houghton on 12/06/2016 08:42:32