Scratcho - Page 8 - RC Groups
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Jan 10, 2013, 02:23 AM
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Well after cutting the wings this plane is now stuffed, happy for you to send it down for me to look after. Lol
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Feb 12, 2013, 06:07 PM
Wind, hill, ... I'm keen ...
AvB's Avatar
No big progress with Scratcho stuff, but since I’ve been posting stuff on other threads I thought I’d bring it all together here to make a bit of an update. On the macro level:

Still had absolutely no success finding a cheap way to get the Scratcho wing, stab, fin moulds cut, and since I refused to pay commercial rates the mould project remained bogged.

I want to keep making some Scratchos, but remember I ruined the original ones when I repaired and polished them. I did try to re-face them but actually made the situation worse and gave up. So I finally decided to get Mike in the UK to cut the fin and stab moulds. Shipping won’t be too bad as they’re not so big. I should have the new fin and stab moulds soon.

Well, I have had a go at making a polyurethane filled wing using a mylar splitter plate. Unfortunately the wing is a failure! But there's hope! ... read on ...

After Christmas I finished building the 2 Scratchos using the experimental full-foam core wings that I worked on prior to doing the half-cores vacuumed in to the skins. It was a lot of work, post-painting and so on and repairing fin and stab that were made in the stuffed moulds. One is sitting waiting to go to a new owner in NZ, and the other I’ve been flying quite a lot.

I’ve got absolutely heaps of flying with the Scratchos. Their toughness means that I can fly in all sorts of spots and conditions.

I totally love flying the light PU version, with the tips crudely hacked back to elliptical shape.

So I took the slightly heavier version and cut its wings and shaped them better, to see how it flew. I reckon it was slicker, better rolls, better loops. So if I do go ahead with cutting SCratcho wing moulds I will redesign the wing with elliptical tips. I was DSing that plane with Eric and Astan at Byron Bay for ages, and finally flew it into the tops of the trees covering the slope at about 130mph and it now needs significant repairs to the leading edge!! It wasn’t built with DS in mind but has been such a superb performer you just have to DS it to see how fast it can flick the turns!! The socked carbon main spar in the blue foam core is so stiff (and aileron/ flap stiffeners too) – I think that’s what gives it the performance.

The newer green/ yellow one, made from the experimental wing, is very responsive but I notice when I DS it that it shimmies and wobbles a bit more than I’m used to, and I think it’s because it’s a light glass wing and has poor torsional rigidity at the tips.

The first PU filled wing is such a success, but the weak point of the technique is that the PU foam does not provide a durable bond of the wing skin seams so the edges are prone to damage and big crashes can tear the skins away from the foam.

So I planned to try using a piece of waxed mylar in between the clamped mould halves as the PU is expanding, so that they can be split apart. The advantage of this would be that areas of dry PU foam could be scraped out around the edges and these areas then filled with a thick bead of expanding epoxy, and the moulds closed again. It should give the best of both worlds … a very quick build method, a stiff and light wing, and good durability. It would also allow trenches to be cut for spars and aileron stiffeners for stronger faster wings, and wiring etc could be put in place too.

On Sunday I spray painted the mould and laid up the wing skins and vac bagged them. Then on Monday a bunch of us did the splitter plate and PU process. There is no time to spare and we had 4 people - one per wing side half, mixing for about 10 seconds then pouring and spreading the liquid over the whole skin with gloved fingers. Then the mylar was laid over one half, and the mould closed and clamped.

The splitter plate worked well, but this experiment wasn’t a success because when I split the moulds apart and removed the mylar splitter plate, there were some significant deficits in the PU foam. As reported in the PU wing thread, I attempted to fix it and join the wing with expanding epoxy around the edges, but for a number of reasons it was pretty much a total failure. The TE is not bonded together, and the LE’s are not great. Worst of all, a good "flex" test and the wing creases quite easily. Very different to the first wing, which is very strong in flex. There are a number of issues that can be rectified next time, but the issue of the poor foam fill during expansion is a worry. I suspect the use of the splitter plate actually reduces the efficiency of the expansion and fill. So the jury is out on whether I proceed further with it.

It was a shame that it failed … I was very excited about the potential of this technique! I spent hours painting the mould and used carbon – I was so sure it would work.
But I console myself with some words I recently heard: "If you're not prepared to have failures you're never going to come up with anything original"!

Having a wing mould to play with has been great, as I’ve been able to not only learn to build, but also experiment with different techniques towards the aim of producing durable, good performance foam-cored wings with minimal work. At one end, very light, and at the other end, capable of being a 280mph DSer that can land in the rocks.

From all the playing around I now feel that the 3 main “build versions” I’ll stick with are:

1) PU wing, expanded in mould as discussed above - very light and excellent for light air slope fun. AUW around 780gm. Probably just stick with one-step PU application and close mould – same as the one I’m flying now.

2) Thicker but comparatively light skin, pre-cut blue foam half cores vac bag bonded into skins and cut flush with hot wire, slots cut for socked spars and stiffeners, edges trimmed for expanding epoxy - all wet up/ poured and mould closed (this is the "Light Scratcho" that flew last year in the US and NZ). AUW around 1050gm. Huge fun on the frontside in 12kts plus, and a brilliant snappy light to moderate air DSer. This really is the “Ducks Nuts” method and the best quality, most fun Scratcho. Everyone who flies or watches that plane is impressed!

3) Much thicker skins, socked spars etc, foam blocks positioned for servos and wiring, but otherwise fully filled with expanding epoxy. This is the more DS oriented version, AUW around 1600gm without ballast. Can be tweaked, eg the rear half of the wing could be blue-foam core to keep the weight down.

So my project for a while to come will be to make another one of each of the above versions, putting together all that I’ve learnt from each of the techniques.

All I can say is – I keep thinking how much easier the design/ prototyping/ mould process would be if I had my own CNC machine! That’s another story. But imagine being able to tweak your wing design and have new Corian moulds in a few days. Or cut out your own perfectly shaped servo trays, control horns, etc. etc. Hmmm.
Last edited by AvB; Feb 12, 2013 at 06:39 PM.
Feb 13, 2013, 12:55 AM
A10FLYR's Avatar
Answered my question. Thx Andrew..
Feb 13, 2013, 03:15 AM
Wind, hill, ... I'm keen ...
AvB's Avatar

Went down for a late afternoon short fly here at Woody Point. Great wind, pretty good angle so the green Scratcho was carving and rolling energetically. It's a really small and not steep slope, so there's a lot of horizontal wind component, and trying to do consecutive loops can leave you blowing back if it slows at the top of the loop. First time it landed gently in a Casuarina tree so it was off with the sandals and a tree climb recovery. Five minutes later it just didn't make that third loop easily and went back too far, and pulling too much elevator it stalled (I think) ... tried to recover but from a fair height it dropped really hard into the top of the hill, nose first and slightly off angle.

Really I should be impressed with the lack of damage. It was a big hard crash, and it's snapped the fuz where it previously broke (in a huge brain fade dumb thumb up at T-Bones). It was only lightly patched so no surprise it broke there. You can see the PU foam inside the fuz too.

Apart from that, and cracked paint running from the inner flap corners it's OK. I am sure the light glass skin has been damaged (you can see a white line), but it's still strong. Under the paint, I can't feel the cracks in the skin. I can bend the wing really hard and it is really solid. I don't think it warrants patching the wing. I like solid foam cores and Ampreg!
Feb 13, 2013, 12:06 PM
Remember... Fly for fun!
Loving the Scratcho testing!
Mar 03, 2013, 05:09 PM
Wind, hill, ... I'm keen ...
AvB's Avatar
57sailplane asked me to post the diagram showing what I meant by undercut tips.

This was sent to me by Larrikin.

The tips are also cut off at an 8 degree angle from the flightline.
Last edited by AvB; Mar 03, 2013 at 05:17 PM.
Mar 03, 2013, 06:19 PM
Plays with Toys
tim_mellor's Avatar
Was just having a read through the thread. I met those guys with the Antonov in my shop last year in March when they were here doing some Joy Flights over our Folk Festival. Noisy beast with a stall speed of not very much

Really interesting thread with the foam filling. Some of the F5B planes of years gone by used to run milled foam cores for their molded wings so they could drop the skin weight and raise the strength.
Mar 05, 2013, 03:48 AM
Dave Pope Manawatu NZ
pope's Avatar
I see you had the same results as we had doing exp. PU with garaths wing, we are going back to hollow molded for a while. just about to start building my cnc so the we will be away. Big learning curve so will take a while. Lucky for us there is a local mate that has built his own cnc so will be able to get help when lost.
Mar 05, 2013, 06:21 AM
Wind, hill, ... I'm keen ...
AvB's Avatar
Dave, I had success this time!

All the best with the CNC venture! Sort it out fast so you can make moulds for us! Make sure you have enough X axis to be able to make one piece wings up to 1.9m or so!
Mar 05, 2013, 01:12 PM
Remember... Fly for fun!
^ +1
Mar 07, 2013, 07:34 PM
Wind, hill, ... I'm keen ...
AvB's Avatar
New CNC cut fin and stab moulds are being cut now. Looking forward to getting back onto making new Scratchos.

Mike is also cutting me a heap of control horns. I shaped up the "perfect" horns for ail/flap and elevator and he's drawn them up. I'm really excited about that! Hand cutting and shaping the 4 wing horns and the elevator horn, and drilling them out etc seems like a small job but takes quite some time, so it will be brilliant to be able to pick them up and fit them.

Gradually making the building process more efficient!
Mar 07, 2013, 08:48 PM
Remember... Fly for fun!
Woot! When does Peter get his?
Mar 07, 2013, 09:24 PM
Wind, hill, ... I'm keen ...
AvB's Avatar
2nd week in April Bernd is travelling to CHCH.
Mar 08, 2013, 01:11 AM
Remember... Fly for fun!
Mar 08, 2013, 02:44 AM
Thermal Wrangler
DrFragnasty's Avatar

Classic Photo

Kudos to the photographer; classic photo.!

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