Thread: Discussion Scratcho
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Old Jan 07, 2013, 06:25 AM
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AvB
Wind, hill, ... I'm keen ...
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Australia, QLD, Woody Point
Joined Nov 2006
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This is interesting … most of you will just go “told you so” but I’ll keep going anyway …

When I designed the Scratcho, it was literally sketched out in chalk on the shed floor and had square tips because 1) I wanted to give it a “Vector-ish” look and 2) construction is so much simpler with a straight taper.

The first Scratcho flew so well, went so fast and DSed so nicely that I thought “when you’re on a good thing stick with it” … ignored the chorus of helpful advice suggesting that I could improve it with elliptical tips, and went ahead with moulding it with the squarish wing shape.

You may remember that I made a really light Scratcho with a wing filled with expanded polyurethane (PU). It was just an experiment but it flew very nicely, until a glitch when it crashed and smashed and split a wingtip. I left it sit until Rhys suggested I should cut the tips short both sides. So I did. I laid an old Caracho tip over it, traced the shape, and for an hour or so I hacked and gouged and sticky taped the tips. The tips look like a real mess, but man that thing flies so damn well. I just love flying it and it’s put in many hours. It feels amazing. So nimble and acrobatic. Even with $3 HXT900 servos for ailerons, elevator and rudder and basic $7 Turnigy MG servos for the flaps. Fantastic.

The other day we flew both the PU Scratcho (Scratchopu??) and the previous light version in strong wind and the lighter one with the crappy curved tips was better, hands down. So since one tip had been slightly chewed by a badly behaved beagle pup anyway, I decided to cut the tips on the other one to see if it changed its behaviour. This is the one that I made with the vac bagged half-cores, which featured strongly in all the NZ flying.

This time I put a bit more effort in and shaped the tips with a belt sander then bogged them up and sanded them to a better foil. It looks messy in the photos, but the shape is smooth.

Today we flew it at T-Bones, and straight away I knew it was definitely a better plane to fly. It just has a more fluid feel to it, and most noticeable is the way it does loops effortlessly compared to when it had square tips. It “flies lighter” than before and feels similar to the PU one, although faster due to its 1100gm vs 800gm. I had a lot of fun with it and flew it for ages … it still DSes beautifully too. It’s definitely the best plane I’ve ever made.

The different tip shape has definitely improved it, but there’s one other factor to consider. When I’ve cut the tips I have cut them at the end of the ailerons, removing the 1 inch fixed outer trailing edge. So the ailerons go right to the tips now. Is the improved performance likely to be due to the curved tips, or the ailerons going to the tips??
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