Scratcho - Page 7 - RC Groups
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Dec 05, 2012, 01:14 AM
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AeroNut45's Avatar
Gees, you don't want to fly that, hang it on the wall.
Very nice.
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Dec 05, 2012, 07:08 AM
Auzzie built planks
timbuck's Avatar
Awesome ...
Now get it to 300.... That paint sure got to help.

Latest blog entry: More colours
Dec 05, 2012, 04:59 PM
Registered User
Can't wait to see the scratcho in action, leaving @ midnight
Dec 05, 2012, 06:26 PM
Wind, hill, ... I'm keen ...
AvB's Avatar
Well, A couple of appropriate sayings: my father in law says "you only need either a good carpenter or a good painter to have a nice looking job".
and the old classic:
"A good paint job covers a multitude of sins".
My crappy building skills are now transformed into a Ferrari style beast!
Dec 06, 2012, 02:50 AM
Remember... Fly for fun!
What Flight you on, Auselectric?
Dec 06, 2012, 03:06 AM
Registered User
Pm sent
Jan 07, 2013, 07:25 AM
Wind, hill, ... I'm keen ...
AvB's Avatar
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??
Jan 07, 2013, 08:25 AM
making my own planes...
decones's Avatar
Nice done AvB,

What's the Light Scratcho AUW and wingspan?
Jan 07, 2013, 11:18 AM
Deniable plausibility
Shedofdread's Avatar
Do bare in mind that with a wing made of straight tapers (and not produced from CNCed masters / moulds) it's far easier to keep the section true than with one that has curvy tips....

Have a Google for 'Liftroll'. It's a spreadsheet for designing planforms and may be of interest.

I'm thinking you'd get the same effect (or maybe better due to improved airfoil fiddelity) with a planform having a single tapered main section and another, smaller, more highly tapered tip panel. Or go 'all in' and move to a 4 tapered wing that approximates an ellipse (Liftroll will REALLY help here).

Jan 07, 2013, 12:02 PM
Remember... Fly for fun!
Cool, Andrew. Must have been hard to cut up a perfectly good wing though...
Jan 07, 2013, 04:18 PM
Auzzie built planks
timbuck's Avatar
Sound great andrew , sctatchochop..
That why I like this hobbie , more you build the more you learn. It's great to have the same plane , buillt differently and to be able to feel the difference straight away.
Small things sometimes make big differences.

Latest blog entry: More colours
Jan 08, 2013, 04:33 PM
Registered User

Muilt section wing panels are the go in full size to. Check out the latest 20m two seater Arcus plan form. Its impressive in the flesh but I do not what the task of doing a refinish of one down the track.

Jan 08, 2013, 04:54 PM
Where is the inspiration
wdeighton's Avatar
I'll Give it a go on my limited knowledge.
but I think some of this has to do with wing tip efficiencies.
And possibly Tip vortices. the more sharp edges you have the more tip vortices you have. this is why models like the erwin has them funny little movable pieces between the aileron tip and fixed trailing edge.

In DS we like to keep the wing strong so your initial design is best. but maybe not most efficient while ailerons are not at neutral.

Drag is based not only on frontal area but plan form. you have reduced your plan form and therefor reduced you drag.

Also by keeping you aileron as the longest, you are also managing the air efficiency over the wing better.

My understanding is that the reason people go for elliptical wings is they don't have such strong tip vortices. and what you have done is going closer to elliptical than square.

Am am sure there are some in accuracies in what I have said. But its my understanding, and its a start point.

I think Avb is looking to the future where he has a cnc cut mould and not just vac bagged.

Will's 2pence
Jan 09, 2013, 12:38 AM
608 km/h!
josh18's Avatar
Originally Posted by Shedofdread

I'm thinking you'd get the same effect (or maybe better due to improved airfoil fiddelity) with a planform having a single tapered main section and another, smaller, more highly tapered tip panel.

That's interesting, cause Avb's older designs were just like that! Its like going full circle!
Andrew, to you think it was the airfoil or the planform or both that makes the scratcho fly so much better than your previous designs? It certainly seems to have 'IT", it would be good to work out why this is, then you could put that into your future designs to keep that feel but be even better.
Cheers- Keep it up!
Jan 09, 2013, 01:27 AM
Wind, hill, ... I'm keen ...
AvB's Avatar
I think it's a combination of things that simply thru good luck have worked well together!

In priority I'd say the biggest things that made the Scratcho so much different and better to the SC 1, 2 and 3 are:

1) HN1038 foil. So much faster and slipperier than the old RG15. And it can still "thermal" and responds excellently to camber. Excellent frontside, excellent in DS.

2) Planform ... yes, I have no proof but I do think the planform of the wing and the overall airframe shape works really well for some reason. I looked at several planes that I thought were simple designs but tracked really nicely in DS (Vector, Flying Fish, etc) and compared them to the things I liked about the SC2, and sketched out shapes on the shed floor till I thought it looked good! Definitely a "TLAR" approach but I think it worked.

3) Tips ... I know they are squarish in shape but they are cut off at the 8 degrees angle, and undercut at the ends (as per Helmut Quabeck's concept from the 80's) , and from that the tip shape of the plug was sanded to a very teardrop smooth rounded shape, which I think helped to reduce drag at the tips.

4) Tail .. again I have no proof but I think that the stab position somehow struck it lucky. It's done as a simple glued-on X-tail, stuck straight onto the boom. Compared to T-Tail you'd think it might be adversely affected by being right in the wing wash but it seems to be perfect. Really great tracking and elevator feel.

5) Toughness! All the SC planes were made with the primary aim of being light enough to be really fun, but very tough to survive hard knocks. Or be crashed into a tree DSing like Asto did at Cass, and with 45 mins fixing a snapped boom and wing crunch, it's back doing 152mph the next day! I know that's nothing to do with planforms etc but tough planes are just so valuable to your all-round sloping experience. It means more stick time and better flying skills and less frustration in the shed.

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