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Old Mar 04, 2012, 05:03 PM
AvB
Wind, hill, ... I'm keen ...
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Australia, QLD, Woody Point
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Building a heavy Scratcho for US DS trip.

Help ... I'm making a heavy Scratcho to take to the US, and I'm not sure how much lead to add to the wings.

It's my homebrew all round fun sloper which DSes really nicely. It has a 1.6m (64") span and a HN1038 foil. There are 4 flying so far - 2 of them are the "SCratchobuck" or "Timbuckerized Scratcho!" wings which were the ones uses to pioneer making molded wings filled with the Ampreg F230 foam.

They've been really successful as all round fun slopers and excellent DSers. Not a record breaking plane but just tremendous fun for ripping around at insane speeds, banging hard turns, acros and inverted, and the most amazing "Ninja stars" that anyone's ever seen.

The "average" Scratchos perform well at at 1.6kg AUW. I recently made a very light one at 1.1kg which is superb. The Scratchobucks are 2.1kg empty and 2.5kg with ballast. They all seem to perform well and haven't shown any tendency to tip stall or other bad habits. The only adverse effect has been that if we hold the Ninja Star manoeuvre too long we can induce a flat spin that it won't recover from!

The max speed so far is 185mph and it would be good to see how well it can go in big conditions. My building skills are pretty basic - fin and stab are essentially flat sections ... and so far the Scratchos haven't had servo covers or wipers. I'm hoping someone will help me to get these things done for the USA Scratcho!

The one I'm making now is going to get the Scratchobuck method, and I am going to put lead shot into the wing half before pouring the Ampreg foam in, so I can spread the shot along the span. It has really thick spar caps, an added rear spar, and will be very stiff.

How much additional weight do you reckon I should put in? I was thinking of 1kg? But I really don't know what's a fair thing!

Are there any "rules of thumb" for this?
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Last edited by AvB; Mar 05, 2012 at 05:54 PM.
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Old Mar 04, 2012, 07:28 PM
PB of 733kph
ALEX HEWSON's Avatar
Christchurch, New Zealand
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Put ballast tubes in the tips. That way you can adjust it and try different weights
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Old Mar 04, 2012, 08:47 PM
AvB
Wind, hill, ... I'm keen ...
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Nup. I'm keeping it simple. I have bitten off more than I can chew and am not sure if I'm gonna have time to get it all done. Quick and dirty for this one. The lead's going in with the Ampreg and staying there!
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Old Mar 04, 2012, 09:09 PM
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Have a look in the D60 thread and see how much that has. Mine feels good with what it has
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Old Mar 04, 2012, 09:17 PM
Can you DS it??
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Let me know if you need any servo covers. I'll send you some.
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Old Mar 04, 2012, 09:31 PM
AvB
Wind, hill, ... I'm keen ...
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Hmm, the search function in RCG doesn't seem to be working for me. But I googled and found a thread with Joe Manor saying that 100 - 110oz was the best AUW, and that he likes 10oz per tip.

That's a surprise because 110 oz is around 3.1kg. So perhaps 1kg extra mass will be too much for the Scratcho.

Leo, I'd love some servo covers. I do have some odds and sods here, only 2 of which will work. I run the control rods direct, so they need to have the tunnel running back towards the horn. I've been fitting fairly tall horns previously but this makes the control rod too high for most servo covers so I will use lower holes on the arm and the horns this time. I'll PM you my address.
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Old Mar 04, 2012, 09:41 PM
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Saw a post that someone used 12oz per tip and they liked it. I think you should go for 8oz personally
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Old Mar 04, 2012, 11:34 PM
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United States, CA, Los Angeles
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Hi Andrew,

I'd say it depends on what you are wanting from it... And are you using snapflap or not? Assuming you have +/-320in^2 of area, an AUW of 100-120oz will put you in good territory for HN1038 at 3.5sec circuits. If no snapflap, go closer to 100oz if snapflap closer to 120oz. Aside from aero performance reason, whatever you add to the tips will only help you with stability and bending so go for as much as you are comfortable without sacrificing the ability to fly (land) in lighter air. One strategy would be CG it with some removable noseweight and not add any permanent tip lead until you are here and we have a forecast for the big winds.

Looking forward to your trip! Got your gun charged and ready for you...

Spencer
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Old Mar 04, 2012, 11:38 PM
Can you DS it??
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Australia, NSW, Berowra
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I've flown D60's with 12, 6 and 0oz in the tips, and I gotta say that the 6 was my favourite. My heavy one with 12oz in the tip is fast and rock solid, but it needs quite a bit of wind to fly frontside. Once it's moving in the loop, it's the same as all my other planes.

My light one with nothing in the tips definitely wags until it going over 130mph.

Pete's medium weight one with 6oz in the tip is the best compromise for our conditions. If you had to have only 1, 6oz would be it.

I make every effort not to compromise, so I have a heavy 12oz version and a light 0oz version!

Yes, 3.1kg sounds heavy, but that's about what mine flies at.

hmmmm..... maybe that means I need to have a 6oz one as well??
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Old Mar 05, 2012, 12:50 AM
AvB
Wind, hill, ... I'm keen ...
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Yes Rhys and I have medium weight Scratchos (2.55kg ballasted or 80 oz).

Making this one is really just a bit of fun to see how a really solid one goes in big conditions. I'm not interested in how this one goes in medium air ... If I can get it there and get a big day I'd like to see how a heavy Scratcho feels in 50 kts, and what speed it does. I think it will top out at around 250 but who knows. And it will be fun finding out!

I think I'll go for 100 oz without fuse ballast, which means I'll need to add 350gm to each tip.
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Old Mar 05, 2012, 02:25 AM
AvB
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Well we've done it now. Mould is all clamped up now and in the heat tent, so this case is now closed! Rhys dropped round after work and helped me. I chickened out a bit and added 350gm to each wing tip. I mixed up 500mL of Ampreg, which should have been enough ... but it just didn't look enough when we spread it into the mould halves! So I ended up mixing up 250mL more! This wing is definitely going to be very heavy and solid! The lead shot is placed in a thick band behind the LE, and is spread over a length of about 20cm from the tip.
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Old Mar 05, 2012, 02:41 AM
AvB
Wind, hill, ... I'm keen ...
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P.S. yeah judging by the amount of froth oozing out of the sides of my mould I may have gone a leeeetel beeeet overboard with the Ampreg.
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Old Mar 05, 2012, 04:18 AM
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looking forward to seeing this popped out opf the mold. I think you may have to do a removal video
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Old Mar 05, 2012, 06:02 PM
AvB
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Alex I was hoping to escape video etc but I can't let you down so we got a short bit of vid this morning. Since this thread has now changed to show some of the build I've renamed it accordingly.

The wing is really excellent. I used Frekote mould release this time ... first time, so it was a bit scary but it's certainly much easier than waxing, less work, and worked beautifully.

Another positive is that despite using way too much F230, it didn't bleed under the skin ... except near one tip, and this would have happened because I accidentally lifted the skin away from the mould there when I was trimming the skin. (very hard to avoid that!).

It weighs 1.94 kg and it balances perfectly! We must have done a good job of distributing the lead shot in the tips.

Heavy Scratcho wing just out of mould (2 min 18 sec)
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