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Capiche Woes

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Jeffrey Cottrell 204/04/2020 22:20:45
93 forum posts
14 photos

Hi Guys, could use some help
Just finishing off a Mini-Capiche from the Malcolm Corbin plan. First fixed wing build for many a year, and first plan build for even longer.
It's been battling me all the way, but finally got to the stage where I can assess the c/g and found a possibly terminal issue.
Placed all the gear as per plan, servos, battery, motor etc.
Come to measure the c/g and it's so far back it's a joke. Quick measure tells me I might need 163g (5.75 oz) in the nose to get it to balance. Bear in mind I am using a 2650 3s battery and that only weighs 215g, I am a bit reluctant to add that much excess weight to the nose. Quick calculation of wing loading gives me 16.61 oz/sq ft without the nose weight, and 19.46 with.
So, any thoughts, ideas?
Thanks
Jeff

john stones 104/04/2020 22:30:05
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11456 forum posts
1516 photos

Can you move servos forwards, failing that can you extend nose/cowl and use motor to help, lively little thing the Capiche, lighter the better.

i12fly04/04/2020 23:47:35
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632 forum posts
22 photos

I extended the nose on mine to avoid nose weight. Made a radial cowl too. It came out at 16.1 oz/sq ft. Flew it with 220W total weight 37 oz. Not sure if I increased the span slightly, but it turned out at 44.5 ins.

When I make a sports model I always leave the nose long then adjust the front former/motor position after doing a rough balance check on the full assembly before finishing the nose and covering.

Very nice flier, very robust, it survived a couple of silly 'arrivals'. Very light, very strong.

I even made some ski's and flew from snow. Good fun!

I scrapped it eventually because I used standard solarfilm, which kept wrinkling in the sun and after many tightening sessions it became tatty. I'd certainly consider making another but would cover the underside in a dark colour and a light topside as orientation could be a bit deceptive.

Edited By i12fly on 04/04/2020 23:48:35

kc05/04/2020 00:00:10
6427 forum posts
173 photos

The only way to get a good flying model will be to extend the nose or reduce the weight at the tail. Make a temporary lashup with rubber bands, tape etc to see how far forward the motor and Lipo would need to be to balance. Then you will know if it's practical. Much easier than re doing the tail end with lighter balsa or lightweight covering.

Chris Walby05/04/2020 06:41:32
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1200 forum posts
299 photos

I don't know as I haven't owned one, but will it really be a dog at 20 oz/sq ft?

I built a Renaissance which comes out at 19.9 oz/sq ft and its a total pussy cat to fly and the pilot is a bit of an animal!

20190801_182212.jpg

I fly IC warbirds and if its below 35 oz/sq ft its a real bonus....just don't ask what the pusher Vulcan comes out at, but the best description would be "flying manhole cover".

IMHO, build it, set the C of G in the right place and go fly (when restrictions allow"

Jeffrey Cottrell 205/04/2020 18:51:52
93 forum posts
14 photos

Hi Guys
Thanks for all your ideas, much appreciated.
Going to try moving the motor and LiPo forward and see where I get.
Wish me luck
Jeff

kc05/04/2020 19:51:00
6427 forum posts
173 photos

My guess is moving the lipo forward 1 inch or 2 at the most will balance without lead and hardly change the looks

Jeffrey Cottrell 205/04/2020 20:24:08
93 forum posts
14 photos

Hi kc
Thanks for the thought, but it took a lot more than that.
I moved the LiPo from its plan position about mid wing, and moved it hard against F1. That's about 4".
Also moved the motor forward some 15mm.
Now it balances where it should, but those changes surprised me and also introduced some issues.
Trying to work out why the c/g was so far back, but can't see it. Didn't use excessively heavy material for the tail feathers. Only thing I can see is I used metal clevises for the tail controls and these are at the rear end, but can't see that making so much difference.
Anyway, it balances but I do have some concerns.
To retain the battery in its new position means I have to do a job I always hate, of trying to fit an internal support structure into a completed and covered airframe.
Having 'sausage' fingers doesn't help.
Also I have the motor backplate mounted on some 3mm stand-offs. Switched these from 20mm to 35mm, but it does leave a long overhang from F1 to the prop.
Not sure if this will be an issue, and if so what to do about it.
Thinking cap on, methinks.
Jeff

i12fly05/04/2020 22:34:49
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632 forum posts
22 photos

Thinking back, when I built mine I left the sides long by about 20mm at the front. But when I did the rough CoG test, strapping on the motor and F1 etc with rubber bands and guessing the weight of covering for the back end I found it wasn't enough and had to extend the sides a further 15 or 20mm. So it doesn't surprise me, I used snakes and metal clevices too. Perhaps Malcolm Corbin used 'contest' grade balsa on the back end of the original? It may be worth adding a new bulkhead F0 behind your motor and build a support box behind it attached to F1. You could then dispense with the stand-offs

cymaz05/04/2020 23:03:09
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9197 forum posts
1186 photos

1 1/4 lb/ft square. I’d say that’s good. A prolific club builder had a few rules of the thumb which kept him in good stead.

  • he’d reckon on 1 1/2 lb/ft square
  • 25% cg for first flight
  • 2t glow hs needle setting, 1, 1/2, bit for luck

I’ve followed the advice and it’s served me well.

kc06/04/2020 09:22:27
6427 forum posts
173 photos

Normally the Lipo goes right near the front bulkhead in most models, however in the past before Lipo people used Nicad packs which were so heavy they went under the wing. I reckon that your design is old that it was designed ( actually modified from ic as an alternative ) to use nicads. Hence the big change in battery position.

All that matters is that CG is correct and weight is minimal........also it looks good!

Jeffrey Cottrell 206/04/2020 20:31:09
93 forum posts
14 photos

Hi kc

Would've thought the design was not old enough to use Nicads, but you got me thinking. On the plan it shows a line drawing of the motor, and that looks suspiciously like a can motor.

So, nicads and can motor would certainly explain the component placement.

Anyway, planning on a mount to move the LiPo as far forward as possible and the motor forward as well.

On the plan it shows 70mm from F1 to the prop driver. With the longer stand offs this comes out to 80mm, so only 10mm further forward.

Shouldn't look out of proportion.

Glad I left the cowl until I'd decided on side and down thrust. Means I won't have to re-do it, phew.

The battle continues

Jeff

Bruce Collinson06/04/2020 21:34:13
515 forum posts

A small Great Planes Sukhoi came with several sprues of horns, clevises etc. I didn’t fancy them and “upgraded” to M2 clevises, then couldn’t get the c of g even close. Replaced by the makers’ plastic fittings, the balance was spot on.

Just a thought.

BTC

Jeffrey Cottrell 207/04/2020 12:44:31
93 forum posts
14 photos

Hi Bruce

Same thought occurred to me. I had to use metal clevises at the tail end. However the difference in c/g was too great.

Bear in mind I had to move the LiPo some 100mm forward and the motor by 15mm to get it to balance right.

That's too far for just those clevises.

Anyway, in the process of building a new mount for the battery so should be ok.

We'll see

Jeff

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