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Tim's Skywriter Build

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Lucas Hofman23/03/2017 06:55:23
596 forum posts
316 photos

I will give it a try and leave it to you guys to judge if it is an improvement. Bit of a folly, since the inner struts will have a AL to AL joint. Maybe not if I can make the lugs from 1 mm brass bent in a fork shape. Still far away.

Lucas

Tim Ballinger23/03/2017 19:09:37
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545 forum posts
229 photos

Wings pretty much finished so as an excuse for more bench flying I thought I would have a weigh in and cg check.Currently 1.85 kg (4lb 2 oz) and 30 % chord . The battery can still move forward if needs must so with only the radio/servos and covering missing i am hopeful of managing without ballast.

Tim

Tim Ballinger23/03/2017 19:10:44
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545 forum posts
229 photos

Oh and the photo of where I am at...

img_0945.jpg

Dwain Dibley.23/03/2017 19:16:59
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1268 forum posts
1265 photos

Very tidy Tim,

Looking forward to seeing it covered...One of my most favorite parts of a build.

D.D.

Lucas Hofman24/03/2017 07:41:22
596 forum posts
316 photos
Posted by Tim Ballinger on 23/03/2017 19:09:37:

Wings pretty much finished so as an excuse for more bench flying I thought I would have a weigh in and cg check.Currently 1.85 kg (4lb 2 oz) and 30 % chord . The battery can still move forward if needs must so with only the radio/servos and covering missing i am hopeful of managing without ballast.

Tim

30% chord on which wing Tim? If is is the upper you are good. Covering is quite heave (I guess 200-300 gram added) and most of it is behind the CG.This probably means rudder and elevator servo's should go as far forward as they can go.

Lucas

Tim Ballinger24/03/2017 07:46:53
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545 forum posts
229 photos

Top wing Lucas. Yes plan is for rudder/elevator servos right up front. I left the snakes as long as I could and will trim accordingly.

Tim

Tim Ballinger26/03/2017 15:39:28
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545 forum posts
229 photos

Since I was installing servos I also commissioned the esc (60A). Hence I could not resist checking the power output.

So the 3548/05 consumed 47A and produced 700W on an 11x8 using a 4S. Pretty much the manufacturer spec.

Might not be spending too much time on full throttle !

Tim

Lucas Hofman27/03/2017 07:09:09
596 forum posts
316 photos

What is the kv on that motor Tim? 700W sound a lot to me for a 2kg plane. But you can always set the endpoint of the throttle to 80 or 90% to tame it a little.

Lucas

Tim Ballinger27/03/2017 08:07:39
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545 forum posts
229 photos

Lucas, it's a 900KV .

Tim

Lucas Hofman30/03/2017 21:31:59
596 forum posts
316 photos

A quick run of motocalc suggest you will nearly be able to do vertical ascents!

Lucas

Tim Ballinger30/03/2017 22:11:04
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545 forum posts
229 photos

Do you think I'll need to cover it first ? devil

Lucas Hofman31/03/2017 07:32:21
596 forum posts
316 photos

Not for the verticals. But you landings will be easier with covering on the wings too .

Tim Ballinger31/03/2017 11:21:24
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545 forum posts
229 photos

Completed the radio installation, connected the top and bottom ailerons and everything appears to move ok so that's it for a few weeks while I am away. Then its final sanding , covering and a bit of cockpit detailing at the end of April beginning of May.

Tim

img_0947.jpg

img_0946.jpg

Lucas Hofman31/03/2017 11:27:55
596 forum posts
316 photos

Nearly a pity to cover it Tim. What is the black stuff on the cockpit coaming?

And what weight are you on now?

Lucas

Martin Dance 131/03/2017 11:52:22
197 forum posts
33 photos

Very nice build Tim, as Lucas says what weight have you reached now? I'm glad you're away gives me a chance to catch up between sessions on the allotment. So when you get back I should have a finished Skywriter and the start of the annual glut of courgettes!

Tim Ballinger31/03/2017 11:57:15
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545 forum posts
229 photos

Lucas,

She's 2.16kg . That's with all radio/servos/battery & esc.

CG is at 35 % or 30% if I move the battery under the motor a bit more. See what the covering does but may struggle a bit without some ballast.

The black stuff is soft automotive fuel tube slit down its length.

Tim

Lucas Hofman31/03/2017 12:08:48
596 forum posts
316 photos

I hope to come this far before breaking for the summer, then she can hang from the ceiling uncovered until autumn comes. The fueltube looks very good, I will get some for the coaming.

Your weight is inline with Lindsay's. He needed some ballast, but then he used heavy covering. You can still make lightening holes in the elevators and rudders if you want. It saved (on mine) 12 grams, which corresponds with 40 grams lead up front.

But then, is 30-35% too far forward? The whole lower wing is further back. 35% sounds about right to me to get the average on 25%.

Lucas

Tim Ballinger31/03/2017 13:32:44
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545 forum posts
229 photos

Funnily enough I was just contemplating if putting holes in the elevators rudders would be a hedge against ballast.

As far as cg position goes I was only going on Lindsay's write up which aimed at 70mm or 30 % of top wing. I did not read of him needing to change that so probably only flying will tell us more.

Tim

Tim Ballinger31/03/2017 13:40:02
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545 forum posts
229 photos

Guys ,

Found this on cg for stagger wings....

https://www.rcgroups.com/forums/showthread.php?519833-CG-on-biplanes-with-forward-top-wing

Tim

Tim Ballinger31/03/2017 14:01:35
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545 forum posts
229 photos

So if my sums are correct and assuming a 10% larger area for the upper wing (and hence a cg that is 0.45 of the distance between the 30% MAC point top and bottom wings) then the required cg is 93mm from the le of the top wing or 40%of the top wing MAC.

Anyone care to check that ?

Tim

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