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Old Sep 07, 2009, 02:02 PM
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Tiger moth wing tips

Hello,

I'm building this Tiger Moth, from plans found on the web. They aren't really clear with the wingtip.. to me it looks like its either just one big chunk of balsa there or a thick shaped sheet. It seems a bit wierd to me, as the rest of the wnig is very light to slap a solid piece there. Should I build some custom framing there instead, or is the solid wingtip there for a reason?

Also, this is the top wing, not very likely to bump into the ground. well.. maybe but then you have a lot bigger problems to deal with.

Attached are some photos that will show what I'm talking about:
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Old Sep 07, 2009, 02:07 PM
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What is the cross section of the view? I imagine it could be a thin sheet perpendicular to the tip rib down the centerline. Also that wing is not all that light.. that is how most wings for glow are built iwth a rear spar and shear webs between ribs, etc... Is it electric or glow?

If it is to be solid balsa, I usually hollow out the middle with a dremel
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Old Sep 07, 2009, 02:15 PM
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Quote:
Originally Posted by Thermalin
I imagine it could be a thin sheet perpendicular to the tip rib down the centerline.
That's where my money goes. The covering material angles down to the tip.

You can also cut lightening holes into the tip material.

EJWash
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Old Sep 07, 2009, 02:21 PM
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Originally Posted by Thermalin
What is the cross section of the view? I imagine it could be a thin sheet perpendicular to the tip rib down the centerline. Also that wing is not all that light.. that is how most wings for glow are built iwth a rear spar and shear webs between ribs, etc... Is it electric or glow?

If it is to be solid balsa, I usually hollow out the middle with a dremel
Thanks for your reply!

The original plan says electric, though this is clearly built for glow. And I intend to power it with glow. Engine is yet undecided, though I wonder if a .90 four stroke is too big for this. It's 1500mm wing span. There is no cross section on the plans at that point. I think the plan calls for a thin sheet perpendicular to the centerline just as you say. I'll take a photo again. (Yeah, i have a computer at my workbench. I'm that kind of geek)

I wish I had a dremel. They are over 100$ in Sweden, and I really wonder if they are THAT good...
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Old Sep 07, 2009, 02:24 PM
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Quote:
Originally Posted by EJWash1
That's where my money goes. The covering material angles down to the tip.

You can also cut lightening holes into the tip material.

EJWash
How thick would you make this sheet? Panel length is approx 700mm (I can't do inches for you US people, at least not in my head, sorry). I tried with a piece of miscut 6mm balsa, that seemed a little big to me.

Also, you really are helpful, thanks a lot! I started a build thread over at rcuniverse, with little response, strongly considering starting one here too since you all seem nice
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Old Sep 07, 2009, 02:44 PM
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You can probably get away with using the same thickness wood as the ribs.

Looking at the straight-on pic that you provided, if you will either have to taper down the leading and trailing edge to the thickness of the wingtip. One thing that you may want to consider to give a nicer and "beefier" wing tip is to lay balsa on top and bottom of the tip (1/4" square stock should do) and then sand it to match the contour of the leading and trailing edge shape.

EJWash
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Old Sep 07, 2009, 02:50 PM
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Originally Posted by EJWash1
You can probably get away with using the same thickness wood as the ribs.

Looking at the straight-on pic that you provided, if you will either have to taper down the leading and trailing edge to the thickness of the wingtip. One thing that you may want to consider to give a nicer and "beefier" wing tip is to lay balsa on top and bottom of the tip (1/4" square stock should do) and then sand it to match the contour of the leading and trailing edge shape.

EJWash
Oh yeah, the leading and trailing edge is not done yet. Nothing's sanded not shaped yet.

You think I'll get away with 2mm balsa (erm, thats 0.08 inches, erm 1/16 and then some more). Wow. I like your suggestions, I think I'll go with those.

As soon as I've built the other part of the wing, I like to go ahead when I hit a problem. Letting the solution brew a bit in my mind always makes me more confident when it comes to actually implementing it.

Actually, all my building follows this pattern:

1. Think about something and worry that I'm not doing it right for at least an hour
2. Build like a maniac until next snag hits.

Edit: Some of the ribs are 1/8" thick, maybe thats a bit more reasonable thickness to go with, 2mm seems a bit too flimsy
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Old Sep 07, 2009, 03:28 PM
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3/32" is a bit more than 0.09", so you should be fine with 2mm. If you go with the 1/8", you should definitely cut the lightning holes, and you can use 1/8" on the top and bottom balsa to the tip as I mentioned before.

EJWash
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Old Sep 07, 2009, 05:14 PM
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I looked at a few Moths over at airliners.net. How scale are you going?

EJWash
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Old Sep 07, 2009, 05:17 PM
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30 four cycle is what you want really.
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Old Sep 08, 2009, 01:39 AM
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Quote:
Originally Posted by EJWash1
I looked at a few Moths over at airliners.net. How scale are you going?

EJWash
I'm going as scale as the plans are. I'm not that proficient as a scratch builder yet, and though I will try my best at doing things as scale as I can I'm not about to change anything structural that I don't know how it impacts the final plane.

I will try to make some scale detailing, I hope to make a cockpit and put a decent pilot in there. As for the general plan, I'm happy if it looks like a moth and I get a working airplane and building practise. I hope that made sense.

Do you have any suggestions for me?
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Old Sep 08, 2009, 01:41 AM
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Originally Posted by vintage1
30 four cycle is what you want really.
Will that work for a 1500mm span? Sounds a little weak, but then again I know that many tend to overpower their planes.

I think the plan calls for either electic or a 10cc two stroke. I really dont want to put a two stroke in there, for the ugly muffler and the nasty sound. As far as I could figure out, a 10cc is about a .60 size, no?
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Old Sep 08, 2009, 04:33 AM
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Quote:
Originally Posted by Dervall
I'm going as scale as the plans are. I'm not that proficient as a scratch builder yet, and though I will try my best at doing things as scale as I can I'm not about to change anything structural that I don't know how it impacts the final plane.

I will try to make some scale detailing, I hope to make a cockpit and put a decent pilot in there. As for the general plan, I'm happy if it looks like a moth and I get a working airplane and building practise. I hope that made sense.

Do you have any suggestions for me?
This is what I don't understand frankly.

A lightbuilt electric 60" bipe should need at most maybe 400 watts. Maybe a 30 4 stroke, yet people put in huge engines..

Maybe its to get somewhere near scale prop sizes. I.e. that 60 4-cycle is really putting out less than half what it could, because its deliberately crippled with a massive prop?


My 60" monoplanes weigh 2lb 10oz all up and fly on less than 200W - and fly darned well.

A typical '40 trainer' at 60" that is probably putting OUT 700W on the right prop weighs 5lb..why?

My conclusion is that IC engines are inefficient brutes on 'normal' props and produce a lot less than they claim in real life circumstances.

And people routinely overbuild and put in larger engines than they need to with IC.

IF you can get that IC engine to turn something like a 15-20" prop at 3-5k RPM, then its in the ballpark.

Turning a 12x6 at 10K rpm is way overpowered.
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Old Sep 08, 2009, 06:04 AM
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Originally Posted by vintage1
This is what I don't understand frankly.

A lightbuilt electric 60" bipe should need at most maybe 400 watts. Maybe a 30 4 stroke, yet people put in huge engines..

Maybe its to get somewhere near scale prop sizes. I.e. that 60 4-cycle is really putting out less than half what it could, because its deliberately crippled with a massive prop?


My 60" monoplanes weigh 2lb 10oz all up and fly on less than 200W - and fly darned well.

A typical '40 trainer' at 60" that is probably putting OUT 700W on the right prop weighs 5lb..why?

My conclusion is that IC engines are inefficient brutes on 'normal' props and produce a lot less than they claim in real life circumstances.

And people routinely overbuild and put in larger engines than they need to with IC.

IF you can get that IC engine to turn something like a 15-20" prop at 3-5k RPM, then its in the ballpark.

Turning a 12x6 at 10K rpm is way overpowered.
Thanks for your input! I'm not the experienced builder I wish I was, and when I've built kit airplanes I've went with the midrange of what was suggested in the instructions. I've yet to fully grasp everything concerning engine sizes and propeller sizes.

I have a feeling that you're right though, that most planes are way way overpowered. I have no desire to make the plane a rocket with a way to big engine. My goal is to have something fairly relaxing to fly.

I kinda want a fourstroke, since I like the sound/smell and have loads of fuel and all the starting equipment I need. I'm not really dead set on this either, I could be persuaded to go for an electric setup. It would certainly cut on the mess and inconvenience of a glow engine.. it all depends. I'm very happy that I can get input from all you guys
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Old Sep 08, 2009, 06:31 AM
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Quote:
Originally Posted by Dervall
Thanks for your input! I'm not the experienced builder I wish I was, and when I've built kit airplanes I've went with the midrange of what was suggested in the instructions. I've yet to fully grasp everything concerning engine sizes and propeller sizes.

I have a feeling that you're right though, that most planes are way way overpowered. I have no desire to make the plane a rocket with a way to big engine. My goal is to have something fairly relaxing to fly.

I kinda want a fourstroke, since I like the sound/smell and have loads of fuel and all the starting equipment I need. I'm not really dead set on this either, I could be persuaded to go for an electric setup. It would certainly cut on the mess and inconvenience of a glow engine.. it all depends. I'm very happy that I can get input from all you guys
I have to say that a decent slow revving 4-cycle sounds very good in a biplane.

It also sounds ridiculous in a spitfire!

A Gypsy moth IIRC tirns at about 1500 rpm flat out on a 4 cylinder engine, and is of course a 4 stroke. So the same sort of sound as a single cylinder doing 4000 RPM.

I found this article..its fascinating. basically how to set up a glow engine to run much larger props at much lower RPM.
The solution is fundamentally :

- shim the head to lower compression ratio
- use onboard glow drivers to maintain the plug temp.

www.cbhmodels.co.uk/OVERPROP.pdf

I think if I was more conversant with those sorts of engines, then a 60 4 cycle derated to run a 15" prop or so at 4-5000RPM would be my target.

My own titchy moth runs an 8x6 at about 6000 RPM WOT, but its only 36" span ;-) Prop is near scale. Flies at about 3000 RPM for cruise.
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