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Author Topic: Comet P38 Lightning - BUILD  (Read 4857 times)
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Rich Moore
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« on: April 18, 2018, 04:48:57 PM »

Here we go. I aim to be more ready for Kitscale next year and I have made a good start by choosing the model. The Comet P38 Lightning. As usual, I will try to build true to the plan as closely as possible. It is a good size at 34" span. Minimal stringers, but I think this could look really nice. Silver tissue all over.
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Comet P38 Lightning - BUILD
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faif2d
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« Reply #1 on: April 18, 2018, 05:28:39 PM »

I have started this one 3 times.  Never finished a single one! If I remember it uses 3/32 stringers, you need to use light wood for those.
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Don McLellan
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« Reply #2 on: April 18, 2018, 06:57:46 PM »

Very, very cool airplane.  On my build list.  Watching with great interest.
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PB_guy
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« Reply #3 on: April 18, 2018, 08:50:30 PM »

I bought the Comet kit back about 1970 intending to build it as a C/L stunter with redesigned wing ribs and twin .049 Cox engines. But University got in the way. I still have the kit tucked away. I thought about building it for rubber scale, but something has to be done about not having the wing interfere with the rubber motor where it passes through the nacelles. I was thinking about making a former that incorporated the nacelle former and spar in a single piece of 1/64th ply reinforced with balsa supports. But I think that it would probably violate the 'kit scale' concept for contests.
ian
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Rich Moore
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« Reply #4 on: April 19, 2018, 02:40:51 AM »

The rubber through the spar is an interesting problem and will likely force a deviation from the plan involving some thin ply (judges won't see this). I hadn't noticed the 3/32" stringers, but there are only 4 on each side. It's the low number of stringers that bothered me but I decided that, if that was a problem to me, I should build Richard Falconers P38 from Aeromodeller plans and do something else for Kitscale.

The other question rattling around my head is whether I should contra-rotate the props or not.
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ZK-AUD
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« Reply #5 on: April 19, 2018, 03:29:38 AM »

I say yes to contra-rotating props.  As for the spar issue have a look  at the wing joiner on the Ted Evans Jaguar. It's on Outerzone and maybe here too.   That's what i would do but glue the inner and outer sections of the main spar between the ply leaves. 
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ZK-AUD
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« Reply #6 on: April 19, 2018, 03:33:42 AM »

Actually doesn't the rubber run a bit below the wing?  I'd do the jaguar thing anyway for more clearance
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billdennis747
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« Reply #7 on: April 19, 2018, 03:49:25 AM »

Or just drop the peg. I had this conversation with J O'D and he was surprised I worried about it. A  nice universal joint like a Gray hook takes care of the angle. Look at the sidethrust angles on some rubber duration models, especially those trimmed for right/left.
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« Reply #8 on: April 19, 2018, 04:27:18 AM »

Before you commit to the Comet P-38 take a quick look at the 1940 Guillows P-38:

http://www.hippocketaeronautics.com/hpa_plans/details.php?image_id=3454

The same issues with the rubber interfering with the wing are there but the infills on the sides of the boom around the wing might allow you to delete the trailing edge and the lower spar where they pass through the booms. I don't know if that's allowed in the kit scale rules but it might be worth a look.
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Rich Moore
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« Reply #9 on: April 19, 2018, 06:07:44 AM »

Thanks. The Jaguar thing is along the lines of what I was thinking. The motor clearance isn't disastrous as it is, but it would be good to gain some space in there. Just dropping the peg is a simple solution and might be enough, but I want a structural solution at the build stage to remove any risk of requiring any retrospective bodging.

I think I will try making up contra-rotating props along the lines of blades formed around a suitable diameter cylinder fixed using dowels in a balsa hub. Or this could be a good use of yoghurt pots.
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DHnut
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« Reply #10 on: April 19, 2018, 06:29:04 AM »

Rich,
        One of our club members George Fay has made many twins over a long period of time and has used contra rotating props almost exclusively to good effect and simplified trimming a lot. It also means directionally they are a lt more stable and often fly in either directionwithout any bother. There was a series of articles by Doug McHard in Aeromodeller on his experiences with trimming twins that wouls be worth looking at. Not sure of the dates but probably in the 1980's
Ricky 
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Rich Moore
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« Reply #11 on: April 19, 2018, 08:04:34 AM »

As I understand it - downthrust on the inside of the desired circle. Care required to tame the tendency for power stall. I expect there may be a fine balance to be had between initial power climb and a nice long cruise, so experimentation with motor section and length. It's the long motor I anticipate I'll need for a nice cruise that'll give me clearance problems.
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yagua
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« Reply #12 on: April 19, 2018, 08:11:30 AM »

...... but the infills on the sides of the boom around the wing might allow you to delete the trailing edge and the lower spar where they pass through the booms.
I was thinking something about that line. Extend down the upper part off formers 3 and 4 and move them a bit forward (or back) to fully contact ribs C and D and get rid off those spars and trailing edge  Huh
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Indoorflyer
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« Reply #13 on: April 19, 2018, 03:44:16 PM »

I think I will try making up contra-rotating props along the lines of blades formed around a suitable diameter cylinder fixed using dowels in a balsa hub. Or this could be a good use of yoghurt pots.

And rotation would be "blade tip outboard, over the top".  On full scale twins, this was apparently done, in part, to "counter" the circulation of the trailing wingtip vortices. Sounds good to me. I don't fly twins--models or full scale...
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Graham Banham
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« Reply #14 on: April 19, 2018, 06:56:32 PM »

Rich,

I wouldn’t worry about rubber clearance: as it stands looking at the plan it clears the wing underside anyway, and it’ll be a smooth wing underside surface the rubber comes into contact with, if at all. It’s not as if there’s any low point along the boom centrelines due to a dihedral break for example. If anything, do as Bill suggests and drop the peg a bit, but i dont think you have to re-engineer the wing spar set-up, especially as you’re intending it for indoor kit scale with comparatively little rubber.

However, you might want to sheet infill the bottom surface of the wing where it crosses the booms, to prevent any stray rubber knots splitting the wing covering and potentially cracking the lower spars at this point.

Richard C and i had a similar possible problem in reverse when we built versions of his Brewster Buccaneer/Bermuda plan: that’s almost mid wing, but if the rubber ever did slap the top surface of the wing where it passed through the fuselage, it never affected anything.

Incidentally, are all the spars on the wing bottom surface? Hmmmm........ wouldn’t leave it like that, regardless of kitscale fidelity to the plan rules...
« Last Edit: April 19, 2018, 07:08:59 PM by Graham Banham » Logged
Rich Moore
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« Reply #15 on: April 20, 2018, 09:57:46 AM »

Sheeting the underside of the wings where they pass through the booms sounds like an excellent idea - thanks for that. The upper spars run out only as far as the outside of the booms. This is a recipe for upward curving wings so will require the addition of upper spars.
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Oldsgt
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« Reply #16 on: April 20, 2018, 11:41:29 AM »

You got me curious about the Guillows P-38 so I pulled mine off the shelf (unbuilt) and had a look. According to the drawings the motor does pass under the the wing with good clearance. I like the idea of sheeting the wing area the motor passes thru so I'll try to remember to do that when I get around to building it. A piece of plastic tube may work as well depending on the weight.
Mike
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dieterperiperi
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« Reply #17 on: April 23, 2018, 04:36:13 PM »

I started the Comet P38 a while back (long term project ). I too found through line of sight that the rubber should clear under the wing. But might have been a mistake in my scaled down 21” ws build.

BTW. Flying Bulls will have their P38 at the Farborough Airshow this summer. Im going and hope to get some inspiration to finish mine

Good luck with build Rich
Keep us posted
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Re: Comet P38 Lightning - BUILD
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Rich Moore
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« Reply #18 on: February 04, 2019, 12:13:55 PM »

I have had a bit of a break from modelling whilst moving house, alongside running a business, alongside a full time job, alongside doing a degree. Anyway, I am getting the aeromodelling itch again, so hopefully I'll find 5 minutes to build this before the end of April. So much for being more ready than last year, but I anticipate cutting balsa within the next 4 weeks.

There, I've said it, now I'll have to do it...
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« Reply #19 on: February 04, 2019, 02:05:19 PM »

No wonder I don't get any sympathy for my "plight"!

Look forward to what you scratch out of that itch!
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Monz
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« Reply #20 on: February 04, 2019, 02:41:38 PM »

I have had a bit of a break from modelling whilst moving house, alongside running a business, alongside a full time job, alongside doing a degree. Anyway, I am getting the aeromodelling itch again, so hopefully I'll find 5 minutes to build this before the end of April. So much for being more ready than last year, but I anticipate cutting balsa within the next 4 weeks.

There, I've said it, now I'll have to do it...


It's alive! IT'S ALIIIIVVVVE!!!

There'll be about eight minutes of trimming before the Nats begins, so you should be good Wink
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Rich Moore
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« Reply #21 on: February 04, 2019, 03:31:26 PM »

8 minutes? Buckets of time then...
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charlieman
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« Reply #22 on: February 09, 2019, 01:29:41 PM »

Is there a link to British Kit Scale rules?

This American cousin, wants to know!

I traded off a Comet P-38 kit I held for years. Too many libetries taken withscale outlines, detail etc., for my taste.  Hwever, the prospect of such a twin has ALWAYS intrigued me.

TIA- charlie
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Squirrelnet
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« Reply #23 on: February 09, 2019, 01:36:57 PM »

Here's a link to the BMFA site

https://bmfa.org/Downloads/Contest-Rule-Books?EntryId=2802

You want the scale rule book, Indoor Kit Scale is 6.4.11 Outdoor Kit scale is different rules
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Pete Fardell
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« Reply #24 on: February 09, 2019, 02:20:32 PM »

8 minutes? Buckets of time then...
Easily enough. Probably only need about half that really.
(Massive model, twin rubber motors, confined space with 4 solid walls. Nothing to go wrong there at all.)
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