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designing a twin - 10/30/2002 6:46:10 PM   
Unstable



Posts: 1588
Joined: 2/7/2002
From: South Bound Brook, NJ, USA
Status: offline 		Im not getting much response in the Scratch build forum so I'll try here (woohoo a twin forum)

Designing my own twin about 50" wingspan, 10" cord, NACA 2412 airfoil. gonna try and power it with 2 .25s (want to keep cost down) and a twin tail

I got a basic design for the wing (spacing for the ribs drawn out etc,) but I'm not sure on a few things like how to mount the nacelles to the wing and how to do the tail (and size I need)

anyone have any good pics or plans showing the mounting of a nacelles?

any suggestions on the tail?

here is a quick render of kinda what I'm thinking of

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Unstable Boy
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designing a twin - 10/31/2002 3:11:07 AM   
BMatthews



Posts: 8945
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		Go bigger, 50 inches is normal for ONE 25..... unless you want a model that qualifies for the prop speed forum.....

I'd say even for normal day to day designing you'd want 750 to 800 sq in in a 60 to 65 inch span model for 2 x 25's. This would still be very sporty but would carry the extra load of a twin and the size would have more presence in the air.


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Proudly wasting balsa since 1965.

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Re: designing a twin - 10/31/2002 4:38:00 AM   
DICKEYBIRD



Posts: 2668
Joined: 4/8/2002
From: Collierville, TN, USA
Status: offline 		[QUOTE]Originally posted by Unstable
but I'm not sure on a few things like how to mount the nacelles to the wing and how to do the tail (and size I need)
[/QUOTE]
Hi guy!

I don't know 'bout a guy that would drive from New Jersey all the way to Memphis, TN just to fly all day in the friggin' rain. Must be some kinda unstable feller!

Milton Dickey here, another unstable feller that stood out in the rain judging youse guys. We enjoyed having you!

Now back to the original question. I designed my "Twin Lizzie 2" with the nacelles configured as "mini fuselages" so they could be easily removed for fuel tank and throttle servo access. I just built hardpoints into the wing for the mount screws with standard ply plates and dowels in the front.

The h/stab has an Eppler 168 airfoil which is a 12.5% thick symmetrical so that I had more room to build in the 90 deg. bellcrank and pushrods to drive the twin rudders.

I've attached a pix that hopefully shows what I'm talking about. You probably don't need all the little tiny sticks and elaborate structure I used....my therapist says I must have "Insufficient Fiddlesticks Syndrome" since the voices in my head keep saying over & over: "You can NEVER have too many little sticks"

Oh, BMatthews is spot on, you need to go bigger! The 'Lizzie has 408 sq. in., almost 53 in. span with 2 .049's and flies great!

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designing a twin - 10/31/2002 4:38:31 AM   
DICKEYBIRD



Posts: 2668
Joined: 4/8/2002
From: Collierville, TN, USA
Status: offline 		'nuther one

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designing a twin - 10/31/2002 4:39:09 AM   
DICKEYBIRD



Posts: 2668
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From: Collierville, TN, USA
Status: offline 		'nuther

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Re: Re: designing a twin - 10/31/2002 8:31:14 AM   
BMatthews



Posts: 8945
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		[QUOTE]Originally posted by DICKEYBIRD

..........I designed my "Twin Lizzie 2".............my therapist says I must have "Insufficient Fiddlesticks Syndrome" since the voices in my head keep saying over & over: "You can NEVER have too many little sticks"
[/QUOTE]


I've seen pics of that somewhere on the web. A fine job you did of it too. Kieth Laumer would be proud to shake your hand I suspect.

As for the stick'itis it's a common degenerative condition amongst us free flighters or those that are free flighters at heart but are in denial......

PS: I almost forgot, getting back to the engine pods. The big thing is to be sure your center section can take the vibration and loads. Top and bottom spars with vertical grain shear webbing and a former glued up to the rear face is a good rear guide. And a spruce or plywood vertical web glued up to the back side of the leading edge that runs from nacelle to nacelle right through the center can also incorporate the nacelle formers that are should be there. That gives you two formers to build from and ensures the center section has some hardwood to take the stress of the engine drive and vibration. Finish up the D box and you're there. To add some additional stifness I'd also web between the spars and the leading edges using flat webs running from the spars to the back side of that leading edge hardwood doubler. These webs would use fore and aft oriented grain. Just like spar shear webs but lying flat and locking the leading edge and the two spars into a box. In fact you could use two of them in a shallow V arrangement. One from the upper spar to the leading edge doubler and the other from the lower spar to the leading edge doubler. These, along with the spar shear webs, would lock that whole center section into a very strong triangle shaped D box. The sheeting to shape the airfoil would then just be icing on the cake.

< Message edited by BMatthews -- Oct 31 2002 3:44AM >


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Bruce-
Proudly wasting balsa since 1965.

Free Flighters go that extra mile........

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designing a twin - 10/31/2002 6:53:03 PM   
Unstable



Posts: 1588
Joined: 2/7/2002
From: South Bound Brook, NJ, USA
Status: offline 		[QUOTE]Originally posted by BMatthews
Go bigger, 50 inches is normal for ONE 25..... unless you want a model that qualifies for the prop speed forum.....[/QUOTE]
ok, bigger it is (time to get back on autocad and slap on a few more ribs)

[QUOTE]Originally posted by DICKEYBIRD
I don't know 'bout a guy that would drive from New Jersey all the way to Memphis, TN just to fly all day in the friggin' rain. Must be some kinda unstable feller! [/QUOTE]


[QUOTE]Originally posted by BMatthews
Top and bottom spars with vertical grain shear webbing and a former glued up to the rear face is a good rear guide. And a spruce or plywood vertical web glued up to the back side of the leading edge that runs from nacelle to nacelle right through the center can also incorporate the nacelle formers that are should be there. That gives you two formers to build from and ensures the center section has some hardwood to take the stress of the engine drive and vibration. Finish up the D box and you're there. To add some additional stifness I'd also web between the spars and the leading edges using flat webs running from the spars to the back side of that leading edge hardwood doubler. These webs would use fore and aft oriented grain. Just like spar shear webs but lying flat and locking the leading edge and the two spars into a box. In fact you could use two of them in a shallow V arrangement. One from the upper spar to the leading edge doubler and the other from the lower spar to the leading edge doubler. These, along with the spar shear webs, would lock that whole center section into a very strong triangle shaped D box. The sheeting to shape the airfoil would then just be icing on the cake.
[/QUOTE]
so basically you are saying "add wood till nothing moves"? (im going to have to do a bunch of scetching now to figure how to do all that )

about what size spars should I use, right now the plans are drawn up with top and bottom 1/8x1/4 hardwood along the length and doubled up from nacelle to nacelle is this enough?(saw a .40 size bipe with that so i figured it would be a good start point)

anyone have any plans for anything like this that I can look at? (Im really a visual type person, I grasp things better when im looking right at them)

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designing a twin - 10/31/2002 11:12:12 PM   
BMatthews



Posts: 8945
Joined: 10/4/2002
From: Burnaby, BC, CANADA
Status: offline 		A lot will depend on how thick your wing is. I have an electric glider that uses the same 1/4x1/8 spruce spars with about 1 1/8 spacing top to bottom and front and rear 1/16 webs. I hit a glitch or a wind shear in turbulent conditions one day and it came straight down for about 200 feet before control was restored and then it did a heart wrenching and very tight 1/2 loop to straight up before my mind could catch up. I've flown that model on and off for 5 more years with no problem. AUW is 56 oz. so you can see how strong this is. But if the wing was thinner then you'll want wider spars. Oh, and be sure the spruce is very straight grained I've seen then with terrible grain runout and it'll fail in a heartbeat when stressed.


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Proudly wasting balsa since 1965.

Free Flighters go that extra mile........

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designing a twin - 11/2/2002 12:31:47 AM   
Unstable



Posts: 1588
Joined: 2/7/2002
From: South Bound Brook, NJ, USA
Status: offline 		maybe I'll up it to 1/4" square just to be safe

I dont think it will add too much weight and I would rather be safe and heavy then light and in peices besides I can always trim it out on latter versions (I plan on doing several of these to fine tune it before "final" version is done.)

I still havent started on the fuse yet.. I probably will just make a "stick" for the first one until i get it flying close to what i want. then I'll start one makeing it pretty

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designing a twin - 11/2/2002 1:48:14 AM   
MinnFlyer



Posts: 19610
Joined: 4/22/2002
From: Willmar, MN, USA
Status: offline 		That nacelle looks fine for an .049, but I'd want something stronger for anything bigger. Here's what I would do...

Make two ribs like this for each wing out of lite ply. The outsides could later be sheeted with 3/32 or 1/8 balsa, then build the firewall, top, bottom, sand to shape, and viola!

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designing a twin - 11/2/2002 2:35:51 AM   
DICKEYBIRD



Posts: 2668
Joined: 4/8/2002
From: Collierville, TN, USA
Status: offline 		I agree with you on the 1/4" (HARD balsa or spruce) spars. I would shear web between the spars with 1/4" endgrain balsa in the center 2 bays, transition to 3/16", then 1/8" then 3/32 progressively out to about 2/3 span. If you pick some good medium balsa for your top & bottom D-Tube sheeting and build in decent hard points to mount your nacelles, I don't think you'll need to add any additional strengthening. That D-tube will have great torsional strength and resistance to loads and vibes from your 2 engines. Picked an airfoil yet?

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designing a twin - 11/20/2002 7:09:30 PM   
Unstable



Posts: 1588
Joined: 2/7/2002
From: South Bound Brook, NJ, USA
Status: offline 		wow... i just found somethign that cuts a ton of my work

[URL=http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=1788464159&ssPageName=ADME:B:EOAB:US:6]http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=1788464159&ssPageName=ADME:B:EOAB:US:6[/URL]

all I have to do now is take the basic design and bash out a twin tail.

its good to find that someone else has done most of the work for you

although I will end up designing a full new plane I now have something that I can go off of for testing and structural design.

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designing a twin - 11/20/2002 10:57:30 PM   
Robby



Posts: 2071
Joined: 2/8/2002
From: SheCarGo, Sillynoise, IL, USA
Status: offline 		Unstable
That is actually a kick ass plane... How do I know... Easy.. Check
this out:
http://www.****/forums/showthread.php?threadid=3495

Will take a few to load, big pics..
The kit calls for .25 motors.. Last year I had .46's and this year I
went with .40 LA's...
The plane is VERY strong.. Mine has been through the wringer
a few times.. On mine it has cables to motors for throttle from a
single servo.. I would suggest going one servo per motor.. Hitec
HS81 is a good one for that..

Robby

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designing a twin - 11/20/2002 11:40:50 PM   
Unstable



Posts: 1588
Joined: 2/7/2002
From: South Bound Brook, NJ, USA
Status: offline 		glad to see I made a good choice.

First I'm going to replicate all the parts so I can build more than one and use bits for my scratch build. (will probably not use any of the original wood)

I should be getting the kit soon... yay

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Unstable Boy

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