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Old Aug 29, 2009, 02:57 PM
Don't worry about it!
j.m.'s Avatar
Raleigh NC
Joined May 2008
1,368 Posts
Idea
Composite D Box Design

I've always liked the look of a composite d ox wing with a thin carbon trailing edge. I have never flown or built anything like this, but from what I have read they rival bagged foam ships.

I have been searching over the internet for construction methods, and have only found dated techniques. I round a few articles in the late 1995-early 1996 Quiet Flight Int'l magazines on spar and rib construction for a composite d box wing, as well as multiple methods for making the actual d box. These methods were nearly identical to the methods used here>
http://www.gregorie.org/freeflight/cf_wing/cf_wing.html

These methods certainly appear to make a nice wing, but I would like to put some ideas forth that use more 'conventional' construction materials and techniques that should result in a structure with the same or more strength, and considerably less weight.

I have noticed that in both articles above, the balsa ribs were simply butt jointed to the spar with little reinforcement. I would think this would be a weak point in the wing if it were to be winch launched because the wing area supported by the ribs is far greater than the d box, and the load from the ribs would need to be transfered to the spar throuhg the weak butt joint.

My idea is to use a vertical grain foam (hi load 60, spyder foam) with carbon spar caps and slots in the shear webs for the ribs to be continuous from the LE to TE. This way the butt joint would be eliminated. It the spar assembly was then wrapped in kevlar tow, I am certain it would be lighter than a conventional vert grain balsa shear web spar, and most likely stronger too. (this spar design is basically identical to Mark Drela's Supra spar, except without the composite sock.)

I would like to construct a 2m RES wing with 1/8" contest balsa ribs spaced every 4" with a thin carbon trailing edge.

Any thoughts or comments on these methods/ ideas?

Jon McLendon
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Old Aug 29, 2009, 06:58 PM
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USA, GA, Marietta
Joined Oct 2000
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John,

Last winter I built a wing close to what you are talking about. I used foam core carbon capped spars wrapped with fiberglass. I too left the foam about a quarter inch long and epoxied the ribs into the slot. Wing turned out great, but I'll warn you, it is a lot of work.


Ric
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Old Aug 29, 2009, 11:21 PM
Don't worry about it!
j.m.'s Avatar
Raleigh NC
Joined May 2008
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Yeah, I was expecting quite a bit. Good thing I like to put my time into things.

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Old Sep 01, 2009, 11:42 PM
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USA, CA, Selma
Joined Sep 2004
394 Posts
As far as your concern about the weakness of the joint between the ribs and the spar. The wing derives very little strength by the butt jointed ribs. The majority of the strength comes from the fact that the D-Box shell overlaps the spar caps and the thin carbon rib cap strips overlap both the trailing edge and the spar from the rear. Sometimes they are put into cutouts in the leading edge shell and sit directly on the spar caps or they may be put over on top of the shells. The balsa in the ribs both in front of the spar and behind is mostly just keep the carbon caps apart to maintain the airfoil shape.

John
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Old Sep 02, 2009, 02:38 AM
working to the closest cm
jirvin_4505's Avatar
brisbane australia
Joined Nov 2002
2,127 Posts
d box details

Quote:
Originally Posted by RicVaughn
John,

Last winter I built a wing close to what you are talking about. I used foam core carbon capped spars wrapped with fiberglass. I too left the foam about a quarter inch long and epoxied the ribs into the slot. Wing turned out great, but I'll warn you, it is a lot of work.


Ric
Ric this sounds interesting.

Do you have a more detailed description or even some photos?

cheers jeff
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