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Fuselage Box vs Keel (Read 181 times)
Reply #8 - Oct 24th, 2018 at 10:12am

staubkorb   Offline
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Another thing to keep in mind is to make sure that the same edge of the material is in the same plane as the other stringers.  A sheet of "A" grain balsa has "C" grain on the thin edge - different bending characteristics.
 

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Reply #7 - Oct 18th, 2018 at 9:00pm

alfakilo   Online
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Good idea! Never thought of that but it makes sense!
 
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Reply #6 - Oct 18th, 2018 at 5:42pm

pb_guy   Offline
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One way to make 'banana' fuselages less likely is to strip your stringers/longerons from a sheet of balsa and keep the ends identified. Use the same length and orientation from separate strips that are adjoining. It is more likely that they have the same density and bending characteristics this way.
ian
 
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Reply #5 - Oct 18th, 2018 at 4:45pm

alfakilo   Online
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Excellent info! Thanks everyone for your input. One of these days I'll have to try the box method again now that I've reacquired a taste for bananas!!
 
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Reply #4 - Oct 16th, 2018 at 10:50am

MKelly   Offline
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pb_guy wrote on Oct 16th, 2018 at 12:10am:
One advantage of the box structure is that it can be lighter if you use lighter wood for the formers, since all the strength is built into the box. With the 'keel' method, you seem to use heavier formers than are really necessary for strength to keep from breaking them while adding stringers.
YMMV
ian


I've been trying to put my thoughts together on this question and I think Ian captured a good bit of them in his post above.  The T-28 has the former-on-box fuselage construction, and I really like the way it came out.  The box lets you keep the fuselage interior open all the way to the corners, which gives room for lots of wound-up rubber from a long motor without getting things hung up on structure.  The former segments can all have the grain long-wise, which makes the fuselage much more crush-resistant (and as Ian said I think you can use much thinner and lighter wood for the formers).  I also think the combination of the inner box and outer stringers makes each quadrant of the fuselage something of a box beam, more rigid than curved stringers over half-shell formers.

However, the box fuselage requires more thought during design and more steps during construction.  The designer and builder have to be very careful to get the box accurately sized or the formers won't come together properly at the corners, and if the builder likes to use glue liberally it may come out heavier than a half-shell fuselage because you've got more and longer glue joints.

I've got a couple of aircraft in the back of my mind for design-build candidates, and based on how the T-28 went together I'm leaning towards box-former construction for them.  That said, there are a lot of plans and kits on my bucket list that use half-shell construction...

Mike
 
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Reply #3 - Oct 16th, 2018 at 8:42am

New Builder   Online
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My thought is that the keel and former method is useful with several aircraft designs, mostly the WWII stuff and the air racers with round or nearly round fuselages. I built a fixture to accommodate these barrel shaped fuselages and it works very well to hold the fuselage in place while adding the stringers. Not all the stringers can be added as the crossing members used for clamping blocks for the formers are in the way but the last few stringers can be added after the fuselage is out of the fixture. The other benefit is the reduction in parts. The entire box is now gone and all the parts that were to be added are now collected into their respective units and in place at the same time. As to weight, I build from plans and strip my own wood so can choose what I want for the keel and formers. On the other side, the box does add a measure of strength and torsion resistance. With careful selection of wood either could come out about the same weight.
Mike
 

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Reply #2 - Oct 16th, 2018 at 12:10am

pb_guy   Offline
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There is nothing that prevents building a basic box structure and then adding formers to create the body shape. That is the way a lot of Joe Ott's models were done - using the 'Ott-O-Matic' formers.
The main advantage of the keel and former method is that it builds the fuselage in one step instead of two.
One advantage of the box structure is that it can be lighter if you use lighter wood for the formers, since all the strength is built into the box. With the 'keel' method, you seem to use heavier formers than are really necessary for strength to keep from breaking them while adding stringers.
YMMV
ian
 
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Reply #1 - Oct 15th, 2018 at 7:15pm

Sky9pilot   Offline
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As far as I've been able to tell, it boils down to one's preference in building or designing.  How about it fellow modelers/designers?
 

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Oct 15th, 2018 at 5:59pm

alfakilo   Online
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Is there an advantage (structural or otherwise) to using the box building method rather than a keel/former method?
 
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