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What weight balsa should I use - 11/7/2012 4:46 PM   
Scratchie



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In addition to lightening holes, I’m thinking about replacing some of the factory-supplied balsa in my Royal B-17 kit with lighter balsa to achieve a light wing loading, less than 30 oz/sf. What weights are typically shipped from the factory, and more importantly, what weight balsa should I use for fuselage bulkheads, ribs and sheeting? The plane will be fully glassed using ¾ oz cloth and water-based poly.
Thanks


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RE: What weight balsa should I use - 11/8/2012 8:34 PM   
BMatthews



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Status: offline 		First thing to do is buy a cheap digital weight scale so you can measure the weights of the kit wood. The scale replaces your "feel" that you don't have yet for the weigth of the woods. Over time as you work with the different densities of balsa you'll get better at guaging the density by feel and appearance.

What weight of wood depends SO much on where the part is located and what the shape is that it's tough to say use "this" for "that". But generally on something like this which is fully sheeted and the fuselage is planked I'd say you want to go with 6 to 8 lb density wood for the sheeting and planking. And preferably more towards the 6lb side of things. 6 lbs/cu foot is pretty much the high side of what is considered to be Contest grade wood.

For fuselage bulkheads around the wing area I'd say go reasonably hard. These are the parts that connect the wing spars to the fuselage so you want to spread out the localized point forces and provide a good strength path for the wing mounting points. Also the harder grades for these heavier load bearing components will produce stronger glue joints.

But as you go towards the nose and tail the loads are considerably less so you can use lighter stock.

Tail surfaces and outer wing panel components can and should be built from light stock as well. And depending on the design some harder and smaller reinforcing parts such as leading edge joiners or mid span hinge spar doublers can provide good resistance to damage at the more highly loaded center line joints.

There is more to picking wood than just the density though. Even and "flat" non stressed grain goes a long way to helping produce flat and stable wings that will be warp resistant. And for those parts that are going to take some of the span loadings or other long slender load bearing components it's often not a bad idea to cut out a small sample and break the wood to see how much long fiber content the sheet stock has. The more splinter'y and fibrous the break the stronger the wood. The softer grades of sheet stock can often end up snapping off clean almost like breaking glass. Such wood is best used for smaller parts that won't need to withstand a lot of structural loads. Look for the more fibrous wood for longer strips that will be loaded in flight or during ground handling.

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RE: What weight balsa should I use - 11/8/2012 9:03 PM   
Scratchie



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I'm fairly conservative and was going to use the kit-supplied plywood for fuse bulkheads in the wing area, and kit-supplied wood for wing spars/joiners.  Since the plane will be fully glassed and flown in a scale-like manner (electric), should I keep the wing/fuse/tail surface sheeting the same thickness as in the kit and just replace the original with lighter wood, or can I go thinner?
In general, where are the real weight savings other than balsa vs. plywood? Does putting a medium amount of holes in ribs and things make that much of a difference? As you and tell, I always have just built kits according to the plans without too much modification (except for scale features). I generally keep a very close eye on weight, and for this project consider a 1-2 oz. weight savings per feature significant. In fact,  I may take the aluminum-supplied cowls (over 2 oz. each) and try use them to make fiberglass molds.


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RE: What weight balsa should I use - 11/8/2012 10:03 PM   
TFF


 

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Status: offline 		Instead of getting lighter wood for the bulkheads cut 1/2 of it off the insides. Any ply doubler, swiss cheese it. Contest 1/16 sheeting will work for the wings and tail. Any carved part if you cant get it to 1/8 inch thick or thinner, make glass parts, of a single layer of 1.5 cloth, and I hate to say it but the cowls too. I built a Royal Spit Jr years ago when I was a kid; it was not all that heavy but with no retracts and no heavy details. I have never seen a Royal B17 fly but I have seen a Stafford B24 fly. It could be slowed down in the air for scale flight but T/O and landing was pretty hot to make sure it had speed. It had every scale do-dad though.

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RE: What weight balsa should I use - 11/9/2012 8:21 AM   
Scratchie



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I'm omitting working bomb bay doors, rotating turrets and any other bells or whistles. I’ll only have retracts if I hit an acceptable wing loading (< 30 oz.).  I want a nice flyer, and this build is a precursor to a much larger plane to be done later.  Anyway, I have a lot of full-scale fiberglass experience on/in wood boats, so I’m thinking I can make my own cowls and use the aluminum ones as molds. Use 1.5 oz. cloth with resin? How do you keep the fiberglass from sticking to the mold, Vaseline or something like that? Also, would you do more than one layer of fiberglass for strength?


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RE: What weight balsa should I use - 11/12/2012 12:08 AM   
BMatthews



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Status: offline 		I've never seen the plans for the Royal kit so I can't tell you if it would be suitable to change the wood thickness. I can't even tell you if it's worth buying lighter wood since I don't know what desity of wood is in your kit. I'm afraid that's where you and your judgement come in.

I can tell you that putting all the "flying" kit contents onto a scale will provide a hint as to the final weight. Weigh the kit contents that will go into the model and then add on the motors, pack and a fudge factor for the final finishing. It'll give you a pretty good idea of the final wing loading.

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RE: What weight balsa should I use - 11/12/2012 12:11 AM   
Scratchie



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Status: offline 		I'm just getting ready to do that.  It seems that if I can make replacements to the aluminum cowls I'll save 4-6 oz.

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RE: What weight balsa should I use - 11/13/2012 3:34 AM   
BMatthews



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Status: offline 		And that's how it starts. Save a few ounces here, a few there and before you know it you've got a model which comes in on the low side of the acceptable wing loading range for that size range. And starting out that way always stacks the deck in favour of a successful and fun to fly model.

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RE: What weight balsa should I use - 11/13/2012 5:08 AM   
Scratchie



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Status: offline 		That's the plan - no white knuckles!

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RE: What weight balsa should I use - 11/13/2012 3:27 PM   
sensei



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Status: offline 		If you are really serious about building a light airplane then you could learn allot from this thread. http://www.rcuniverse.com/forum/m_5153525/tm.htm

Bob

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RE: What weight balsa should I use - 11/13/2012 4:57 PM   
Scratchie



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Status: offline 		Thanks, I'm getting close to commiting to this one, I did an inventory of parts and started to plan out where lightening holes will be placed.  Once I'm sure I can make good fiberglass cowls I'll probably start the project. 

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RE: What weight balsa should I use - 11/13/2012 6:44 PM   
fritzke


 

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Status: offline 		Here is a handy density chart page:
http://www.rfcafe.com/references/general/balsa-weight-sheet.htm
Dave

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RE: What weight balsa should I use - 11/14/2012 4:30 AM   
BMatthews



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Status: offline 		For an electric remember that a lot of the stuff can be cut down drastically. Again I have not seen the plans for the Royal kit but from what I've read they tend to be "buckets O' wood". So cutting down or tapering many of the parts and adapting ribs and formers to suit could save considerable weight.

On their own lightening holes do not do as much good as simply using lighter density wood. And you can't cut lightening holes in leading and trailing edges which are simply bigger than needed. However you CAN use reduced size stock and shim ribs or make new ones.

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