I've noted the different weights of balsa that I find in my balsa orders of regular non-contest balsa. I am scratch building a small profile plane at the moment and for the fuselage, I've tried to use the heavier balsa pieces due to their strength. I did note that the lighter pieces would not bode well in place of this heavier balsa.

My question is, do you set aside heavier and lighter balsa for different builds? I could see this lighter stuff working well for a smaller 1/2A build but don't know where you would want to use this really lightweight balsa?

Appreciate any input.

somegeek

I build enough Peanuts to make having some good really light balsa around worth while. But then, I'm in love with little sticks and glue.

Yes, balsa varies widely in density. And balsa strength is highly linked to the density. In my own building each part is made from sheet stock that is appropriate in weight and strength for the job it does.

Although it is not good to rely on the balsa density for strength on something like a profile fuselage. I'd strongly recomend using carbon strips on each side or better yet useing side frames like the depron foamies do in order to use good design rather than rely on strong, and heavy, materials.

Google for "balsa density chart" and you'll get a bunch of hits for charts that can help you determine your sheet stock densities.

Great info - thanks for this.

I do have some 1/4" carbon fiber ribbon... think I will epoxy strips down each side of the fuselage.

Thanks,
somegeek

[quote]ORIGINAL: somegeek
My question is, do you set aside heavier and lighter balsa for different builds? I could see this lighter stuff working well for a smaller 1/2A build but don't know where you would want to use this really lightweight balsa?

Absolutely!-balsa selection is the basis for effective sratch building. Not only by selection of weight, but also by cut.....straight, random or quarter grain-or as it is expressed in the USA.....A-grain, B-grain and C-grain stock. All my models are built by first selecting the wood for both weight and cut. To that end-at any one time I probably have several thousand dollars of balsa wood in stock-ranging from 4-5lb cu ft (v.light) up to 10-12 lb (heavy).
Given that I am largely a competition F/F flier, with only an occasional foray into radio-I probably spend more time and effort in balsa selection than the average R/C flier. Ditto for C/L competition flyers-especially those flying team race-where the wing wood selection will determine whether you end up with a winner or an also ran.

To give some concrete examples, among the many contest classes I fly are P-30, Coupe d'Hiver, and Hangar Rat. The first two have minimum airframe weights of 40grams and 70 grams (less rubber) respectively. poor or indifferent wood selection can result in a model 50% overweight-and a performance not much better than a flying brick. Hangar Rat is a very simple indoor class-the first one i built-out of the scrap box-weighed 10.2g-the second, doing some reasonably careful wood selection-but not using any ndoor grade wood-was 6.5g-thats a nearly 40% weight difference-simply on wood selection. The difference is a model that flies on 3/32" rubber rather than 1/8"-and takes a lot more turns for longer flight times.

Yes wood selection is important-and so is glue-and covering material choice for weight control.

The Indoor fliers go a step further-and select for stiffness as well as density-and in the tiny tiny %s of balsa suitable for Indoor wood-it costs more than gold on a weight basis!

'ffkiwi'

With building a smaller 1/2A kit for the first time and being mindful of the adage that Raindave stated (don't find one way to save an ounce, find twenty ways to save a gram) I've given a lot more thought to shedding grams on my last and current build then I have before on other builds. Also, comparing balsa weights and seeing two 1/4" square stock pieces of equal length, side by side with one weighing double that of the other, my eyes were opened.

Thanks for the info!

somegeek

Here is one I cobbled up on my lunch hour.
Now, will someone check my math?
Could be helpful for picking sheets at the LHS.
You will need a good scale though!
Dave
Show your work for extra credit:
There's 1728 cubic inches per foot, 1 lb/cu foot = .00926 oz/cubic inch.

And now I see I listed it as pounds per foot SQUARED instead of CUBED, DOH!
The numbers are still valid though, just a little off on the description.
I KNEW I'd regret posting this.

Some fast and dirty calculator work on the 1/32 x 2 shows it to be good. I guess we can trust you and your 'puter....

OK, now give us a version where the oz are converted to grams, smartypants....

(meanwhile Bruce rushes to print the chart and get it ready for laminating into a wallet card....)

Very light contest grade in your 1/8", 3/16" & 1/4" thicknesses is great for 1/2A glow projects. You can put more shape and style into a model by selecting thicker wood and airfoiling the surfaces.
1/16" contest grade is too fragile for me.

Grams? Maybe you Canadians can think in grams, but south of the 38th it's still yards, pints and slugs.
A gram is something you dunk in milk, or make a pie crust out of!
Dave

Ye grams , its much easier to calculate lots of 10 rather than ODD No's , but I guess some are slow to see the light

If you spent as many hours as I did trying to learn the English system....
my eyes are calibrated to see 1/8ths of an inch of difference, millimeters would take lots of retraining.

Weigh everything now, never used to. Really helps keep the weight down, no question.

Have a digital postal scale, it can give ozs, and a button push converts to grams.

Old shop in H.S. never even brought up metric back then.

I switch to grams when using my kitchen scale since that's more granular when weighing light items.

Oh... I still think in terms of pints. I actually think a lot more clearly with a pint in my hand.... well it seems more clear to me at least.....

If you share the spreadsheet file out I could probably multiply all the columns by the grams factor or set up a set of shadow columns (not a big spreadsheet brain so I'm not sure if that's the right term) over to the right hand side that uses the oz columns and just multiplies the numbers by the conversion. Then we'd have both.

True words.
A good scale is indispensable if you want to grade your balsa, and build stronger, lighter planes. Nowadays there are incredibly cheap and accurate electronic scales that will weigh fractions of grams.

It pays off to play with the balsa bits in your hands, and bend them in various directions to get a feel for the stiffness. It’s easy to compare the stiffness of sticks and sheets of same size by bending in your hands. Sheet stiffness can be measured by holding the sheet on end on a scale and pushing down until the sheet buckles. Once it buckles the reading will be more or less constant and it will give an indication of the stiffness of that sheet.

The density of balsa can vary wildly not just between trees, but also within a block, or a sheet.
A strong lamp is helpful to check for hard and soft streaks in balsa sheets.
The photo below shows a 1/16th sheet with a calculated density of 7 lb/cu.ft. The more translucent areas are probably 5-6 lb, and the darkest streaks on the right side could well be over 12 lb.

It´s entirely possible to take an 8 lb, 1/8” x 4” x 36” sheet , slice up 30 sticks, and get sticks that vary from 6 lb to 12 lb with not a single one at 8 lb.

Think about it folks-it would be not just odd-but downright suspicious if a sheet of wood stripped DID give strips all the same density. Remember that sheet came from a block, and was cut either tangentially, radially or at an angle through the annual growth rings. For the density to be identical through the sheet, the growth rate would have to be constant regardless of the year, the time of year,the actual weather and season, and regardless of the orientation of the tree. That's not just a big ask-that's a biological impossibility!

'ffkiwi' (wearing his scientist's hat!)

I've noticed hard spots in 1/16" sheeting... didn't think about stripping it and sorting by weight. Good idea.

somegeek