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Old Mar 10, 2013, 01:54 AM
KNS
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Balsa weight/density

I am - as it so happens - planning ahead the next model to build, while others are still on the launchway.Inevitably, this "has" to be a Valkyrie (Goldberg). I am amazed at his final weight below 5 pounds for a FF design of 10 ft span. Which makes me wonder if what we call "soft","light","medium","hard","very hard" is actually something different nowadays.

Can anyone please inform me on what - in 1938 (year of the publication of the Valkyrie plan) - the different balsa densities actually referred to? Anything pre-war that was different with balsa wood by then compared to todays wood?

Imperial or SI measures welcome.

Here is what I usually find for Balsa sold presently in Switzerland, but the ranges indicated are large and the number of annual growth rings (as a measure of wood quality) is never indicated.

Ultra-Soft below 70g/dm3
soft 70-125 g/dm3
medium 125-175 g/dm3
hard 175-250 g/dm3
extra hard above 250g/dm3
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Old Mar 10, 2013, 04:17 AM
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OK, here's some info from a "Model Aircraft" (magazine) plans catalogue, from the mid-fifties.
You can take the range shown as being from "very soft" to "very hard".
Note that the fractions shown in the body of the table are in ounces.
I think it's probably true that our idea of "soft", "medium" or "hard" may have shifted a bit over the years, but not excessively, and in any case, it will always be somewhat subjective, depending on your modelling culture, etc.
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Old Mar 10, 2013, 11:59 AM
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With a model that size and wing area I wouldn't really worry too much about a little extra weight. At 5 lbs it would not be happy in the wind. Beside, going with contest balsa, it will probably triple the balsa cost.
Look at how sail planes fly. They add ballast when the wind is up, or they want to go a little faster. I had a small model that flew so,so, or ok. I wanted to use it to carry a camera, another 4oz. I wasn't sure it was going to carry the extra weight up front, but carry it, it did. Surprise for me, the model actually flew better with the added weight.
John
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Old Mar 10, 2013, 02:26 PM
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I'm pretty sure the numbers you are using wouldn't have been much different in 1938. It's quite rare to find a usable piece of wood at less than 4 lbs (64gm/dm3)per cubic foot or so. (I've only ever found one piece like that.) I suspect that was true in 1938 as well.

The Complete Model Aircraft Manual, by Edwin Hamilton, says that balsa averages 6 lbs per cubic foot, so perhaps the light stuff was easier to find back then. The edition I have is from 1938.

I know the design has a gazillion pieces, but have you tried estimating the volume of wood in it? That might give you a clue as to density of balsa.

I suspect that the weight and the number of growth rings are closely correlated.

When I look at this picture (from elsewhere in RC Groups) of an uncovered Valkyrie, it looks like mostly air! So maybe 5 lbs isn't so surprising, particularly with no radio on board. It's not an especially light weight for an RC glider of the same span, though of course those have less wing area and skinnier fuselages.

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Old Mar 11, 2013, 12:19 AM
KNS
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Thank you for all your replies - much appreciated! This was exactly the type of info I was looking for, and thanks a lot also for the encouragement. There are only so many planes one can build, and this one just looks right to be among them .
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Old Mar 11, 2013, 07:57 PM
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If you can weigh your balsa sheets then here is a chart (stolen from the Free Flight ML) which will tell you the density of your balsa.

http://www.indoorduration.com/BalsaCalc2.asp
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Old Mar 15, 2013, 10:37 PM
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Density is part of the story. Stiffness is strongly correlated with density, but there can be a large amount of variation, even in the same density of balsa. So it pays to flex the pieces a little if you want to use the stiffer stuff. I suspect strength is strongly correlated with stiffness too, but you don't have to break the stuff to flex it.
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Old Mar 31, 2013, 05:59 PM
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Refer to Bob Meuser on Balsa

Most of the "growth rings" we see in balsa are not that at all but rather they are "medullary rays". The annual rings in balsa must be spaced 1-2 inches apart as balsa is a very fast growing tree. For a more complete discussion about this wonderful hardwood species see Bob Meuser's article in Model Aviation of June 1976. On the availability of light wood there was a time when good light wood was very hard to find. It seems LNG tanker building was going at a fevered pace and light balsa was preferred for those ships. Needless to say the model trade was lower on the list than shipbuilders.
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Old Apr 10, 2013, 04:39 AM
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Interesting thread - as a dedicated balsa-builder I'm always scouring the model shops for good quality stock. Is it just me or do others find that you just simply can't get it these days, or very rarely anyway.

Maybe it's just my aging memory, but I seem to recall that back in the 70's the hobby shops always had great quality wood, of all grades and densities, whereas now they A) either don't even sell wood (or very small range) or B) what wood they do have is always on the heavy side, with none of that elusive 'light but firm' grade I've always looked for.

I can't imagine the balsa trees have changed over the last 40 years...or have they? Is this current global ecology producing a different standard of wood? Food for thought.

..that Valkyrie is stunning! It looks like the wing has been woven, rather than built. Transparent covering only please!

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Harry
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Old Apr 10, 2013, 05:09 AM
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Quote:
Originally Posted by Harry Rule View Post
Maybe it's just my aging memory, but I seem to recall that back in the 70's the hobby shops always had great quality wood, of all grades and densities, whereas now they A) either don't even sell wood (or very small range) or B) what wood they do have is always on the heavy side, with none of that elusive 'light but firm' grade I've always looked for.
Regards
Harry
I don't think it is your memory Harry. Apart from the fact that there are now very few model shops worthy of the name in the UK - i.e. shops that cater for the model BUILDER as opposed to the model BUYER - their wood stocks are pretty pathetic. Back in the day I used to do a regular circuit of several "local" shops and trawl through the wood using a set of scales, buying what was good when it was available rather than buying for a specific build. It was very rare to come away from a visit without several sheets of the now elusive "6lb/cu ft or less and springy" wood. Last time I was in the UK I visited a shop which did at least have some wood in stock and left empty handed, there wasn't a single piece there that I would have wanted to incorporate in any model. Some good flooring quality 1/8" sheet though!

Having said that, I order my wood from Balsa Cabin these days and find that their "light" grade is pretty good and, if you are willing to wait and pay a bit more, their "contest grade" excellent.
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Old Apr 10, 2013, 06:11 AM
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Originally Posted by Sundancer View Post
I don't think it is your memory Harry. Apart from the fact that there are now very few model shops worthy of the name in the UK - i.e. shops that cater for the model BUILDER as opposed to the model BUYER - their wood stocks are pretty pathetic. Back in the day I used to do a regular circuit of several "local" shops and trawl through the wood using a set of scales, buying what was good when it was available rather than buying for a specific build. It was very rare to come away from a visit without several sheets of the now elusive "6lb/cu ft or less and springy" wood. Last time I was in the UK I visited a shop which did at least have some wood in stock and left empty handed, there wasn't a single piece there that I would have wanted to incorporate in any model. Some good flooring quality 1/8" sheet though!

Having said that, I order my wood from Balsa Cabin these days and find that their "light" grade is pretty good and, if you are willing to wait and pay a bit more, their "contest grade" excellent.
Yes balsa has become a problem. My flying buddy Marc von Vaerenberg does build many of his big scales using the stuff but Albatros shop here does still offer good service with balsa. I remember when I was going to take over Svenson Models that prices, previously adequate, did start swinging out of the pan. Reason for this were the big container ship companies buying up huge stocks that they used for isolating the ship walls ! Light and very floating. I also remember from a magazine some easy built device to measure your balsa planking. It made use of a weight, one did place the plank in and put a certain weight on it so the plank would bend down and you could read from a scale, a bit like a paper weight.
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Old Apr 10, 2013, 09:02 AM
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Finding good wood

Harry Rule wrote:

"I can't imagine the balsa trees have changed over the last 40 years...or have they? Is this current global ecology producing a different standard of wood? Food for thought."

As best I can determine 95% of balsa still comes from Ecuador but rather than searching the jungle for mature trees it seems most is now plantation raised, sometimes with other crops. I believe with more consistent quality. A tree is ready for harvest in 3 to 4 years. Shipbuilding, wind turbine blade makers and other industries seem to want what we call "good" wood. In spite of that Sig Manufacturing Company (USA) how has connections that seem to assure a good supply of quality Ecuadorian wood. They tell me their last shipment was the best they have seen in years.
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Old Apr 11, 2013, 05:32 AM
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I get my mail-order balsa now from Inwood Model Supplies; they're pretty good at sending me the right grades when I specify it and the price is good too. That's another beef I have with model shops these days; even if you can get the wood you want, the prices can be high.

I haven't gone over to the dark side of Depron yet though..though I did build one model from it purely as a test and was quite impressed. Only balsa dust will do for me though!

Regards
Harry
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Old Apr 11, 2013, 01:11 PM
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Originally Posted by plenny View Post
Harry Rule wrote:

"I can't imagine the balsa trees have changed over the last 40 years...or have they? Is this current global ecology producing a different standard of wood? Food for thought."

As best I can determine 95% of balsa still comes from Ecuador but rather than searching the jungle for mature trees it seems most is now plantation raised, sometimes with other crops. I believe with more consistent quality. A tree is ready for harvest in 3 to 4 years. Shipbuilding, wind turbine blade makers and other industries seem to want what we call "good" wood. In spite of that Sig Manufacturing Company (USA) how has connections that seem to assure a good supply of quality Ecuadorian wood. They tell me their last shipment was the best they have seen in years.
Yes Ecuador. I was offered ambasador status of Ecuador as to facilitate balsa import for the Svenson production ! For a long time I did have a full size balsa tree trunc in my workshop. By the way, balsa is a hardwood despite the misleading fact it is the lightest kind of wood. All tree's with leaves are hardwoods. Tree's with needle leaves are softwoods. Nowadays
those ships probably do not make huge use of the wood any more but it was a fact at the time, I am talking around 1978.
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