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Building Tips and technical articles. => Building techniques => Topic started by: Andrew Tinsley on June 11, 2011, 11:43:21 AM
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Hello Everyone,
Getting hold of contest balsa is getting difficult and very expensive. To get round this problem, I have been using 8-12 pound balsa and simply halving the thickness, i.e. if the plan calls for 1/8 wood, then I use 1/16. This helps enormously to get the weight down. So far the planes built like this have shown nothing in the way of weakness.
Am I about to wake up to something nasty, or do people think that this is a valid dodge to beat the shortage and expense of contest grade balsa?
Regards,
Andrew.
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Where have you been trying to get wood from? Contest wood is not cheep, but, in my personal view its worth every penny. I think its very hard to use heavier wood-even in smaller sizes, and get away with a model thats suitably light.
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Hello Steve,
I live in the UK and although its possible to get the good stuff, its either mail order or drive one heck of a long way to get it. I like to see what I buy so mail order is not really on. By the time you have factored in the gas (approx four times US prices!), the contest stuff gets even more expensive.
I don't have much trouble in keeping models light, so that really isn't a problem. I am just wondering about long term failure, so far, I seem to have avoided this problem, wondered if other people have had experience of this type of cheating?
Regards,
Andrew.
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Smart engineering is the key words in this case. For instance, you can use 3-4lb. balsa for wing ribs, but they are really too soft, and easily break or deform. Use 6-8lb. wood of the same thickness, and remove more in the way of lightening holes. The result is often parts weighing the same, and are more durable.
Figuring where the stress is going to occur, and engineering the parts to take the loads will allow the use of dimensionally thinner stock.
I often substitute 3/32' 6-8lb. balsa, for lighter 1/8" thick balsa for fuselage sides and bulkheads. I've also used 1/16" 8+ lb, balsa for fuselage sides along with using about 30% more bulkheads that are also from the same stock.
Many folks over build trying to make the plane survive the mishaps that occur. Usually the result is an overweight plane that won't perform well enough to avoid the mishaps in the first place.
So, to qrap this up, I was taught and still believe that a good stunter will turn to balsa dust when crashed, if it doesn't, then some parts were overbuilt, and could have been lighter.
y1 S?P H^^
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Hello Everyone,
Getting hold of contest balsa is getting difficult and very expensive. To get round this problem, I have been using 8-12 pound balsa and simply halving the thickness, i.e. if the plan calls for 1/8 wood, then I use 1/16. This helps enormously to get the weight down. So far the planes built like this have shown nothing in the way of weakness.
Am I about to wake up to something nasty, or do people think that this is a valid dodge to beat the shortage and expense of contest grade balsa?
Shortage? You can get it, I saw about half a semi-trailer's worth of it, mostly 4-5 lb, just a few months ago. We were discarding wood that is better than you can get from the major suppliers.
But putting that aside, sure you can reduce the size and use stronger wood. The only issue is that twice the weight is not twice as strong. Since I have mostly 4 lb wood, I am sometimes looking for good 6-ish# wood just for this reason. For instance, do I really think it's a good idea to sheet a stab with 3.8 lb 1/32?
Brett
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I've always maintained that air is lighter than wood. So it's not a bad idea to just use less if you use harder/heavier. As Brett notes, it's seldom in halves as 8-12 pound wood is not twice as strong as 3-4 pound wood. But in general you can get away with just using less.
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We send it to him. LL~ LL~ LL~ Right now I can't remember his name,but a gentleman used to come to VSC with his Old Time plane in a little wooden case. It was built mostly of spruce/basswood. It was also take-apart and very light. H^^
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I agree with John! LOL!! If a stunter bites the dust and doesn't turn into dust, it was overbuilt. ;D There are places where strength is needed, but I have seen models that appeared to have been built to withstand flying through a barn door when it was closed......
And 6lb 3/32nd" wood is plenty for almost any stunt plane's fuselage sides, especially with a proper engine crutch. Many have used (and DO use) 1/16th" behind the wing for the sides. :o
Big Bear
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Don't build to survive crashes, but do build to survive normal wear and tear. Repairs always add weight. If your repair adds 2 oz, and you could have avoided making that repair by building it stronger by one ounce, you should have done so, IMO. I'm a believer in a practical plane/engine/prop setup.
Experience with Gary G's Yatsenko Yak 54 at NW Regionals has reinforced that belief. That plane is clearly too fragile to be practical for most folks. Light, yes. I think it was 51 oz with the Yatsenko .76...why? One LG leg's cute plug-in socket was broken out of the fuselage when the engine kicked back while starting. If the two sockets had been connected across the fuselage width with a tube "spar", and a little more CF had reinforced the attachment to the fuselage, it wouldn't have broken. Certainly no more than two ounces added, and probably easily under one ounce. D>K Steve
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The wood strength is mostly proportional to the density. It runs all the way from 4-6lb. balsa up to basswood, maple, and oak. Really low density balsa is not as strong per/lb as slightly heavier wood. But once you get past 6lb/cu.ft. or so, the strength is pretty much proportional to density. The other thing to keep in mind is wood selection. Getting the grain correct can have as much effect on overall strength as density. Aircraft grade spruce is selected mainly for the number of growth rings/inch. For the same density, wood with more growth rings is significantly stronger. For our use, pick C grain wood for parts that need to be stiff(ribs, bulkheads, solid stabs, trailing edge sheeting, etc). Select A grain for parts that need to bend- leading edge sheeting and forming sheeting. B grain can be used for fuse sides, bulkheads, thin solid tail surfaces, etc.
Just as important, the amount and type of glue is just as important. It is very easy to add several ounces to a 60 inch plane by using too much of the wrong type of glue in the wrong places.