Understanding Balsa grades..

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retiredVTT
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Understanding Balsa grades..

Unread post by retiredVTT » Tue Oct 27, 2015 5:13 pm

http://fly2build.com/SF%2014-03%20Balsa%20Wood.pdf

Good info here on selecting Balsa grades.

Bill
Bill
HEFA #5
MAAC #13708

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JohnOSullivan
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Re: Understanding Balsa grades..

Unread post by JohnOSullivan » Wed Oct 28, 2015 8:05 pm

Balsa is a unique material. as soft as S##t or as hard as nails. It has to be chosen carefully to get the most out of it. Grain direction is as important as weight. "C" grain is good for components which require stiffness across the grain such as sheet tailplanes. It resists warping very well. It is not much good for stuff like spars, due to a short cross grain which can lead to brittleness across the spar. "A" grain is straight grained and is ideally suited to spars and situations where curved sheeting (top sheeting of a wing or fuselage sheeting) is required.
In my free flight days, I used to scour a new delivery of balsa to my hobby shop and go through the whole batch and chose about 1% to suit my needs. In recent years in R/C I have mellowed and am not as picky with my balsa.

Another thing that bugs me is "LIGHTENING HOLES" (aka "Weakening Holes"). They are "Daddy Cows KAKA" ! Too often kits are designed with lightening holes in the most ridiculous places (e.g in the noses of sailplanes where lead ballast is needed). Because of the linear grain of balsa they usually severely weaken the structure especially in ribs.
Their actual weight saving is a fallacy in 90% of their application. A few years ago Bill Foster made a very nice High wing Monoplane (can't remember the name, but he planted it on the top of the Pylon at Shannon). It is still active and he should bring it out more often. It had lightening holes in the ribs. I did a calculation on the actual weight saved on the model by using lightening holes. It amounted to the weight of a large Deans male and female connector. ( much less than an ounce) This was on a 5 lb or so model. Was it worth the weight saving in compared to the loss of structural integrity?

Proper choice of balsa both in density and grain really pays off.
John
MAAC #5401 L
MACI (Ireland) IRL#26

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