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Old Dec 29, 2010, 11:33 AM
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in persuit of low wing loading
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Boston, Mass
Joined May 2001
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Estimating Skeleton Weight

Ok, now an example to help estimate the weight of a part before building one. However, if you keep notes over time you'll develop your own measurements and rules of thumb.

Let's take my Airbender wing as an example. The wing span is 18 inches and wing area is 102 square inches. 3mm Depron weighs 0.08g per square inch, from the table in post #3. So, if the wing were made from solid 3mm foam it would weigh about 8.15g.

My skeleton wing uncovered weighed 2.25g. This works out to about 27% of the weight of the solid foam wing. This is what I refer to as the "skeleton ratio". For those cutting by hand I think a rule of thumb of about a 35% skeleton ratio is more realistic, and really very little different. Of course, the more foam you leave in your design the higher the Skeleton Ratio will be.

So, for a particular plane, take your wing area, times the skeleton ratio you think you will be able to achieve, and multiply it times the weight per square unit of the foam you will be using and this gives you an estimated weight for the skeleton.

Next, estimate the weight from the covering. In the case of my Air Bender I used the WES 2um film, which weighs 0.00142 g/sq.in, and multiplied by 102 square inches gives a weight for the covering of 0.15g.

Add the weight of the covering to your weight for the skeleton and you have the weight of your wing, or other part. There is, of course, the weight from the 3M77 Adhesive. But, it's marginal if you dust it on, which is all that's needed, and the adhesive weight can be ignored.

So, to recap for my Air Bender:

Skeleton Wt = Area x SkeletonRatio x FoamWt
= 102 x 0.27 x 0.08
= 2.2g

Covering Wt = Area x MylarWt
= 102 x 0.00142
= 0.15g

Total Est Wing Wt = Skeleton + Covering
= 2.35g

Use this same technique for tail surfaces and fuselage and related pieces such as side stiffeners and you can estimate how much you can save on the weight of your airframe. If CF rods are to be used to add strength back in then they should be factored in.
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Last edited by Gordon Johnson; Jan 01, 2011 at 09:59 PM.
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