Weight, why and how to keep it down

jander14indoor
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Weight, why and how to keep it down

Post by jander14indoor » August 6th, 2009, 3:16 pm

Since we have a dedicated board for WS now, I thought it might help to organize by kicking off topics around the typical WS questions. Content of string to be driven by subject line (I hope).

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Re: Weight, why and how to keep it down

Post by carneyf1d » August 8th, 2009, 2:09 am

Why: lighter planes fly longer.
How: weigh each piece of balsa. before building find appropriate weights for each item of the plane. prop, MS, wing, boom, stab, etc. Add em up and get to the min weight as close as possible. Then build to those weights

Thanks for creating each individual category Jander. Seems a lot more organized and a lot more beneficial for SO teams.

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Re: Weight, why and how to keep it down

Post by calgoddard » August 8th, 2009, 4:08 pm

Thanks Jander and Carney. It's a good idea to breakdown the WS topic into different subject matter threads.

On this thread, how about discussing the need to make certain parts of the plane from stiffer, heavier wood, such as the leading edge spar? Or maybe this discussion belongs on the wood selection thread.

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Re: Weight, why and how to keep it down

Post by carneyf1d » August 8th, 2009, 10:54 pm

Heavier doesn't always mean stiffer. The leading edge does need to be stiff though. Read up on SC calculations for more information. I'm sure this will be covered in the wood selection forum.

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Re: Weight, why and how to keep it down

Post by gh » August 10th, 2009, 6:09 am

Well, a lot of new flyers don't understand just how minimal a plane should be, and overbuild by far. So just for the newbies, I think the most important information for newbies here is the the finished plane must be very close to the rules' mass limit in order to be competitive.

Most elements of your plane will be made of very light balsa wood. The construction of the larger surfaces (wings and horizontal stabilizers, etc.) can not be solid sheets of wood, even very thin sheets, as that would be too heavy (and also illegal, depending on the rules). Instead, balsa frames covered with a thin material (very light paper or plastic) are used. Frame elements are most commonly 1/16″ × 1/16″ square sticks of balsa.

Finally, do not try to overcompensate for what may seem like an overly fragile choice of materials, for example by using a lot of glue. Glue can be a very big part of a plane's weight, as it is much denser than balsa. A small amount of glue can dry faster than a lot of glue, and so may form to be stronger, but definitely at least more efficient for its weight.

Besides, a WS is fairly resilient in flight because it's light, but not so light that it can destroy itself. With the right glue and choice of balsa, and of course proper construction, a WS plane's frame is quite flexible and robust, and can survive scrapes against a ceiling, bumps against a wall, and of course some hard landings. What it can not survive, however, is improper handling and transportation, the biggest causes of damage that planes suffer from.
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Re: Weight, why and how to keep it down

Post by jander14indoor » August 10th, 2009, 2:36 pm

blue cobra wrote:Where could one find a good scale?
To build to 7.0 gm minimum, you need a scale that can weigh accurately down to hundredths of a gram.

You can find all kinds of suitable digital scales on-line with that level of accuracy for prices ranging from $12 to $200+++. The cheap ones tend to have very small pans and limited maximum weights. Good for parts, a little awkward for the whole plane. For only a little more, you can have a very nice size pan and a reasonable max capacity.
Examples:
http://www.dealextreme.com/details.dx/sku.15002
http://www.oldwillknottscales.com/searc ... -01g-0-05g

You can find used quad-beam balances that weigh to .01 gm resolution used on E-bay for $35 or so. Probably digital ones too.

You can make your own very adequate balance using lengths of music wire for counterweights. Music wire has a very consistent diameter, thus, consistent weight per length. With very fine wire, you can easily manufacture 0.01 gm accurate pieces with a simple ruler and wire cutters.With those you can get resolution in the 0.001 gm range with a little patience.
Examples:
http://www.gryffinaero.com/models/ffpag ... lance.html
http://www.gryffinaero.com/models/ffpag ... scale.html

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Re: Weight, why and how to keep it down

Post by wlsguy » August 11th, 2009, 7:25 am

jander14indoor wrote:
blue cobra wrote:Where could one find a good scale?
To build to 7.0 gm minimum, you need a scale that can weigh accurately down to hundredths of a gram.

The cheap ones tend to have very small pans and limited maximum weights. Good for parts, a little awkward for the whole plane.

I have one of the small pan scales and made a plane stand to be able to weigh everything.
Basically the stand looks like the one in the photo at http://gallery.scioly.org/details.php?image_id=1645
I just made the base small enough to fit on the scale (without the pan). Once it is zeroed, it works great.
Because it is small (and battery powered), I can put it in my pocket when I go to the hobby shop to weigh the balsa sheets.

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Re: Weight, why and how to keep it down

Post by blue cobra » August 16th, 2009, 4:25 pm

Should a calibration weight be purchased, or is it unnecessary?
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Re: Weight, why and how to keep it down

Post by robotman » August 16th, 2009, 5:21 pm

i got a scale similar to those for towers and the calibration weight was fairly useful they can get out of sync if you change the batteries or sometime when you transport them so it is good to have.
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Re: Weight, why and how to keep it down

Post by jander14indoor » August 17th, 2009, 3:42 am

blue cobra wrote:Should a calibration weight be purchased, or is it unnecessary?
My good electronic scale came with one, haven't used it much except rarely just to check things.

Technically, yes. Proper scientific (and engineering practice) is to check or calibrate all measuring devices against known good standards on a regular basis. Your calibration weight is the known good standard in this case. Of course, it has its own calibration procedure and schedule, ad nauseum back to national standards bureaus.

Practically, modern instruments are pretty good and you can probably get away with it for a while and not calibrate (check) your equipment. Still, a bad habit to develop if you are pursuing a technical career.

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