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Old Jan 26, 2003, 08:37 AM
Registered User
Deer Park, Tx, USA
Joined Feb 2001
422 Posts
Building Light?

Seems my electric planes built from plans always come out on heavy side. If the article mentions 16 oz AUW, mine will end up 18 oz. I don't "beef up" anything, am careful with the CA, don't use epoxy, hollow things out and use oracover. I'm wondering if not weighing the wood is step am missing. I buy the balsa from LHS for consistency, that is, medium density for spars, other load bearing items and low density for sheeting, etc. But, I don't weigh the wood.

What else am I missing? Only way seem able to hit target weights is to extensively modify the model's attributes.
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Old Jan 26, 2003, 08:47 AM
RIP Ric
Andy W's Avatar
Marietta, GA
Joined Jun 1999
43,312 Posts
Yikes - hobby store wood is probably the worst you could get - you may come across a light sheet every now and then, but you can bet anyone else in the area that builds will pop over when a new batch arrives and steal the best stuff!
Order online - plenty to choose from - and select contest grade. You'd be amazed at the difference..
..a
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Old Jan 26, 2003, 09:00 AM
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Haslemere, Surrey, UK
Joined Jul 2001
91 Posts
Russell
I agree re hobby shop wood - and remember that what one person regards as medium another may see as hard and vice versa. Only one way forward - get a decent set of gram scales and weigh every sheet and strip before using them. Sounds a bind but amazing how quickly it goes once you start. Make a quick conversion table so that you can at a glance determine that a sheet of, say, 1/4 x 3 x 36 weighing x is z lbs/cu.ft.
A reasonable assumption as towhat hard, medium, etc means (but as I say, folk do have differing views) is:

Hard 12-16 lb/cu/ft
medium 8-12 lb/cu/ft
medium-light 6-8 lb cu.ft
Light -under 6 lb
Hope this helps
I thnk you will be surprised at how little hobby store stock is under 10-12 lbs.
Mike
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Old Jan 26, 2003, 12:03 PM
Visitor from Reality
United States, VA, Arlington
Joined Dec 1996
12,788 Posts
Hi Russell
Old saying from the control line aerobatic guys - "It is easier to take a gram off 100 pieces, than to take 100 grams off a finished model". Not sure what a gram looks like in the wild, but it sure is true.

See above - get a good scale that weighs in grams. Mine was under $30.00 from Wal Mart, of all places. I've also used a $2.00 balance found in a flea market! It's as much comparative as actual weight - faced with two sheets of wood, which is the lighter?

You don't have to blow your year's hobby budget on a scale

I buy balsa in bulk - Lonestar, Superior, Balsa USA etc - and use what matches in a project. The heavy stuff always gets used, for jigs if nothing else, or I do use a lot of stripwood in my projects, which comes off heavy sheets. Mike's figures are fine - you'll get to be able to guesstimate closely with practice.

If forced to actually buy wood over the counter, I'll head to a hardware store 2 miles from my house rather than the nearest LHS some 10 miles away. That hardware store always keeps its "Midwest" balsa rack stocked, and it's always better wood than the LHS keeps (I have more balsa in stock than many HS these days anyway). Let's not discuss the price of that Midwest wood though, fair makes my eyes water...

If you build small, try using yellow woodworker's glue vice CA. CA is like epoxy - it doesn't evaporate, so adds its own weight. Aliphatic glues evaporate solvent off, so can add less weight. Much cheaper, but you need to let it dry.

Try Solarfilm Lite for covering - I've got it on a 500 square inch aerobatic 300W model, and it's done fine for a season's use. Even better on my Switchback, of course, plus it is pretty close to opaque (you don't want to look inside my models )

Hope that helps.

Regards

Dereck
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