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Old Nov 30, 2001, 02:47 PM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,439 Posts
Building Light

It seems like a good idea to start a thread where we can post techniques for building light. For example, I recently started using Super Phatic glue (from Dave Lewis) instead of CA. It is supposed to be much lighter. I plan to do a couple of tests to see how much lighter, and will post what I learn.

The other thing I've been doing is borrowing techniques from the rubber power folks. I've got Don Ross' book on Flying Model Airplanes, which gives a number of good techniques we can use here. I've also got Lew Gitlow's book on Indoor Flying Models, which has some more techniques, and have ordered his other book "Indoor Flying Scale Models". The point is I'm trying to learn and absorb techniques that will help me build light, but scale electric micro planes.

I also put in an order for "indoor" balsa from Indoor Model Supply http://members.aol.com/indoorMS/IMS.htm. I'll post if I find that the balsa is better and lighter.
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Old Nov 30, 2001, 05:44 PM
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Pat Daily's Avatar
United States, VA, Chesterfield
Joined Mar 2001
5,411 Posts
Gordon

Building light is the only way! I have been a free flight scaler for most of my life, and when got into small electric rc, I started building heavier. Well, I have discovered you don't need hardwood spars (my first two scratch efforts for electric rc had pine spars) or longerons. Balsa works just fine. Go very light on wheels--they add a lot of weight for what they do. The area that I concentrate on is the "box" that holds all the gear (and I still make it too heavy). Luckily, I got a couple of decent scales and weigh everything -- amazing how fast balsa can add up. I still think the biggest bang for the buck will come with improvements in batteries--that is were most of the weight is anyhow. So to get around that, I think CO2 is worth looking at. Besides, I love C02 and have since I built a Sig 29er with CO2 in 1973.

Pat Daily
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Old Nov 30, 2001, 07:23 PM
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Houston Texas
Joined Mar 2001
743 Posts
One way you can cut out 4 plus grams is going down to three cells, both Ralph and I have been flying on three cells. But the next goal is one cell with a DC-DC board. The lighter the electronics the the lighter the air frame can be. Guess it all works hand in hand.
Philip
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Old Dec 19, 2001, 10:34 PM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,439 Posts
balsa density

I just calculated density for six sheets of 4x36x1/16 "contest grade" balsa that I ordered from National Balsa. The thickness ranged from 0.075 to 0.066, with 1/16 being 0.0625. The density ranged from 6.4 to 11.5. Only one sheet had a density less than 8. In addition, the assumption that each sheet was exactly 1/16th would have resulted in density measurements that were on average too high by 0.80. I got a similar range of densities for 1/32nd balsa in the same order.

As a completely unscientific sample (I don't have that much other balsa laying around) I weighed one each 1/16th and 1/32nd for SIG and Midwest balsa. The SIG had densities in the 9-10 range, while the Midwest was in the 13-14 range. This is not to slam balsa vendors, but just to show there are differences.

Finally, Indoor Model Supply in Oregon sells "indoor" balsa. I ordered some peanut balsa packs a while back to sample ($25 minimum order on their full sized balsa). Each pack consists of different cuts of balsa (A, B, C) and different densities. In one pack, the lightest sheet had a density of 5.9. This makes me think that if I put in an order for indoor balsa in full sized sheets from them that I would probably get pretty light balsa.

An alternative is to go to the LHS (which in my case carries Midwest) and weigh a whole lot of balsa to hopefully find that rare sheet of low density balsa.

Dave Robelen recommends using balsa in the 4 to 6 pound per square foot range for indoor/micro models. Hmm...

This building light stuff always has one wrinkle after another to learn about. But, "we choose to build small planes, not because it is easy, but because it is hard". (sorry, shades of JFK).
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Old Dec 21, 2001, 07:39 AM
Faroe Islands/Germany
epilot's Avatar
Joined Nov 2000
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For me, the key to building light is to use foam. I use both Depron, beaded styrofoam and pink/blue foam. Pink/blue foam is easy to carve and sand and is also easily cut into sheets as thin as 1mm. I use a foam friendly contact adhesive for most of the glueing (UHU Por or Bison).

I just got a precision scale and the model in the "sub ounce model" thread has an AUW of only 24 grams. Only the struts are balsa.

Michael
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Old Dec 21, 2001, 08:07 AM
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Houston Texas
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Michael,

How do you cut your Pink/Blue foam into 1mm sheets?

Thanks
Philip
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Old Dec 21, 2001, 08:15 AM
Faroe Islands/Germany
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Joined Nov 2000
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I hang my hot wire cutting bow on a piece of board and place some spacers under the wire to acheive the desired thickness. A couple of screw in the board prevent the cutting wire from moving. Then just slice away - very easy. Sheets need to be lightly sanded to remove the "angel hair" .

Drawing attached (hopefully)

Michael
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Old Dec 21, 2001, 09:23 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,439 Posts
Michael,
As you may or may not know, we are Depron starved in the US. It's almost impossible to get. You are inspiring me to maybe take the plunge and make or buy a foam cutting bow. Maybe the answer is cut our own foam to get thin stuff over here.
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Old Dec 21, 2001, 09:44 AM
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Houston Texas
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Thanks Michael,

Gordon,

Making a hotwire bow is very easy, all I did is took a 1" wide piece of wood and glued two wooden dowels to it. After the glue has set, then while wrapping Nicrome wire around the two dowel ends, kind of compress the two dowels, this will put a good amount of tension on the wire when uncompressed.

Philip
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Old Dec 21, 2001, 10:53 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,439 Posts
What do you use as a power source? Believe it or not, my brother and I had a foam cutting bow when we were kids that used a model train transformer. Also, where do you generally get nicrome wire?
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Old Dec 21, 2001, 11:20 AM
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Rotten Robbie's Avatar
Kent, Wa.
Joined Feb 2001
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Model train dealers that crry Woodland Senics should have the nicrome wire. Wooland Senics has a line of foam cutting tools.

But I don't use foam, so what do I know.

I build light RBFF airplanes and convert them to EPRC. Like the Sig 29er.

Robbie
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Old Dec 21, 2001, 11:27 AM
Faroe Islands/Germany
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Joined Nov 2000
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I have a fairly large variable powersupply, but I'm sure a train regulator can be used if it can handle the current.

You can use thin piano wire instead of nichrome. I got a length of nichrome wire from a fellow modeller years ago and it is still in use. With a 12volt battery current draw is app. 4 amps and this gives a good temperature for cutting so a PSU is not really necessary.

HWC is NOT a black art although many people seem to think so. I have built several foam models where templates were used to cut fuselage parts - see the Hellcat and GeeBee on my homepage: http://fly.to/epilot

Michael
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Old Dec 21, 2001, 12:17 PM
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Houston Texas
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Foam cutting is not all that bad, but cutting 1mm sheet, guess I will need some practice, think if I set it up like yours should work out better.

Gordon,
I use a 12v battery charger, the one you would use for you car, all ready had one around the house, just found another use for it. For Nicrome wire, I looked every where for it, except for my Local hobby shop, they sell it in small packages for under 2 dollars. Still using my original piece

Philip
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Old Dec 21, 2001, 04:13 PM
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Wiltshire
Joined Sep 2001
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Building light is about wood-selection and accurate joints that need a minimum of glue.

I get most of my wood from the local model shop.

I take electronic-scales with me and look for wood that weighs less than 10gm. for every 1mm. thickness for a 4" x 36".

1/16"....1.6mm therefore must weigh 16gms or less.
1/8".......3.2mm must weigh 32gms or less.
etc. etc.

This should give you 6.5 lbs. wood or better.

Hold the wood up to the light to check for even density.

If I see a nice peice of wood I buy it and add it to my stash. There is decent wood to be had....but it needs searching out.


Steve
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Old Jan 15, 2002, 09:25 AM
in persuit of low wing loading
Gordon Johnson's Avatar
Boston, Mass
Joined May 2001
6,439 Posts
I recently got an order of contest-grade balsa from Superior Balsa. I measured thickness and calculated balsa density. Here is what I found. The first value is the size (thickness), followed by the number of sheets in my sample, the average density, and the minium density.

Size, #shts, Avg, Min
1/32, 7, 6.0, 5.3
1/16, 7, 6.1, 4.9
3/32, 3, 4.9, 4.8
1/8, 3, 6.3, 6.2

Overall, I'm happier with the densities I received when paying for contest grade brom Superior Balsa.

I had previously ordered contest grade from National Balsa, but only a few sheets were in the 6 pound density range. I have also heard (from Ralph Bradley) that he has good results from Lone Star Balsa.
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Last edited by Gordon Johnson; Jan 15, 2002 at 09:44 AM.
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