Foam Prime Ribs anyone?

[John Carrodus]

During LD here in Auckland, Kiwiland, it's been hard getting good balsa. I have built several good stunt ships with this stuff before and I am now in the process of building my own hybrid design for a Spitfire. ( Stealing the wing of a Calamity Jane and messing with it a wee bit - adding my own fuz and tail assembly , for an LA 40 RR version . )
To illustrate how cheap and useful this supermarket meat tray ( unused) material really is, I have attached some photos of a far from finished Spitamity main wing so far.
For what it's worth if there are other folk out there frustrated by lack of strategic supplies!
Cheers
John Carrodus

[Mark wood]

Nice. I have been down this avenue myself. It works. Maybe not as well as we'd like but when you are in need and just gotta get that building and flying fix in... Well, you know, ya just gotta do something for it.

I like it.

[John Carrodus]

Thanks Mark.
Here's the picture I forgot to post.
Cheers
John C

[Craig Beswick]

John,
a second Spar? Does that not just defeat the purpose?

What is the weight of the wing?

Craig

[Dan McEntee]

A number of years ago, During one of the earlier balsa crunch periods,  one of our club members some of you may know, Bob Arata, built a Tucker Special using foam ribs with balsa cap strips and the fuselage has block foam instead of balsa blocks. Finished in silk span with colored tissue trim and looked pretty nice and flew well also.
  Type at you later,
    Dan McEntee

[John Carrodus]

Craig - An excellent question. My answer is - I haven't the foggiest idea! Its in the original plan, and the designer was a dentist so I kinda had faith in his eye for stuff structural. ( Let's face it - they are only mining engineers with a biology degree )
However, building any wing with a curve is a challenge to keep true. I think that the second spar actually turns the guts of the wing into a torsion box to resist flexing? You engineers out there will have no doubt already run the numbers.
I also tend to overbuild and ask for forgiveness later buy cutting and drilling out 'fat' where it's adding too much strength and un-needed weight. The wing so far weighs in at 410 grames = approx 34 ozs ( A bit heavy?) Wing span 56 inches. Hmmm any ideas?? Skinny her down somehow?

[Brett Buck]

The wing so far weighs in at 410 grames = approx 34 ozs ( A bit heavy?) Wing span 56 inches. Hmmm any ideas?? Skinny her down somehow?

   410 grams = 14.5 ounces. Not great, but also not a show-stopper.

     Brett

[Craig Beswick]

A guy at my club, I think passed now, used Pizza box cardboard to make ribs.

They used one of his old planes as a trainer for newbie. Sure enough, it came down and there they were for all to see, Pizza Hut!

Craig

[John Carrodus]

Brett
You are correct. Just a tad over 14 oz! Don't know where the hell my phone calculator got 34 from?! Good help is so hard to find these days and NEVER trust a machine.  <=Where's my pencil and paper. By the way the wing is 56 inches span.
Cheers
John

[katana]

I've often thought why isn't foam used more often in plane construction? There is the obvious point that it is more labour intensive with hot wire cutters etc. but for its use would be the density aspect - good light foam has a density around 1.5 - 2lbs cu ft and contest balsa at about 4lbs cu ft? So you could use 1/8" thick ribs in lieu of those fragile 1/16" ones and benefit from slightly better rigidity due to section size? Plus at least you'll be putting what will become landfill to a good use and saving the rainforests - well until you pile it into the ground and try burying it yourself 

[Craig Beswick]

I just don't see the weight saving in foam ribs. My last 58" wing weighed in at 360 grams and the ribs weighed 22 grams trimmed and ready for installation. That was out of 6lb x 1/16" wood.

Balsa harvesting does not impact the rain forest. Nearly all balsa is new growth plantation trees and they mature in 4 - 5 years!!

Craig
Ps. Foam as a base for a composite finish on a fuselage perhaps?

[John Carrodus]

Hi again Craig and Katana.
I use flat foam food trays to save balsa which is hard to get sometimes, especially good stuff and particularly at the moment here in NZ where many parts are still under some kind of annoying lockdown. I didn't do it to save weight initially, although this would be one spin off. The stuff I use is reasonably high density, cheap as chips ( less than 50c a tray) reasonably stiff and tough. It appears to absorb energy and bounce back in a prang rather than shatter or split. In a few crude strength tests, it appears to be up there with all but the hard stuff. When capped with balsa the wing is stiffened up even more, although not needed in smaller models as it is fairly thick.  I use Gorilla Glue which foams up slightly to form its own gusset around the joints. The foam also expands the glue into any gaps and locks in tight. Be careful though with painting. Some paints chomp through it like a hot knife through butter. Anyone using foam has first hand knowledge of this I would bet! Also don't linger to long with a heat source when tightening up heat shrink covering.