[ATM] Re: ATM Digest, Vol 6, Issue 20

[artbianconi@blast.net]

Howard Hughes must have known something we don't. The Spruce Goose 
was built almost entirely of plywood and had concentrations of 
dynamic stress far beyond that of any telescope I've ever seen. Ditto 
the famous Mosquito bomber of world war two. Built up wood spars 
continue to be the most popular load carrying members in the wings of 
the most highly stressed of all aircraft: aerobatic airplanes.

Although not often spoken of as such, wood is Natures original 
composite material. Properly insulated from the ravages of moisture, 
it will last a long time. All building materials have there weak 
points. In more modern composites using glass or carbon fiber, the 
weakness is poor compressive strength. In wood it's deterioration of 
the matrix from moisture or loss of geometric stability if not 
sealed. A properly designed structure which allows for wood's 
limitations will be no weaker and often stronger  than other 
materials and certainly lighter than cardboard. And it can be light 
too. Again, Howard Hughes Spruce Goose, which was for many years, the 
largest aircraft every built. A violin is very light. Every try to 
twist the body on a violin?

In 1979, I helped build an all wood fuselage for a 200 mph Formula 
One pylon racer. It was pulled along by a highly modified 200 cubic 
inch Continental aircraft engine. The wood we skinned it was with was 
0.060" thick mahogany veneer backed by 0.375" square longerons of 
sitka spruce spaced about every 12" around three eliptical bulkheads. 
They were built up from sitka spruce and gave it it's 20" by 30" 
median cross section. It weighed 35 pounds and we were self critical 
of that weight feeling we could have made it lighter. There was 
little about the design that was novel. Others have used similar 
construction for other aircraft of that type. David Harbor has 
designed and built a number of successful OTA's and fork mounts using 
thin plywood and they were strong and light.

I agree that building a wood OTA of the size being considered is a 
formidable task. There are far stronger and far lighter materials 
today we did not have back then. I agree too that anyone that can 
lift a bulky device that size could probably run for Governor of 
California. I also agree that an open truss offers distinct 
advantages.

There are a number of arguments detering you from using wood as a 
primary structure in a tube that 
size. Unless it's a bad design or built poorly, It's ability to 
handle stress concentrations is not likely to be 
one of them.

Art Bianconi

On 15 Jun 2004 at 22:10, atm-request@atmlist.net wrote:

Don't use plywood as this won't have the basic strength to resist the 
local stresses.

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