Monday

Re: Monday

Ken Oakley-Sunny St. Cloud, Fla.

Sitka Spruce?????

Early day for part 1

Gary Smyth

Answer: SITKA SPRUCE

Ken gets a gets a gold star for part one.

A spruce tree will grow to heights of 200 feet and higher with a diameter of 8 feet or more. A tree of this size will have taken 400 years or more to reach this dimension. A spruce tree will not yield usable aircraft lumber until it is at least 5 feet in diameter. Even with this size tree only 5% or less of the resulting lumber will be of the quality necessary for aircraft construction. With this in mind it is easy to understand why we often have a deficit in aircraft grade lumber. Prior to and during World War II large forests of the Sitka were depleted.

It is the standard against which all other woods for aircraft are judged. It is easily worked, uniform in texture, resistant to rotting, and has no odor. It can also be obtained in clear, straight-grained pieces having very few defects.

Part two. What are the two criteria for wood for aircraft?

Re: Monday

Alex Y

I'm guessing that the two specs you are looking for are grain slope and rings per inch, although specific gravity is also a key spec.

Here's a reference that is almost as old as I am:

http://www.aircraftspruce.com/catalog/pdf/13-14900.pdf

Re: Early day for part 1

Ken Oakley-Sunny St. Cloud, Fla

Howard Hughes Spruce Goose Flying Boat was constructed mainly of Sitka spruce. It is also an eccellent wood for guitar building.

You got it. Gold for you.

Gary Smyth

Alex,

Thanks for posting mil-spec 6073. I went looking for it last night but it was removed from the site for violation of some sort on that site. Anyway you are correct. The ring minimum is six per inch, and a grain that runs straight with a variation of no more than a ratio of 15:1.

Looking at the face of a board the growth rings should not slope upward or downward more than the specified amount, "usually" 1 inch in a 15-inch length of the wood. This slope of grain is usually termed diagonal grain. CHART OF WOODS FOR AIRCRAFT USE

TYPE OF WOOD

STRENGTH

MINIMUM RINGS PER INCH

MAXIMUM GRAIN SLOPE

Sitka Spruce

100%

6

1:15

Douglas Fir

Exceeds Spruce

8

1:15

White Pine

85-96%

6

1:15

Western Hemlock

Slightly exceeds spruce

6

1:15

For those still with me, there is no guaranteed aircraft certified wood. Each piece even if it meets the minimum spec must be signed off by a AI (Airframe Inspector) and a A&P (Airframe and Powerplant) mechanic. www.eaa.org can supply additional information.