Douglas fir vs sitka spruce

[k***@gmail.com]

Subject: Douglas fir vs sitka spruce
Date: 13 Feb 1995 01:47:49 -0700
Keywords: wood
I have been thinking a bit about wood planes. And have often
heard that douglas fir can be used in place of spruce in certain areas of
a plane in order to reduce costs. Just what areas?
Spars?
fuselage?

According to the FAR's if fir is up to spec it can be used in any area that
spruce is used.

Wing ribs?
From what I remember, (if that is right) douglas fir of the same
dimentions, is heavier, and slightly stornger than spruce.

Fir is about 25% heavier and about 21% stronger than Sitka Spruce.

I am just wondering in which areas of construction douglas fir could be
used, whithout violating some reasonable conventions of safety. And also
just how much you would save in money and gain in weight, if you used as
much douglas fir as would be resonable. Its just that spruce seems so

Cost savings varies, but if you can find a good deal on fir it can save you
up to 80%.
Gordon Nichol
Steve Eldredge

Some years ago, I built a Corben Jr. Ace. Open cockpit, fabric covered sport plane from 1929. I read everything the EAA had on wood and wood substitutions. In the end, I found my local lumber yard had some terrific Doug Fir in long lengths with tight, straight running grain.
I used it for the spars, I ripped it into the 1/4" x 1/4" strips I needed to build the ribs and everything else where Sitka was called for.
The FAA inspector told me it was way better than almost all the Sitka he's seeing these days.
Go ahead and use it.
Brian
Michigan

[c***@snyder.on.ca]

Post by k***@gmail.com

Subject: Douglas fir vs sitka spruce
Date: 13 Feb 1995 01:47:49 -0700
Keywords: wood
I have been thinking a bit about wood planes. And have often
heard that douglas fir can be used in place of spruce in certain areas of
a plane in order to reduce costs. Just what areas?
Spars?
fuselage?

According to the FAR's if fir is up to spec it can be used in any area that
spruce is used.

Wing ribs?
From what I remember, (if that is right) douglas fir of the same
dimentions, is heavier, and slightly stornger than spruce.

Fir is about 25% heavier and about 21% stronger than Sitka Spruce.

I am just wondering in which areas of construction douglas fir could be
used, whithout violating some reasonable conventions of safety. And also
just how much you would save in money and gain in weight, if you used as
much douglas fir as would be resonable. Its just that spruce seems so

Cost savings varies, but if you can find a good deal on fir it can save you
up to 80%.
Gordon Nichol
Steve Eldredge

Some years ago, I built a Corben Jr. Ace. Open cockpit, fabric covered sport plane from 1929. I read everything the EAA had on wood and wood substitutions. In the end, I found my local lumber yard had some terrific Doug Fir in long lengths with tight, straight running grain.
I used it for the spars, I ripped it into the 1/4" x 1/4" strips I needed to build the ribs and everything else where Sitka was called for.
The FAA inspector told me it was way better than almost all the Sitka he's seeing these days.
Go ahead and use it.
Brian
Michigan

Just make sure it meets all the other requirements - good close
straight grain etc.

[s***@gmail.com]

According to the FAR's if fir is up to spec it can be used in any area
that spruce is used.

There is no FAR that states this. What Spec ?? So far as I am aware there is no aeronautical spec for Douglas Fir (well there is an obscure Australian Spec that is about 70 years old but not widely known or used).

[s***@gmail.com]

Fir is about 25% heavier and about 21% stronger than Sitka Spruce.

That is over simplifying the issue. Sure it is heavier, no problem with that ... generally about 32 lb/cu ft compared to 28 lb/cu ft for Sitka Spruce (15% heavier).

However, strength is another matter. With wood there are many different strength values that we are concerned with, each of which is important for different failure modes of the structure. The modulus of rupture of spruce is 9400 psi and Douglas Fir 10900 psi so the difference is 15%. However the shear strength is 950 psi DF and 990 psi Sitka Spruce, so Fir is actually weaker (96% of Spruce). Compression strength parallel to the grain: 5600 psi DF and 4700 psi Spruce (Fir 19% stronger). Fibre stress at the proportional limit: DF 5900 psi and Spruce 5300 psi (Fir 11% better). See ANC-18 for a full set of mechanical properties.

Provided that you have a structure that will not be critical in shear then you could safely substitute Douglas Fir for Spruce. Unless you know what you are doing just use the same section sizes and accept the weight gain.

With any wood the challenge it to select consistently good timber. This is where you need a spec. Spruce is graded IAW MIL-S-6073. As mentioned above, there is no equivalent spec for Fir so you are on your own. The spec deals with details such as permissible growth ring spacing, grain slope, spiral grain, density, knots and pin holes etc.

Douglas Fir has been used in plenty of aircraft. Some examples of homebuilt aircraft that were designed to use Fir are the Woodstock Sailplane, the Jodel D-18 (Fir was also used in some of the certificated Jodel's).

Bud Evans also wrote an article on the use of 'non certified' wood in Sport Aviation Jan 67 which takes a more pessimistic view.

Finally don't confine yourself to Douglas Fir when considering alternatives to spruce. Northern White Pine (TEAM Minimax etc), Hoop Pine are also widely used substitutes for spruce.