The reason that you can't find an answer to this question is that there is
no simple answer short of a structural analysis or load test of the
airplane. However I will do the best I can to answer in 10 minutes or less
......

For starters lets compare the properties of Douglas Fir and Sitka Spruce
(from ANC-18 at 15% moisture content which is typical)

Sitka Spruce
Douglas Fir % Difference
Density 28
33 17%
Fiber Stress Prop Limit (Bending) 5300 5900
11.3%
Modulus of Rupture 9400 10900
15.9%
Modulus of Elasticity 1380
1480 7.2%
Fiber Stress Prop Limit (Comp) 3530 4220
19.5%
Max Crushing Strength 4700 5600
19.1%
Compression Perp to Grain 740 1020
37.8%
Shear Strength Parallel to Grain 990
950 -4.0%
Tensile Strength Perp to Grain 170
140 -17.6%

(units are psi or lb/cu ft)

(I hope the format of the table is preserved when I post this ... )

OK so what does this mean:

1. The weight increase is approx 17%.

2. The most important of the material properties is the Fibre stress at
proportional limit for bending. This only increases by 11.3% so clearly the
Fir is not as efficient as the spruce. i.e. the increase in strength is not
in the same proportion to the weight increase. Based on this (which would be
a measure of the bending strength of the spar) I would only increase the
limit load factor by 11%. The modulus of rupture is a measure of the
ultimate strength of the spar and based on this, the ultimate load factor
could be increased by 15.9%.

3. The Modulus of elasticity increases by only 7.2%. This is the primary
material property that will control the strength of structure where buckling
instability is the primary mode of failure. In general terms the buckling
strength would be linear with modulus and so you would only increase the
load factor by 7.2% based on this consideration without a more detailed
investigation to establish if buckling is infact critical in the structure.

4. Depending on which airplane we are talking about, the shear strength of
the timber may or may not be important. If the shear strength of the timber
is approached at limit load or if any of the critical failure modes in the
structure are due to shear (unlikely) then the use of Fir would result in a
4% reduction in load factor !!!

5. The glue area in joints is unchanged and thus the strength is these
joints is largely unchanged so based on this consideration I would not
increase the load factor at all unless I ran the numbers to understand what
sort of margins the glued joints have. They may have sufficient margins to
enable them to tolerate an increase in shear stress etc but you don't know
without a stress analysis or load test.

6. The same is true of the plywood spar webs and skins. These are not
changed by the switch to Fir. Can the plywood shear webs in the spars handle
the increase in shear stress ?? Can the wing skins handle the increase in
shear / tensile and compressive stresses ??? Can the fuselage and tailplane
skins handle the increased stress ??? You will only know if you do the
analysis or run a static load test. All of these components will have to
sustain higher stress levels.

7. Same goes for the metal components and the hardware (bolts etc) ... spar
attachment fittings, engine mount etc. All need to be restressed for the
higher load factors because their strength has not been increased by the
Fir.

8. One of the primary modes of failure in wooden aircraft structures is bolt
bearing in the wood. The max crushing strength of the timber is the primary
controlling factor here. This has increased by 19% as a result of the switch
to fir and so the bearing allowables would permit an increase in the load
factor of approx 19%.

OK I could go on but my 10 minutes is almost up ...

You get the picture. If you don't understand what you are doing don't muck
with the design and build it as per the plans or contract someone who does
know what they are doing to run the numbers for you if you are serious about
your well being.

Several final comments. I have been involved with the construction of a
number of aircraft from Douglas Fir in lieu of other timbers. There are
other considerations ... more practical ones. First it is no more easy or
cheap these days to get good straight grained, knot free Fir than it is
spruce. Second fir is far more prone to splitting and so you may need to
modify some of the manufacturing methods a little.

Finally if the wood is to be used in an airplane then it needs to be
selected and graded to an aeronautical specification (just as aircraft grade
spruce is graded to MIL-S-6073 or B.S.2V.37 or B.S.2V.38).

I am aware of only one such standard for Douglas fir ... B.S.V.36. I have
actually developed my own spec that I use for these projects for various
reasons.