Douglas Fir

[XL-C-100]

For those of you out there who built their wing spars from doug fir, has anyone weighed them and gotten some numbers on the weight penalty vs spruce? I am in the process of building mine now and pretty much have my mind made up on DF but I was just curious.

[rockiedog2]

sorry I don't have that but IMO any weight penalty is too much. I would go with spruce and look for somewhere else to save the $
there are a lot of charts etc for lumber weight at google that would give you an idea of the diff

[Dan_]

Here's a piece on it...

[XL-C-100]

Ok, I did some math on how many cubic inches of wood are in the spars and came up with 1094 divide that by 1728 cubic inches in a foot and you get 0.61 cubic feet of wood for the spars. I estimated a little on some of the dimensions but that should be pretty close.

Average dried weight for Sitka Spruce is 28lbs/cuft,Douglas fir is 35lbs/cuft. When you do the math it comes out to 4.88lbs of weight if you keep all the dimensions the same. That is significant but you do gain over 20% in strength. Hmmm?

[keithv1954]

Initially I was also leaning towards Douglas fir, but gave it up for 2 reasons. Firstly I read somewhere that it is slightly more difficult to work on, and secondly it is IMO a bit too heavy. I am now leaning towards hemlock. it is only marginally heavier than spruce, but marginally stronger too. Much lighter than DF. I am a first time builder, so don't read too much into my opinion on this. Another member on this group, Bill Utt, mentioned he has used DF on his wings.

[rockiedog2]

>>>4.88lbs of weight if you keep all the dimensions the same. That is significant but you do gain over 20% in strength. Hmmm?

the 20% is unneeded strength and the 4.88# is dead weight. 4.88# is HUGE. Do that all over the plane or really not very many places and you got a dog. And whatever the spars weigh you're stuck with it. I've been getting weight off mine just about ever since it was built. But cant do that to the spars...35/28# cu foot is a 25% increase! No sir no way I would do that to mine

I don't know what the diff in price is but I'd just pay it for the spruce and eat sardines and viennies for a while. but thats what I eat anyway

It's just really hard to understand on first time project like the Eagles how important every little bit of weight is. Some have a diff idea of it

>>>Another member on this group, Bill Utt, mentioned he has used DF on his wings.

I love Utt but ask him what his plane weighs sorry Utt

[bill utt]

Yep, Spencer is dead on!
With Close inspection of my a/c, it should be apparent that it is only similar to the DE.
The design parameters are more in line with the X-AIR H model with a Jab. 2200.
Last W/B was 640lbs. Will soon have better numbers available to be chastised and raked over the coals about. LOL
Different airfoil, different wing cord. Most everything is different.
The DE was used as a guideline due to its simplicity and ease of modification.
Whether it is a "DOG "or not , we shall soon see.
The engine is not of standard configuration either. Even the oiling system is a little different.
100% EXPIERAMENTAL
Might turn out to be a yard ornament!
Best regards,
Merry Christmas!!!
Bill

[rockiedog2]

Merry Christmas Utt and all...

the Eagles due their simplicity and good performance on low power sometimes seem to tend to cause the builder to relax a bit about weight when in reality the opposite is required for a good performing plane. Every little bit of extra weight in the empty weight reduces the climb rate of an already low climb rate plane and that reduces the margin for outclimbing ordinary downdrafts. Max power and min weight...perfectly tuned and fanatical min weight.
yeah I know that's pretty basic and most all of us know that but it seems we lose sight of it anyway. I thought I did but didn't really realize the importance of weight when building mine. But have learned...

well that's my idea of it.

Spencer

[grdev]

So has anyone used DF? What was the weight gain? How much can you down size the DF?

[Steve]

What was the weight gain?

Pounds per cubic foot weight Table for a variety of wood species:  http://www.engineeringtoolbox.com/wood-density-d_40.html

See attached .pdf documents re: wood in aircraft with comparison table at end of 1st document & brittleness test rig in 2nd document:

[keithv1954]

I have worked with hemlock for home-built projects. 
It is lighter than douglas fir , very strong,  and is a less expensive alternative to spruce.
A bit heavier than spruce though.

[grdev]

The DF I found has very little run out of the gain and averages 17 rings per inch Actual weight for 1" x1" piece is .017777 oz. per inch. A 13' 1" sq cap weighs 2.773 pounds and those are real world numbers I just weighed. That would mean both rear and forward spars would come in around 11 pounds. the ribs for one wing weigh 4.2 pounds. Looks like with out the leading edge ,web,  comp struts covering and paint I'm in the 15 + range per wing using DF spar caps How does this compare to Sitka spar caps in the real world? Is it really worth down sizing the DF spar caps?
thanks for the information Steve, been looking all over for it.

[grdev]

I have my spars built with the exception of the plywood webbing. I had calculated with the DF my weight would be around 11 pounds per wing. The actual weight for the spars, front and rear, one wing is 11.4 pounds. Another way  I'm going to try and save some weight is by using an old torque wrench I had for my tail wheel spring. I cut the 3/8" drive off with my diamond ceramic tile saw. the rod just slips in a 1/2" tube. Will order a shopping cart wheel this coming week.