Spars

[Hank Jarrett]

Re: Spars

Check out NACA report No. 354. http://ntrs.nasa.gov/archive/nasa/ca...1993091423.pdf

To make a long story short,
Spruce is lighter per cubic foot than Doug Fir (DF)
DF is STRONGER per cubic foot than Spruce
DF is HEAVIER than Spruce
DF is LESS EXPENSIVE than Spruce by volume, strength and weight

None of this matters. What matters is you can make a DF spar the same dimensions as a Spruce spar and it will be much stronger (and heavier).

You can REDUCE the size of a DF spar so that it has the same strength as Spruce and it will only be a LITTLE heavier, but the ribs and other parts won't fit on it.

You can SELECTIVELY shape a DF spar so that it fits in the same space as a Spruce spar and it will be the SAME STRENGTH AND WEIGHT, but you better be ready for the FAA with a good analysis showing how you removed the wood in the right places. This is possible because COST drove Taylorcraft (and almost everyone else building light planes) to use PLANKS for spars. If you MILL the spar to an "I" beam shape by removing wood from the web area you don't loose any significant strength, but reduce the weight A LOT. The machining costs would be outrageous for a factory without CNC milling machines, but with modern tools it might be worth it so you could use much less expensive materials.

OH BOY! A trade study! Who wants to finance it?

Hank

[cvavon]

Re: Spars

Kevin get external load permit fly to spruce with dad's 170 and hall it back to ky

[crispy critter]

Re: Spars

I need material for one rear spar blank.....around $200ish for the wood and nearly twice that for shipping!

[otrcman]

Re: Spars

Now I know I should use spruce but fir is being pushed now also. Yes I understand it is a little heavier and I might get a splinter but the FAA says it's cool so looking for your thoughts

Could you provide some reference in which the FAA is pushing the use of fir ? This would not be consistent with the published guidance in the FAR's or in advisory circular AC 43.13.

In your FAR/AIM book you will find paragraph 43.13(a), which states in part, "Each person preforming maintenance, alteration, or preventative maintenance on an aircraft, engine, propellor or appliance shall use the methods, techniques, and practices prescribed in the current manufacturer's maintenance manual...." So, step one in any repair is to follow the manufacturers type design data.

What if there is no data available from the manufacturer? FAA provided for this possibility with the publication of advisory circular AC 43.13. The front page of AC 43.13 (para 1. PURPOSE.) states, "This advisory circular (AC) contains methods, techniques, and practices acceptable to the Administrator for the inspection and repair of nonpressurized areas of civil aircraft, only when when there are no manufacturer or maintenance instructions."

AC 43.13 does contain a table of wood properties, including those of spruce and fir. But that does not imply an open license to make such substitutions.

The FAR's tell us to follow the manufacturer's data. In the case of Taylorcraft spar replacement, we do have manufacturer's data. None of this says that you CAN'T use fir for a spar, but you would need a valid reason for doing so and would have to receive approval for the design change via an approval from your FSDO.

Dick

[cvavon]

Re: Spars

The $900.00 was the 4 slabs of wood ply wood for doublers spar varnish rib nails and 2 qts of primer now it was 4 or 5 years ago but if the price doubled thats 1800 less then 2000

[Bird]

Re: Spars

That is why I am asking around, I have just started to reasearch and am only seeing prices of just over 2 grand for a set at the cheapest.

[cvavon]

Re: Spars

Bird why don't you drive down to Atlanta to AC spruce and get spruce blanks and plywood for doubler 4 years ago that cost me $900.00 you are closer then i am to spruce 5 hr trip one way and they will let you pick out your wood from stack the ones we got were grate

[Bird]

Now I know I should use spruce but fir is being pushed now also. Yes I understand it is a little heavier and I might get a splinter but the FAA says it's cool so looking for your thoughts