Bumb Question - Upper/Lower Spar Flange Bars

N722SH · #1 07-02-2006, 11:11 AM

After much delay, I am finally getting around to counter sinking all the holes in the wing spar flanges. (BTW - I?m building a ?9) The instructions say to ??mark the orientation of the spar?and study the plans until you understand how the spars are installed. You do NOT want to build a wing upside down!? Well, after studying the plans and staring at the spar for a while, I ?understand? how the spar is installed, but don?t ?understand? how the spar was built!

While ?studying? the spar and plans, I noticed that the upper and lower spar flange bars (the thick stepped bars that run along the top and bottom of the forward side of the wing spars, W-906 B & D) are in the opposite position from what I would expect. The bottom flange bar is shorter and gets thinner sooner than the top flange bar. This is counter intuitive from what I would expect - at least to me! I would think the longer and thicker flange bar would be on the bottom (bottom of the spar loaded in tension) with the shorter and thinner bar (loaded in compression) on the top.

Anyone else noticed this? I?m just a ?dumb? engineer, but what am I missing here?

Scott Harding
RV-9 ? Slowing working on wings
Sugar Land, TX

mlw450802 · #2 07-02-2006, 03:15 PM

Quote:

Originally Posted by N722SH

After much delay, I am finally getting around to counter sinking all the holes in the wing spar flanges. (BTW - I?m building a ?9) The instructions say to ??mark the orientation of the spar?and study the plans until you understand how the spars are installed. You do NOT want to build a wing upside down!? Well, after studying the plans and staring at the spar for a while, I ?understand? how the spar is installed, but don?t ?understand? how the spar was built!

While ?studying? the spar and plans, I noticed that the upper and lower spar flange bars (the thick stepped bars that run along the top and bottom of the forward side of the wing spars, W-906 B & D) are in the opposite position from what I would expect. The bottom flange bar is shorter and gets thinner sooner than the top flange bar. This is counter intuitive from what I would expect - at least to me! I would think the longer and thicker flange bar would be on the bottom (bottom of the spar loaded in tension) with the shorter and thinner bar (loaded in compression) on the top.

Anyone else noticed this? I?m just a ?dumb? engineer, but what am I missing here?

Scott Harding
RV-9 ? Slowing working on wings
Sugar Land, TX

Perhaps the design consideration is for buckling, not yield. Tensile and compressive yield stresses are very similar and if that were the only design criterion, both top and bottom would likely be the same.
-mike

N722SH · #3 07-02-2006, 09:43 PM

Quote:

Originally Posted by mlw450802

Perhaps the design consideration is for buckling, not yield.

I thought about that, but the rivets and plate nuts go in the spar flange, which is on the aft side of the spar web - the side opposite the spar flange bars. The bars don't interfere with the riveting/bucking of the wing skins and are inside/beneath the tank skins.

Scott

mlw450802 · #4 07-03-2006, 05:50 AM

Quote:

Originally Posted by N722SH

I thought about that, but the rivets and plate nuts go in the spar flange, which is on the aft side of the spar web - the side opposite the spar flange bars. The bars don't interfere with the riveting/bucking of the wing skins and are inside/beneath the tank skins.

Scott

Just in case you thought I misspoke, I meant classical column buckling (i.e. Euler or Johnson type as appropriate), not rivet bucking.

-mike

N722SH · #5 07-03-2006, 07:27 AM

Quote:

Originally Posted by mlw450802

Just in case you thought I misspoke, I meant classical column buckling (i.e. Euler or Johnson type as appropriate), not rivet bucking.

-mike

Mike,

You are right, I did miss read your original post - Sorry.

I do think you are correct that the larger upper spar flange bar is to prevent Buckling. I have no desire to "re-engineer" the airplane, but wondered if anyone out there in RV land with experience in wing structures had an explanation for the difference in size of the upper and lower spar flange bars beyond the obvious.

Thanks - Scott