Main Spar tube thickness?

[Tree top pilot 13]

OK I have gotten hold of a local supplier {less than a three hour truck run} that has the 2.5" dia. 6061 T-6 tube. They have .065" and .125" wall thickness. They also have titanium, aluminum, delrin sheet, and 4130 sheet stock in 18" X 36" X .050" for me to fabricate a new set of lift strut attatch fittings. I wish they had .089" as I seem to have read somewhere that this was optimal tube size on most gross around 1000 lds. for this type of Kitfox/Avid wing design. My kit calls for .065" and no stiffener internally , but the gross on this aircraft will be under 575 lds. Just wondering what spar tube thicknesses are deployed and if they have internal brace parts. BTW I did find a media mail parcel today on the front porch....build manual. Interesting read tonight. Still no answer on the tele or e-mails....guess I am on my own from here out. The shop is near the S.C./ N.C. border if anyone in the area needs materials. Well worth a drive accross the state for less than 1/4 of the shipping fees. Website.....http://www.stockcarsteel.com/

And they have prices on line !

Edited 23 Dec 2011 by Tree top pilot

[dholly 718]

Avid used both .065" and 0.83" x 2-1/2" o.d. spars, see HERE.

Kitfox used/still uses .065" x 2-1/2" spars

Both used spar stiffeners of various material and design depending on model

My Mk-IV had 1/4" x 16" ply stiffeners, my Avid+ had 1/4" x 18" ply stiffeners, my KF3 had 4' extruded aluminum I-beam stiffeners

If you source new spar material, I suggest .065" for weight considerations. Plenty strong for your M.T.O.W. as is, but I would still add a simple 1/4" x 16" or 1/4" x 18" ply stiffener like the pic attached (Note: I added some hardwood quarter round molding to stiffen my stiffeners).

[Tree top pilot 13]

Mnat thanks ! Will fab up a set soon...good tip on the 1/4 round addition. A length of , say , 24" spread the load well? Nice looking attatch fittings ...will be making a set of these soon. Interesting rib lightening holes in the links....I will post a pick of the rib I have...has more of a trusswork cutout than oval holes though. I would tend to think that the oval hole would stand more compressive loads. Still not too keen on riveting a flattened tube to the spar for a drag tube....still going to make some brackets like the Kitfox uses for this...using the tube as supplied {.500" O.D.}.

[dholly 718]

The Avid (all models) and the KF1-3 ribs used the oval shaped cutouts. When the KF went to the new Riblett airfoil for models 4-1050 and up, they changed to the truss style cutout. I imagine the increased webbing of the Avid ribs would prove stronger, however, there is a point of weakness in the Avid rib design that the KF 1-3 airfoil resolved (at least in my opinion). In the pics below, note that the Avid HH rib (w/blue tape on capstrip) has a break in the webbing at the rear spar... see how the capstrip rests directly on the spar. The rear spar hole through the KF1-3 rib web is positioned slightly different and allows contiguous web material between the spar and top capstrip. That HAS to be stronger iMO.

In any event, many folks have been led to believe that the KF1-3 and the Avid HH/STOL ribs are identical but clearly that is not so. While the profile along the top of both ribs is identical from the very front spar cutout to the end of the flaperon hinge tail, the under camber of the lower rib profile is greater by ~1/4" on the Avid airfoil. That, in addition to the different web design, results in a thicker airfoil section at the rear spar fo the KF1-3 airfoil. Note: the Avid A - Mk-IV wing uses (2) wood stringers vs. (1) aluminum stringer for the KF1-3, my Avid+ uses (2) aluminum stringers. The yellow rib is my KF 4-1200 'truss style' rib for comparison. I be very curious to learn see how your RR rib profile compares to the KF1-3 profile if someone near you has one laying around.

Edited 23 Dec 2011 by dholly

[Tree top pilot 13]

Here is a pic of my ribs and the flap/aileron rib. Total estimated cord length is 46". There is no rib portion to encapsulate the spar tube at the rear location either. I did not get the fiberglass leading edge material for the flaps or the ailerons...will probably use 1/16" acft ply for that chore....unless I come up with something lighter. I figure I can steam it ,bend over a form and leave it for a few weeks until it takes a set. There are a lot of things I just don't get...OK the wing design is long proven strong and safe....but what if you made aluminum ribs...and faired the wing together using 2024 aluminum sheet for leading edge? Get a bit closer to a production type, lighter structure? I may invest some time in making a set of dies for doing the ribs...if there may be an intrest in this route of construction. This leads me to want to build a fuse jig...if only my job duties did not get in the way of my dreams/hobbies.

On a side note...man I am really diggin those attatch fittings....got to make a set soon. Trying hard not to covet. I really appreciate craftsmanship in any part...not many do. I take pride in my work and try to convey that into whatever I make/build. A friend of mine once said....you are only as good as the last job you turned out.... You think that if I use a sponge to apply West epoxy to the ribs to seal them up it would be too heavy? I relish the idea of sealing them up and adding a strength factor. Your thoughts? The stringers are .500" T-6 ally. 2 false between each rib.

Edited 23 Dec 2011 by Tree top pilot

[Tree top pilot 13]

Main spar tubes !!!! And a few other goodies picked up today......4130 sheet stock, kevlar fiber, carbon. Now I can start to build something. The guys at the supply house let me pick through the pile of tubing to get exactly what I wanted.....concentric and not gouged or scratched. Primo tubes for spars. Also picked up a 48" X 96" sheet of 5052 H32 aluminum to make trailing edge with.....the plywood as shipped for the task weighed better than 12 lds. !!! This thing is going on a diet. Now just need to figure out what to make the aileron/flap leading edges of. Steaming the acft. ply I have didn't work out so well....not good enough to use. May have to buy the fiberglass if someone would sell it to me.

Edited 27 Dec 2011 by Tree top pilot