Strut angle conundrum

[astjp2]

Re: Strut angle conundrum

If properly torqued, they clamp up...the attach fittings are only sheetmetal that is bent....

The bolt is NOT supposed to clamp up the fitting on the strut. The standard for pinned joints is that the bolt should be able to rotate (yea, it doesn't say how much torque you may need). Squeezing the lugs together is NOT a good idea. They were designed for a bolt bearing load, not a bending load plus bolt bearing load.

You also would have to put a bending load on the strut itself which would significantly reduce the long column buckling load capacity. Under a snow load or negative "G" the strut would fail much sooner. Not a huge risk for how our planes are flown, but not a good idea.

[Hank Jarrett]

Re: Strut angle conundrum

Is there a torque requirement on the bolt? Not the nominal torque for that size bolt, but a factory spec for torquing it. It is NOT normal to torque a bolt in a pin fitting. It is SUPPOSED to be able to rotate when loaded so the loads transfer directly down a line between the pins (why you also do NOT want the strut to bow). If you torque the bolt there is a moment introduced into the joint that makes the design indeterminate. Engineers DO NOT like indeterminate joints. It is almost impossible to analyze, especially with the tools available in the 40s.
Unless you have a spec from the factory saying to torque the bolt you SHOULD NOT do it.

[Hank Jarrett]

Re: Strut angle conundrum

Is there a torque requirement on the bolt? Not the nominal torque for that size bolt, but a factory spec for torquing it. It is NOT normal to torque a bolt in a pin fitting. It is SUPPOSED to be able to rotate when loaded so the loads transfer directly down a line between the pins (why you also do NOT want the strut to bow). If you torque the bolt there is a moment introduced into the joint that makes the design indeterminate. Engineers DO NOT like indeterminate joints. It is almost impossible to analyze, especially with the tools available in the 40s.
Unless you have a spec from the factory saying to torque the bolt you SHOULD NOT do it.

[astjp2]

Re: Strut angle conundrum

You can argue that with the FAA, it would get a nominal torque at the strut and only snug on the spar.

Is there a torque requirement on the bolt? Not the nominal torque for that size bolt, but a factory spec for torquing it. It is NOT normal to torque a bolt in a pin fitting. It is SUPPOSED to be able to rotate when loaded so the loads transfer directly down a line between the pins (why you also do NOT want the strut to bow). If you torque the bolt there is a moment introduced into the joint that makes the design indeterminate. Engineers DO NOT like indeterminate joints. It is almost impossible to analyze, especially with the tools available in the 40s.
Unless you have a spec from the factory saying to torque the bolt you SHOULD NOT do it.

[PA1195]

Re: Strut angle conundrum

Which end of the strut is being discussed?

Edit: If it's the outboard/wing attach end then look closely at my Post #40 pics of the ends. The threaded/rear end of the bolt is visible in all four and appears to assume the same position in the spar brackets. But for #3, that looks like one of those 8* strut ends, the bolt looks like it passes each bracket the same distance from each end. I can reexamine that plane and take more pics if desired.

Gary

I'll make it easy:

[Dick Smith]

Re: Strut angle conundrum

I noticed when I tried installing my first set of Alaska Airframes sealed struts, that the excess angle of the upper-end lug, coupled with the rigidity of the bracket channels, which had the rectangular steel plate welded to them, didn't allow the hole in the lug to align with the holes in the channel pieces. This problem went away when Airframes built the second set to match the angle of the original struts. In fooling around with the first problematic set, I tried aligning the bracket holes with the relatively fixed hole of the lug, by using a tapered rod. You could see right off the bat that the movement that would be required was way out of bounds...forcing things to line up and then just hammering a bolt through wouldn't work. Today I fooled around with an old piece of front spar and two spare channel brackets that are bolted together with the spar between them, but without a rectangular plate welded to them (as used in the planes that are powered with the C85 via STC and F-19's. I noticed that the 2 channel brackets had a bit of "give" in them, as opposed to the boxbeam affect
that is created when the flat plate is welded in place and the channels are squared-up and snugged via the spar and the related bolts. I guess what I'm thinking is that the older planes without the flat plate welded on, allow a bit more flex and are not so 'touchy" regarding near perfect alignment, whereas the higher power rigs probably had the upper attach brackets built in a jig or at least were built to be squared-up and rigid...and consequently don't allow much, or any, movement. I know we all have made things "fit" via tapered pins and a bit of judicious "tapping"! Still mystified by which planes or run of planes would accept the "over-angled" strut lugs. On mine it was a "Go" or "No Go" deal.

Anyhow, I was thinking that the channel bracket with the flat plate welded on might make even relatively minor angle-misalignment more of a pain. Oh, another thing...my wings appear to have a slight backsweep! I guess I'll have to get out the stringline.

(I included a picture of an old rough-looking spar/bracket, that didn't have the flat plate welded on...and a picture of one that does have it.) Dick

[PA1195]

Re: Strut angle conundrum

I wonder if this is an example of the "which came first the chicken or the egg" scenario for F-19's (and possibly BC12D-4-85 and Model 19's that preceded the F-19?). As in Taylorcraft built the wings with the stronger welded channel brackets then found on some planes the standard offset struts didn't fit the hole alignment...so they modded the struts rather than change the wing's fittings? Maybe a few wings got done one way before they could correct the build design.

Not sure what to make of the wing backsweep, as in why like trammeled wrong? Will take another good look at the F-19 plane #3 in my pics to see if the wings sweep back. I didn't think so but may have missed that. FWIW plane #2 above is also an F-19, the rest are earlier models.

Gary

[PA1195]

Re: Strut angle conundrum

Checked the 4 T's again today for markedly visible sweepback...none noted for any including the F-19 T's below.

So here's what a 2* (left pic) and 8* (right pic) upper strut look like on F-19's. I have no idea why the difference.

Left is F-19 F-104; right is F-19 F-143:

[astjp2]

Re: Strut angle conundrum

Shitty tolerances and poor quality control

[PA1195]

Re: Strut angle conundrum

Shitty tolerances and poor quality control

Seems to be the case. But what fix is developing a new set of struts versus tig welding a couple of heavy washers over correctly line bored and revised wing fittings and using the original strut offset?

Now if they show up as needed on non-F-19's then there's something else to consider.

Gary

[Dick Smith]

Re: Strut angle conundrum

Gary,
Went back over to the plane...crawled up on a snowcovered sawhorse and re-checked the "backsweep" I observed yesterday. By lining my eye up with the leading edge and moving my head aft relative to the wing...I saw that both leading edges appear to lay on a straight line. The llittle bit of dihedral deceived my brain and I would have bet you that the wings had a bit of backsweep! By moving my head aft, the perspective changed and the "straightline" effect appeared. I guess things are not always as the appear. Dick

[PA1195]

Re: Strut angle conundrum

Well Dick you braved the slippery tippy stuff for the good of the group! Ladders and age don't mix I hear. I looked again today at the local 4 and couldn't see any visible differences in sweep. I'm sure they all fly ok despite any warts.

Thanks again for helping Dave M from Fairbanks in his pursuit of that local Taylorcraft. I hope he finds what he wants and joins the Tribe.

Gary

[astjp2]

Re: Strut angle conundrum

I am hearing a whole lot of worry on here over 6° that is a minor misalignment of the strut attach hole

[Guest]

Re: Strut angle conundrum

No worry, just curious about what causes it.

And we are free to worry/discuss as much as we want.

That aside who forced you to read it?

[Guest]

Re: Strut angle conundrum

Gary, Dick and all,

Thanks for your efforts.

I have been tied up the last few days doing Stewarts Systems EkoPoly topcoat on a wing while we had some 70 degree days here in North Carolina.

I confess that for a few days it was starting to make sense to me but at this point after my fiddling and what you guys found it still makes no sense to me in that no specific explanation describes it well.

That is other than somebody somewhere (factory or field) screwed up and some parts (strut, fittings, wings...) don't fit right and the strut has to be modified to bring it all together.

I suppose that the small number/percentage that AA sells of the 8 degree strut could be used as evidence that it is a situation that is based on exceptions rather that the norm. Also I think AA (Alaska Airframes) is the only fabricator making 8 degree struts so that may further the theory that it is a small % and a result of anomalies somewhere.

Dave R