Strut angle conundrum

[PA1195]

Re: Strut angle conundrum

Forgot my camera at the plane today but did look at my strut ends...almost straight/2* and there's enough end exposed to measure the offset. I did look along my trailing edge and it's straight across the fuselage and easily observed. Pics tomorrow unless I forget again.

Wing chord is about +3.8* relative to the level. Designed for max lift over drag for the 23012 airfoil when level: See P. 28 https://ntrs.nasa.gov/archive/nasa/c...9930083935.pdf and https://ntrs.nasa.gov/archive/nasa/c...9930091603.pdf P. 436.

Gary

[Guest]

Re: Strut angle conundrum

Forgot my camera at the plane today but did look at my strut ends...almost straight/2* and there's enough end exposed to measure the offset. I did look along my trailing edge and it's straight across the fuselage and easily observed. Pics tomorrow unless I forget again.

Wing chord is about +3.8* relative to the level. Designed for max lift over drag for the 23012 airfoil when level: See P. 28 https://ntrs.nasa.gov/archive/nasa/c...9930083935.pdf and https://ntrs.nasa.gov/archive/nasa/c...9930091603.pdf P. 436.

Gary

Thanks Gary, that is good info.

Pondering it I was first inclined to say that it must mean then that the spar attach fitting hinge line is skewed +3.8 degrees to the horz. stab. (ie level).

But I think that is not a necessary condition. The cord may be skewed +3.8 degrees to the horz. stab. (ie level) and the hinge line (hinge line and stab are parallel).

Worst case though would be that the hinge line is skewed +3.8 to the horz. stab. (level) so the wings will sweep back as dihedral increases.

I don't believe (at this moment) this is the case, if it is though the dihedral may play a part in sweeping the front spar back in some condition that I don't understand yet.

I want to spend some time thinking about that.

Laughed out loud at "70 is the new 60" that's a good one!

Dave R

[Guest]

Re: Strut angle conundrum

I calculate that to make 6 degree difference over the length of that strut end fitting (i.e. the the last 1.8" at the top of the strut where the bolt goes, same as AA's pictures) that the end of the fitting must be swept back about .16" more on one style than the other and about .08" at the bolt hole.

Looking at AA's pictures they seem to agree.

Also the spar does not have to be swept back to create this issue, only the front spar strut attach fittings do.

A few odd things occur at the strut attach fittings; 1) you need to use a special short compression strut, 2) the plywood plates on the spar need to be the correct thickness, 3) the feet of that strut often have to be ground a little bit at the perimeter so it will sit down flat in the strut fitting. 4) there is a tube and wire that goes between the front spar strut attach fitting and the rear spar fitting.


Thoughts on items above;

1) Using the wrong compression strut will push the spars farther apart causing the front spar to be forward too much and that strut end to be angled forward instead of backward so that seems like a "false lead".

2) The plywood being too thin would cause the issue we see.

3) I wonder if the foot not being ground would cause the spar to twist in such and way to move it's bottom aft and cause this issue? I bet it could. The rubs against the stamped curved edge of the strut fitting and depending which edgeee rubbed worse it could twist the spar.

4) If that tube was too short, over tightened? or missing shims/washers it could pull the attach fittings closer together causing the issue we see.

Dave R

[PA1195]

Re: Strut angle conundrum

Interesting analysis Dave. The Factory must have done the optional strut ends to compensate for potential errors?...or, did they find it easier to offer the two strut shapes that allowed for differing build-up techniques and materials? Like using up excess parts, or compensating for a lack of wing component inventory.

Has anyone been able to focus on the date of production or model versus strut shape?

Gary

[Guest]

Re: Strut angle conundrum

Interesting analysis Dave. The Factory must have done the optional strut ends to compensate for potential errors?...or, did they find it easier to offer the two strut shapes that allowed for differing build-up techniques and materials? Like using up excess parts, or compensating for a lack of wing component inventory.

Has anyone been able to focus on the date of production or model versus strut shape?

Gary

You raise a good thought in my mind ; How could the factory have even tolerated such a situation? It makes no sense to me. Mistakes being made in the field would not be planned for but rather prevented.

Yet these strut angle differences appear to me a production practice if you believe that the strut angle differences on old struts are NOT a result of damage. When I look at some of the 8 degree struts I think I see bulging that could be a result of damage.

I am confused.

Maybe I'll relax a join and Trump protest for the evening .

Dave R

[PA1195]

Re: Strut angle conundrum

Hmmm...assuming rearward strut deformation via wing impact. Plane is fixed and would the bent-back strut ends still fit a properly rebuilt wing? Probably not. If bent as such, were they simply recycled in the after-parts market?

How about "Taylorcraft had a bent-up jig E-I-E-I-O" And on that jig they built some planes, etc...once built how hard to fix an airframe? Change strut design. Best measure first across the airframe fitting to fitting for front and back attachments.

There is a huge moon out tonight (closer to the earth since 69 years ago; 14% larger) so maybe the troops like us are a bit wingy.

Gary

[Guest]

Re: Strut angle conundrum

They did have defective fuselage jig for a period of years.

They discovered it in the early '80s.

It had spent time in a salvage yard prior to that and it was assumed to have been ok before being in the yard. Mrs. Feris told me that.

However it may have be defective even before it got to the yard??

If such a thing had any bearing on this then a stream of serial numbers would have 8 degree struts.

No moon here just rain, I could still be luny though.

Dave R

[PA1195]

Re: Strut angle conundrum

"it was assumed to have been ok before being in the yard"

Can't ignore the unknown and assumed possibility. How did they determine it was different? Fuselage prints?

Wonder where it varied? Wing attach points, strut attach points, or ???

Maybe if enough are interested we could all have a look at our struts and estimate the offset variability? Build a database of sorts.

If so then the question I have is what's to be considered a normal offset? I'm going to assume nothing to 2* as starters.

I don't believe my "normal offset" suggestion makes for a better airplane. It's just an airframe difference that can be adjusted for via different angles of the outer strut.

Gary (wife and dog are luny during the full moon...and one barks louder than usual)

[Dick Smith]

Re: Strut angle conundrum

When the strut AD came out, I followed the discussion here on the forum and ordered a set of new sealed struts from Airframes Alaska, which is located about 220 miles north of us.
I drove up to Birchwood Airport and picked up a new set to install on my 1947 BC12-D, which I've owned since about 1974. When I brought the struts home, I primed and painted them and set about installing them. Just holding the new front struts next to the old strut still mounted on the plane, I could see that the angle on the upper tang on the new one seemed to be a bit more angled than the old one, but assumed the approved, STC'd strut would fit. In any case, I pulled the old strut off and tried to fit the new one...no way...when one end was installed and a bolt inserted, the other end would not begin to line up properly. I called the company and explained the apparent misalignment problem. They said they had used Taylorcraft struts as a pattern for the struts they were selling and were surprised at the problem. I told them I laid the new, front struts on the floor, upper end of each strut touching the other, ...then I flipped one strut over, so one was pointing "forward" and the other was pointed "backwards"...then slid the two, wide, upper tangs together, side by side and slid a new bolt through them...snugging them together with a nut. Since each tang was was welded at the same angle, I assumed, the "mirror" image effect should, when the tangs were snug and parallel, make the struts lay on a straight line relative to each other. I stretched a string from one end of the "end-matched" (but "flipped") assembly to the other and taped it down on the ends. The visual effect was that the string stretched down the centerline of the lashup. Long story short, I figured out a way to remove my old struts, which came off easily when the bolts were driven out...and performed the same "test" with the string. My old struts showed the same straight-line effect. I then took one of my old struts and did the same "test", pairing it with one of the new struts...bolting them together and the string told the "Story" very plainly. I called the company and explained my "farmboy" test. They said they (the owner and his chief engineer) would be down to Kenai in a day or so. I met them at the hangar where my two sons worked and demonstrated the "String Test" first using my old struts and then their new struts and finally one of mine coupled with one of theirs. They said, "we see what you mean". They took my old struts and the faulty set back to their operation and built me a new set, which fit perfectly. They used my old ones to correct the jig. If you look at their website and open the section explaining how to measure your struts, you will see a yellow strut next to one of their green-primered struts. That yellow one is my old one...and you can see the angular difference. I have both of my old yellow struts out in my shed and have new ones on the plane now.

I'm not sure exactly how this problem developed. I do know that Airframes Alaska builds really good stuff. There are SuperCub guys that buy a lot of stuff from them and many of the Cub guys have more money than you can imagine in their planes and they continue to buy from them. One comment that was made in this thread mentioned the oddball jig that the Ferris planes or some of them, may have been connected to. I didn't ask the Airframes guys if their original pattern struts came off of a Ferris plane...wasn't aware of the "jig" issue possibilty at the time. In any case, Airframes Alaska made the effort to look at my situation and made it right. And maybe we are a little closer to understanding what happened.
Hope this helps someone. Dick

[Guest]

Re: Strut angle conundrum

When the strut AD came out, I followed the discussion here on the forum and ordered a set of new sealed struts from Airframes Alaska, which is located about 220 miles north of us.
I drove up to Birchwood Airport and picked up a new set to install on my 1947 BC12-D, which I've owned since about 1974. When I brought the struts home, I primed and painted them and set about installing them. Just holding the new front struts next to the old strut still mounted on the plane, I could see that the angle on the new one seemed to be a bit more angled than the old one, but assumed the approved, STC'd strut would fit. In any case, I pulled the old strut off and tried to fit the new one...no way...when one end was installed and a bolt inserted, the other end would not begin to line up properly. I called the company and explained the apparent misalignment problem. They said they had used Taylorcraft struts as a pattern for the struts they were selling and were surprised at the problem. I told them I had laid the new, front struts on the floor, lower end of each strut next to the other, in the relative position as they would fit on the plane...then I flipped one strut over, so one was pointing "forward" and the other was pointed "backwards"...then slid the two, wide, lower tangs together and slid a new bolt through them. Since each tang was welded at the same angle, I assumed, the "mirror" image effect should, when the tangs were snug and parallel, make the struts lay on a straight line relative to each other. I stretched a string from one end of the "end-matched" (but "flipped") assembly to the other and taped it down on the ends. The visual effect was that the string stretched down the centerline of the lashup. Long story short, I figured out a way to remove my old struts, which came off easily when the bolts were driven out...and performed the same "test" with the string. My old struts showed the same straight-line effect. I then took one of my old struts and did the same "test", pairing it with one of the new struts...bolting them together and the string told the "Story" very plainly. I called the company and explained my "farmboy" test. They said they (the owner and his chief engineer would be down to Kenai in a day or so. I met them at the hangar where my two sons worked and demonstrated the test using my old struts and their new struts and one of mine and one of theirs. They said, "we see what you mean". They took my old struts and the faulty set back to their operation and built me a new set, which fit perfectly. They used my old ones to correct the jig. If you look at their website and open the section explaining how to measure your struts, you will see a yellow strut next to one of their green-primered struts. That yellow one is my old one...and you can see the angular difference. I have both of my old yellow struts out in my shed and have new ones on the plane now.

I'm not sure exactly how this problem developed. I do know that Airframes Alaska builds really good stuff. There are SuperCub guys that buy a lot of stuff from them and many of the Cub guys have more money than you can imagine in their planes and they continue to buy from them. One comment that was made in this thread mentioned the oddball jig that the Ferris planes or some of them, may have been connected to. I didn't ask the Airframes guys if their original pattern struts came off of a Ferris plane...wasn't aware of the "jig" issue possibilty at the time. In any case, Airframes Alaska made the effort to look at my situation and made it right. And maybe we are a little closer to understanding what happened.
Hope this helps someone. Dick

Hi Dick,

Thanks for the info.

I have seen other FAA/PMA parts fabricated from reverse engineered used parts that turned out incorrect because the used part that was copied was damaged.

In some cases I have the actual drawing and comparing the FAA/PMA part to the drawing makes it clear that the FAA/PMA part is not correct. That's not a critique but merely evidence that such things happen.

It also makes me wonder if the 8 degree strut and plane combinations are a result or damage and/or some field assembly mis-configuration rather than factory differences.

You used a word that I don't know (lashup) can you elaborate about what it means?

Thanks, Dave R.

[Robert Lees]

Re: Strut angle conundrum

I bought ten sets (yes, ten!) from Airframes Alaska in early 2008, for ten different BC12D aircraft here in the UK, and they all fitted.

Rob

[Guest]

Re: Strut angle conundrum

I bought ten sets (yes, ten!) from Airframes Alaska in early 2008, for ten different BC12D aircraft here in the UK, and they all fitted.

Rob

Hi Rob,

How many 8 degrees and how many 2 degrees sets did you purchase?

Dave

[Robert Lees]

Re: Strut angle conundrum

None or all...I just ordered 10 sets, with no spec because I didn't know there was a difference (and I am still not aware of the nuances).

Nine (9) of the aircraft were post-war 1946 BC12D. One (1) of the aircraft was a Nov 1941 BC65 fuselage, but not completed & registered until Nov 1945. They still all fitted.

Rob

[Dick Smith]

Re: Strut angle conundrum

You used a word that I don't know (lashup) can you elaborate about what it means?

Thanks, Dave R.[/QUOTE]
----------------------------------------
Dave, I've always used the “lashup” term to refer to something that is temporarily cobbled together to accomplish a purpose or in this case demonstate a point...it works, sort of...but it's really “quick and dirty”! Dick

[Guest]

Re: Strut angle conundrum

Thanks Dick, I learned a new word today

I am taking a Mandarin Chinese class so I learn new words but not so much in English.

Dave R