Building Fin

I'm setting up my Fin for welding and there seems to be an issue with the length of one of the ribs that I got from Airframes? Laying out to Piper drawing 40592 fin assembly. All dimensions are accurate but the lower rib is too short? Two possibilities - 1 is I enlarge the slot on the rib and slid the rib north so it remains parallel with the top rib but places the rib about 3/4 of an inch higher than specified or 2, I angle the rib so it connects the post and leading edge but it will not be parallel to the top rib which doesn't seem right. I have a photo but I'm not sure how to post. Perhaps I've got the wrong drawing? (northland) http://www.flickr.com/photos/38941687@N07/3579540732/
http://www.flickr.com/photos/38941687@N07/3579540656/

The distance from Fin post to front attachment tube is stated as 28 - 5/8ths. If this was shortened than the rib would fit. Can I do this instead and adjust at the stab attach point on the screw tower?

Cheers.

J,

The drawing is a Piper drawing and SHOULD be correct - it was not altered by Northland.

Does the rib measure 24 15/16" long? If it does, then the rib is the right length. If not, and you have the time - have them get you a new rib - don't mess with the engineering if you don't have to.

Double check that your measurements are center of tube to center of tube at the bottom.

Assuming that the measurements are correct there, moving the stab attach point would be a slippery slope - one that may add up to all sorts of measuring problems.

Bushwhacker Air said:

J,
Assuming that the measurements are correct there, moving the stab attach point would be a slippery slope - one that may add up to all sorts of measuring problems.

Click to expand...

Boy, I'll say! Been there - there were several pain-in-the-neck, unforeseen consequences of inadvertently moving the stab tower only 5/16" aft.

Chris,

The measurements are from center tube to center and it is indeed correct at 28 - 5/8. The rib is 24 and 7/8ths long. That 16th should not be making the difference. I could move it up so that the bottom of the rib is at the correct measurement, not the top as per drawing. That puts me around 1/2 an inch off from specs. I don't think that would make that much structural difference here. I have used a .045 wall leading edge tube which stiffens that up a bit. It does mean that I have to A: shorten the diagonal brace to fit through the slot in the rib or B: elongate the slot so that the diagonal is the correct length along with it's attach point on the fin post. That seems to weaken the rib however. Any advise?

12 Geezer said:

Bushwhacker Air said:

J,
Assuming that the measurements are correct there, moving the stab attach point would be a slippery slope - one that may add up to all sorts of measuring problems.

Click to expand...

Boy, I'll say! Been there - there were several pain-in-the-neck, unforeseen consequences of inadvertently moving the stab tower only 5/16" aft.

Click to expand...

wow, like lack of total travel then, hit at up position on front tower tubes early.....

Yup, travel stops had to be redone, but the worst was the elevator bellcrank wanted to interfere with the longerons where they vee together. It worked out ok, but would have been easier to cut off the tower and put it back in the correct position right away!

Okay, so moving the attach point on the tower is a bad idea. I have discovered that the lower brace is too short also. I would have assumed that Airframes knows what they are doing? when they sent me the ribs for this along with the Piper channel. Anyway, I cut a new piece of channel the correct length and raised the lower rib to fit and had to enlarge the slot so the brace would fit in it's correct position. I plan on re-enforcing the rib around the cutout slot. As I mentioned before, I had beefed up the leading edge with a thicker wall section so moving the rib slightly higher should not make a structural difference. Forgive my ignorance, I am only a sculptor who learned to fly and wants to drive a cub - late in life!

Jt... If your happy with the mod (sounds good to me) great. If not, let me know and I will make you a new rib. I got pretty good at making them last summer for my tail feathers; everything from scratch even the bender. Lets just say this is from a wood carver to a sculptor with the same desires in life. Let me know if you need it made, gratis on my part.

Marty57

www.xanga.com/martyfeehan

12 Geezer said:

Yup, travel stops had to be redone, but the worst was the elevator bellcrank wanted to interfere with the longerons where they vee together. It worked out ok, but would have been easier to cut off the tower and put it back in the correct position right away!

Click to expand...

yeah that tower to back stab attach point is one of the few dimensions that must be exactly right or you bring all sorts of evil to bear on yourself....

You guys are princes among men! Marty, thanks for the offer on the new rib, that goes way over - above and beyond! I will stick to the mod and proceed.

Cheers everyone!