Horizontal Stabilizer skin oil canning

[billy]

I own a 1946 Cessna 140, assembled from a basket case in 2010. It is a great flying little plane. I had a concern when I first started the assembly about the horizontal stabilizer flexing. I joined this web sight back then for information on the build up and it was very helpful. I remember this subject of the stabilizer being discussed back then on this forum, and it seemed to be a common issue. I have to ask the many owners on this sight, is this a common issue? I borescoped as best I could, the area it seems the flex is coming from, and found no cracks in the stringer or the bulkhead. The flex (oil canning) seems to be mainly in the center under the vertical stabilizer, and up to two stringers outboard. All the rivets are tight and in tact. I did a search on the forum before this post and found a little discussion about this subject under Rear strut condition from June 16 2020, one member, 6597, said as far as oil canning " some do some don't". I have not flown this aircraft in a while, and flew again recently. I do not hear any pops in flight, however, these are very noisy acft. I really do not want to take the skin off to inspect it for cracks if its a common issue. I even seem to remember one gentleman posting that he had reskinned his stabilizer and it still oil canned. Any input on this subject would be appreciated!

[billy]

You know, since I posted this I have observed quite a few aircraft from this era and before. Allot of them had upper and lower stabilizing rods on the stabilizer. Newer aircraft have used stringers between the spars,or ribs to add support. This has neither? I wonder if it's a simple design flaw. Also on the post I mentioned above someone posted about downward force in flight rather than an upward force. I understand that you are talking about the air coming from the top of the wing down on the tail. But I may be wrong , but would this be the case on descent and at Stall? Interesting thought, I was thinking a flying tail would have the same forces as the wing? Again any input, especially on the stabilizer flex, I know there's many owners, anyone's input would be appreciated.

[2066]

While it prob has little effect on oil-canning, there's a stab reinforcement that was left out of some early SNs as I recall, so be sure that it's in there (...best I recall, you can almost "squeeze" the leading edge inboard and "feel" if it isn't?). Mine does have the oil-can...right side only, and has had for the 37 years owned/flown. It never gets any worse and I've watched IAs frown at it many times over the years @ annuals. Me, too, but I'm almost begining to relax . It is noisy when taxiing, especially with a solid TW, so difficult to "hear" any popping. The scary thing over the years has been to watch a few owners "grab" the stab to move their 120 or 140 the same way they did their old Pipers -- if that's been done, once or repeatedly, there may be an issue that can't be seen. ...but, yep, from my limited experience and view, it may be "the way it is" for much of the fleet(?). There, that didn't help at all, did it . Mac

[6643]

on the post I mentioned above someone posted about downward force in flight rather than an upward force. I understand that you are talking about the air coming from the top of the wing down on the tail.

The tail is a negative lift surface: it pushes the rear of the plane down to counteract the normal tendency for the nose to drop. This happens because the center of gravity is forward of the center of lift, to give the plane stall resistance and "automatic" stall recovery. The only time the tail provides lift is when you put the plane into a dive.

[billy]

Mac, John, I greatly appreciate both your inputs, and it does help,also puts my mind at ease a little. Very interesting about the tail, it makes perfect sense that it's called a stabilizer, and power off off the nose will drop so normal flight would be a downward force. I always thought it was opposite of that, thanks John. I will check into the spar reinforcement you mentioned Mac, as well. It is of some comfort to know it's not uncommon, considering I can not find anything wrong with it. Thank you both.

[6597]

Here are some pics of that reinforcement piece, at least you'll know what to look for.

[billy]

Thank you David, by looking at the pictures you sent it appears this is a channel that fits top and bottom of the leading edge under it's lip or overlap of the stabilizer. Is this a service bulletin if it's not installed? I know it's not an AD unless I am overlooking one of them. I will look an see next time I am out at my hangar. Thanks again!

[atypicalguy]

I recently saw a video on this reinforcement strut. I cannot seem to locate it, however. Does anyone know the video I am referring to?

Thanks,
Karl
90083

[atypicalguy]

Seems like this reinforcement would be a good idea to add at some pioint if anyone has a part number.

[6597]

https://www.univair.com/cessna/parts/u0 ... a-channel/

From Univair, 0432160