No slow Super Cub

I'd think they're a
consequence of simple inch dimensions.

frequent_flyer said:

What is the source of these numbers? Does anyone really believe that Piper specified angles to 0.001 degree on anything they ever built? What is the use of such a specification without an associated tolerance?

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It's not a Piper specification. It's document compiled by Gordon Mandell, the author of the alternate rigging instructions in AC 43-16. It's a linear representation of the twist of a cub wing defined as the length between the fuselage center line and the wing tip. You can make your own spread sheet by dividing 2.5 by 211.25 and applying that to any station along the wing starting from the fuselage centerline. You can assume things like Gordon got the definition of the wing and the 2.5 degrees from Piper.

Jerry

PA-22/20-160 said:

The source is right at the bottom of the letter

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No, that states who compiled the data not who derived or specified it.

I'm pretty sure it was Gordon. The basis of this document was Atlee's rigging advice to use a 3/8" block to set washout on sawhorses. Jerry can correct me but I think the 3/8" block works on a leveled plane? When building my -12 I was so confused I didn't know what to do. I think we ended up using a 1-3/8" block. No idea why but that's the number I remember. If memory serves back then Atlee made different tanks for -18s and -12s because he believed the washout (twist) was different. Gordon set it straight that the washout was the same and that only the base AOI number was different. Lots of planes are out there rigged with 3/8" of total washout and I've heard they fly great. Also memory based, but I think the 3/8" twist was what the J3 used? I know diddley about J Cubs so I don't have a clue. Steve Kracke can fill in the details about Atlee vs Gordon. They had some history.

I recall Mandell's notes from many years ago, but failed to find them here. A dead link .PDF appears to lead there, but? They are worth looking at.

Gary

AC 43-16 appears to have been cancelled so have been unable to find out what it says. I was familiar with how wings are built with an offset block under the rear spar. If the block was specified as 0.3750 +/- 0.0005 inch then I can see some justification for the twist being specified to 0.001 degree. If the block is specified as 3/8 inch with no tolerance then specifying twist to 0.001 degree is nonsense.

The twist built into the wing is only one of the errors that has to be considered when measuring wing surface angle relative to longitudinal fuselage datum. We have already been told that Piper introduced errors over 1 degree in the positioning of the fuselage spar attach fittings.

Just a retired engineer commenting on unreasonable specifications. I don't pretend to have any expertise in rigging Cubs. I have no reason to change the rigging on my FX-3. It flies straight, it flies fast, and it flies slow.

frequent_flyer said:

AC 43-16 appears to have been cancelled so have been unable to find out what it says.

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https://www.faa.gov/aircraft/safety/alerts/aviation_maintenance/media/1997/1997_08_Alert.pdf

Gary

frequent_flyer said:

No, that states who compiled the data not who derived or specified it.

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You would have to know Gordon Mandell. He is one of the most qualified, knowledgeable and common sense FAA engineers I've ever known or worked with. He was a great help to me in acquiring several STCs. You would like him, as he is very much like you in his attention to details.

I THINK Piper's spec for the 18 is a 3/8 block and for the 12 1-3/8. I know those are correct numbers, but I don't know for sure whether they are Piper original specs. The difference is due to the different wing incidence. With the fuselage level, the -12 incidence is negative and the -18 incidence is positive. Hence the different outboard rib angle with the fuselage leveled.

Edit: I agree re too many significant figures in that degrees spec!:lol:

PA18 rigging instructions, and alternate instructions approved per AC 43-16.


20181116105333.pdf (fadodge.com)