Extended Leading Edge rib stitching

It might have nothing to do with the LE scallop as to if a wood spared J3 was sweeter to fly then a metaled spared one, it might be because the wood winged J3 was lighter, not sure, I've only flown a dozen or so Cubs from 37 hp to a 180 hp CC and I think the less then 550lb, 37 hp E2 flew the sweetest of them all. Am I allowed to say that on SC.org :lol:

Glenn

The best flying J3 I flew was a 1938 with a 65 Lycoming solo. Add the passenger and it was the scariest. :lol: I'm gonna stick with light and stock on my own SC wings.

Some time ago, maybe a year or two, somebody here posted a link to an engineering report of wind tunnel testing of scalloped vs non-scalloped wings. I wish I could find it again. My recollection of the conclusion is that the scallops had minimal to no effect on stall AOA, or lift or drag as a function of AOA. I remember that conclusion because I was a little bit surprised. I THINK it was another thread on extended leading edge skins.

This topic has gone way beyond my original question, however, it has answered another question which I have had. Why extend the leading edges? & why do some folks see performance improvements and some are disappointed? My guess is that the first person who installed extended leading edges did it to correct the effects of loose fabric. The technique used to iron the fabric controls whether it sags or scallops, aft of the LE. I have found that if you iron between the ribs first, the fabric will shrink in the shortest direction first, chordwise. You will get a scallop or depression, that cannot be raised, similar to a dope job on cotton or linen. If you iron on top of the ribs first, the fabric will shrink spanwise first with the ribs preventing the scallop by holding the loose material up. Once the fabric is tightened over the ribs it defines the shape so that when you shrink between the ribs the fabric comes up to near the level of the rib cap.

I had a low fabric wing airplane, Cessna T-50, that you could see the fabric rise up between the ribs. It didn't raise very much, maybe an 1/8". The change in airfoil was minimal.

As far as increased stiffness of the wing is concerned, why? Why would you want the wing stiffer? Have there ever been any structural problems related to the wing stiffness on the cub? Have you ever seen an airliner wing in rough air? It moves up and down many feet. Something could break if it were too stiff.

I do not see any advantage to an extended leading edge other than someplace to set down the gas can. My wings came with it so I am using it. If they didn't I would not have put it on.

12 Geezer said:

Some time ago, maybe a year or two, somebody here posted a link to an engineering report of wind tunnel testing of scalloped vs non-scalloped wings. I wish I could find it again.

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See my post about 1/2 way down the first page.

John Scott

skywagon8a said:

.. it has answered another question which I have had. Why extend the leading edges? .....

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one other huge benefit is if you have beat up old ribs with repair patches on them, the extended LE and a little silicone under it where needed makes the wing look smooth and like new......

Had I been allowed to extend my leading edge metal I'd have done so simply for improved durability when removing snow. I could have been more aggressive (less timid?) when sweeping the wings. That's a practical advantage in my world.

SB

Longwinglover said:

12 Geezer said:

Some time ago, maybe a year or two, somebody here posted a link to an engineering report of wind tunnel testing of scalloped vs non-scalloped wings. I wish I could find it again.

Click to expand...

See my post about 1/2 way down the first page.

John Scott

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Thank you John. That's the one. Dunno how I missed that, but I sure did.

Looking at the curves on the last page of the report, it appears the scalloped wing might have a gentler stall - at least the lift curve is continuous through a larger AOA for the fabric wing, though maybe the veneer wing wasn't tested at those higher AOA'S. Anyway, that data does support the idea that the scalloping of the fabric doesn't matter a whole lot. The drag coefficient on the scalloped is a little higher at moderate angles of attack, like in climb, but not much.

Interesting topic - -

12 Geezer said:

Longwinglover said:

12 Geezer said:

Some time ago, maybe a year or two, somebody here posted a link to an engineering report of wind tunnel testing of scalloped vs non-scalloped wings. I wish I could find it again.

Click to expand...

See my post about 1/2 way down the first page.

John Scott

Click to expand...

Thank you John. That's the one. Dunno how I missed that, but I sure did.

Looking at the curves on the last page of the report, it appears the scalloped wing might have a gentler stall - at least the lift curve is continuous through a larger AOA for the fabric wing, though maybe the veneer wing wasn't tested at those higher AOA'S. Anyway, that data does support the idea that the scalloping of the fabric doesn't matter a whole lot. The drag coefficient on the scalloped is a little higher at moderate angles of attack, like in climb, but not much.

Interesting topic - -

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Variable camber. Fat wing for takeoff and climb. Thin wing for high speed cruise.

This is good stuff...I'm currently building up an old cub and researching the nuances of extending the leading edge skins...going back to these old posts is part of the education. Thank you for your input and the article...

Longwinglover said:

We went through all this before and it was someone like Crash or Jerry Burr that originally linked us to this 1927 NACA report that says the scalloping of the fabric has no effect on the amount of lift produced by a wing.

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930087647_1993087647.pdf

John Scott

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Wow! Ten years ago! Anybody notice the guy who went from 85 mph cruise to 110 mph with extra tin leading edges the only change? LoPresti should have hired him!

When you spend lots of money on a performance mod you are guaranteed to feel you have made a significant improvement in performance.

skywagon8a said:

My leading edge skin is extended 12-1/2" aft of the spar on the top and stops at the spar on the bottom. How is everyone attaching the fabric to the bottom cap of the rib in this area where you can't get a full stitch in place. Do you just run the stitch around the lower cap? or do you glue it in this area and start the stitching at the 12-1/2" location?

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I didn't remember starting this thread. I glued the fabric to the extended leading edge using the Stewarts system with a thin layer of felt between the fabric and leading edge aluminum. I had no reservations in doing this as fabric has been glued/doped to wood wing surfaces for decades without issue. The lower fabric was installed and shrunk first. Then the stitches were tied to the bottom rib cap back as far as the trailing edge of the extended leading edge. This made it easy to get my hand inside to maneuver the needle. Then the top fabric was installed and shrunk with normal stitching completing the attachment. I can't say whether this was the proper way or not, except that it worked and has been satisfactory. The Cub flies great.

Saw this on Facebook








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My -12 today, in steady-state level flight. The sun is low, so it's easy to see the contours on the top of the wing when zoomed in. The fabric is very slightly convex. Maybe 1/4" Maybe? I would argue that the fabric scallops we see on the ground are a non-issue. I did a hard turn (2 g?) to the left and Andrew, the photographer in his -12, saw very little if any change in the fabric contour from that in level flight.

Edit: Wrong thread. I'll reference this post from the correct thread.

skywagon8a said:

I glued the fabric to the extended leading edge using the Stewarts system with a thin layer of felt between the fabric and leading edge aluminum. I had no reservations in doing this as fabric has been glued/doped to wood wing surfaces for decades without issue.

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Yup, amazing how that's worked for some really fast aircraft for a long time with no issues! I did the same, substituting a layer of heavy fabric for the felt. Been doing it that way for years.
John