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Thorp T211 corrugated wing skins

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BBerson

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I think inside beading (Piper Cherokee) is the way to go. The metal is stretched more with outside beads around the nose because the radius is larger.
 

Marc Bourget

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It was interesting reading the speculation on forming the wing skin. I was mentored by John for 15 plus years and we talked about the Scooter frequently as various buyers for the Scooter Project approached John because the Type Cert went with the project. Dealing with such issues was frequently discussed. Yes, the "beads" allowed John to eliminate the wing ribs. The original Scooter wings were pressed and some difficulty was originally encountered as the leading edge portion of the bead or corregation would stretch while the trailing edge would fold. It was worked out however. Keep in mind, there was a tremendous capacity for stretch forming at the time. Those machines have been scrapped for the most part.

Onward and upward
 

Turd Ferguson

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Corrugations you said?
Your pics are of the Bowlingxwind at OSH. The latter pic was right after arrival and later in the week it was moved up in front of the control tower. Here's an inflight pic on the way home that yr and the last known pics of the Bowlingxwind rotting away at a farm in Indiana.

The corrugations were not structural, it was just a skin over a tube structure. And while the plane flew, it was nothing to write home about. Certainly a neat one-off plane though....
 

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skyscooter

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The original Scooter wings were pressed and some difficulty was originally encountered as the leading edge portion of the bead or corregation would stretch while the trailing edge would fold. It was worked out however.
When I bought my T211 kit in 2002, Venture Light Aircraft was offering a "smooth skin" version with no bead skins additional ribs for a lower price. This was because of the difficulty that they were still having making the beaded skins for the wing and horizontal. I preferred to get the original beaded skin version, and the owner of the company offered me a deal to get the beaded skin option at the same price if I bought a version that added a second leading edge rib that allowed using shorter span piece of skins that were cut down from scrapped, longer pieces. That's what I ended up building.

I suspect much of the difficulty in forming beaded skins could have been avoided if Thorp would have used the Seabee approach and stopped the beads in the leading edge radius area. This link shows a Seabee wing while the restorers were reinstalling the skins:

 

BBerson

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I bought a version that added a second leading edge rib that allowed using shorter span piece of skins that were cut down from scrapped, longer pieces. That's what I ended up building.
Not sure what you meant here? Do you mean a separate beaded nose skin?
I see the photo in post 13 has one leading edge rib.

Edit, I guess you added a rib because it had one end splice.
 
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skyscooter

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There was not a separate beaded nose skin added. The post 13 photo shows two leading edge ribs, highlighted in the picture below. I was told that the outboard leading edge rib in the picture was added by Thorp during the early days of the T211 (or maybe when it was the T11 version at the time) because the original leading edge skin was continuous from root to tip and the rib was added to allow the skin to be spliced there. This reduced risk of making a very long skin that would be ruined if a bead was bad late in the process. As I think I mentioned earlier, I was told that the tooling formed a single bead at a time in the skin.

The inboard leading edge rib was the one that was added to my kit to allow the use of two shorter leading edge skins that were spliced there.
 

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cluttonfred

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This seems like an interesting concept to explore if the fundamental challenge is the forming of the beads in a consistent and low-risk manner. It could be done with a massive hydraulic press but that seems impractical for homebuilt or even low-volume factory production. What are the alternatives? Could you use a CNC-controlled welder to weld beads to the skin held in a jig? What about external ribs riveted on to the skin so that each rib becomes a little triangular tube? And those are sticking with aluminum, if you move to preformed composite skins things get even more interesting, but then you'd probably need a composite structure underneath as well because of thermal expansion differences, and that's a whole new can of worms.
 

skyscooter

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It probably wouldn't need a rib if the fastener spacing was close enough to equal the strength of an uncut skin. I'm glad it has one, because it helps hold the contour better and it is a more structurally sound splice.

I was out flying it this afternoon, and took this picture after I landed to show where all but the very inboard ribs are located. There are four full chord ribs, including the second from the end where the original leading edge skin splice is located. The leading edge only rib location is where the skin is spliced. They put an extra leading edge rib for the splice on each wing panel, and are identical to the leading edge rib further out. The aft skin is the original length out to the outboard splice rib. It has contour in it due to the upper surface curve of the NACA 4415 airfoil, but I seem to recall that it was rolled up when it was put in the box that the kit came in.

T211 wing TV.JPG
 

TFF

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Is there a noticeable speed difference between the ribbed skin and smooth, at this speed range?
 

BBerson

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Hard to believe a curved beaded skin can be rolled. If it can, it would make an ideal fast build kit with no ribs like the Seabee.
 

skyscooter

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I'm not aware of any speed difference with it without the beads.

It was a fast building wing.
 

Tiger Tim

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I wonder what that Bowling was skinned with. Did Bowling make a corrigating roller? NOS Ford trimotor skins? Factory surplus from one of the heavy jet builders? Was every shed on his block missing its roof?
 

skyscooter

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Hard to believe a curved beaded skin can be rolled.
The upper skin beads have some, but not a lot of curvature as viewed from the side, and the most of the skin doesn't have beads so it is easy to roll. I probably just sort of looked like a barrel with a slight bulge due to the beads when it was all rolled up.
 

BBerson

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sort of looked like a barrel with a slight bulge due to the beads when it was all rolled up.
That's what I was thinking. The D-cell nose sections could be boxed in 4 foot sections, perhaps folded at the nose with the radius. The flatter main skins could be rolled up.
 

Riggerrob

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When considering drag, corrugated skins work best on wing and horizontal tail surfaces.
OTOH corrugations on fuselage and vertical fin can produce little drag or lots of drag, depending upon airspeed and angle of attack ... because they are no longer parallel to the relative wind. Corrugations can only parallel airflow at one angle of attack.
Note that most modern Cessnas and Cherokees still use beaded skins on flaps, ailerons and horizontal tail. Cherokkes retain external stiffeners under their cabin floors. These stiffeners are aluminum L-channels riveted to the outside of the belly and aligned parallel to the centre-line/airflow.

Most manufacturers abandoned beaded fuselage skins by 1940. Ford quit building corrugated tri-motors circa 1930, while Junkers continued building corrugated Ju-52 tri-motors until 1945, even though they were considered obsolete compared with smooth-skinned Douglas transports.
When Shorts built Skyvans, they oriented corrugated fuselage skins vertically and covered them with smooth sheets of aluminum to minimize drag. Corrugated fuselage skins also minimized the number of stiffeners - and rivets - to stabilize thin, flat fuselage sides.
Zeniths are among the few kitplanes with perfectly flat fuselage skins. Those skins tend to oil-can, making noise in the short run and metal fatigue in the long run. A friend flew his Zenith 601 for 600 hours per year, but had to replace fuselage side skins when they developed cracks and "smoking rivets."
Murphy avoids those fatigue problems by pressing shallow bends in fuselage skins. Those bends do limit strength - on the vertical axis - but also eliminate a few stiffeners and rivets.
 
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