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Thorp T211 corrugated wing skins

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handprop

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Hey guys, In my quest for learning about the stress in wings I remembered the good ol Thop designs. On some of his airplanes like the T211 he used corrugated aluminum for the skins. What was the reason for this? Is that because it helps with skin buckling and stress or did it have something to do with the manufacturing process. Mike
 

lr27

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djschwartz

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The idea was to reduce the number of ribs to save weight and manufacturing cost. Rolling corrugations into the skin was supposed to be cheaper than forming and attaching ribs. I suspect it was draggy. As Orion has explained in other threads, airflow is not exactly straight from front to back over much of the wing. There is a spanwise component even on an unswept wing, especially near the tip. Also, the corrugations completely eliminate the ability of the skin to carry any of the bending loads of the wing and may even have made it less stiff torsionally as these loads are approximately at 45 degrees to the span or chord. In fancy engineering terms the corrugations turn the skin into an anisotropic material, one that has more strength and stiffness in one direction than another. This can be an advantage in some cases but it also can make stress analysis more complicated.
 
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orion

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And the corrugation increases the whetted area so although it may reduce the need for as much internal structure, overall it may be a significant aero-based penalty.
 

addaon

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Another plane with corrugated skin is the SR-71, although there you can overcome the aero penalty by sheer brute force.
 

bmcj

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Another plane with corrugated skin is the SR-71, although there you can overcome the aero penalty by sheer brute force.
Yes, but I believe what you percieve as corrugations is actually a ridge pattern placed to allow for thermal expansion.

Bruce :)
 

DaveK

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And the fuel tanks don't seal completely until the metal warms up and the expansion closes the gaps. Leaks JP-7 all over the place on the ground.
 

skyscooter

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Yes, the T211 skin corrugations go around the leading edge. Here's a picture of the leading edge that I took while flying my T211. I was told that the beads were formed one at a time seem to recall that they were maybe a pain to make. Part of the problem was that you have most of the leading edge beaded when one of the beads would tear, ruining the part.

They eventually split the design into shorter sections by adding a leading edge rib at the splice, and thereby reducing the skin length. The Seabee had corrugated skins too, but the beads stopped short of the leading edge radius allowing the skins to be hydroformed. The leading edge radius was formed after that.LH LE in flight.jpg
 

skyscooter

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In addition to the picture that I posted above, here's an earlier link showing the T211 wing skin beads, and a picture of one of my wings when I was building it:

 

BBerson

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Thanks for that other link. So the "tool" for beading, was that like a mold for one bead ? Or some sort of industrial beading machine?
I was thinking about going with the internal Piper Cherokee beads. I could make a beaded leading edge skin mold, say 4 feet long. And hammer the beads in by hand. The whole thing would be covered by fabric. (just a beaded D-Cell)
 

TFF

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The kit company bought the old factory tooling from the thorp fire sale. I imagine there was some tooling this that was not part of the sale. Heavy presses that could stamp out a ten foot bead. They probably got the press mold but to buy a press was probably out for the kit makes. Splicing the parts probably got the tooling under control.
 

skyscooter

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Thanks for that other link. So the "tool" for beading, was that like a mold for one bead ? Or some sort of industrial beading machine?
I got my kit from Venture Light Aircraft in 2002. My recollection is that they owned the tooling and parts inventory at the time. The VLA owner told me that they had to make beaded skins using the tooling, and had help from an older guy who had worked for I believe Tubular Aircraft in the 60's when they made most of the parts for an attempted production run. My recollection was that the leading edge bead tool would press one bead at a time, so I my guess is that the skin LE was roll formed or otherwise bent to put the LE radius in. After that they pressed one bead in at a time using a tool that had a male and female bead profile.

Originally it was about 8-10' long, but was split into two sections to reduce the risk to wrecking a LE towards after most of the beads were put in. VLA also offered smooth skinned versions at a lower price with additional ribs at more conventional spacing. I was able to get a ribbed kit for the same price as the smoothed skin version because he added another splice rib and made up of two shorter skins that were otherwise going to be scrap because they had a blown out bead.

My understanding is that IndUS got all the bead tooling to make the T211 when they bought the Type Certificate, parts, tooling etc. They eventually came out with a Jabaru powered LSA version as well as offered TC'ed versions with the 0-200. I don't know how many they built before they shut down about 10 years ago.

The wing was very easy to build and does attract attention as something different. Not related to the beads, but it is a nice plane to fly with great visibility! Not sensitive at all on the controls for such a small plane.
 

BBerson

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Ok, that is sort of a drawing process in a big press. The 12 foot sheet would get about 2" shorter with each bead stamp and only one bead per press cycle. Thanks.
 

Riggerrob

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A reverse alternative is the wing segments used on Bede 4 kitplanes. Bede molded his wing segments in fibreglass. Each segment included top and bottom skins and a rib all molded in a single piece. Bede ribs were all internal. This created smooth wing skins with joints that were barely visible, ergo no additional surface drag. Bede wing segments overlapped by a few inches and overlaps were bonded with epoxy. IOW an outer wing segment nested over an inner wing segment.

Bede's wing segments are probably impossible to deep-draw in sheet aluminum, but the concept might be possible with shallow ribs ... little more than T sections .... L or C sections rolled into the ends of wing skin segments. Probably want to squeeze shallow ribs almost flat where they cross spars and rivet them directly to sheet aluminum spars. With a bit of fore sight, you could even figure out how to solid rivet them.

Hmmmm? If wing skins are almost flat ... aft of the main spar .... how deep can you make rib webs? Can you make rib webs deep enough to meet in the middle of the wing?

This reminds us of the way that Messerschmitt 109 fuselage segments were made from sheet magnesium with ring bulkheads "rolled" into the front and back faces of each fuselage segment.

Sorry folks, that was just my mind wandering about possibilities. I can sketch the new concept in Solidworks, but do not have the sheet metal tools to make samples.
 

Victor Bravo

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When you talk about these innovative or clever ideas, rolled skin corrugations, rolled bulkheads, etc. it very very quickly becomes a discussion about mass production.

If I have my facts straight, mass production is the primary reason why the Seabee was designed the way it was. Same for the Ercoupe. Mass production was the reason the Me-109 was designed the way it was. Mass production is probably the reason Cessna control surfaces have corrugations. The Blanik and Pilatus glider fuselages had external flanges for joining the fuselage halves, not because it was aerodynamically cleaner (it isn't) but because it made production easier in large numbers.

It's about reducing man-hours of riveting and reducing parts count far more than it is about anything else.

Which means that when you bring this discussion over to the homebuilder level, for an airplane built in the garage... a lot of features that you would put into a production airplane are not valuable or negative value for a one-person hand-built airplane.
 
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