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Mustang II/Thorp S-18 folding wing option info

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gschuld

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May 9, 2007
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422
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Toms River, New Jersey
I am exploring the idea of implimenting an aft folding wing similar to what is available for the Thorp S-18, Mustang II and a bunch of others.

Video of foling Mustang II wing below:

I am particlarly interested in this style of folding wing. It appears to be very useful in that it is always supported(being hinged) on one end so one person can easily do everything themselves with minimal effort and worry about dropping an edge, etc. during handling. I consider the removable wing option far less useful as it really requires two people to handle the process. I have read a bunch of material and opinions on the viability of folding wings, and there are many valid points. There seem to be many levels or degrees of wing stowage/transportability for airplanes. My focus is more with a system that allows wing folding on every flight, not just a once a year seasonal trip to and from the airport for the winter. In some places of the country, hanger space is nearly impossible to get. Waiting lists are open ended and can go on open ended for many years. I've seen hanger rents(for small planes!) go for $800/month and up. It's no suprise that some people have reluctantly been interested in truely convenient folding wings for transport. There are good oppuntunities for people who have folding wing planes to share the hanger space of people who are lucky enough to already have a hanger space. Two planes will not fit, but a plane with folded wings (7'6" wide by 19' long) can easily fit along side. In my area, this appears to be one of the few viable options. Rolling a plane in and out of an enclosed 8.5' x 20-24ft car trailer every time would not be preferable to me, although it would certainly fit easily and not be too difficult. According to all the airports in a 50 mile radius of me(and well beyond I'm sure) the waiting lists for hangers is expected to be 5 years minimum, and yes I am currently on several waiting lists. I can think of about a dozen people though that would be thrilled to share hanger rents at about 1/4 to 1/3 of the total hanger rent(as the plane would be taking up such miniman space of the hanger), and I could move in right away, no problem. The airports have no problems with two active planes in one hanger, so I have to consider this as being a potential logistical necessity. Also when traveling, which I hope to do some of, I understand that many airstrips will have inside hanger room available for a plane that measures only 7'6" x 19' x 5'6"(taildragger). I have spoken with a Mustang II pilot a while back with the folding wing option and he has had great luck hangering his plane at airports when traveling (many time for free) when there is no room available for a "full sized" plane, and would be charged for it if it was available. As I will building a wood/composite airplane, inside hanger space when away is a fairly big deal to me. Anyway that's where I'm coming from.










Above is the folding wing option on the mustang II. These seem to be fairly common and proven setups. I know the Whitman Tailwind, the Zodiac 701, the Thorp T-18/S-18, among many others appear to use the same basic folding system. I am interested in feedback from any one who has experience with folding wings or generally any opinions on the subject. I am searching for good reeference material, articles, etc detailing the specifics of the designs.

If starting from scratch, is there a particular folding(back) wing design for a low wing plane that works better/more user friendly then others for a starting point? Obviously, a well designed system needs to handle the control linkages well for the aileron,flaps(likely split flaps in my case), the ease of conneting/disconnecting the main attach bolts, leaving an aerodynamically clean joint of the assembly when connected, etc.

I am interested in any info on the subject:).

George
 
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GESchwarz

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Oct 23, 2007
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I'm going with the Grumman Sto-Wing design originated by Leroy Grumman back in the 1930's. It was so successful that they used it on many models including the Wildcat, Hellcat, Avenger, Tracker and Hawkeye. I've studied every one of these planes up close and personal and have quite a photo archive, including exploded view drawings of the Hellcat wing. The slickest control linkage is where there is actually no connection at all; for a push-pull system you just have two 180 degree bellcranks that have push pads at the end of each arm and when the wing extends, these pads just press together and you are ready to go. I had begun designing my mechanism and building scale mockups, and with each itteration it became more like what Grumman has been using all along. So now my design is just like the Grumman all the way even down to the sloped main spar web.

With this design you just have one locking pin at the rear spar and a tiedown point at the wing tip to elevator to keep it from swinging. No monkeying around with control connections.
 

GESchwarz

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Here's a sampler of photos. The secret to this hinge geometry is as simple as it is effective, which makes it in engineering vernacular, elegant. Leroy came up with this using a gum eraser and a paper clip…

Imagining yourself as the airplane and your shoulders are the wing root. Using your right index finger, place it over your left shoulder, pointing towards the horizon, aft 45 degrees. From there, point upwards about 50 degrees. This is the hinge axis. The 45 degrees shouldn’t be fooled with. But the up angle of approximately 50, you can adjust that angle to vary the amount of twist about the lateral axis of the wing and the angle, relative to the horizon, at which the wing will be in the stowed position. I can post drawings later, as I haven’t scanned them yet and I’m short on time tonight.

The two angles given are what I'm using on my design. The spar web is in direct alignment with the hinge axis. That axis in the side view comes out to about 26 degrees from vertical.

There are many nuances to this design that I can clue you in on should you decide to go this route.

Personally I'd never fold any other way. Not only for nostalgia, but it is just the coolest way, it surprises everyone who sees it happen and it's all done with a simple but clever compound angle.
 

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gschuld

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May 9, 2007
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Toms River, New Jersey
GESchwarz,

The Grumman Stow-wing design sure has had its success. There is much to like about it. One of my concerns is that I will have all my fuel in the stub wings between the main and rear wing spar. I have a 45" cabin width(I finally backed off my 48" width) with a 7'6", so I have roughly 22" of space be between fuselage sides and outer wing stubs for the fuel tank. Building with the Stow-wing design would cut into that space by a good bit. Plus it would screw with the basic KR wing attach architecture more than I am comfortable with. Messing with the wing attach system makes me very nervous as it is. I figure that the mustang II type system basically utilizes the same attachment point locations and types than the standard KR multiple bolt arrangement.
http://www.youtube.com/watch?v=qqTer7pqHvk

It is the "coolest" folding wing though IMHO. I've always liked watching old movies where those Hellcats, etc warbirds would swing out their wings on the flight deck with that fluid/twisting motion:). Don't get me thinking about it...:lick: ...:devious:.

George
 

GESchwarz

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Thanks for that Hellcat video, lots of P-47 stuff too. Lots of people rave about the P-51, but I'd take the P-47. One hole in the P-51 coolant system and it's game over for you...I could go on and on but I won't.
 

LGM

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Jul 4, 2008
Messages
52
George,

Here's a link to a guy who as folding wings on a KR. His method uses some ply slider blocks on the ribs and a length of tube. It also uses the original KR wing attach fittings and seals over the wing gaps similar to the mustang in your pics.

My KR2: Home Page: Once there, click on "folding wing brackets" and "wing gap seals" to see how he did it.

Good luck,

L.G.

Just noticed the link says My KR2... it's not "MY" KR2, it belong to Phil Matheson and I believe it's Aussie based.
 
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GESchwarz

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Ya, if you want to replicate the Grumman Sto-wing hinge, the simplest and best way is to design from scratch and do it just the way Grumman did it. I originally tried to design the hinge to fit to a standard vertical-web spar and it was going to require 4 rather complicated metal working masterpieces to joggle the load paths...something you don't want to have to do and it gets kinda heavy. The KR design, although not as slick in it's operation, is very simple and lightweight, which are excellent reasons for going that route.

Because I'm designing from scratch, I don't have the added obsticals that you have of having to adapt to existing structure. I have this life-long facination with the Grumman folding wing, so once I do this, my life will be nearly complete. So if I've done nothing else, I will be able to die with a smile on my face.
 

GESchwarz

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I'm curious, how many G's is the KR-2 rated for? The reason why I ask is that those wing attach brackets sure seem thin to me. Also. what are the material dimensions at the cross section where the attach bolt is? I'm assuming the material is 1/8" 4130 steel.

What bolts are used what is the shear strength?
 

gschuld

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Toms River, New Jersey
The KR-2 was rated for +7, -4 Gs at the designed weight of 900lbs. Unfortunately, there is no real data to support what the KRs have turned into. Most of the modern KRs have been built larger, have bigger more powerful engines, larger wing area, etc. and file around 1300+lbs as their gross weights. The wing attach fittings, 4130 1/8" has remained the same if I am recalling correctly. I do know of a KR that was built without any "beefing up" from the original plans, running a strong 0-200 engine, that routinely pegs 6 gs on his G-meter flying at 1300lbs poking holes in the sky with big grins, on the dead stock and plans built wing attach fitings. There are also KRs out there that have flown consistantly over 1400lbs with any structural problems. Not that these are endorsements in any way. In reality, modern KRs are basically the result of backyard engineering, with the help of some talented aeronautical engineers for guidance occasionally. There have been no formal structural evaluations that really relate to what people are building now that I am aware of. It is mostly a "well his plane didn't come apart after doing THAT:ponder::para:, so I'll probably be OK" kind of deal. Not particularly comforting, but at least there are whole bunch of continuing "test flights" demonstrating a very trouble free basic structural design. There has never been any type of WAF failure on any KR according to the records. I believe the main spar bolts are 3/8" high grade and the rear spar 5/16". Don't quote me on that. I could find out for you fairly easily if you want to know for sure though.

Below are results of a very unscientific wing spar/WAF destruction test on an abandoned KR project with a few captions.




We were treated to a KR2 spar test. This boat looked like it was at least twenty years old, and it must have spent much of it out in the weather. Many of the spar verticals showed signs of separation from the spars, and there were other fitup problems. Most didn't think it would take much punishment, but we were amazed at how much the spar bent before anything started cracking...

Both of these spars were pretty straight before the load was applied. In this first stage of the test, the jack was outboard of the WAFs (wing attach fittings), and the outboard spar caps separated (due to the plywood web failing) before anything happened with the WAFs. After that, the jack was moved to the end of the center section of the spar.

The shear webbing between the spar caps was the first to go, and it still took a lot of abuse before the spar cap itself broke, right where it enters the fuselage. This was way more than 7g of force. It could be argued that the setup was less than perfect, but we all walked away with a new appreciation for the wing attach fittings and the integrity of the spar and fuselage system. If there's anything to be learned from this, it's that Birch is better than Mahogany for spar shear webs.



Hope this helps some,


George
 

Dieselav8tor

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North California
George, Thank you for the Beautiful pics. I am in the pre build stage of a plansbuilt KR2S (gathering the money and materials). This gives me a lot more confidence in what to me looked a mite weak, I was thinking about building the WAFs a bit longer and out of 1/4" steel. Now however I will just build to the plans! Thanks again.
Bart
 

jbinikos

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Feb 16, 2011
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Location
carlsbad california
GE Schwarz,... just joined, have been looking for the Grumman folding wing plans for some time now. I understand you have made youre own from the full scale aircraft. Any chance you are willing to sale me a copy ? thanks John
 

Puggo1

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May 19, 2020
Messages
6
Hi,
does anyone have a set of Thorp Sunderland S-18 folding wing plans for sale?
thanks
Graham
 

Riggerrob

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Sep 9, 2014
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Canada
Wing fold method depends upon configuration, cost, weight and stowage space.

Stowage space means the maximum sized, folded airplane you can stow in a given hangar.

Length, width and height are all important when considering folding method.
Width for trailering must be kept less than 8 feet in most jurisdictions. If you only have to fold it narrow enough to fit into a standard T-hangar (e.g. 35 feet or 10 metres) then you only need to fold outer wing panels (ala. Stemme motorglider). Stemme outer wing panels disconnect, then swing so far aft (180 degrees) that they overlap the aft fuselage when stowed.
Height is important when folding into a glider trailer, but less important when stowing in a hangar. Too tall a trailer might tip over when hit by cross-winds on the highway.
If height is not an issue, then simply folding outer wing panels straight upwards (F4U Corsair, Grumman Tracker or Onex) is often the simplest solution. This can be as simple as adding hinges to the top edge of a wing fence (rib that extends above the top skin). Onex uses clever, over-lapping wing spars to spread flight loads.

Biplanes (Vickers Vimy) and high wings (Zenith 701) often just hinge about the rear spar and use struts to stabilize wings during the folding operation.

Width of the center section can seriously impact empty weight becuase the farther outboard the joints, then lighter the hinge fittings can be. For example, early Grummans (e.g. Wildcat and Hellcat) folded wings close to fuselage to maximize the number they could stow in hangar decks below the flight deck. Avenger moved hinge lines farther outboard so that wide main landing gear remained bolted to a wider wing centre section.
Later they realized how heavy fold mechanisms can be, so moved them outboard until they folded barely small enough to fit in ships' elevators (e.g. Grumman F8F Bearcat). The farther outboard a joint is, the lighter it can be built.
Early Grummans (Wildcat, Hellcat and Avenger) need a main wing spar web that tilts back about 45 degrees ... best on a clean-sheet design.
Grummans also stow wings trailing edge upwards where it is easy for snow and rain to enter.
I would prefer a wing-fold system that stows wings trailing edge downwards (e.g. Stitts Playmate) to reduce the risk of contamination inside the wings. Stitts cheats by using lift struts to stabilize wings while folding.

Finally, some gliders have suffered structural damage while trailering, so it might be wise to separate structure components needed for flight from rubber-mounted fold mechanisms. This concepts make sit easier to retrofit fold mechanisms because you do not need to worry about fold mechanisms carrying flight loads.

If folded length is not a big issue, you can simply pivot a high wing to lay longitudinally along the top of the fuselage (CV-22 Osprey or Backyard Flyer).

Note: all these systems still need some sort of ground lock pins or ropes to prevent wings from flapping while stowed.
.
.. just the ramblings of an old navy flight deck crew member.
Master Corporal (retired) Rob Warner, HMCS Athabaskan, HMCS Iroquois, CD, BA, private pilot, jump wings, Master Parachute Rigger, etc.
Sea King helicopters, CF-18 fighters, etc.
 

rv7charlie

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Nov 17, 2014
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Jackson
The BD4 has a slightly different take; used on a high wing a/c. This one uses the round main spars as part of the mechanism.
Downloadable pdf here:
Redirect Notice
Figures 29 & 29a.

The Dyke Delta folds up, more like a Corsair, but the hinge points are above the wing so that the wings can fold 'flat' on top of the fuselage.
Redirect Notice

Charlie
 

Riggerrob

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Yes,
Dyke Delta is a great example of simple folding wings. Its flight span is almost 24 feet, but it quickly folds to less than 8 feet for trailering. Dyke's outer wing panels over-lap on top of the fuselage.
The only additional equipment are a couple of fixed hinges that extend above the wing separation line. One builder deleted those exposed hinges, but later regretted his omission. Small exposed hinges have little affect on cruise speeds, but they do make it easier to find hangar space.
Dyke wings are so deep at the separation line that simple shear pins - inserted into the top and bottom of the main spar - are enough to carry flight loads.
Dykes still need a few simple, jury struts for trailering.

Grumman Tracker is a far fancier variation on that folding scheme with hinges slightly offset so that the left outer wing panel lays just aft of the right wing panel. Both outer wing panels lay flat on top of the fuselage to reduce height in the (below decks) hangar.
Unfortunately, Trackers have a far more complex locking scheme with a dozen-or-so, hydraulically actuated locking pins connecting the respective top and bottom skins.
 
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