Geodetic structural design

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BBerson

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I have been thinking about geodetic wing skin for a light plane design. It seems to me that geodetic strips are much stiffer than a typical thin aluminum wing skin that easily buckles and therefor geodetic would be lighter and stiffer than a thin skin.

Wood geodetic strip is used in several designs but I want to use metal strips.

The Vickers Wellington used rolled formed aluminum channels but these would be difficult to make.
Any thoughts on how to make simple metal geodetic strips?
Your thoughts about geodetic structures in general would be appreciated also.
 

Tom Nalevanko

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Dean Sigler gave a great presentation at the Sept. '07 eSoaring homebuilders gathering in Tehachipi on this topic. It seems to be his research topic and maybe he will write a book. A lot of the work happened with homebuilders in Oregon. I recently got a CD with both PowerPoint and PDF versions of an expanded presentation and also some paper articles for $10 or $15 to cover costs. I don't know if this offer is good outside of the eSoaring community.

My recent question on the geodesic Thalman T-4 originated from Dean's work.

You can contact Dean at muchcatfur@comcast.net

I think that there may be some possibilities for this approach using the pultruded carbon fiber strips and rods. Still noodling on this...
 

BBerson

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Thanks Tom,

I emailed Dean to ask if he had heard about homebuilts with metal geodetic and he replied with "nothing happening that he knew about". There was one plane that used steel tube back in the 1930's.

I guess I will be doing some pioneer work here.

Just ordered a book called: "Wellington, The Geodetic Giant", this book might have some insights into geodetic theory.

BBerson
 

Tom Nalevanko

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Hi BB (sorry don't know your first name),

Thanks for pointing out this book. I checked it out and it looks interesting. Please let us know if there is any good design analysis and maybe I will get a copy myself.

Blue skies,

Tom
 

Jeremy

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An interesting idea. What about using off-the-shelf composite pultrusion instead of wood or metal?

This stuff is very light, stiff and readily bonded, plus it's available quite easily too.

Jeremy
 

BBerson

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Using carbon would seem like a logical choice to consider but I am building with metal spars and ribs so riveted metal geodetic "skin strips" make more sense in my case.

I just got the book "Wellington, The Geodetic Giant" yesterday. I was disappointed in that the book has almost no reference to geodetic structures. It is a book about wwII stories. Has a few good photos but not much better than the photos on the web.

I think geodetic is relatively simple in principle, it is similar to a lattice truss and the lattice truss is really just a warren truss with extra diagonals.
Just google "lattice truss" for a picture of a lattice bridge structure.
 

Topaz

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Go looking for references to a man named Buckminster Fuller. He pioneered a lot of work in such structures. If you're lucky, you may find some technical material on their design. His work was often in a related type called 'geodesic', but IIRC, he did some work in geodetics, too.
 

BBerson

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Yeh, Fuller is most famous for his geodesic dome. His dome is made of struts assembled in triangles where the Wellington has square holes. I don't think it matters which design is used as both are rigid. In the case of the Wellington the triangle structure is embedded in the larger view same as a lattice truss that also has square holes.

I think I have method of construction all figured out. I will build a wing and see how it works.
BB
 

orion

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I've looked all over the place in an attempt to find a good pictorial example of this construction method but so far have not come up with anything except this line drawing of a Wellington fuselage:


The geodesic form is reasonably well suited for flight application but it is important to keep in mind that its use was developed as a stop-gap measure primarily for application of less than favorable materials, where using full sheets would have resulted in a dramatic weight penalty. The lattice framework allows the use of relatively weak materials within large structures where the sizable cross sectional moment of inertia results in relatively low stresses and high stiffness, even with a flexible and relatively weak material such as wood. But then in time of war, when optimal materials are scarce, ingenuity is often the key word of the day.

If one looks at the application from a structural standpoint, it is relatively obvious that the structural stability and integrity improves as the lattice framework is applied with smaller gaps. The closer the strips are together, the thinner the material can be. And of course the limit of this is where the material strips are right next to each other, essentially covering the structure as with a single sheet. But then the single sheet can be quite thin and thus we arrive at a very efficient assembly, that being the monocoque structure.

But one of the advantages of the lattice framework is that the construction method is quite flexible and thus can form compound curvature surfaces, something that is of course difficult to do with a isotropic material like aluminum.

The graphite rods are an interesting idea but the key question to the application is the method of attachment. To get a good homogeneous structure you'll most likely have to wrap each intersection with a resin soaked tow of material, sort of like tying a raft together. Seems like a lot of work.
 

BBerson

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Thanks for posting the drawing.
Here is the best I have found on the Wellington structure, it is an old film and even shows the incredible slow STOL landing of the twin bomber. Click pause on the internal structure for a good look.

I think the C shaped channel frames are about 3 inch X 1 inch and are made on a rolling machine from sheet aluminum (I have some photo's of the rolling dies). But I don't know the thickness of the channels.

It seems these diagonal channels replace what would be vertical formers and stringers in a Boeing bomber. And since they are diagonal, no skin is required either. Imagine the weight savings.
But they reached the limits of fabric covering at 300mph and that is the main reason the geodetic was abandoned for aluminum skin.

I see the advantage of geodetic for light and ultralight designs with fabric.
No advantage for fast planes that require smooth skin.
BB
 
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Topaz

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I believe someone posted pictures (or links to pictures) around here of a Mignet-style Flea done entirely as a geodetic structure. I can't quite recall the topic of conversation, though. Check around here and on the web - I'm sure I've seen those photos around here somewhere.
 

Topaz

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Yep, that's it, in post #15 in that thread.

Seems like a good place to get started. Contact the guy and ask how he did it.
 

BBerson

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That is a good photo of geodetic using wood strips.

I am going to build a wing using corrugated aluminum strips instead of wood strips. As this is an experimental structure , I will need to test the wing for torsional stiffness of course.
But first I need to build it...
I will probably need some assistance with calculating torsion loading for the test when the time comes.
Thanks for your interest.
BB
 

103

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Thanks for posting the drawing.
Here is the best I have found on the Wellington structure, it is an old film and even shows the incredible slow STOL landing of the twin bomber. Click pause on the internal structure for a good look.

I think the C shaped channel frames are about 3 inch X 1 inch and are made on a rolling machine from sheet aluminum (I have some photo's of the rolling dies). But I don't know the thickness of the channels.

It seems these diagonal channels replace what would be vertical formers and stringers in a Boeing bomber. And since they are diagonal, no skin is required either. Imagine the weight savings.
But they reached the limits of fabric covering at 300mph and that is the main reason the geodetic was abandoned for aluminum skin.

I see the advantage of geodetic for light and ultralight designs with fabric.
No advantage for fast planes that require smooth skin.
BB
A little fisrt hand report arrived in my inbox form Eric Clutton this morning.

" I flew in Wimpies at OTUs and they were usually hand me downs from Bomber Command , well used of course! In the fuselage, everywhere the geodetic members crossed there was a rivet and they all seemed to be rotating, some clockwise and some anti-clock, so the whole fuselage kind of writhed in flight and the walkway down to the rear gunner had a rope with stanchions which was highly necessary. The top surface of the wings were like a quilted mattress and they also twisted about a bit but the aircraft flew quite well and it was almost impossible to break one although they could be bent. I once had one bounce over me when it landed short in the field just off the end of the runway but I threw myself flat and watched that bloody great wheel go over me and the short fence before it hit the runway tail first then staggered off to dispersal without apparent damage. A grand old machine !" ....Eric Clutton 11/25/19
 
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