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Geodesic Airframes

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Geodesic Airframes

Old 24th Feb 2011, 02:02
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Geodesic Airframes

As I understand it, a geodesic structure is based on straight lines drawn along a curved surface. There were a couple of British bombers based on this design concept by Barnes Wallis and they were remarkably sturdy though the skeleton looked like some kind of metal basket.

Did they use conventional stringers/longerons and spars? Also is there any commonality in concept between a geodesic frame and a honeycomb structure?
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Old 24th Feb 2011, 05:25
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http://www.avsim.com/pages/0409/FCS/...Brooklands.jpg
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Old 24th Feb 2011, 13:22
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DN de G Allen

Try and find a slim book called the Systematic relaxation of Constraints by D N de G Allen. Written in the 1930's for airship designers this is the tour de force and succinct with it.

Last edited by enicalyth; 24th Feb 2011 at 13:23. Reason: speeling poleece
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Old 24th Feb 2011, 13:36
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the frames were made of wood at first...this is where they used a two pack glue, plyurethane of the fist time. Now I am guessing here but the airframes specically using this process was the Halifax and also the Hurricane but not sure. The wood used was ASH...as from an ash tree.
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Old 24th Feb 2011, 14:15
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An interesting Barnes Wallis design of fabric-covered duralumin channel section. Used by Vickers (in no date order) on the Wellesley, Windsor, Warwick and, most famously, the Wellington. The earlier R100 airship also utilised the helical weave construction.
Amazing structural strength.
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Old 25th Feb 2011, 01:35
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the hurricane is/was a semi conventional steel tube fuselage, with wooden formers, and fabric covering
Hawker Restorations Limited Restoration of Hurricane G-ROBT
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Old 26th Feb 2011, 21:19
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The Wellington had external stringers to give a smooth surface for airflow. Despite the incredible ability to absorb battle damage, geodesic construction was dropped on later designs as fabric ripple at speeds over 300mph increased drag.

This gives a pretty good idea of the structure:

Wellington cutaway image by chris7421 on Photobucket

It appears that there were spars, longerons and formers, although they look smaller than one would expect to find on a semi-monocoque structure.

I made an experimental geodesic leading edge for a large, ultra-lightweight UAV using 0.6mm pultruded carbonfibre rod. The resulting structure was very stiff and weighed almost nothing. It is definitely worth revisiting for appropriate applications.

Honeycomb structures use the inner and outer skins to transmit loads. The honeycomb converts gives each skin depth to resist buckling. I would say that they are quite different.
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Old 27th Feb 2011, 01:01
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Hmm. It is not difficult to imagine that not only are geodesics and CFRP similar, for purposes of discussion here, they are identical.

Strength/Weight was always a driver in Aviation, hence Spruce rather than Fir, and Titanium rather than Steel. When the paradigm became Moot, we went back to solids. Howard Hughes developed an early form of prepreg using Birch veneer and Urea or Phenolic shear layers, enormously strong, and absent the need to "relieve" structure to minimum weight to maintain Strength.

The "Skeleton" of steel tube covered with fabric became obsolete when prestressed Aluminum panels and minimal structure could provide a three dimensional operant of what the Tube Fabric structure only "suggested".

Inside every solid and shaped prepreg panel of Carbon reinforced Plastic is a ghost of the Honeycomb, and the "Geodesic".

The secret to strength has always been "Two Phase", being a matrix enveloped in some solid. Bamboo is a perfect analogue in Nature of Rutan's best efforts.

eg: Fibrous/Lignin and Carbon Fibre/Resin. Same-o same-o.

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Old 27th Feb 2011, 08:43
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Looking at the Wellington cutaway, it is a lattice, two-dimensional in some areas and curved into a cylinder in others. Neither is truly geodesic in the sense of being a self-defining surface of linked rods. It is only in two-way curved surfaces that the geodesic structure becomes rigid.

The added rigidity in the Wellington comes from having a very thick skin, anyone with mechanical knowledge will recognise
(bd^3)/12
which is the moment of inertia of a beam of width b and depth d, the stiffness being proportional to the cube of d.

In the context of a lattice structure Bearfoil is quite correct in drawing a parallel with honeycomb materials where the separated skins provide the stiffness and the honeycomb simply defines the gap between skins.

PS - Apologies for lapsing into lecturer mode, will try not to in future!

Last edited by mike-wsm; 27th Feb 2011 at 09:42. Reason: apology added
 
Old 27th Feb 2011, 08:50
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You learn something every day. I always thought it was geodetic. Well well.
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Old 27th Feb 2011, 14:02
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Barnes Wallis' original patents for fuselage, wings and airframe all use the word "geodetic", and personally I'm inclined to go with his version. He was cleverer than me!

I think he first published a journal paper on the science of it sometime in the mid 1930s, probably in "aircraft engineering", but I can't find a reference to that - I have seen the paper whilst browsing in a university library some years ago.

Wouldn't it be nice if we could all access old British patents and RAE TNs online in the same way we can US papers and NACA reports.

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Old 27th Feb 2011, 15:03
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But his US Patent 1,956,480 line 10 says geodesics.
 
Old 27th Feb 2011, 19:04
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So the reasons geodetic airframes were abandoned were because the skin would ripple at high airspeed? Why didn't they just use a metal skin over a fabric skin?

Were there any other reasons why geodetic airframes were abandoned?
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Old 27th Feb 2011, 19:59
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The use of 'geodesic' design was very much a Barnes Wallis personal mission. He used geodesics in the design of airships around 1930, aircraft like the 1935 Wellesley and 1938 Wellington, and a radiotelescope in 1955. Other designers did not go that route.

Another more practical reason is tooling. Geodesic aircraft structures require special production tooling. Once a factory has invested in geodesic production tooling they would be more willing to incorporate geodesics in further similar designs.

Interesting to note that the two aircraft mentioned above are named for the same man. It was a Mr Arthur Wesley b1769 who changed his name to the more impressive sounding Wellesley in 1798, and, as his career advanced, changed his name again, this time to Wellington, around 1810 and became Duke of Wellington a few years later..
 
Old 27th Feb 2011, 20:00
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Jane-DoH,

The original geodetic arframes, Wellington, Watwick etc; were all covered in fabric and as has already been said the fabric would start to pant at higher speeds.
The early Vicker Vikings had a conventional fuselage of stressed skin, but the wings were fabric covered. They then came out with a modification (Mod 508???) where the fabric was removed and sheet metal riveted on. The later ones and the Vallettas all had sheet metal over the same structure but from the factory.
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Old 27th Feb 2011, 22:47
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Jane-Doh

So the reasons geodetic airframes were abandoned were because the skin would ripple at high airspeed? Why didn't they just use a metal skin over a fabric skin?

Were there any other reasons why geodetic airframes were abandoned?
It may not be the reason and the story may well be apocryphal, but I always understood that when a Wellington was tested as a potential glider tug for D-Day, it worked well. However, when they measured the fuselage on its return ....... they found it to be considerably longer and somewhat slimmer than before the test.

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Old 28th Feb 2011, 02:07
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mike-wsm,

What kind of tooling do they require that is not required for traditional aircraft fabrication? Is it hard stuff for an aircraft manufacturer to procure in those days?
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Old 28th Feb 2011, 03:15
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The application for this approach was narrow, dependent entirely on the radius of the Sphere needed, and Airframes are not conducive to "round". One needs a tube, a planform or two, and some cruciform slabs. The flatter the structure, the more strength was sacrificed by the shallow curve needed, and the material and intense labor needed to assemble became disqualifying. Each triangle needed to close itself with gusseting shared by its partner, and the edges of the tri were supposed to be mitred as "compound" surfaces. This is an enormous amount of work, and by the time methods caught up with demands, Howie was pressure cooking Birch trees and Phenolics into compund panels of immense strength.

One of my favorite a/c besides the Largest a/c ever made (The Birch Goose) is the Mosquito. If God had wanted Plastic airplanes, he would have made Carbon Fibre trees.

bear

Please don't use the word "Geodetic" here, it is an entirely different animal. Bucky would not approve.

If you think Hughes was no genius, walk through IKEA and see his brilliant application of Duramold with Baltic Birch (FinnPly) shapes made into nomadic furniture, wagons, and pieces that are their own shipping crates before they become Cabinets.

Phthallates, acrylates, and God's own laminates. Go bang on the skin of an F-16 where the wing blends into the Fuse. 'A' Sharp, if it passed QA. Geodesic construction was retro less than ten Years after the Wrights flew, imo.

Last edited by bearfoil; 28th Feb 2011 at 03:29.
 
Old 28th Feb 2011, 03:59
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bearfoil,

is it possible to have a geodetic fuselage and a non geodetic wing? Or would the whole hybridization not work?

As for Howard Hughes pressure cooking birch trees and phenolics into compound panels -- is this a very very primitive composite?
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Old 28th Feb 2011, 11:12
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Originally Posted by Jane-DoH
What kind of tooling do they require that is not required for traditional aircraft fabrication? Is it hard stuff for an aircraft manufacturer to procure in those days?
There is a reference to this in wiki under Barnes Wallis. Tooling in this context would include assembly jigs, generally made in-house to meet the needs of any design. It is part of the cost and time-scale of any project, you don't just build the product, you have to produce the tools and the jigs to hold things as they are assembled.

The Wellington took a lot of (wo)man-hours to build, you can see the complexity in this pic:


 

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