Vickers Wellington question

[Stephan Wilkinson]

As far as I know, the Wellington and its single-engine predecessor, the Wellesley, were the only aircraft (other than some rigid airships) to use geodetic construction, yet accounts of the airplane always rave about how fabulous Barnes Wallis's geodetic framework was.

Question: why weren't more aircraft designed in this manner, then? I'm assuming it was either because the methodology was too complex and time-consuming, or monocoque construction was lighter and better.

Does anybody have a better answer? I'm writing about the airplane for Aviation History magazine, to which I'm a frequent contributor.

Stephan Wilkinson

 

[HoHun]

Hi Stephan,

Welcome to the board! You probably don't remember, but we "met" on AVSIG about 12 years back when you were writing the article on tailsitter VTOLs - I actually sent you a copy of a Flug Revue article on the (pretty fantastic and never built) German "Triebflügel". By a lucky coincidence, I even managed to get hold of the Air Space November 1996 to see your finished article. Good thing it was the cover story - made it easy to spot!

>Question: why weren't more aircraft designed in this manner, then? I'm assuming it was either because the methodology was too complex and time-consuming, or monocoque construction was lighter and better.

I'm sort of a Wellesley fan - despite its looks, I believe it's a rather remarkable aircraft! - and my impression always has been that geodetic construction was as weight efficient as true monocoque construction, and it could be seen as a compromise between a conventional tube frame with no lead-bearing skin and a true monocoque with a load-bearing skin covering the entire surface. However, I can offer no citations for this impression, so I'm afraid I can't really contribute to your research.

Regards,

Henning (HoHun)

 

[Graeme]

The Wellesley, Wellington, Warwick and Windsor all used the geodetic structure. Despite the inherent strength of the design they were all covered by fabric, which as performance requirements increased, became less than adequate.

Boscombe Down reports show that later Marks of the Wellington displayed reduced performance and increased fuel consumption attributed to "dirty and continually tearing fabric of the airframe." Early Warwicks crashed when "fabric detached from the wings." The Establishment "insisted on improved attachment to the geodetic structure."

The Windsor's fabric tended to 'balloon' at high speed, necessitating the utilisation of wire-backed heavyweight fabric. This in turn reduced the maximum speed by 25mph.

The Windsor required a main wheel unit under each engine owing to the "considerable wing span and weight outboard of the inner nacelles". However as Francis K Mason (The British Bomber) points out, this undermines Dr Barnes Wallis' assertion that the geodetic structure was weight for weight stronger than conventional spar and rib structure with metal stressed skin, when you consider the Avro Lincoln.
The Lincoln had two main wheels despite being heavier and having an aspect ratio about 11% greater than that of the Windsor.

He sums up the Windsor with the following observation; "The wisdom of employing fabric on a modern, high-performance, high-altitude bomber in hot, humid conditions has been doubted ever since."

The days of fabric were over.

 

[Stephan Wilkinson]

Wow, what a coincidence. The Web really is a small world!

I think the geodetic construction must have been vastly more difficult to build than a monocoque--after all, I built a monocoque airplane myself (a Falco) in my barn, but I doubt I could build a Wimpy. My amateurish conclusion at this point is that the Wellington was a fascinating, imaginative dead end, I hate to say...particularly since the Brits still think Barnes Wallis's work in that regard was pioneering.

Stephan

 

[Stephan Wilkinson]

Oh, and by the way, HoHun, I actually built a model of the Triebflugel after initially hearing from you.

Stephan

 

[Stephan Wilkinson]

Graeme, thank you for the info. That makes great sense, and I hadn't considered the fabric factor.

Stephan

 

[HoHun]

Hi Stephan,

>Oh, and by the way, HoHun, I actually built a model of the Triebflugel after initially hearing from you.

Hey, that's great! When I sent you the article, I was afraid you'd be disappointed because it was fairly far out from your main topic, and the article did not have much detail, and "just some model photographs". Sounds like you found it quite interesting anyway!

Regards,

Henning (HoHun)

 

[HoHun]

Hi Stephan,

>I think the geodetic construction must have been vastly more difficult to build than a monocoque--after all, I built a monocoque airplane myself (a Falco) in my barn, but I doubt I could build a Wimpy.

Another coincedence: I just chatted with a guy who had built a "Kiebitz" ultralight ( Kiebitz (Flugzeug - Wikipedia) ), and when he showed me the construction of the aluminium-tube fuselage frame, I thought of the Schütte-Lanz airships which featured similar connection pieces ("node sheets" in literal translation), albeit in a much larger scale.

Node sheets of the Kiebitz:

http://www.kiebitzflieger.de/Knotenbleche2.JPG

(http://www.kiebitzflieger.de is a nice site, but they hide the full URL so I can't link to the page displaying the pictures of the fuselage in progress.)

I'm not sure about the details of geodetic construction, though - the Schütte-Lanz node pieces were admittedly more complex than the Kiebitz sheets, so maybe Barnes Wallis had added another level ...

I think I have two "Profile" booklets somewhere that have quite some detail on the astonishing Wellesley - I'll have a look to see what I can find about geodetic construction in them.

Regards,

Henning (HoHun)

 

[merlin]

Although prototypes of the Windsor (a new, four-engined, geodetic heavy bomber) were ordered; the siting of 20mm canon (rear of engine nacelles) failed because of the flexibility of the geodetic airframe made it impossible to aim them accurately!
Work on the aircraft stopped after one of the prototypes crashed.
The Warwick was a failure not because of the structure but the specification - two Vulture engines.
What geodetics gave was a structure that was able to absorb more battle damage than a conventional aircraft, but it was a specialised manufacturing process - only those tooleed for it could do it.

 

[HoHun]

Hi again,

>I think I have two "Profile" booklets somewhere that have quite some detail on the astonishing Wellesley - I'll have a look to see what I can find about geodetic construction in them.

From "Profile Aircraft 256 - Vickers Wellesley variants" by Norman Barfield:

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The slim oval-section fuselage of the Vickers Type 253 (G.4/31) biplane incorporated a unique Wallis-invented structural concept. his consisted of four light-alloy (Duralumin) longerons (in sections with screwed lightweight joints as in the M.1/30) with spiral channel section members wrapped around them and running in opposing directions to describe a lattice formation.

This so-called "geodetic"-type fuselage represebted a significant advance over his lightweight structural concept of the M.1/30. (The wing of the G.4/31 biplane was of conventional Vickers construction as described later.)

Wallis' objective was to achieve a fundamental reduction in weight and a consequent improvement in the strength/weight ratio for a fabric-covered structure. This departure from the monocoque principle of airframe structural design - which had been established for more than a decade - was a direct outcome of his airship design experience - from the arrangement that he had adopted for the "wire netting" that contained the gas bags of the R.100.

At this point it is appropriate to describe the principle of geodesy and its adaptation to aircraft contstruction. A geodetic line is defined as the shortest line between two points on the surface of a sphere and is known in global navigation as a "great circle" route. As adopted by Wallis in lattice form to aircraft structural design, this principle resulted in an ideal form of load balance and "fail-safe" combination.

In the tubular fuselage of substantially single-degree curvature, the arrangement consisted of two helices running in opposite directions but joined at each intersection such that one set of members was in tension while the other was in compression and the curved diagonal latticework (along the lines in which the principal forces set up in flight could be shown to act) absorbed all loads by stress equalization. Consequently, the resulting structural system replaced the conventional primary and secondary members with a system of main members only which was self-stabilizing and did the work of the shell of a normal monocoque without the need for internal load-carrying structure. The Wallis system also dispensed with stressed-skinning - and fabric covering was used.

The chief advantages of this type of structural design were the ability to adopt a near ideal streamlined shape while simultaneously providing maximum unobstructed internal space.

Hitherho untried in aerodynes (heavier-than-air aircraft), the adoption of the Wallis geodetic system was a bold step in the much smaller and more complex structure of a fixed-wing aircraft - especially in such a vital new tender. The accomodation of cut-outs, such as cockpits doors, gun turrets and - later, in the Wellington - bomb compartments, needed specially reinforced boundary frames. Nevertheless, Vickers continued to finance the system through the difficult and expensive development stages and the reward for their confidence is now a highlight of military history.

[...]

Wallis believed that high speed and long range could best be attained by the monoplane with a carefully shaped low resistance form of wing and body. He advanced that the essential features should be: An aspect ratio at the wing root of not more than about 17%; great stiffness and torsional rigidity of the wing and fuselage; and, very light structure weight. He rasoned that the weight of a wing of constant area and wing loading varied at a rather greater rate than the square of aspect ratio when the thickness/chord ratio at the wing root was held constant. However, prevailing structural design techniques meant that a wing of aspect ratio nine would weight approximately three times as much as one of aspect ratio six. This, Wallis said, was the reason why most of the stressed-skin type monoplanes at that time did not exceed an aspect ratio of six or seven. In his opinion, progress in performance was thus rapidly becoming blocked by structural limitations, and to get out of this impasse the success of the geodetic technique in the biplane fuselage led Wallis to adopt this unique concept for the entire airframe of his monoplane design. Its much cleaner shape promised substantially reduced drag and the thick-section high aspect ratio wing an outstanding lift/drag ratio as well as the much lower weight.

[...]

A measure of the strength/weight ratio of the geodetic system was given by the official record of tests carried out by teh R.A.E. on the fuselage of the Wellesley; and then compared with an orthodox type of structure of identical specification requirements. They showed that the geodetic structure, although only two-thirds of the weight, was more than twice as strong in flexure and nearly twice as stiff in torsion as in the conventional type.

[...]

Extolling the virtues and the evident success of his unique concept in creating an aircraft of exceptional load-carrying capacity and radius of action, Wallis wrote:

"The Wellesley is constructed on an entirely new principle - the Vickers-Wallis 'Geodetic' system.

"All parts of the structure are formed as geodetics in the streamline shape of the fuselage, and also in the curved profile of the wings . . . This method of aeroplane construction is the most important contribution to aircraft engineering since the completion of the first successful metal aircraft. For example, it permits each wing to be hollow and entirely free from any kind of obstruction - the additional space thus gained can be utilized for extra tankage or other loads, and the complete structure is one of extreme lightness combined with great strength and rigidity, thus making possible a range and load carrying capacity that has hitherho been considered unattainable."

[...]

With a disposable load of around 1 1/3 times its own weight the Wellesley carried twice the average percentage disposble load of its contemporaries. Addtionally, its high aspect-ratio wings decreased take-off run and increased cruising range.

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Regards,

Henning (HoHun)

 

[Graeme]

The stratospheric Wellington Mk VI with its 'pressure pod', clearly illustrates the surrounding geodetic framework.

 

[Martin W]

Although prototypes of the Windsor (a new, four-engined, geodetic heavy bomber) were ordered; the siting of 20mm canon (rear of engine nacelles) failed because of the flexibility of the geodetic airframe made it impossible to aim them accurately!

Lack of rigidity was another disadvangate of the basket weave framework. It is for the same reason that Wellington/Warwick could not be employed as glider tugs.

Also, the prviously mentioned balooning of fabric covering over the latticework on the wings was aerodynamically much less efficient than the similar effect on spar-and-rib structure. It must have had a marked effect on performance.