Geodetic airframe

A geodesic (or geodetic) airframe is a type of construction for the airframes of aircraft developed by British aeronautical engineer Barnes Wallis in the 1930s. It makes use of a space frame formed from a spirally crossing basket-weave of load-bearing members.[1] The principle is that two geodesic arcs can be drawn to intersect on a curving surface (the fuselage) in a manner that the torsional load on each cancels out that on the other.[2]

Early examples

18th-century American warships "Diagonal rider" in their construction.

The "diagonal rider" structural element was used by Joshua Humphreys in the first US Navy sail frigates in 1794. Diagonal riders are viewable in the interior hull structure of the preserved USS Constitution on display in Boston Harbor. The structure was a pioneering example of placing "non-orthogonal" structural components within an otherwise conventional structure for its time. As the "diagonal riders" were included in these American naval vessels' construction to reduce the problem of hogging in the ship's hull, and did not make up the bulk of the vessel's structure, they do not constitute a completely "geodetic" space frame.

Calling any diagonal wood brace (as used on gates, buildings, ships or other structures with cantilevered or diagonal loads) an example of geodesic design is a misnomer. In a geodetic structure, the strength and structural integrity, and indeed the shape, come from the diagonal "braces" - the structure does not need the "bits in between" for part of its strength (implicit in the name space frame) as does a more conventional wooden structure.

Aeroplanes

Wellington Mk.X HE239 of No.428 Sqn. RCAF, illustrating the geodesic construction and the level of punishment it could absorb while maintaining integrity and airworthiness.
A section of the rear fuselage from a Warwick showing the geodesic construction in duralumin. On exhibit at the Armstrong & Aviation Museum at Bamburgh Castle.

The earliest-known use of a geodesic airframe design for any aircraft was for the pre-World War I Schütte-Lanz SL1 rigid airship's envelope structure of 1911, with the airship capable of up to a 38.3 km/h (23.8 mph) top airspeed.

Barnes Wallis, inspired by his earlier experience with light alloy structures and the use of geodesically-arranged wiring to distribute the lifting loads of the gasbags in the design of the R100 airship, evolved the geodetic construction method (although it is commonly stated, there was no geodetic structure in R100).[3] Wallis used the term "geodetic" to apply to the airframe and distinguish it from "geodesic" which is the proper term for a line on a curved surface, arising from geodesy.

The system was later used by Wallis's employer, Vickers-Armstrongs in a series of bomber aircraft, the Wellesley, Wellington, Warwick and Windsor. In these aircraft, the fuselage was built up from a number of duralumin alloy channel-beams that were formed into a large framework. Wooden battens were screwed onto the metal, to which the doped linen skin of the aircraft was fixed.

The metal lattice-work gave a light structure with tremendous strength;[1] any one of the stringers could support some of the load from the opposite side of the aircraft. Blowing out the structure from one side would still leave the load-bearing structure as a whole intact. As a result, Wellingtons with huge areas of framework missing continued to return home when other types would not have survived; the dramatic effect enhanced by the doped fabric skin burning off, leaving the naked frames exposed (see photo). The benefits of the geodesic construction were partly offset by the difficulty of modifying the physical structure of the aircraft to allow for a change in length, profile, wingspan etc.

See also

References

Notes

  1. ↑ 1.0 1.1 Buttler, p.93
  2. ↑ Buttler, p.94
  3. ↑ Murray, p.34 and p.44

Bibliography