Avro 531 Spider

The Avro 531 Spider was a British fighter aircraft built by Avro during the First World War. It did not proceed past the prototype stage.

Design and development
The Spider was a sesquiplane with a largely conventional configuration, but it used Warren truss-type interplane struts, hence the appellation "Spider". In tests, the aircraft demonstrated exceptional performance, handling, and pilot visibility. By the time it flew, the War Office had already selected the Sopwith Snipe for mass production.

A second, refined version, the 531A was apparently never completed, but some of its components seem to have been used to build a derivative design, the 538. This had standard interplane struts and was intended as a racing aircraft. It was never used for this purpose, however, since it was discovered that it had a faulty wing spar, so the Avro firm used it as a hack instead from May 1919 to September 1920.