 In this case, the company took advantage of the large number of war-surplus Wolseley Viper engines left over from Royal Aircraft Factory S.E.5a production.In 1921, one of these was married to a standard 504K airframe for evaluation under the designation Avro 551.Trials proved encouraging, but an extra fuel tank was installed in the upper wing as a result of the Viper's higher rate of fuel consumption, and changes were also made to the aileron design.