There were also difficulties in finding skilled workers due to the remoteness of the location, and a large proportion of the workers were local people who had to be trained. Conditions in the unheated airship shed were also poor: the roof leaked, ice formed on the girders in winter, and condensation caused corrosion of the airship's duralumin structure, so that the girders had to be varnished. For three years the assembly work was close behind that of the designers, and the progress of the design work was the determining factor in speed of construction. Airframe Since wind tunnel tests showed that a 16-side transverse section had about the same drag as a circular one, both R100 and R101 used a smaller number of longitudinal girders than previous airships to simplify stress calculations. Even so, the calculations for the transverse frames required hand computation that took two or three months to produce a solution for each frame. The thoroughness of the stressing calculations was a consequence of new Air Ministry criteria for the strengths required of airships, formulated after the catastrophic structural failure of R38 in 1921. Fewer longitudinal girders resulted in larger unsupported panels of fabric in the envelope, and flight trials were to prove that the R100's covering was barely adequate. The envelope of R101 was also unsatisfactory and a failure in its cover was possibly a cause of its crash. Barnes Wallis created the frame of the airship using only 11 standard components. The 16 longitudinal girders were formed of three tubes each, formed from strips of Duralumin wound into a helix and riveted together. These connected 15 polygonal transverse frames, which were held in shape by wire bracing connected to a central longitudinal girder running the length of the ship. A further consequence of the new rules for airframe stress design was that a new way of harnessing the lifting force of the gasbags had to be found. Wallis's solution to this problem later led to his innovative geodesic airframe fuselage and wing design for the Wellesley, Wellington and Windsor bombers. The elevators were aerodynamically balanced but the rudders were unbalanced. When the designers learned that R101 had been fitted with servo motors at a substantial cost in weight and money they thought that they had made a mistake and rechecked their calculations. They eventually concluded that their calculations had been correct: when R100 was flown the