.No detail was too small to consider, even the
humble rivet. Mitchell had designed an absolutely smooth surface for
the metal skin of his fighter. This meant that every single rivet hole
had to be countersunk to take the flat-headed flush rivets. But were
they all essential? With tens of thousands of holes to make on every
single aircraft, much time and labour (and cost) could be avoided
during production by the use of conventional dome-headed rivets. To
establish what drag would be created by domed heads, the design
team went out shopping. Several bags of dried split-peas were bought
from a local grocer and one was glued to the head of every single rivet,
creating the effect of a domed head. Flight testing revealed a loss of 22
mph on the top speed. By progressively scraping off row after row of
peas, the lost speed was regained, while the critical areas that needed
flush riveting were identified: the rest could stay dome-headed.