I have a question about the characteristic dimension.
Q1 The drag on an airship is known to depend on the air density, ρ , dynamic viscosity, μ , speed relative to the air , V , and a characteristic dimension, L . An airship is to operate at 20 m/s in air at standard conditions. A model is constructed at 1/20 scale and tested in a wind tunnel at the same air temperature to determine drag. (a) Perform a dimensional analysis to determine the criterion which should be considered to obtain dynamic similarity.
like the question said for the characteristic dimension L, How can i know what dimension is L.