  We present a detailed photometric study and measurements of spiral arm pitch
angles for a sample of 50 non-barred or weakly barred grand-design spiral
galaxies selected from Sloan Digital Sky Survey. In order to find pitch angles,
we used a new method based on the window Fourier analysis of their images. This
method allows us not only to infer the average pitch angle, but to obtain its
value as a function of galactocentric radius as well. Our main results are as
follows:
  (1) Spiral arms of most galaxies cannot be described by a single value of the
pitch angle. About 2/3 of galaxies demonstrate pitch angle variations exceeding
20%. In most galaxies in the sample their pitch angle decreases by increasing
the distance from the centre.
  (2) Pitch angle variations correlate with the properties of galaxies - with
the shape of the surface brightness distribution (envelope-type or truncated
disc), and with the sign of stellar disc colour gradient.
  (3) More luminous and bright bulges produce more tightly wound spiral arms,
that is in agreement with current models for spiral arms formation.
