The FAA is usually a very good place to start. Example: http://www.faa.gov/regulations_policies/orders_notices/index.cfm/go/document.information/documentID/1020684
The FAA is where I started, with AIM 7-3-3(a), for example:
"The strength of the vortex is governed by the
weight, speed, and shape of the wing of the generating
aircraft. The vortex characteristics of any given
aircraft can also be changed by extension of flaps or
other wing configuring devices as well as by change
in speed. However, as the basic factor is weight, the
vortex strength increases proportionately. Peak
vortex tangential speeds exceeding 300 feet per
second have been recorded. The greatest vortex
strength occurs when the generating aircraft is
HEAVY, CLEAN, and SLOW."
I've never heard that only swept-wing planes generate measurable vortices, and was just wondering what the source might be.
- oldman