I have given countless lectures over the years to a great variety of groups. They include the Smithsonian Museum in Washington DC; the US Air Force Museum at Wright-Patterson AFB, OH; the National Geospatial Intelligence Agency (NGA) in Chantilly, VA; and the Royal Aeronautical Society (RAeS) in London.
When appearing before volunteer groups or societies, I charge only for expenses.
Here is a link to a podcast of the lecture that I gave to the RAeS in London in 2010: