Hello there! As your question is related to looking for identification/information regarding military personnel, our Guide on Military Identification may be of use to you. It provides a number of different resources, including how to request service records from a number of national agencies around the world, as well as graphical aids to assist in deciphering rank, unit, and other forms of badges or insignia. While the users here may still be able to lend you more assistance, hopefully this will provide a good place to start!

Check out [this page] (http://www.antiquesimagearchive.com/items/D9733123.html)

It identifies this pin as being a "US Army Air Service airship pilot's wings with dirigible or zeppelin profile".

With a bit of research it should be easy to find out more about this branch of the service.

Ok, I'm kind of worried about posting because I am not an expert in this field, but I did do some research for you starting with the links posted by jschooltiger. I cannot answer "Yes" to all of the questions in the rules, but perhaps I can further point you down the path to finding what you are looking for. If that is not allowed, I won't be upset if a mod removes this reply.

This is listed as an Airship Pilot Badge on wikipedia's Obsolete badges of the United States military page. According to William K. Emerson on his site (page 5), this is indeed an Airship Pilot Badge that was established on October 14, 1921 and abolished in 1940. He makes note that these were heavily reproduced for collectors.

Further, United States Army Medals, Badges and Insignia by Col. Frank Foster (pg. 56) also labels these as Airship Pilot Wings agreeing that they were created in 1921, but also adds that they were awarded to graduates of the Air Corp Balloon and Airship School and the pilot needed at least 200 hours of flying time, 75 of which were solo.

The school itself was at Scott Field, Illinois during this time, and was nine to ten months long according to Aviation in the U.S Army 1919-1939 (starting on pg. 59). This passage also mentions the creation of the Airship Pilot rating, which this badge belonged to. Starting on page 371, this book goes into more details on the end of the airship wing. In 1933 there were 20 officers and 650 enlisted men in the lighter-than-air branch (although there were 73 trained pilots across the service serving in other roles), and by the time WW2 broke out it had shrunk to 10 officers and 350 enlisted men.

I hope that helps get you started. If this badge did belong to your grandfather (or his father) and wasn't a reproduction, it appears he had a relatively unique and interesting role in the U.S military.