This badge is shown and discussed in Malcolm Hobart's 'Badges and Uniforms of the Royal Air Force'. I don't have access to it right now, but can check later to see what it says, but it is definitely Royal Naval Air Service. Today ebay is showing an other rank's pilot's badge (repro):
http://cgi.ebay.co.uk/RNAS-Other-Ran...742.m153.l1262
There are also a couple of repro officer's badges like the one illustrated in the previous posts.