A post by GTX_Admin with a lot of pictures of the Fairey Battle with various mods, got me thinking: which aircraft in history has had more test-bed derivatives that any other? Some manufacturers have used a single aircraft with which they often attached engines; Pratt and Whitney Canada used a Viscount and a B-720 for years, but those are in a different class. They were strictly used for testing new engines and their descendants. I'm talking about modifying an aircraft some way so as to find out how the whole thing behaved, from front to back, inside and out. Performance, handling, range, altitude, etc. Here are the pictures from that thread, and I know there are more, but my mind (such as it is) was sparked by this post so that's the example I'll use.
So, which one? I'll also count out aircraft that went through many changes during an operational lifetime, like the Spitfire. Let's leave it just for aircraft that have been used as test-beds more than others.
Someone should really release some conversion kits (maybe 3D printed) for the various Fairey Battle Engine Testbeds:
Wright Cyclone engine:
Napier Sabre:
RR Exe:
Fairey Monarch or Prince 4 (also see images above):
Bristol Taurus:
Bristol Hercules: