I wanted to share something that i thought was interesting, and want to get your input.
We've all heard of B747s with a 5th engine, L-1011's with a 4th engine, etc but they've always been 'inoperative', just hanging there for the purpose of transporting for maintenance.
I stumbled on this image of an engine mounted on the rear fuselage of a Boeing 707, specifically reg: N70700 which was the actual PROTOTYPE Dash 80.

It appears that it's an operational engine (note the S exhaust duct, re-directing hot gasses over the horizontal stabilizer).
The image didn't come with any captions, so I'm not sure what the purpose was. Any ideas?
My 'guess' is that it has something to do with designing the 727. I can see that the engine pod is tufted, so maybe they were testing airflow characteristics around the pod? But if so, why would they need the engine to be operative.. just to test airflow characteristics?
Thoughts?