My flight instructor made a comment when talking about complex airplanes that "never over-square an engine" is a good rule of thumb but secondary to POH instructions. The POH, (I presume) is the best-informed source of how the engine was intended to operate.
He then went on to give an example: Supposedly, since the Doolittle Raid had to launch their B-25B Mitchels 200 miles further out to sea, they ran their engines over square which changed their fuel consumption such that they were still able to pull of the mission.
So here are my questions:
- Did the pilots during the Doolittle raid run their engines over square?
- By what mechanism would running over square change fuel consumption that dramatically? Is that unique to the B-25B's engines?
- If there is such a big fuel consumption benefit, why is the "never over-square an engine" rule so prevalent? (Seems like this competes with operating the engine at "best economy"?)
Image of B-25B launching off the USS Hornet: