Glad Yorky Towers (not surprisingly, from Coventry) finally mentioned the Gooney Bird, saving me the trouble.
But, just for the record, we used also to do it in the Dart-Herald on hot days, provided the runway was long enough; always with water-methanol injection for extra power. Like the F27 and others, it's to enable a decent RTOW when the second-segment climb gradient (WAT) would otherwise be a problem. It gave us a better payload out of Southampton, but not out of Guernsey or Jersey (too short).
Lookforshooter, perhaps you are not old enough to remember large piston aeroplanes that got airborne in half the runway length; then struggled to clear a 100-foot hill one mile beyond the airfield boundary?
The prototype piston (Leonides) Herald was lost at Khartoum, because it failed to climb through an inversion layer. Does anyone know whether it was using flap for take-off?