Tommy Cookers wrote:the VDT was intended at speed to reduce the TC exhaust recovery path and open a exhaust jet path ?
Yes, the help use the exhaust thrust. But also eliminate the engine mounted supercharger, relying on turbos only for supercharging, freed up several hundred hp.
Tommy Cookers wrote:the airframe changes (eg needed to the B50) were judged to be excessive
On R-4360 VDT was tried on a B-50. But the flight engineer had to monitor and control it constantly.
Tommy Cookers wrote:and ..... when fuel quality was raised CRs were usually held and mep/boost raised
this should have increased the exhaust thrust even with the usual simple exhaust systems
Higher boost improved performance at low altitude. That is the critical altitude or full throttle height was lower at a higher boost level, above which the extra boost could not be maintained.
Exhaust thrust was far more important at high altitudes, where engine power was reducing and propeller efficiency was lower.
Tommy Cookers wrote:so how much exhaust jet thrust (or equivalent hp) would eg a 2000 hp Mustang or Spitfire have had when high and fast ?
Not sure. There are figures about.
We can say that the Mosquito picked up ~10mph by changing to ejector exhausts.
Tommy Cookers wrote:what did the Merlin 1940 'ejector exhausts do ? (these were then dropped) increase crankshaft power ? increase jet effect by entrainment ?
The aime was to get more exhaust thrust, though the end shape was sometimes modified to reduce exhaust glare for night flying.
The ejector exhausts were refined throughout the war.