Hi Davparlr,
>I am not sure as I do not have any data on the DB 603E and its supercharger implementation. The low ceiling, however, tends to imply that the aircraft is more tuned to lower altitude performance (25k and below).
The aerodynamical advantage of the Do 335 is evident from the height differential for roughly equal top speed since at low altitude, it is handicapped by the greater drag of the denser air. If you look at speed over altitude diagrams of WW2 fighters, speed invariably increases with altitude until the full through height of the engine is reached. (Since many engines had two supercharger speeds, they had two full-throttle heights, giving the familiar "sawtooth" look of so many curves.)
In short, if you provide the Do 335 with an engine of equal power but greater full throttle height that matches those of the fighters it's compared against, it will be faster than it was at 21000 ft, where it was as fast in terms of true airspeed as the others at 25000 ft.
(The low ceiling you observed is caused not only by the engine characteristics, but also by the relatively small wing.)
>As for 500 mph, I think that the piston powered aircraft was pretty well reaching its perfomance limit in speed and several aircraft claimed to have flown 500 mph, all of which I am skeptical of, if nothing more than lack of proper instrumentation.
I absolutely agree on the value of caution. In fact, I'd have been disappointed if the 500 mph figure I suggested would not have been met with skepticism
All I can say is that I have calculated it carefully, and don't mean it to be the final word on the type anyway. I just thought quoting an actual figure I personally consider realistic would be better than simply writing "It would have been very, very fast"
>By the time the Do-335 began flying, the XP-72 had been cancelled due to changing need for a escort fighter and not an interceptor, and most likely the promised performance of the upcoming jets.
Quite right - in fact, there was some disappointment among Luftwaffe officers assigned to the secret Do 335 project when they learned it was not going to be a jet aircraft. The technological landslide is also evident from some later Do 335 spin-off paper projects that showed the rear engine being replaced by a turbojet, a configuration which resembles the later (and actually built) Ryan Fireball fighter.
Regards,
Henning (HoHun)