Hi Bolillo_loco,
>I get the opinion that some believe that at 20,000 ft from low to medium ias that if you dove a 38 it would just compress and go into the ground or break up in mid air, but then again I could be wrong.
I'm beginning to share Gripen's impression that you're falling victim to selective perception when it comes to the P-38. Here's what I actually wrote:
"Any P-38 attempting to go to the P-51's maximum possible Mach number would end up as a cloud of debris even before hitting the ground - dive brakes or not."
The advantage of a P-38 in a dive is better initial acceleration (i. e. higher specific excess power at slow speed). The disadvantages are lower top speed, lower critical Mach number, and a negative pitching moment in a dive.
What happens if a P-38 goes into a diving contest with the P-51 from equal, low speed? First, it will have the advantage of higher specific excess energy and accelerate faster. Knowning that, the P-51 pilot will put his nose down sharpy. At high altitude and low speed, the thrust to weight ratio of a propeller fighter may be 1:5 (high estimate in favour of the P-38).
If you're going straight down - which the Mustang pilot will - the "thrust to weight ratio" of gravity is 1. The benefit of propeller thrust is comparatively small that way, and the benefit of the P-38's higher specific excess power is even smaller. What's more, diving straight down will result in rapid acceleration so that the P-38 will leave its region of superiority very quickly. Besides, with increasing speed, the thrust provided by the propeller will drop, making it pretty much insignificant in comparison to the "thrust" provided by gravity.
In short, the "better initial dive acceleration" observed in a low G pushover is a good estimate of specific excess power for this flight situation, but in an all-out combat dive, it means little.
Now we have two aircraft going steeply down at high speed. As soon as each of them reaches the level top speed for the altitude, it starts losing energy. The P-38 is slower, so its acceleration will begin to fall back behind that of the P-51. Soon, the P-38 will reach its critical Mach number, which means a dramatic drag rise, so it will fall back behind the P-51 even more. Extending the dive recovery flaps now will add yet more drag, and let the P-51 pull away even further. When the P-38 comes up against the maximum allowable Mach number with dive brakes extended, it can't hold the dive angle or it will enter a negative-G loop overstressing the structure. So the P-38 will have to break off chase of the P-51 that's far ahead by now anyway.
Even worse: The steeper the dive is, the more allowance the P-38 pilot has to make for the additional acceleration during the pull-out process. The above sequence is probably only possible for fairly shallow dives - in a vertical dive, the P-38 pilot would have to begin the pull-out very early to make sure he doesn't exceed the maximum Mach number with dive brakes extended in the pull-out. Due to the uncontrollable nose-down moment this would stop the pull-out and push the plane into the dive again - and into self-destruction.
The P-38 certainly was a very successful fighter in several theatres, but with regard to high-speed diving, it was simply crippled. Not due to bad design, I'm sure - in fact, the P-38 was a very advanced and sophisticated aircraft in many areas.
Widewing summed it up pretty well:
"So, if you find fault in the aerodynamics, you do so only with the hindsight advantage of 65 years of development in the science of aerodymanics since Johnson first penned the concept in the Fall of 1936. "
The British aerodynamicist Sir Morien Morgan stated the same in greater detail:
"Looking back, I think that the greatest problem at the end of the 1930s was that it was extremely difficult to visualize what combat would be like in the new monoplane fighters; the only air fighting experts we had were from the First World War and that had been twenty years earlier. I think we all paid too much attention to the behaviour of an aeroplane flying on a calm sunny day and harmonizing the controls so that they could do nice aerobatic displays; we seemed to miss the importance of handling at speeds around the maximum permissable, in fast dives. Before the war, I remember, people thought that it was rather an academic exercise to scream downhill at one's maximum permissable speeds."
Regards,
Henning (HoHun)
