The aircraft is fairly heavy and, until mach effects appear, fairly streamlined so air has little ability to impose significant resistance to keep the plane from picking up speed very fast with gravity pulling it down combined with the thrust of the aircraft's propellers and exhaust system (which is little as it has a turbocharger).
The propellers produce enough thrust to have allowed speeds of over 400 mph in the prototypes, around 395 in mass production models (extra weight, some drag produced by the bullet proof glass), and up to around 420 mph in later variants, gravity effectively provides (at least in a 90-degree dive) 1g of acceleration, turning every pound of aircraft into a unit of thrust pushing downwards at a little under 9.81 m/s every second, with little resistance.
The aircraft's critical mach number is about 0.65 (airflow hits mach 1 on some part of the wing) drag divergence mach is around 0.67-68 (airflow is now supersonic on some parts of the wing, shockwave strength reaches a point where it can drain enough energy out of the airflow to cause some separation), which causes an increase in drag, as well as a decrease in control effectiveness. The aircraft's loss of control occurs at around 0.74 mach. While gravity still is pulling it down, the drag from the airframe has gone up, and the efficiency of the propellers are affected by airspeed and mach numbers (which is why flying up high helps to a point, but mach effects do start to kick in and, past a certain point, it'll reduce the available net thrust) and by this point the net thrust of the propellers are less than the drag produced by the aircraft. I'm not sure if 0.74 is terminal velocity (a speed at which drag and thrust equal each other, gravity in this case is thrust as well), but going straight down without a means of recovery is quite terminal!
Since the speed of sound is affected by temperature, as the airplane reaches warmer air, some control effectiveness is restored, but propeller thrust also increases for the same reason (not much, but with gravity doing it's thing, it's enough to get you back to 0.74), so once the control is restored one would want to start pulling back, at first with everything you got, then progressively less (as the aircraft comes out of the dive, it will stop accelerating as gravity is being countered by lift to a degree, propeller thrust is fairly low, overall airflow over the aircraft is significant, and lift actually increases drag) to avoid over stressing the airframe (once it slows down enough, you'll regain full control and be well above the maneuvering speed, yanking the stick-all the way back above the maneuvering speed risks snapping off the wings).
Long message short: The plane accelerates very quickly into mach-tuck! The P-38's problems with mach tuck had to do with two things
- Thick wings: This is largely due to the high aspect-ratio (8.26), which favors a thick structure to avoid excessive aeroelastic flexing
- The junction between the fuselage and the wings
- The wings are convex on top, which causes the airflow to accelerate over them
- The fuselage is convex in shape, causing the airflow to accelerate to either side it
- The two combine together to produce an unusually high velocity airflow (I'm surprised this isn't thought of as an early example of area rule issues)
Eventually filleting was used to reduce this problem, though the critical mach number was 0.65 after the modification...
An airplane like the Spitfire has less mass, which means it won't pick up speed quite as fast, but it can ultimately tolerate a higher mach number, so it will accelerate a lot longer before it runs into mach tuck or airframe destruction: While this does sort of contradict what we were all taught in school that all objects fall at the same speed, this fails to factor in
- Air resistance: A heavier object is not affected as much by the mass of the air. If not raindrops would probably kill us.
- Gravity: This only applies with objects of significant mass, but technically they would both attract each other, and as a result would hit faster than if only one attracted the other.
