Udet,
It was nothing about the engine that caused the problem. The DB engine was excellent. The issue was with the cooling system design.
An airfoil involves what is referred to as the boundary layer. This is a layer of air molecules that are relatively stationary with respect to the wing (or other airfoil surface). Molecules near the wing move with the wing, then as the distance from the wing increases, there are increasing levels of exchange with fast moving ambient air molecules (though this is still relatively small), and then there is a point further from the wing where there is ambient airflow (ie: still air through which the wing is passing). The faster the plane goes (within the subsonic realm - rules change for transonic/supersonic flight) the thicker the boundary layer.
The issue arises when the air flowing across the bottom of the wing encounters the scoop, which creates a back-flow resistance to the air flow. This tends to lift the boundary layer away from the wing and create boundary layer ingestion, which is turbulent air entering the scoop which diminishes cooling efficiency. As speed increases, the problem gets worse, until finally it becomes chronic and the boundary layer lifts up and flows completely over the scoop, creating a vacuum in front of the scoop. It then slaps down to fill the vacuum, then builds up again and repeats the cycle. Air flow into the scoop is non-existent when the flow is over the scoop, and extremely turbulent when it slaps down and does enter the scoop. This greatly diminishes cooling efficiency.
You can visualize what is going on by taking a 2 liter soda bottle, filling it most of the way, and then pouring it out into the sink. Start pouring slowly and the water will flow smoothly out of the bottle. Increase the angle and it flows faster and faster until suddenly it starts "gulping" and the rate of flow is greatly diminished.
In the late 30's they had no experience with this phenomena and so neither the Bf109 nor the Spitfire scoop designs take it into consideration. it came to light on the P-51 purely by accident, because the scoop size and position was such that the test pilots could hear the boundary layer collapsing and making a loud "popping" sound beneath their seats. The solution was to remove the scoop inlet from the wing surface and snorkel it forward:
The 109 had no mechanism to deal with this issue (Bf109G-10):
It was not an issue on the E series, they didn't go fast enough to encounter it except in dives where it was not such a big issue (as power was reduced significantly anyway). It started to become a slight issue on the F series, and it was a significant issue on the G series but was never dealt with, probably because there really is no good solution for wing scoops.
The British simply increased the scoop height to ingest the boundary layer at higher speeds, accepting the added drag this created at all speeds:
Spitfire Mk. I (only has 1 scoop)
Spitfire Mk. IX
Spitfire Mk. XIX
So it's nothing to do with the quality of the engine, it has to do with the cooling system design. At high speeds the cooling system simply gets bypassed by the boundary layer separation effect. The Bf109G/K has no remedy for this issue.
As for pilot comments, P-51 pilots consistently claim they could easily out-turn 109's at speed, and FW's too but less easily. Pilot plane bias has to be considered as well of course. But kill stats really are meaningless because the great majority of the time the victim never saw the plane that shot him down or did not see it until it was too late. This does not always mean a "bounce" of an unaware target (though often it does), it can also mean they were engaging an opponent (either offensively or defensively) when another opponent they were unaware of shot them down.
By 1944 the Bf109 was long in the tooth. Its design had never considered 400 mph class level speeds, and its cooling system was not made for such speeds. With the cooling system barely functional, the radiators overheated rather quickly.
BTW: the Soviet Fighter Tactics book did not become available to anyone but Soviet military personnel until after the fall of the Soviet Union in the 1990's. It was created to teach Soviet pilots how to fight the German planes in late 1943 and was based upon both combat experience and testing of captured aircraft. This was not "propaganda" to defame the 109.
In general, it appears the Germans considred 2 minutes at high speed enough for combat needs. The FW190A series was limited to about 3 minutes of top speed performance by design. The very skilled pilots of the Luftwaffe' were probably able to make this work for them most of the time.
=S=
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