Smooth airflow into a radiator is a major issue. Belly radiator scoops ingest air from the turbulent, slow-moving boundary layer. Underwing radiators (aka. Spitfire and Me109) suffered from similar turbulent airflow.
Fortunately, there are two ways to ensure smooth airflow into radiators, carburetors, oil coolers, etc. If intakes are far enough forward, the boundary layer is still smooth. That is why so many WW2-vintage airplanes sported chin radiators. Leading edge radiators (Whirlwind, Mosquito, later Fireflies, etc.) were so far forward that they always ingested smooth air.
The only way P-51 Mustangs' belly radiator could invest smooth air was by "raising" the intake above turbulent boundary layers.
P-51's other advantage was a huge radiator face that transferred heat from slow-moving air. Slow speed equals low drag.
Spitfire and Me 109 tried similar diverging radiator inlets in their wings, but they could never bury large enough radiators inside shallow wings. P-51 radiators were buried in the aft fuselage to reduce profile drag. Finally, Meredith Effect only produces a few percentage points of thrust in one corner of the envelope. In practical terms, P-51 radiators reduced cooling drag to almost zero.
Similarly, later marks of Mosquitos sported chin engine intakes for smoother airflow.
Jet fighter engine intakes resemble P-51 intakes with ramps far enough away from the fuselage that boundary layers are never invested into engines. Those intake ramps vent turbulent boundary layers away from the engine.
Similarly, most modern turboprops use pitot inlets.
Fortunately, there are two ways to ensure smooth airflow into radiators, carburetors, oil coolers, etc. If intakes are far enough forward, the boundary layer is still smooth. That is why so many WW2-vintage airplanes sported chin radiators. Leading edge radiators (Whirlwind, Mosquito, later Fireflies, etc.) were so far forward that they always ingested smooth air.
The only way P-51 Mustangs' belly radiator could invest smooth air was by "raising" the intake above turbulent boundary layers.
P-51's other advantage was a huge radiator face that transferred heat from slow-moving air. Slow speed equals low drag.
Spitfire and Me 109 tried similar diverging radiator inlets in their wings, but they could never bury large enough radiators inside shallow wings. P-51 radiators were buried in the aft fuselage to reduce profile drag. Finally, Meredith Effect only produces a few percentage points of thrust in one corner of the envelope. In practical terms, P-51 radiators reduced cooling drag to almost zero.
Similarly, later marks of Mosquitos sported chin engine intakes for smoother airflow.
Jet fighter engine intakes resemble P-51 intakes with ramps far enough away from the fuselage that boundary layers are never invested into engines. Those intake ramps vent turbulent boundary layers away from the engine.
Similarly, most modern turboprops use pitot inlets.
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