Quote:
Originally Posted by winkosmosis
That's what it was like when it was completely blocked off with no exit. No exit means no air going in. The grill is just acting like a parachute with air going in a certain distance and spilling back out
frank lee and I were arguing about this months ago.
The grill opening is small, the outlet is small. That means air slows down quite a bit going through the BIG engine compartment. Let's say the open cross section of the engine bay is 4x that of the inlet. That means the air would slow down to 1/4, and have 1/16 the drag as if it were full speed.
That would explain why the outlet vent on that plane reduces drag so much... Because of the small cowl opening, engine bay drag is low, and it acts as a tube to let air through.
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Internal ducting is a black art, but an important one for minimum drag. NACA did a lot of research on this during WWII. Once the cooling air gets in, it needs to slow down and increase pressure to reduce its transit drag, but importantly to have time for its higher pressure air to absorb engine heat. Once having done so, it needs to re-enter the slipstream in as efficient a pathway as possible.
The inlet should be at stagnation point on the nose, where pressure is greatest for ram-air effectiveness, then exit at the point of max. speed and therefore lowest pressure along the sides or bottom. Nature has perfected this, as seen with fast-swimming fishes.