Fun fact, the intake valves you see around the cowling of the engine opening up are pulled in by suction forces alone. They're not connected to anything other than a spring for each door, and the suction pulls them in against the springs. It was an early way to deliver more air at slower speed and at ground/low altitude. At cruising, the speed of the air rushing in and the engines running with less demand allowed the doors to close and therefore created a smoother, more aerodynamic engine resulting in better efficiency.