Why does a plane create a delayed sound after the airplane has already passed over you?
OK so phrasing the question is difficult. To explain some friends and I were at a park near an airport and we noticed the planes would pass over our heads and ten seconds later when no other plane was in sight there would be a weird new lingering noise of the plane that had just passed. Is it a lingering wake of noise from the plane that had passed?
You may be hearing the sound of the wake vortex coming off of the wing flaps. It depends on the particular aircraft, some are more noticeable than others but trailing off of the outboard edges of the wing flaps is a wake vortex. That is where the air has built up in front of the flaps and spills over the sides of the flap on the downwind side, creating a vortex. Air is a fluid. Air is not a liquid but it IS a fluid. Sometimes in high humidity conditions you can actually see the little tornadoes coming off of the flaps. The flaps are extended to slow the plane down. They create drag. It's like opening a barn door in a stiff breeze. There is lots of surface area to get caught by the wind and it concentrates in front of that door area until it can force it's way around. These little tornadoes can last for several seconds and can be dangerous to the next plane landing. That's why you will sometimes hear the tower controller warn the next plane like this, "Caution wake turbulence of departing heavy jet." If you were close enough to the landing aircraft this might have been what you heard. The wake vortex. Pretty neat. You don't hear that every day.
RC Airplane Park Flyer Compilation (With Crashes)
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