Over how large an area does an airplane in flight increase the pressure on the ground underneath it?
Answers
Air is pressurized by the engines. Turbofan engines compress intake air with a series of vaned rotors right behind the fan. At each stage of compression, the air gets hotter, and at the point where the heat and pressure are highest, some air is diverted. Some of the hot, high-pressure air, called bleed air, is sent to de-ice wings and other surfaces, some goes to systems operated by air pressure, and some starts its journey to the cabin. The cabin-bound air has to be cooled first in an intercooler, a device like a car radiator that sheds the heat to the ambient air scooped aboard for that purpose. From there the air travels into the airplane’s belly, where air packs cool it further using air cycle refrigeration. An air cycle cooler is perhaps the simplest air conditioner ever invented, because it doesn’t need a refrigerant as an intermediate fluid to dump heat. The air packs compress the incoming air to heat it before sending it to another intercooler to dump the heat to the outside. The air then expands through an expansion turbine, which cools it the way blowing with your lips pursed results in a cool flow of air. (Test the principle by blowing with your mouth wide open to see how warm the air would be if it weren’t compressed and then allowed to expand.) Now the air is ready to mix with air from the cabin in a mixer, or manifold, that adds the new air to the recirculating cabin air, which is moved by fans. To maintain a comfortable temperature for the passengers, automatic systems regulate the mixture of heat from the engines and cold from the air packs. To maintain the pressure in the cabin equal to that at low altitude, even while the airplane is at 30,000 feet, the incoming air is held within the cabin by opening and closing an outflow valve, which releases the incoming air at a rate regulated by pressure sensors. Think of a pressurized cabin as a balloon that has a leak but is being inflated continuously. On the ground, the airplane is unpressurized and the outflow valve is wide open. During preflight, the pilot sets the cruise altitude on a cabin pressure controller. As soon as the weight is off the main wheels at takeoff, the outflow valve begins to close and the cabin starts to pressurize. The airplane may be climbing at thousands of feet per minute, but inside the cabin, the rate of “climb” is approximately what you might experience driving up a hill. It might take an average airliner about 20 minutes to reach a cruise altitude of, say, 35,000 feet, at which point the pressurization system might maintain the cabin at the pressure you’d experience at 7,000 feet: about 11 pounds per square inch. Your ears may pop, but the effect is mild because the climb rate is only 350 feet per minute. When the airplane descends, the pilot sets the system controller to the altitude of the destination airport, and the process works in reverse Read more at https://www.airspacemag.com/flight-today/how-things-work-cabin-pressure-2870604/#AFUFxMHLERRSKHB2.99