How Planes Work

Airplanes fly when the movement of air across their wings creates an upward force on the wings (and thus the rest of the plane) that is greater than the force of gravity pulling the plane toward the earth. The physics behind this phenomenon was first described by Daniel Bernoulli, an 18th century Swiss mathematician and scientist who studied the movement of fluids. Bernoulli discovered that the pressure exerted by a moving fluid is inversely proportional to the speed of the fluid. In other words, fluid pressure decreases as fluid speed increases, and vice versa.

The Bernoulli Principle

Airplanes fly when the movement of air across their wings creates an upward force on the wings (and thus the rest of the plane) that is greater than the force of gravity pulling the plane toward the earth.

The physics behind this phenomenon was first described by Daniel Bernoulli, an 18th century Swiss mathematician and scientist who studied the movement of fluids. Bernoulli discovered that the pressure exerted by a moving fluid is inversely proportional to the speed of the fluid. In other words, fluid pressure decreases as fluid speed increases, and vice versa.

The same principle applies to moving air. The faster that air moves through a space, the lower the air pressure; the slower it moves, the higher the pressure. Aircraft wings are designed to take advantage of that fact and create the lift force necessary to overcome the weight of the aircraft, and get airplanes off the ground. The undersides of wings are more or less flat, while their tops are curved. In addition, wings are slanted slightly downward from front to back, so air moving around a wing has a longer way to travel over the top than it does underneath. The air going over the top moves faster than the air going underneath, and the air pressure above the wing thus is lower than it is under the wing, where slower moving air molecules bunch together. The pressure differential creates lift, and the faster the wing moves through the air, the greater the lift becomes, eventually overcoming the force of gravity upon the aircraft.

The Phases of Flight

Push-Back and Taxi-Out.

This first phase of flight, after all doors have been secured, involves the movement of the aircraft away from the terminal jetway and along taxiways to a runway. A motorized vehicle called a tug sometimes is used to push the aircraft back from its gate. At some airports, certain aircraft are permitted to power back. This means that following engine start at the gate, the thrust reversers are used to literally back the aircraft away from the gate. The aircraft then moves under its own power along the taxiways. Since aircraft are designed primarily for flight, and are not ground vehicles, they are taxied at very low speeds. Push-back occurs only when the pilot has clearance to do so from Air Traffic Control, which monitors all aircraft movements during taxi.

Takeoff and Climb

When ready for takeoff, and cleared by Air Traffic Control to proceed, the pilot or first officer of an aircraft releases the brakes and advances the throttle to increase engine power to accelerate down the runway. Once aligned on the runway, steering the aircraft is normally accomplished by using foot pedals that manipulate the nose wheel until the speed is sufficient enough that wind rushing by the rudder on the aircraft tail makes nose wheel steering unnecessary.

As the aircraft gains speed, air passes faster and faster over its wings and lift is created. Instruments onboard the aircraft display this airspeed, which equals not only the speed of the plane relative to the ground, but also the speed of any wind that may be blowing toward the aircraft (aircraft normally take off headed into the wind). When the airspeed reaches a certain predetermined point known as rotation speed, the pilot manipulates panels on the tail of the aircraft to rotate the nose of the plane upward. This creates even stronger lift and the plane leaves the ground.

Rotation speed, abbreviated VR, is one of three important airspeed settings calculated before every flight. The others are V1 – the speed beyond which a safe stop on a runway is no longer possible; and V2 – the minimum speed needed to keep a plane flying should an engine fail after the aircraft surpasses V1. Some of the factors affecting VR and V2 are the weight of the aircraft, the air temperature and the altitude of the airport. The heavier the aircraft, the more lift, and thus speed is needed to get it off the ground. Aircraft also need to go faster to fly on a hot day than on a cool day. Hot air is less dense than cool air and less density produces less lift for the same speed. Similarly, the higher the altitude, the less dense the air. Aircraft need more speed to leave the ground at a place like Denver than at a place like New York, with all other factors such as weight being equal. Some of these factors also are important in calculating V1, although the key factor is the length of the runway that is being used.

Most large jets leave the ground at about 160 miles per hour and initially climb at an angle in excess of 15 degrees. The angle of a plane’s wings to the air flowing around them is extremely important to maintaining lift. If the so-called angle of attack is too severe, the flow of air around the wings becomes disrupted and the plane loses lift.

To make an aircraft more aerodynamically efficient, the wheels on which an aircraft rolls when it is on the ground are retracted into a cavity in the belly of the plane after it is airborne. There is less drag (wind resistance), and an aircraft can fly faster when its landing gear is retracted.

Cruise

Once a plane is in the air, it continues to climb until it reaches its cruising altitude, which is determined by the pilot and must be approved by Air Traffic Control. At this point, power is reduced from the setting that was needed to climb, and the aircraft maintains a consistent, level altitude. To fly level, the weight of the aircraft and the lifting force generated by the wings are exactly equal.

There is no standard altitude for cruising. Generally, it is around 35,000 feet, but that can vary considerably depending on length of flight, weather conditions, air turbulence and the location of other planes in the sky. Cruising speeds are at a constant mach number, about 82 percent of the speed of sound. This translates to a groundspeed of about 550 miles per hour, although that too can vary considerably with headwinds, tailwinds and other factors.

During flight, pilots normally follow designated airways, or highways in the sky, that are marked on flight maps and are defined by their relationship to radio navigation beacons, whose signals are picked up by the aircraft. Some jets also have inertial navigation systems onboard to help pilots find their way. These computer-based systems calculate the plane’s position from its point of departure, by closely tracking its heading, speed and other factors after it leaves the gate. Some aircraft also are capable of using signals from a constellation of satellites to pinpoint their position. This is known as the Global Positioning System. Commercial aircraft are increasingly using it. GPS enables aircraft to operate, with the permission of Air Traffic Control, to operate safely off predetermined airways. This capability makes for more efficient operations and adds capacity to the aviation system.

Pilots control and steer aircraft in flight by manipulating panels on the aircraft wings and tail. Those control surfaces are described in greater detail later in this chapter.

Descent and Landing

In this phase of a flight, the pilot gradually brings the aircraft back toward the ground, by reducing engine power and speed, and thus the force of the lift. The so-called final approach begins several miles from the airport. By this point, Air Traffic Control has put the aircraft in a sequence to land, carefully separating it from all other aircraft headed for, or leaving, the same airport. The landing gear is lowered, slowing the plane further. In addition, panels at the trailing edge of the aircraft’s wings, known as flaps, are manipulated to increase drag and thus reduce speed and altitude. Other panels, known as elevators, and the rudder are used (as they are throughout the flight) to steer the plane and keep it on the localizer (heading) and glideslope (glidepath), the continuous radio signals the flight crew will follow to the end of the runway.

Airline aircraft generally are traveling at about 120 miles per hour relative to the ground when they touch down. The flight crew then slows the aircraft quickly with several actions: pulling back on the throttles, raising yet another set of panels on the top of the wings, called spoilers, that disrupt airflow and increase wind resistance, reversing the thrust of the engines, and, of course, applying the brakes.

Taxi-In and Parking

The final phase of a flight is a reverse of the first phase. The aircraft is driven at slow speed under its own power onto the taxiway and from there to a gate. Since most gates are equipped with moveable jetways, or covered ramps, aircraft generally are parked under their own power.