How do airplanes work?

In real-world aviation, aeroplanes fly because their wings turn airflow into lift; most use a propulsion system to create thrust, while control surfaces change attitude and direction. In straight-and-level powered flight at constant speed, lift balances weight and thrust balances drag. Pilots vary these forces to accelerate, climb, turn, descend, take off and land.

What keeps an aeroplane in the air?

An aeroplane remains in steady, level flight when its lift equals its weight and its thrust equals its drag. Four forces determine what happens next:

Balanced forces do not mean the aircraft is stationary; they mean it is not accelerating. If thrust exceeds drag, it accelerates. If the forces or their directions change, the aircraft can climb, descend or turn.

A common oversimplification is that lift must remain greater than weight throughout a climb. Extra force is needed to establish the climb, but once a constant climb speed and angle are reached, the forces rebalance along the new flight path. Part of the aircraft's thrust may also support its weight.

How do aeroplane wings create lift?

A wing creates lift by producing a pressure difference around itself and deflecting air downwards. Lower pressure usually develops over much of the upper surface, while higher pressure acts below it. The resulting aerodynamic force has an upward component, and the air receives an equal downward change in momentum.

The wing's shape helps, but its angle of attack is critical. This is the angle between the wing's reference line and the oncoming relative airflow, not simply its angle to the horizon. Increasing angle of attack normally raises lift until airflow begins separating from the surface.

The basic lift relationship is L = 0.5 × rho × V² × S × CL, where air density is represented by rho, V is airspeed, S is wing area and CL is the lift coefficient. Speed, density, wing configuration and angle of attack therefore all affect how much lift is available.

The familiar claim that air particles travelling over and under a wing must meet at the trailing edge simultaneously is false. There is no equal-transit-time rule. Pressure distribution and downward momentum are two descriptions of the same aerodynamic process, not competing explanations.

What causes an aeroplane to stall?

A wing stalls when it exceeds its critical angle of attack and extensive airflow separation causes a sharp loss of lift. Low airspeed makes a stall more likely because the wing must usually operate at a higher angle of attack to support the same weight, but low speed is not the actual cause.

An aircraft can stall while climbing, descending, turning or flying faster than its published one-g stall speed. Ice, contamination, damage, configuration and manoeuvring load can also reduce the available margin. Recovery begins by reducing angle of attack; the precise control inputs and limitations must follow the approved procedure for that aircraft.

How does an aeroplane engine create thrust?

An aircraft propulsion system creates thrust by accelerating a mass of air or exhaust rearwards. A propeller behaves like a set of rotating wings, while a jet engine compresses air, adds energy through combustion and accelerates air through its fan and exhaust system.

• Piston and electric aircraft normally use an engine or motor to turn a propeller.
• Turboprops use a gas turbine to drive a large propeller through a gearbox.
• Turbofans use a turbine core to drive a fan; in a high-bypass airliner engine, the fan supplies a large share of the thrust.

Our detailed explanation of how piston, turboprop and jet aircraft engines turn energy into thrust covers the machinery behind each type.

Thrust must overcome drag. Induced drag is closely associated with producing lift and is greatest when the aircraft is slow and working the wing hard. Parasite drag from the fuselage, undercarriage, antennas and skin friction becomes increasingly significant at high speed.

How does a pilot control an aeroplane?

A pilot controls an aeroplane by using movable surfaces to change aerodynamic forces and rotational moments around three axes. The primary controls are:

• Ailerons control roll by changing lift near the outer portions of the wings; some aircraft also use spoilers to assist.
• Elevators or stabilators control pitch by changing the aerodynamic force at the tail.
• The rudder controls yaw and helps keep turns coordinated.
• Flaps and slats increase maximum lift for lower-speed take-off and landing, usually with additional drag.
• Trim removes sustained control pressure; it does not create a separate mode of flight.

Our guide to how each aircraft control surface changes the aeroplane's motion explains these devices in more detail.

To turn, the pilot rolls the aircraft into a bank. This tilts the lift vector, creating a horizontal component that curves the flight path. Maintaining altitude then requires greater total lift, while rudder is used as necessary to counter yaw and keep the turn coordinated. We explain the force balance in our coverage of why banking turns an aircraft and how rudder keeps the turn coordinated.

Small aircraft may connect the controls to the surfaces through cables or pushrods. Larger aircraft commonly use hydraulic assistance, and fly-by-wire designs send electrical commands to computers and actuators. That changes how an input reaches a surface, not the underlying aerodynamics; see how fly-by-wire computers interpret and protect control inputs.

How do take-off and landing work?

Take-off uses thrust to build enough airflow over the wings for flight, while landing removes height and speed in a controlled way before the undercarriage supports the aircraft again.

Take-off

1. Configure the aircraft: The crew sets the required high-lift devices and calculates speeds from weight, runway conditions, wind, air density and aircraft performance.
2. Accelerate: Thrust moves the aircraft along the runway while the wheels still support most of its weight.
3. Rotate: At the calculated rotation speed, the pilot raises the nose, increasing wing angle of attack and lift.
4. Climb away: Once the wheels no longer need to support the aircraft, it accelerates or climbs and the undercarriage and high-lift devices are retracted according to the aircraft's procedure.

There is no universal lift-off speed. A heavier aircraft, short runway, high temperature, high elevation, tailwind or contaminated surface can materially change the required distances and speeds.

Landing

1. Establish the approach: The pilot manages pitch, power and configuration to maintain a stable path and target speed.
2. Configure: Extending flaps, slats and undercarriage permits a slower approach but adds drag.
3. Flare and touch down: Near the runway, the pilot reduces the descent rate and places the main wheels on the surface.
4. Decelerate: Spoilers reduce residual lift so the wheels carry more weight, allowing the brakes to work effectively. Reverse thrust may assist where fitted, but it is not available on every aircraft.

Can an aeroplane fly if its engine stops?

A fixed-wing aeroplane can glide after losing thrust because an engine failure does not remove its wings or their ability to make lift. The aircraft descends to maintain airspeed, with a component of its weight supplying the energy needed to overcome drag.

Pilots initially establish the appropriate glide speed, choose a landing area and work through the aircraft-specific failure procedure. Glide performance depends on aerodynamic efficiency, configuration, wind and damage; extended undercarriage, flaps or a windmilling propeller can shorten the distance available.

A multi-engine aircraft may be able to maintain altitude after one engine fails, but that depends on weight, altitude, temperature, configuration and remaining engine performance. A gliding aircraft cannot remain airborne indefinitely without rising air or restored power.

What other systems make the whole aeroplane work?

Wings and propulsion explain flight, but an operational aeroplane also needs structures, controls, energy supplies, instruments and ground equipment working together.

• The structure transfers lift, weight, thrust, landing and pressurisation loads through the wings, fuselage and tail. Wings are designed to flex within approved limits rather than remain perfectly rigid.
• Hydraulic, mechanical or electrical systems move flight controls, brakes, undercarriage and other equipment.
• Electrical generation and batteries power instruments, communication, lighting and control systems, with redundancy appropriate to the aircraft.
• Flight instruments and avionics show attitude, airspeed, altitude, position, engine condition and warnings.
• Environmental systems provide ventilation, heating and, where required, cabin pressurisation. Pressurisation makes high-altitude flight tolerable for occupants; it does not create lift.
• Undercarriage, brakes and steering support and control the aircraft while it is on the ground.

Which common explanations of flight are wrong?

• Engines do not normally hold an aeroplane up. Wings provide most of the lift in a conventional aircraft; engines provide the energy needed to overcome drag and maintain or change the flight path.
• Pulling back does not guarantee a climb. It increases pitch and usually angle of attack. Without enough energy, pulling farther can cause a stall instead.
• The rudder is not the main turning control in normal flight. Ailerons establish bank, the tilted lift vector turns the aircraft, and rudder keeps the manoeuvre coordinated.
• A stall is not an engine failure. It is an aerodynamic condition caused by excessive wing angle of attack.
• Autopilot does not use different physics. It commands the same control surfaces a pilot uses, based on sensor feedback and selected targets.