How Jetliners Fly

Commercial jetliners - like all airplanes - fly according to basic physical principles:

• Airplane Parts
• Aerodynamics
• Propulsion

Pitch
To maneuver, a jetliner uses control surfaces attached to its wings and tail. Moving the elevators up or down makes the jet's nose rise or drop (pitch axis).

Yaw
Moving the rudder side to side swings the nose left or right (yaw axis).

Roll
Using the ailerons banks the airplane left or right around it's roll axis.

Flaps and Slats
Jetliner wings have leading and trailing edge devices (slats and flaps) that extend during takeoffs and landings (low speed flight) and retract for cruise (high speed flight). The landing gear also retracts in flight to reduce drag. Jets also have swept wings. Wing sweep lets them fly closer to the speed of sound than airplanes with straight wings.

Spoilers
Jets fly so efficiently, in fact, that they need spoilers to help them slow down and lose altitude. Spoilers are hydraulically actuated panels on the tops of the wings. Pilots sometimes call them "speed brakes" or "lift dumpers," reflecting their function. On landing, they deploy even higher to make sure the airplane stays on the ground.

Aerodynamic Lift
Four forces come into play during a jetliner flight:

• Lift — The upward force generated by the wings.
• Gravity — The downward force opposing lift
• Thrust — The forward force produced by the engines
• Drag — The rearward pull as air resistance seeks to slow the airplane down

Wings
An airplane wing has a distinctive shape called an airfoil. Seen from the side, an airfoil is curved on top and relatively flat on the bottom.

As a wing moves through the air, it splits the airflow. Some of the air passes above the wing and some below. But because the wing's upper surface is curved, the air rushing over the top is forced to speed up.

The result is decreased air pressure above the wing. In contrast, the air flowing below the wing is slower and its pressure is higher.

The wing is lifted because the air pressure on the top of the wing is lower than the air pressure on the bottom. The faster the airplane flies forward, the more of this aerodynamic "lift" it creates. Flight occurs when the force of lift exceeds the weight of the airplane.

Note:
Aerodynamic lift is based on the Bernoulli principle, which states that the pressure of a flowing fluid decreases as its velocity increases. Daniel Bernoulli
(1700-1782) was the first person to define the fundamental relationship between pressure, density and velocity in fluid flows.

Glide
Finally, jetliners are highly efficient gliders. Even without engine thrust, they can glide about 20 feet forward for each 1 foot of altitude they lose. If all engine power were lost at cruise altitude, a typical jet could glide more than 100 miles (160 kilometers) before landing at sea level.

Propulsion
A jet engine uses a fan on the front to suck in air, mixes it with burning fuel, and blasts the expanded air-gas mixture out the back. In the process, it turns a turbine in the back of the engine that spins the fan in the front. Because every action has an equal and opposite reaction, a jetliner surges forward as its engines push air backward.

Fanjets
High-bypass-ratio fanjets are the most reliable engines ever developed. Fundamentally simple, fanjets are fuel efficient and quiet turbine engines. They feature continuous combustion and smooth rotation, unlike the internal-combustion engine of a car, truck or bus. A fanjet engine has three sections:

• Fan unit and compressor section
  
• Combustion chamber
• Turbine section

The compressors pressurize air and feed it aft. Most goes around the engine core through a nozzle-shaped chamber. The rest goes through the engine core where it mixes with fuel and ignites. The hot expanding combustion efflux passes through the turbine section, spinning the turbine as it exits the engine.

The spinning turbine turns the engine shaft. The rotating shaft spins the fan on the front of the engine. The fan compresses more air and keeps this continuous cycle going.