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How airplanes use moving air

An adaptation of the lesson posted by Prof. P. M. Sarun, Applications of Fluids and Its Motion — Part 5: Aeroplanes, IIT (ISM) Dhanbad. This is a new explanation for this learning game, not his original wording or an endorsed version.

Air may be invisible, but it has mass and can push on objects. An airplane flies by changing how that air moves around its wings and through its engines. To understand flight, follow the air: Where does it go? How does its speed change? What pushes on it?

1. A wing changes the air’s motion

A wing has a shaped cross section called an airfoil. Its shape and angle help turn passing air downward. To change the air’s motion, the wing must push on it. The air pushes back on the wing. This creates lift.

Lift acts across the incoming airflow. In ordinary level flight, it acts upward and balances weight. Air pressure also helps explain this force. Pressure is the push caused by air particles striking a surface. During normal lifting flight, pressure is generally lower above the wing than below it. The combined pressure forces push the wing upward.

These explanations fit together. The pressure pattern acts on the wing and turns the air. You do not need to choose between “pressure makes lift” and “the wing pushes air down.” They describe the same event from different viewpoints.

Watch for this myth: Air above and below a wing does not have to reach the back edge at the same time. Equal travel time is not a rule of flight. Also, a curved wing can make lift at zero geometric angle of attack.

2. The wing’s angle must be measured against the air

Draw a straight line from an airfoil’s front edge to its back edge. This is its chord line. The angle between that line and the incoming airflow is the angle of attack.

That is different from the airplane’s angle above the ground. The airflow seen from the moving airplane is called relative wind. A gust can change its direction even when the airplane’s nose does not move. An upward gust can increase angle of attack; a downward gust can decrease it.

Within a useful range, increasing angle of attack generally increases lift. But there is a limit. Lifting the nose farther is not a promise of more lift forever.

3. Smooth airflow has a limit

Air close to the wing is slowed by friction. This thin, slowed region is the boundary layer. A smooth, tapered shape helps the air keep following the surface.

As the air travels toward the back of a wing, it may meet rising pressure. This is an adverse pressure gradient: pressure pushes against the direction the air is trying to move. If the boundary layer loses too much forward motion, the airflow can pull away from the surface. This is flow separation.

When a wing exceeds its critical angle of attack, separation can spread over the upper surface. Lift drops, and drag increases. This is a stall.

A stalled wing is not a dead engine. A wing can stall while the engine runs normally. An airplane can also glide after losing engine thrust if its wings still have useful airflow. Slow flight can lead to a high angle of attack, but a stall is not defined by one speed for every airplane.

4. Making lift costs energy

Drag resists an airplane’s movement through the surrounding air. Skin-friction drag comes from air rubbing against its surfaces. Pressure drag comes from the pressure pattern around it. A large separated wake can greatly increase pressure drag.

Induced drag is linked to making lift with a wing that has tips and a limited span. The wing gives the air downward motion, and that moving air carries energy away. The airplane needs a source of energy to keep overcoming drag.

For a given lift and speed, a longer wingspan can spread the downward push across more air and reduce induced drag. But more wing surface can add friction drag, and larger wings have other design costs. Designers balance these tradeoffs rather than simply choosing the biggest wing.

5. Wings can change shape—and leave a wake

Flaps move near the back edge of the wing. Slats move near the front edge. They help the wing produce lift at lower speeds by changing its shape and airflow. Slotted designs can help keep air moving over the wing or flap. These changes usually add drag too.

Spoilers are panels that rise from the top of the wing. They disturb airflow, reduce lift, and add drag. After touchdown, less lift means more of the airplane’s weight rests on its wheels. That helps wheel braking.

Pressure differences also cause air to curl around the wingtips. The airplane leaves spinning trails called wingtip vortices. They can disturb another aircraft even when they are invisible. Winglets can improve efficiency, but they do not erase wake hazards.

6. How lifting flow gets started

The lesson shows airflow changing as a wing begins to make lift. At first, the air would need to make a sharp turn around the back edge. That pattern does not last. A swirling region called a starting vortex is shed into the wake as the flow develops.

In the later, established flow, air leaves the sharp back edge smoothly. This is called the Kutta condition. The air turns downward overall, and the wing makes lift. The starting vortex is different from the continuing vortices behind the wingtips.

7. A propeller is a set of rotating wings

A propeller blade is an airfoil. As it turns, it changes the air’s pressure and motion. The propeller pushes air backward. The air pushes the propeller and airplane forward. This forward force is thrust.

The tip of a blade travels farther in each turn than a section near the center. It therefore moves faster. Blades are twisted so different sections can meet their local airflow at useful angles. Blade pitch can also change on some propellers to suit different conditions.

A propeller can stall if its blades meet the air at too large an angle. It also twists the air, which pushes back with an opposite twisting force called reaction torque. Opposite-turning propellers can help balance this effect.

Very fast blade tips can create shock waves—sudden changes in pressure and other air properties. These can increase drag and noise. This is one reason propellers become less useful at very high flight speeds.

8. A jet engine adds energy to flowing air

A simple turbojet has five main steps. The inlet or diffuser slows incoming air and raises its pressure. The compressor then does work on the air, raising its pressure further.

Next, fuel burns in the combustion section, adding heat to the already compressed air. Hot gas passes through a turbine, giving up some energy. A shaft carries that power to the compressor. Finally, the gas expands and speeds up through the nozzle.

The engine changes the momentum of the air and exhaust. It pushes gas backward; the gas pushes the engine forward. It does not have to push against the runway to make thrust.

9. Why turbofans move so much air

A turbofan uses a turbine-driven fan to move a large amount of air. Some goes through the engine core. Much goes around the core as bypass air, which also helps produce thrust.

For a similar forward push, giving a smaller speed increase to more air can waste less energy than making a small amount of air move much faster. This helps explain the usefulness of high-bypass turbofans on airliners. The exact share of bypass air depends on the engine design.

10. Ramjets and scramjets use fast incoming air

A basic ramjet has no rotating compressor. Its inlet uses fast incoming air to provide compression. It then burns fuel and sends hot gas out through a nozzle. It needs forward speed to work well.

A ramjet slows its combustion airflow below the speed of sound. In a scramjet, that airflow stays faster than sound. Fuel must mix, ignite, and burn while the air rushes through. Keeping a stable burning region is a major challenge.

The idea to remember: Wings and engines both change the motion of air. Wings mainly make lift; engines mainly make thrust. Pressure, airflow direction, and energy connect the whole story.