What are the four courses of flight?

Four forces affect an airplane while it is flying: weight, thrust, drag and lift.

You’ve probably seen an airplane flying at some point. But have you ever wondered how an aircraft flies? The answer is easy – with a little physics! Flight is all about forces and movement, which can be explained using physics.

Let’s start with the forces. There are four forces that act on things that fly. These are weight, lift, thrust, and drag. Each of these plays a key role in keeping an aircraft in the air and moving forward.

The first of the four forces exerted on aircraft is weight. The weight of an object is the force on the object due to gravity. Certain objects in space, including planets like the Earth, exert a force that attracts objects toward itself. In the case of the Earth, “toward itself” means “down toward the ground.” The force exerted on a body due to gravity can be expressed using the equation:

F = mg

Where F is the force in (N), m is the mass of the object in kg and g is the acceleration due to gravity. When doing this calculation, it is best to use the unit for gravity in N/kg:

g = 9.81 N/kg

In terms of the four forces acting upon an aircraft, weight is measured as the F in the above equation. However, we usually use the symbol W when specifically talking about weight. Substituting W for F above we get:

W = mg

From this equation, we can see that when we talk about ‘weight’ we are actually talking about how much force is acting on a mass due to gravity. This force, as mentioned above, also has a direction. We could call it “down”.

When we stand on the ground, we push down on the Earth and the Earth pushes up against our feet with the same amount of force in the opposite direction. This is an example of Newton’s Third Law). Newton’s Third Law states that for every action, there is an equal and opposite reaction. We usually abbreviate this upward force of the Earth as Fn.

Forces acting on people sitting and standing on the ground 

 

Any type of flying machine experiences weight. This weight is always in the direction of the Earth, no matter which way the aircraft is travelling. It is very important to know the weight of an aircraft before flight. Too much weight can cause an aircraft to fly poorly. Heavy aircraft may need higher takeoff speeds and longer runways. They also may not be able to fly as far or as high.

From left to right: Weight directional arrow on a hot air balloon, United Airlines 777 airliner and CF-18 Hornet jet fighter 

 

If an aircraft is being pulled down toward the Earth by gravity and its own mass, how does it stay in the air? The answer is the second force, lift. Lift refers to the force that an object needs to overcome its weight.

Lift is an upward force caused by air moving over a wing. A wing or blade, such as that of a propeller, rotor, or turbine as seen in cross-section has a special shape called an airfoil.

As a wing moves through air, the air splits and flows both over and under the wing. The difference in the movement of the air on top of the wing and below the wing generates lift. There are two explanations for the causes of lift: deflection and differences in pressure.

Diagram showing the movement of air above and below an airfoil and the forces it generates – Text Version

 

Deflection

As air passes along a wing, some of the air is directed downward. This is called deflection. Once again Newton’s Third Law is in action. Here, the ACTION is air pushing downward under the wing, and the REACTION is the wing moving upwards.

When the leading edge of the wing points upward, such as when the aircraft is climbing, it creates a positive angle of attack. Angle of attack is the angle between the and the direction of motion. Since air is being deflected downward by the wing, there is lift.

The opposite is also true. When the wings point downward (a negative angle of attack), there is less lift and the aircraft goes down.

When the leading edge points upwards, there is a positive angle of attack 
 – Text Version

 

Pressure Differences

Lift can also result from differences in pressure. These differences occur above and below the wing as air moves past the wing.

Air pressure is measured by dividing the force of the air molecules by the area that the air molecules are in. When air moves over a wing, the layer of air is squeezed into a smaller area. As a result, the speed of the air increases and the pressure of the air decreases. The opposite occurs below the wing. The air is squeezed less, resulting in slower moving air that has higher pressure.

Air flow and pressure above and below an airfoil  – Text Version

 

Lift can be explained using Bernoulli’s Principle. It states that “as the speed of a movin increases, the pressure within the fluid decreases.” Since the force pushing up from the high pressure air is greater than the force pushing down from the low pressure air, there is lift in an upward direction.

Earl flew in hot air balloons. These lighter than air (LTA) vehicles could easily go up and down, but once in the air, they were at the mercy of the wind. A pilot had no way to steer the balloon.

Not long after they were invented, people began to think of ways to make balloons go in the direction they wanted. To accomplish this, they needed a way to push the balloon forward. This pushing is known as thrust. Like lift, thrust is another type of reaction force that can be explained using Newton’s Third Law.

Propellers

In 1784, Jean-Pierre Blanchard attached a hand-powered propeller to a balloon, which is the first recorded use of propulsion by a hot air balloon. People tried many other forms of propulsion in the 1700s and early 1800s, and it wasn’t until 1852 that Henri Giffard created an airship which used an engine to turn a propeller.

September 19, 1900 – Alberto Santos-Dumont demonstrating his No. 4 airship in Paris, France (2015) by Guy Jones (1:13 min.)

Propellers are rotating blades which may be found at the front or back of an aircraft. If they are on the front, they are called tractors. If they are at the back, they are called pushers.

A propeller consists of two or more blades connected together by a hub.

Propellers were initially made of wood but nowadays many are made of metal.

Early propellers were turned by hand, pedalled by foot, or powered by steam engines. Today, propellers are powered by either internal combustion engines or jet engines (see below).

Parts of a propeller (Let’s Talk Science using an image of a P-51D Mustang from – Text Version

 

Each of the blades of a propeller is shaped like an airfoil. When they turn, they act as spinning wings. As the propeller turns, it pulls slow air towards itself and pushes fast air out behind itself. This generates a force directly behind the propeller – the action – that pushes the aircraft forward – the reaction.

Action, reaction and air speeds caused by a moving propeller 
– Text Version

Rotors 

Rather than propellers, helicopters use a set of rotary wings called the rotor. A rotor is made up of two or more rotor blades. Helicopters typically have two rotors. These are the main rotor, which is located at the top of the aircraft and the tail rotor, which is located at the back of the aircraft.

Location of rotor blades, main rotor and tail rotor on a helicopter 
 – Text Version

Unlike a propeller, a rotor produces both lift and thrust. In order to fly in a particular direction, the pilot changes the of the rotor blades. This makes the rotor tip in a given direction. The helicopter will then move in that direction.

Rotors allow helicopters to take off and land vertically, as well as hover. This makes them useful for search and rescue, firefighting, and medical transport.

Arrows showing direction of action and reaction due to moving rotor  from 

Jet Engines

Many modern aircraft have replaced engines turning propellers with jet engines. These engines create thrust by:

  1. pulling air into the engine,
  2. mixing the air with fuel,
  3. igniting the fuel/air mixture, and
  4. pushing the hot air out of the back of the engine at high speed.

As with the propeller, the jet engine pushes out air at a higher speed than the air entering the engine. This causes the aircraft to move forwards.

How thrust is produced by a turbojet engine 

Drag

The fourth and final force of flight is called drag. Another term for drag is air resistance. Like other fluids, air can resist, or try to stop the movement of an object through it. This is similar to how water behaves when you try to walk or swim through it. The same is true for aircraft. Air resists the movement of aircraft through it. This resistance counteracts thrust and slows down forward motion.

There are two main types of drag: parasite drag and lift-induced drag.

Parasite Drag

Form drag is drag that is caused by the shape of an object travelling through a fluid. Some shapes, such as the airfoil shape, move fairly smoothly through air. The air moves neatly above and below the shape without creating a lot of behind it. However, other shapes do not move smoothly through air. Shapes like the sphere and the flat plate create a lot of turbulence behind them. This turbulence slows down their movement.

Air movement around objects of different shapes 
– Text Version

 

Early aircraft such as the Curtiss 1911 Model D had a lot of form drag, especially from vertical parts such as wing struts. Over time, advances in aerodynamics and materials has led to much more streamlined designs, such as the SR-71 Blackbird.

Left: Curtiss 1911 Model D. Right: SR-7

Surface friction drag occurs whenever an object moves through a fluid. The roughness of the surface affects how much the fluid slows down the movement of the object. This is because the rough spots cause turbulence. Surface drag is actually a form of To reduce surface friction drag, aircraft are designed to be as smooth as possible.

This P51 Mustang has little surface friction drag 

Lift-Induced Drag

The other main type of drag is lift-induced drag. This type of drag is a result of lift. The greater the lift, the greater the lift-induced drag.

When a wing is roughly parallel to the airflow the air tends to flow smoothly past the wing. As you increase the angle of attack, though, you begin to get more unstable air behind the wing. This is due to the shape of the wing becoming more like the flat plate mentioned under form drag.

The top airfoil has a small amount of lift-induced drag and the bottom airfoil has a large amount of lift-induced drag 

There comes a point where the angle of attack becomes so great that the wing is no longer able to generate lift. This is known as the critical angle of attack. At this point, the aircraft stalls. Many modern aircraft have warning systems that alert the pilot if the aircraft is about to stall.

So to summarize, there are four forces that keep an aircraft in the air and moving forward: weight, lift, thrust, and drag. But if you think about it, that means an aircraft is falling, rising, moving forward, and being pulled back – all at the same time! Scientific innovations over the centuries have allowed us to keep these four forces in balance so that we can fly aircraft from one place to another.