In order for an airplane to remain in level, steady flight, what must happen to the four forces?

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Four forces acting on an aircraft (Let’s Talk Science)

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.

Weight

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”.

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 (Sources: hot air balloon - Keith Pomakis [CC BY-SA] via Wikimedia Commons, airliner and jet from Scott Taylor. Used with permission). Image - Text Version

Shown, from left to right, are photographs of a hot air balloon, a large commercial airliner and a fighter jet. Directional arrows and text are overlaid on the photo. The green hot air balloon appears to be floating in a clear blue sky. Below the balloon is a red arrow pointing downward beside the word "weight", indicating that weight is a downward force, and the direction of the force is perpendicular to the ground. The large twin-engined airliner, appears to be flying in a level position and travelling from left to right. Below the centre of the aircraft is a red arrow pointing downward beside the word "weight".

The fighter jet appears to be flying almost straight up into the sky. Below the rear part of the aircraft is once again a red arrow pointing downward with the word "weight" beside it.

Lift

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.

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.

Pressure Differences

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

Lift can be explained using Bernoulli’s Principle. It states that “as the speed of a moving 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.

Thrust

Early 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

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.

Rotors

Jet Engines

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 (Let’s Talk Science using an image by MikeRun [CC BY-SA 4.0] via Wikimedia Commons). Image - Text Version

Shown are four drawings used to describe how air flows around objects of different shapes. The air flow is represented by thin blue lines with small arrows that are point to the right. In each diagram, the lines initially come towards the shape in parallel. The top left drawing shows how the air would move around a thin rectangular object in a vertical position. The lines indicate that some air would be blocked by the object, but that most air move over and below the object. Directly behind the object are a number of curved lines. These indicate turbulent air, which occurs directly behind the object. The top right drawing shows how the air would move around a circular object. The lines indicate that some air would be blocked by the object, but that most air move over and below the object. Directly behind the object are a few curved lines indicating turbulent air. There are fewer turbulence lines than in the drawing of the rectangular object. The bottom left drawing shows how the air would move around an airfoil. The lines indicate that almost all air would move over and below the object. As the airfoil is parallel to the direction of airflow, there are no turbulence lines.

The bottom right drawing shows how the air would move around a thin rectangular object in a horizontal position. The lines indicate that almost all of the air would move over and below the object. The airflow lines are almost undisturbed by the object and there is no turbulence.

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-71 Blackbird (Sources: Left: Scott Taylor. Used with permission. Right: Public domain image from NASA via Wikimedia Commons). Image - Text Version

Shown are two photographs of aircraft in flight. The photo on the left is of an early biplane. This early aircraft looks a lot like a box kite. It is made from wood and fabric with wires connecting the two sets of parallel wings. A pilot sits in the open near the engine.

The photo on the right is of a modern stealth aircraft. It is very flat and angular, with a long flat fuselage and a single set of sleek wings.

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.

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.