What Makes an Aircraft Fly?

Have you ever wondered how an enormous machine can effortlessly travel through the air? How does an aircraft fly? Despite appearances, they are not magical objects that levitate. For the laws of physics to hold true, there must be a force that supports the weight of the aircraft. This is the aerodynamic force generated by the wings (Fig.1).

How does an aircraft fly?
Fig.1: Weight and aerodynamic force acting on an aircraft. 

How is aerodynamic force generated?

This lifting force is made possible by the wing’s airfoil shape, and that is precisely what makes an aircraft fly. There are several theories about how airfoils generate lift, and even among very experienced aerodynamicists there are different schools of thought. The most commonly accepted explanation is that force is created through Bernoulli’s principles.

The pressure at the tip of the aerofoil is static. As we move over the chambered upper surface, there is a region of lower pressure over the wing (see Figure 2) due to the geometry of the airfoil. Assuming the bottom is flat, the pressure on the bottom side will be close to ambient pressure. So, with high pressure on the bottom and low pressure on top, the result is lift, or a force pushing upward on the airfoil.

This makes aircraft fly
Fig.2:  Pressure profile around airfoil.

How does an aircraft stay level?

To stay steady in the air an aircraft, like any other object, requires balanced forces. The wings generate lift, but the wings alone are not enough to maintain a balanced center of gravity. This is where the tails come in handy. The tails provide additional forces (though lift) to balance the whole aircraft (see figure 3). The book Flight Dynamics by Cook (2013) provides a very good overview of flight dynamics.

Mechanics of flight

An airplane can steer along 3 axes. Motion along each axis is known as pitching, yawing and rolling motion (Fig.3, 4 & 5 respectively). An aircraft is essentially equipped with control surfaces which allow it to rotate along each of these axes. Let’s look at these control surfaces in more detail:

Elevator: As seen in Fig.3, the elevator is located on the tail. When lift produced by the elevator is adjusted, the pitching moment of the aircraft changes. This is how the pitch is controlled. The elevator is used to maintain or change the altitude of the aircraft.

Aircraft 3 - Lift
Fig.3: Elevator actuated for pitching motion.

Rudder: The rudder is important for the aircraft to maintain lateral stability. It allows the pilot to fight side winds that the airplane might encounter. The rudder IS NOT used to turn an aircraft (although many people mistakenly think so based on the way it looks).

Aircraft 4 - Side Force
Fig.4: Rudder actuated for yawing motion.

Aileron: The ailerons (Fig.4) are located on the wings. They are used to roll the aircraft by creating a differential in lift between the starboard and port wings. A steady roll is what turns the craft. This is why, when taking off or landing, you can often see the ground from your window. The aircraft is rolling, hence it is turning!

Aircraft 5 - Aileron rolling
Fig.5: Aileron actuated for rolling motion.




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