What is something that most schools teach incorrectly, and that most professionals in the field don’t fully understand?
I’m sure you’ve been taught what makes airplanes fly, but I’m willing to bet that you were given an incorrect explanation, or you don’t fully understand it. Why? Even academics in the field of aerodynamics don’t agree on an explanation for one of the great marvels of the 20th century – what makes airplanes fly – or more specifically, how does lift work?
Most of us are so accustomed to seeing an aircraft fly that we take for granted the modern miracle that it is. Consider the Boeing 747-8 – its maximum takeoff weight is 433 tons, the fact that something so heavy can stay airborne is an incredible feat of engineering. But how is it possible? Well, there’s the simple answer, and there’s the complicated answer (which is still incomplete!). The fact is, that when you get down to the details – aerodynamic lift is something that no one fully understands or agrees on – yet.
The balance of forces (i.e. the simple answer)
Most of us are taught in middle school that forces are what make airplanes fly. This explanation is true, but it’s just not the complete answer. So here’s how the explanation goes – there are four fundamental forces in flight: lift, drag, weight and thrust, and the balance of these forces make airplanes fly.
An aircraft, like a car or train, is built in sections or components, and each component has a specific function. For example, the cockpit or flight deck houses the controls and is where the pilots fly the aircraft from, the fuselage often contains the payload such as passengers and cargo, the engines or propellers provide the forward thrust to propel the aircraft forward, and the wings of the aircraft produce the necessary lift to support everything in flight, and the tail provides the stabilizing force to keep the plane level.
So what are the different forces?
Weight is the downward force on an aircraft caused by gravity acting on its mass. It always acts vertically downward toward the center of the earth.
Lift is the force created by the wings to support the weight of the plane and overcome the effect of gravity. The design of the wings and the flying characteristics of the airplane cause the wings to produce lift.
Drag is the backward aerodynamic force caused by friction from moving forward through the air, as well as the penalty for providing lift.
Thrust is the forward force generated by the aircraft’s engines; there are many types of engines, but typically smaller aircraft use propellers powered by piston engines or turbine engines, whereas larger airliners or high-performance military airplanes use jet engines.
When an aircraft is taking off, the thrust is greater than the drag, which is why it accelerates, and the lift is greater than the weight, which is why it gains altitude. When the aircraft is cruising through the sky, the thrust and the drag are equal, which is why the aircraft maintains a constant airspeed. The lift and the weight are equal, which is why it stays at the same altitude. Simple right?
Not quite. The main problem is that we haven’t explained why the wings create lift, which takes us to the complicated answer.
Air pressure and momentum (i.e. the complicated answer)
Would it be frustrating to hear that there are several explanations for what makes airplanes fly, or more specifically, how do they generate lift? Even better, the most widely accepted explanations are still not complete, and proponents of each often disagree as to the true explanation of how lift is generated by wings.
Probably the biggest problem is that we still do not have a complete understanding of the physics behind fluid mechanics, of which air flow and aerodynamics are just a subtopic. Think you can solve it? The Clay Mathematics Institute offered $1 million to anyone who can make “substantial progress” towards a mathematical explanation of the higher level equations behind fluid mechanics, known as the Navier-Stokes equations.
Conservation of momentum
The first widely accepted explanation for how lift is generated, is that it is the result of the redirection of air downward by a wing. It’s also the theory of lift supported by NASA. As the wing approaches the oncoming flow of air, the shape and the orientation of the wing drive the air downward.
Since the air has been pushed downwards (known as downwash), there is an opposing force upwards on the wing, generating lift.
When we examine the airflow around a there’s a few a wing, we can see the airflow split as it meets the leading edge of the wing. At this point the air flow above the leading edge narrows, and the flow below expands. This is not due to the aerofoil shape, but due to the nature of the airflow around the wing, in fact, the result is similar for a flat wing, or a flat plate. The narrower airflow above the wing has a higher velocity than below the wing, resulting in a lower pressure above the wing than below, the pressure differential then creates lift.
The right answer
Both the momentum and air pressure answers adequately explain the process of lift generation by a wing, the explanations are complementary and sufficient on their own. The problem is that for both explanations, there are still gaps and further questions. The further down the rabbit hole we go with either explanation, the more complicated and academic they become – and the more disagreements occur. However, in answer to what makes airplanes fly – we can still correctly answer that it is as a result of air pressure acting on the wings and conservation of momentum.
Did you learn something new today? Or do you have another answer for what makes airplanes fly? We’d love to hear from you in the comments below if you agree or if you’ve got a different view.