Why do you need to know this?

The mystery of the how aircraft flies lies in the art of positioning the aircraft (and by extension the wing) in such a way that the flow of air over the wings generates lift. I am not joking – positioning your aircraft is the only thing needed, along with the thrust from the engines. To understand what position is best in a particular situation to produce enough lift force, you need to understand how a wing generates lift, and the effect that angle of attack has on lift.

I’ve read tons of articles online, watched many videos and have read a few books, but most fail to make someone understand what lift is and how the angle of attack actually affects it, both in a practical sense as well as a theoretical one. Even a few student pilots don’t have a good understanding of this. While they can fly based on their instinct, we unfortunately don’t have that luxury, and so we need to have a good grip on this conceptually.

The role of the airfoil

Most textbooks have an incorrect explanation for this, and chances are you have also read it. It says that due to a wing’s curved upper surface and a flat lower surface, the wind has to travel faster above the wing than the wind below to reach the other end at the same time.

Thus the speed of wind above the wing is more than the speed of the wind below it. So by Bernoulli’s principle, low speed means high pressure and high speed means low pressure. The air pressure above the surface is lower than the air pressure below it. So as the air moves from a region of high pressure to lower pressure, it applies a force upwards.

Even though it is repeated widely, it is incorrect; you’ll get to know why later. One of the reasons is that if that’s the case, then how do planes fly upside down? Yes, they do, watch this.  If this explanation was correct, then flying upside down would not be possible, as the air pressure below the surface would be less than the air pressure above the surface, thereby having a downward force, which is probably not something that you want to be acting on your aircraft.

This explanation is incorrect. Although the speed of the airflow on the upper surface is faster, there is no rationale for why it would want to reach the end of the wing at the same time the air on the surface reached there. There’s simply no reason for that, thereby making the premise of the explanation flawed. And although Bernoulli’s principle does has some part to play in how a wing generates lift, in the words of the internet, this ain’t it chief.

So, here’s the correct explanation: air moving above the wing has a natural tendency to move straight, but due to the curvature of the upper surface of the wing, which is inclined downwards, it stretches out the air into a larger volume, reducing the air pressure. Also, the inclined surface of the lower surface of the wing pushes the air into a smaller space, thereby increasing pressure. So an upward force is applied on the wing due to this pressure difference.  

There’s a problem, however. If you’ve ever wanted to become a pilot you might recognize that this does not explain why aircraft travelling at slower speeds have to hold a higher attitude. Another mechanism is at play here (and one which is actually partly just a different explanation of the above phenomenon) is the net turning of air caused by the wing. Let’s explore that next.

Pushing the air downwards

To take off, the pilot has to speed up the airplane. Once, it is moving at considerable speed (ever wonder why this is the case?), he pitches the aircraft up, pointing its nose upwards. Now, the wing of the aircraft makes an angle relative to its direction of motion. This means that as air flows over the wings, it experiences a net change in velocity relative to the aircraft, which is to say it gets pushed (and pulled) downwards.

Newton’s Third Law says that every action has an equal and opposite reaction, so a reaction force will act on the wing in the opposite direction, ie upwards. This is what the lift force is – the reaction force due to the change in speed and direction of the oncoming air caused by the wing. Note that it is very important to understand here that both the upper and lower surface of the wing contribute to changing the direction of the oncoming air. Even when the wing is perfectly level or pointed towards the ground, it is still generating lift because the force comes not from air molecules hitting the bottom of the wing but rather from the net change in velocity they undergo.

Now, what role is the angle of attack playing here? The angle of attack is the angle made by the wing with the relative wind.

You might be wondering what relative wind is. It isn’t that complicated; in fact, you’ve also experienced it. Remember that hot sunny day, when you were running from {Placeholder for a nice joke}. And suddenly, you experience a cool breeze flowing past you. Although there was no wind blowing, your motion relative to the air makes you feel a wind relative to you. This is called relative wind. The air is stationary with respect to the ground, but moving in the opposite direction you are running relative to you. It is just a matter of relative motion.

The angle of attack dictates how much air passing by the wings – both above and below it – is being turned. Even though the mass and velocity of the oncoming air is the same, a greater angle means that the change in velocity (remember we’re talking in terms of a vector quantity, so both direction and magnitude) it will experience will be greater. Since the change in velocity imparted to the air is greater, the corresponding reaction force exerted by it on the wing is more.

Based on this information, most people would say that greater the angle of attack, greater the downward push and thus more lift! Unfortunately, they would be wrong. You see, the world of fluid dynamics is very counterintuitive. Increasing the angle of attack at first does have the intended effect of increasing lift, but only up to a point. At about 15-20 degrees of angle of attack (depending on the aircraft and external conditions), the wings will actually start losing lift rapidly. This phenomenon is known as a stall. Let’s explore why it happens.

Why do we stall at a very high angle of attack?

As you know, the shape of the wing is such that it causes a net turning of the relative wind. As the angle of attack increases, the air flowing over the top of the wing is being deflected downward at a greater and greater angle, but is nevertheless still sticking to the surface of the wing. However, there comes a critical point at which if the angle of attack is increased further, the airflow over the top of the wing separates from its surface. Now, the air above the upper surface of the wing is a turbulent, disorderly stream that is not being directed in any particular direction, and so is not generating lift. The air hitting the bottom of the wing simply does not contribute enough lift to keep the aircraft airborne, and at such high angles of attack the amount of drag caused by it is also huge. So, this causes the aircraft to lose lift and fall.

Another way you could look at this would be to realize that this flow separation essentially changes the entire shape of the wing in terms of how it behaves. If you look at the flow lines in the above diagrams, in the case where the wing is stalled, it is essentially a giant blocky triangle, which is not something you want to have as your wing.

Stalls usually cause most aircraft to drop like a rock, with the only way to safely recover being to pitch down, increasing airspeed and re-establishing stable airflow over the wing. This is where knowing the mechanics of what exactly is happening is important – an inexperienced pilot would try to increase the angle of attack in an attempt to increase lift, further worsening the situation.

Variation of Angle of Attack with Speed

Now that you know the emergence of lift force, we shall move on to understand the relation of the angle of attack with the speed of your aircraft.

This is purely common sense. As you speed up your plane, the amount of air being washed down per minute increases. So as the amount of air being washed down increases, the reaction of that downward push also increases. So the angle at which you need to deflect the air decreases, and by extension the angle of attack you need to maintain the same amount of lift also decreases. This is why aircraft are basically level at cruise altitudes and speeds.

When your plane is at a slower speed, the amount of air being washed down per minute decreases. This calls in for a need for a nose-high position, i.e. high angle of attack. This is why aircraft need to hold a relatively high angle of attack when coming in to land.

You can use the terms ‘high angle of attack’ and ‘low speed’ and ‘small angle of attack’ and ‘high speed’ interchangeably. That’s what most pilots do.

Assignments on lift and angle of attack

  1. What are flaps and slats? What is their role?
  2. What is the ground effect? How does it affect aircraft design?
  3. What is the effect of angle of attack on the drag produced by the wing? What causes this? Explain in terms of both the pressure differential and net turning of airflow mechanisms.
  4. Lighter aircraft can go to much more extreme attitudes (even vertical in the case of military jets), but large commercial aircraft try to stay mostly level. Why?

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Further reading