Understanding Lift and Bernoulli’s Principle in 5 Minutes

Lift: It’s the foundation of aviation, yet it’s not really given a lot of time in the spotlight. Well, that’s what we’re going to change here! 💡

We’ve heard many different types of explanations for lift over the years, with some of them being close to complete nonsense.

Yeah, it’s great to keep things simple, but let’s keep things rooted in reality as well. So today, we’re going to talk about how Bernoulli’s Principle works, how that explains lift, and what other factors influence all of this! 🔭

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What is Bernoulli’s Principle?

Bernoulli’s Principle applied to aerodynamics tells us that:

Let’s unpack this. The first thing to understand is that this principle assumes that air is an “ideal fluid”. This simply means that:

• It’s not compressible (it’s volume can’t be increased or reduced by changes in pressure)
• It has no viscosity (it doesn’t stick, or create friction with itself or any surface)

Keep in mind that this is technically not correct, but is generally assumed in most literature for subsonic airspeeds (less than Mach 0.4 in general), as the effects of this are very small.

Now that this is out of the way, let’s look at the actual principle.

Imagine a tube that has airflow going through it:

Bernoulli’s Principle states that if you measure the sum of static and dynamic pressure throughout this tube, this value will be constant! We call this the total pressure. This can be used to create this slightly simplified equation:

If we insert the variables for the dynamic pressure, we get:

So, what does any of this really mean? Well, it simply shows the total pressure remains the same throughout the stream tube, no matter what happens to the other 2 pressures.

This is also related to the Law of Continuity, which explains that if the area of a tube reduces, airflow velocity increases.

So, when the tube becomes more narrow, the velocity (and therefore dynamic pressure) increases, and the static pressure reduces.

This little exchange keeps the total pressure constant!

To summarise, if either static or dynamic pressure increases, the other one decreases – and vice versa. Total pressure will always remain the same.

So what does any of this have to do with lift?

That’s exactly what we’ll cover next!

What is Lift?

Lift is defined as:

So, Bernoulli’s Principle links directly to this.

As we’ve covered earlier, if dynamic pressure (i.e airflow velocity) increases, the static pressure drops.

The top half of the wing’s shape creates one of the stream tubes discussed earlier.

This makes the stream tube more narrow on top of the wing, compared to below the wing.

As it becomes more narrow, velocity increases, and therefore the static pressure decreases, as we mentioned earlier.

The pressure difference between the area below and above the wing, creates a force pushing the wing up: we now have lift! ⬆️

While this concept is one of the main contributors to lift, it isn’t the whole picture, so let’s move on to what else is part of it.

What other Factors Influence Lift?

To fully understand lift, let’s have a look at the lift equation:

As you can see, there’s more to it that just reducing pressure above the wing. Let’s go over each variable, and its relationship with the outcome of lift.

Density

Rho is the air density, which dictates how much air the wing can actually ‘work with’ to create lift.

Pressure directly affects density, so does temperature.

Pressure increases ⬆️ – density increases ⬆️
Temperature increases ⬆️ – density decreases ⬇️

This is why performance goes down a lot when your flying environment gets hotter or higher (or more humid): density altitude, which we’ve covered here!

The Lift Coefficient

Then, there’s the lift coefficient (CL)

CL basically represents the ability and effectiveness of the wing to produce lift.

It’s mainly influenced by the shape of the wing, the angle of attack, and something called a Reynolds Number. A Reynolds number is determined by the air velocity, viscocity, (we’ll cover this one in another article).

In general, the higher the angle of attack, the higher the CL value, up to the point of stall where CL reduces massively.

This is because of a few things, but mainly:

• Air velocity on top of the wing increases, further reducing pressure as per Bernoulli, increasing CL
• As the angle of attack increases, so does the amount of deflected air. Newton’s 3rd law comes in here: For every action there is an equal and opposite reaction! This also increases CL.

So, that’s a summary on CL. Still with us? Let’s move on to the next variable!

True Airspeed (TAS)

It’s not just that airspeed increases lift, it increases lift by the square of airspeed.

This means that if you double your airspeed (or blade speed for rotors), it quadruples the amount of lift!

Wing or Surface Area

This one is fairly straight forward: bigger wing means more lift!

The relationship is proportional, so doubling the wing area means double the amount of lift.

Both the effect of Newton’s 3rd law (deflecting air down), and Bernoulli (reducing pressure above the wing), can act across a larger area, which means an increase in lift.

Conclusion

Lift is one of those basic things that doesn’t get enough attention throughout a pilot’s career.

Understanding Bernoulli’s Principle is crucial for anyone that wants to really understand lift. At the same time though, it’s important to recognise that this principle is part of a broader set of factors influencing lift, including air density, lift coefficient, airspeed, Newton’s 3rd law, and wing surface area.

We don’t publish all our Notes from the Cockpit (like this one) publicly, some are shared only by email. Get the next one sent straight to your inbox ⤵️