Helicopter Rotor Blade Aerodynamics: The Basics Step by Step

Black magic, wishful thinking, and beating the air into submission: all terms that have been used at times to describe helicopter rotor blade aerodynamics.

But it really doesn’t have to be that mysterious 🔮

We’ll go through the basics of all the aerodynamic forces acting on a helicopter blade, step by step, without all the fluff that usually comes with it.

What exactly is the relationship between the different forces and angles, and where does rotor thrust come from exactly?

Let’s answer it all in just one vector diagram💡

Ready? Let’s start from the beginning ⤵️

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What is a Vector Diagram?

A vector diagram gives a sideview perspective of a helicopter blade, which we can use to look at all the forces on a rotor blade in one overview. It’s nothing fancy, just a handy tool to keep everything as simple as possible.

We use vectors to show a force, which look like arrows. A vector has two qualities:

↗️ Direction (where the arrow points to)
💥 Magnitude (the length of the arrow)

We’ll go from the very first step (the rotational flow), all the way to the final result: rotor thrust.

All the diagrams we discuss here are based on a hover in still air.

We’ll build up the entire aerodynamic vector diagram step by step, so it’s hopefully easy to follow! ✅

The Rotational Flow

It all starts with rotational flow. Our purple blade rotates, creating rotational flow (also called Vr), which hits the leading edge of each individual blade:

A higher rotor RPM ⬆️ means a higher Vr ⬆️
A lower rotor RPM ⬇️ means a lower Vr ⬇️

We know that with a constant angular velocity (i.e rotor RPM), Vr would be much greater at the tip compared to the root though:

The Pitch Angle

Each blade has a chord, which is:

Which looks like this:

The difference between the chord and the rotational flow is called the pitch angle (or blade angle):

Keep in mind that the pitch angle does not actually influence the direction of the rotational flow (Vr). Vr is always parallel to the rotor disc’s plane of rotation! 💨

The Induced Flow

As the blades turn, they force down air. This is called Induced flow:

It acts perpendicular to the rotational flow:

The Relative Airflow

We can add up the rotational flow and induced flow, to give us the relative airflow:

The size and angle of the relative airflow depends on the magnitude of the induced flow and rotational flow.

Let’s look at an example. If we keep the rotational velocity the same, but reduce induced flow massively (like when you gain translational lift), look at what happens to the relative airflow:

The relative airflow comes in at a much ‘lower’ angle. Why is this relevant? Well, this impacts the angle of attack ⤵️

The Angle of Attack

The angle of attack is defined as:

In addition to this, we also have the inflow angle, which is the difference between the relative airflow and the rotational velocity. So we end up with two different angles here:

As you can see:

If one increases, the other must decrease ⬇️ And vice versa.

If we reduce the induced flow while rotor RPM remains constant (like when gaining translational lift), we massively increase our angle of attack, like this:

Remember the CL-alpha graph that we covered in our lift article?

Higher angle of attack = more lift. This is also what causes dissymmetry of lift in helicopters. Talking about lift…

The Lift and Drag

So where does the lift point towards here? Most people’s initial thought would be “well UP of course”, but weirdly this isn’t quite right.

The lift vector always acts perpendicular to the relative airflow, like this:

Unfortunately, we can’t have lift without drag due to air resistance and many other variables we’ll cover in the future.

Drag acts in the same direction as the relative airflow, also perpendicular to lift:

The Total Reaction

If we combine lift and drag, we get what’s called the total reaction:

The angle between the total reaction and the lift is determined by the lift to drag ratio. The higher this ratio, the smaller the angle is (which is a good thing).

If you image the drag to be huge, you’ll see in the image above that the total reaction will tilt backwards, increasing the angle between the lift and total reaction.

The Rotor Thrust and Rotor Drag

And then finally, the force we all came for: Rotor Thrust!

Rotor thrust is simply the component of the total reaction that is perpendicular with the rotational flow (and therefore the plane of rotation of the blades). “But isn’t that lift?” Not quite! Remember, lift is perpendicular to the relative airflow.

For the rotor drag (you’ve probably guessed it already), it’s the horizontal component of the total reaction:

And there we have it, all the forces and airflows acting on a blade.

Conclusion

We will elaborate on all of these forces and flows acting on a blade in future articles, but we hope that this has either refreshed your memory, or made things a little easier.

Please feel free to reach out if you have questions, feedback, or topic suggestions. Our contact details are in the About section.

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