The Aerodynamics of Autorotation Without the Headache

Autorotation: It’s the final safety net for any helicopter pilot. It can make the difference between a big fireball and a landing you walk away from without a scratch. 🔥

Unfortunately, the aerodynamic side remains a tricky topic for pilots. What doesn’t help is the hundreds of textbooks that all reference fancy aerodynamic papers and impressive jargon, that give you a headache after page 2.

Today, we’re going to do the opposite, but without dumbing it down to the point where nothing makes sense anymore.

Let’s look at what exactly is going on with the aerodynamics during autorotation! 💨

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What is Autorotation?

Autorotation is a flight condition that allows helicopters to keep their rotor disc turning at a constant RPM (revolutions per minute) without engine power. We can use this to safely land any helicopter after a critical power failure!

Boom 💥 Engine failure, so now what?

🔸 First, we lower the collective to reduce the pitch angle of all blades simultaneously. This massively reduces the drag that acts on each blade, and creates a large rate of descent.

🔸 Second, this rate of descent creates an airflow that is now going through the disc from below. This flow creates the force required to keep the rotors at a constant RPM.

🔸 Third, we now have three regions across the disc that need to be carefully managed by the flight crew:

1. The stalled region
2. The driving region
3. The driven region

We’ll dive further into these in the next section.

🔸 Fourth, as the ground gets closer we convert our rate of descent into a decelerative force that slows us down, and allows for a (hopefully) smooth landing! We have now landed without an engine turning the blades.

But what exactly is going on to make this a possibility? Well, remember our breakdown of a normal vector diagram? ⤵️

Helicopter Rotor Blade Aerodynamics: The Basics Step by Step

Well, now we have a vector diagram where some things have to change to keep the rotor blades turning without a working engine!

So what do the rotor regions we mentioned above do exactly, and why are they so important? Let’s take a look ⤵️

The Three Aerodynamic Rotor Regions

As we said earlier, we have three rotor regions to carefully manage if we want to keep our rotor RPM constant. The stalled, driving, and driven regions:

Let’s go over what each of these do exactly, and use vector diagrams again to show why they’re different. The main variable that makes these regions so different is the amount of lift vs the amount of drag they produce.

The Stalled Region

This is the region nearest to the root. As the name suggests, this region of the disc is completely stalled and is therefore pretty useless.

So why is only this part of the disc stalled? 💡

At any given rotor RPM, the airspeed at the root is much lower than at the tip. This is because the leading edge near the root travels much shorter distances in one rotation compared with the tip.

When we add the rate of descent airflow to this, we get a relative airflow that is coming from well below the disc, causing an angle of attack that is way too big. Like this ⤵️

This results in this section of the disc generating more drag than lift. Note how the total reaction points towards the trailing edge (to the left).

This creates an overall “winning” drag force that acts in the opposite direction of where we want the blade to go to keep our RPM constant!

If we increase the pitch angle of the disc by raising collective, we increase the size of this region, and therefore we reduce the rotor RPM.

You with us so far? ✅

Let’s move on to the next region.

The Driving Region

The driving region results from exactly the right combination of blade velocity and rate of descent airflow. The result is an optimal lift to drag ratio.

You can see in the vector diagram below that the total reaction points towards the leading edge. This means it’s providing us with the force we need to keep things going: it is driving the rotor RPM.

Note the orange vector called “Driving force” in the image below, it’s also called autorotative force.

The main take-away here is that this force is what replaces the engine(s) during autorotation. It’s a component of the total reaction acting towards the leading edge of the blade.

Without this force, autorotation is not possible!

Without this region, autorotation is not possible. But we have one more region left…

The Driven Region

The driven region is located at the area near the outer edge of the rotor disc. What is the main significance here? Well, we’re at the far end of the blade here, so two things:

🔸 We have a high induced flow because of tip vortices.

🔸 We have a large amount of drag because of the high airspeed.

These two combined result in a huge amount of total drag here. The driven and stalled regions both create a total reaction that is pointing towards the trailing edge, like this:

This means that they both slow down the disc and reduce rotor RPM if we don’t manage them properly.

Summary of the 3 Rotor Regions

So to summarise, let’s have a look at this picture below. As you can see, we have five points along the blade:

1️⃣ Is the stalled region, with a force acting towards the trailing edge.

2️⃣ Is the point where the stalled region transitions into the driving regions because the airspeed has increased enough.

3️⃣ Is the driving region.

4️⃣ Is the point where the driving region transitions into the driven region, because the airspeed is increasing too much, combined with the increased induced flow.

5️⃣ Is the driven region.

Now, 2️⃣ and 4️⃣ are not stationary. They constantly move depending on our collective setting, airspeed, and many other variables.

As long as area 3️⃣ is able to compensate for 1️⃣ and 5️⃣, we will have a constant rotor RPM!

How Does Airspeed Influence Autorotation?

All of the stuff we’ve talked about so far is based on one pretty big assumption: the disc is in still air.

In reality of course, this is almost never the case. Whether we have a headwind component or have put on speed during the autorotation, there is usually at least some amount of airflow going through the disc “horizontally”.

So how does the amount of airspeed influence the dynamics we’ve discussed so far?

There are three main factors here. We’ll go over each one separately, and then show the combined effect of all three together afterwards.

Factor 1

The airflow that comes from below during an autorotation can’t go through the disc uninterrupted. There’s a rotor disc in the way after all!

What this means is that there’s a ‘buildup’ of air trying to get through from below. This buildup further reduces the amount of air that can get through.

If we increase speed, this buildup reduces, and will increase the flow going through the disc, which increases the inflow angle. ⬆️ inflow angle = ⬆️ angle of attack = ⬆️ rotor thrust. This decreases the rate of descent!

Notice that in autorotation, because the airflow comes from below the disc, an increase in inflow angle causes an increase in angle of attack. In powered flight (as shown in our previous article), an increase in inflow angle has the opposite effect on angle of attack because the airflow comes from above the disc.

Factor 2

Pushing the cyclic forward results in the disc tilting forward. This forward tilt causes a reduction in overall angle of attack across the disc.

This reduction in angle of attack reduces rotor thrust, and therefore increases the rate of descent.

Factor 3

Forward flight creates an increase in horizontal flow across the disc. This component gets added to the relative airflow hitting the disc, and as you can see in the image below, reduces the angle of attack and therefore rotor thrust:

Putting These Factors Together

What we end up with when we increase forward speed during autorotation is a disc with three regions that have moved towards the retreating side of the disc. So for anti-clockwise rotating blades, it looks like this:

These three factors all play a part in how the helicopter behaves when changing airspeed while in autorotation. Keep in mind: factor 1 increases rotor thrust, factors 2 and 3 reduce it.

In general though, factor 1 tends to win over factor 2 and 3 at low speeds. Factor 1 = factor 2 + 3 around the speed that gives our minimum rate of descent. And then finally, at higher speeds, factors 2 and 3 combined are higher than factor 1.

That probably reads a bit difficult, so to visualise this, here is the graph that shows this relationship (which is a similar looking graph to another you’re probably familiar with: the power curve):

Conclusion

Autorotation remains a tricky topic for many pilots. We’ve covered the three regions during autorotation, and where they get their names from.

Increasing speed during an autorotation has three main effects, that are all working on the disc simultaneously. If you put them all together, you get a relationship that looks very similar to the power curve that you’re probably already familiar with.

In the future, we’ll cover the factors that influence Rotor RPM during autorotation. For any questions, please feel free to reach out!

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