Why Are the Airbus H160’s Rotor Blades Shaped Like Boomerangs? 🪃

Have you ever looked closely at the H160’s main rotor blades?

They have a distinctive double-swept shape, thanks to a clever piece of engineering Airbus calls Blue Edge technology. It looks like this:

To understand it, we need to look at a few problems helicopter engineers have been trying to solve for decades.

We’ll be referencing two fascinating research papers from the Blue Edge development programme, exploring both noise reduction and aerodynamic performance.

Let’s get into it!

1️⃣ The problem: Rotor Blades fly through the vortices of other blades

We all know rotor blades produce lift.

But as they do, air curls around their tips, creating concentrated swirling flows called tip vortices.

Nothing unusual there. Airplanes create them too.

But here’s the problem:

➡️ Those vortices don’t immediately disappear.

They remain in the rotor’s wake, and under certain conditions, the next blade comes around and passes very close to (or through) the vortex of the blade in front of it.

Suddenly, that blade experiences a rapid change in local airflow, angle of attack and aerodynamic loading.

That’s called Blade-Vortex Interaction (BVI).

And it’s caused plenty of headaches for aerospace engineers!

BVI is responsible for much of that iconic helicopter blade slap, particularly during descent, when the rotor’s wake influences the following blades.

2️⃣ A large part of the blade can encounter that disturbance at once (not good!)

Imagine a relatively straight rotor blade approaching a tip vortex.

Depending on the geometry, the blade and vortex can end up almost parallel.

Not ideal.

Why?

Because instead of different sections encountering the disturbance at different moments, a large portion of the blade can experience it almost simultaneously.

That creates rapid changes in aerodynamic loading.

And those changes generate acoustic pressure disturbances.

When the resulting sound waves reach someone on the ground at nearly the same time, they can reinforce each other.

The result:

A really loud, sharp blade slap.

3️⃣ What if we changed how the blade meets the vortex?

This is where Blue Edge technology gets interesting.

Rather than trying to eliminate the vortex, Airbus explored another idea.

What if we changed the blade’s shape?

That’s how we got to the iconic double sweep.

The outer section sweeps forward and then backward, creating that unusual boomerang-like appearance.

This shape changes how the blade encounters a vortex.

Instead of a strong interaction occurring across most of the blade at once, it happens differently along its span.

The resulting acoustic disturbances no longer combine in the same way.

This is where acoustic phasing comes in:

When sound waves reach an observer at roughly the same time, they can reinforce each other.

Change the blade’s shape ➡️ you change that timing.

The waves become less synchronised, reducing the intensity of the resulting blade slap.

Pretty clever, right?

The engineers weren’t just trying to make a quieter rotor.

They also had to make sure these unusual blades could produce lift efficiently across the helicopter’s flight envelope.

Because reducing noise is great, but not if you need substantially more power to fly!

4️⃣ You can also change where the noise goes

Here’s something we hadn’t appreciated before reading the research.

Noise doesn’t radiate equally in every direction.

BVI can send particularly strong acoustic energy in certain directions, depending on the blade’s motion and interaction geometry.

And the backward-swept section of Blue Edge changes that too.

During the research project, the strongest BVI noise radiation shifted towards the advancing side of the rotor.

Why does that matter? 👀

Because the blade’s velocity relative to an observer affects how strongly the sound is amplified.

Changing the direction of the strongest noise can therefore reduce what reaches certain areas on the ground.

So these blades aren’t just reducing noise.

They’re also changing where it goes!

Unfortunately, the noise complainer is always right in aviation! 😅

5️⃣ And then there’s still the vortex itself

There’s another detail worth mentioning.

Look closely and you’ll notice Blue Edge doesn’t just sweep forward and backward.

It also gradually tapers towards the tip.

The researchers explain that this helps produce a smoother blade-loading distribution, reducing the strength of the tip vortex itself.

Remember, the vortex is a consequence of producing lift.

Change how that lift is distributed along the blade, and you can influence the vortex left behind.

And here’s where the aerodynamic side becomes interesting too.

The blade’s shape, twist and loading distribution also influence how much power the rotor needs to produce lift.

That’s particularly important in hover, where rotor efficiency has a massive influence on performance.

So the Blue Edge development programme wasn’t simply about making the blades quieter.

It was also about optimising how efficiently they produce lift.

And the resulting design tackles several things at once:

🔸 Changes how the blade encounters a vortex.

🔸 Changes how acoustic disturbances combine.

🔸 Changes where the strongest noise travels.

🔸 Helps reduce vortex strength.

🔸 Incorporates aerodynamic design changes aimed at improving rotor efficiency.

That’s quite a lot of engineering hidden inside one strange-looking rotor blade!

6️⃣ So why not just sweep every rotor blade?

Because there’s a catch.

Changing a rotor blade’s shape can create new problems.

Think about what a helicopter rotor has to deal with.

On the advancing side, higher relative airspeeds can introduce compressibility effects.

On the retreating side, lower relative airspeeds can require higher angles of attack to produce sufficient lift.

And in the hover?

The rotor is operating in its own induced airflow.

Three very different aerodynamic environments!

Now add enormous centrifugal forces, bending loads, vibration and aeroelasticity.

Oh, and the blades need to flex, but not too much.

Suddenly, changing their shape doesn’t sound so simple anymore.

Improve one characteristic, and you might accidentally make another worse.

The challenge is reducing noise, improving efficiency where possible, and avoiding compromises elsewhere in the flight envelope.

And that brings us to the really interesting question.

Did the engineers actually manage to achieve both?

7️⃣ But did it actually work?

Engineers fitted Blue Edge blades to a full-scale EC155 demonstrator and compared them with conventional straight blades:

And the results were pretty impressive.

During a 60 knot approach at an 8° descent angle, they measured:

🔸 Up to 5 EPNdB less noise at certain ground microphone positions.

🔸 Over 9 TPNdB reduction at another measurement point.

🔸 A greater than 60% reduction in the modelled area enclosed by one selected noise contour.

Now, what on earth are EPNdB and TPNdB? 😅

They’re both ways of measuring how noisy an aircraft sounds to people on the ground.

➡️ EPNdB (Effective Perceived Noise Decibels) considers the overall noise event, including its frequency characteristics, prominent tones and duration.

➡️ TPNdB (Tone-Corrected Perceived Noise Decibels) looks at perceived noisiness at a particular moment, including an adjustment for prominent tones.

So they’re measuring slightly different things, and those two reductions came from different microphone positions.

That last figure doesn’t mean 60% less noise. It just means a substantially smaller geographical noise footprint at that particular contour level.

Still, pretty impressive!

But what about aerodynamic efficiency?

During hover testing, researchers reported an approximately 7% improvement in maximum rotor figure of merit compared with the reference EC155 blades.

Figure of merit is essentially a measure of how efficiently a rotor produces lift for the power it consumes in hover.

So a higher figure means better hover efficiency.

And flight testing also confirmed improved hover performance, particularly at higher pressure altitudes.

Interestingly, in level forward flight, the Blue Edge blades required roughly the same power as the conventional blades.

So the efficiency gains weren’t necessarily equal across the entire flight envelope.

But that’s still a pretty impressive combination!

Less noise during descent, improved efficiency in hover, and comparable power requirements in level forward flight.

Not bad for ‘just’ changing the shape of a rotor blade!

These were EC155 demonstrator results, not production H160 measurements, but they helped validate the technology later adopted for the H160.

💭 The Bigger Picture

What we find fascinating about Blue Edge is how simple the underlying idea sounds.

The rotor creates vortices.

Those vortices can interact with following blades.

And those interactions can create particularly strong noise.

So rather than eliminating the vortices, engineers changed how the blades interact with them.

They also influenced how the sound combines, where it travels and how the vortex develops.

All while working to improve aerodynamic efficiency.

Achieving all of that while maintaining the structural and handling characteristics of a helicopter is anything but simple!

That’s what makes the H160’s unusual-looking rotor blades such a fascinating piece of engineering 🪃 🚁

Research references:

Gervais, M. & Gareton, V. (2011). Analysis of Main Rotor Noise Reduction Due to Novel Planform Design: The Blue Edge Blade. 37th European Rotorcraft Forum.

Rauch, P. et al. (2011). Blue Edge: The Design, Development and Testing of a New Blade Concept. American Helicopter Society 67th Annual Forum.