The Fenestron is a fascinating type of anti-torque system. Single-rotorhead helicopters have to deal with the same issues that come with pushing a rotor disc around. There are a lot of different ways to deal with these issues though. We covered the NOTAR system in a previous article. Today we’ll do some comparing: Conventional tail rotors vs the Fenestron!
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What is a Fenestron?
Fenestrons are shrouded tail rotors that offer operational benefits compared to conventional tail rotors. They are essentially tail rotors shrouded by a vertical fin, to improve efficiency and safety.
Let’s take a little trip back in time to 1968. The Fenestron was designed by two guys named Paul Fabre and René Mouille, for the second prototype of the Gazelle. It received official certification in 1972, so Fenestrons have been utilised for quite a while now!
Tail rotors are pesky little things, hard to see, spin around at crazy speeds, and often cause things to go from 0 to 100 if anything goes wrong. It was initially designed to improve safety for ground crew, and also to protect the tail rotor in forward flight and in hazardous areas and confined areas. Perfect for operations like HEMS or SAR (like the Dauphin in the picture below).
While the Fenestron was not really providing enough thrust for bigger helicopters in the early days (due to reasons we’ll cover later), it is now used in many bigger helicopters including the new Airbus H160. The H160 is definitely not a small helicopter, boasting a MTOW of about 6000 kg (about 13000 lbs for our American friends).
Airbus has invested lots of time and energy into the design and performance of Fenestrons as the years have gone by. Take the H160 Fenestron for instance. It looks and performs very differently compared to the early designs. It now has a massive 1.2m diameter and is tilted about 12º to increase payload and stability, as it provides vertical thrust as well.
Why do helicopters need a tail rotor?
So why do helicopters need a tail rotor in the first place? Let’s quickly go over the basics first. As we know, helicopter engines turn the blades either clockwise or anticlockwise, depending on the manufacturer and helicopter type. For now, let’s look at a clockwise rotating disc:
As Newton’s taught us with his third law: for every action (force) there is an equal and opposite reaction. So in this case, the fuselage of the helicopter wants to spin anti-clockwise:
We call this the torque effect, and is the reason we require some form of anti-torque: a tail rotor, or any other anti-torque system such as a Fenestron or NOTAR system.
The tail rotor or Fenestron’s purpose is to provide this anti-torque thrust towards the left to keep the nose of the helicopter pointing in a constant direction, both in forward flight and in the hover.
How does a Fenestron work?
When it comes to the actual working principle, Fenestrons work in a very similar way to normal tail rotors.
The pitch link is the component that transfers pedal input to pitch change of all the Fenestron blades simultaneously. If we raise collective, we increase the amount of anti-torque required, so the pitch angles on all fenestron blades will have to increase as well.
The other main parts are the drive shaft (which is driving the blades) and the stators, which we’ll discuss later.
Remember the lift formula? Total surface of a blade and its speed are both components that result in increased lift. Due to the small surface of all Fenestron blades, we require more of them to make sure we get enough tail rotor thrust. This is the reason they have a lot more blades than conventional tail rotors.
In addition to this, Fenestrons run at a much higher RPM compared to conventional tail rotors. To give the perfect example let’s have a look at the EC145 C2 and compare it to the H145 / EC145D2. The C2 has a conventional tail rotor:
While the D2 and H145 have a Fenestron:
The RPM of the tail rotor on the C2 is roughly 2150 RPM. This is a lot lower compared to the Fenestron RPM of roughly 3150 RPM (about 50% higher RPM!).
This again, is to make sure the thrust output remains high enough to support the anti-torque required to lift an EC145 from the ground. Smaller blades need more speed to achieve the same amount of lift and therefore thrust.
What are the benefits of a Fenstron?
So what are the benefits then? Let’s break them down!
Tail rotor protection
The most obvious one is the fact that the entire tail rotor is completely shrouded now. This results in a safer working environment for ground crew, and is beneficial for special operations such as HEMS when flying near obstacles.
Tail rotors are noisy, very noisy.. Luckily certain Fenestrons have been proved to be quieter than conventional tail rotors, and extensive research has gone into their noise profile. The main reasons it’s quieter are:
1) Instead of using straight tail rotor blades, the newest one have 2 slight bends, which results in all of the noise not being amplified on the same frequency. Instead, the noise signature gets spread out across multiple frequencies. This makes it sound quieter.
2) An acoustic liner. What’s this? It’s a sheet inside the fan duct that absorbs sound waves to reduce the noise emitted by the Fenestron.
3) Stator vanes. These are the ‘blades’ that you can see inside the Fenestron duct that never move. One of the benefits of these is a reduced amount of noise.
This might not be very obvious, as the huge fin probably makes you think there’s more drag to account for. But remember how tip vortices are generated? It’s the mixing of air between an area of low pressure above a wing, with the high pressure below the wing.
We don’t want this mixing to occur, which is why you see lots of passenger planes with winglets nowadays, they stop this mixing and reduce vortices.
The problem is that you can’t just give a rotor blade a winglet. Some designers have tried this, and you might find main rotor blades that do tip down slightly, but there are more reasons for that which we covered here.
By completely blocking the area at the end of the blades, air on each side of the tail rotor blades cannot mix. The big fan around the blades essentially acts as a giant winglet, quite clever! This massively improves thrust efficiency.
Less components and linkages
Fenestrons rely less on linkages and components with a certain amount of life cycles. Of course they are still subject to regular maintenance, but they have less points of failure compared to conventional tail rotors.
Tail rotor redundancy
This one’s a biggie. If you lose input or power to the tail rotor, conventional helicopters will start spinning depending on their speed. Even in forward flight, there will likely not be enough anti-torque to keep a constant heading.
With a Fenestron however, the vertical fin will generate a large amount of anti-torque thrust in forward flight. It’s basically a giant wing, generating lift in the same direction as the tail rotor thrust! This gives pilots more options to deal with emergencies, which often leads to better outcomes.
What are the downsides of a Fenestron?
While Fenestrons fix some of the problems that we usually have to deal with for tail rotors, they unfortunatley come with some downsides as well.
Fenestrons are, overall, less powerful than conventional tailrotors. Anyone who has flown both an R22 and a Cabri G2 has probably discovered this fact quite abruptly. They are more efficient, but need a larger surface area to achieve the same amount of thrust without the help of the vertical fin they are shrouded by.
The elephant in the room though is the way Fenestrons handle. Pilots who are used to conventional tail rotor systems often find that the ‘feeling’ of controlling yaw is less responsive or takes more input to achieve the same result.
Airbus has published a very interesting bulletin that explains why this is. Have a look here.
To (over)simplify it, have a look at the graph below. It’s not exactly accurate due to simplification, but it will get the point across. We have plotted the tail rotor thrust to the amount of pedal input:
What it comes down to is the fact that most of the tail rotor output only becomes available in the last quarter of pedal input. Looking at the graph, the tail rotor line is almost linear (it isn’t in real life), while the Fenestron thrust increases more at the far end of pedal travel.
This means that the initial amount of pedal travel will not achieve as much as the last bit. In other words, you’ll have to get used to using more pedals in general.
This can especially be tricky in less stable aircraft without any autopilot aid such as a Cabri G2. As you gain and lose speed the vertical fin’s anti-torque thrust changes rapidly.
This makes the issue worse as well compared to other helicopters who do not have such a massive fin helping with anti-torque thrust.
I’ve had many students who’ve had a lot of difficulty adjusting to the amount of pedal required as you lose translational lift and more collective is required. The fact that the vertical fin won’t help with anti-torque in the hover makes this issue even worse.
The Fenestron is currently the signature feature of Airbus Helicopters. It provides a lot of benefits but also comes with some drawbacks.
As a pilot, knowing how your tail rotor system responds to yaw input is vital, and will make sure you can anticipate and compensate for the way it handles differently than you might be used to in other helicopters.