Tail rotor systems are one of the most crucial components onboard a helicopter. While pilots can deal with tail rotor emergencies, there are variables that could be extremely hard to deal with and manage.
This is why the structural design and manufacturing of tail rotor systems is taken extremely seriously amongst aircraft manufacturers. Many implementations and designs have surfaced over the last few decades. Manufacturers are getting more creative every year to minimise the amount of energy getting lost to anti torque devices.
From conventional tail rotor systems:
To Fenestrons (most airbus helicopters, including the new H160 pictured here at an event):
To NOTAR (most MD Helicopters):
And even the complete removal of an anti-torque tail rotor system in the case of dual rotor systems (The Chinook for example) or co-axial rotors such as on the new S-97 Raider currently being developed by Lockheed Martin. It is an ongoing struggle between efficiency, power, weight, safety, and energy.
While some systems seem more straight forward than others, the concept of NOTAR (which stands for no tail rotor) is patented by MD Helicopters, and is poorly understood across the industry. So how exactly does it work, and what are the benefits and considerations?
Well, let’s start with 2 effects that you’ll need to understand first: The Coanda Effect and the Magnus Effect. But before we get into that, let’s remind ourselves why we need a tail rotor in the first place. Ready?
NEWONT’S THIRD LAW
This is where our troubles begin. If only this wasn’t a thing, then we wouldn’t even need this silly tail rotor. Newton’s first law states:
“When one object exerts a force on a second object, the second one exerts a force on the first that is equal in magnitude and opposite in direction.”
Complicated? Nah, just imagine swimming: you push the water backwards, and then the water therefore pushes you with the same amount of force but opposite direction (forwards)! The same applies to the blade system of a helicopter. If we have a look at a helicopter with an anti-clockwise rotating blade system:
The blades turn anti-clockwise, but they push themselves in that direction away from the fuselage. This causes the fuselage to turn in the opposite direction: clockwise.
This is where the tail rotor (or anti-torque thrust) comes in. We need to provide the helicopter with a stable heading if we want to make it fly properly, so we need a force pushing the tail towards the right:
This is where things get interesting though, because the way manufacturers choose to generate this required anti-torque thrust is varied as discussed before. For NOTAR, it uses a combination of the Coanda and Magnus Effect which we’ll go over first, to make sure you don’t run away in confusion!
THE COANDA EFFECT
The Coanda effect is named after Henri Coandă, who discovered it. We have left the funny looking ‘a’ out of ‘Coandă’, which is technically not correct, Henri will not be pleased).
Henri defined the Coanda effect as:
“The tendency of a jet of fluid emerging from an orifice to follow an adjacent flat or curved surface and to entrain fluid from the surroundings so that a region of lower pressure develops.”
That’s a lot of fancy words. Not to worry though, it simply means that whenever we have a stream of airflow getting close to any sort of smooth surface (straight or curved), it has a tendency to follow this surface and in addition create a low pressure area just above this surface (and therefore thrust).
This is super handy for us, as it is basically the same concept as lift explained by Bernoulli’s theory, which is also created by a pressure differential (low pressure above the wing, high(er) pressure below the wing). We can use this lift to create an anti-torque thrust! This is exactly what NOTAR aims to do. Let’s zoom in on how it works exactly.
Imagine an existing amount of airlflow (downwash for instance):
Fast flowing air has a tendency to ‘steal’ air molecules from it’s surrounding still air, which leaves a low pressure area behind. This happens whenever a stream of air speeds up. For instance, if you put a piece of paper between two hands and blow just over the top of it, the whole paper will lift, that’s comparable to this:
Now, these low pressures areas will get airflow flowed into them from the ambient atmosphere, which is trying to equalise the pressure constantly. However, if we put a surface on one side of this airflow, this low pressure area is ‘trapped’ and cannot be equalised by surrounding air (as there is none!). This will suck the air towards this surface and the air will tend to stick to it, tadaaaa: the Coanda effect.
This now means we have flowing air over a surface, with a remaining low pressure area on the other side of the airflow. This creates suction (or thrust) towards this remaining low pressure area.
Even if the surface bends downwards, the flow will tend to stick to it. Now let’s talk about the other contributing effect that is relevent for the proper explanation of NOTAR:
THE MAGNUS EFFECT
The magnus effect is often overlooked when people explain the NOTAR technology. The Magnus effect is named after Heinrich Gustav Magnus, the German physicist who investigated it (no complicated letters this time, yay!).
It is best explained by imagining a cylinder in airflow:
Now, the airflow will tend to stick to the surface (as discussed before), creating an ‘equal’ amount of low pressure area and lift effects into the upwards and downwards directions. Not too useful yet right?
But now let’s make the cylinder turn anti-clockwise from our perspective. This has 2 main effects:
- The air gets deflected downwards, creating thrust towards the top
- The top part of the airflow is sped up by the cylinder, reducing pressure, increasing lift (Bernoulli’s principle)
This, again, is very useful for us as we can direct this thrust in the direction needed to stop the fuselage from turning in the opposite direction to the rotor system. So how does any of this relate to NOTAR? Let’s talk about it!
HOW NOTAR WORKS
Let’s get to why you’re actually here: NOTAR! Now that you understand the principles behind the technology, let’s cover how they all link together and result in a working system for countering torque on a helicopter fuselage.
There are a few components that all need to work together:
- The NOTAR fan inside the tailboom
- The gap in the tailboom supplying airflow
- The Vortex Generators
- The thruster at the end of the tailboom
- The Vertical Stabilisation Control System (VSCS)
THE NOTAR FAN AND THRUSTER
The NOTAR fan is located in the back of the fuselage. It operates at a constant RPM but at variable pitch. The pedals are linked to a hydraulic actuator that controls the rotation of the direct jet thruster and at the same time change the blade pitch angle of the NOTAR fan.
This is to make sure it maintains a constant air pressure in the tail boom as the thruster nozzle opens and closes. The amount of opening and closing depends on the amount of anti-torque thrust required by the pilot by using pedal input.
Apart from providing air to the thruster, the NOTAR fan provides air to the gap along the tailboom, which is discussed further below. One thing to note here is that the thruster only provides roughly 30% of the required anti-torque thrust, the rest is all provided by the tailboom itself!
THE TAILBOOM GAP
As shown in the picture, you can see a small gap going horizontally across the tailboom section of the aircraft. This is to let NOTAR fan airflow through, which will add energy to the right hand side of the airflow coming down from the rotor, and creates the Coanda and Magnus effect. Normally, the airflow situation across the tailboom looks like this:
However, air from the NOTAR fan comes through the tailboom and gets split into 2: one section goes to the thruster and the other part gets expelled through this gap.
This extra airflow on the right hand side of the tailboom creates a more pronounces Coanda and Magnus effect (just like the rotating cylindarm the airflow is sped up on 1 side). The situation will now look like this, the air sticks even longer, gets sped up, and gets deflected towards the left, creating a thrust to the right:
Even conventional tail rotor helicopters use this principle. They quite often have something called a ‘strake’ on one side of the tailboom, which distorts the airflow on one side, and therefore creating a small thrust towards the other side due to the relative airflow velocity difference.
THE VORTEX GENERATORS
So what are these funny looking objects on the tailboom? They are called vortex generators. Vortex generators delay airflow separation. They affect the airflow in such a way that it has an easier time sticking to the surface even more. This increases the efficiency of the tailboom anti-torque and puts less pressure on the thruster to deal with anti-torque.
They are not just used within the NOTAR system, they are widely used on wind turbines, aircraft wings, and anything else that relies on flow sticking to a surface. How does it do this? Let’s dive into it!
Consider an airflow going across as wing, at some point, we have an airflow separation point (the point where laminar airflow no longer sticks to the wing and becomes turbulent):
The reason for this separation is that the flow closest to the actual surface slows down so much that it is no longer able to freely flow over the wing due to friction:
If we can give it some extra speed (or energy), we can delay the separation point. This is where vortex generators come in.
We can turn a laminar flow into a turbulent one by installing a vortex generator. This will mix the faster flow (further away from the wing) with slower airflow (closer to the wing). This makes the entire airflow that almost ‘touches’ the surface a lot faster, and therefore delays the separation point!
Now so far all of the theory applies to both hovering and forward flight. But the VSCS only works above roughly 60 KIAS. The vertical fins with control surfaces need airflow to become actually useful and are therefore not usable in the hover. Part of the system is a fly-by-wire actuator that is looking at how much collective input is used. More collective means more required anti-torque and therefore more deflection of the vertical fins.
It is trying to anticipate collective usage, so that it can compensate in time at high speed and therefore minimise the power required by the NOTAR fan. This then leaves more power available for the main rotor! The remaining portion of the system is a fly-by-wire yaw damping function that uses yaw gyro/lateral accelerometer signals to avoid aerodynamic side effects like dutch roll (we might cover this in a future article).
So there we have it: NOTAR! There will be articles in the future that dive dive deeper into individual principles discussed here, as it can be a slippery slope at times! Keep sending in your questions, feedback, and requests for future articles, and fly safe!
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