Brief interludes will precede the
main feature; please
do not adjust your set.
EXT: Outside an engineering workshop. Two mechanics, Steve and Jake, are leaning against one wall and smoking. One of them knows more than the other.
Jake: "Why dun't you like tail rotors?"
Steve: "Ahh, they're a pain in the arse. They do my 'ed in."
J: "Yeah, why though?"
S: "
Dangerous, man. You gotta worry about
tail rotor strikes when you're flyin'."
J: "What, the tail rotor hits someth-"
S: (interrupting) "Yeah. 'Coupleyears back I was on a training op, just dropped off some
pax and loaded six. Took off, got to about fifteen feet and moved forward about fifty to
hover, and the
tach drops to like
five thousand.
J: (raises eyebrows)
S: (nodding) "Yeah. Anyway we're dropping so I try to pedal turn back to the
LZ.. there's trees in front of meIcan'tsee
owt. Some of 'em in the back must've seen the warning light.
(pauses)
Four of 'em jump out while I'm still turning."
J: (shakes his head, looks down) "Jesus."
S: "Tail rotor hits the ground. I'm only a few feet up, it starts spinning left,
tail hits these trees and grounds the main
rotor. Spins more and the boom comes off, pulls the
engine off its mounts and rips the whole
gearbox out.
(pause)
We were okay but it's a joke, man. Total writeoff. With twin rotors or
notar you can
maybe get awaywi'bumping the tail, but wi'a tail rotor you've got no chance. It's always gotta be the complicated stuff though. Idiots."
J: "Is it time for lunch yet?"
S: "
Shut up, man."
HELICOPTER ATTACKS AND KILLS LOGGER
HORROR OF TAIL ROTOR MUTILATION
A helicopter killed a 43-year old man yesterday, devastating a family.
The evil Bell Jet Ranger helicopter spun its razor sharp rotor blades menacingly at logging camp chief Carl Johnson as he walked alongside it, after talking to its pilot.
Then without warning it sliced up his face and chest with its tail rotor blades as he tried to duck underneath it. The father of four was left bleeding to death while the pilot screamed for help. A short time after emergency services arrived, Johnson was pronounced dead at the scene.
The helicopter made no effort to help its victim and showed no regret for its callous and senseless act of violence.
The National Transportation Safety Board today launched a probe into the attack. The Jet Ranger is being held for questioning.
Oil Workers killed in Helicopter Crash
Three passengers, including an Aberdeen oil worker, and two crew have been killed in a helicopter crash 115 miles north-east of Rio de Janeiro off the coast of Brazil.
The helicopter was being used for a crew-change operation when its tail rotor struck one of the antennas on the Toisa Mariner tanker and broke off. The helicopter lost control and crashed into the sea quickly sinking to a depth of 900 meters. There were no survivors.1
NOTAR® = No Tail Rotor
Hopefully the reader gets the idea from these (ahem) excerpts that a tail rotor, while not necessarily a bad thing, is one more item in the plethora of weaknesses and complications that every single-rotor helicopter possesses.
"A helicopter is a collection of spare parts flying in formation
around an oil leak, waiting for metal fatigue to set in."
What tail rotors do
Brief background on why tail rotors or other anti-torque systems are essential for most helicopters should be useful here.
A helicopter flies by using an engine to rotate a set of rotor blades with aerofoil cross-sections. These create lift. In accordance with Newton's third law of motion, in turning the rotor blades the engine and the rest of the helicopter - which the engine is attached to - 'wants' to turn in the opposite direction. If this effect were not counteracted a helicopter would spin uncontrollably the moment it left the ground.
A tail rotor is another, smaller set of rotor blades mounted sideways on the end of the helicopter's tail boom. When this rotor spins it exerts a turning force on the fuselage that counteracts the torque generated by the main rotor, keeping the helicopter's heading stable. The amount of force applied by the tail rotor can be varied with the pilot's rudder pedals, allowing for yaw control.
Drawbacks of Tail Rotors
Despite the tail rotor's popularity as an anti-torque system (I would hazard a guess that 85-90% of all helicopters use a tail rotor for torque control) it is probably the most mechanically complex of those available. At least one extra gearbox is required to transmit power from the engine(s) to the tail rotor, which may sap up to 30% of the engines power - this is often referred to as "wasted" power because it doesn't provide any lift (except in isolated cases like the UH-60 Black Hawk, which has an angled tail rotor to provide some extra lift). Furthermore the weight of the helicopter is increased by the attendant control systems for the tail rotor blades, which are concentrated in the helicopter's tail boom. This has a consequent increase in drag (and reduction in manoeuvrability), as well as the physical length of the tail boom complicating manoeuvring or parking the helicopter.
The tail rotor is also not without its hazards, as touched on above. They are rarely shielded (some shielded designs do exist but most designs, if shielded, would not work correctly), creating a significant danger to personnel operating around them. There are many recorded incidents of passengers exiting a helicopter after landing and then walking back along the fuselage into the still-spinning tail rotor, with obvious consequences.
A tail rotor is also as fragile as it is vital to the stable flight of a helicopter it is attached to. This makes operating in enclosed areas that much more problematic and adds another vulnerability to combat helos; most helicopters can be downed if something solid gets in the way of the spinning tail rotor. According to the United States Army, twenty per cent of all peacetime helicopter mishaps are caused by tail rotor failure (for whatever reason). If you're in an airborne helicopter that encounters Loss of Tail rotor Effectiveness (LTE) you're probably screwed; see Black Hawk Down and How to survive a helicopter mishap for reference.
Finally, tail rotors are loud. Various sources estimate that on average, more than half of all the noise produced by a helicopter is caused by interaction of the tip vortices from the two rotor discs.
NOTAR
A tail rotor is no longer a necessary evil for helicopters with a single main rotor. NOTAR is a proprietary system, entering developing with McDonnell Douglas around 1974 and in use from the early 1980s. NOTAR was developed to have the same effect as a tail rotor but with less of the life-threatening side-effects. NOTAR systems are lower-maintenance, quieter, more reliable and far safer for pilots and ground personnel than tail rotor systems.
While sources are sadly lacking on how the idea came about, the basic principle behind the NOTAR system is fairly simple: to use a helicopter's tail boom as a 'sideways wing' to exert a turning force on the helicopter fuselage, much like a tail rotor does. If you were to look at a NOTAR-equipped helicopter (if you've seen the movie Speed you've seen one) you might notice that its tail boom is rather fat and cylindrical. There is a reason for this, as there is for the Lancaster bomber-style vertical stabilisers.
The major components of a NOTAR system are:
- A hollow, cylindrical tail boom
A cylinder made of composite material, with two horizontal slots at 70° and 140° from vertical on the side facing the retreating main rotor blades. If the main rotor blades rotate counter-clockwise (as do those of all NOTAR-equipped helicopters in existence at the time of writing), the slots are on the pilots right. We'll come back to these in a few paragraphs' time.
- An air intake
A small grille or array of slots, on the top of the helicopter fuselage between the main rotor mast and the root of the tail boom (these slots may, alternatively, encircle the root of the tail boom):
__|_|__ <---Rotor mast
___/ \___
___ | --_ <---Air intake
\ |____--_______
| _\ <--- Tail boom
/ ______\
_/ /
/
--____________/
/ \
/___________\_____/
Through this intake, air is drawn into the tail boom so the rest of the system can operate.
- A compressed air fan
This is mounted in the root of the tail boom and sucks in air through the air intake, blowing it along the tail boom. By using variable pitch fan blades an air pressure of 0-5 pounds per square inch is maintained inside the tail boom. The pitch of the fan blades is indirectly controlled by the pilot's rudder pedals.
- A compressed air jet.
This is mounted on the far end of the tail boom, and is a set of variable-pitch slots (like a window blind) for air blown through the tail boom to exit through. Any air that doesn't go out of the circulation control slots comes out here. The pitch of these slots is also linked indirectly to the pilot's rudder pedals.
How NOTAR works
A NOTAR-equipped helicopter provides torque and yaw control by using the tail boom like a sideways wing, whose lift counteracts the turning force being exerted on the fuselage by the rotor system. Before continuing a brief explanation of how a wing works will be useful.
A wing has an aerofoil cross-section (see Yurei's excellent helicopter writeup for some diagrams of these). When a wing moves forwards through the air (blunt end first) it deflects the air flowing over it. The 'Coanda Effect' aerodynamic principle dictates that fluid substances tend to follow the contours of curved surfaces they come into contact with. This means that air contacting an aerofoil will tend to follow its curves and be forced downwards, due to the shape of these curves. In forcing the air downwards the wing, in accordance with Newton's third law of motion, is forced upwards. With sufficient speed and/or wing area the wing and attaching parts (e.g. an aeroplane or a helicopter) will rise.
Now, how to make a helicopter's tail boom work like a wing? It is done using the downwash from the main rotor. Ever seen film of a helicopter hovering over water or long grass? Those rotor blades move a lot of air. Obviously some of this air is going to pass over the helicopter's tail boom. The diagrams below show tail boom cross-sections. Diagram A shows how air passes roughly symmetrically over the tail boom of a 'normal' helicopter; diagram B shows roughly how we need air to pass over the tail boom, if we are to use it to counteract the main rotor torque:
xxxxxxx xxxxxxx
xxxxxxx xxxxxxx
xxxxxxx xxxxxxx
xxxx xxxx xxxx xxxx
xxxx xxxx xxxx xxxx
xxxxx_____xxxxx xxxxx_____xxxxx
xxxx--- ---xxxx xxxx--- ---xxxx
xxx-- --xxx xxx-- --xxx
xxx- -xxx xxx- -xxx
xxx| |xxx xxx| |xxx
xx| A |xx xx| B |xx
xxx| |xxx xxx| |xxx
xxx- -xxx xxx - -xxx
xxx-- --xxx xxx -- --xxx
xxx---_____---xxx xxx ---_____---xxx
xxxx xxxx xxx xxxxx
xxxxx xxxxx xxx xxxxx
xxxx xxxx xxx xxxxx
xxxx xxxx xxx xxxxx
xxx xxx xx xxxxxxxxx
xxxxx xxxxxxxxxxx
xxxxxxxxxxx xxxxxxxxxxxx
xxxxxxxxx xxxxxxx
xxxxx xxxx
xxx xx
A NOTAR system needs to create the airflow shown in diagram B. If it does this then the tail boom will force air in one direction and in doing so be forced the opposite direction. These mixtures of forces can get confusing; the following points set out how they apply to a modern helicopter:
- The helicopter's main rotor rotates anti-clockwise. This means that when aloft, the helicopter fuselage 'wants' to turn clockwise.
- To counter this, the NOTAR tail boom must exert an equal anti-clockwise yawing moment on the fuselage.
- To exert this force, the tail boom must force air clockwise, to the pilot's left; in doing so it will itself be forced to in the opposite direction, counteracting the torque of the main rotor.
How is this achieved? Most of it is done with the circulation control slots mentioned earlier. The slots are angled downwards; they allow low-pressure air to exit downwards from the tail boom. This air interacts with the (high-pressure) downwash from the main rotor, causing the downwash to follow the curve of the tail boom further than it would do otherwise. This is called 'circulation control'. In effect, the air exiting through the slots causes the rotor downwash to behave as if the tail boom were a different shape. If we add the slots to the earlier diagram it illustrates this:
xxxxxxx
xxxxxxx
xxxx xxxx
xxxx xxxx
xxxxx_____xxxxxx
xxxx--- ---xxxxx
xxx-- -- xxx
xxx- - xxx
xxx| Slot ----> SA xx
xx| |A xx
xxx| | A xx
xxx - Slot ----> SAA xx
xxx -- -- A xx
xxx ---_____--- A xx
xxx A xxx
xxx xxxx
xxx xxxx
xxx xxxx
xxx xxxx
xxxx xxxx
xxxxxxx
xxxxxxxxxx
xxxxxxxxx
xxxx
xx
'A' is the low-pressure air from the tail boom slots. 'x' is the rotor downwash. The downwash flows over the low-pressure air, which causes it to hug the curve of the tail boom for longer than it would do otherwise. It results in air being forced to the left of the tail boom, much as an aerofoil forces air downwards; this creates lateral lift to the pilot's right, providing about half of the anti-torque force. The remainder is provided by the jet thruster at the end of the tail boom. By varying the pitch of the outlet vents and blades of the compressed air fan the NOTAR system also provides yaw control, by varying the strength of the air jet. A small amount of anti-torque force is also provided by the helicopter engine exhaust tailpipe, which is pointed in the same direction as the jet thruster.
In-Flight Control
We're not quite there yet. The method and systems described above allow a NOTAR-equipped helicopter to provide torque control in hover, but these alone are insufficient to do the same in forward flight. This is because as speed increases, less and less of the rotor downwash flows over the tail boom until a point is reached where the circulation control slots have no effect at all. Therefore, a method of supplementing the anti-torque system is needed (the jet thruster still works in forward flight).
If you were to look at an example of every different helicopter model with NOTAR, one thing they all have in common is a large H-shaped set of vertical stabilizers on the end of the tail boom (numbered below):
____
| 3 |
|____|
/ /| <--- 4
________ / /_|_
\ __/_ / \
_\ | 1 | /|||| |
\____|____|/_||||_/
| 2 |
|____|
...in fact, a great many tail rotor-equipped helicopters also have a
vertical stabilizer and use it to take some
load off the tail rotor in forward flight. In this case they are used to turn the force of the rotor downwash into anti-
torque force. The reason a NOTAR helicopter has four vertical surfaces is to balance the load on the tail boom, since having the circulation control slots reduces its strength. One curious thing about these stabilizers is that none of them are mounted at quite the same angle; this can be seen when standing behind a NOTAR-equipped helicopter (the
Speed DVD unfortunately does not have any useful frames for this). This irregularity is present because of the different angles at which air and downwash approaches the
empennage when the helicopter is flying forwards.
These vertical surfaces are all active, moving depending on factors such as airspeed, pilot controls and the helicopter attitude. They have to be active because of the widely changing flight regimes a helicopter can be flying in; a static surface (like the vertical stabilizer on an aeroplane) would only provide a fixed anti-torque force and be of no use in many situations. During autorotation, for example, (when the helicopter is aloft with no power to its engine) static vertical stabilizers would still exert a turning force on the fuselage even though there is no torque from the rotor system to counteract!
Design of factors such as the size, angle and placement of the vertical stabilizers had to be completed through many prototypes. The airflow approaching the empennage during flight is so unpredictable that no computer modelling would be a useful substitute for flight testing. Apparently over fifty different tail configurations were tested for the first NOTAR helicopter (the McDonnell Douglas MD 520N - a development of the OH-6 Cayuse/AH-6J Little Bird chassis). Even when other models were re-engineered with NOTAR systems they still required further prototyping and flight testing (and sometimes modification to thus-far-developed control systems) before the control was satisfactory. The Prouty sources below give more detail on these design processes.
In summary, NOTAR systems do the same job as a tail rotor; although they still require a long tail boom they do away with the spinning tail rotor, thus are a lot safer, quieter and more durable. Try sticking the tail boom of a helicopter with a tail rotor into a tree and you'll get the idea.
Slurred words are intentional.
Sources:
- The Sun; its inimitable house style.
- Vietnam Helicopter Pilots' Association; "Information on helicopter 67-17659" via Google cache (site down at time of posting); <http://tinyurl.com/scr1>
- info@kulikovair.com; "NOTAR Helicopter Design"; <http://www.kulikovair.com/Notar.htm>
- Prouty, R.W.;
- "Aerodynamics: Vertical Tail Surfaces for NOTAR"; <http://www.aviationtoday.com/reports/rotorwing/previous/1100/11aero.htm>
- "Controlling NOTAR Vertical Tails"; <http://www.aviationtoday.com/reports/rotorwing/previous/1200/1200aero.htm>
Eastern Atlantic Helicopters Ltd; "The NOTAR System"; <http://www.easternatlantic.co.uk/info.htm>
aerospaceweb.org; "Helicopter Yaw Control Methods"; <http://www.aerospaceweb.org/question/helicopters/q0034.shtml>
(Author unknown); "MD600N Features and Benefits"; <http://www.mdhelicopters.com/Rotorcraft/pdfs/md600_Tech/600techd_Features.pdf>
Rand, IngerSoll; "History and Devolopment" (sic); <http://home.wanadoo.nl/helicopter/development.htm>
helicoptersonly; "Antitorque Systems"; <http://www.helicoptersonly.com/maneuvers_3AntiTorque.html>
1: Contractors Unlimited (author unknown); "Oil Workers killed in Helicopter Crash"; <http://www.contractorsunlimited.co.uk/news/030710a.shtml>