Rotary Wing Terminology
Most helicopters the engine turns a shaft that connects to an input quill on the transmission; the main rotor mast comes straight out of the top of the transmission and the tailrotor driveshaft connects to an output quill 90 degrees out from the mast.
Spinning the rotor which has an aerofoil section causes lift, allowing the helicopter to rise vertically or hover.
Tilting the spinning rotor will cause flight in the direction of the tilt.
There are many terms associated with rotary wing flight and it is important for a student to become familiar with them to understand the mechanics of rotary wing flight.
Main Rotor System
Root: The inner end of the blade where the rotors connect to the blade grips.
this type of rotor system pivots around the trunion to allow for blade flapping
Swash plate
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The swash plate assembly has two primary roles:
The swash plate assembly consists of two plates -- the fixed and the rotating swash plates -- shown above in blue and red, respectively.
Controls
Collective: The up and down control. It puts a collective control input into the rotor system, meaning that it puts either "all up", or "all down" control inputs in at one time through the swash plate. It is operated by the stick on the left side of the seat, called the collective pitch control. It is operated by the pilots left hand.
The collective lets you change the angle of attack of the main rotor simultaneously on both blades.
Cyclic: The left and right, forward and aft control. It puts in one control input into the rotor system at a time through the swash plate. It is also known as the "Stick". It comes out of the centre of the floor of the cockpit, and sits between the pilots legs. It is operated by the pilots right hand.
Pedals: These are not rudder pedals, although they are in the same place as rudder pedals on an airplane. A single rotor helicopter has no real rudder. It has instead, an anti-torque rotor (Also known as a tail rotor), which is responsible for directional control at a hover, and aircraft trim in forward flight. The pedals are operated by the pilots feet, just like airplane rudder pedals are. Tandem rotor helicopters also have these pedals, but they operate both main rotor systems for directional control at a hover.
Here are some of the component parts that make up a helicopter. While this is an example of one specific helicopter (UH-1C), not all helicopters will have all of the parts listed here. Some of this may be a bit more of the same old stuff we have just discussed, but it will show everything as it relates to everything else on the aircraft and the location of each component. Just mouse over the grey spots to see the explanation of the parts of the helicopter below.
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The Tail Rotor
The tail rotor is very important. If you spin a rotor using an engine, the rotor will rotate, but the engine and the helicopter will try to rotate in the opposite direction. This is called TORQUE REACTION
The tail rotor is used like a small propeller, to pull against torque reaction and hold the helicopter straight.
By applying more or less pitch (angle) to the tail rotor blades it can be used to make the helicopter turn left or right, becoming a rudder. The tail rotor is connected to the main rotor through a gearbox. When using the tail rotor trying to compensate the torque, the result is an excess of force in the direction for which the tail rotor is meant to compensate, which will tend to make the helicopter drift sideways. Pilots tend to compensate by applying a little cyclic pitch, but designers also help the situation by setting up the control rigging to compensate. The result is that many helicopters tend to lean to one side in the hover and often touch down consistently on one wheel first. On the other hand if you observe a hovering helicopter head-on you will often note that the rotor is slightly tilted. All this is a manifestation of the drift phenomenon.
This picture illustrates how the helicopter moves when using the appropriate controls. Up and Down movements are controlled by the "Collective". Side to Side and Forward and Back motions are controlled by the "Cyclic". Lateral control (Also called directional control or "Yaw") is achieved by using the "Foot Pedals".
Dissymmetry of lift
One cannot begin to talk about the mechanics of helicopters until the problems associated with rotary wing aerodynamics are understood. When the first rotary wing pioneers started trying to make a helicopter fly, they noticed a strange problem.
The helicopters rotor system would generally work just fine until one of two things happened: Either the aircraft began to move in any given direction, or it experienced any sort of wind introduced into the main rotor system. Upon either of these events, the rotor system would become unstable, and the resultant crash would usually take the life of the brave soul at the controls. The question then was; Why does this happen? The answer is what we refer to today as "Dissymmetry of lift".
What "Dis-Symmetry of lift" means is, when the rotor system is experiencing the same conditions all around the perimeter of the rotors arc, all things are equal, and the system is in balance. Once the system experiences a differential in wind speed from any angle, it becomes unbalanced, and begins to rotate. Take for instance forward flight. Imagine a two bladed rotor system spinning at 100 MPH.
The blade moving toward the forward end of the aircraft is going 100 MPH forward, and the blade moving toward the back of the aircraft is travelling at 100 MPH in the other direction. This is just fine when the aircraft is not moving or is in a no wind condition. It is experiencing 100 MPH of wind in all directions, so it is totally in balance. Once the aircraft moves forward, it begins to change this balance. If we travel 10 MPH forward, then the forward moving, or advancing rotor blade, is experiencing 110 MPH of wind speed, and the rearward, or retreating blade, is experiencing only 90 MPH of wind speed.
When this happens, we get an unbalanced condition, and the advancing blade experiencing more lift wants to climb, while the retreating blade experiences less lift and wants to drop. This is where we get the term "Dis-Symmetry of lift". The lift is not symmetrical around the entire rotor system.
How do we compensate for this situation? We compensate by allowing the rotor to flap. By allowing the advancing blade to flap upward, and the retreating blade to flap downward, it changes the angle of incidence on both rotor blades which balances out the entire rotor system. As you can see in this simple graphic, there are a few ways to allow for blade flapping.
One is to allow the blades to flap on hinges (Articulated rotor system). Another way is to have the whole hub swing up and down around an internal bearing called a trunion (Semi-rigid rotor system). Unfortunately, we can not compensate completely for dis-symmetry of lift by using blade flapping. Once the aircraft gets to a certain airspeed, and the rotor had flapped as much as it possibly can, then "Retreating blade stall" may be experienced. In retreating blade stall, the retreating blade can no longer compensate for dis-symmetry of lift, and the outer portions of the blade will "Stall".
This situation, when not immediately recognized can cause a severe loss of aircraft controllability. This is a major airspeed limiting factor for helicopters. For many years, aeronautical engineers have tried to figure ways to eliminate this problem and increase the forward airspeed for single rotor helicopters. Although many breakthroughs have been made, the manufacturers of single rotor helicopters are usually not willing to change the entire design on their products because of the extra costs involved for little airspeed payoff. Most have resigned themselves to slower airspeeds for their aircraft, at a lower cost and less maintenance.
The main rotor hub, where the rotor's drive shaft and blades connect, has to be extremely strong as well as highly adjustable. The swash plate assembly is the component that provides the adjustability.
Counter-Rotation Vs Contra-Rotation
One thing that people often get confused with is the difference between "Contra-Rotation" and "Counter-Rotation". The terms are used incorrectly more than you could possibly imagine in books, manuals, and on web sites. I wanted to take this opportunity to clear up the difference between the two.
As you can see by the first diagram, "Counter-Rotation" is where there are two individual shafts driving two propellers or rotors in different directions. Although we have chosen to show this example on a CH-47 Chinook from a top view, it is exactly the same on a twin engine airplane that has one propeller turning one way, and one turning the opposite way (Like on a P-38 "Lightning"). Sometimes, as in the case of the CH-47, the rotors will mesh, so the synchronization of the systems is crucial.
On airplanes, where the propellers do not mesh it is not as critical that the systems are in synch. In an airplane, if the systems are out of synch, it can put undue stress on the airframe, and cause harmonic vibrations throughout the airframe. You can usually hear an airplane that has the engines out of synch, as it will make a varying strobe like sound.
Each propeller in an airplane counter rotating system has its own set of mechanical controls to vary the pitch of the blades. Often it is a hydraulic system, but in some cases (Like the P-38), other means can be employed such as electric power. In a helicopter, both rotors are manipulated by one set of controls for the pilot.
contra-rotation
"Contra-Rotation" is where the propellers or rotors are mounted "Co-Axially", meaning one in front of (or on top of) the other on the same axis. Usually, the drive mechanism is a single source, but the direction of rotation is spilt by a gearbox to drive the two systems in opposite directions. This is usually done to reduce the "P" factor or "torque" in a turn. While we have chosen to show this example in the form of a Royal Navy Fairey Gannet, it also applies to helicopters (Like on the Soviet "Hokum").
The main use for this on a helicopter is that it negates the need for a tailrotor (Anti-torque rotor) to maintain directional control at a hover. It also tends to relieve some of the effects of retreating blade stall as both sides of the aircraft have advancing rotor blades.
In an airplane, one set of controls will adjust the pitch of both propellers at the same time. Usually, it is done by varying hydraulic pressure in the propeller hubs. In a helicopter, both rotors are manipulated by a single set of pilot controls as well, but two sets of control tubes working off of two alternately rotating swashplates are needed to adjust the rotors at the individual hub.
The Forces At Work
There are many forces at work when a helicopter flies, and many are specific to helicopter flight. We will touch on some of these briefly. We all know about lift, drag, gravity, and thrust, so discussion of these would not really be necessary. I would rather talk about specific conditions experienced exclusively in rotary wing flight. Here are some examples.
Translating Tendency
Translating tendency is defined by the textbooks as: The tendency for a single rotor helicopter to drift laterally, due to tail rotor thrust. One may not think about how much thrust is produced by the tail rotor, but we must remember that the tail rotor has a 6 to 1 rotational ratio to the main rotor system.
It actually spins 6 times faster than the main rotor, so it can compensate for the torque of the main rotor without the need for a massive tail rotor span. The thrust it produces tends to push the aircraft sideways at a hover. We compensate for this by adding left cyclic control inputs (On American Helicopters, the opposite in foreign manufactured aircraft, because their rotor systems turn the opposite way from ours). This makes the helicopter hang left skid, or wheel, low at a hover. If you ever see an American helicopter hovering, you may notice this left side low condition. If you ask a helicopter pilot how he is doing, and he answers, " Left skid low", that means everything is normal.
Settling With Power
Settling with power can be a dangerous condition that any pilot may face, and if he or she is not on their toes, it may cause a serious uncontrollable situation. Settling with power is basically when the helicopter settles into the rotor wash produced by its own main rotor system. It requires 3 key elements to occur, and these conditions should be avoided in combination with one another.
These are: A near zero airspeed, up to 100% power applied, and a better than 300 foot per minute rate of descent. Once you have all of these situations in occurrence, the aircraft will settle in its own down wash from the rotor system. The only way to recover is to gain forward airspeed and allow the rotor system to fly into "Clean air". Once the rotor system is clear of the rotor-wash, it will become efficient again, and the settling with power conditions will cease to exist.
This can become a real problem at an out of ground effect hover (Above 10 feet from the ground), and during landings.
'Settling With Power' or 'Settling in your own downwash' is a dangerous situation that any rotary wing machine can experience. The term "Vortex Ring State" is used to describe the actual swirling of the air within the rotor system itself that causes "Settling With Power". Vortex Ring State can begin to occur when you have 300 Feet per minute (FPM) as a rate of descent. Pilots need to be aware of the situation and avoid it at all costs.
Dynamic Rollover
Another dangerous condition for a helicopter pilot to experience is called dynamic rollover. It is again, where you have a series of conditions that combine to make a dangerous situation. Once again, 3 key elements make up this hazardous condition. They are: A pivot point, a rolling moment, and weight equal to thrust at some time during the manoeuvre. What actually happens is that the helicopter, which is still on the ground, will start to roll over on its side using one skid, or wheel, as the pivot point.
Once the aircraft starts to roll, a downward collective movement is the only thing that will stop the forces in action from flipping the aircraft on its side. By reducing the collective, the thrust to weight ratio decreases, which allows the aircraft to settle back down in a level attitude. If this is done on sideward sloping terrain, a collective reduction performed too quickly can cause the aircraft to roll over on the other side, down the slope. Care must be exercised when performing slope operations, but dynamic roll over can occur on the flattest of surfaces if the pilot becomes complacent.
Once that force is no longer applied, the weight of the aircraft will get it started sliding and, depending on the slope, could pick up so much speed that it crashes severely at the bottom of the hill. The ones that have wheels and brakes could slide also depending on the degree of slope and condition of the ground.
Other reasons not to attack a slope from the front or back is that the tail boom may strike the hill before the skids do (Again, depending on the degree of the slope) or the rotor system may impact the hill before the skids do. Usually, if the standard 8 degrees of slope are used as a maximum, then a sideward approach to the slope will have the skids touching before the rotor system. Care should be used when passengers depart the aircraft on a slope as they may walk into the rotor if they go up hill. Always brief the passengers to leave the aircraft on the down slope side of the aircraft.
A rotating body acts like a gyroscope and the forces that act upon the gyroscope require some adjustment to allow for the rotation itself. A spinning body will take inputs placed at one part of the cycle of rotation and react later in the cycle of rotation. Now without getting too technical, the main thing to remember here is that with the rotation comes some extra planning. If you want a control input to take effect, you just have to be a little ahead of where you want it to happen. In this case, 90 degrees before the spot where the action you desire is to take effect is where you have to plan to put it into the system. Input is placed in one location and as the blade swings 90 degrees more in the direction of rotation, the desired effect will be realized.
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