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. 

Tilting the spinning rotor will cause flight in the direction of the tilt.

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.

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.

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

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.

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 above, 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" 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 including lift, drag, gravity, and thrust. However there are specific conditions experienced exclusively in rotary wing flight.

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.