THE HELICOPTER PAGE

# Helicopters: How They Work

 Web www.helicopterpage.com

### Dis-symmetry of lift

Now that we have discussed terminology a bit, let's begin exploring the realm of rotary wing flight. One can not begin to talk about the mechanics of helicopters until they discuss the problems associated with rotary wing aerodynamics. 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 "Dis-Symmetry 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 traveling 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.

## Counter-Rotation Vs Contra-Rotation

One thing that people often get confused with is the diffference 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 stobe 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 modified racing P-51 airplane, 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 alternatly rotating swashplates are needed to adjust the rotors at the individual hub.