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.

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 break-throughs 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.  The reason we are discussing this here is because both Counter-Rotating and Contra-Rotating rotors solve the problem of Dis-Symmetry of lift by providing a balance of two advancing and two retreating rotor blades. This also allows for faster forward air speeds by eliminating most of the effects of retreating blade stall at faster forward air speeds. 


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 sync. In an airplane, if the systems are out of sync, it can put undue stress on the air frame, and cause harmonic vibrations throughout the air frame. You can usually hear an airplane that has the engines out of sync, 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 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 alternately rotating swashplates are needed to adjust the rotors at the individual hub.

Integrity, Respect, Honor