Knowing that the fineness ratio of your rotor blade is its thickness as a percentage of its chord length may be something with which you can impress your friends at parties, but what are the aerodynamic basics we need to be familiar with in the early stages of helicopter training?
Let’s start with the helicopter hovering motionless in the air. By manipulating the cyclic, collective and foot pedals, the pilot can move in any direction and rotate the airframe about any of three axes: longitudinal, lateral and vertical. Describing to your party friends how each flight control moves the airframe is not that difficult, but the airframe is also subject to not so obvious forces in all of these same directions, making helicopter control much harder to grasp or explain. Learning helicopter flight is frustrating, and should not be compared with playing a video game.
To maintain height over the ground, you should control blade pitch (feathering) with the collective lever and hold position with cyclic and pedals. The engine is used to keep the rotational speed of the rotors at a fixed setting. Because engine and rotor speeds remain constant, the term “power” is commonly used to describe collective movements.
The hovering helicopter has a sideways drifting tendency from two forces. The torque from the circular rotation of the rotors twists the front of the airframe in the opposite direction. To prevent this, the tail rotor anti torque thrusts in the same sideways direction at the rear. To correct, the pilot tilts the rotor disc with the cyclic to the side opposite the airframe drift. The airframe rolls slightly about the longitudinal axis and the skid gear on the side opposite the drift hangs lower. Airframe roll as well as internal loading should be carefully considered when lifting up and landing from the hover.
To climb into forward flight from the hover, tilt the rotor disc ahead with forward cyclic and apply more collective to keep the airframe from settling. More collective means more torque, requiring more tail rotor pitch and more lateral cyclic to prevent airframe drift. As you accelerate to climb speed, the increasing airflow along the sides of the airframe has a keel or weathervane effect, and you will need less anti torque and lateral cyclic.
The flight control movements needed to tame the unstable helicopter are subtle and sensitive, especially at low speeds. Learning to juggle five balls in the air is probably on par with helicopter training, which most of us probably didn’t realize when we signed up.
As you leave the hover you will move through translation into forward flight. The speed increase is referred to as a transition, but translation is an aerodynamic event of its own. A thorough knowledge of what occurs at translation, and the implications of entering and leaving forward flight at translation, is crucial to safe helicopter flight. Many accidents occur each year because the intricacies of translational lift are not understood.
Translation occurs at a speed faster than your uncle Boris can run, but slower than a fast running pace, depending on the wind. At and ahead of translation the helicopter begins to acquire added lift from the speed of forward flight, much as a paper airplane does as it leaves your hand. At the same time, the blades sweeping past the advancing side of the helicopter gain more lift than the blades sweeping rearwards on the other side. This lift dissymmetry is neutralized when the advancing blades flap up, reducing lift slightly, and the retreating blades flap down, to increase lift. Remember that the helicopter wing is a rotating disc above made up of blades that change pitch, like Venetian blinds opening and closing.
The lift differential from pitching and flapping originates at the sides of the airframe, but isn’t fully realized until the blades arrive at the front and rear because rotating objects are subject to a 90-degree phase lag or gyroscopic precession. As the airframe accelerates past translation, the pilot needs to advance forward with the cyclic to maintain a constant attitude along the lateral axis. Nose up flap back occurs when you don’t compensate with the cyclic. The rotor disc moves through the air more like a coin wobbling on a table than the flying saucer movement of a Frisbee.
Early designers were not so much concerned with their creation’s ability to hover, but with the uncertainty of what would happen during the first transition through translation into forward flight, because of the more complex aerodynamics that come into play.
This discussion was not intended as a magnum opus on the topic of helicopter flight theory, but rather a rendering of the basic principles directly related to the early learning stages of helicopter flight control.