The Aerodynamical Effects Specific to The Helicopter

Due to its specific aerodynamics, a helicopter is subject to a series of possible aerodynamical effets. A pilot has to knwo them either in the purpose of using them, or to know how they are impacting his pilotage

• the ground effect. The 'ground effect' consists into turbulences which are yielded by the plane of lift (which, for a helicopter, is the plane of the main rotor). When used for the benefit of the pilotage the ground effect allow, at the very low altitudes or heights, to increase the lift of the helicopter! Such a turbulent air is produced due to that the ground is interfering with how the air is moving along the main rotor's plane. That occurs because the helicopter is at a very feeble distance from the ground, which is about one rotor's height. The ground effect still increases when as low as a half rotor's height, or about 3 ft above the ground. Those turbulences, technically, as far as a plane and not a helicopter is concerned, are constituting the so-called 'induced drag', which is the air moving from the high pressure area, on the lower side, or 'intrados' of the wing, to the low pressure area on the upper side, or extrados. The induced drag is part of the drag, one of the four forces determining the flight with the lift, the weight, and thrust. The motion from the wing's intrados to the extrados is yielding swirls of air, as the proximity of the ground further is reducing those and turning them into some air useable for lift! The induced drag is increasing when the aircraft's speed is decreasing. As far as a helicopter is concerned, the turbulences, which are producted by the blades rotation determining the lift plane, are further added with the interaction between the ground and the induced downwash airflow (which is the air flowing downwards through the rotor). The proximity of the ground is reducing the speed of that flow. That decrease in speed of the downwash airflow still improves the ground effect, for a given pitch inclination angle. With that closeness to the ground, contrarily to a situation of stationary flight in altitude, the induced downwash airflow is decreasing less the angle of incidence of the rotating blades. In terms of the pilotage of a helicopter, the ground effect is mostly used for a stationary flight at a low height (just before the takeoff, for example, when the helicopter has to remain put above the takeoff point or just before a landing), or for a slow, low height, short-distance motion. In the case of the stationary flight, the pilot will have to have his gaze looking at at least about 50-65 ft ahead of the helicopter so to be able to check his pitch and roll, altitude and heading. Just take the horizon, high buildings or trees, etc. like visual clues which means vertical and horizontal reference points! You will need a 70-75 percent torque to keep into a stationary flight in the ground effect. The altitude is controlled through small pressions upon the collectif as the heading -or where the nose of the aircraft is pointing to- through the rudder, and any possible motion through the cyclical. A slow, low height, short-distance motion, with a speed of at most 20 kts and a altitude of 27 ft maximum is called a 'translation into the ground effect'. It also requires a torque of 70-75 percent. In both cases, when a height of 3ft above the ground is exceeded, that torque value has to be increased by 10 percent
• the yaw effect. The yaw effect for a helicopter is the tendency it has, during a stationary flight, to shift in the direction of where the tail rotor is pushing to. On a helicopter like the Bell 206B JetRanger III that shift is occurring towards the right. Such a shift, most of the time, is compensated for by default through how the helicopter is deviced, with, for example, the main rotor's pylon inclined or the cyclical slight left-tuned. Even with such built-in corrections, it is possible that a helicopter pilot have, for a stationary flight or a climb, which are maneuvers requiring a full power, to input some cyclical left to compensate for the yaw effect
• air biting effect. The 'air biting effect' is the fact that the nose of the helicopter is rising and that the aircraft begins a vertical climb. That effect is due to a horizontal increase of the airflow along the main rotor's plane, and generally from the aft to the rear. That increase of the airflow may be yielded either, when in a situation of climb, by a strong increase of the collective (which thus is increasing the relative speed of the airflow relative to the rotor's plane), or, when in a situation of a stationary flight, by the presence of wind, which generates a relative airflow. The air biting effect occurs generally at a speed between 10 and 15 kts. The bite in the air is hinted to by a low frequency vibration. For a same collective setting, the helicopter's lift is rapidly increasing! The air biting effect, per se, is theoretically what makes a helicopter begin to climb when the speed is reaching 10-15 kts. It is the too much of air biting which is nefarious. As the nose tend so yaw left and to slightly raise, you will have to push the cyclical forwards and to the left and to have some rudder to the right as far as the heading is concerned. Such actions are the same than those to use when the helicopter, after a takeoff, is beginning the climb!
• the transversal effect. The transversal effect is a aerodynamical effect which occurs during a flight, or during a stationary flight with the presence of wind. It consists into a decrease of the lift for the main rotor's rear part! That effect appears at the low speeds as the air which is flowing through the rear part of the lift plane generated by the main rotor is flowing more slowly there that it does in the aft part of that plane. The airflow in the rear part further, is moving vertically (upwards). Thus, when the rotor's blades are passing in the rear part of the rotor's plane, their angle of incidence is decreasing and so their lift does
• the dissymetrical lift. The dissymetrical lift is a aerdynamical effect specific to a helicopter which occurs in a forward flight, or during a stationary flight with the presence of wind. The dissymetrical lift consists into that a part of the rotor's plane does not yield lift anymore. The most known occurrence of that condition is the so-called backwards blade stall situation. This is one of the dangerous situations for a helicopter, as it is treated in details with our 'Other Dangerous Situations in a Helicopter' tutorial. Some helicopters, due to how they have been deviced, are default-compensated against the dissymetrical lift as they are featuring blades' flaps, which act upon the angle of incidence of the blades