question: Why, if you over just above ground in a helicopter, would the earth not move beneath you?
comment: It's because the atmosphere is rotating along with the earth. It's why we don't experience 1,000 mph winds constantly. Same reason why a bee can fly in a moving car, or you can toss a ball in a moving car: the air is moving along with the thing.
passages: The pilot tries to perfectly balance a helicopter so that the fuselage remains horizontal in hovering flight, with no cyclic pitch control needed except for wind correction. Since the fuselage acts as a pendulum suspended from the rotor, changing the center of gravity changes the angle at which the aircraft hangs from the rotor. When the center of gravity is directly under the rotor mast, the helicopter hangs horizontal; if the CG is too far forward of the mast, the helicopter hangs
   Helicopters which lose power do not simply fall out of the sky. In a maneuver called autorotation, the pilot configures the rotors to spin faster driven by the upward moving air, which limits the rate of descent. Very shortly before meeting the ground, the pilot changes the momentum stored in the rotor to increase lift to slow the rate of descent to a normal landing (but without extended hovering).
  
  Section::::Rapid descents.:Dives.
   As a payload is lifted up a space elevator, it would gain not only altitude, but horizontal speed (angular momentum) as well. The angular momentum is taken from the Earth's rotation. As the climber ascends, it is initially moving slower than each successive part of cable it is moving on to. This is the Coriolis force: the climber "drags" (westward) on the cable, as it climbs, and slightly decreases the Earth's rotation speed. The opposite process would occur for descending payloads: the cable is tilted eastward, thus slightly increasing Earth's rotation speed.
   The effect of ground elevation depends on the density of the ground (see Slab correction section). A person flying at 30 000 ft above sea level over mountains will feel more gravity than someone at the same elevation but over the sea. However, a person standing on the Earth's surface feels less gravity when the elevation is higher.
  
  The following formula approximates the Earth's gravity variation with altitude:
  
  Where
  
  BULLET::::- is the gravitational acceleration at height above sea level.
  
  BULLET::::- is the Earth's mean radius.
  
  BULLET::::- is the standard gravitational acceleration.
   The reduction in velocity near the surface is a function of surface roughness, so wind velocity profiles are quite different for different terrain types. Rough, irregular ground, and man-made obstructions on the ground can reduce the geostrophic wind speed by 40% to 50%. Over open water or ice, the reduction may be only 20% to 30%. These effects are taken into account when siting wind turbines.
   As the helicopter starts moving into undisturbed air, a portion of the disk is in clean, unaccelerated air, while the remaining portion of the rotor disk is still working on descending air. The part of the disk working on clean air therefore sees a higher angle of attack than the portion of the disk which is working on descending air. This causes a difference in lift between the section in clean air and the section in descending air. The result is that the portion in clean air develops more lift, and the disk tilts as a result. Which way the disk tilts depends on a couple of factors:
   An example of how beneficial this additional lift can be is: a helicopter might be slightly overloaded for take off so that it cannot hover in ground effect. Liftoff can still be achieved if the helicopter has enough of a straight runway to make a "running take off" where the pilot will slowly accelerate the helicopter across the ground on its landing gear until translational lift speed is achieved, extra rotor-disc lift is produced, and the aircraft will begin to climb.
   BULLET::::- if the velocity is parallel to the rotation axis, the Coriolis force is zero. (For example, on Earth, this situation occurs for a body on the equator moving north or south relative to Earth's surface.)
  
  BULLET::::- if the velocity is straight inward to the axis, the Coriolis force is in the direction of local rotation. (For example, on Earth, this situation occurs for a body on the equator falling downward, as in the Dechales illustration above, where the falling ball travels further to the east than does the tower.)
   For helicopters in hover, the center of mass is always directly below the rotorhead. In forward flight, the center of mass will move forward to balance the negative pitch torque produced by applying cyclic control to propel the helicopter forward; consequently a cruising helicopter flies "nose-down" in level flight.
  
  Section::::Applications.:Astronomy.
   BULLET::::- Low-level jet streams. When a nocturnal low-level jet forms overnight above the Earth's surface ahead of a cold front, significant low level vertical wind shear can develop near the lower portion of the low level jet. This is also known as nonconvective wind shear since it is not due to nearby thunderstorms.
  
  BULLET::::- Mountains. When winds blow over a mountain, vertical shear is observed on the lee side. If the flow is strong enough, turbulent eddies known as "rotors" associated with lee waves may form, which are dangerous to ascending and descending aircraft.
   Circuits used to feed sensitive audio/video production equipment or measurement instruments may be fed from an isolated ungrounded technical power system to limit the injection of noise from the power system.
  
  Section::::Power transmission.
  
  In single-wire earth return (SWER) AC electrical distribution systems, costs are saved by using just a single high voltage conductor for the power grid, while routing the AC return current through the earth. This system is mostly used in rural areas where large earth currents will not otherwise cause hazards.
   BULLET::::- As seen from the vantage point of the "earth" (e.g. by a spectator), the rotor (acting like a wind turbine) decelerates the air and drives the wheels against the earth, which it accelerates imperceptibly.
  
  BULLET::::- As seen from the vantage point of the "air stream" (e.g. by a balloonist), the wheels impede the vehicle—"decelerating" the earth imperceptibly—and drive the rotor (acting like a propellor), which "accelerates" the air and propels the vehicle.
   By conservation of mass, the mass flow rate is constant across the slipstream both upstream and downstream of the disk (regardless of velocity). Since the flow far upstream of a helicopter in a level hover is at rest, the starting velocity, momentum, and energy are zero. If the homogeneous slipstream far downstream of the disk has velocity formula_6, by conservation of momentum the total thrust formula_7 developed over the disk is equal to the rate of change of momentum, which assuming zero starting velocity is:
   In 1690, Pierre Varignon assumed that all bodies are exposed to pushes by aether particles from all directions, and that there is some sort of limitation at a certain distance from the Earth's surface which cannot be passed by the particles. He assumed that if a body is closer to the Earth than to the limitation boundary, then the body would experience a greater push from above than from below, causing it to fall toward the Earth.
   BULLET::::- On a rotating planet such as Earth, the strictly gravitational pull of the planet (and other celestial objects such as the Moon, the Sun, etc.) is different from the apparent net force (e.g., on a free-falling object) that can be measured in the laboratory or in the field. This difference is the centrifugal force associated with the planet's rotation. This is a fictitious force: it only arises when calculations or experiments are conducted in non-inertial frames of reference, such as the surface of the Earth.
   Ground resonance
  
  Ground resonance is an imbalance in the rotation of a helicopter rotor when the blades become bunched up on one side of their rotational plane and cause an oscillation in phase with the frequency of the rocking of the helicopter on its landing gear. The effect is similar to the behavior of a washing machine when the clothes are concentrated in one place during the spin cycle. It occurs when the landing gear is prevented from freely moving about on the horizontal plane, typically when the aircraft is on the ground.
  
  Section::::Causes and consequences.
   At some point up the cable, the two terms (downward gravity and upward centrifugal force) are equal and opposite. Objects fixed to the cable at that point put no weight on the cable. This altitude (r) depends on the mass of the planet and its rotation rate. Setting actual gravity equal to centrifugal acceleration gives:
  
  On Earth, this distance is above the surface, the altitude of geostationary orbit.
   The inability to descend in true vertical fashion and unresolved issues as the craft approach the ground, known as “adverse ground effect”, seemed to eliminate consideration of this craft as a VTOL contender. As they investigated the vehicle's behavior as it approached the ground, and became subject to the “ground effect”, they found:
   In amusement parks, pendulum rides and centrifuges provide rotational force. Roller coasters also do, whenever they go over dips, humps, or loops. When going over a hill, time in which zero or negative gravity is felt is called air time, or "airtime", which can be divided into "floater air time" (for zero gravity) and "ejector air time" (for negative gravity).
  
  Section::::Linear acceleration.
   A good earth connection is normally a 6 m stake of copper-clad steel driven vertically into the ground, and bonded to the transformer earth and tank. A good ground resistance is 5–10 ohms which can be measured using specialist earth test equipment. SWER systems are designed to limit the voltage in the earth to 20 volts per meter to avoid shocking people and animals that might be in the area.
   A forward CG will not be as obvious when hovering into a strong wind, since less rearward cyclic displacement is required than when hovering with no wind. When determining whether a critical balance condition exists, it is essential to consider the wind velocity and its relation to the rearward displacement of the cyclic control.
  
  Section::::Incorrect weight and balance in helicopters.:CG aft of aft limit.
  
  Without proper ballast in the cockpit, exceeding the aft CG may occur when:
  
  BULLET::::- A lightweight pilot takes off solo with a full load of fuel located aft of the rotor mast.
   The bottom of the enclosure, if it is not the ground or trays of dirt, needs be a surface more solid than screening. A snail placed in a wire-mesh-bottom pen will keep crawling, trying to get off the wires and onto solid, more comfortable ground.
   Section::::Flight.:Transition from hover to forward flight.
  
  As a helicopter moves from hover to forward flight it enters a state called translational lift which provides extra lift without increasing power. This state, most typically, occurs when the airspeed reaches approximately , and may be necessary for a helicopter to obtain flight.
  
  Section::::Flight.:Forward flight.
   Section::::Performance Parameters.:Displacement.
  
  Displacement is calculated as discharged liters per minute, it is related with the drilling hole diameter and the return speed of drilling fluid from the bottom of the hole, i.e. the larger the diameter of drilling hole, the larger the desired displacement. The return speed of drilling fluid should reach the requirement that can wash away the debris and rock powder cut by the drill from the bottom of the hole in a timely manner, and reliably carry them to the earth surface. When drilling geological core, the speed is generally in range of 0.4 to 1.0 m^3/min.
   BULLET::::- "Medium Earth orbit" (MEO) satellites are closer to Earth. Orbital altitudes range from above Earth.
  
  BULLET::::- The region below medium orbits is referred to as "low Earth orbit" (LEO), and is about above Earth.
  