question: Why does it get harder for helicopters to climb the higher they get?
comment: The elicopters climb moving a certain amount of air downwards. As you go up, the air become less dense, so to move the same volume of air they must do more work
passages: Helicopter manufacturers try to reduce this differential effect (that is, aim for more equality of lift along the blade length). This has two main aspects:
  
  BULLET::::1. tapering a blade toward its tip, which reduces its surface area, in turn reducing its lift;
  
  BULLET::::2. twisting the blade (commonly called geometric twist) so that the blade root near the hub presents a higher angle-of-attack, thus higher lift.
  
  When the helicopter is travelling forwards with respect to the atmosphere, a further phenomenon comes into play, dissymmetry of lift.
   The lowest NOE flying is by helicopters because they have lower speeds and more maneuverability than fixed-wing aircraft, particularly fast-jets. Helicopters can fly at treetop levels or even below the height of surrounding trees where there are clear areas (such as in river gullies), flying under wires (such as electricity cables) rather than over them. Attack helicopters can hide behind trees or buildings, "popping up" just enough to use their (rotor mast-mounted) radar or other sensors and then minimally exposing themselves to launch weapons. The required altitude depends on the type of terrain, such as low vegetation or low buildings, tree canopy layer of less than , presence of tall buildings or concrete pylons with heights of up to international standard, or emergent tree layer canopy and lattice steel electricity pylons to being present. Over populated areas with tall radio transmission towers, an altitude of AGL applies.
   When dissymmetry causes the retreating blade to experience less airflow than required to maintain lift, a condition called retreating blade stall can occur. This causes the helicopter to roll to the retreating side and pitch up. This situation, when not immediately recognized can cause a severe loss of aircraft controllability.
  
  Section::::Analysis.
   The collective pitch control or "collective" is located on the left side of the pilot's seat with a settable friction control to prevent inadvertent movement. The collective changes the pitch angle of all the main rotor blades collectively (i.e. all at the same time) and independently of their position. Therefore, if a collective input is made, all the blades change equally, and the result is the helicopter increasing or decreasing in altitude.
   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.
   Most helicopter designs compensate for this by incorporating a certain degree of vertical "flap" movement of the rotor blades. When flapping, a rotor blade will travel upward during its advance, creating a lesser angle of attack (AOA) and therefore lesser lift. When the blade retreats, the blade falls downward again, increasing the AOA and therefore generating greater lift.
  
  There are three general designs. The earliest, and by far, least common design today, is the fully rigid rotor system; the blades are rigidly fixed to the rotor hub but made of a flexible material that allows some degree of flap.
   tends to cause the rotor disc to pitch up. As the angle of
  
  attack increases, the center of pressure moves forward.
  
  If it moves ahead of the pivot point, the pitch of the
  
  rotor disc decreases. Since the angle of attack of the
  
  rotor blades is constantly changing during each cycle
  
  of rotation, the blades tend to flap, feather, lead, and
  
  lag to a greater degree.
  
  Section::::Limitations and hazards.
   The collective pitch control, or "collective lever", is normally located on the left side of the pilot's seat with an adjustable friction control to prevent inadvertent movement. The collective changes the pitch angle of all the main rotor blades collectively ("i.e.," all at the same time) and independent of their position. Therefore, if a collective input is made, all the blades change equally, and as a result, the helicopter increases or decreases its total lift derived from the rotor. In level flight this would cause a climb or descent, while with the helicopter pitched forward an increase in total lift would produce an acceleration together with a given amount of ascent.
   In contrast, low-g conditions can be disastrous for helicopters. In such a situation their rotors may flap beyond normal limits. The excessive flapping can cause the root of the blades to exceed the limit of their hinges and this condition, known as mast bumping, can cause the separation of the blades from the hub or for the mast to shear, and hence detach the whole system from the aircraft, falling from the sky. This is especially true for helicopters with teetering rotors, such as the two-bladed design seen on Robinson helicopters.
   Wind velocities increase at higher altitudes due to surface aerodynamic drag (by land or water surfaces) and the viscosity of the air. The variation in velocity with altitude, called wind shear, is most dramatic near the surface.
   As the airspeed increases without an increase in height, there comes a point where the pilot's reaction time would be insufficient to initiate a flare, and prevent a high-speed ground impact. Each increase in height, increases the pilot reaction time. This is the reason the bottom right part of the H–V curve has a shallow gradient. If above ideal autorotation speed, a pilot can avoid the deadman's curve by flaring, converting airspeed into height, and increasing rotor RPM through coning.
   BULLET::::- 7 November 2010 – A Eurocopter AS 350 of Fishtail Air crashed when it was on a rescue mission on the North ridge of Mount Amadablam at an elevation of 6,350 meters. A five-member investigation team was formed on 9 November 2010 and its started its investigation one day later. The final report stated that the main cause of the accident was the inability of pilots to notice and arrest the lateral drift of the chopper to its left due to lack of ground reference until the main rotors came into contact. The accident was covered by the Swiss documentary "The mountain rescuers in the Himalayas".
   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::::- Reduced take-off power hampers an aircraft's ability to climb. In some cases, an aircraft may be unable to climb rapidly enough to clear terrain surrounding a mountain airport.
  
  BULLET::::- Helicopters may be forced to operate in the shaded portion of the height-velocity diagram in order to become airborne at all. This creates the potential for an uncontrollable descent in the event of an engine failure.
  
  BULLET::::- In some cases, aircraft have landed at high-altitude airports by taking advantage of cold temperatures only to become stranded as temperatures warmed and air density decreased.
   This effect was first discovered when many accidents with Bell UH-1 and AH-1 helicopters occurred. These particular helicopters simply crashed without any obvious cause. Later, it was found that these accidents usually happened during low terrain flight after passing a ridge and initiating a dive from the previous climb.
  
  Articulated and rigid rotor systems do not lose controlling forces up to 0"g", but may encounter this depending on their flapping hinge offset from the mast. However, dangerous situations, as with a teetering rotor, may not occur.
  
  On fixed-wing aircraft
   As the climb progresses, the rate of climb decreases as thrust reduces due to reducing air density. A gradual climb improves forward visibility over the nose of the aircraft.
  
  Aircraft also climb when flying in a zone of rising air, but since such zones are unpredictable and inconveniently located, and since most are poorly adapted to passive climbs of this type, only gliders attempt such climbs on a regular basis.
  
  The opposite of a climb is a "descent".
  
  Section::::"Normal" climb.
   Improved rotor efficiency resulting from directional flight in a helicopter is called translational lift. The efficiency of the hovering rotor system is greatly improved with each knot of incoming wind gained by horizontal movement of the aircraft or surface wind. As the incoming wind produced by aircraft movement or surface wind enters the rotor system, turbulence and vortices experienced in hovering flight are left behind and the flow of air becomes more horizontal. While transitioning to forward flight at about 16 to 24 knots, the helicopter goes through effective translational lift (ETL). The rotor blades of the helicopter become more efficient as forward airspeed increases. Between 16 and 24 knots, the rotor system completely outruns the recirculation of old vortices and begins to work in relatively undisturbed air.
   Air vortices can form around the main rotor of a helicopter, causing a dangerous condition known as vortex ring state (VRS) or "settling with power". In this condition, air that moves down through the rotor turns outward, then up, inward, and then down through the rotor again. This re-circulation of flow can negate much of the lifting force and cause a catastrophic loss of altitude. Applying more power (increasing collective pitch) serves to further accelerate the downwash through which the main-rotor is descending, exacerbating the condition.
  
  Section::::Other examples.:Vortex rings in the human heart.
   In forward flight, there is no upward flow ("upflow") of air in the hub area. As forward airspeed decreases and vertical descent rates increase, an upflow begins because there are no airfoil surfaces in the mast and blade grip area. As volume of upflow increases, the induced flow (air pulled or "induced" down through the rotor system) of the inner blade sections is overcome and the blades begin to stall near the hub. As the inner blade sections stall, a second set of vortices, similar to the rotor tip vortices, form in the center of the rotor system, and combined with the outer set of vortices, result in severe loss of lift. The failure of a helicopter pilot to recognize and react to the condition can lead to high descent rates and ground impact.
   BULLET::::- Atmosphere — In order to minimize gravity drag the vehicle should begin gaining horizontal speed as soon as possible. On an airless body such as the Moon this presents no problem, however on a planet with a dense atmosphere this is not possible. A trade-off exists between flying higher before starting downrange acceleration, thus increasing gravity drag losses; or starting downrange acceleration earlier, reducing gravity drag but increasing the aerodynamic drag experienced during launch.
   Helicopter pilots are most commonly taught to avoid settling with power by monitoring their rates of descent at lower airspeeds. When encountering settling with power, pilots are taught to apply forward cyclic to fly out of the condition or lowering collective pitch. While transitioning to forward or lateral flight will alleviate the condition by itself, lowering the collective to reduce the power demand decreases the size of the vortices and reduces the amount of time required to be free of the condition. However, since the condition often occurs near the ground, lowering the collective may not be an option; a loss of altitude will occur proportional to the rate of descent developed before beginning the recovery. In some cases, vortex ring state is encountered and allowed to advance to the point that the pilot may lose cyclic authority due to the disrupted airflow. In these cases, the pilot's only recourse may be to enter an autorotation to break the rotor system free of its vortex ring state.
   This kind of aerial maneuver originated in the Swiss Alps. In 1970, a mountain guide with Air Zermatt performed the first longline mountain rescue on north face of the Eiger. This mission forever changed mountain rescue operations 
  
  Section::::System Components.
  
  The HFRS system has multiple components which ultimately safely suspend a rescuer under the helicopter (extended below the skids) with two distinct actions required to release the load, and load bearing redundancy. The components include:
   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.
   The wind blows faster at higher altitudes because of the reduced influence of drag. The increase in velocity with altitude is most dramatic near the surface and is affected by topography, surface roughness, and upwind obstacles such as trees or buildings. However, at higher altitudes, the power in the wind decreases proportional to the decrease in air density. Rendering significantly less efficient power extraction by the wind turbines, requiring for a higher investment for the same generation capacity at lower altitudes.
   Vertical wind shear above the jet stream (i.e., in the stratosphere) is sharper when it is moving upwards, because wind speed decreases with height in the stratosphere. This is the reason CAT can be generated above the tropopause, despite the stratosphere otherwise being a region which is vertically stable. On the other hand, vertical wind shear moving downwards within the stratosphere is more moderate (i.e., because downwards wind shear within the stratosphere is effectively moving against the manner in which wind speed changes within the stratosphere) and CAT is never produced in the stratosphere. Similar considerations apply to the troposphere but in reverse.
  