Low-G condition
Encyclopedia
Low-g condition is a phase of aerodynamic flight
where the airframe is temporarily unloaded. The pilot—and the airframe—feel temporarily "weightless
" because the aircraft is in free-fall
or decelerating vertically at the top of a climb. It may also occur during an excessively rapid entry into autorotation. This can have a disastrous effect on the aircraft, particularly in the case of helicopters, some of which need the rotor to constantly be under a non-zero amount of load.
Most smaller airplanes and all gliders have no problems with 0g conditions. In fact, it can be enjoyable to have zero gravity in the cockpit. To produce 0g, the aircraft has to follow a ballistic flight path, which is essentially an upside down parabola.
This is the only method to simulate zero gravity for humans on earth.
In helicopters
In contrast, low-g conditions can be disastrous for helicopter
s. In such a situation their rotor
s may flap beyond normal limits into a condition known as mast bumping, which can cause the rotor to droop and shear the mast, 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 helicopter
s.
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 0g, 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
Low-g conditions can also affect fixed-wing aircraft
in some instances, mainly by disrupting the airflow over the wings, making them difficult or impossible to control via the aerodynamic surfaces.
The controllability of an airplane by the control surfaces only depends on airspeed. So, if one keeps airspeed, control is retained. Usually the controllability is even increased, because there is no need to produce lift. However, despite increased lift, g-forces will hinder aerobatic flight as velocity increases, making the compromise between speed and g-forces necessary. 0g forces are a minimal problem for fixed wing aircraft, but there are exceptions, including, but not limited to, airplanes with gravity-fed fuel systems, as well as airplanes with low airspeed. An airplane with low airspeed at 0g will stall until airspeed is recovered.
, for example. NASA
has the Vomit Comet
. One upside down parabola simulates 0g for about 25 s, in the following normal parabola the aircraft is loaded with 2g.
Flight
Flight is the process by which an object moves either through an atmosphere or beyond it by generating lift or propulsive thrust, or aerostatically using buoyancy, or by simple ballistic movement....
where the airframe is temporarily unloaded. The pilot—and the airframe—feel temporarily "weightless
Weightlessness
Weightlessness is the condition that exists for an object or person when they experience little or no acceleration except the acceleration that defines their inertial trajectory, or the trajectory of pure free-fall...
" because the aircraft is in free-fall
Free-fall
Free fall is any motion of a body where gravity is the only force acting upon it, at least initially. These conditions produce an inertial trajectory so long as gravity remains the only force. Since this definition does not specify velocity, it also applies to objects initially moving upward...
or decelerating vertically at the top of a climb. It may also occur during an excessively rapid entry into autorotation. This can have a disastrous effect on the aircraft, particularly in the case of helicopters, some of which need the rotor to constantly be under a non-zero amount of load.
Effects
In smaller airplanesMost smaller airplanes and all gliders have no problems with 0g conditions. In fact, it can be enjoyable to have zero gravity in the cockpit. To produce 0g, the aircraft has to follow a ballistic flight path, which is essentially an upside down parabola.
This is the only method to simulate zero gravity for humans on earth.
In helicopters
In contrast, low-g conditions can be disastrous for helicopter
Helicopter
A helicopter is a type of rotorcraft in which lift and thrust are supplied by one or more engine-driven rotors. This allows the helicopter to take off and land vertically, to hover, and to fly forwards, backwards, and laterally...
s. In such a situation their rotor
Helicopter rotor
A helicopter main rotor or rotor system is a type of fan that is used to generate both the aerodynamic lift force that supports the weight of the helicopter, and thrust which counteracts aerodynamic drag in forward flight...
s may flap beyond normal limits into a condition known as mast bumping, which can cause the rotor to droop and shear the mast, 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 helicopter
Robinson Helicopter
The Robinson Helicopter Company, based at Zamperini Field in Torrance, California, is the largest manufacturer of civil helicopters in North America.-History:...
s.
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 0g, 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
Low-g conditions can also affect fixed-wing aircraft
Fixed-wing aircraft
A fixed-wing aircraft is an aircraft capable of flight using wings that generate lift due to the vehicle's forward airspeed. Fixed-wing aircraft are distinct from rotary-wing aircraft in which wings rotate about a fixed mast and ornithopters in which lift is generated by flapping wings.A powered...
in some instances, mainly by disrupting the airflow over the wings, making them difficult or impossible to control via the aerodynamic surfaces.
The controllability of an airplane by the control surfaces only depends on airspeed. So, if one keeps airspeed, control is retained. Usually the controllability is even increased, because there is no need to produce lift. However, despite increased lift, g-forces will hinder aerobatic flight as velocity increases, making the compromise between speed and g-forces necessary. 0g forces are a minimal problem for fixed wing aircraft, but there are exceptions, including, but not limited to, airplanes with gravity-fed fuel systems, as well as airplanes with low airspeed. An airplane with low airspeed at 0g will stall until airspeed is recovered.
Use in space agencies
Every space agency uses a modified passenger jet (especially stronger engines) to simulate a low-g condition. The ESA uses an Airbus A300Airbus A300
The Airbus A300 is a short- to medium-range widebody jet airliner. Launched in 1972 as the world's first twin-engined widebody, it was the first product of Airbus Industrie, a consortium of European aerospace companies, wholly owned today by EADS...
, for example. NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
has the Vomit Comet
Vomit Comet
A Reduced Gravity Aircraft is a type of fixed-wing aircraft that briefly provides a nearly weightless environment in which to train astronauts, conduct research and film motion pictures....
. One upside down parabola simulates 0g for about 25 s, in the following normal parabola the aircraft is loaded with 2g.