Low-g condition

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 some horizontal turning maneuvers. This can have a disastrous effect on the aircraft, particularly in the case of helicopters, some of which need the airframe to constantly be under a non-zero amount of load.

Effects

In smaller airplanes

Most smaller airplanes and all gliders have no problems with 0"g" conditions. In fact, it can be enjoyable to have zero gravity in the cockpit. To produce 0"g", 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 Helicopters. In such a situation their rotors may move beyond the 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 semi-rigid rotors, such as the two-bladed design seen on the Robinson Helicopters and some other light helicopters.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 out that it mostly happened during low terrain following flight while 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

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, one has perfect control. 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. 0"g" 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 0"g" 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 A300, for example. NASA has the Vomit Comet. One upside down parabola simulates 0"g" for about 25 s, in the following normal parabola the aircraft is loaded with 2"g".