Measuring G Forces

An accelerometer is the main tool used for measuring G Force. The prime focus of this page is to shed light on how to measure g force. Read on.

An accelerometer is a damped mass on the end of a spring in its simplest form. It is capable of measuring the distance the mass has moved on the spring along the axis or in a particular direction. Accelerometers are often standardized to measure g-force along one or more axes. For example, a stationary, single-axis accelerometer is adjusted so that its measuring axis is horizontal. Therefore, its output will show G Force measurement to be 0 g, and will continue to be 0 g if it is placed in a vehicle moving at a constant velocity on a level road. But if the automobile driver brakes sharply, the accelerometer will give a reading of about −0.9 g, which corresponds to a deceleration. However, the jerk due to a change in motion in the vehicle and gravity pull of the ground on the accelerometer should not be looked at as the same thing.

But if the accelerometer is turned around by 90°, so that its axis points upwards, it will calculate G Force to be +1 g upwards even though the vehicle is still stationary. Here, the accelerometer is exposed to two forces: the gravitational force and the ground reaction force of the surface it is placed on. Remember that the accelerometer can measure only the latter force, due to mechanical interaction between the accelerometer and the ground.

During a free fall in an airplane the accelerometer does not calculate the earth’s force of gravity. The reading given by the device is the acceleration it would have if it were solely subject to that force. The accelerometers are designed to measure only the mechanical components of accelerations, and thus calculate G Force directly.

Let us look into the three-axis accelerometer, for calculating G Force. It will give the output zero-g on all three axes if it is dropped. Otherwise it is put into a ballistic trajectory, which is also known as an inertial trajectory, so that it experiences "free fall”. This is what the astronauts experience while in orbit. Some popular amusement park rides offer several seconds at near-zero g force. “Vomit Comet” of NASA also offers near-zero g for about 25 seconds at a time.

Measuring G Force in a single-axis accelerometer adjusted in an airplane so that its measurement axis is vertical, will give a reading of +1 g when the plane is stationery. This is the "g-force" applied by the ground. But when the airplane is flying at a constant altitude, the accelerometer will continue to give the G Force measurement of 1 g. Now, the g-force is provided by the aerodynamic lift, acting in place of the ground to prevent the plane from free-falling. Under these conditions, the upward force which acts upon the pilot’s body is the normal value of about 9.8 newtons per kilogram (N/kg). This is provided by his seat, which is supported by the lift of the wings. If the pilot pulls back on the stick so that the accelerometer gives a reading of 2 g, the g force calculation acting upwards on him will become double to 19.6 N/kg.