Factors determining g force

[Speedbird744]

What are the factors that determine how high the g force is on takeoff in a heavy jet like the 747-400?

[Pielander]

'g' force is centripetal acceleration caused by changing the direction of motion of a body. (Newton's law states that a body will continue to move at constant speed in a straight line unless pushed). On rotation, for example, the aircraft moves in an arc rather than a straight line, so it is constantly being pushed into changing direction. Centripetal acceleration is given by the formula:

a = v^2/R

where:
a = centripetal acceleration
v = speed
R = radius of the turn
(All in S.I. units)

So, the only things that affect 'g' force on rotation are the speed of the plane and the radius of the rotation (or how hard the pilot pulls back).

Hope this doesn't patronise you.

Pie

[Zeke]

Simply from spooling up to rotation going along the runway….

Acceleration = Force/Mass

Force = Thrust – [Drag +Friction]

Mass = Aircrafts mass (not weight)

During the roll down the runway, the nett Force is decreasing, thrust decreases, drag increases, and friction decreases, and the mass decreases (fuel burn).

The jet thrust decreases slightly during the roll, the magnitude of [Drag +Friction] increases during the roll, [drag increases as lift increases, friction decreases as lift increases].

The major factor effecting acceleration is the mass of the aircraft, a 10% increase in mass would give a 9% decrease in acceleration, a 5% increase in takeoff velocity, and greater than 20% takeoff distance.

A down sloping runway would give have the aircraft appearing to have additional thrust, and reduced friction force, 1% slope would increase the thrust term by 1%.

Higher density altitudes reduce the thrust produced by the engines, therefore reduced acceleration, 1000 ft increase in density altitude give about a 2% decrease in acceleration, and about 7% increase in takeoff distance.

The outside air temperature also effects acceleration as the high ambient temperature lead to higher compressor inlet temperatures, the smaller the temperature rise available in the engine, the less thrust you can produce.

:rolleyes: