# Negative to a Positive Gravitational Force

1. Dec 28, 2009

### EA6BMech

I am with an aircraft squadron and we have a conditional inspection for aircraft that pull an excessive amount of "G's". The inspection is to check significant parts of the aircraft that might crack or buckle under certain amounts of stress. There is an arguemt in the shop about going from a negative "G" (Forces pulling in one direction) to a positive "G" (Forces pulling in another direction). If the aircraft is flying and is pulling -2 G's then changes direction and pulls +3 G's... How many "G's" were pulled. Some say 5 G's and others say 3 G's. Can someone help us out and maybe provide a reference for this issue? Thanks!

2. Dec 28, 2009

### Astronuc

Staff Emeritus
The change in force would correspond to a change of ~5G, but when going from -2g to +3g, the significance of the sign is the direction of force.

Take a beam fixed at two ends. Pushing up with 2g (-2g) would cause if to bow up. The top would be in tension and the bottom in compression at the point of loading. Then press down with 3g (+3g) and the beam bows downward, and the top is in compression and the bottom in tension under the point of load.

So with the reversal in acceleration, some parts will experience a change from compression to tension, or change in amount of tension, and a reversal of the direction of shear.

Is one referring to stationary parts or moving/rotatig parts?

3. Dec 28, 2009

### EA6BMech

We would be moving because we are flying in an aircraft. And just for simplicity we would just be going up and down. The debate is if you were flying in a negative G but then you change direction to put positive G on the aircraft. I do understand that a "G" force (- or +) is just directional (ie up or down such as on the "suspension beam" you used for a visual). Does this help?

4. Dec 28, 2009

### EA6BMech

Here is something else I would like to add just to get the point across. If you start at no G's and go to 3 G's then go from -2 G's to +3 G's... Are you still only putting 3G's of force on the aircraft?

5. Dec 28, 2009

### Lsos

The "G" is a measure of acceleration. If the plane was accelerating at -2Gs and then goes to +3Gs...then the maximum acceleration was +3G. If you plot a graph of velocity vs. time, and you derive the acceleration, the maximum acceleration will be +3G. If you look at the forces experienced by the airplane, the maximum will be 3x the force of gravity.

I can see how somebody might be tempted to add the accelerations and come up with 5Gs, but it doesn't work that way. You might as well accelerate at 3Gs for ten seconds, and then claim that you actually accelerated at 30Gs because 3x10 is 30....