How can you tell if the Acceleration of your Craft is positive or negative?

@Toinkove that's a great analogy, pretty close to what I would've used myself. It's basically how a high school physics teacher would introduce the topic of Newtonian physics to class(along with some kind of visual demonstration involving a toy car rolling down sloped track).

To add a bit for clarity, acceleration(and deceleration) is always positive because it requires some type of force effecting the body. An object falls toward the ground because of gravitational force(downward acceleration), the air in it's path imparts drag force(upward acceleration), and when the two balance each other then acceleration is 0(terminal velocity). In the absence of these forces(or any others), then an object would never have any acceleration. This is all regardless of velocity or direction of motion/force applied.

Acceleration is a vector quantity
A=F•m
since F is a directional force(vector), m is mass(scalar), and a scalar times a vector is always a vector in the same direction but of a different length.
Velocity is also a vector and typically represents the path of motion of an object(assuming some outside force doesn't change that), so it's common to accept any acceleration that increases velocity length as positive "acceleration" and any acceleration that decreases velocity length as negative "deceleration". Acceleration can be in any direction though, so it doesn't necessarily mean the velocity vector's length(the actual "speed" of the object) will change with a given acceleration vector, the direction of velocity is instead effected.

If the angle measures between the two vectors
[Angle(v)(a)]
Is less than 90°, then it's likely acceleration, and if the angle is greater than 90°, then it's likely deceleration, but only if the length of v changes respectively.

PS, I realize there's a lot more to it than that, especially since I'm leaving out angular moment and rotational acceleration, but that's waaaaaay beyond anything I'm capable of explaining in a reasonable manner

Also, yes, I am dumb and respond to random posts with entire essay sized comments XP

@DMNK99 if you have a dot product, it will show you how well the two vectors align. I suggested this since you asked for determining the sign value and that is the only was I can think of doing it. Sadly, this means that you will almost never get the full magnitude of acceleration, but it will be close.
When I say normal, I mean the magnitude is being constrained to 1. The operation is basically this in FunkyTrees:
dot(FlightData.Acceleration,normalise(FlightData.SurfaceVelocity))
As I stated earlier, this should give you an acceleration close to the actual value with a sign depending on whether or not it opposes or adds to the current velocity. If this doesn’t work, I can always submit a demo craft.

@DMNK99 No need to divide by timespan if you just want to know if it's positive/negative.

Sorry I can't help with the actual problem at hand, I just don't use Vizzy. But I'm not surprised it always reads Acceleration as positive because this is actually how it works.

@DMNK99 think of it this way .... if you're driving forward, foot on the gas peddle, you're accelerating forward. take foot off the gas, your acceleration forward starts to decrease (what most call deceleration). eventually the car will just be coasting along at a constant speed (or velocity) that would be zero acceleration. But when the car begins to start sowing down (in forward velocity), That's now acceleration to the rear (still have a forward velocity that is decreasing, but rearward acceleration). Acceleration (and velocity) is a vector, always has a numeric unit, like 10 KPH, and a direction, forward/backward/up/down. And is never negative. At least that's the physics aspect of it.

@LeMagicBaguette Can you tell me, how to do that correctly? I used this dot function from the math section put the acceleration in the left slot and the velocity in the right one. I displayed the result in comparison to the acceleration variable and I don’t know, what it gave me there, but it’s not nearly the acceleration. And what did you mean with normal? Thanks.

Dot product of acceleration and velocity (normal).

@PointBreak Thanks, I tried that, but this isn’t really working, since the wait command doesn’t seem to function very precisely.
Because I have to divide the Result of v1-v2 by this timespan.

@Toinkove Yes with positive I meant increasing velocity and with negative decreasing. What did you mean with changing direction? Let’s say a Craft is falling straight down to the center of a Planet, accelerating by it’s Gravity. Then I turn the Engines on which acceleration is greater than that Gravity, then the velocity would decrease, but the direction would still stay the same. Ok after it reaches 0, it would accelerate up again and then change it’s direction. But until that point it wouldn’t. Or did I got anything wrong?

Maybe check the velocity every 0.1s and do v1-v2

Maybe cause Acceleration is Always positive! That's what my physics teacher was always scolding us about! It's the vector (or direction) that changes! Ok well not always positive, it can be 0. instead of positive or negative, maybe increasing or decreasing is what's necessary here?