Previously I’d mentioned that there are a number of different factors that influence an aircraft’s stability. Today I want to talk about a major one, which is the location of the center of gravity (CG).
I want to start here because understanding the influence of CG on stability provides some fundamental principles that make understanding other influences a lot easier.
We define the CG as a single location on the vehicle through which the full weight of the aircraft acts. Essentially, if you were to represent the force of gravity (i.e. weight) acting on the aircraft as a single down-facing arrow, that arrow would start at the CG and point downwards from it.
We can do something similar with the lift force acting on the aircraft. Each component of the vehicle—wing, tail, fuselage—is generating some amount of lift, but to make things easier we can sum all of these contributions up into a single lift vector, i.e. a single arrow pointing up from the aircraft. We place that lift vector similarly to how we placed our weight vector, basically at the center of lift of the full airplane.
What’s often confusing is that this center of lift vector has multiple names, some of which are used in other situations, so it’s really important to be clear on what precisely to call it. We can refer to it as:
- The aerodynamic center of the aircraft: the single point at which we can assume all the forces and moments are acting
OR
- The neutral point: as no matter the angle of attack of the aircraft, the moment about this point is the exact same—it is neutrally stable
This neutral point is a single, unchanging location on the aircraft for a given design, entirely independent of your CG.
If this is the point where your lift vector is pushing you up, and that lift does change with angle of attack, while your weight vector is at a different location and pulling you down—then it probably makes sense that your airplane is going to want to rotate about its center of gravity more or less, depending on where that CG is relative to the neutral point…right?
Exactly.
But now we need to figure out where the CG needs to go.