So we’ve established that our center of gravity needs to be in front of our neutral point for us to have a good flight and not have our nose pitch up until we stall. Great!
But where does it actually need to be?
This is where the true business of aircraft design comes in, and also where working alongside other disciplines is key.
Our actual, physical center of gravity is going to depend on all the things going in the aircraft: autopilot, engine, servos, all the way down to the snakeskin on your electrical harnesses. And these things need to go in certain places to actually be useful—it doesn’t help to have all your servos stacked in the nose of the plane when they’re meant to be moving your elevators.
So we only have so much wiggle room when it comes to the physical mass properties of the aircraft. If our CG can’t move much, then, what do we do?
We flip our thinking, and design our aircraft to move the neutral point instead. If the neutral point is a single fixed location unique to an aircraft design—well, change the design, and the neutral point moves!
The other stability influences I’m going touch on are what impact the neutral point’s location. Once you have a configuration locked down, the actual location of your CG tweaks your stability—but we needed the full understanding of what the neutral point represents, and how the CG and neutral point interact, in order to get to the fun design part.
What’s our target, though? Where should the neutral point be, exactly?
Aerodynamicists like to have fancy names and numbers for basic algebra, so the term for the distance between the neutral point and the CG is called the static margin.
Because this term describes the moment caused by lift acting at a distance from the CG, it is effectively a direct measure of an aircraft’s pitch, or longitudinal, stability. The larger the margin, the more stable the airplane.
But if you remember what I said in a previous email, we only want a certain amount of stability, because too much can lead to problems. Typically, a crewed transport aircraft will have a static margin of 5%-10%. This means that the distance between the CG and neutral point is between 5% and 10% of the wing chord (cross-sectional length). If your wing chord is two feet, your desired distance between these two points is from 1.2 inches to 2.4 inches. (For my metric friends, a one-meter wing chord would have a static margin of 5 to 10 cm.)
So we know where the CG and neutral point have to be relative to each other. But we still don’t know what ideal CG location to give our mechanical team!
For any aircraft configuration, we can basically assume that the wing generates the vast majority of our lift. Our tail and fuselage have small contributions, but because the wing is generating the bulk of our lift, our neutral point/aerodynamic center/center of lift will be somewhere in the wing. That is, if you sliced the airplane like a loaf of bread at the aerodynamic center, you’d have to slice along the entire span of the wing.
If we know our neutral point is somewhere within the wing, and our CG needs to be a certain distance in front of it, that has substantially narrowed down our design condition.
This is where the iterations begin: the aero designer chooses a target CG to design for, the mechanical team comes back with an achievable CG, and the aero designer adjusts to accommodate.
But what can they even adjust? What are their knobs to turn?
Turns out, a whole lot.