Now that we’ve gotten the fundamental physics out of the way, we can talk about the fun part: actually shaping our aircraft! I appreciate all of you who have stayed with me so far, especially if you’re not as big of an airplane geek as I am.
So to quickly recap last week (and bring any new readers up to speed—welcome!):
- We can simplify the aerodynamic forces and moments on an airframe by assuming they all act at a single location for a given aircraft: the neutral point.
- To ensure a stable aircraft, we need our center of gravity (CG) to be located in front of the neutral point.
- We only want a certain amount of stability, so that we can still maneuver our aircraft appropriately. We can quantify this stability using the distance between the CG and neutral point, the static margin.
One of the primary ways to move your neutral point around is by changing the location of the wing and tail on the fuselage.
If we’re moving the wing and tail independently, we should probably know the aerodynamic contribution from each. At this point we’ll move from talking about the aerodynamic center of the aircraft, to talking about the aerodynamic center of the wing and aerodynamic center of the tail.
Note that I’m not calling it the neutral point of the wing or tail—the neutral point exists only for the full vehicle.
The majority of wings, when evaluated in isolation, will generate a negative (nose-down) pitching moment. The lift they create also causes a negative pitching moment about the CG.
To balance this, the tail is placed so that it generates negative lift, which creates a positive (nose-up) pitching moment and balances out the wing. Neat!
From here, the effect of moving the wing or tail is relatively straightforward: moving the wing or tail forward or back pulls its lift contribution in the corresponding direction, creating a larger or smaller moment about the CG. Moving the surface closer to the CG creates a smaller moment; moving it further away creates a larger one.
However, things can start to get a little quirky depending on your configuration. There are three main cases I can think of that pose some interesting complications to the standard wing-tail configuration.