Yesterday I talked about how the location of the wing and tail impact the stability of an airplane, and how the tail’s purpose is to balance out the moment generated by the wing.
For most conventional aircraft, the tail generates negative lift. In these aircraft, the wing’s aerodynamic center is at or behind the center of gravity of the entire vehicle.
Remember, we’re rotating around the CG: if the only forces/moments are from the lift of the wing and tail, that means we need the tail’s lift and moment to balance out the wing’s lift and moment. So, we end up needing negative lift on the tail.
In some cases though, the aerodynamic center of the wing will actually be in front of the CG of the aircraft. This shakes things up a bit.
A handful of the configurations I’ve worked on have had this more unique configuration: the lift of the wing is acting in front of the CG and generates a nose-up moment. To maintain a stable aircraft, the tail needs to create an opposing, nose-down moment from its location behind the CG. The tail generates positive lift here, not negative.
This has led to some minor disagreements with team members who are also private pilots: the aircraft they’ve flown all had CGs forward of the wing’s aerodynamic center, so they’ve learned as a general rule that the tail always needs to generate negative lift.
They were confused, and a little concerned, when I talked about our UAV’s tail generating positive lift—until we both realized it was because the UAV’s CG was actually behind the wing’s aerodynamic center! Somehow, despite working with that airframe for five years, this hadn’t been obvious to me until I had someone making me prove that it made sense.
This is a great example of why it’s so important to understand the actual physics behind any aircraft design choices, and the in-flight behaviors they cause—especially if you’re working with unconventional, even unique, configurations.