Generalized force expression for this "airplane" oscillator system?

I am reading a book on aircraft dynamics (Blakelock Aircraft and Missiles) where the author presents the following system on page 408:

Lagrange’s equation is given by:

$$
frac{d}{dt}left(frac{partial T}{partialdot q_j}right)-frac{partial T}{partial q_j}
+frac{partial F}{partialdot q_j}+frac{partial U}{partial q_j}=Q_j,~j=1,2,dots,n,
$$

where $T$ is the kinetic energy, $U$ is the potential energy (including internal strain energy), $F$ is one half the rate at which energy is dissipated, and $Q_j$ is the generalized external force acting on the $j$th station, with $q_j$ representing the $j$th station’s generalized coordinate.

In this case, the author splits each force $P_j$ in the above diagram into a gravitational force $P_{g_j}$ and an aerodynamic force $P_{a_j}$. The gravitation force is of course included in the potential energy term, whereas the aerodynamic force is included in $Q_j$. Furthermore, because the system is symmetric, only half of it is considered. With this in mind, the author says that:

$$
Q_0=P_{a_0}+P_{a_1}+P_{a_2},~
Q_1=P_{a_1},~
Q_2=P_{a_2}.
$$

My question is why is the generalized force $Q_0$ equal to the sum of all the aerodynamic forces? I don’t see how the center station (the airplane fuselage) is different from the other stations. In particular, I would have said that $Q_0=P_{a_0}$.