# What is the purpose of an inboard aileron? [duplicate]

I've noticed that larger Boeing aircrafts (such as the 777, 767, 747, etc) are equipped with inboard ailerons. What is their purpose and why hasn't Airbus incorporated them on the A330 or the A340?

• the question is actually having 2 parts. The need of inboard aileron (duplicate) and why Airbus' designs are not including it (new one). I will propose to not close the question but reformulate it making the link to the answer to the first question – Trebia Project. May 5 '15 at 8:53
• Airbus aircraft have no inboard ailerons because they move the flaps slightly differently: Whereas Boeing aircraft move them perpendicular to the hinge line, Airbus planes move them straight back. This avoids any interference between flaps on wing sections of different trailing edge sweep and the need for filling the gap with a flaperon. – Peter Kämpf May 9 '15 at 11:14

As far as I know, the inboard ailerons are used for high speed maneuvering. The rolling moment induced by the ailerons is linearly dependent with the distance to the center of gravity (e.g. the farther out, the stronger the moment, the harder the roll). As the aileron is an aerodynamic surface, the force induces is equal to: $$F_{aileron} = c_l(\alpha) \frac{1}{2} \rho V^2 S_{aileron}$$. The moment induced is equal to this force, multiplied by the distance to the c.o.g.: $$M_{aileron} = y_{aileron} \cdot c_l(\alpha) \frac{1}{2} \rho V^2 S_{aileron}$$ If you're flying in cruise , and you want to make a smooth turn, you need a relatively small $M_{aileron}$.
However, the $V^2$ term, is already very large. So we want to make the rest of the terms relatively small.
We could do this with outboard ailerons: Use the outboard aileron with a large $y_{aileron}$, combined with a small $c_l(\alpha)$, usually the outboard $S_{aileron}$ is fixed by the low speed control requirements. However, you can provide $M_{aileron}$ more accurate if you use the inboard ailerons (with a small $y _{aileron}$). The smaller distance makes the moment a bit smaller, and less susceptible to small deviations.