**2.3.4.4 Determination of the aerodynamic force**

After unknown circulations *i* are obtained, velocity at every point of the flow field is known, and we can use them for the determination of aerodynamic forces that act on the blade. The calculation aerodynamic force is necessary for the defining of the blade position at the next moment of time. The total aerodynamic force is calculated as the sum of forces acting on all panels. Aerodynamic force acting on a single panel can be defined by:

$$
\vec{F}\_i = \rho \vec{V}\_{\oplus} \times \Gamma\_{i(\mathcal{G}.)} \stackrel{\rightarrow}{BC}
$$

where *BC* is bound circulation vector and effective circulation can be defined by using:

$$\Gamma\_{i(\mathcal{cf.})} = \Gamma\_i + \frac{1}{V\_{\infty}} \frac{\partial}{\partial t} \int\_{LE}^{M\_i} \mathcal{\gamma} dl = \Gamma\_i + \frac{1}{V\_{\infty}} \frac{\partial}{\partial t} \left( \sum\_{k=1}^{i-1} \Gamma\_k + \frac{\Gamma\_i}{4} \right)$$

where *Mi* denote point at quarter-chord position on the *i*-th panel.

After determination of aerodynamic forces, moment of aerodynamic forces *MA* , necessary for blade flapping equation, can be calculated as sum of moments acting flapping hinge.
