\begin{document} In this paper we show that the oscillatory motion of a wing in an incompressible inviscid fluid can determine the apparition of a propulsive force. In the framework of the linearized theory the dimensionless lifting surface equation for oscillatory wings is :% $$ \frac \varpi {4\pi }\int \int_D^{*}\tilde f(\xi ,\eta )\exp (-i\tilde \omega (x-\xi ))\left( \int_{-\infty }^{x_0}\frac{\exp (i\tilde \omega u)}{% (u^2+\varpi ^2(y-\eta )^2)^{3/2}}du\right) d\xi d\eta = $$ $$ =-\left( \frac{\partial h(x,y)}{\partial x}+i\tilde \omega h(x,y)\right) , $$ where $Re[\tilde f(x,y)\exp (i\omega t)]$is the pressure coefficient, $% \omega $ is the frequency of the oscillation, $\widetilde{\omega }$ is the reduced frequency, and $z=h(x,y)\exp (i\omega t)$ is the equation of the wing. Employing adequate quadrature formulas, we discretize the integral equation and we obtain the values of $\tilde f$ in the nodes of the grid. For certain oscillatory delta wings we calculate the drag coefficient and we notice that if $\omega $ surpasses a critical value, the drag coefficient becomes negative i.e. there appears a {\it propulsion force}. \end{document}