The mechanical behaviour of silica aerogels is studied by the three points flexural technique. The Young's modulus (E), the fracture strength (S) and the toughness KIC are investigated as a function of the different preparation parameters. It is demonstrated that the evolution of mechanical characteristics as a function of the apparent density follows scaling laws. The evaluated exponents are 3.7, 2.6 and 1.6 for E, S and KIC respectively. The experimental data are discussed in terms of the percolation theory.