Please click here first for some illustrations (otherwise it might be difficult to get my point across). The aerodynamic lift on the wing of an airplane (airfoil) is generally explained by the argument that the faster speed of the air along the top of the wing leads to reduced air pressure there and hence produces a lift (Bernoulli's Law). Using this argument, one should also expect a lift for a symmetric wing profile as shown in Fig.1. However, if one considers the problem from a microscopic point of view, one comes to a different conclusion: upward and downward forces should exactly cancel for a symmetric wing profile. This is easy to see if one simplifies the situation and replaces the curved wing surface by two plane sections (Fig.2): If the wing is stationary, the pressure on all parts of the wing is identical, i.e. there is no lift. If the wing is moving in the indicated direction, the front half of the upper wing surface experiences an increased pressure because of the increased speed and number of air molecules hitting it (due to the orientation of the surface, this creates a downward force). On the other hand, the rear half experiences a reduced pressure because the of the reduced speed and number of air molecules hitting it (creating a lift). Overall, there is consequently no lift, but only an anti-clockwise torque. It is obvious that an overall lift is only achieved if the rear section of the wing has a larger area than the front section, i.e. one would get the maximum lift for the profile in Fig.3 (and this is (schematically) actually how airplane wings seem to be designed (see for instance http://www.zenithair.com/kit-data/ht-87-6.html). On the other hand, the reverse situation (Fig.4) should lead to a downward force, although Bernoulli's Law would again predict a lift. Note: the above arguments assume that the lower surface of the wing is always parallel to the velocity vector, i.e. the pressure acting on it is unchanged; by varying the 'attack angle' of the wing the amount of lift can of course be arbitrarily be changed and one could even generate a lift for the bottom image.