BECAUSE OF THE COMPLEXITY of predicting the forces on an airfoil resulting from penetration of a traveling blast wave, a simple approximation tha t is frequently made is to represent the blast wave as a traveling gust, neglecting the associated overpressure. The resulting lift-growth function can be conceived of as resulting from two causes: growth of circulation and a noncirculatory part which, in incompressible flow at least, can be attributed to mass. The circulatory lift reaches a steady-state value whereas the apparent mass effect has a finite impulse. Those who have used the traveling gust representation of the blast wave have invariably been aware of the associated overpressure and the diffraction of the blast wave about the airfoil. The force associated with the undiffracted overpressure is proportional to the area of the airfoil subtended by the blast a t any instant, and thus yields a finite impulse which is proportional to the volume. For a thin airfoil this impulse is negligible. The force associated with the diffraction of the blast wave, on the other hand, is not negligible, and it is not clear how much of this diffraction is accounted for b}^ the traveling-gust representation. I t is the purpose of this note to show tha t when a blast wave of any magnitude is represented by a traveling gust and the impulse is computed using apparent-mass concepts, the result is equivalent to accounting for diffraction of the blast wave. Since the familiar lift-growth functions for a traveling gust include apparent-mass effects as well as the circulatory lift growth, these functions are applicable directly to the blast-penetration problem because the diffractive loading has already been taken into account. Criscione and Hobbs, using a simplified model based on shocktube investigations estimate the diffractive loading on a thin airfoil by a shock wave (see Fig. 1). Inasmuch as the airfoil is thin and stationary, forces both due to undiffracted overpressure and growth of circulation are absent. The model of Criscione and Hobbs has subsequently been partially justified theoretically by Ehlers and Shoemaker. > Assuming for simplicity tha t all shock and rarefaction waves travel at the speed of the incident wave (an assumption slightly different from tha t made in reference 1), the average pressure differential is