The common references to choose a racquet usually do not include the frame shape, as we have seen for years with professionals and amateurs. Even professionals do not know how much their fundamentals change from using different shapes for their frames. Players choose racquets using weight, overall shape, size, number of strings, and, not very often, swing weight (inertia) as reference. We have never seen anybody talking about Drag Force.
D = Cd x P/2 x S x V²
Where Cd = racquet drag coefficient, p = air density, S = racquet surface area, V = racquet speed, and D = Drag force.
Players never heard of it. Different shapes have different aerodynamics resistance coefficient (Cd). On a rectangular frame shape, this coefficient is much higher than an airplane wing frame, over 25 times. The lower the coefficient the easiest the racquet will cut through the air. It may sound like and advantage, but it damages the quality of baseline strokes. The speed of racquet head is so high that players have to break the movement and change the direction of the racquet trajectory to keep the ball in court, to be consistent and to have control over the ball. It is similar to a very high inertia racquet, where the player cannot control the racquet head because it is too heavy. Check our previous post.
Even though this is a very short explanation for a very complex and technical knowledge, the conclusion for your tennis lessons is that if a player wants a heavy baseline stroke with lots of control, he has to choose frames as close as possible to a rectangular shape. Those frames, together with a well-balanced weight and inertia, allow fluidity and continuity of the movement, furthermore, a follow through in the same direction you hit the ball.