I am relatively new to aerodynamics and am curious to know what difference it makes on an aircraft - lift, drag, weight etc - which of the two airfoils is used on the wing.
the lift generation mechanisms of these two wing designs are quite different.
Insects use completely flat wings to great advantage. They generate both thrust and lift by shedding vortices in a flight regime in which the wing is operated right at the threshold of flow separation. Dragonflies operate there, and can accelerate from zero to 10 meters/second in one second or less, fly inverted, and execute unbanked turns at full speed.
Cambered wings are used by birds and airplanes and generate lift using (your choice) models based on vorticity and rotational flow, the bernoulli effect, or van Karman vortex shedding.
The droplet shaped airfoil has multiple advantages.
- It provides space for storing stiffening structures. The flat plate foil is very thin, so would need to be quite heavy to maintain stiffness, as stiffness is proportional to the square of the thickness.
- It provides space for storing fuel. This is important to reduce weight on modern cantilever aircraft. Distributing weight along the wings reduces the bending caused by the heavy fuselage during flight. On the ground, the extra weight (and bending) from the fuel is little, since the fuel is not as heavy as the fuselage!
- It provides good performance across a range of angles of attack. (Angle of attack is the difference between the direction of motion and the direction you're facing.) The round leading edge allows air to smoothly flow over it when this angle is high, for example during climb or slow flight.
- It gives gentle stalls. (A stall is when increasing the angle of attack does not increase lift force anymore, and instead decreases it.)
Flat plate wings will stall suddenly and violently as soon as the angle of attack needed to stall is reached, as the flow separates across the whole wing at once. (Flow separation is when a region of air moving in reverse develops on the wing, causing the airflow on the wing to flow around this region, which creates less lift and more drag.)
But the greater curvature on the top of the teardrop foil causes flow separation to happen at the rear of the wing first, as angle of attack continues to approach the value needed for stall, warning the pilot that any further increase will stall the plane.
But for supersonic applications, the teardrop foil does not work very well. At these speeds, the extra drag from the bluntness begins to outweigh any other benefit. So, an thin, sharp foil, not quite a flat plate, but pretty close, is used.