Let's talk flaps. We all use them, we check them during the preflight and run-up and most aircraft have them. Why?

 

There are many variations of flaps, but for this discussion I will discuss the ones applicable to almost all of the aircraft in the SFS fleet. The Cessna 152/172 fleet has Fowler flaps and the SportStar has split flaps. The main difference in these flap systems is the way it affects the aerodynamics of the wing and pitch characteristic of the aircraft.

 

The Fowler flap, named after it's inventor, not only comes down into the airstream, but also moves back on the flap tracks. This backward motion is the key difference between the two flaps discussed here and helps explain why there is a dramatic pitch change with flap deployment on the Cessnas and virtually none on the Sports Star. As the Fowler flap is deployed, it increases the wing chord (distance from the wing leading edge to trailing edge) as it moves backwards and camber (curvature) as it moves down. The increase in chord effectively gives the wing a greater surface area to generate lift while the increase in camber increases the lift coefficient - the ability of the wing to generate lift as a function of the airflow. This combination reduces the stalling speed of the aircraft. BUT...as discussed in last months article, any surface generating lift also generates induced drag. As a rule of thumb, on the Cessna Fowler flap system anything past 20 degrees of flap setting generates more drag then lift. This is why maneuvers requiring maximum lift such as short or soft field takeoffs use no more then 20 degrees of flaps but in landing, when we want to have a slow controllable airspeed, we use full flaps, making use of the added drag to keep the speed down as we pitch down on the glide path to the runway. 

Blue Skies,