Form Drag

                                                                     Form Drag

Form drag is experienced in the surface of the aircraft when the streamlined airflow separates and becomes turbulent. To experiment form drag keep a flat plate on the streamlined airflow, now the pressure before the plate is atmospheric and the pressure after it is below the atmospheric which results in sucking effect behind the plate and vortices are formed.

It is mandatory to delay the separation point of airflow and this is achieved by altering the shape or streamline of the given object. When the airflow changes its direction rapidly form drag experienced is higher.

A fairing is fitted around the fixed undercarriage leg to reduce form drag to considerable extent. The resultant form drag depends on the length and maximum thickness of the streamlined object. The ratio of length to maximum thickness is called the fitness ratio.

Fitness ratio = a/b

At subsonic speeds fitness ratio of streamlined shapes is 4to 1 and gives least form drag, but these parameters may vary without increasing drag.

Airflows around the streamlined object and separate at a point where it no longer continues to be streamlined, which is called separation point. As the increase in angle of attack separation point moves steadily forward towards leading edge. It moves further forward when the form drag is greater.