Is induced drag essentially nothing more than a specific type of form drag?
No, by definition it isn't.

There is drag. Drag is caused by different physical phenomena. According to the cause, it is classified to:

Induced drag

Induced drag is side-effect of generating lift over finite wing span.

Lift is an upward force that the air exerts on the wing. By principle of action and reaction (Newton's third law of motion), the wing exerts downward force of equal magnitude on the air, and since the air is free to move, that force accelerates the air downwards.

Now there are two equivalent arguments why this causes drag:


In the frame of reference of the wing, the wing is not doing any work (because it is not moving), so it can't change kinetic energy of the air, and therefore can't change its total velocity. Since the vertical component of that velocity increases, the horizontal component has to decrease and this requires forward force from the wing. The backward reaction to that force is the induced drag.
In the frame of reference of the oncoming air, the kinetic energy does increase. Therefore the wing has to do some work on the air, which means applying force in direction of its motion. The backward reaction to that force is the induced drag.


The explanations are both equally true, just using different frame of reference.

Induced drag decreases with (square of) speed (for constant lift), because at higher speed there is more air to accelerate, so it only needs to be accelerated by less.

Induced drag is also independent of cross-section. If you cancel lift by moving elevator or aileron up, the cross-section may not change at all (the flap might still be behind the wing), but the induced drag will vanish.

By the way, it follows from this that induced drag of wing at 90 degrees is zero, because it is not producing any lift. The form drag is, of course, huge.

Form drag

Form drag is caused by imperfect pressure recovery behind the body. At the front of the object moving through fluid the fluid slows down, which comes with increase of pressure. In ideal laminar flow the fluid closes behind the object with the same slow speed and same high pressure. But as the object moves faster, the inertia causes the flow to separate and form a turbulent area behind the object where the pressure remains low. This pressure difference cause the form drag.

The form drag depends on cross-section and shape of the object. It always increases with speed.

Interference drag

When complex structure, like an aircraft, is analysed, the form drag of each component (wing, fuselage, empennage etc.) is first analysed independently. But as the components are assembled together, the pressure fields around the parts negatively affect each other and this increase in drag is then called interference drag.

Skin drag

Skin drag is caused by the friction between the surface of the object and the fluid. The skin drag depends mainly on wetted surface of the object. It increases with speed in similar way to the form drag, so for practical purposes it is usually combined with the form drag into one term, the parasitic drag.

Wave drag

As the object moving through fluid approaches the speed of sound, the flow exceeds it as it tries to avoid the object and forms a shock wave. Behind the shock wave the pressure is much higher and this increase is not balanced by any increase on the aft part of the object, therefore causing large amount of drag.

This kind of drag only appears as the flow speed approaches speed of sound, but then it quickly grows larger than the other forms.