Author's note. This post turned out to be wrong. I have described Induced Drag - not Parasitic Drag. However I'll leave it up a) as a lesson in induced drag and b) as a reminder that the ATPLs are full of little traps like this to trip up the unwary... and I certainly just fell for it!
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You are probably aware of the concept of Drag, being that force which opposes thrust. The Engines Push, (or pull), the aircraft forwards and the drag tries to hold the aircraft back.
The TOTAL drag is comprised of two elements:
Form Drag and
Parasitic Drag.
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Form Drag is, (as the name suggests), due entirely to the effects of a solid body ploughing through the air. The precise components which make up form drag are mainly to do with displacement of the airflow, friction across the skin and localised turbulence around protrusions or other irregularities on the skin surface.
Parasitic Drag is a bit more complex, but is entirely a function of the
LIFT generated by the aerodynamic surfaces, (primarily the wings of course). There is no such thing as a perfect wing, because there are so many different factors which determine its design. It can only be described as the best compromise for all the factors required.
But one thing they all have in common is that they are joined to the airframe at the root and end in a tip which is essentially in "Free Air". In general terms, Lift is generated by the contour of the upper surface which is curved in an
"Aerofoil" shape.
In the cruise, the wing presents itself to the oncoming air in cross section, with the airflow meeting the leading edge at an optimum angle, determined as the point where the ratio between lift and total drag is at a maximum. LD Max in fact, (hence my name!)
As the airflow accelerates over the top surface faster than the bottom surface, it creates a pressure differential where the air pressure on the top of the wing is lower than on the bottom. This is LIFT. Ideally, the
Lift will equal the
Weight, (Straight and Level Flight), and the
Thrust will Equal the
Drag, (Constant Speed).
However, since the wing is unattached at one end, the higher air pressure
UNDER the wing will be drawn towards the lower air pressure
ABOVE the wing, and this encourages something called
"Spanwise Flow", where a proportion of the air flows from the root towards the tip. As it traverses the wing tip, it causes turbulent flow called
"Wing Tip Vortices", which are
MASSIVE currents of spiralling air which trail behind the aircraft much like a wake.
These vortices contain a LOT of energy, (enough to turn another aircraft over if too close behind!), and by the law of conservation of energy, we can see that any nett loss of energy must come from the engines! Thus the mechanics of LIFT conspire to oppose the thrust - in other words it is a type of DRAG.
Thus when an aircraft is
HEAVY, it needs more lift from the wings AND when it is
SLOW, it needs a greater angle of attack to generate the required amount of lift, and which departs from the optimum angle required for LD Max.
This type of drag is called "Parasitic" because it is dependant entirely on the amount of lift being generated, and is at the very worst when the aircraft is slow and heavy, i.e. during takeoff! Next worst is when landing!
There are some devices employed to minimise parasitic drag, the most obvious being the current trend for upturned wingtips. Other devices include "wing fences", which are exactly as the name suggests, and which are maybe a couple of centimetres high, running from the leading to trailing edge. Several of these can be deployed in different positions along the span of the wing. Whatever device is used, the objective is to discourage the
spanwise airflow, minimise wingtip vortices and reduce the amount of parasitic drag.
Some simplifications were necessary for a concise answer, so don't anyone "slam" me for a few generalisations!
Hope that helps