This is the total sum of all drag generated by the aircraft itself. The form of the fuselage, wings, skin friction and interference of air flowing along parts all create drag holding the aircraft back. As air is a viscous medium and it acts like a fluid flowing around the aircraft structure and it also tends to stick to the surface.
This small layer is called the boundary layer and can it be laminar or turbulent in flow causing drag when lift is generated. The amount of drag depends on the form or shape, surface condition and the speed of the aircraft.
The aircraft builder has some limited influence in these forms of drag but by clever thinking he can reduce this as much as possible and count on more performance or fuel savings. We have two basic forms of parasite drag and we will take a look into each of them.
When air flows over a surface its viscosity tends to keep the air to adhere to that surface creating a small area where the speed of the air slows down from the free air speed to almost stand still. This is the boundary layer and the airflow can be laminar or turbulent.
The initial flow over a surface starts out to be laminar. It is like layers of air moving over each other with increasing speed and this layer is only about 2 mm thick, or thin if you like. Aircraft like Lancair / Pipistrel use this laminar airflow over the wing (chord wise) to be able to fly so fast. It requires though, that the wing be kept as clean as possible as rain, small bugs, dent and such are capable of disrupting this layer and create drag.
After a certain distance along the chord the flow becomes turbulent and all layers of air start to mix with each other creating small eddies or swirls. The point where that happens is the transition point and the turbulent layer becomes much thicker, about ten times and it contains a lot of energy. It is this energy that vortex generators try to use to enhance/re-energize the airflow so it sticks longer to the wing with larger angle of attacks (slow flight).
A laminar airflow means that layers of air are smoothly flowing over the surface and each other with low skin friction drag as a result.
The angle of attack (AOA) of a wing determines when this turbulent boundary layer becomes detached from the wing (at the separation point) and then it creates even more drag. And when it does, sometimes it even reverses flow going forward!
This consist of skin friction drag and form drag. Skin friction drag results from a turbulent airflow over any surface, be that wings or fuselage. Form drag is caused by the form an object has and resulting drag is a force created by the forward facing surface into the airflow.
With increasing speed of an aircraft, the amount of air flowing past it also increases and inside the boundary layer there are larger changes in velocity. This causes more shear and more drag. The shape of a wing determines where the laminar flow becomes turbulent, you will find it usually around the point of maximum wing thickness.
While easily done for the wing but for the fuselage it is quite difficult keeping the airflow laminar, and skin friction is a larger factor with increasing speeds.
Maintaining a laminar airflow means paying attention to the surface condition (smoothness) of the lift producing parts. But there are also huge advantages in keeping the whole of the aircraft as smooth as possible. Regular washing and waxing does really help in that regard.
Imagine a flat plate perpendicular to the airflow creating a lot of form drag. Now place that plate parallel to the airflow and form drag is almost nil, but in this situation skin friction drag plays the bigger part in the total drag from this plate.
Streamlining reduces form drag. Gradual curvatures smoothly guide the air around the aircraft without creating turbulence so that form drag is kept to a minimum. A good example are the wheel pants on the landing gear, these can increase cruise speeds up to 5 kts.
At some point in time it was discovered that parasite drag was greater than the sum of skin friction and form drag. It seemed that when airflows from various surfaces (wing and fuselage) meet, the result will be more drag from these interfering air flows.
By filling these turbulent areas with a fairing the interference drag is much less or even zero. This way the designer of the aircraft controlled the pressure differences and the airflows blends together more smoothly and as a result reducing the turbulent wake and drag.