To preface, I know about ~nothing about aerodynamics, but have been reading through wiki pages half the morning, but am still not quite sure how to answer a question. Basically my question is concerning the turbulent flow that happens behind an aircraft, maybe just focusing on the wing for now. My understanding is that laminar flow typically takes place on regular (say commercial) aircraft wings, and a colleague of mine (who a long while back majored in Aerodynamics) stated that the turbulent flow *behind* the aircraft was very important. Specifically he was citing the pressure differential between the front and end of the wing, and stated that it was a considerable factor relative to thrust. My understanding is that turbulence generates lower pressure than laminar flow behind the wing (wrong maybe), such that in that regard turbulent flow would effectively slow down a plane more than laminar flow. Alternatively there is the effective cross section of a wing, maybe also described as the flow displacement extending beyond the physical wing. (I can't remember what your term is for it). Turbulent flow in general should lower this is my understanding. Creating early turbulent flow is one way, and turbulent flow from behind the wing the other possibility. So the heart of my question is actually which of the two processes (or maybe something else), is the dominant force on planes (obviously drag is dominant, but next leading in order). I'd guess it would have to be geometry and speed dependent, so any guidelines would be useful. Thanks in advance. Hopefully my post is comprehensible -- I don't feel comfortable using a lot of the standard terminology.