Assuming the pilot applies the appropriate crosswind flight controls (see below) during takeoff and rotation, and releases those flight controls after the aircraft leaves the ground, in what direction will the aircraft's ground track diverge from the extended runway centerline as the aircraft accelerates from takeoff velocity?
On a recent practical test, I had a DPE insist to me that the aircraft, as soon as it breaks ground, will immediately begin drifting downwind. We then had an extensive discussion, in the oral portion of my examination, as I am of the (I hesitate to use the word "opinion" because this is basic physics of which I have absolutely no doubt), but let me say I attempted to explain to the DPE that in the scenario described, the aircraft would immediately weathervane into the crosswind, to eliminate the sideslip it was in, and thereafter, as it accelerated, its ground track would deviate upwind of the extended runway centerline, not downwind.
I am posting this because I am curious as to the extent of this misconception among the general aviation community. If DPE, certified by the FAA to administer Practical Tests, has this incorrect idea I wonder how many others do not understand the basic concept that wind is not pushing on aircraft in flight, it is but a mathematical concept representing a translation of values from one frame of reference (the surface of the earth) to another (moving) frame of reference, (the airmass around the aircraft). Also, I thought that a discussion, here in this forum would be useful and educational for all.
NOTE. By proper crosswind takeoff controls, I mean of course, downwind rudder (opposite the crosswind), to keep the aircraft tracking down the centerline, and upwind aileron (into the wind), to prevent the crosswind from lifting the upwind wing ahead of the downwind wing at rotation. These cross-controls effectively create a slip at rotation, and allow the aircraft to rotate and liftoff with the fuselage aligned with runway centerline even though, (within the moving airmass) it is flying sideways (the fuselage is misaligned with the relative wind). Of course, as soon as the cross controls are released, the fuselage will rotate (weathervane) into the crosswind and the sideslip angle will be eliminated.
Because so many readers do not understand the basic physics underlying the concept of wind, (which is that in the airmass, there is no wind) I have decided to present this thought experiment.
Imagine you are flying North towards an airfield with a North-South Runway. You are on extended runway centerline, but there is a strong crosswind from the East. You want to track along the runway centerline directly towards the runway, so you establish the appropriate crab angle by turning the aircraft into the wind.
At point A, then, the aircraft ground track is now Due North, perfectly aligned with the runway centerline. The aircraft heading, of course, is slightly to the right of North, by whatever crab angle is required to counteract the movement of the air mass (the "Wind").
Now, at Point B, you decide you might want to land. To minimize the sideloads on the landing gear (or for whatever other reason you may imagine), you want to align the fuselage of the aircraft with the runway, WITHOUT changing your ground track. So you add in the appropriate amount of left rudder to bring the nose to the left so as to align the aircraft fuselage with the runway heading (North). Now, at point C, the aircraft heading, (AND THE AIRCRAFT GROUND TRACK), are aligned with the runway. They are both True North. Because the aircraft is in a left sideslip (or left forward slip), however you want to describe it), adverse yaw due to the dihedral effect will cause the aircraft to slowly turn, (change its heading) very slightly, to the left. To stop this you add a small amount of right bank, which will require some right aileron to maintain. The aircraft is now in a cross-controlled sideslip, but it is stably tracking across the ground due North, with its fuselage and heading aligned due north with the runway.
Now, and here's the question that determines if you understand this or not. Halfway down the runway you neutralize the controls. You are simply returning the flight controls to the same place they were at Point A though Point B. What will happen?
The answer, is that after neutralizing the controls, the aircraft, in EVERY WAY YOU CAN POSSIBLE EXAMINE, is in EXACTLY the same aerodynamic conditions it was in from Points A to Point B.
So, if you still maintain that the aircraft will immediately take up a ground track to the left as is depicted in the diagram along path A, and not simply weathervane into the relative wind coming from the right of its nose, and continue to track straight ahead, along path B, you are wrong, but more than that, you have to be able to explain what is different about the situation after neutralizing the controls at Point D, than between Point A and Point B.
The only thing that can possibly explain someone maintaining this belief is the false concept that the wind is Pushing on the side of the aircraft, and that this (fictitious and non-existent) force is what causes wind drift.
Nothing could be further from the reality. There is no force. Bodies only change their velocity when a force is applied. $F=ma$ is true, always was true, and always will be true. Since "wind" is fictitious, it cannot exert a force on anything free to move in the airmass. Since there can be no force from wind (it is only an abstraction representing the difference between measurements in two frames of reference moving with respect to one another, there can be no change of velocity due to a wind. The law of inertia says that bodies will always continue to move at a constant velocity unless a FORCE is applied. Without a force, the aircraft will continue to maintain a constant inertial velocity.