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. enter image description here 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.

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    $\begingroup$ Are you actually asking a question, or do you just want people to tell you you're right? $\endgroup$ – Dan Hulme Jun 21 '18 at 13:48
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    $\begingroup$ Gutsy move arguing with a DPE, and I'm curious if you've validated your theory empirically. Anyway, to make this question clearer, it might help to spell out exactly what you believe the "appropriate crosswind controls" are. It's not clear - to me! - what what technique you're describing, and "weathervaning" is usually used to describe something that happens on the ground, not in the air. $\endgroup$ – Pondlife Jun 21 '18 at 14:00
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    $\begingroup$ @CharlesBretana - If you remove the "I am posting this not because I am in doubt as to the answer" paragraph, it will be a good question and will invite answers. This site not being a forum, is not the best place for open-ended discussions, the chat however is. (Don't shoot the messenger.) You can also then submit your own detailed answer. $\endgroup$ – ymb1 Jun 21 '18 at 14:47
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    $\begingroup$ The DPE is right, and this is incredibly easy to verify. On your next flight take off in a cross-wind and then center the controls. The aircraft weathervanes into the wind, yes, but look at the runway and notice that you are physically ground-tracking in the downwind direction. The only way to track upwind is to turn more into the wind, your aircraft will drift with the air mass it is flying in. This is basic piloting, and exactly why you need to take wind into account when computing headings to a destination. $\endgroup$ – Ron Beyer Jun 21 '18 at 14:57
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    $\begingroup$ Let us continue this discussion in chat. $\endgroup$ – Charles Bretana Jun 21 '18 at 15:46

The DPE is correct, the airplane will drift in the direction of the wind.

If you are piloting a boat across a river, and point the bow at the opposite side of the river with the rudder amidships then the boat will float in the direction of the current and stay where you pointed it. The boat is not going to turn upstream by itself because the water exerts equal force against the bow and stern.

The same physics is at work in an airplane, air exerts an equal force on the front and rear of the airplane, it's not going to somehow blow harder on the tail.

  • $\begingroup$ "it's not going to somehow blow harder on the tail" Well, technically, I suppose it can, but hopefully not for long enough to matter for the purpose of what OP is describing. Think downdrafts, windshear, and similar weather phenomena which can cause very distinct shifts in wind passing through various boundary layers (both vertical and horizontal). $\endgroup$ – a CVn Jun 21 '18 at 14:00
  • $\begingroup$ Building turbulence can yaw you around for sure, as I have experienced plenty of times @MichaelKjörling, but as you say never for very long. I may refine this answer a bit, I wanted to keep it simple. $\endgroup$ – GdD Jun 21 '18 at 14:04
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    $\begingroup$ @CharlesBretana, your position contravenes physics and aerodynamics as taught and my own 15 years of piloting with hundreds of crosswind departures, however I'd be happy to accept it's wrong if you can put together a case for it. Or perhaps you haven't expressed your question in a way the community understands, either way I'd say you have some work to do to prove your point. $\endgroup$ – GdD Jun 21 '18 at 14:32
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    $\begingroup$ I know my physics @CharlesBretana, and I know why the rudder is at the back. I think what you are describing is actually happening due to your own control inputs and/or gyroscopic/slipstream effects creating unbalanced flight, which is potentially hazardous in slow flight. Transitioning between crossed controls and balanced flight is a skill that comes with practice, concentrate on keeping the ball in the middle. $\endgroup$ – GdD Jun 21 '18 at 14:59
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    $\begingroup$ @CharlesBretana, releasing the controls will not center the ball, you need rudder to do that! Please, please be willing to learn from others and accept you may not understand this fully. In any case extended discussion is discouraged, feel free to open a chat with the community. $\endgroup$ – GdD Jun 21 '18 at 15:13

You drift downwind unless you turn into it. In a normal crosswind takeoff, say with the wind from the left, at rotation, you have left aileron to keep the left wing from lifting and right rudder to maintain runway heading during the rotation. You are taking off in a sideslip. As soon as you are clear, you center the controls to take out the sideslip while still on the runway heading and you now are in ball-centered coordinated flight, on runway heading through an air mass moving from left to right.

The airplane won't weathervane into the surface wind because it's no longer anchored to the ground; it weathervanes into its own relative wind as it normally does. You are now on runway heading in coordinated flight in an air mass moving from left to right. The airplane's relative wind is straight ahead if the ball is centered. Where is the airplane's track? To the right of the heading. You will see it as a drift downwind of the runway centre line as you climb out. There may be a very brief weathervaning tendency in the instant following weight off wheels as inertia resists the lateral drift, but that is quickly overcome.

The proper technique on a VFR takeoff is to perform a coordinated turn into the crosswind once airborne to establish a crab angle that will allow you to maintain the runway track.

The only time you don't do this crab correction is on an IFR takeoff where you are given a departure heading. You maintain that heading and let ATC worry about any drift.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Federico Jun 28 '18 at 5:06

I believe I see where the confusion is so I will attempt an answer to the question.

This Wikipedia article describes the weathervane effect:

Aircraft on the ground have a natural pivoting point on an axis through the main landing gear contact points [disregarding the effects of toe in/toe out of the main gear]. As most of the side area of an aircraft will typically be behind this pivoting point, any crosswind will create a yawing moment tending to turn the nose of the aircraft into the wind.2[3]

The keyword here is that there is a pivoting point which allows rotation. When the plane has a pivot point it actually acts as a really good weathervane because more of the force from the wind is exerted on the large vertical stabilizer and much less force is exerted on the nose. See this cool picture of a DC-3 acting as a weather vane at Yukon Transportation Museum:

enter image description here

However when there is no pivoting point (the plane is in the air) things now change. The side forces on the vertical stabilizer will still be greater than the nose but with no fixed point of rotation the airplane will slip downwind (hence the term slip in aerodynamics) so the ground track would move downwind (hence the DPE is right I am afraid). The nose would eventually line up with the relative wind (if the forces remain constant) but not before the plane is blown off course. The rudder is essentially used to yaw the plane into the relative wind (which with a crosswind would be angled away from your ground track) to prevent the slip from happening and preventing you from being blown off course. The technique to use the rudder to correct for the crosswind is called crabbing and is essentially a way to stay in coordinated flight when there is a crosswind to your ground track.

In general the rudder is used to align the nose with the relative wind and bank is used to turn the aircraft in relation to the ground track. When the nose is aligned to the relative wind the plane is in coordinated flight.

  • $\begingroup$ in the air, any body has a natural pivot point, about which any & all applied moments will cause the body to rotate. This point is called the center of Gravity. The law of conservation of momentum requires that unless an external force is applied, the total momentum must remain constant. The total momentum is the product of the velocity of the body tines the mass of the body. If the CG was allowed to change it's velocity the total momentum would be changing. Without an applied force, that would violate the momentum conservation law. $\endgroup$ – Charles Bretana Jun 22 '18 at 0:05
  • $\begingroup$ When a force is applied all across the surface of a body in free space, (like from a relative wind not aligned with the fuselage), it's change in momentum from that force affects the trajectory of the CG, its attitude, or orientation is affected by the relative distribution of the forces - forces applied in front of the CG will cause a rotational moment away from the direction of the applied force, force applied behind the CG will cause the nose to rotate n the same direction as the applied force. $\endgroup$ – Charles Bretana Jun 22 '18 at 0:07
  • $\begingroup$ in the air, the CG IS the "Fixed point of rotation". Your statement "... use the rudder to correct for the crosswind is called crabbing and is essentially a way to stay in coordinated flight when there is a crosswind ..." is wrong. The rudder MIS-aligns the aircraft with the relative wind. It aligns the aircraft with some ground reference line that is not the same as the natural ground track when the air mass is moving laterally across the ground under the ground track. Use of the rudder (except when used to correct for adverse yaw), creates uncoordinated flight, not coordinated flight! $\endgroup$ – Charles Bretana Jun 23 '18 at 14:07

Your opinion is a theory. DPE is correct. In the real world, you lift off, crab into the wind and will still move downwind of the centerline. You see this over and over, no airplane ends up on the upwind side of the extended centerline.

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    $\begingroup$ Of course some do when they overcompensate. I witnessed one which strayed into a glider area this way and collided with a glider, right after take-off. $\endgroup$ – Peter Kämpf Jul 27 '18 at 13:04
  • $\begingroup$ No, an aircraft with 5 degrees of crab in a wind requiring only 3 degrees of crab, will move UPWIND. You cannot seriously believe that aircraft will move downwind of a fixed ground reference when it is heading, (pointed), well upwind of that ground reference line. But your statement above indicates you don't even understand the real issue here. It is not whether an aircraft in a "crab" will move upwind or downwind, It is whether an aircraft will weathervane into an existing relative wind when it breaks ground on liftoff, and by how much will it weathervane. $\endgroup$ – Charles Bretana Jul 28 '18 at 13:24
  • $\begingroup$ You might as well state that an aircraft will move "downwind"of its ground track. Statements like this or like yours only indicate a failure to understand the fundamental principals of physics (not a theory) behind what is nothing more than a transformation between two moving frames of reference. $\endgroup$ – Charles Bretana Jul 28 '18 at 13:35

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