What is the physics of crosswind landings for large airplanes?

Question

This always confuses me from a physics perspective. Upon touchdown, what is the effect of the wind on a large airplane/airliner (Boeing 737 and above)? Below are three cases I put together to make it easier to explain my confusion:

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No decrab, the wind acts on the tall fin yawing to plane further into the wind at touchdown, but the pilot anticipates it and counters this by pushing the rudder downwind (video).

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The pilot decrabs, overcooks it (rudder remains deflected), and the correction is rudder upwind initially (video). Based on the previous analysis, the wind would have helped to correct it.

You see in the video the second time they are facing upwind, the pilot lets the wind correct it before settling on rudder downwind like the previous example.

So I have two cases that say eventually it's rudder downwind, and this case in particular shows the yaw effect on the fin (with and without using the rudder).

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What I normally read: the wind pushes the aircraft, and the correction is rudder upwind. This AOPA article for instance, shows a drawing with the wind force pushing a [small-er] plane downwind.

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Apologies if I butchered the crosswind landings. To summarize:

When the wind acts on the tall fin upon touchdown, is there a yaw effect, or does the wind push the airplane downwind? And what is the rudder correction to be anticipated?

Answer 1

When the wind acts on the tall fin upon touchdown, is there a yaw effect, or does the wind push the airplane downwind? And what is the rudder correction to be anticipated?

Let's assume a strong crosswind at 90° from the right with a 747 on a dry, uncontaminated runway for illustration purposes.

Once the airplane is firmly on the runway, the wind is acting on the whole airplane, with the crosswind pushing it downwind, and given the greater side force aft of the c.g. (especially because of the tailplane) than the side force forward of the c.g., there is at the same time a net yaw effect wanting to rotate the airplane about the c.g. into the wind.

Perhaps its best to think of the rudder pedal displacement not just as where you want the rudder, but also what you want the nose gear to do. While there were some 747s that had no connection between the nose gear steering and the rudder pedals, most did (at least the ones I flew). Now think of what you have to do to get the airplane where you want it, pointed in the direction you want, and actually traveling in the direction you want.

What you want to wind up with is the airplane's longitudinal axis on the runway center line, nose pointed straight down the center line, and the airplane actually moving straight down the center line. Let's say you've succeeded in getting the airplane to that state. To keep it, in our strong 90° right crosswind,you would use right aileron to keep the wind from getting under the upwind wing so to speak, and some left rudder. How much would depend on whether you had rudder pedals that connected to the nose gear steering, your speed, your weight and when you started using the tiller for steering. I forget the details of the connection between the nose gear steering and the rudder pedals, but it's not linear, and as I remember you can have the nose gear tracking straight ahead even though you are using some rudder.

On a 747, once you've slowed down to tiller speed, you would use it, and on a dry, uncontaminated runway, the main and wing gear will keep you from moving sideways and the nosegear will control your direction of movement, at least in the strongest of winds I operated in, say 40-45 knots or so.

Chances are, once you are firmly on the runway after touchdown, you will not be exactly on the center line nor pointed precisely down the runway. Your first and second illustrations are dealing with getting there from a less than ideal touchdown, and if I understand them, they are correct. If to the right of the center line, you can allow a drift leftward. If to the left, you don't have a lot of room to spare so you need positive action to get to the center line.

Answer 2

Bottom line, wind does not "push" on the aircraft. The most accurate and consistent way to think about wind is that it is simply motion of the entire air mass with respect to the ground. Until the wheels hit the runway, the aircraft does not "feel" wind. It is flying in the air mass exactly the same (with respect to the air mass) irrespective of how fast the airmass moves across the ground. Landing on an aircraft carrier deck is a good analogy to make this clear. If there were 60 knots of wind, but the boat is traveling with the wind at 60 knots across the ocean, then a sailor standing on the deck would "feel" no wind, and the aircraft landing on the deck would behave, in every respect, exactly as it would if landing at a land-based runway in calm wind.

The issue is simply what direction the aircraft is pointing in, and the consequences of hitting the runway with the inertia of the aircraft velocity relative to the ground pointing in a different direction than the direction the wheels are pointing in. It's exactly the same problem you would have if you executed a ski jump and landing with the skis pointed 20 degrees off to one side of the direction you were moving in. When the skis hit the snow at an angle to the direction they are moving across the snow (or the aircraft wheels hit the runway cocked off to one side of the runway alignment, there will be a sideload placed on the wheels, that will exert a force on the aircraft that may cause it to flip over, or damage the undercarriage. In fact, in some aircraft (The B-52 is an example), the wheels were mounted on casters that allowed them to free wheel from left to right, and which allowed the aircraft to land, safely, with the fuselage misaligned (but the wheels aligned), with the aircraft ground track.

So, what the pilot is doing when he adds rudder just before touchdown is, simply, making an attempt to align the fuselage with the aircraft ground track so that these sideloads on the landing gear will be minimized. If he does this too soon, and does not cross control to establish and maintain a slight bank opposite the sideslip angle, the rudder will induce a slight turn rate (change of heading) that, if not corrected, will cause the aircraft to turn, and then drift, downwind. This is why this rudder input is executed at the last possible moment, just before touchdown. If done earlier, the upwind wing must be lowered by cross controlled aileron, to prevent this downwind heading change and drift.

But the objective is simply to get the aircraft fuselage aligned with the ground track, which, if the approach is flown properly should be aligned with the runway centerline. Once the wheels hit, of course, friction of the wheels with the ground will keep the aircraft ground track moving down the runway in a straight line.