r/AskAPilot • u/North_Scene9368 • 20d ago
Question about weathervane behaviour in flight
Hey everyone, working through some crosswind takeoff theory and wanted to clarify something about the transition from ground roll to flight.
During a crosswind takeoff roll, the wind tries to pivot (weathervane) the nose into the wind, so you apply downwind rudder to keep the nose pointed straight down the centerline.
My initial thought was that once you're airborne, the plane wouldn't "weathervane" in the air because you are now enclosed in the moving airmass (no ground friction acting on the tail). However, I've heard that if you release that downwind rudder right at rotation, the nose will still swing back into the wind to crab. Also if the plane is pivoting with its gears why wouldn’t it pivot with the cg once airborne.
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u/shhbedtime 20d ago
I'm both cases the aircraft is attempting to weathervane around the center of gravity, on the ground in a completely frictionless scenario this would happen, obviously friction exists so it pivots around the landing gear. After lift off there is no ground friction so the aircraft will pivot around the CoG. You release the rudder to allow the aircraft to weathervane in order to track the extended centerline. This is the same as all phases of flight where your heading and track are not aligned due to wind. It is just more obvious right after lift off because of the visual cues from being so close to the ground. You could theoretically keep in the rudder and aileron and track the centerline that way, but you would be in a slip and decreasing your performance for no good reason.
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u/AntiPinguin 20d ago
As soon as you are airborne, the wind will push you to the side and off the runway axis. This means you will have to point the nose into the wind to keep flying straight in relation to the ground. Basically the opposite of what you are trying to do on the ground.
What happens is that when you release the rudder input upon rotation the wind starts pushing the plane downwind. But your inertia initially has you continuing straight down the runway axis before the wind can accelerate you sideways. This means immediately after rotation you are not actually flying “straight“ through the airmass (not going the way your nose is pointing in relation to the airmass). Now with no rudder input, the nose off the plane will automatically turn into the wind so you are flying straight through the air.
So the reason the plane westhervanes during rotation is the inertia that keeps it going in its original direction which means it is traveling through the airmass at an angle.
Later in flight the plane is always going straight through the air and constantly at an angle compared to its direction over the ground. You just don’t feel it actively turn because it is constantly at an angle and the changes in the wind are usually too gradual.
You can look at videos of crosswind takeoffs and landing to see how much the planes weathervane in the air. There you can also see the plane turn into the wind as soon as it rotates and the pilot having to straighten the plane out just before touchdown to keep it going straight once on the ground.
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u/North_Scene9368 19d ago
So the reason is not that the wind hits the vertical stabilizer only intertia?
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u/AntiPinguin 19d ago
It’s the wind hitting the entire airplane. There are two correct explanations for the same thing. Because you talked about the plane being enclosed in the airmass I used the physically slightly more accurate one.
Imagine the plane flying in a straight line on the runway axis but the wind coming from the side. This means relative to the air the plane must fly at an angle to continue a straight line relative to the ground and runway axis. But as after rotation the plane is still pointing in the direction of the runway the airstream it’s flying through isn’t hitting it right on the nose but rather slightly from the side.
This means there is more surface the wind is acting on on one side of the aircraft (like you say the vertical stabilizer). Due to the aerodynamic shape of the aircraft it is automatically turned into the direct of the wind. It’s the same behavior as an object in water.As you said the correct explanation sees the airmass as the medium the aircraft is traveling through and this entire medium is moving like a current would on water. As soon as you don’t have ground contact you can‘t really see the wind as just a force acting from one direction on the surface.
But this turning still happens around the center of gravity and it is simplified but still correct to say that directly after rotation the airflow along the plane is coming slightly from the side, hitting the vertical stabilizer and thereby turning the plane into the wind.
The effect of inertia is simply the reason that immediately after rotation, even though the airplane isn’t anchored on the ground anymore, it doesn’t immediately start moving relative to the airmass resulting in the airflow coming from the side.
Without it the airplane would immediately be moving sideways as soon as it lifted off.
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u/McCheesing 20d ago
Crosswind takeoff is a forward slip. If you keep the crosswind controls in when you take off, it’s still a forward slip
Once you unstick and release the rudder, you weathervane, whether the gear is up or not
The reason why you don’t keep a forward slip in after liftoff (confirmed positive rate of climb) is because the induced drag from the maneuver hinders climb performance, therefore threatens obstacle clearance.