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Originally Posted by moxiepilot  The max x-wind is determined by the surface area of the rudder because that surface yaws the aircraft. However, the surface area of the rudder is not the only condition. It would also have to be based on max gross weight because the rudder must be able to yaw the mass of the aircraft. |
Your logic is really addressing the maximum
possible crosswind, not max
demonstrated, as Midlifeflyer said. Even for the max possible crosswind, gross weight isn't really a factor for rudder authority for two reasons:
1) If the increased mass were located at the CG, there would be no difference in the yawing capacity of the aircraft, since it yaws *around* the CG. What you would really be interested in is the distribution of mass, not its absolute quantity. This is addressed by the concept of
moment of inertia. A large moment of inertia means that the mass is distributed toward the extremities of the object, rather than at the center. (Think of a dumbbell.)
2) Even if there were a high moment of inertia (resistance to yawing), it would merely affect the
acceleration of yaw, not the maximum quantity of yaw achievable. The aircraft would respond sluggishly to the rudder, but it would respond.
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This is why a 747 rudder the size of a cessna still only allows a max x-wind of 38 kts.
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The use of the rudder during a crosswind landing is to neutralize the tendency of the vertical stabilizer to weathervane the aircraft. Since the 747 has a very large vertical stabilizer, it needs a very large rudder to neutralize it.
For those of you flying transport category aircraft, how many of these actually have an operating limitation on crosswind vs a max demonstrated? (Assuming uncontaminated runways.)