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#1
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Weathervaning
Just a quick question...
During a crosswind landing, for instance a left crosswind, you'd lower the left, upwind wing to counter the right drift induced by the crosswind. You'd also use some right rudder to keep the nose straight and prevent it from "weathervaning". Is this "weathervaning effect" caused by your leftward relative motion due to the left bank OR by the rightward crosswind ITSELF? Personally, I think that the former applies. The rightward crosswind only displaces the airplane to the right. Only the relative motion of the airplane with respect to that airmass would induce the weathervaning effect. I presume that the airplane does not know, aerodynamically, of the left crosswind. Next thing I was wondering, which is related to the above: say you're dead on centerline on landing, and all of a sudden a crosswind from the left starts blowing. The effect would be that you should only be displaced to the right of runway centerline. Your airplane nose would still be parallel to the centerline. Do you agree? Alex |
#2
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As far as the plane is concerned, there is no such thing as wind until the
wheels touch the ground. When you are in a hot air balloon, you don't feel any wind at all because you are being carried along with it. Same with the plane in a steady wind. If the wind is changing speed or direction quickly, the plane will feel it momentarily because its inertial prevents it from moving instantaneously. In the simple case of a steady crosswind, there is no wind aerodynamically. The fact that the ground is sailing along sideways is irrelevant, until the wheels touch that is. You don't want the wheels to touch while you are moving sideways so you bank. This angles the lift to the side which pulls the plane in that direction. You bank until the plane is being pulled sideways by its wings at the same speed as the wind. There is now a wind blowing sideways on the rest of the plane, including the rudder. The rudder, having lots of leverage way back there, tries to turn the plane into the wind. The effect you are counteracting with the pedals is caused by the bank; not by the wind. -- Roger Long |
#3
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It's from the wind striking the vertical stabilizer and pushing the nose
into the wind. Kobra "Koopas Ly" wrote in message m... Just a quick question... During a crosswind landing, for instance a left crosswind, you'd lower the left, upwind wing to counter the right drift induced by the crosswind. You'd also use some right rudder to keep the nose straight and prevent it from "weathervaning". Is this "weathervaning effect" caused by your leftward relative motion due to the left bank OR by the rightward crosswind ITSELF? Personally, I think that the former applies. The rightward crosswind only displaces the airplane to the right. Only the relative motion of the airplane with respect to that airmass would induce the weathervaning effect. I presume that the airplane does not know, aerodynamically, of the left crosswind. Next thing I was wondering, which is related to the above: say you're dead on centerline on landing, and all of a sudden a crosswind from the left starts blowing. The effect would be that you should only be displaced to the right of runway centerline. Your airplane nose would still be parallel to the centerline. Do you agree? Alex |
#4
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Your airplane nose would
still be parallel to the centerline. Do you agree? No, the airplane is pushed harder from the tail because of the vertical stabilizer. Therefore the nose turns into the wind. Kobra "Koopas Ly" wrote in message m... Just a quick question... During a crosswind landing, for instance a left crosswind, you'd lower the left, upwind wing to counter the right drift induced by the crosswind. You'd also use some right rudder to keep the nose straight and prevent it from "weathervaning". Is this "weathervaning effect" caused by your leftward relative motion due to the left bank OR by the rightward crosswind ITSELF? Personally, I think that the former applies. The rightward crosswind only displaces the airplane to the right. Only the relative motion of the airplane with respect to that airmass would induce the weathervaning effect. I presume that the airplane does not know, aerodynamically, of the left crosswind. Next thing I was wondering, which is related to the above: say you're dead on centerline on landing, and all of a sudden a crosswind from the left starts blowing. The effect would be that you should only be displaced to the right of runway centerline. Your airplane nose would still be parallel to the centerline. Do you agree? Alex |
#5
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The tail does not know what direction the wind is blowing if the airplane is in
the air. So therefore it DOES NOT push the nose into the wind. Do You fly? It's from the wind striking the vertical stabilizer and pushing the nose into the wind. Kobra |
#6
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"Koopas Ly" wrote in message
m... Is this "weathervaning effect" caused by your leftward relative motion due to the left bank OR by the rightward crosswind ITSELF? Personally, I think that the former applies. Yes, the former applies. It doesn't have anything to do with the crosswind, and has everything to do with the airplane's sideways motion through the airmass. The vertical stabilizer tends to orient the airplane into the relative wind, and in a slip, you are trying to maintain an orientation at an angle to the relative wind. Rudder is necessary to counteract the vertical stabilizer's normal force. (Oversimplifying, of course, since there are other forces involved that act in a variety of directions, including both with and against your rudder input). Next thing I was wondering, which is related to the above: say you're dead on centerline on landing, and all of a sudden a crosswind from the left starts blowing. The effect would be that you should only be displaced to the right of runway centerline. Your airplane nose would still be parallel to the centerline. Do you agree? I disagree. Because of inertia, a change in the air mass's momentum will momentarily not be compensated for by the airplane's configuration. Until the airplane "catches up" with the air mass, the relative wind is from the left, and will cause a temporary yaw force turning the airplane to the left. The force will gradually diminish as the airplane accelerates in the direction of the new movement of the airmass. The airplane will remain in this orientation unless the pilot adjust for it (and of course, the pilot most likely will). Pete |
#7
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#8
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Great question!!
"Koopas Ly" During a crosswind landing, for instance a left crosswind, you'd lower the left, upwind wing to counter the right drift induced by the crosswind. You'd also use some right rudder to keep the nose straight and prevent it from "weathervaning". This is a proper xwind landing conditon and is a slip (forward vs side is debatable). You are banking left into the xwind to counteract the drift and get the a/c to track down the centerline. You apply enough righ rudder to align the nose with your direction of movement (i.e. the centerline). I would not call this weathervaning. Is this "weathervaning effect" caused by your leftward relative motion due to the left bank OR by the rightward crosswind ITSELF? Personally, I think that the former applies. The rightward crosswind only displaces the airplane to the right. Only the relative motion of the airplane with respect to that airmass would induce the weathervaning effect. I presume that the airplane does not know, aerodynamically, of the left crosswind. I'll leave weathervaning undefined but the last line is correct - as long as the a/c is airborne, it does not know of the left crosswind. The xwind is relative to the ground and if you are not touching the ground, then you might as well be at 30,000feet Next thing I was wondering, which is related to the above: say you're dead on centerline on landing, and all of a sudden a crosswind from the left starts blowing. The effect would be that you should only be displaced to the right of runway centerline. Your airplane nose would still be parallel to the centerline. Do you agree? Yes & No. The the a/c will weathervane into wind. That is, the nose will tend to point into the wind. The wheels represent the vertical axle of the weather vane. Assume 1 wheel (glider) on the CG. You could grab a wingtip and rotate with little effort. The wind hits the side of the aircraft. End of of the a/c with the most side area will be rotated downwind. The tail, like feathers on an arrow, will always be that end. Now the whole a/c will also tend to be displaced left to right. But imagine same single wheel and try to grab a wingtip and drag the a/c against the friction of the wheel. You won't be able to budge it. So, the net effect is that the weathervane effect will be immediate and dominant with weight on the wheel and low speed. At higher speeds and lighter weights, the drift will take over. What happened to weather vaning? It is still their but the forward motion of the a/c changes the net wind vector and the a/c ends up 'weathervaning' closer to straight ahead than towards the xwind. A gllider tow from a dead stop in a xwind demonstrates all that very nicely. As soon as you start rolling, balanced on one wheel, the glider will rotate (vane) right into the wind. So downwind rudder is required at first. As speed picks up, both the rudder becomes more effective and the relative wind vector shifts towards your intended direction of flight so less downwind rudder is required. As the wing starts lifting the weight off the wheel, the glider starts drifting downwind with the tire scrubbing sideways. Dipping the upwind wing fixes that and you leave the ground in a slip. As soon as the wheel is off the ground, you can basically let go of the controls for a second and the plane will assume an immediate wings level crab relative to the ground and off you go. This is less complicated than it sounds but you have to do it a few times before you can do it right. Until that time, the pull of the towrope on a quickly accelerating tow plane will keep you out of the weeds while you regain your dignity. (BTW, all that happens in reverse on landing but again, the stars are favorable and decreasing energy tends to mask problems at the end. Alight gracefully and complain about sudden 'turbulence' on rollout. On a powered trike (C150), the friction of 3 wheels and a favorable configuration of CG and CP will make all of that invisible in light xwinds. Conversely, a J3 will amplify all of the above and the weeds await the unwary. |
#9
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"Robert Moore" wrote in message
. 7... There is NO weathervaning effect until the wheels touch the ground. Of course there is. There is "weathervaning" any time the relative wind is not parallel to the longitudinal axis. Banking an airplane (putting a wing down) causes it to turn. Actually, in some aircraft, banking an airplane causes it to turn in a direction *opposite* of the bank. Rudder is necessary in a turn, to make sure the turn is coordinated and is as efficient as possible. Many airplanes, without the use of rudder, will simply slip sideways without any change in heading at all. You use opposite rudder simply to keep it from turning due to the bank. That's one way to look at it. But think about why the airplane is turning due to the bank. The yaw occurs because bank alone causes a slip, which causes the relative wind to come from an angle to the vertical stabilizer, which causes yaw. That yaw can be described as "weathervaning" and the rudder is used to counteract it. Pete |
#10
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Koopas Ly wrote: During a crosswind landing, for instance a left crosswind, you'd lower the left, upwind wing to counter the right drift induced by the crosswind. You'd also use some right rudder to keep the nose straight and prevent it from "weathervaning". No, I would use right rudder to keep the nose straight. Is this "weathervaning effect" caused by your leftward relative motion due to the left bank OR by the rightward crosswind ITSELF? Weathervaning is *always* caused by the wind. That's the source of the name of the effect. Next thing I was wondering, which is related to the above: say you're dead on centerline on landing, and all of a sudden a crosswind from the left starts blowing. The effect would be that you should only be displaced to the right of runway centerline. Your airplane nose would still be parallel to the centerline. Do you agree? Depends on my airspeed. If I'm close to touchdown, my aircraft will turn into the wind. Regardless of speed, it will also drift. George Patterson If you're not part of the solution, you can make a lot of money prolonging the problem. |
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