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Wsherif1, posted 28th July 2002 :... The 200+ mph force of the rotating vortices striking the FIN, (vertical stabilizer and the rudder), BROADSIDE resulted in an INSTANTANEOUS left YAW! …….
Wsherif1, posted 2nd August 2002 NTSB and Rudders: The F Meter When the vertical stabilizer and the rudder were struck broadside by the forces of the 747's clockwise rotating vortices the resulting instantaneous yaw maneuver was initiated before the pilot had a chance to react to what would have been erroneous flight instrument indications anyway. The pilots were just along for the ride!
Wsherif1, posted 4th August 2002 …. Dr. AA Wray of NASA affirms that in smooth air aircraft wake turbulence can persist for extended periods of time. I had a severe wake turbulence encounter 45 miles behind another 707, in smooth air! …
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I have been thinking on and off over the past month about the above posts and the various responses from Wino, Stator Vane, and Belgique, and finally got around to wondering what would happen with a much LOWER vortex X-wind than 200 mph, like maybe 100 kts.
I should start by saying that I am NOT a commercial pilot, and have NO training or qualifications in aerodynamics or aircraft structures, and that this post is really an attempt to encourage people who really know their stuff into following a train of thought and providing guidance to the rest of us.
The train of thought goes as follows: AW&ST published a diagram (on p. 25, AW&ST for Jan 21, 2002) showing the results of their calculations of how the side forces on the fin/rudder of an A300-600 varies with sideslip angle at 250 kts. One of the three curves in that diagram was for a centered rudder. It showed that the fin/rudder design strength limit is reached at a sideslip angle of 10 degrees, and that the design ultimate limit is reached with a sideslip angle of 15 degrees. I have plotted these results in a different form in Fig. 1 below.
Since the a/c cannot be held in a sideslip on a fixed heading with the rudder centered, AW&ST assumed that the sideslip was established using the rudder and other controls, and then the rudder was suddenly centered. At a maximum of 39 degrees/sec, this might be done on an A300 rudder in less than half a second.
Now suppose instead of sideslipping, you fly along normally on a fixed heading and with the rudder centered, and then you suddenly meet a wake vortex which effectively provides a X-wind. To the a/c this feels like it was initially flying along normally and then suddenly it is flying at a yaw angle due to the vortex X-wind, as shown on the right in Fig. 2. The relative wind comes at an angle to the nose and is the vector sum of the speed of the plane, S, and the speed of the vortex X-wind, X. In both Figs. 1 and 2, S=250 kts. The calculated stresses on the a/c for a given sideslip angle are as shown in Fig.1. While a sideslip is not the same as a yaw, and neither can be held at a constant heading with the rudder centered, I am going to assume for the purposes of this discussion that that the forces on the fin are roughly equivalent for similar sideslip/yaw angles. If this is accepted, then Fig. 1 can be re-drawn in terms of “Angle of Relative Wind Off the Nose” and “X-wind” as shown in Fig. 2.
Fig.2 indicates that the fin/rudder design strength limit is reached at a sudden X-wind speed of about 44 kts, and the design ultimate limit is reached at a sudden X-wind speed of about 66 kts. A lot less than 200 mph.
A possibly interesting aspect of all this is that a vortex X-wind speed of 200 mph MIGHT be LESS stressful because relative wind would then be at an angle of 35 degrees, at which angle-of-attack the coefficient of lift might be much lower than at 10-15 degrees, and consequently the force might be LOWER than at a LOWER X-wind speed.
Now what happens if the a/c is flying along normally (fixed heading, rudder centered, and no yaw) and then it suddenly meets a wake vortex X-wind speed of, say, 60 kts (i.e. where the stresses on the a/c are more than design but less than ultimate), and the pilot has NO TIME TO REACT? The a/c is aerodynamically in a yaw, but the autopilot and yaw dampers, being gyro-based, think the a/c is flying straight and level, so they initially do nothing, and the rudder initially remains centered. One of the quotes above says that the X-wind will start a yaw and roll by direct action on the fin/rudder, which I assume the autopilot and yaw-damper will then try to counter. The numbers in the AW&ST diagram referred to above for cases where the rudder is not centered (not shown in Figs. 1 or 2) suggest that if the angle the rudder then turns through is equal to or less than the angle that the plane turns through (as might be expected of a gyro-based correction system), then the net result will be a DECREASE in the stress on the fin/rudder. So the autopilot/yaw damper response should NOT be an additional hazard.
To summarize this (possibly erroneous) train of thought:
(a) It looks from the AW&ST calculations that a wake vortex X-wind of about 66 kts would stress the fin/rudder to its design ultimate stress (at which level the fin/rudder might deform permanently, but it should NOT immediately detach). The AW&ST article says that the FAA requires that the aircraft must be able to withstand the ultimate limit stress for three seconds, permanent distortion being allowed. However, I assume that a X-wind of 100 kts. might result in a different outcome.
(b) The responses of the auto-pilot yaw-damper combination at X-wind speeds of 66 kts. or less are likely to immediately lower the stresses on the fin/rudder.
(c) I cannot even guess what a 200 mph X-wind would do, because that corresponds to a 35 degree yaw, at which angle of attack the fin/rudder coefficient of lift might be lower than at 10-15 degrees, and the forces might therefore be LOWER than at a LOWER X-wind velocity. 
And, of course, the vortex wind might not be a direct X-wind, but come at an angle other than 90 degrees.
To repeat my initial note, I am NOT a commercial pilot, and have NO training or qualifications in aerodynamics or aircraft structures. This post is an invitation to people who really know their stuff to follow a train of thought and respond with correction, amplification, confirmation, or whatever they may feel is appropriate.
Meanwhile, I think I will temporarily retire into my underground bunker ……….. 