Windshear Question
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Join Date: May 2009
Location: Geelong, Victoria
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Windshear Question
Eg. You are descending down the ILS into a headwind of 20kts, suddenly the head wind changes to 10kts (windshear). Your inertia will keep try and maintain your original ground speed thus your IAS is now 10kts lower. Thus Lift/Drag are now lower, thus the anticlockwise (downward pitching) moment created from lift is now reduced. Why does the nose pitch down when theoretically it should pitch up due to the unbalanced moments about the CG?
Hi P_B
Yes, the lift vector would be slightly smaller leading to a nose-up trim change.
But, as you say, the drag vector would also be smaller, leading to a nose-down trim change (assuming low wing-mounted engines).
The airflow over the tailplane would also be less, leading to a nose-down trim change. This would probably have the largest effect of all, hence a nose-down pitch.
Eventually, the airspeed would increase back to the pre-windshear value and any transient trim effects would disappear.
I have to say that I haven't really noticed these effects in practice. I guess we just 'fly the bird' and control any trim changes instinctively.
Hope this helps!
Yes, the lift vector would be slightly smaller leading to a nose-up trim change.
But, as you say, the drag vector would also be smaller, leading to a nose-down trim change (assuming low wing-mounted engines).
The airflow over the tailplane would also be less, leading to a nose-down trim change. This would probably have the largest effect of all, hence a nose-down pitch.
Eventually, the airspeed would increase back to the pre-windshear value and any transient trim effects would disappear.
I have to say that I haven't really noticed these effects in practice. I guess we just 'fly the bird' and control any trim changes instinctively.
Hope this helps!
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Eg. You are descending down the ILS into a headwind of 20kts, suddenly the head wind changes to 10kts (windshear). Your inertia will keep try and maintain your original ground speed thus your IAS is now 10kts lower. Thus Lift/Drag are now lower, thus the anticlockwise (downward pitching) moment created from lift is now reduced. Why does the nose pitch down when theoretically it should pitch up due to the unbalanced moments about the CG?
Suppose instead of windshear, you did it all yourself. You trim the aircraft at 120 and record all the trim values, wheel inputs, etc, then slow down to 110kts. Now, suddenly put all the controls back to the values at 120kts. One of the things you'd have to do would be push forward on the stick, to get it back to the 120kt position. So the a/c will pitch down.
