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Under banking in a Descending Turn
southafricanairways created the topic: Under banking in a Descending Turn
I understand that during a climbing turn the aircraft can tend to over bank because of the higher AoA and more distance covered by the outside wing.
I'm just not sure why or how an aircraft has the tendency to under bank during a descending turn. Would you be able to explain this to me?
bobtait replied the topic: Under banking in a Descending Turn
It's hard to generalize about bank behaviour during a descending turn. In a climbing turn, the outside wing has both the highest angle of attack and the highest relative airflow, The combined effect of extra speed and higher angle of attack causes the tendency to overbank in a climbing turn. In a descending turn, the outside wing has the higher speed since it travels a greater distance than the inside wing (this effect is really quite small when you consider the percentage difference in the circumference of the circles traveled by each wing), but the inside wing has the highest angle of attack.
Whereas most people can easily accept the outer wing traveling faster, it's not so obvious when we consider the angle of attack. Most students find it easier to understand by carrying out a simple experiment. Take a model aeroplane and hold it above your head with a moderate angle of bank relative to the floor.
Now turn your body and make it do a descending turn while keeping the bank the same relative to the floor. You have to continuously roll your wrist into the turn to prevent the bank from decreasing. In other words, the aeroplane must continually roll about its longitudinal axis as the turn progresses. So the inside wing is continually moving downwards and so is meeting the airflow at a higher angle of attack. In a descending turn, the inside wing has the greatest angle of attack, while the outside wing travels faster, so the effects tend to cancel out usually with no tendency to to overbank or sometimes a tendency to underbank depending on the aircraft design and the amount of bank used.
If you repeat the experiment by holding the model below your waist and then making it do a climbing turn, you will find that the aeroplane must continuously roll out of the turn to keep the bank the same. This means that the outside wing has a greater angle of attack in a climbing turn.
You can also use vectors as in the figure below. Both wings lose exactly the same height, but, in descending turn, the inside wing travels less distance so it has the highest angle of attack.