If a positive angle of attack (AoA) is needed to produce lift, then how is it created when a plane is in a 30 degree nose low dive?
How does it not stall? At this attitude the AoA has to be negative, right?
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3$\begingroup$ Hi Zach Barker, welcome to aviation.stackexchange.com. I have reformulated your question a bit so it is clear to everybody what you mean. Can you check if I have correctly interpreted your question. If not, please use the edit function to improve it. $\endgroup$– DeltaLima ♦Nov 9, 2022 at 9:15
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3 Answers
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The angle of attack is the angle between the chord of the wing and the incoming airflow.
Let's look at the case where the nose is 30 degrees down as you mention in your answer. The wing chord is typically mounted at a small angle (angle of incidence) with the fuselage center-line (x-axis of the body frame).
- Let's assume the AoI is 2 degrees.
- The nose is 30 degrees below the horizon,
- the chord line of the wing is pointing (30 - 2 =) 28 degrees below the horizon.
- the aircraft is traveling on a path 31 degrees below the horizon.
The angle of attack is the angle between the incoming airflow (from 31° below the horizon) and the wing's chord line (28° below the horizon) is still a positive (31° - 28° = ) 3 degrees.
At 3° AoA, the wing produces lift and will not stall.
answered Nov 9, 2022 at 9:32
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An aircraft produces lift in respect to air, so the AoA has to be measured in respect to air. The -30° that you are considering are measured in respect to earth, not air.
This picture (source) depicts this attitude in a simple way:
Let's suppose that the airplane is flying with am horizontal speed of 100 and a vertical speed of -70 (unit of measurement is not important). Here -70 means that the airplane is sinking (dive). Now, these combination of horizontal plus vertical speed gives an airflow with an angle of $\arctan(-70/100)=-35°$ in respect to earth (the blue line in the picture)
The aircraft has an attitude of -30° ("chord line" in the picture) but it is invested by an airflow of -35°. Its AoA is the difference between these two angles: (-30)-(-35)=5°. So, in this particular case, even if the attitude is -30° the AoA is actually +5°.
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Because the pitch attitude of the aircraft, relative to the horizon, is irrelevant. The angle that is important is the angle between the wing and the direction (relative to the wing) of the airflow impacting the wing (which is the exact opposite of the direction the aircraft is moving through the air, relative to the air).
