Zephyr: A High Altitude Long Endurance Unmanned Air Vehicle ...

3.5 Angle of Attack SensorThe angle of attack (AoA) is the angle between the relative wind and the longitudinalaxis of the plane, see figure 10.LongitudinalaxisWinddirectionAngle of AttackFigure 10. Diagram to show the angle of attack.The angle of attack is important as it helps to generate the lift. When the wind hits thefront of the plane some of the air flows under the plane while some flows over theplane. As the wind flows over the top of the plane its velocity increases and thus thepressure decreases. This creates an uneven pressure distribution around the plane andforces the aircraft upwards towards the low pressure. The greater the angle the largerthe lift coefficient and the more the aircraft rises.At the critical angle of attack the air no longer flows smoothly over the wing and thiscauses a dramatic drop in lift. The air becomes turbulent meaning that it no longerlifts the aircraft. This causes the aircraft to rapidly accelerate downwards entering aspin which can cause it to break. This is not desirable, so the angle of attack has to becarefully monitored during flight to ensure the aircraft stays within the limits.46