What is the lowest speed at which a full-size fixed-wing aircraft can fly without stalling?
• Even when an aircraft is in straight level flight, it is not horizontal. There is an “angle of attack” between the longitudinal axis (the gangway) of the aircraft and the airflow, so the leading edge of each wing is higher than the trailing edge. This means that the air is deflected downwards – resulting in a reactive force with an upward component (lift) and a backward component (drag).
Forward motion is needed to generate lift. The faster the aircraft is travelling, the smaller the angle of attack needed to generate the lift which is required to counter the aircraft’s weight. As the aircraft slows down, the required angle of attack must increase to maintain lift. But increasing the angle of attack also increases drag, which slows the aircraft further, reducing lift. The stall speed is the speed at which increasing the angle of attack can no longer be used to generate lift, so the aircraft descends.
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Stall speed is slowest when the aircraft is in straight level flight. When an aircraft turns it has to bank and so generates less lift, leading to an increase in stall speed. Increasing the payload or pulling up from a dive also increases stall speed, as does increasing the drag when the aircraft is in a landing configuration with its undercarriage down. For example, the stall speed of a Boeing 737 in straight level flight is about 220 kilometres per hour, compared with about 295 kilometres per hour when coming in to land.
Air France 447 tragically crashed into the Atlantic on 1 June 2009. Even though all components of the data flight recorder were recovered on 2 May this year, the crash remains unexplained. It is thought that icing of the , which calculate airspeed from the pressure of the outside air flow, generated erroneous airspeed data. According to a report published on 27 May by the French Bureau of Enquiry and Analysis for Civil Aviation Safety, this led the pilot to increase the angle of attack, stalling the aircraft. The mystery is that the aircraft stayed in a stall throughout the 3.5 minutes of descent from an altitude of 38,000 feet, with an angle of attack consistently around 35 degrees. Without the benefit of daylight the crew had to rely on instruments in which they appear to have lost faith.
Mike Follows, Willenhall, West Midlands, UK