If the CP moves aft, a diving moment referred to as “Mach tuck” or “tuck under” is produced, and if it moves forward, a nose-up moment is produced. Movement of the wing CP affects the wing pitching moment. Thus, an increase in downwash decreases the horizontal tail’s pitch control effectiveness since it effectively increases the AOA that the tail surface is seeing. The nose-up and nose-down pitch control provided by the horizontal tail is dependent on the downwash behind the wing. Airflow separation produces a turbulent wake behind the wing, which causes the tail surfaces to buffet (vibrate). The loss of lift due to airflow separation results in a loss of downwash and a change in the position of the center pressure on the wing. heavy)) to 10,000 feet and then at a specified en route climb airspeed (about 330 if a DC10) until reaching an altitude in the “mid-twenties” where the pilot then climbs at a constant Mach number to cruise altitude.Īssociated with “drag rise” are buffet (known as Mach buffet), trim, and stability changes and a decrease in control force effectiveness. This describes what happens when the aircraft is at a constant KCAS with increasing altitude, but what happens when the pilot keeps Mach constant during the climb? In normal jet flight operations, the climb is at 250 KIAS (or higher (e.g. All the while, the KCAS for stall has remained constant at 152. Simultaneously, the speed of sound (in KCAS) has decreased from 661 to 574 and the Mach number has increased from 0.23 (152 KTAS divided by 661 KTAS) to 0.50 (287 KTAS divided by 574 KTAS). Thus, for our jet transport aircraft, the stall speed (in KTAS) has gone from 152 at sea level to 287 at FL 380. At sea level, the speed of sound is approximately 661 KCAS, while at FL 380 it is 574 KCAS. Thus, as the aircraft climbs in altitude with outside temperature dropping, the speed of sound is dropping. A decrease in temperature in a gas results in a decrease in the speed of sound. Taper has the desirable effect of reducing the root bending stress by shifting the lift inboard, but it has been argued by some noted designers, including John Thorp and Karl Bergey, that an untapered rectangular planform is best for aeroplanes of less than 6,000 pounds gross weight.Another factor to consider is the speed of sound. The ratio of tip chord to root chord is called the taper ratio. The wings of jet airliners, which are highly optimized for efficiency, are far from elliptical in shape. A number of factors influence induced drag, however, and as a practical matter a wing of elliptical planform, like that of the Supermarine Spitfire fighter of World War II, is not necessarily the most efficient.
In theory, induced drag is at its minimum when the span wise distribution of lift is elliptical. The wings of fast subsonic craft such as jet airliners tend to be swept in order to delay the onset of these shock waves. This soon leads to the shock stall, causing a rapid increase in drag.
Small, low-altitude general aviation planes typically have aspect ratios of six or seven airliners of 12 or more and high-performance sailplanes of 30 or more.Īt speeds above the critical Mach number, the airflow begins to become transonic, with local airflow in some places causing small sonic shock waves to form. This is best achieved with a high aspect ratio, and high-performance types often have this kind of wing.īut other considerations such as light weight, structural stiffness, manoeuvrability, ground handling and so on often benefit from a shorter span and, consequently a less efficient wing. It is therefore desirable for a wing to have the least area compatible with the desired lift characteristics.
VURDS THAT FLY AT SUBSONIC SPEED SKIN
The drag of a wing consists of two components: the induced drag, which is related to the production of lift, and the profile drag, largely due to skin friction which is contributed to by the whole wing area. A higher aspect ratio gives a higher lift/drag ratio and so is more efficient. The aerodynamic efficiency of a wing is described by its lift/drag ratio, with a wing giving high lift for little drag being the most efficient. They are related by the aspect ratio, which is the ratio of the span, measured from tip to tip, to the average chord, measured from leading edge to trailing edge. The span and area of a wing are both important to the lift characteristics. A Spitfire built in 1945 shows off its straight elliptical wing
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