In this situation, without reliable AOA information, a nose-down pitch attitude with an increasing airspeed is no guarantee that recovery has been affected, and up-elevator movement at this stage may merely keep the aircraft stalled. In the pre-stall and immediate post-stall regimes, the lift/drag qualities of a swept wing aircraft (specifically the enormous increase in drag at low speeds) can cause an increasingly descending flight path with no change in pitch attitude, further increasing the AOA. If the horizontal tail surfaces then become buried in the wing’s wake, the elevator may lose all effectiveness, making it impossible to reduce pitch attitude and break the stall. The T-tail, being above the wing wake remains effective even after the wing has begun to stall, allowing the pilot to inadvertently drive the wing into a deeper stall at a much greater AOA. ![]() The stall situation can be aggravated by a T-tail configuration, which affords little or no pre-stall warning in the form of tail control surface buffet. This is the primary reason for the development of the T-tail configuration on many turbine-powered aircraft, which places the horizontal stabilizer as far as practical from the turbulence of the wings. 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. Transonic flow, where both sub- and supersonic flow may exist, is also usually treated as a distinct regime. A common demarcation is subsonic and supersonic flow, where the latter has airspeeds greater then the speed of sound. Thus, as the aircraft climbs in altitude with outside temperature dropping, the speed of sound is dropping. Aerodynamics may be categorized as either low- or high-speed, depending on where the fluid behavior changes. A decrease in temperature in a gas results in a decrease in the speed of sound. Blade tip speed is one of the limiting factors to the size of rotors, and also to the forward speeds of helicopters (as this speed is added to the forward-sweeping (leading) side of the rotor, thus possibly causing localized transonics).Another factor to consider is the speed of sound. However, as this puts severe, unequal stresses on the rotor blade and may lead to dangerous accidents if it occurs, it is avoided. ![]() Transonic speeds can also occur at the tips of rotor blades of helicopters and aircraft. During transonic flight, the plane must pass through this large shock wave, as well as contend with the instability caused by air moving faster than sound over parts of the wing and slower in other parts. When an object such as an aircraft also moves at the speed of sound, these shock waves build up in front of it to form a single, very large shock wave. Shock waves move through the air at the speed of sound. Severe instability can occur at transonic speeds. ![]() Transonic cloud forms around shock wave as aircraft passes through the sound barrier.
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