Par 3-2 Page 43 adequately trained to operate them safely. This training cannot be complete until pilots are thoroughly educated in the critical aspect of aerodynamic factors pertinent to Mach flight at high altitudes.
3-3. AIRCRAFT AERODYNAMICS AND PERFORMANCE. Pilots who operate aircraft at high speeds and high altitudes are concerned with the forces affecting aircraft performance caused by the interaction of air on the aircraft. With an understanding of these forces, the pilot will have a sound basis for predicting how the aircraft will respond to control inputs. The importance of these aerodynamic forces and their direct application to performance and execution of aircraft maneuvers and procedures at altitude will be evident. The basic aerodynamics definitions that apply to high-altitude flight are contained in paragraph 1-4.
a. Wing Design.
(1) Lift. The wing of an airplane is an airfoil or aircraft surface designed to obtain the desired reaction from the air through which it moves. The difference in curvature between the upper and lower surfaces of the wing generates a lifting force. Air passing over the upper wing surface moves at a higher velocity than the air passing beneath the wing because of the greater distance it must travel over the upper surface. This increased velocity results in a decrease in pressure on the upper surface. The pressure differential created between the upper and lower surfaces of the wing lifts the wing upward in the direction of the lowered pressure. This lifting force is known as lift. Lift may be increased, within limits, by:
• Increasing the AOA of the wing or changing the shape of the airfoil; changing the geometry (e.g., aspect ratio).
• Increasing the wing area.
• Increasing the free-stream velocity.
• A change in air density.
(2) Critical Mach Number (Mcr). Mcr is the speed at which the flow of air over a portion of a specific airfoil design reaches Mach 1. This results in the formation of a shock wave and drag divergence.
(3) Wing Sweep. Sweeping the wings of an airplane is one method used by aircraft designers to delay the adverse effects of high Mach flight and bring about economical cruise with an increase in the Mcr. Sweep allows a faster speed before critical Mach is reached when compared to an equal straight wing. This occurs because the airflow now travels over a different cross-section (camber) of the airfoil. This new cross-section has less effective camber, which results in a reduced acceleration of airflow over the wing, thus allowing a higher speed before critical Mach is reached.
(4) High-Speed Laminar Airflow Airfoil. Another method of increasing the Mcr of an aircraft wing is through the use of a high-speed laminar airflow airfoil in which a small leading edge radius is combined with a reduced thickness ratio. This type of wing design is more tapered with its maximum thickness further aft, thus distributing pressures and boundary layer air more evenly along the chord of the wing. This tends to reduce the local flow velocities at high Mach numbers and improve aircraft control qualities.
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(5) Vortex Generators and Boundary Layer Energizers. Several modern straight-wing, turbojet aircraft use another design method to delay the onset of Mach buzz and obtain a higher MMO. These aircraft sometimes incorporate the use of both vortex generators and small triangular upper wing strips as boundary layer energizers. Both systems seem to work equally well, although the boundary layer energizers generally produce less drag. Vortex generators are small vanes affixed to the upper wing surface, extending in height approximately 1 to 2 in. This arrangement permits these vanes to protrude through the boundary layer air. The vortex generators deflect the higher energy airstream downward over the trailing edge of the wing and accelerate the boundary layer aft of the shock wave. This tends to delay shock-induced flow separation of the boundary layer air, which causes aileron buzz, and thus permits a higher MMO.
(6) Lift Characteristics Related to Swept Wings. The lift characteristics of straight-wing and swept-wing airplanes related to changes in AOA are more favorable for swept-wing airplanes. An increase in the AOA of the straight-wing airplane produces a substantial and constantly increasing lift vector up to its maximum coefficient of lift and, soon thereafter, flow separation (stall) occurs with a rapid deterioration of lift. By contrast, the swept wing produces a much more gradual buildup of lift.
(7) Airflow and Airspeed. Regardless of the method used to increase the critical Mach number, airflow over the wing is normally smooth. However, as airspeed increases, the smooth flow becomes disturbed. The speed at which this disturbance is encountered will be determined by the shape of the wing and the degree of sweep. When the aircraft accelerates, the airflow over the surface of the wing also accelerates until, at some point on the wing, it becomes sonic. The aircraft March number at which this occurs is the Mcr for that wing.
b. Jet Engine Efficiency.
(1) Fuel Economy. The efficiency of the jet engine at high altitudes is the primary reason for operating in the high-altitude environment. The specific fuel consumption of jet engines decreases as the outside air temperature decreases for constant revolutions per minute (rpm) and TAS. Thus, by flying at a high altitude, the pilot is able to operate at flight levels where fuel economy is best and with the most advantageous cruise speed. For efficiency, jet aircraft are typically operated at high altitudes where cruise is usually very close to rpm or exhaust gas temperature limits. At high altitudes, little excess thrust may be available for maneuvering. Therefore, it is often impossible for the jet aircraft to climb and turn simultaneously, and all maneuvering must be accomplished within the limits of available thrust and without sacrificing stability and controllability.
(2) Compressibility. Compressibility also is a significant factor in high-altitude flight.
The low temperatures that make jet engines more efficient at high altitudes also decrease the speed of sound. Thus, for a given TAS, the Mach number will be significantly higher at high altitude than at sea level. This compressibility effect, due to supersonic airflow, will be encountered at slower indicated speeds at high altitude than at low altitude.
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c. Controllability Factors.
(1) Stability. Stability is the inherent flight characteristic of an aircraft to return to equilibrium after being disturbed by an unbalanced force or movement.
(2) Controllability. Controllability is the ability of an aircraft to respond positively to control surface displacement and to achieve the desired condition of flight.
(3) Air Density. At high-flight altitudes, aircraft stability and control may be greatly reduced. Thus, while high-altitude flight may result in high TAS and high Mach numbers, calibrated airspeed is much slower because of reduced air density. This reduction in density means that the AOA must be increased to maintain the same coefficient of lift with increased altitude. Consequently, jet aircraft operating at high altitudes and high Mach numbers may simultaneously experience problems associated with slow-speed flight such as Dutch roll, adverse yaw, and stall. In addition, the reduced air density reduces aerodynamic damping, overall stability, and control of the aircraft in flight. Swept-wing and airfoil design alone, with boundary layer energizers such as the vortex generators described earlier, has reduced the hazardous effect of the problems described below. However, these problems are still encountered to some extent by the modern turbojet airplane in high-altitude flight.
(a) Dutch Roll. Dutch roll is a coupled oscillation in roll and yaw that becomes objectionable when roll, or lateral stability, is reduced in comparison with yaw or directional stability. A stability augmentation system is required to be installed on the aircraft to dampen the Dutch roll tendency when it is determined to be objectionable, or when it adversely affects control stability requirements for certification. The yaw damper is a gyro-operated autocontrol system installed to provide rudder input and aid in canceling out yaw tendencies such as those in Dutch roll.
(b) Adverse Yaw. Adverse yaw is a phenomenon in which the airplane heading changes in a direction opposite to that commanded by a roll control input. It is the result of unequal lift and drag characteristics of the down-going and up-going wings. The phenomena are alleviated by tailoring the control design by use of spoilers, yaw dampers, and interconnected rudder and aileron systems.
(4) Results of Supersonic Flow. Supersonic flow over the wing is responsible for:
(a) Shock Waves. The formation of shock waves on the wing, which results in drag rise.
(b) Aft Shift. An aft shift in the center of lift resulting in a nosedown pitching moment called Mach tuck.
(c) Airflow Separation. Airflow separation behind the shock waves resulting in Mach buffet.
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(5) G-Force and Safety Margins. In general, this discussion has been confined to normal level, unaccelerated 1.0 G-flight, when rotating or maneuvering about the pitch axis;
however, acceleration of G-forces can occur while maintaining a constant airspeed. As G-forces increase, both the aircraft’s aerodynamic weight and AOA increase.
(a) Maneuvering Flight G-Loading. The margin over low-speed stall buffet decreases, as well as the margin below Mach buffet, because of the increased velocity of the air over the wing resulting from the higher AOA. This, in effect, could lower the aerodynamic ceiling for a given gross weight.
(b) Non-Maneuvering Flight G-Loading. Increased G-loading can also occur in non-maneuvering flight because of atmospheric turbulence or the lack of fine-touch skill by the pilot. Pilots flying at high altitudes in areas where turbulence may be expected must carefully consider acceptable safety margins necessary to accommodate the sudden and unexpected vertical accelerations that may be encountered with little or no warning. How wide is the safety margin between low-speed and high-speed buffet boundaries for an altitude and weight in a 30 bank? The answer may be easily determined by reference to the Cruise Maneuver/Buffet Limit Chart for a particular aircraft. For example, in a typical jet aircraft, the 1.0 G buffet-free margin at flight level (FL) 350 is 135 knots (kts); at FL 450, this speed is reduced to a mere 26 kts. Thus, the safety margin in airspeed spread diminishes rapidly as the aircraft climbs and leaves little room for safety in the event of an air turbulence encounter or accidental thunderstorm penetration.
(6) Thunderstorm Penetration. If a thunderstorm cannot be avoided, follow high-altitude thunderstorm penetration procedures and avoid over-action of thrust levers. When excessive airspeed buildup occurs, pilots may wish to use speed brakes. The use of aerodynamic speed brakes, when they are part of the lateral control system, may change the roll rate any time there is a lateral control input.
(7) Information. For detailed information concerning the operation of specific turbojet aircraft, refer to the aircraft’s AFM.
Advisory Circular Feedback Form If you find an error in this AC, have recommendations for improving it, or have suggestions for new items/subjects to be added, you may let us know by contacting the General Aviation and Commercial Division (AFS-800) at 9-AFS-800-Correspondence-Mail@faa.gov or the Flight Standards Directives Management Officer.
Subject: AC 61-107B CHG 1, Aircraft Operations at Altitudes Above 25,000 Feet Mean Sea Level or Mach Numbers Greater Than .75.
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