Par 3-2 Page 43 Consequently, a maximum cruising flight altitude must be selected that will allow sufficient buffet margin for the maneuvering necessary and for gust conditions likely to be encountered.
Therefore, it is important for pilots to be familiar with the use of charts showing cruise maneuvering and buffet limits. Flightcrews operating airplanes at high speeds must be 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) Induced 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 profile of an aircraft wing is an excellent example of an efficient airfoil. 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 induced lift. Induced 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). The pilot may have only varying degrees of control over these factors. Thus, the pilot must keep in mind that an aircraft will obey the laws of physics just as precisely at its high-speed limits as it does during a slower routine flight. So, regardless of wing shape or design, Mach range flight requires precise control of a high volume of potential energy without exceeding the critical Mach number (MCR). The MCR is important to high-speed aerodynamics because it 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 airfoil 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,
Page 44 Par 3-3 which results in a reduced acceleration of airflow over the wing, thus allowing a higher speed before critical Mach is reached. Sweep may be designed either forward or rearward; the overall effect is the same. However, rearward sweep appears to be somewhat more desirable, since it has presented fewer problems to manufacturers of models of GA aircraft in terms of unwanted design side effects. In effect, the wing is flying slower than the airspeed indicator indicates and, similarly, it is developing less drag than the airspeed indicator would suggest. Since less drag is being developed for a given IAS, less thrust is required to sustain the aircraft at cruise flight.
(a) There is a penalty, however, on the low speed end of the spectrum. A swept-wing airplane has a higher landing/stall speed when compared with a similar straightwing aircraft. This means higher touchdown speed, longer runway requirements, and more tire and brake wear as opposed to a straight-wing design. A well-stabilized approach with precise control of critical “V” speeds is necessary. Swept-wing drag curves are approximately the reverse of the lift curves. A rapid increase in drag component may be expected with an increase of AOA with the amount being directly related to the degree of sweep or reduction of aspect ratio.
(b) The extension of trailing edge flaps and leading edge devices may, in effect, further reduce the aspect ratio of the swept wing by increasing the wing chord. This interplay of forces should be well understood by the pilot of the swept-wing aircraft. Raising the nose of the aircraft to compensate for a mild undershoot during a landing approach at normal approach speeds will produce little lift and may instead lead to a rapid decay in airspeed, reducing the margin of safety.
(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.
(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 sweptwing 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
Par 3-3 Page 45 separation (stall) occurs with a rapid deterioration of lift. By contrast, the swept wing produces a much more gradual buildup of lift with no well-defined maximum coefficient, the ability to fly well beyond this point, and no pronounced stall break.
(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.
c. Controllability Factors.
(1) Static Stability. Static 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
Page 46 Par 3-3 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.
(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 FL 350 is 135 kts; at FL 450, this speed is reduced to a mere 26 kts. Thus, the safety
Par 3-3 Page 47 (and 46) 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 Airplane Flight Manual (AFM).