THE ANGLE OF ATTACK INDICATOR AND THE MIRROR LANDING SYSTEM

The usual errors during the takeoff and landing phases of flight involve improper con­trol of airspeed and altitude along some desired flight path. Any errors of technique are ampli­fied when an adequate visual reference is not available to the pilot. It is necessary to provide the pilot with as complete as possible visual reference field to minimize or eliminate any errors in perception and orientation. The angle of attack indicator and the mirror land­ing system assist the pilot during the phases of takeoff and landing and allow more consistent, precise control of the airplane.

THE ANGLE OF ATTACK INDICATOR. Many specific aerodynamic conditions exist at particular angles of attack for the airplane. Generally, the conditions of stall, landing ap­proach, takeoff, range, endurance, etc., all occur at specific values of lift coefficient and specific airplane angles of attack. Thus, an instrument to indicate or relate airplane angle of attack would be a valuable reference to aid the pilot.

When the airplane is at high angles of attack it becomes difficult to provide accurate indica­tion of airspeed because of the possibility of large position errors. In fact, for low aspect ratio airplane configurations at high angles of attack, it is possible to provide indications of angle of attack which are more accurate than indications of airspeed. As a result, an angle of attack indicator can be of greatest utility at the high angles of attack.

A particular advantage of an angle of attack indicator is that the indicator is not directly affected by gross weight, bank angle, load factor, velocity, or density altitude. The typical lift curve of figure 6.3 illustrates the variation of lift coefficient, CL, with angle of attack a. When a particular aerodynamic configuration is in subsonic flight, each angle of attack produces a particular value of lift coefficient. Of course, a point of special interest on the lift curve is the maximum lift coefficient, CLmax. Angles of attack greater than that for CLjaax produce a decrease in lift coefficient and constitute the stalled condition of flight. Since CLjnax occurs at a particular angle of attack, any device to provide a stall warning should be predicated on the function of this critical angle o( attack. Under these conditions, stall of the airplane may take place at various airspeeds depending on gross weight, load factor, etc., but always the same angle of attack.

In order to reduce takeoff and landing dis­tances and minimize arresting loads, takeoff and landing will be accomplished at minimum practical speeds. The takeoff and landing speeds must provide sufficient margin above the stall speed (or minimum control speed) and are usually specified at some fixed per­centages of the stall speed. As such, takeoff, approach, and landing will be accomplished at specific values of lift coefficient and, thus, particular angles of attack. For example, assume that point A on the lift curve is defined as the proper aerodynamic condition for the landing approach. This condition exists as a particular lift coefficient and angle of attack for a specific aerodynamic configuration. When the airplane is flown in a steady flight path at the prescribed angle of attack, the resulting airspeed will be appropriate for the airplane gross weight. Any variation in gross weight will simply alter the airspeed necessary to provide sufficient lift. The use of an angle of attack indicator to maintain the recom­mended angle of attack will insure that the airplane is operated at the proper approach speed—not too low or too high an airspeed.

In addition to the use of the angle of attack indicator during approach and landing, the instrument may be used as a principal reference during takeoff. The use of the angle of attack indicator to assume the proper takeoff angle of attack will prevent both over-rotation and excess takeoff speed. Also, the angle of at­tack indicator may be applicable to assist in control of the airplane for conditions of range, endurance, maneuvers, etc.

THE MIRROR LANDING SYSTEM. A well planned, stabilized approach is a funda­mental requirement for a good landing. How­ever, one of the more difficult problems of perception and orientation is the positioning of the airplane along a proper flight path dur­ing approach to landing. While various de­vices are possible, the most successful form of glide path indicator applicable to both field and shipboard operations is the mirror landing system. The function of the mirror landing system is to provide the pilot with an accurate visual reference for a selected flight path which has the desired inclination and point of touch­down. Utilization of the mirror system will allow the pilot to position the airplane along the desired glide path and touch down at the desired point. When the proper glide path inclination is set, the pilot can be assured that the rate of descent will not be excessive and a foundation is established for a successful landing.

The combination of the angle of attack in­dicator and the mirror landing system can provide an excellent reference for a landing technique. The use of the angle of attack indicator will provide the airplane with the proper airspeed while the mirror system refer­ence will provide the desired flight path. When shipboard operations are conducted without the mirror system and angle of attack indicator, the landing signal officer must provide the immediate reference of airspeed and flight path. The LSO must perceive and judge the angle of

attack (and, hence, airspeed) and the flight path of the landing aircraft and signal correc­tions to be made in order to achieve the desired flight path and angle of attack. Because of the field of orientation available to the LSO, he is able to perceive the flight path and angle of attack more accurately than the pilot with­out an angle of attack indicator and mirror landing system.