OF FLYING

While the previous chapters have presented the detailed parts of the general field of aero­dynamics, there remain various problems of flying which require the application of princi­ples from many parts of aerodynamics. The application of aerodynamics to these various problems of flying will assist the Naval Aviator in understanding these problems and develop­ing good flying techniques.

PRIMARY CONTROL OF AIRSPEED AND ALTITUDE

For the conditions of steady flight, the air­plane must be in equilibrium. Equilibrium will be achieved when there is no unbalance of force or moment acting on the airplane. If it is assumed that the airplane is trimmed so that no unbalance of pitching, yawing, or rolling moments exists, the principal concern is for

the forces acting on the airplane, i. e., lift, thrust, weight, and drag.

ANGLE OF ATTACK VERSUS AIRSPEED. In order to achieve equilibrium in the vertical direction, the net lift must equal the airplane weight. This is a contingency of steady, level flight or steady climbing and descending flight when the flight path inclination is slight. A refinement of the basic lift equation defines the relationship of speed, weight, lift coefficient, etc., for the condition of lift equal to weight.

or

FE=17.2y^

where

V= velocity, knots ([TAS)

VE = equivalent airspeed, knots (EAS) W= gross weight, lbs.

T=wing surface area, sq. ft.

WjS= wing loading, psf v = altitude density ratio Cb = lift coefficient

From this relationship it is appreciated that a given configuration of airplane with a specific wing loading, W/S, will achieve lift equal to weight at particular combinations of velocity, V, and lift coefficient, CL. In steady flight, each equivalent airspeed demands a particular value of Cb and each value of Cb demands a particular equivalent airspeed to provide lift equal to weight. Figure 6.1 illustrates a typical lift curve for an airplane and shows the relation­ship between CL and a, angle of attack. For this relationship, some specific value of a will create a certain value of CL for any given aero­dynamic configuration.

For the conditions of steady flight with a given airplane, each angle of attack corre­sponds to a specific airspeed. Each angle of attack produces a specific value of CL and each value of Ci requires a specific value of equiva­lent airspeed to provide lift equal to weight. Hence, angle of attack is the primary control of airspeed in steady flight, If an airplane is es­tablished in steady, level flight at a particular airspeed, any increase in angle of attack will result in some reduced airspeed common to the increased CL. A decrease in angle of attack will result in some increased airspeed com­mon to the decreased CL. As a result of the change in airspeed, the airplane may climb or descend if there is no change in power setting but the change in airspeed was provided by the change in angle of attack. The state of the airplane during the change in speed will be some transient condition between the original and final steady state conditions.

Primary control of airspeed in steady flight by angle of attack is an important principle. With some configurations of airplanes, low speed flight will bring about a low level of longitudinal stick force stability and possi­bility of low airplane static longitudi­nal stability. In such a case, the “feel” for airspeed will be light and may not furnish a ready reference for easy control of the air­plane. In addition, the high angles of attack common to low speed flight are likely to pro­vide large position errors to the airspeed indi­cating system. Thus, proper control of air­speed will be enhanced by good “attitude” flying or—when the visual reference field is poor—an angle of attack indicator.

RATE OF CLIMB AND DESCENT. In order for an airplane to achieve equilibrium at constant altitude, lift must be equal to weight and thrust must be equal to drag. Steady, level flight requires equilibrium in both the vertical and horizontal directions. For the case of climbing or descending flight condi­tions, a component of weight is inclined along the flight path direction and equilibrium is achieved when thrust is not equal to the drag. When the airplane is in a steady climb or descent, the rate of climb is related by the following expression:

fPa-Pr

ЖС^-ЪЪ&Ц^рг-)

where

RC=rate of climb, ft. per min.

Pa = propulsive power available; h, p.

Pr=power required for level flight, h. p.

W= gross weight, lbs.

From this relationship it is appreciated that the rate of climb in steady flight is a direct function of the difference between power avail­able and power required. If a given airplane configuration is in lift-equal-to-weight flight at some specific airspeed and altitude, there is a specific power required to maintain these conditions. If the power available from the powerplant is adjusted to equal the power required, the rate of climb is zero (Pa—Pr=0′). This is illustrated in figure 6.1 where the power available is set equal to the power required at velocity (A). If the airplane were in steady level flight at velocity (A), an increase in power available would create an excess of power which will cause a rate of climb. Of course, if the speed were allowed to increase by a decreased angle of attack, the increased power setting could simply maintain altitude at some higher airspeed. However, if the original aerodynamic conditions are maintain­ed, speed is maintained at (Л) and an increased power available results in a rate of climb. Also, a decrease in power available at point (A) will produce a deficiency in power and result in a negative rate of climb (or a rate of descent). For this reason, it is apparent that power setting is the primary control of altitude in steady flight. There is the direct correlation between the excess power ([Pa—Pr), and the airplane rate of climb, RC.

FLYING TECHNIQUE. Since the condi­tions of steady flight predominate during a majority of all flying, the fundamentals of flying technique are the principles of steady flight:

(1) Angle of attack is the primary control of airspeed.

(2) Power setting is the primary control

of altitude, i. e., rate of clirhb/descent.

With the exception of the transient conditions of flight which occur during maneuvers and acrobatics, the conditions of steady flight will be applicable during such steady flight condi­tions as cruise, climb, descent, takeoff, ap­proach, landing, etc. A clear understanding of these two principles will develop good, safe flying techniques applicable to any sort of airplane.

The primary control of airspeed during steady flight conditions is the angle of attack. However, changes in airspeed will necessitate changes in power setting to maintain altitude because of the variation of power required with velocity. The primary control of altitude (rate of climb/descent) is the power setting. If an airplane is being flown at a particular airspeed in level flight, an increase or decrease in power setting will result in a rate of climb or descent at this airspeed. While the angle of attack must be maintained to hold airspeed in steady flight, a change in power setting, will necessitate a change in attitudeto. accommodate the new flight path direction. These princi­ples form the basis for "attitude” flying tech­nique, i. e., "attitude plus, power equals per­formance," and provide, a background for good instrument flying technique as well as good flying technique for all ordinary flying conditions.

One of the most important phases of flight is the landing approach and it is during this phase of flight that the principles of steady flight are so applicable. If, during the landing approach, it is realized that ;the airplane is below the desired glide path, an increase in nose up attitude will not insure that the airplane will climb to the desired glide path. In fact, an increase in nose-up attitude may produce a greater rate of descent and cause the airplane to sink more below the desired glide path. At a given airspeed, only an increase in power setting can cause a rate of climb (or lower rate of descent) and an in­

crease in nose up attitude without the appro­priate power change only controls the airplane to a lower speed.

REGION OF REVERSED COMMAND

The variation of power or thrust required with velocity defines the power settings neces­sary to maintain steady level flight at various airspeeds. To simplify the situation, a gener­ality could be assumed that the airplane con­figuration and. altitude define a variation of power setting required (jet thrust required or prop power required) versus velocity. This general variation of required power setting versus velocity is illustrated by the first graph of figure 6.2. This curve illustrates the fact that at low speeds near the stall or minimum control speed the power setting required for steady level flight is quite high. However, at low speeds, am increase in speed reduces the required power setting until some minimum value is reached at the conditions for maximum endurance. Increased speed beyond the con­ditions for maximum endurance will then increase the power setting required for steady level flight.

REGIONS OF NORMAL AND REVERSED COMMAND. This typical variation of re­quired power setting with speed allows a sort of terminology to be assigned to specific regimes of velocity. Speeds greater than the speed for maximum endurance require increas­ingly greater power settings to achieve steady, level flight. Since the normal command of flight assumes a higher power setting will achieve a greater speed, the regime of flight speeds greater than the speed for minimum required power setting is termed the “region of normal command.” Obviously, parasite drag or parasite power predominates in this regime to produce the increased power setting required with increased velocity. Of course, the major items of airplane flight performance take place in the region of normal command.

Flight speeds below the speed for maximum endurance produce required power settings

which increase with a decrease in speed. Since the increase in required power setting with decreased velocity is contrary to the normal command of flight, the regime of flight speeds between the speed for minimum required power setting and the stall speed (or minimum control speed) is termed the “region of re­versed command.” In this regime of flight, a decrease in airspeed must be accompanied by an increased power setting in order to main­tain steady flight. Obviously, induced drag or induced power required predominates in this regime to produce the increased power setting required with decreased velocity. One fact should be made clear about the region of reversed command: flight in the “reversed" region of command does not imply that a decreased power setting will bring about a higher airspeed or an increased power setting will produce a lower airspeed. To be sure, the primary control of airspeed is not the power setting. Flight in the region of re­versed command only implies that a higher airspeed will require a lower power setting and a lower airspeed will require a higher power setting to hold altitude.

Because of the variation of required power setting throughout the range of flight speeds, it is possible that one particular power setting may be capable of achieving steady, level flight at two different airspeeds. As shown on the first curve of figure 6.2, one given power setting would meet the power requirements and allow steady, level flight at both points I and 2. At speeds lower than point 2, a deficiency of power | would exist and a rate of descent would be in­curred. Similarly, at speeds greater than point 1, a deficiency of power would exist and the I airplane would descend. The speed range be­tween points 1 and 2 would provide an excess of power and climbing flight would be pro­duced.

FEATURES OF FLIGHT IN THE NOR­MAL AND REVERSED REGIONS OF COM­MAND. The majority of all airplane flight is conducted in the region of normal command,

e. g., cruise, climb, maneuvers, etc. The region of reversed command is encountered primarily in the low speed phases of flight during takeoff and landing. Because of the extensive low speed flight during carrier operations, the Naval Aviator will be more familiar with the region of reversed command than the ordinary pilot.

The characteristics of flight in the region’of normal command are illustrated at point A on the second curve of figure 6.2. If the airplane is established in steady, level flight at point A, lift is equal to weight and the power available is set equal to the power required. When the airplane is disturbed to some airspeed slightly greater than point A, a power deficiency exists and, when the airplane is disturbed to some air­speed slightly lower than point A, a power excess exists. This relationship provides a tendency for the airplane to return to the equili­brium of point A and resume the original flight condition following a disturbance. Also, the static longitudinal stability of the airplane tends to return the airplane to the original trimmed CL and velocity corresponding to this CL. The phugoid usually has most satisfactory qualities at low values of CL so the high speed of the region of normal command provides little tendency of the airplane’s airspeed to vary or wander about.

With all factors considered, flight in the region of normal’ command is characterized by a relatively strong tendency of the airplane to maintain the trim speed quite naturally. How­ever, flight in the region of normal command can lead to some unusual and erroneous impres­sions regarding proper flying technique. For example, if the airplane is established at point A in steady level flight, a controlled increase in airspeed without a change in power setting will create a deficiency of power and cause the airplane to descend. Similarly, a controlled decrease in airspeed without a change in power setting will create an excess of power and cause the airplane to climb. This fact, coupled with the transient motion of the airplane when the angle of attack is changed rapidly, may lead to the impression that rate of climb and descent can be controlled by changes in angle of attack. While such is true in the region of normal com­mand, for the conditions of steady flight, pri­mary control of altitude remains the power setting and the primary control of airspeed re­mains the angle of attack. The impressions and habits that can be developed in the region of normal command can bring about disastrous consequences in the region of reversed com­mand.

The characteristics of flight in the region of reversed command are illustrated at point В on the second curve of figure 6.2. If the air­plane is established in steady, level flight at point B, lift is equal to weight and the power available is set equal to the power required. When the airplane is disturbed to some air­speed slightly greater than point B, an excess of power exists and, when the airplane is dis­turbed to some airspeed slightly lower than point B, a deficiency of power exists. This relationship is basically unstable because the variation of excess power to either side of point В tends to magnify any original dis­turbance. While the static longitudinal sta­bility of the airplane tends to maintain the original trimmed CL and airspeed correspond­ing to that CL, the phugoid usually has the least satisfactory qualities at the high values of CL corresponding to low speed flight.

When all factors are considered, flight in the region of reversed command is characterized by a relatively weak tendency of the airplane to maintain the trim speed naturally. In fact it is likely that the airplane will exhibit no inherent tendency to maintain the trim speed in this regime of flight. For this reason, the pilot must give particular attention to precise control of airspeed when operating in the low flight speeds of the region of reversed command.

While flight in the region of normal com­mand may create doubt as to the primary con­trol of airspeed and altitude, operation in the region of reversed command should leave little

doubt about proper flying techniques. For example, if the airplane is established at point В in level flight, a controlled increase in air­speed (by reducing angle of attack) without change in power setting will create an excess of power at the higher airspeed and cause the airplane to climb. Also, a controlled decrease in airspeed (by increasing angle of attack) without a change of power setting will create a deficiency of power at the lower airspeed and cause the airplane to descend. This rela­tionship should leave little doubt as to the primary control of airspeed and altitude.

The transient conditions during the changes in airspeed in the region of reversed command are of Interest from the standpoint of landing flare characteristics. Suppose the airplane is in steady flight at point В and the airplane angle of attack is increased to correspond with the value for the lower airspeed of point C (see fig. 6.2). The airplane would not instanta­neously develop the lower speed and rate of descent common to point C but would approach the conditions of point C through some tran­sient process depending on the airplane char­acteristics. If the airplane characteristics are low wing loading, high L/D, and high lift curve slope, the increase in angle of attack at point В will produce a transient motion in which curvature of the flight path demonstrates a definite flare. That is, the increase in angle of attack creates a momentary rate of climb (or reduction of rate of descent) which would be accompanied by a gradual loss of airspeed. Of course, the speed eventually decreases to point C and the steady state rate of descent is achieved. If the airplane characteristics are high wing loading, low L/D, and low lift curve slope, the increase in angle of attack at point В may produce a transient motion in which the airplane does not flare. That is, the increase in angle of attack may produce such rapid re­duction of airspeed and increase in rate of descent that the airplane may be incapable of a flaring flight path without an increase in power setting. Such characteristics may neces­sitate special landing techniques, particularly in the case of a flameout landing.

Operation in the region of reversed command does not imply that great control difficulty and dangerous conditions will exist. However, flight in the region of reversed command does amplify any errors of basic flying technique. Hence, proper flying technique and precise control of the airplane are most necessary in the region of reversed command.