Pilot Training - Theory of Flight: Equilibrium, Stability and Control

118. Reference axes.-The motion of the airplane may be studied with reference to three axes fixed in the airplane with their origin at the center of gravity:

a. The longitudinal or X axis is drawn through the center of gravity parallel to the relative wind.

b. The lateral of Y axis is perpendicular to the X axis and is horizontal when the airplane is on an even keel.

c. The vertical or Z axis is drawn through the center of gravity perpendicular to both the X axis and the Y axis.

119. Angular motion.-In flight the airplane may not only move from one point in space to another (motion of translation), but it may also rotate about its center of gravity. In studying, the motion of the airplane, of interest are not only translational velocities and accelerations but also rotational velocities and accelerations. (See fig. 1.)

a, Roll-Angular movement about the X axis is called "roll" and is considered positive when the airplane banks to the right.

b. Pitch.-Angular movement about the Y axis is called "pitch", positive if the nose of the airplane rises, negative if it falls. A positive pitching moment or a stalling moment is a moment which tends to make the nose of the airplane rise. A negative pitching moment or a diving moment is a moment which tends to make the nose of the airplane fall.

C. Yaw.-Angular movement about the Z axis is called "yaw", considered positive when the turn is to the right.

120. Angle of incidence.-The angle of incidence is the angle between the chord of the wing and an arbitrary reference line fixed in the airplane. This reference line is usually drawn through the center of gravity parallel to the crankshaft of the engine and called the longitudinal axis of the airplane. This axis usually coincides with the X axis as defined in paragraph 118 when the airplane is in level flight at normal cruising speed. The angle of incidence is built into the airplane structure and must not be confused with angle of attack. The angle of incidence is usually so designed that at normal cruising speed in level flight the longitudinal axis of the airplane is horizontal.

121. Relative wind.-In calm, still air, the direction of the relative wind is opposite to the flight path of the airplane referred to the ground. When the wind blows, the path of the airplane referred to the ground is not the same as the path referred to the moving air. The aerodynamic forces on the airplane are always a function of the relative wind and not of the ground wind.

122. Forces acting on complete airplane in level flight.-In unaccelerated flight, three fundamental equations of mechanics must be satisfied.

åH = 0   åV = 0    åM = 0

The horizontal forces in level flight are the propeller thrust acting in a forward direction and the sum of all the drag forces acting to the rear. For equilibrium,

Thrust=total drag

The vertical forces in level flight are the force of gravity (equal to the weight of the airplane), the lift of the wings, and the tail load (usually a down load). For equilibrium,

 Lift =  weight+ tail load.

The center of gravity is taken as the origin of moments. The third requirement for equilibrium (fig. 97) is such that the sum of all moments must equal to zero.

(Thrust x c) + (drag X b) + (tail load x e) = (lift X a).

Of the forces acting on the airplane, the lift and weight remain nearly constant, the thrust can be changed by manipulation of the engine throttle, the drag forces are a function of the speed of flight. The center of gravity can be shifted only by a redistribution of weights.

The manipulation of the elevators by the pilot places at his disposal a powerful means of changing the tail load, and since the moment arm of the tail load is comparatively long, large changes in tail load moment can be made at will by the pilot. The equilibrium of the airplane is maintained by the adjustment of the tail load moment to neutralize the moments of the remaining forces in such manner that the condition åM=0 is satisfied.

123. Equilibrium in climb.-In a climb, the center of pressure at the larger angles of attack moves forward thus decreasing the moment arm of the lift force. A corresponding decrease in tail load moment is required for equilibrium. The fundamental forces may be resolved into components parallel to and perpendicular to the relative wind, and the three equations of equilibrium become

Thrust_H = drag + weight_H

Lift + thrust_v= weight_v + tail load

(Thrust_H x c) + (thrust_v x f) + (drag x b) +(tail load x e) = Iift x a

where the subscripts H and V indicate the components of the forces parallel to and perpendicular to the relative wind respectively.

124. Equilibrium in glide.-In a glide without power, the component of the weight parallel to the relative, wind becomes the motive power, and the equations of equilibrium may be written

Weight_H = drag

Lift= weight_v + tail load

(Drag x b) + (tail load x e) = (lift x a).

It will be observed that the cotangent of q, the angle of inclination of the flight path to the horizontal is,

If the best gliding angle be defined as the flattest gliding angle, the angle of best glide occurs at the speed where the L/D ratio for the complete airplane, is a maximum. Since the L/D ratio is not affected by the air density, the angle of best glide, is the same for all altitudes. The further conclusion may be drawn that, since the L/D ratio is purely an aerodynamic characteristic of the external shape of the airplane, the angle of best glide is the same regardless of the total weight of the airplane, that is, whether with full load on board or empty. The speed of best glide is affected by air density and loading conditions, but not the gliding angle.

125. Equilibrium in dive.-In a vertical dive there is no resultant lift force, but a large down load on the front spar of the wing and an up load on the rear spar require a down load on the tail surfaces for balance. The equations of equilibrium may be written

Weight=drag

Lift_R= lift_F + tail load

(Lift_F x a_F) + (Lift_R x a_R) = (drag x b) + (tail load x e)

In a vertical dive, the velocity will accelerate until the drag equals the weight. The velocity at which a state of equilibrium is reached is the terminal velocity. Due to the decrease of air density with altitudel the terminal velocity for a given airplane will be greater at high altitudes than at sea level.

126. Effect of throttle setting on balance.-In general, a change in throttle setting disturbs the balance of the airplane and requires a readjustment of the stabilizer or elevator setting. The factors contributing to the change in balance are many.

a. Unless the line of action of the thrust force passes through the center of gravity, the thrust moment will change with throttle setting.

b. The slipstream velocity changes with throttle setting. Since the tail surfaces are usually located in the slipstream, any change in slipstream velocity directly affects the load on the horizontal tail surfaces and a corresponding change in the tail load moment occurs.

c. The change in speed of the airplane with change in the engine power available results in a shift of the center of pressure of the air forces on the wings with consequent changes in the moments of these forces.

d. The angle of downwash e is the angular deflection of  the airstream due to the lift produced by the wings. At slow speeds the angle E is larger than at high speed. Any change in speed, the result of a change in throttle setting, will then produce a change in the angle of downwash e. As the tail surfaces of the conventional airplane are located in the downwash, any change in E contributes to a change in tail load moment.

e. Unless the line of action of the parasite drag passes through the center of gravity, the change in drag produced by change in speed will result in change of the parasite drag moment.

1. In some airplanes the disturbing factors may tend to neutralize each other, so that the effect of change of throttle setting on balance will be small. If the disturbing factors cumulate, the balance of the airplane will be greatly influenced by throttle setting, requiring large changes in the setting of the horizontal stabilizer or use of the elevator controls to bring the airplane in to equilibrium. It is a problem of design to adjust these factors so that the flight characteristics of the airplane will be satisfactory.

127. Definition of stability.-A body is said to be in stable equilibrium if, when slightly disturbed from a condition of equilibrium, forces or moments are developed of a character such that they tend to restore the body to its original state. In a condition of neutral stability, after disturbance, the body tends neither to return to its original state nor to move further from it. A state of equilibrium is said to be unstable when after disturbance, the forces and moments tend to move the body further away from its original state.

Thus a round ball at the bottom of a convex surface is in stable equilibrium, for if it is disturbed it will roll back to its original position. If the ball is on a flat level surface it is in neutral equilibrium, for if displaced from its original position it neither returns to its original position nor will the disturbance tend to increase. If the ball is balanced at the top of a, concave, surface it is in unstable equilibrium, for if disturbed the forces acting on the ball tend immediately to increase the disturbance

128. Static and dynamic stability.-The conditions discussed in paragraph 127 are the conditions of static stability. Dynamic stability treats with the oscillations that are set up as the result of a system of restoring forces or moments when a body is disturbed.

A pendulum, when disturbed from its position of rest, is acted upon by the forces of gravity tending to restore it to its original position of rest. The system is statically stable. But the pendulum does not return to its original position and stop, it oscillates. Ordinarily the, forces of friction damp out the oscillations, the amplitude of each oscillation is less than the one before, the system is not only statically stable but dynamically stable. In the extreme case where the forces of friction are so large that the pendulum returns slowly to its position of rest without oscillation, its dynamic stability is dead beat.

If, at each swing of the pendulum, a small force is applied as the bob starts on the downward swing, the damping forces, of friction may be counterbalanced and the pendulum will swing indefinitely. Such a system is statically stable, but has, neutral dynamic stability.

It is quite possible that the force applied to the bob at each end of the swing may be larger than that necessary merely to counterbalance the forces of friction, in which case the amplitude of the oscillations will increase. Although this system is statically stable, it is dynamically unstable.

129. Motion of airplane.-The motion of the airplane may be classed as statically stable or unstable, and if statically stable may in addition be classed as dynamically stable or unstable. This classification may be made for the motion of the airplane about each of the three principal axes, the longitudinal or pitching motion, the lateral or rolling motion, and the directional or yawing motion.

130. Requirements for longitudinal stability.-In normal level flight, the airplane is balanced longitudinally by adjustment of the horizontal tail surfaces. If the attitude of the airplane then be slightly disturbed by a gust or by temporary manipulation of the elevator control, the longitudinal motion, if not checked by further manipulation of the elevator control, may proceed in one of five ways:

a. The airplane may return to its original attitude without oscillation. (Statically stable, and dead beat dynamically stable.)

b. The airplane may show no tendency to change its new attitude. (Neutral static stability.)

c. The airplane may oscillate with decreasing amplitude until it assumes its original flight attitude. (Statically and dynamically stable.)

d. The airplane may oscillate with increasing amplitude. (Static stability, dynamic instability.)

e. The angle of attack may continue to increase or continue to decrease depending on the sense of the original disturbance. (Static instability.)

The airplane in its longitudinal motion may be statically and dynamically stable but be unstable in roll or yaw. However, in its pitching motion, the cases of dynamic instability are extremely rare.

The oscillations damp out with greater or less rapidity according to the airplane design. But there are frequent cases in service aircraft of static instability in pitch, practically without exception the result of faulty or ill-considered design. Such airplanes require constant effort on the part of the pilot to maintain steady flight.

131. Pitching moment curves.-It is common practice to run a wind tunnel test on the model of the complete airplane to obtain pitching moment curves. This test represents quite accurately the condition of flight with power off, and by estimating the effects of the thrust moment and slipstream the designer is enabled to determine the longitudinal stability characteristics of a proposed airplane while it is still in the drafting board stage. The wind tunnel test of the model is run for several different settings of the horizontal stabilizer. The pitching moments about the center of gravity of the full scale airplane are then plotted as ordinates against angle of attack as abscissae. The angle of attack at which the curve cuts the X axis is the point where the equation åM=0 is satisfied and is the angle of attack for equilibrium in flight for the particular stabilizer setting. The criterion for static stability is that the slope of the moment curve be negative. At all points where the slope is positive, static instability is indicated. At points where the curve is parallel to the X axis, neutral stability is indicated. The steeper the slope of the curve, the greater the change of pitching moment when the attitude of the airplane, that is, the angle of attack, is disturbed, and consequently the larger the restoring moments. A brief visual inspection of the curve of pitching moments is sufficient to disclose a great deal of information about the stability and flying characteristics of an airplane.

a. The general type of pitching moment curve shown in figure 102(1) is illustrative of excessive stability which is undesirable for the reason that excessive forces are required to change the attitude of the airplane.

b. The general type of pitching moment curve shown in figure 102(4) is illustrative of a type of instability which has acquired the descriptive title "catastrophic instability". The airplane is stable at the higher angles of attack but is extremely unstable at high speed or in a dive.

c. The general type of pitching moment curve shown in figure 102(3) is illustrative of a. type of instability that is characteristic of some airplanes in the cruising speed range. Steady flight at cruising speeds is difficult to maintain in an airplane of this type. The airplane has a tendency to "hunt" and requires constant effort on the part of the pilot for control.

d. The type of pitching moment curve shown in figure 102@ is in general the most desirable. The pitching motion is stable so that the airplane tends to maintain a constant flight attitude, but the forces required for longitudinal control throughout the flight range are small and within the physical capabilities of the pilot.

132. Factors influencing longitudinal stability.-a. The chief factors influencing longitudinal stability are as follows:

(1) The larger the ratio of horizontal tail area to wing area the greater the stability.

(2) The less the center of pressure of the wings shifts with angle of attack, the greater the stability of the complete airplane.

(3) The greater the distance from the tail surfaces to the center of gravity the greater the degree of stability.

(4) The greater the aspect ratio of the horizontal tail surfaces the greater the degree of stability.

(5) Downwash has a very marked adverse effect on stability.

(6) The type of airfoil section used for the horizontal tail surfaces influences stability.

(7) The vertical location of the center of gravity with respect to the wing chord has marked influence on stability. The tendency of a center of gravity location above the wing chord is for the system to be less stable than where the center of gravity is below the wing chord. The prevalent conception that this tendency is due to a pendulum effect is, however, entirely erroneous.

(8) The horizontal location of the center of gravity with respect to the wing chord has a marked influence on stability. The general tendency in modern design is to fix the location of the center of gravity in the range 25 to 30 percent from the leading edge of the mean aerodynamic chord. Movement of the center of gravity toward the trailing edge leads to instability.

(9) The slipstream and the thrust moment have their effects on stability. The stability characteristics of airplanes vary to a considerable degree with power on and power off.

b. In service aircraft, a distribution of the load carried in a manner other than the designed distribution may so shift the center of gravity of the complete airplane that very undesirable flight characteristics are developed. A slight shift in center of gravity location to the rear may change the pitching moment characteristics from the type shown in figure 102(2) to those shown in figure 102(3). For this reason, many aircraft of the cargo carrying type have loading distribution schedules posted for the information of the pilot who is required to supervise the loading of the aircraft in such manner that its flying characteristics may not be dangerously impaired.

133. Lateral stability and directional stability.-The motions of yaw, roll, and sideslip are all so interrelated that lateral stability and directional stability can only with great difficulty be considered independently. The same forces that produce motion in roll also produce motion in yaw, although these forces are frequently more effective in their results on the one motion or the other. Static lateral stability may be defined as the characteristic of the airplane which tends to restore the airplane to an even keel after it has been tipped sideways. The airplane may return to an even keel and then tip to the other side, the motion being oscillatory in the same sense as oscillations in pitch. Should the oscillations damp out, the lateral motion is dynamically stable. In the lateral motion, the restoring moments are due to the

resulting sideslip which occurs when the airplane is tilted. The sidewise component of the relative wind when sideslip occurs may be utilized to secure lateral stability as follows:

a. By raising the wing tips so that a dihedral angle is formed by the wing, the lower wing acquires additional lift thus producing a restoring moment. This is illustrated in figure 104.

b. Distribution of a larger vertical fin area above the center of gravity than below it places the center of resistance of the fin area in a sideslip above the center of gravity and a restoring moment is thus produced when sideslip occurs.

c. The concentration of a large fin area in the vertical tail surfaces far to the rear of the center of gravity produces a yawing moment in sideslip and the airplane acts much like a weather vane. As the yawing effect produced by the vertical tail surface heads the airplane into the wind, one wing moves forward faster than the other and thus a lifting force is produced tending to increase the bank. The increased bank produces increased sideslip, the cycle repeating itself and resulting in a motion known as spiral instability.

d. Directional stability is secured by so distributing the vertical fin area that the resultant air forces in sideslip act to the rear of the center of gravity and so force the nose of the airplane into the wind. As indicated in c above, excessive directional stability combines with the rolling motion to produce spiral instability. The proper adjustment of vertical fin area to secure directional stability and to avoid spiral instability is a problem that the designer must successfully solve if the flight characteristics of the airplane are to be satisfactory.

e. Sweepback in modern aircraft is a device which the designer uses principally to secure the proper horizontal location of the center of gravity with respect to the mean aerodynamic chord. It incidentally has an effect on directional stability since in a sideslip the drag forces on the two wings are unequal and a stable restoring moment in yaw is produced. Conversely, a negative sweepback has an unfavorable effect on directional stability.

134. Dynamic instability in yaw and roll.-A form of dynamic instability in roll and yaw is occasionally observed which is the result of the vertical tail surfaces being blanketed by the fuselage, usually of the large cargo type. Due to the deflection of the airstream by the fuselage past the vertical tail surfaces, the airplane is unstable in yaw for a limited range of angles. As the airplane yaws, the vertical tail surfaces move out past the dead air space directly behind the fuselage and a stable restoring moment is produced. The resultant motion is a constant hunting in direction and a constant rolling motion from side to side due to the unbalanced lift forces on the wings as the airplane oscillates.

135. Flutter and buffeting.-a. Wings with large unbraced overhang and cantilever monoplane wings are subject to large deflections at the wing tip. As the center of pressure shifts along the chord, the spars deflect by unequal amounts with resultant changes in angle of attack of the portion of the wing toward the tip. This tends to reduce the effectiveness of the aileron control since the change in center of pressure location produced by operation of the ailerons produces a torsional deflection of the wing tip and a change in angle of attack.

b. The torsional deflection of the wing tip induced by aileron operation in some cases results in a dynamically unstable oscillation known as "wing flutter". As the wing deflects, the inertia of the aileron causes its trailing edge to lag behind the motion of the wing, producing a change in effective camber of the wing and a shift of the center of pressure with further wing deflection. The elastic properties of the wing cause it to vibrate, and when the inertia characteristics of the aileron control system cause it to vibrate at the same period as the wing structure, an unstable oscillation of large amplitude is rapidly built up which will disintegrate the wing structure unless checked. The methods used to eliminate this form of instability are as follows:

(1) Design the wing structure for a large degree of torsional stiffness.

(2) Balance the aileron statically about the hinge line by placing balancing weights in the portion of the aileron ahead of the hinge.

(3) Place the aileron control system under considerable initial tension to remove all lost motion.

c. In flight, torsional oscillation or "wing flutter" usually occurs at the higher speed ranges or in a dive. The remedy available to the pilot is promptly to reduce speed and fly at the larger angles of attack until a landing can be effected.

d. A similar oscillation or flutter occurs in propeller blades. In this case the exciting force is supplied by the engine, usually the impulse of the explosion in each cylinder. When the period of the exciting force coincides with the elastic period of vibration of the propeller, large deflections are produced which eventually result in propeller failure. The remedy is to design the engine so that the magnitude of the exciting forces is small and of such frequency that resonance with the natural periods of vibration of the propeller will not occur within the operating speed range.

e. Excessive engine vibration has been known to furnish the exciting force resulting in flutter of wings in much the same manner as aileron flutter. The remedy is to reduce engine speed to avoid the resonant period and the flutter will die out.

f. The unstable airflow behind the wing at stalling angles of attack has in some airplanes furnished a variable exciting force resulting in a vibration of the tail surfaces known as "buffeting". Surfaces located in the propeller slipstream may also be excited to excessive vibration by the periodically varying changes in slipstream velocity due to each individual propeller blade.

g. The number and variety of the phenomena that can be classed as cases of dynamic instability are large and require attention and correction in every new type of aircraft.

136. Control.-Control of the motion of the airplane is secured through manipulation of any one or all three sets of control surfaces, that is, the elevator, the rudder, and the ailerons. Control should be positive and effective throughout the flight range. Control and stability are interdependent. Where stability is excessive, powerful controls are necessary to effect, changes in attitude. It was formerly considered necessary deliberately to design longitudinally unstable airplanes for types where extreme maneuverability was an essential requirement. A better understanding of the problem, however, has shown that there is no necessity for the sacrifice of a moderate degree of stability to secure service aircraft that will perform with the required degree of maneuverability.

137. Design of control surfaces.-a. A prime factor in securing easy and effective control is to design the control system so that large air loads are the result of a small amount of physical exertion by the pilot. By hinging a relatively small movable flap along the rear edge of a larger fixed surface, a change in the setting of the flap has the effect of changing the effective camber of the entire surface. The major part of the air load is carried by the fixed surface and relatively small loads are transmitted to the control system by the movable flap. This method of designing control surf aces has become standard for nearly all aircraft.

b. The hinge moments required to move the control flaps may be further reduced by placing a portion of the surface of the movable

control flap ahead of the hinge line. The air loads ahead of the hinge line and behind the hinge line tend to balance each other.

c. The use of trimming tabs to balance the airplane is coming into increased use as a substitute for adjustable stabilizing surfaces. The

tab is a small adjustable surface fixed at the trailing edge of the movable control surface. The air load on the tab produces a moment about the control hinge and deflects the control surface, producing a control moment but without the exertion of effort on the part of the pilot. The tabs may be adjustable on the ground only, as is usually the case with aileron tabs, or may be adjustable at the will of the pilot in flight, as is frequently the case with rudder and elevator control tabs.

d. The flettner control is a trimming tab adjustable by the pilot in flight. By this means the rudder control may be adjusted to relieve the pilot of the constant strain of correcting for an unbalanced yawing moment, or the elevator tab may be used to trim the longitudinal balance of the airplane for different loading conditions or speeds of flight.

e. The aileron trimming tab eliminates the necessity for the wash-in or wash-out of angle of incidence to counteract the effects of enngine torque, and is a much simpler method of securing lateral balance.

138. Longitudinal control.-The horizontal tail surfaces must be designed to give adequate longitudinal stability as well as to meet the requirements of control. The tail surfaces are also most effective in damping the pitching oscillations. The distribution of area between the fixed stabilizer and the movable elevator is adjusted so that the smallest elevator feasible with adequate control is used. This arrangement results in minimum control forces. The stabilizer-elevator combination must produce pitching moments sufficient to pull the airplane out of a steep dive and to get the tail down in landing.

139. Directional control.-Directional control is effected by the rudder-fin combination, the same considerations in general governing the distribution of the total area as in the case of the stabilizer-elevator combination. The slipstream as it leaves the propeller tends to follow a helical path, and consequently exerts a greater pressure on one side of the rudder-vertical fin than on the other. A yawing moment is the result which may be offset by use of a trimming tab or by offsetting the leading edge of the vertical fin. In multimotored aircraft, a large, yawing moment is produced when one engine becomes dead. In such aircraft, the rudder control must be sufficiently effective to restore the balance. Provision by means of a flettner control to relieve the pilot of the strain of supplying a continuous control force to the rudder in order to maintain straight flight in cases of engine failure is becoming standard practise. The rudder control is considerably more effective at high speed than at low speed. in particular at speeds near the stall, the rudder control loses its effectiveness to a marked degree.

140. Lateral control.-The aileron control is most effective at high speed. As the stalling speed is approached the aileron control becomes sluggish. Accompanying the rolling effect of the ailerons is a yawing moment due to a redistribution of the drag on the wings. Moving the aileron down increases the drag on that wing, the resultant yaw causes the other wing to move faster tending to produce greater lift and consequent banking. This tendency operates against the normal aileron action and at stalling speeds may result in reversed aileron control action. The Frise balance now widely adopted for ailerons is so hinged that the nose of the lifted aileron drops below the wing surface and produces a drag on that wing. The tendency is to counterbalance the drag produced by the other aileron which is pulled down and thus counteract the tendency to reversal of the aileron control.

141. Autorotation.-At speeds less than the stalling speed the airplane becomes unstable laterally and the action of the aileron control is reversed. Any disturbance tending to increase the angle of attack on one wing decreases the lift on that wing causing a rolling moment. The increased angle of attack is accompanied by an increase in drag producing a yawing moment. The combined yawing and rolling motions result in a spin with the nose well down where equilibrium is established at a rate of descent and speed of rotation depending upon the characteristics of the airplane. Many airplanes, if permitted to spin in the normal manner for a sufficient period of time, develop a flat spin. The immediate difference between the normal and the flat spin is the attitude of the airplane. Whereas the nose is pointed well down in the normal spin, with the preponderant motion one of roll, in the flat spin the nose approaches the horizon and the motion becomes one of nearly pure yaw. Airplanes having flat spin characteristics do not evidence them at the outset of the spin, but develop the flat spin from the normal spin. In the flat spin the aerodynamic forces on the elevators are very large and the control surfaces become largely ineffective. Recovery is always slow and difficult and frequently impossible.

142. Factors affecting spins.-An airplane must first be stalled before it will spin. Some airplanes will not spin for the reason that the elevator control is so ineffective that the airplane cannot be

forced to a stalling angle of attack. By choice of wing section and the design and arrangement of gap, stagger, and decalage, the peak of the lift curve of the wing may be so flattened that spinning is difficult. The tendency of the airplane to spin depends upon the mass distribution, the shape of the wing structure, the position of the center of gravity, the area of the exposed fuselage, and the vertical tail group and its distance from the center of gravity. Since the flat spin occurs at angles of attack approaching 90°, the shape of the lift and drag curves at angles up to 90° may be expected to influence flat spinning tendencies and characteristics. Such has indeed proved to be the case.

143. Recovery from spins.-The National Advisory Committee for Aeronautics has made flight tests to study the spinning characteristics of various airplanes for a period of many years. For example over 900 spin tests were made with one airplane whose spinning characteristics were vicious at times due to modification in the load distribution. As a result of these, tests, the conclusion was reached that no method of control manipulation for recovery from prolonged spins is infallible for all airplanes. Since dangerous spins develop from the normal spin, prolonged spinning should be prohibited. The following rules have been found generally applicable to recovery from spins:

a. During the spin before recovery is attempted, the ailerons should be neutral and the elevator and the rudder controls should be held all the way with the spin.

b. When applying controls for recovery from a vicious spin, the rudder should be briskly moved to a position full against the spin and later, after at least one-half additional turn is made, the elevator should be moved to the full down position.

c. In a vicious spin, the applied controls should be held for at least five turns before attempting any other measure for promoting recovery.

144. Wing tip stalling.-a. The turbulent airflow that occurs when the wing stalls may not occur simultaneously at all points along the wing. The tapered wing now favored for cantilever monoplanes in many designs stalls very abruptly at the wing tip with consequent loss of aileron control. The use of the aileron to lift the dropping wing usually makes matters worse and spin develops rapidly unless the airplane is inherently stable against spinning.

b. The use of a considerable degree of static longitudinal stability, thus providing a, definite warning of the approaching stall through the backward movement, position, and forces on the control column, together with a gradually developing stall secured by allowing the upper or lower wing of a biplane to stall first, or the use of monoplanes with little or no taper and "poor" wing-fuselage junctures which tend to bring about a gradually developing stall beginning at mid span are measures that insure that the stalled condition will develop progressively after a reasonably definite warning. Furthermore, these measures result in maintenance of lateral control owing to the fact that the essentially effective parts of the wing system remain unstalled even after the angle of attack has exceeded that of maximum lift.

c. Modem design trends toward high wing loadings and landing speeds; the use of efficient high speed airfoils having less desirable stalling characteristics; the use of highly tapered monoplane wings; the low wing position which contributes to reduced longitudinal stability; the use of "good" wing-fuselage junctures; and, finally, high lift devices add to the dangers of tip stalling and vicious section stalls resulting in sudden large and usually unsymmetrical loss of lift. The worst offenders may give no indication of the approaching stall which, when it occurs, is manifested by a sudden uncontrolled rolling dive, that results from a sudden loss of lift on one wing and a simultaneous loss of lateral control.

d. The airplane, which stalls without warning is a menace in the hands of the less skillful pilot, and must be flown with such liberal margins of safety by even the most skillful Hot that full advantage cannot be taken of its performance characteristics.