Aerodynamics of Manouevring Flight

Three Axes of Flight

All manoeuvring flight takes place around one or more of the three axes of rotation. They are called the longitudinal, lateral and vertical axes of flight. The common reference point for the three axes is the airplane’s centre of gravity (CG), which is the theoretical point where the entire weight of the airplane is considered to be concentrated. Since all three axes pass through this point, you can say that the airplane always moves about its CG, regardless of which axis is involved. The ailerons, elevator, and rudder create aerodynamic forces which cause the airplane to rotate about the three axes.

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Longitudinal axis
Now consider what happens when you apply control pressure to begin a turn. When you deflect the ailerons, they create an immediate rolling movement about the longitudinal axis. Since the ailerons always move in opposite directions, the aerodynamic shape of each wing and its production of lift is affected differently.
One of the first things you will learn during flight is that the rolling movement about the longitudinal axis will continue as long as the ailerons are deflected. To stop the roll, you must relax control pressure and return the aileron to their original or neutral position. This is called neutralizing the controls.

Roll movement about the longitudinal axis is produced by the ailerons.

LATERAL AXIS
Since the horizontal stabilizer is an airfoil, the action of the elevator (or stabilator) is quite similar to that of the aileron. Essentially, the chord line and effective camber of the stabilizer are changed by deflection of the elevator.

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Pitch movement about the lateral axis is produced by the elevator (stabilator).

Movement of the control wheel fore and aft causes motion about the lateral axis. Typically, this is referred to as an adjustment to pitch, or a change in pitch attitude. For example, when you move the control wheel forward, it causes movement about the lateral axis that decreases the airplane’s pitch attitude. A decrease in pitch attitude decreases the angle of attack. Conversely, an increase in pitch attitude increases the angle of attack.

VERTICAL AXIS
When you apply pressure on the rudder pedals, the rudder deflects into the airstream. This produces an aerodynamic force that rotates the airplane about its vertical axis. This is referred to as yawing the airplane. The rudder may be displaced either to the left or right of centre, depending on which rudder pedals you depress.

Yaw movement about the vertical axis is produced by rudder

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FORCES ACTING ON A CLIMBING AIRPLANE
When you transition from level flight into a climb, you must combine the change in pitch attitude with an increase in power. If you attempt to climb just by pulling back on the control wheel to raise the nose of the airplane, momentum will cause a brief increase in altitude, but airspeed will soon decrease.

An airplane climbs because of excess thrust, not excess lift.

The amount of thrust generated by the propeller for cruising flight at a given airspeed is not enough to maintain the same airspeed in a climb. Excess thrust, not excess lift, is necessary for a sustained climb. In fact during a vertical climb, the wings supply no lift, and thrust is the only force opposing weight.

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FORCES ACTING ON A DESCENDING AIRPLANE
Let’s continue our discussion by considering the forces of weight, lift, thrust and drag as they affect a descending airplane. If you are using power during a stabilized descent, the four forces are in equilibrium. During the descent, a component of weight acts forward along the the flight path. As speed increases, this force is balanced by an increase in parasite drag.

In a descent, a component of weight acts forward along the flight path.

During a power-off glide, the throttle is placed in an idle position so the engine and propeller produce no thrust. In this situation the source of the airplane’s thrust is provided only by the component of weight acting forward along the flight path. In a steady, power-off glide, the forward component of weight is equal to and opposite drag.

CONSTANT AIRSPEED DESCENT
Once you have established a state of equilibrium for a constant airspeed descent, the efficiency of the glide will be affected if you increase drag. For example, if you lower the landing gear, both parasite and total drag increase. To maintain the airspeed you held before the landing gear was extended, you have to lower the nose of the airplane.
You can also increase drag by descending at a speed that creates more drag than necessary. Any speed, other than the recommended glide speed creates more drag. If you descend with the speed too high, parasite drag increases; and if you descend with speed too slow, induced drag increases.

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GLIDE ANGLE AND GLIDE SPEED
During a descent, the angle between the actual glide path of your airplane and the horizon usually is called the glide angle. Your glide angle increases as drag increases, and decreases as drag decreases. Since a decreased glide angle, or a shallower glide provides the greatest gliding distance, minimum drag normally produces the maximum glide distance.

The way to minimize drag is to fly at an airspeed that results in the most favourable lift-to-drag ratio. This important performance e speed is called the best glide speed. In most cases, it is the only speed that will give you the maximum gliding distance. However, with a very strong headwind, you may need a slightly higher glide speed, while a slower speed may be recommended to take advantage of a strong tail wind.

LIFT-TO-DRAG RATIO
The lift-to-drag ratio (L/D) can be used to measure the gliding efficiency of your airplane. The airspeed resulting in the least drag on your airplane will give the maximum L/D ratio (L/D max), the best glide angle, and the maximum gliding distance.

The higher the value of L/D max, the better the glide ratio.

GLIDE RATIO
The glide ratio represents the distance an airplane will travel forward, without power, in relation to altitude loss. For example, a glide ratio of 10:1 means that an airplane will descend one foot for every 10 feet of horizontal distance it travels. Since the throttle is closed in a power-off glide, the pitch attitude must be adjusted to maintain the best glide speed.

In the event of an engine failure, maintaining the best glide speed becomes even more important. This is especially true for a power failure after becoming airborne. Promptly establishing the correct gliding speed attitude and airspeed is critical. For a loss of power during flight, using the right speed could make the difference between successfully gliding to a suitable area or landing short of it.

If a power failure occurs after takeoff, immediately establish the proper gliding attitude and airspeed.

EFFECT OF WEIGHT ON THE GLIDE
Variations in weight do not affect the glide angle, provided you use the correct airspeed for each weight. Normally, optimum, or best, glide speeds are given in the pilot’s operating handbook (POH) for typical weight ranges. A fully loaded airplane requires a higher airspeed than the same airplane with a light load. Although the heavier airplane sinks faster and will reach the ground sooner, it will travel the same distance as a lighter airplane as long as you maintain the correct glide speed for the increased weight.

An airplane’s maximum gliding distance is unaffected by weight, but the best glide airspeed increases with weight.

FORCES ACTING ON A TURNING AIRPLANE
From our discussion about the three axes of rotation, you learned that ailerons control roll movement about the longitudinal axis, and the rudder controls yaw movement about the vertical axis. Coordinated turns require you to use both of these flight controls. You use the ailerons to roll into or out of a bank and, at the same time, you use the rudder to control yaw.

The horizontal component of lift causes an airplane to turn

Before your airplane turns, however, it must overcome inertia, or its tendency to continue in a straight line. You create the necessary turning force by banking the airplane so that the direction of lift is inclined. Now, one component of lift still acts acts vertically to oppose weight, just as it did in straight-and-level flight, while another acts horizontally. To maintain altitude, you will need to increase lift by increasing back pressure and, therefore, the angle of attack until the vertical component of lift equals weight. The horizontal component of lift, called centripetal force, is directed inwards, towards the centre of rotation. It is this centre seeking force which causes the airplane to turn. Centripetal force is opposed by centrifugal force, which acts outwards from the centre of rotation. When the opposing forces are balanced, the airplane maintains a constant rate of turn without gaining or losing altitude

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ADVERSE YAW
When you roll into a turn, the aileron on the inside of the turn is raised, and the aileron on the outside of the turn is lowered. The lowered aileron on the outside increases the angle of attack and produces more lift for that wing. Since induced drag is a by-product of lift, you can see that the outside wing also produces more drag than the inside wing. This causes a yawing tendency towards the outside of the turn, which is called adverse yaw (Figure 1-36).

The coordinated use of aileron and rudder corrects for adverse yaw when you roll into or out of a turn. For a turn to the left, you depress the left rudder pedal slightly as you roll into the left turn. Once you are established in the turn, you relax both aileron and rudder pressures and neutralize the controls. Then, when you want to roll out of the turn, you apply coordinated right aileron and rudder pressure to return to a wings-level attitude.

The basic purpose of the rudder on an airplane is to control yawing.

OVERBANKING TENDENCY
During your initial flight training, you will learn how to manoeuvre the airplane through coordinated use of the controls. As you enter a turn and increase the angle of bank, you may notice the tendency of the airplane to continue rolling into an even steeper bank, even though you neutralize the ailerons. This overbanking tendency is caused by the additional lift on the outside, or raised wing. This adds to the lift, and the combined effects tend to roll the airplane beyond the desired bank angle. The overbanking tendency is most pronounced at high angles of bank. To correct for this tendency, you will have to develop a technique of holding just enough opposite aileron, away from the turn, to maintain, to maintain your desired angle of bank. Overbanking tendency exists, to some degree, in almost all airplanes.

LOAD FACTOR
So far in the discussion you have looked at the combination of opposing forces acting on a turning airplane. Now it’s time to examine load factors induced during turning flight. To better understand these forces, picture yourself on a roller coaster. As you enter a banked turn during the ride, the forces you will experience are very similar to the forces which act on a turning airplane. On a roller coaster, the resultant force created by the combination of weight and centrifugal force presses you down into your seat. This pressure is an increased load factor that causes you to feel heavier in the turn than when you are on a flat portion of the track.

The increased weight you feel during a turn in a roller coaster is also experienced in an airplane. In a turning airplane, however, you must compensate for the increase in weight and loss of vertical lift, or you will lose altitude. You can do this by increasing the angle of attack with back pressure on the control wheel. The increase in the angle of attack increases the total lift of the airplane. Keep in mind that when you increase lift, drag also increases. This means you must also increase thrust if you want to maintain your original airspeed and altitude. An airplane in a coordinated, level turn is in a state of equilibrium, where opposing forces are in balance. This is similar to the state of equilibrium that exists during unaccelerated, straight-and-level flight.

During turning manoeuvres, weight and centrifugal force combine into a resultant which is greater than weight alone. Additional loads are imposed on the airplane, and the wings must support the additional load factor. In other words, when you are flying in a curved flight path, the wings must support not only the weight of the airplane and its contents, but they also must support the load imposed by centrifugal force.

The load factor imposed on an airplane will increase as the angle of bank is increased.

Load factor is the ratio of the load supported by the airplane’s wing’s to the actual weight of the aircraft and its contents. If the wings are supporting twice as much weight as the weight of the airplane and its contents, the load factor is two. You are probably more familiar with the term “G-forces” as a way to describe flight loads caused by aircraft manoeuvring. “Pulling G’s” is common terminology for higher performance airplanes. For example, an acrobatic category airplane may pull three or four G’s during a manoeuvre. An airplane in cruising flight, while not accelerating in any direction, has a load factor of one. The one-G condition means the wings are supporting only the actual weight of the airplane and its contents.

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A positive load factor occurs when centrifugal force acts in the same direction as weight. Whenever centrifugal force acts in a direction opposite weight, a negative load is imposed. For example, if you abruptly push the control wheel forward while flying, you would experience a sensation as if your weight suddenly decreased. This is caused by centrifugal force acting upward, which tends to overcome your actual body weight. If the centrifugal force equaled your actual body weight, you would experience a “weightless” sensation of zero G’s. A negative G-loading occurs when the centrifugal force exceeds your body weight. In rare instances, you may experience, a rapid change in G-forces. For example, in extremely turbulent air, you might be subjected to positive G’s, then negative G’s and sometimes sideward G-forces. Side-ward G-forces are called transverse G-forces.

LOAD FACTOR AND STALL SPEED
Earlier you learned that you can stall an airplane at any airspeed and in any flight attitude. You can easily stall an airplane in a turn at a higher-than-normal speed. As the angle of bank increases in level turns, you must increase the angle of attack to maintain altitude. As you increase the angle of bank, the stall speed increases (Figure 1-38).

Actually, stall speed increases in proportion to the square root of the load factor. If you are flying an airplane with a one-G stalling speed of 55 knots, you can stall it at twice that speed (110 knots) with a load factor of four G’s. Stalls that occur with G-forces on an airplane are called accelerated stalls. An accelerated stall occurs at a speed higher than the normal one-G stall speed. These stalls demonstrate that the critical angle of attack, rather than speed, is the reason for a stall. Stalls also occur at unusually high speeds in severe turbulence, or in low-level wind shear.

Increasing the load factor will cause an airplane to stall at a higher speed.

LIMIT LOAD FACTOR
When the FAA certifies an airplane, one of the criteria they look at is how much stress the airplane can withstand. The limit load factor is the. Umber of G’s an airplane can sustain, on a continuing basis, without causing permanent deformation or structural damage. In other words, the limit load factor is the amount of positive or negative G’s an airframe is capable of supporting.

Most small general aviation airplanes with a gross weight of 12,500 pounds or less, and nine passenger seats or less, are certified in either the normal, utility, or acrobatic categories. A normal category airplane is certified for nonacrobatic manoeuvres. Training manoeuvres and turns not exceeding 60 degrees of bank are permitted in this category. The maximum limit load factor in the normal category is 3.8 positive G’s and 1.52 negative G’s. In other words, the airplane’s wings are designed to withstand 3.8 times the actual weight of the airplane and its contents during manoeuvring flight. By following proper loading techniques and flying within the limits listed in the pilot’s operating handbook, you will avoid excessive loads on the airplane, and possible structural damage.

In addition to those manoeuvres permitted in the normal category, an airplane certified in the utility category may be used for several manoeuvres requiring additional stress on the airframe. A limit of 4.4 positive G’s or 1.78 negative G’s is permitted in the utility category. Some, but not all utility category airplanes are also approved for spins. An acrobatic category airplane may be flown in any flight attitude as long as its limit load factor does not exceed six positive G’s or three negative G’s,

A key factor for you to remember is that it is possible to exceed design limits for load factor during manoeuvres. For example, if you roll into a steep, level turn of 75 degrees, you will put approximately four G’s on the airplane. This is above the maximum limit of 3.8 G’s for an airplane in the normal category. You also should be aware of the conditions specified for the maximum load limit. If flaps are extended, for instance, the maximum load limit normally is less. The POH for the airplane you are flying is your best source of load limit information.

MANOEUVRING SPEED
An important airspeed related to load factors and stall speed is the design manoeuvring speed [Va].This limiting speed normally is not marked on the airspeed indicator, since it may vary with total weight. The POH and/or a placard in the airplane are the best source for determining Va. Although some handbooks may designate only one manoeuvring speed, others may show several. When more than one is specified, you will notice that Va decreases as weight decreases. An aircraft operating at lighter weights is subject to more rapid acceleration from gusts and turbulence than a more heavily loaded one.

Any airspeed in excess of Va can over stress the airframe during abrupt manoeuvres or turbulence. The higher the airspeed, the greater the amount of excess load that can be imposed before a stall occurs. Va represents the maximum speed at which you can use full, abrupt control movement without over stressing the airframe. If you are flying at or below this speed any combination of pilot-induced control movement, or gust loads resulting from turbulence, should not cause an excessive load on the airplane. This is why you should always fly at or below Va during turbulent conditions.

The amount of excess load that can be imposed on the airframe depends on the aircraft’s speed.

The design manoeuvring speed also is the maximum speed at which you can safely stall an airplane. If you stall the airplane above this speed, you will generate excessive G- loads. At or below this speed, the airplane will stall before excessive G-forces build up. By staying at or below Va you will avoid the possibility of over stressing or even damaging the airplane.

SPINS
No discussion of the aerodynamics of flight would be complete without considering spins. Your awareness to what causes spins, and how you can avoid them, is very important.

A spin is defined as an aggravated stall which results in autorotation. In order for a spin to develop, a stall must first occur. The spin results when one wing stalls before the other and begins to drop. Although both wings are stalled during a spin, they are both producing some lift. However, the outer (or rising) wing produces more lift than the inner (or lowering) wing. This imbalance in lift contributes to the aircraft’s rolling and yawing motion while it is in the spin.

Many airplanes are prohibited from spin manoeuvres. For examples, airplanes certified by the FAA in the normal category are probing from spins, which is also true of some airplanes in the utility category. Airplanes that are not certified for spins may not be recoverable from fully developed spins.

To enter a spin, an airplane must first be stalled. In a spin, both wings are in a stalled condition.

Although stress loads usually are not severe during a spin, an erratic recovery ma6 impose excessive loads on the airframe that could result in an accelerated stall or structural failure. For example, some airplanes have a placard displayed on the panel which tells you not to enter a spin when passengers are in the rear seats. This is because the passengers move the centre of gravity to an aft position. Recovery from a spin in an airplane with aft loading may be difficult or even impossible.

Specific recovery techniques also vary with different makes and models of airplanes. This is why you should never intentionally spin an airplane without an experienced instructor on board the aircraft. If you enter a spin inadvertently, you should follow the procedure outlined by the manufacturer of your airplane. The following procedure pertains to a general recovery procedure, but it should not be applied arbitrarily, without regard for the manufacturer’s recommendation.

Since an airplane must be in a stalled condition before it will spin, the first thing you should do is to try to recover from the stall before the spin develops. If your reaction is too slow and a spin develops, move the throttle to idle and make sure the flaps are up. Next, apply full rudder deflection opposite to the direction of the turn. As the rotation slows, briskly position the elevator forward of the neutral position to decrease the angle of attack. As the rotation stops, neutralize the rudder and smoothly apply back pressure to recover from the nose-down pitch attitude. During recovery from the dive, make sure you avoid excessive airspeed. This could lead to high G-forces, which could cause an accelerated stall, or even result in structural failure.

LEFT-TURNING TENDENCIES
Propeller-driven airplanes are subject to several left-turning tendencies caused by a combination of physical and aerodynamic forces – torque, gyroscopic precession, asymmetrical thrust, and spiralling slipstream.
You will need to compensate for these forces, especially when you are flying in high-power, low-airspeed flight conditions following takeoff or during the initial climb. If you know what is happening to the airplane, you will have a better idea of how to correct for these tendencies.

TORQUE
In airplanes with a single engine, the propeller rotates clockwise when viewed from the pilot’s seat. Torque can be understood most easily by remembering Newton’s third law of motion. The clockwise action of a spinning propeller causes a torque reaction which tends to rotate the airplane counterclockwise about its longitudinal axis.

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Torque effect is greatest in a single-engine airplane during a low-airspeed, high power flight condition.

Generally, aircraft have design adjustments which compensate for torque while in cruising flight, but you will have to take corrective action during other phases of flight. Some airplanes have aileron trim tabs which you can use to correct for the effects of torque at various power settings.

GYROSCOPIC PRECESSION
The turning propeller of an airplane also exhibits characteristics of a gyroscope – rigidity in space and precession. The characteristic that produces a left-turning tendency is precession. Gyroscopic precession is the resultant reaction of an object when force is applied. The reaction to a force applied to a gyro acts in the direction of rotation and approximately 90 degrees ahead of the point where force is applied.

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 ASYMMETRICAL THRUST
When you are flying single-engine at a high angle of attack, the descending blade of the propeller takes a greater “bite” of air than the ascending blade on the other side. The greater bite is caused by a higher angle of attack for the descending blade, compared to the ascending blade. This creates the uneven, or asymmetrical thrust, which is known as the P-factor. P-factor makes an airplane yaw about its vertical axis to the left.

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P-factor results from the descending propeller blade on the right producing more thrust than the ascending blade on the left.

You should remember that P-factor is most pronounced when the engine is operating at a high- power setting, and when the airplane is flown at a high angle of attack. In level cruising flight, P-factor is not apparent, since both ascending and descending propeller blades are at nearly the same angle of attack, and are creating approximately the same amount of thrust.

P-factor causes an airplane to yaw to the left when it is at high angles of attack.

SPIRALLING SLIPSTREAM
As the propeller rotates, it produces a backward flow of air, or slipstream, which wraps around the airplane. This spiralling slipstream causes a change in the airflow around the vertical stabilizer. Due to the direction of the propeller rotation, the resultant slipstream strikes to the left side of the vertical fin

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GROUND EFFECT
Another significant aerodynamic consideration is the phenomenon of ground effect. During takeoffs and landings when you are flying very close to the ground, the earth’s surface interferes with the airflow and actually alters the three-dimensional airflow pattern around the airplane. This causes a reduction in wingtip vortices and a decrease in upwash and downwash.

An airplane is usually in ground effect when it is less than the height of the airplane’s wingspan above the surface.

WIngtip vortices are caused by the air beneath the wing rolling up and around the wingtip. This causes a spiral vortex that trails behind each wingtip whenever lift is being produced. Wingtip vortices are another factor contributing to induced drag. Upwash and downwash refer to the effect an airfoil exerts on the free airstream. Upwash is the deflection of the oncoming airstream upward and over the wing. Downwash is the downward deflection of the airstream as it passes over the wing and past the trailing edge.

Ground effect reduces induced angle of attack and induced drag.

If you remember how angle of attack influences induced drag, it will help you understand ground effect. During flight, the downwash of the airstream causes the relative wind to be inclined downwards in the vicinity of the wing. This is referred to as the average relative wind. The angle between the free airstream relative wind and the average relative wind is the induced angle of attack. In effect, the greater the downward deflection of the airstream, the higher the induced angle of attack and the higher the induced drag. Since ground effect restricts the downward deflection of the airstream, both the induced angle of attack and induced drag decrease. When the wing is at a height equal to its span, the decline in induced drag is only about 1.4 %; when the wing is at a height equal to one-tenth its span, the loss of induced drag is about 48%.

Ground effect allows an airplane to become airborne before it reaches its recommended takeoff speed.

With the reduction of induced angle of attack and induced drag in ground effect, the amount of thrust required to produce lift is reduced. What this means is that your airplane is capable of lifting at a lower-than-normal speed. Although you might initially think that this is desirable, consider what happens as you climb out of ground effect. The power(thrust) required to sustain flight increases significantly as the normal airflow around the wing returns and induced drag is suddenly increased. If you attempt to climb out of ground effect before reaching the speed for normal climb , you might sink back to the surface.

In ground effect, induced drag decreases, and excess speed in the flare may cause floating.

Ground effect is noticeable in the landing phase of flight, too, just before touchdown. Within one wingspan above the ground, the decrease in induced drag makes your seem to float on the cushion beneath it. Because of this, power reduction is usually required during flare to help the airplane land. Although all airplanes may experience ground effect, it is more noticeable in low-wing airplanes, simply because the wings are closer to the ground.

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Courtesy of : Private Pilot Manual published by Jeppesen Sanderson Inc. 1991, CO, USA.

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