ppl_05_e2
.pdf
ID: 3658
Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
CHAPTER 12: FLIGHT CONTROLS AND TRIMMING
THE FLYING CONTROL SURFACES.
Directional Control about the Normal Axis.
Figure 12.4 The rudder controls the aircraft in yaw, about its normal axis.
The rudder is a symmetrically-cambered, hinged aerofoil section, mechanically linked by cables and rods to the rudder pedals, which are operated by the pilot’s feet. When the rudder pedals are central, the rudder itself is central. Pushing the right rudder pedal forward defects the rudder to the right, generating an aerodynamic force which causes the aircraft to yaw to the right as in Figure 12.5.
Flying controls are designed to work in the instinctive sense.
Figure 12.5 Pushing the right rudder pedal forward causes the aircraft to yaw to the right.
If the left rudder pedal is pushed forward the aircraft will yaw to the left. Because of the positive directional stability created by the fn, the aircraft will tend to return to straight fight when the rudder pedals are centralised. The magnitude of the aerodynamic force produced by defection of the rudder, and, therefore, the effectiveness of the rudder and the amplitude of the yawing movement, is proportional to the amount of defection of the control surface, the airspeed, and the thrust from the propeller;
(slipstream effect).
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Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
CHAPTER 12: FLIGHT CONTROLS AND TRIMMING
The primary
purpose of the rudder is to maintain the
aircraft in balanced flight.
Balanced Flight.
You should note that although the rudder is said to give directional control, this latter expression can be very misleading. The rudder is not used to turn the aircraft and change direction. A turn is initiated by applying aileron in the direction of the turn to select an angle of bank, the rudder being used to balance the manoeuvre, by preventing unwanted yaw. The primary use of the rudder, then, is to enable the pilot to maintain the aircraft in balanced fight, whether in straight or turning fight. In balanced fight, the longitudinal axis of the aircraft is approximately parallel to the relative airfow. The aircraft meets the relative airfow head-on, and the ball of the turn and slip indicator will be in its central position, between the two vertical markers, as depicted in Figure 12.6.
Other uses of the rudder include:
Figure 12.6 The rudder is primarily used to keep the aircraft in balanced flight with the ball
“in the middle”.
•The holding of a sideslip in specialist manoeuvres.
•To balance a coordinated turn, especially to eliminate adverse yaw on entering a turn.
•To steer the aircraft during taxying.
•In the recovery from a spin.
•To overcome asymmetric power effects in the event of engine failure on a multi-engined aircraft.
You have already learnt that directional stability and lateral stability (i.e. stability in the yawing and rolling planes) are inextricably linked. You will not be surprised, then, to learn that pilot-induced yawing movements also have a secondary effect in the rolling plane. This phenomenon is discussed below.
Longitudinal Control about the Lateral Axis (Pitch).
The elevator or stabilator (all-fying tailplane) gives the pilot longitudinal control in the pitching plane about the lateral axis (See Figure 12.7).
Fore and aft movement of the control column or control wheel operate the elevator in such a way as to produce a pitching movement in the natural and logical sense.
If the pilot moves the control column aft, the elevator or stabilator is defected upwards.
This will generate an aerodynamic force acting downwards at the tailplane which will pitch the aircraft’s nose up. Likewise, if the control column is moved forward an aerodynamic force is generated which will pitch the aircraft nose downwards.
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ID: 3658
Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
CHAPTER 12: FLIGHT CONTROLS AND TRIMMING
The elevator
provides control in the
pitching plane
(longitudinal control about the lateral axis).
Figure 12.7 The elevator controls the aircraft in pitch.
Fore and aft movements of the control column, then, control the aircraft in pitch. In turn, the aircraft’s pitch attitude will affect airspeed. A pilot selects a desired airspeed with a combination of pitch attitude and engine power setting. The effectiveness of the elevator or stabilator is proportional to the amount of control defection, the aircraft’s speed, and the magnitude of propeller thrust.
Lateral Control about the Longitudinal Axis (Roll).
The ailerons give the pilot lateral control in the rolling plane about the longitudinal axis. The ailerons are located outboard at the trailing edge of each wing. Control of the ailerons, or roll control, is achieved by lateral movements of the control column or control wheel in the logical and instinctive sense.
To roll the aircraft to the left requires the control wheel to be rotated anticlockwise or the control column to be moved to the left. Movement of the control column or control wheel in the opposite sense will cause the aircraft to roll to the right.
The ailerons are hinged aerofoil sections which move differentially. Movement of the control column either to the left or right will cause one aileron to be defected upwards whilst the other is defected downwards. In a roll to the right, for instance, as illustrated in Figure 12.8, the right (starboard) aileron is defected upwards, and the left (port) aileron is defected downwards.
Figure 12.8 Control in roll is achieved with the ailerons.
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Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
CHAPTER 12: FLIGHT CONTROLS AND TRIMMING
The aircraft rolls to the right because the lift force has been increased on the left wing and decreased on the right wing.
The roll can be stopped by the pilot centralising the control column at the required angle of bank. This action returns the ailerons to their undefected position, removing the differential in lift generated by the wings.
The effectiveness of the ailerons is proportional to the amount of control defection and to the aircraft’s speed through the air. The engine power setting has little or no effect on aileron effectiveness, as the ailerons lie outside the propeller slipstream.
As the ailerons induce a rolling movement about the longitudinal axis, they will also have an effect on the movement of the aircraft about its normal axis for the reasons already examined in the chapter on stability. This secondary effect of the ailerons is covered in the next section.
SECONDARY OR FURTHER EFFECT OF CONTROLS.
There is no secondary
effect of elevator,
though a change in the pitch attitude will lead to a change in airspeed.
The fying control surfaces have both primary and secondary effects. You have just learnt about the primary effects of the controls; here, we deal with the secondary effects.
The Elevator.
There is no secondary effect of the elevator or stabilator, although a change in the aircraft’s pitch attitude will induce a change of airspeed.
The Secondary Effect of Rudder.
In the chapter on Stability, you learned that an aircraft’s rotation about its normal axis (yaw) and its longitudinal axis (roll) are not independent of each other.
A displacement in the yawing plane will always infuence an aircraft’s motion in the rolling plane and vice versa.
You have learnt that the primary effect of rudder is yaw. The secondary effect of rudder is roll in the direction of yaw.
The secondary
effect of rudder is roll in
the same direction as yaw.
If a pilot initiates a yaw with the rudder, one wing will move forward relative to the other. In a yaw to the left, for example, the right (starboard) wing will move forwards and the left (port) wing will move rearwards. The velocity of the airfow over the forward-moving right wing will increase, generating a greater lift force than that produced by the rearwards-moving left wing. The aircraft, therefore, will begin to roll to the left, in the direction of the yaw. Once the yaw is established, the wing being held back presents a shorter effective span to the airfow than the forward wing, further increasing the lift on the forward wing, and, thereby, reinforcing the roll to the left. The roll, will lead to a sideslip which, for the reasons discussed in an earlier chapter, will induce more yaw, which, in turn, will lead to further roll, and so on, until the aircraft enters a spiral dive to the left, if the pilot does not intervene. A spiral dive can turn into a dangerous fight manoeuvre becoming an ever-steepening, evertightening turn. Your instructor will demonstrate a spiral dive to you, and show you how to recover from it, before it becomes dangerous, by frst rolling the wings level with coordinated aileron and rudder, and then easing out of the dive.
The secondary effect of rudder, then, is roll in the direction of the initial yaw.
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ID: 3658
Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
CHAPTER 12: FLIGHT CONTROLS AND TRIMMING
The Secondary Effect of Aileron.
You have learnt that the primary effect of aileron is roll. The secondary effect of aileron is yaw in the direction of roll.
If a pilot selects an angle of bank, to the left, say, without making any further control movements, the aircraft will respond by rolling to the left, about the longitudinal axis. The aircraft, then, will enter a slip to the left, towards the lower wing and the side component of the relative airfow will, consequently, strike the aircraft’s keel surfaces.
Because the keel surface area behind the C of G is greater than the area of the keel surfaces forward of the C of G, the aircraft will yaw in the direction of the lower wing. For the reasons given above, the yaw will induce further roll, which will lead to further yaw and so on, until the aircraft enters a spiral dive to the left, if the pilot does not intervene.
The secondary effect of aileron, then, is yaw in the direction of the initial roll.
Figure 12.9 The secondary effects of both rudder and ailerons induce rolling and yawing actions, in the same direction, which mutually reinforce each other and lead to a spiral dive.
The secondary effect of aileron is yaw.
Unless the pilot intervenes,
the secondary
effect of aileron can lead to a spiral dive.
Adverse Yaw.
There is yet another effect arising from the interrelationship between control in the rolling and yawing planes that it is vital for you to understand, if you are to become a competent pilot. This effect is called adverse yaw. Eliminating adverse yaw demands the coordinated use of the ailerons and rudder, which is an ever-present requirement for controlled rolling movements, especially on the entry to and exit from turns. A balanced roll requires the use of an appropriate amount of rudder, as well as aileron, applied in the same direction and at the same time.
Adverse yaw arises when the pilot attempts to roll using aileron alone, or with uncoordinated movements of the aileron and rudder. It is convenient to examine adverse yaw by considering a pilot attempting to roll his aircraft using aileron alone.
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Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
CHAPTER 12: FLIGHT CONTROLS AND TRIMMING
Adverse
yaw is counteracted by applying
rudder in the same direction of roll.
Figure 12.10 Adverse yaw. Here the aircraft is rolling to the left and yawing to the right. Notice the position of the ball showing that the aircraft is out of balance.
If a pilot rolls the aircraft, to the left, with aileron alone, aileron defection will be such that the left (port) wing lowers and the right (starboard) wing rises. During the rolling movement, then, the right wing is generating a greater lift force than the left wing. As you have learnt, the penalty for increased lift is increased drag. Consequently, as the aircraft rolls to the left, the up-going right wing, generating the greater lift and drag, is held back. So while the aircraft rolls to the left, it yaws to the right, as shown in Figure 12.10.
It is because the yaw is in the opposite direction to the roll that it is given the name adverse yaw. Adverse yaw is the aircraft’s initial response to the rolling movement. If the pilot were to reverse the roll, to the right, the aircraft would yaw to the left. When adverse yaw is present, then, the aircraft begins to wallow around, giving the pilot the impression that he does not have the aircraft under control. What is actually happening is that the aircraft is rolling in an unbalanced way. Note the position of the ball in Figure 12.10. If the aircraft were in balance, the ball would be in the middle. However, the aircraft is rolling to the left but the ball of the turn and slip indicator is fully defected to the left, too, indicating that left rudder is required to bring the aircraft back into balance.
(Of course, if an angle of bank is selected and held, without any further intervention from the pilot, the aircraft will then slip towards the lower wing and begin to yaw in the direction of roll, eventually entering a spiral dive.)
Adverse yaw then is yaw in the opposite direction to the roll. Adverse yaw is the initial reaction to a roll initiated using aileron alone.
Adverse yaw can be eliminated by using an appropriate amount of rudder in the same direction as aileron and at the same time. By applying aileron and rudder together in this way, a pilot is said to be using coordinated control movements. You will doubtless hear your instructor urge you to use “coordinated control movements” several times during your initial training. When you roll with properly coordinated aileron and rudder, the ball of the turn and slip indicator will remain in the middle.
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ID: 3658
Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
CHAPTER 12: FLIGHT CONTROLS AND TRIMMING
Adverse yaw
is eliminated by coordinated
use of rudder
and aileron. That is, applying rudder in the same direction as aileron.
Figure 12.11 Adverse yaw has been eliminated. Here the aircraft is rolling to the left without adverse yaw. The ball’s position “in the middle” shows that the aircraft is in balance.
In short wing-span, high-powered aircraft, adverse yaw is not very pronounced, but pilots must be ready to balance rolling movements with coordinated aileron and rudder, as required. In long-wing span, low-powered aircraft such as motor-gliders, adverse yaw is very pronounced and a high level of aileron and rudder coordination is required, to eliminate adverse yaw.
USE OF THE FLYING CONTROLS IN TURNING FLIGHT.
Having learnt what are the primary and secondary effects of controls, this is a convenient point for you to consider how the fying controls are used to enter, maintain and exit a turn. Before continuing, you may wish to read again the section on turning fight in Chapter 9.
You will recall that, in order to turn, the pilot applies an appropriate amount of bank in the direction in which he wishes to turn.
Figure 12.12 A PA28 established in a turn to the left, with the ailerons centralised.
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Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
CHAPTER 12: FLIGHT CONTROLS AND TRIMMING
In a level turn, i.e. a turn at constant altitude, the total lift force, LT, depicted in Figure 12.13, has to do two jobs: provide the necessary centripetal force to turn the aircraft, and also support the weight of the aircraft (which continues to act vertically downwards) so as to maintain the aircraft in level fight. Consequently, in order to achieve both these objectives, the total lift force in the turn needs to be greater than that required for straight and level fight.
Figure 12.13 In a turn, the lift generated by the wings must continue to support the aircraft’s weight while also providing the turning (centripetal) force.
In a medium turn of about 30° angle of bank, the extra lift for the turn will be provided by the pilot increasing the back pressure on the control column (thereby increasing the wing’s angle of attack and, thus, CL) suffciently to maintain the correct pitch attitude and constant altitude. The increase in angle of attack also causes a rise in drag which will lead to a reduction in airspeed, if thrust is not increased. The reduction in speed is small in a 30°-banked turn, and may be acceptable, but, in steep turns of 45° angle of bank and above, thrust will need to be increased, too, in order to maintain the entry speed because of the increased stalling speed in the turn. Remember, in a 45°-banked turn, the stalling speed of a PA28 Warrior is around 60 knots,10 knots higher than its straight fight stalling speed of 50 knots (See Figure 12.14).
Figure 12.14 A 45°-banked turn. For turns of 45° angle of bank, and above, thrust must be increased to maintain entry speed.
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ID: 3658
Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
CHAPTER 12: FLIGHT CONTROLS AND TRIMMING
Holding the above facts in mind and recalling what we have just learned about the primary and secondary effects of the controls, we consider below the fying control movements involved in turning fight. Remember, these are considerations to bear in mind in the context of your study of Principles of Flight. Your fying instructor will teach you how to fy the turn, in your airborne exercises. One of the main teaching points that he will doubtless repeat several times, is the necessity to keep a good look out in every stage of the turn.
•On rolling into the turn, the control column is displaced laterally in the direction of the desired turn. Remember, if ailerons are used on their own you will get adverse yaw; therefore, rudder must be applied in the same direction as aileron, in order to maintain balanced fight. The amount of rudder required will depend on the rate of roll and the type of aircraft being fown.
•As the aircraft is still rolling into the turn, the pilot increases back pressure on the control column suffciently to increase angle of attack of the wings, through elevator defection, by the amount necessary to provide the extra lift required to support the weight of the aircraft, while also providing the centripetal force which actually changes the aircraft’s direction. The pilot judges the amount of back pressure he needs to apply by holding the correct pitch attitude and maintaining constant altitude.
•For medium turns of up to 30° angle of bank, the small reduction in airspeed resulting from the increased angle of attack, and increased drag, is usually acceptable. This will be about 5 knots in the Warrior for a 30°-banked turn. But for turns of 45° angle of bank, and steeper, the increase in drag is such that thrust must be increased to maintain entry speed.
•When the required angle of bank is reached, the pilot centralises the ailerons and rudder, although very small rudder pressures may be necessary to maintain balanced fight.
•The raised wing, which is on the outside of the turn and, therefore, moving faster through the air than the lower wing, will develop more lift. This situation requires the pilot to apply a small amount of aileron in the opposite direction to the turn, an action which is called “holding off the bank.” However, in the pilot’s perception, he needs merely to maintain the required bank-angle with aileron.
•In the turn, any deviation from the bank angle is corrected with coordinated aileron and rudder movements. In turns steeper than 30°, speed is controlled with power. Altitude is maintained with pitch attitude using elevator. Elevator trim is not normally used in turns, the required back pressure being held by the pilot.
•To exit the turn, the wings are rolled level using coordinated aileron and rudder; (control column and rudder pedal inputs at the same time and in the same direction). At the same time, the back pressure on the control column, required during the turn, is eased off, to maintain an appropriate pitch attitude, and power adjusted as required for speed.
As mentioned earlier, the above considerations are Principles of Flight considerations.
Your instructor will teach you how to fy the turn.
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Order: 6026
Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
Customer: Oleg Ostapenko E-mail: ostapenko2002@yahoo.com
CHAPTER 12: FLIGHT CONTROLS AND TRIMMING
Differential ailerons reduce
the amount of adverse
yaw because the down-going aileron moves through a smaller angle than the upgoing aileron.
Frise ailerons increase
parasite drag on the down
going wing to counteract adverse yaw.
Design Features Aimed at Reducing Adverse Yaw.
Differential Ailerons.
The problem faced by designers in terms of reducing adverse yaw is to counterbalance the extra drag generated by the wing which produces the greater lift.
Figure 12.15 Differential Ailerons.
As drag increases with an increasing angle of attack, one way of reducing the drag on the up-going wing is to make the downward defection of the aileron as small as practically possible. The differential in lift required to induce the roll can still be provided by increasing the upward defection of the aileron on the downgoing wing. The greater upward defection of the aileron on the down-going wing also increases the parasite drag on that wing, further reducing adverse yaw. With this arrangement, known as differential ailerons, adverse yaw is reduced, but not eliminated (See Figure 12.15).
Frise Ailerons.
Another method of reducing adverse yaw is for the aircraft designer to concentrate solely on increasing parasite drag on the down-going wing in order to balance the drag associated with the increased lift on the up-going wing. This aim is achieved by using a type of aileron hinge which causes the leading edge of the up-going aileron on the down-going wing to protrude into the airfow, thereby increasing parasite drag. Conversely, the leading edge of the down-going aileron on the up-going wing remains shrouded, but creates a slot which re-energises the top-surface boundary layer, increasing lift. This type of aileron arrangement is known as Frise Ailerons, after their inventor, Bristol Aircraft designer Leslie Frise. On some aircraft, Frise Ailerons and Differential Ailerons are combined into one system (See Fig 12.17).
Figure 12.16 Frise Ailerons.
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