Flight Controls and High-Speed Aerodynamics

5-43 Dihedral Distance to vertical stabilizer creates stability Pivot point (CG) Elevator stabilizer. control wheel or yoke, the control stick, or on some airplanes, a side stick. On the airplanes shown in Figures 5-70 and 5-71, a control wheel or yoke is used. On the Cessna 182 shown in Figure 5-65, pulling back on the control wheel causes the trailing edge of the elevator to deflect upward, causing an increased downward force that raises the nose of the airplane. Movement of the elevator causes the nose of the airplane to pitch up or pitch down by rotating around the lateral axis. The Cessna 182 control wheel can be seen in Figure 5-68.

On the Piper Cherokee Cruiser PA-28-140 shown in entire horizontal surface, or stabilator, to move, with the trailing edge deflecting upward. The anti-servo tab seen on the Cherokee provides a control feel similar to what would be experienced by moving an elevator. Without this tab, the stabilator might be too easy to move and a pilot could overcontrol the airplane. The ruddervators shown on the Beechcraft Bonanza in trailing edges deflecting upward when the control wheel is pulled back. As the name implies, these surfaces also act as the rudder for this airplane. Flight Controls and the Longitudinal Axis The longitudinal axis of the airplane runs through the middle of the airplane, from nose to tail, passing through the center of gravity. Movement around this axis is known as roll, and control around this axis is called lateral control.

Movement around this axis is controlled by the ailerons, and on jet transport airplanes, it is aided by surfaces on the wing known as spoilers. The ailerons move as a result of the pilot rotating the control wheel to the left or to the right, much the same as turning the steering wheel on an automobile. [Figure 5-68] When a pilot turns the control wheel to the left, the airplane is being asked to turn or bank to the left. Turning the control wheel to the left causes the trailing edge of the aileron on the left wing to rise up into the airstream, and the aileron on the right wing lowers down into the airstream. This increases the lift on the right wing and decreases the lift on the left wing, causing the right wing to move up and the airplane to bank to the left.

In Figure 5-69, an aircraft can be seen doing an aileron roll. Notice that the left aileron is up and the right aileron is down, which would cause the airplane to roll around the longitudinal axis in a counterclockwise direction. Flight Controls and the Vertical Axis The vertical axis of an airplane runs from top to bottom through the middle of the airplane, passing through the center of gravity. Movement around this axis is known as yaw, and control around this axis is called directional control. Movement around this axis is controlled by the rudder, or in the case of the Beechcraft Bonanza in Figure 5-67, by the ruddervators.

The feet of the pilot are on the rudder pedals, and pushing on the left or right rudder pedal makes the rudder move left or right. The trailing edge of the rudder moves to the right, and the nose of the airplane yaws to the right, when the right rudder pedal is pushed. The rudder pedals of a Cessna 182 can be seen in Figure 5-68. Even though the rudder of the airplane will make the nose yaw to the left or the right, the rudder is not what turns the 5-44 Control wheel or yoke Rudder pedals Ruddervators control. Moving horizontal stabilizer (stabilator) on a Piper Cherokee Cruiser P A-28-140 provides pitch control.

airplane. For what is called a coordinated turn to occur, both the ailerons and rudder come into play. Let’s say we want to turn the airplane to the right. We start by turning the control wheel to the right, which raises the right aileron and lowers the left aileron and initiates the banking turn. The increased lift on the left wing also increases the induced drag on the left wing, which tries to make the nose of the airplane yaw to the left. To counteract this, when the control wheel is moved to the right, a small amount of right rudder is used to keep the nose of the airplane from yawing to the left. Once the nose of the airplane is pointing in the right direction, pressure on the rudder is no longer needed. The rudder of a Piper Cherokee Arrow can be seen in Figure 5-70.

Tabs Trim Tabs Trim tabs are small movable surfaces that attach to the trailing edge of flight controls. These tabs can be controlled from the flight deck, and their purpose is to create an aerodynamic force that keeps the flight control in a deflected position. Trim tabs can be installed on any of the primary flight controls. A very common flight control to find fitted with a trim tab is the elevator. In order to be stable in flight, most airplanes have the center of gravity located forward of the center of lift on the wing. This causes a nose heavy condition, which needs to be balanced out by having the elevator deflect upwards and create a downward force. To relieve the pilot of the need to hold back pressure on the control wheel, a trim tab on the elevator can be adjusted to hold the elevator in a slightly deflected position. An elevator trim tab for a Cessna 182 is shown in Figure 5-71.

Anti-servo Tab Some airplanes, like a Piper Cherokee Arrow, do not have a fixed horizontal stabilizer and movable elevator. The Cherokee uses a moving horizontal surface known as a stabilator. Because of the location of the pivot point for this movable surface, it has a tendency to be extremely sensitive to pilot input. To reduce the sensitivity, a full length anti-servo tab is installed on the trailing edge of the stabilator. As the trailing edge of the stabilator moves down, the anti-servo tab moves down and creates a force trying to raise the trailing edge. With this force acting against the movement of the stabilator, it reduces the sensitivity to pilot input. The anti- servo tab on a Piper Cherokee Arrow is shown in Figure 5-70.

Balance Tab On some airplanes, the force needed to move the flight controls can be excessive. In these cases, a balance tab can be used to generate a force that assists in the movement of the flight control. Just the opposite of anti-servo tabs, balance tabs move in the opposite direction of the flight control’s trailing edge, providing a force that helps the flight control move. Servo Tab On large airplanes, because the force needed to move the flight controls is beyond the capability of the pilot, hydraulic actuators are used to provide the necessary force. In the event 5-45 Rudder Anti-servo Tab Elevator trim tab Aileron down Aileron up of a hydraulic system malfunction or failure, some of these airplanes have servo tabs on the trailing edge of the primary flight controls. When the control wheel is pulled back in an attempt to move the elevator, the servo tab moves and creates enough aerodynamic force to move the elevator. The servo tab is acting like a balance tab, but rather than assisting the normal force that moves the elevator, it becomes the sole force that makes the elevator move. Like the balance tab, the servo tab moves in the opposite direction of the flight control’s trailing edge. The Boeing 727 has servo tabs that back up the hydraulic system in the event of a failure. During normal flight, the servo tabs act like balance tabs. [Figure 5-72] Supplemental Lift-Modifying Devices If the wing of an airplane was designed to produce the maximum lift possible at low airspeed, to accommodate takeoffs and landings, it would not be suited for higher speed flight because of the enormous amount of drag it would produce. To give the wing the ability to produce maximum low speed lift without being drag prohibitive, retractable high lift devices, such as flaps and slats, are utilized.

Flaps The most often used lift-modifying device, for small airplanes and large, is the wing flap. Flaps can be installed on the leading edge or trailing edge, with the leading edge versions used only on larger airplanes. Flaps change the camber of the wing, and they increase both the lift and the drag for any given angle of attack. The four different types of flaps in use are called the plain, split, slotted, and Fowler. [Figure 5-73] Plain flaps attach to the trailing edge of the wing, inboard of the ailerons, and form part of the wing’s overall surface. When deployed downward, they increase the effective camber of the wing and the wing’s chord line. Both of these factors cause the wing to create more lift and more drag.

The split flap attaches to the bottom of the wing, and deploys downward without changing the top surface of the wing. This type of flap creates more drag than the plain flap because of the increase in turbulence. The slotted flap is similar to the plain flap, except when it deploys, the leading edge drops down a small amount. By having the leading edge drop down slightly, a slot opens, which lets some of the high-pressure air on the bottom of the wing flow over the top of the flap. This additional airflow over the top of the flap produces additional lift. The Fowler flap attaches to the back of the wing using a track and roller system. When it deploys, it moves aft in addition to deflecting downward. This increases the total wing area, in addition to increasing the wing camber and chord line.

This type of flap is the most effective of the four types, and it is the type used on commercial airliners and business jets. Leading Edge Slots Leading edge slots are ducts or passages in the leading edge of a wing that allow high pressure air from the bottom of the wing to flow to the top of the wing. This ducted air flows over the top of the wing at a high velocity and helps keep the boundary layer air from becoming turbulent and separating 5-46 Stabilizer Elevator Elevator tab Upper rudder Anti-balance tabs Inboard aileron tab Inboard aileron Leading edge slats (extended)Leading edge flaps (extended) Lower rudder Ground spoilers Inboard flap Flight spoilers Outboard flap Balance tab Outboard aileron from the wing. Slots are often placed on the part of the wing ahead of the ailerons, so during a wing stall, the inboard part of the wing stalls first and the ailerons remain effective.

Leading Edge Slats Leading edge slats serve the same purpose as slots, the difference being that slats are movable and can be retracted when not needed. On some airplanes, leading edge slats have been automatic in operation, deploying in response to the aerodynamic forces that come into play during a high angle of attack. On most of today’s commercial airliners, the leading edge slats deploy when the trailing edge flaps are lowered. The flight controls of a large commercial airliner are shown in Figure 5-72. The controls by color are as follows: 1. All aerodynamic tabs are shown in green. 2. All leading and trailing edge high lift devices are shown in red (leading edge flaps and slats, trailing edge inboard and outboard flaps).

3. The tail mounted primary flight controls are in orange (rudder and elevator). 4. The wing mounted primary flight controls are in purple (inboard and outboard aileron).

High-Speed Aerodynamics

Compressibility Effects When air is flowing at subsonic speed, it acts like an incompressible fluid. As discussed earlier in this chapter, when air at subsonic speed flows through a diverging shaped passage, the velocity decreases and the static pressure rises, but the density of the air does not change. In a converging shaped passage, subsonic air speeds up and its static pressure decreases. When supersonic air flows through a converging passage, its velocity decreases and its pressure and density both increase. [Figure 5-74] At supersonic flow, air acts like a compressible fluid. Because air behaves differently when flowing at supersonic velocity, airplanes that fly supersonic must have wings with a different shape.

The Speed of Sound Sound, in reference to airplanes and their movement through the air, is nothing more than pressure disturbances in the air. As discussed earlier in this chapter, it is like dropping a rock in the water and watching the waves flow out from the center. As an airplane flies through the air, every point on the airplane that causes a disturbance creates sound energy in the form of pressure waves. These pressure waves flow away from the airplane at the speed of sound, which at standard day temperature of 59 °F, is 761 mph. The speed 5-47 Plain flap Split flap Slotted flap Fowler flap Airflow Supersonic Converging Decreasing velocity Increasing pressure Increasing density Diverging Increasing velocity Decreasing pressure Decreasing density of sound in air changes with temperature, increasing as temperature increases. Figure 5-75 shows how the speed of sound changes with altitude.

Subsonic, Transonic, and Supersonic Flight When an airplane is flying at subsonic speed, all of the air flowing around the airplane is at a velocity of less than the speed of sound, which is known as Mach 1. Keep in mind that the air accelerates when it flows over certain parts of the airplane, like the top of the wing, so an airplane flying at 500 mph could have air over the top of the wing reach a speed of 600 mph. How fast an airplane can fly and still be considered in subsonic flight varies with the design of the wing, but as a Mach number, it will typically be just over Mach 0.8. When an airplane is flying at transonic speed, part of the airplane is experiencing subsonic airflow and part is experiencing supersonic airflow. Over the top of the wing, probably about halfway back, the velocity of the air will reach Mach 1 and a shock wave will form. The shock wave forms 90 degrees to the airflow and is known as a normal shock wave. Stability problems can be encountered during transonic flight, because the shock wave can cause the airflow to separate from the wing. The shock wave also causes the center of lift to shift aft, causing the nose to pitch down.

The speed at which the shock wave forms is known as the critical Mach number. Transonic speed is typically between Mach 0.80 and 1.20. When an airplane is flying at supersonic speed, the entire airplane is experiencing supersonic airflow. At this speed, the shock wave which formed on top of the wing during transonic flight has moved all the way aft and has attached itself to the wing trailing edge. Supersonic speed is from Mach 1.20 to 5.0. If an airplane flies faster than Mach 5, it is said to be in hypersonic flight. Shock Waves Sound coming from an airplane is the result of the air being disturbed as the airplane moves through it, and the resulting pressure waves that radiate out from the source of the disturbance. For a slow-moving airplane, the pressure waves travel out ahead of the airplane, traveling at the speed of sound. When the speed of the airplane reaches the speed of sound, however, the pressure waves, or sound energy, cannot get away from the airplane. At this point the sound energy starts to pile up, initially on the top of the wing, and eventually attaching itself to the wing leading and trailing edges. This piling up of sound energy is called a shock wave.

If the shock waves reach the ground, and cross the path of a person, they will be heard as a sonic boom. Figure 5-76A shows a wing in slow speed flight, with many disturbances on the wing generating sound pressure waves that are radiating outward. View B is the wing of an airplane in supersonic flight, with the sound pressure waves piling up toward the wing leading edge. Normal Shock Wave When an airplane is in transonic flight, the shock wave that forms on top of the wing, and eventually on the bottom of the wing, is called a normal shock wave. If the leading edge of the wing is blunted, instead of being rounded or sharp, a normal shock wave will also form in front of the wing during 5-48 AL TITUDE IN FEET SPEED OF SOUNDTEMPERATURE (°F) 0 1,000 2,000 3,000 4,000 5,000 6,000 7,000 8,000 9,000 10,000 15,000 20,000 25,000 30,000 35,000 * 36,089 40,000 45,000 50,000 55,000 60,000 65,000 70,000 75,000 80,000 85,000 90,000 95,000 100,000 761 758 756 753 750 748 745 742 740 737 734 721 707 692 678 663 660 664 671 678 684 59.00 55.43 51.87 48.30 44.74 41.17 37.60 34.04 30.47 26.90 23.34 5.51 −12.32 −30.15 −47.98 −65.82 −69.70 −64.80 −56.57 −48.34 −40.11 * Altitude at which temperature stops decreasing.

supersonic flight. Normal shock waves form perpendicular to the airstream. The velocity of the air behind a normal shock wave is subsonic, and the static pressure and density of the air are higher. Figure 5-77 shows a normal shock wave forming on the top of a wing. Oblique Shock Wave An airplane that is designed to fly supersonic will have very sharp edged surfaces, in order to have the least amount of drag. When the airplane is in supersonic flight, the sharp leading edge and trailing edge of the wing will have shock waves attach to them. These shock waves are known as oblique shock waves. Behind an oblique shock wave the velocity of the air is lower, but still supersonic, and the static pressure and density are higher. Figure 5-78 shows an oblique shock wave on the leading and trailing edges of a supersonic airfoil.

Expansion Wave Earlier in the discussion of high-speed aerodynamics, it was stated that air at supersonic speed acts like a compressible fluid. For this reason, supersonic air, when given the opportunity, wants to expand outward. When supersonic air is flowing over the top of a wing, and the wing surface turns away from the direction of flow, the air will expand and follow the new direction. An expansion wave will occur at the point where the direction of flow changes. Behind the expansion wave the velocity increases, and the static pressure and density decrease. An expansion wave is not a shock wave.

High-Speed Airfoils Transonic flight is the most difficult flight regime for an airplane, because part of the wing is experiencing subsonic airflow and part is experiencing supersonic airflow. For a subsonic airfoil, the aerodynamic center, or the point of support, is approximately 25 percent of the way back from the wing leading edge. In supersonic flight, the aerodynamic center moves back to 50 percent of the wing’s chord, causing some significant changes in the airplane’s control and stability. If an airplane designed to fly subsonic, perhaps at a Mach number of 0.80, flies too fast and enters transonic flight, some noticeable changes will take place with respect to the airflow over the wing. Figure 5-79 shows six views of a wing, with each view showing the Mach number getting higher.

The scenario for the six views is as follows: A. The Mach number is fairly low, and the entire wing is experiencing subsonic airflow. B. The velocity has reached the critical Mach number, where the airflow over the top of the wing is reaching Mach 1 velocity. C. The velocity has surpassed the critical Mach number , and a normal shock wave has formed on the top of the wing. Some airflow separation starts to occur behind the shock wave. D. The velocity has continued to increase beyond the critical Mach number, and the normal shock wave has moved far enough aft that serious airflow separation is occurring. A normal shock wave is now forming on the bottom of the wing as well. Behind the normal (MPH) 5-49 A B Normal shock wave Subsonic air Supersonic airSubsonic air Oblique shock Oblique shock Expansion wave Airflow expansion waves.

shock waves, the velocity of the air is subsonic and the static pressure has increased. E. T he velocity has increased to the point that both shock waves on the wing, top and bottom, have moved to the back of the wing and attached to the trailing edge. Some airflow separation is still occurring. F. T he forward velocity of the airfoil is greater than Mach 1, and a new shock wave has formed just forward of the leading edge of the wing. If the wing has a sharp leading edge, the shock wave will attach itself to the sharp edge. The airfoil shown in Figure 5-79 is not properly designed to handle supersonic airflow. The bow wave in front of the wing leading edge of view F would be attached to the leading edge, if the wing was a double wedge or biconvex design.

These two wing designs are shown in Figure 5-80. Aerodynamic Heating One of the problems with airplanes and high-speed flight is the heat that builds up on the airplane’s surface because of air friction. When the SR-71 Blackbird airplane is cruising at Mach 3.5, skin temperatures on its surface range from 450 °F to over 1,000 °F. To withstand this high temperature, the airplane was constructed of titanium alloy, instead of the traditional aluminum alloy. The supersonic transport Concorde was originally designed to cruise at Mach 2.2, but its cruise speed was reduced to Mach 2.0 because of structural problems that started to occur because of aerodynamic heating. If airplanes capable of hypersonic flight are going to be built in the future, one of the obstacles that will have to be overcome is the stress on the airplane’s structure caused by heat.

Helicopter Aerodynamics

The helicopter, as we know it today, falls under the classification known as rotorcraft. Rotorcraft is also known as rotary wing aircraft, because instead of their wing being fixed like it is on an airplane, the wing rotates. The rotating wing of a rotorcraft can be thought of as a lift producing device, like the wing of an airplane, or as a thrust producing device, like the propeller on a piston engine. Helicopter Structures and Airfoils The main parts that make up a helicopter are the cabin, landing gear, tail boom, power plant, transmission, main rotor, and tail rotor. [Figure 5-81] Main Rotor Systems In the fully articulated rotor system, the blades are attached to the hub multiple times. The blades are hinged in a way that allows them to move up and down and fore and aft, and bearings provide for motion around the pitch change axis.

Rotor systems using this type of arrangement typically have three or more blades. The hinge that allows the blades to move