UNIT III. WAT IS WHAT IN AVIATION
Aviation for amateurs
All aircrafts are built with the same basic elements: wings to provide lift, engine(s) to provide motive power, a fuselage to carry the payload and controls, and a tail assembly[29] which usually controls the direction of flight. These elements differ in shape, size, number, and position. The differences distinguish one aircraft type from another.
Aircraft Components
Angle of Attack (AOA). The АОА is angle between the wing and the relative wind. When all else is held constant, an increase in AOA results in an increase in lift. This increase continues until the stall[30] AOA is reached then the trend reverses itself and an increase in AOA results in decreased lift.
Angle of Attack
Ailerons – Located on the outer part of the wing, the ailerons help the airplane turn. Ailerons are control surfaces which are used to change the bank of the airplane, or roll the airplane. As the ailerons hinge down on one wing, they push the air downwards, making that wing tilt up. This tips the airplane to the side and helps it turn. This tipping is known as Banking. They are manipulated from the cockpit by moving the control column (stick) left and right. Right movement rolls the airplane to the right and vice versa. Roll speed is proportional to the amount of stick deflection. Once a desired bank is attained, the stick is centered to maintain the bank.[31]
Airfoil[32] Section – is the cross-sectional shape of the wing. The airfoil section shape and placement on the fuselage are directly linked to the airplanes performance. Bank – The angle between the wings and the horizon, as viewed from the rear of the airplane. An airplane with its wings level has zero degrees of bank.
Bank Angle
Banking – Pushing the control stick in the cockpit to the left or right makes the ailerons on one wing go down and the ailerons on the other wing go up. This makes the plane tip to the left or right. This is called Banking. Banking makes the plane turn. Like a bicycle, the plane tilts or banks as it turns. This process is also called Roll.
Cockpit – Where the pilot sits. All of the controls and instruments are located here.
Control Stick – The ailerons are connected to the Control Stick which is located in cockpit. Pushing the stick to the left or to the right makes the ailerons on one wing go down and the ailerons on the other wing go up. This makes the plane tip to the left or right. This is called banking. This tipping is also called roll.
Drag[33] – One of the four basic principles of flight. Drag is the force encountered as an airplane pushes through the air, which tends to slow the airplane down. There are two types of drag, and an airplane must fight its way through both kinds of drag in order to maintain steady flight.
Profile or parasite[34] drag is the same kind of drag experienced from all objects in a flow. Cars, rocks, and hockey pucks must all overcome profile drag. This type of drag is caused by the airplane pushing the air out of the way as it moves forward. This drag can easily be experienced by putting your hand out the window of a moving vehicle (experienced en masse if your hand encounters something denser than air). The other type, called induced «drag», is the result of the production of lift (you can't get something for nothing!). This drag is the part of the force produced by the wing that is parallel to the relative wind. Objects that create lift must also overcome this induced drag, also known as drag-due-to-lift. Skin friction is a function of the surface area wetted by the airstreams. Any increase in surface area will increase skin friction drag. The other component of profile drag is pressure drag. Pressure drag is a function of the size of the wake behind an object in an airstream; it can be reduced by streamlining the object in order to delay separation of the flow. A side effect of streamlining is an increase in the wetted (exposed) area and hence the skin friction, so it is important to ensure that a net reduction in drag is actually achieved when adding streamlining.
Elevators – the Elevators are movable flaps attached to the horizontal stabilizer used to change the angle of AOA of the wing which will, in turn, change the pitch, moving the airplane up and down. It is operated by moving the control stick forward or backward, which in turn moves the elevator down or up, respectively.
When pilot «moves the stick forward to make the trees bigger and back to make them smaller», it is the elevator that does the work.
Engine – this part of the plane produces thrust or forward movement necessary to sustain flight. Thrust is one of the four basic rules behind plane flight. The engine turns the propeller.
Flaps – located on the inner part of the wing, the Flaps help the plane fly slower. This helps to increase the lifting force of the wing at slower speeds, like during takeoff and landing. These slower speeds make takeoff and landing distances shorter. The Flaps slide back and forth, and are controlled by a lever in the cockpit. Flaps are moved down from a streamlined position to increase the amount of lift produced at a particular airspeed.
Flaps
Fuselage – the Fuselage is the central «body» of the plane. The wings, tail and engines are all attached to it. In a modern passenger airplane, you sit only in the top half of the Fuselage. The Fuselage also houses the cockpit where all the controls necessary for operating and controlling the plane are located. Cargo is also housed in the bottom half of the Fuselage. The Fuselage is generally streamlined as much as possible.
Horizontal Stabilizer – the horizontal stabilizer is a fixed position airfoil that stabilizes the pitch of the airplane. When a wing produces lift, it also develops a force that tries to pitch the airplane forward. The horizontal stabilizer prevents this unwanted pitch from occurring.
Gravity – gravity is the attractive force from the earth that acts upon all mass. It is one of the four principles of flight.
Landing Gear[35]– on conventional aircraft, the Landing Gear consists of wheels or tires with supports (struts) and shock absorbers which help in takeoff and landing. To reduce drag while the plane is flying, most wheels fold up into the body of the plane after takeoff. On many smaller aircraft, the wheels do not fold up after takeoff.
Lift – an upward force that causes an object to rise. In aircraft it may be produced by downward-facing propellers, or by a moving wing with an airfoil shape (the specially curved shape of an airplane wing). Lift is one of the four basic principles of flight. Forces are produced by the wing as the air flows around it. Lift is the part that is perpendicular to the relative wind. The other part contributes to drag.
Pitch[36] – the angle between the airplane's body (lengthwise) and the ground. An airplane going straight up would have a pitch attitude of ninety degrees and one in level flight, about zero degrees.
Pitch
Relative Wind – the direction that the air is going as it passes the airplane relative to the airplane. Relative wind has nothing to do with the wind speed on the ground.
Propeller – this part of the plane produces thrust or forward movement necessary to sustain flight. This turning blade on the front of an airplane moves it through the air.
Roll – roll is the tilting motion the airplane makes when it turns.
Rudder[37] – the Rudder, controlled by the rudder pedals, is the hinged[38] part on the back of the tail which helps to turn the aircraft. It is the vertical part of the tail which controls the sideways movement of the airplane, called the yaw[39]. The least used of all controls; most flying can be safely accomplished without it. (One exception is landing with a crosswind; yaw induced by the rudder must be used to keep the fuselage aligned with the runway and prevent an excursion into the grass.)
Stall – what a wing does when a given angle of attack is exceeded (the stall angle of attack). The stall is characterized by a progressive loss of lift for an increase in angle of attack.
Tail – the Tail has many movable parts. The pilot controls these parts from the cockpit. Included in the parts on the Tail are the rudder and the elevators.
Thrust[40] the force produced by the engines, thrust works opposite of and counteracts[41] drag. Thrust is the forward movement that is necessary to sustain flight. It is one of the four basic principles of flight.
Trim[42] – when the controls are moved from neutral, it takes a certain amount of pressure to hold them in position in the airflow. Trim gets rid of this pressure and effectively changes the «center» of the controls – or the neutral position where there is no stick pressure.
Vertical Stabilizer – the vertical stabilizer is the yaw stabilizer for the airplane; it keeps the nose of the airplane (as seen from above) pointed into the relative wind.
Weight – the force produced by the mass of the airplane interacting with the earth’s gravitational field; the force that must be counteracted by lift in order to maintain flight.
Basic Weight – the weight of the basic aircraft plus guns, unusable fuel, oil, ballast, survival kits[43], oxygen, and any other internal or external equipment that is on board the aircraft and will not be disposed of during flight.
Operating Weight – is the sum of basic weight and items such as crew, crew baggage, steward equipment, pylons and racks, emergency equipment, special mission fixed equipment, and all other nonexpendable[44] items not in basic weight.
Gross Weight – is the total weight of an aircraft, including its contents and externally mounted items, at any time.
Landing Gross Weight – Is the weight of the aircraft, its contents, and external items when the aircraft lands.
Zero Fuel Weight (ZFW) – is the weight of the aircraft without any usable fuel. This is due to structural limitations of aircraft)
Wing – the Wings are the «arms» of the airplane. They provide the principal lifting force of the airplane. They hold the plane aloft by creating lift from the air rushing over them. Like all plane parts, the Wings should be light and strong, but also flexible to absorb sudden gusts of wind.
Yaw – the angle between the fuselage of the airplane and the relative wind as seen from above the airplane. Yaw is the term pilots use to describe the turning left or right of the plane. Yaw is the sideways movement of the plane. Normally an airplane is flown without yaw.
Yaw Wings
Lift is the aerodynamic force that supports an aircraft in flight, due to the airflow over the wings or body. Drag is the resistance a vehicle moving through the air experiences, and pitching[45] moments are a result of aerodynamic forces that make the nose of an aircraft move either up or down.
The shape of a wing looks like an elongated[46] water drop lying on its side. This shape is referred to as an airfoil. Usually the top is curved more than the bottom making the upper surface slightly longer than the bottom. Since air passing over the top and bottom must reach the rear of the wing at the same time, the air passing over the top must not only travel faster, but also changes direction and is deflected downward. This actually results in lift being generated due to a rate of change of vertical momentum and a difference in static pressure between the top and bottom of the wing.
The production of lift is probably the most important topic in the science of aerodynamics. It is a wing’s ability to efficiently produce a force perpendicular to the air passing over it that makes heavier-than-air flight possible.
In the big picture, all wings produce lift the same way – they push down on the air, forcing the air downward relative to the wing. It is this force that we call lift.
Many different types of shapes do this, but the shapes built specifically for this purpose are called «airfoils».
Various Airfoils
The wing makes its «magic» by forcing the air down. Some people like to compare it to water skiing, where water skis and speed are used to force the water down and the skier up. But that analogy tells only part of the story. Most of the time, the top of the wing does the majority of the «pushing» on the air (actually, in this case, «pulling» the air down). The top and the bottom of the wing combine to produce a force, and the part of this force perpendicular to the relative wind is lift. Since the wing not only pushes the air down but slows it down as well, some drag (induced drag) is caused.
The chord[47] line is an imaginary line drawn from the leading edge to the trailing edge of an airfoil. Secondly, the relative wind is the airflow which acts on the airfoil and is parallel to but opposite the direction of flight. The angle between the chord line and the relative wind is called the angle of attack, which is called «alpha». As the angle of attack increases, the change of vertical momentum increases. Additionally, as the angle of attack increases, the coefficient of lift (CL) increases. The result is an increase in lift. However, there are limits to how much the angle of attack can be increased. At some higher angle of attack, the lift coefficient begins to decrease. The angle of attack where the lift coefficient begins to decrease is called the critical angle of attack. Once the critical angle is exceeded, the wing can no longer produce enough lift to support the weight of the aircraft and the wing is said to be «stalled». In other words, the aircraft will stall when the critical angle of attack is exceeded.
Lift and Drag
A wing must be at a high enough AOA to deflect the air downward and produce the desired lift. The pilot uses the elevators to change the angle of attack until the wings produce the lift necessary for the desired maneuver.
Other factors are involved in the production of lift besides the AOA. These factors are relative wind velocity (airspeed) and air density (temperature and altitude). Changing the size or shape of the wing (lowering the flaps) will also change the production of lift. Airspeed is absolutely necessary to produce lift. If there is no airflow past the wing, no air can be diverted downward. At low airspeed, the wing must fly at a high AOA to divert enough air downward to produce adequate lift. As airspeed increases, the wing can fly at lower AOA to produce the needed lift. This is why airplanes flying relatively slow must be nose high (like an airliner just before landing or just as it takes off) but at high airspeeds fly with the fuselage fairly level. The key is that the wings don’t have to divert fast moving air down nearly as much as they do to slow moving air.
As an airplane in flight slows down, it must continually increase its pitch attitude and AOA to produce the lift necessary to sustain level flight. At high AOA, the top of the wing diverts the air through a much larger angle than at low AOA. As the AOA increases, a point will be reached where the air simply cannot «take» the upper curve over the entire distance of the top of the wing, and it starts to separate. When this point is reached, the wing is not far from stalling. The airflow unsticks[48] further up the wing as the AOA increases. The top of the wing still contributes to the production of lift, but not along its entire curve.
As the airspeed slows or as the angle of attack, or both, is increased further, the point is reached where, because of this separation, an increase in the AOA results in a loss of lift instead of an increase in lift. Thus, the wing no longer produces sufficient lift and the airplane that the wing is supporting accelerates downward. This is the stall.
Air density also contributes to the wing’s ability to produce lift. This is manifested primarily in an increase in altitude, which decreases air density. As the density decreases, the wing must push a greater volume of air downward by flying faster or push it down harder by increasing the angle of attack. This is why aircraft that fly very high must either go very fast like the SR-71, capable of flying Mach 3 (three times the speed of sound), or must have a very large wing for its weight, like the U-2.