Aeromodelling

Aerodynamic Center (AC) is an imaginary point on the aircraft wing. When the aircraft wing move through the air, the position of the aerodynamic center remain at the same point regardless of change in angle of attack.

Aerodynamic center is located around 25% of the chord from the leading edge for low speed airfoils. For subsonic flow, it located approximately 50% chord from the leading edge of an aerofoil.

This concept of the aerodynamic centre is useful to designers, because lift force assume located at a fixed point and only among of lift force changes with angle of attack vary.

Center of Pressure is a resultant point where the totals sum of all the pressures that produced on the aerofoil surface. And this pressure resultant point fall along the chord line.

The center of pressure for a flat plate or any symmetrical airfoil stay in one position at the chord line when change in angles of attack.

For a non-symmetrical airfoil, the center of pressure moves alone the chore line as changes in angle of attack.

when increase in angle of attack, the center of pressure moves forward

and the center of pressure moves backward when decrease in angle of attack.

The total weight distribution of an aircraft often assumes it concentrate and act through a single resultant point called the Center of Gravity. The aircraft would be perfectly balanced when it suspended through this point — ‘Center of Gravity’.

Aircraft maneuvering in flight, it rotates about the center of gravity. Therefore, the CG is an important point on an aircraft which significantly affects the stability of the aircraft.

Aircraft designer are very careful of placing those components that vary the aircraft overall weight and moving parts, such as fuel tanks, bombs and landing gear. Reduction of fuel in the tanks, dropping a bomb or lower down and retract of landing gear cause the position of CG shifted. The CG location is crucial for aircraft stability. It is important to place the components within the limit to maintain a stable and controllable in flight.

Forces Acting on an Airplane

During the aircraft in straight-and-level unaccelerated flight, there are fore forces acted on the aircraft. These four main forces acting on an airplane are lift, drag, gravity, and thrust. Maintaining a steady flight of the aircraft requires a balance of the four forces, often described as equilibrium.

To maintain equilibrium the following conditions must be met:

In general view, the vectors of the four forces are always assumed act on a single point. The presentation of the four forces as illustrated in figure above. However, in reality as shown below figure, Weight is acted on the Center of Gravity. Lift is acted at the Center of Pressure. Thrust and Drag are paired to reduce the pitching moment which is created by the lift.

Weight and Drag forces are inherent in any objects that lifted from the ground and moved through the air. Thrust and lift are artificially created forces used to overcome the forces of nature and enable an airplane to fly. The engine and propeller combination is designed to produce Thrust to overcome Drag. The wing is designed to produce Lift to overcome the Weight (gravity).

When the four forces are in equilibrium, the aircraft in motion will tend to keep moving along the same flight path with a constant speed, whether it is flying straight and level, descending or climbing.

If the pilot sets the engine throttle fully, and maintains straight-and-level flight. The aircraft accelerates in a direction; the thrust force is initially greater than drag. That mean the forces are not in equilibrium state. However the aircraft will soon reach the full throttle speed. And the aircraft speed remains constant, where the forces are again balanced.

Control surfaces are the moving parts of an airplane that adjust it flight path. There are several control services that have been standardized and used in most common R/C model aircrafts as well as the real aircrafts. The three primary flight controls are the ailerons, elevator and rudder. Other flight control surfaces like flaps, slots, slats and trim tabs are to improve the flight control and stability of the aircraft.

The angular movement of control surface basically is to change the Angle of Attack of the aerofoil. The Angle of Attack of the aerofoil change is to generate aerodynamic forces (lift and drag forces). This aerodynamic force cause the aircraft rotate about the three imaginary axles which intersected at the center of gravity at right angles to each other.

Ailerons:
Ailerons are the control surfaces that located on the outer trailing edge of each wing. They serve the purpose of adjusting the roll of the airplane. The two ailerons always move in opposite directions to create aerodynamic forces on the wings cause the aircraft rolls about the longitudinal axis. They also are used to turn the airplane, although unless rudder is added the turn will be sloppy.

Elevator:
Elevators are used to control the planes pitch or up down motion. Elevators are hinged to the horizontal stabilizer (tailplane) at the rear side to form a single airfoil. The elevators up and down motion generated a force to press or lift up the tailplane to controls the movement of the aircraft about its lateral axis. Another word it makes the aircraft climb and descend.

Rudder:
Rudder is to control yawing motion of the aircraft. It vertically attach on the vertical stabilizer (tail fin). A small change in angular motion of rudder there is changes in angle of attack of the vertical stabilizer (tail fin). The dynamic forces generated due to angular movement of the rudder to cause the aircraft rotate about its vertical axis.

When the fixed wing aircraft airborne, control surfaces of the aircraft allows pilot to adjust and control the flight attitude. The aircraft rotational motion is in three-dimensional space. It will rotate about the three imaginary axes. These three imaginary axles are intersected at the center of gravity at right angles to each other.

Longitudinal Axis: The imaginary line that extends lengthwise through the fuselage, from nose to tail, is the longitudinal axis. Motion about the longitudinal axis is roll and is produced by movement of the ailerons located at the trailing edges of the wings.

Lateral Axis: The imaginary line which extends crosswise, wing tip to wing tip, is the lateral axis. Motion about the lateral axis is pitch and is produced by movement of the elevators at the rear of the horizontal tail assembly.

Vertical Axis: The imaginary line which passes vertically through the center of gravity is the vertical axis. Motion about the vertical axis is yaw and is produced by movement of the rudder located at the rear of the vertical tail assembly.

Wing Configurations and Shapes

Aircraft wings are built in many shapes and sizes for difference application. It is depending on the desired flight characteristics of an aircraft. Also, wing designed in difference configurations to achieve greater lift, balance or stability in flight.

Here shows a number of typical wing leading and trailing edge shapes

Also figure below shows the common wing forms and configuration.

F-14 Tomcat is a supersonic aircraft with variable geometry wing. This aircraft wing geometry changes according to flying speed by swinging the wings forward and backward.

Enjoy the video

Aspect Ratio

Aspect ratio is an indicator of the general performance of an aircraft wing. In aerodynamics, the aspect ratio of a wing is defined as the square of the span divided by the wing area. It is a measure of how long and slender a wing is from tip to tip.

For “high” aspect ratio aircraft wing indicates long, narrow wings, whereas a “low” aspect ratio wing indicates short and stubby. Higher aspect ratio has the effect of a higher rate of lift increase, as angle of attack increases, than lower aspect ratio wings.

High aspect ratio wing – higher Lift Coefficient
lower stalling angle of attack… eg. Gliders

Low aspect ratio wing – lower Lift Coefficient
high stalling angle of attack… eg. Fighter Jets

However because wings may have varied plan forms it is usual to calculate aspect ratio as:

Aspect ratio = wing span² / wing area

= Wing span / Chord length

Pictures below show the F-15 Eagle with low aspect ratio wing shape and B52 bomber with high aspect ratio wing.

Dihedral Angle

The purpose of dihedral is to improve the aircraft stability during flight. Dihedral angle is added to the wings for later or rolls stability. When the aircraft encounters a slight roll displacement caused by distribute from air stream or a gust of wind. An aircraft wings with some dihedral will naturally return to its original position.

The front view of this wing shows that the left and right wing do not lie in the same plane but meet at an angle. The aircraft’s wing is inclined upward an angle from root to tip. The angle that the wing makes with the local horizontal is called the dihedral angle.

You may have noticed that most large airliner wings are designed with dihedral. The wing tips are farther off the ground than the wing root.

Anhedral Angle

Highly maneuverable fighter planes, on the other hand do not have dihedral. In fact, some fighter aircraft have the wing tips lower than the roots giving the aircraft a high roll rate. A negative dihedral angle is called anhedral

Wing AnhedralAngle remove the effect of Dutch Roll: Oscillatory motion combining roll & yaw with aircraft waddling from side to side.

Drag is the aerodynamic force that opposes an aircraft’s motion through the air. Drag is generated by every part of the airplane and it induced depends on the shape, size, inclination, and flow conditions of the air passing the object.

Drags commonly caused by the airplane exposed parts in the air stream which are not lift producers called as Parasite Drag. The two major contributors to parasite drag are Form drag and Skin-friction drag.

Form drag refer to the resistance which is the air flow past an object, the air stream no longer get a smooth streamline flow. The amount of induced air turbulent and vortices depend upon the shape of the object.

Skin friction drag refer to the skin smoothness of aerodynamically structures determines the resistance of the skin to airflow. If such resistance exists, the stream line of a thin layer is disturbed and affecting the adjacent layers. This form of drag is known as Skin-friction Drag.

Induced drag is the unavoidable by-product of lift as increases the angle of attack. The greater the angle of attack, up to a critical angle, the greater the amount of lift developed and the greater the induced drag.

There are two different types of induced drag

a) Dynamic induced drag

b) Pressure induced drag

Dynamic induced drag is depending on the angle of attack of the aerofoil, when the aerofoil move across the air stream there will be a force backward (induced drag) and a force upward (lift). The amount of force in each direction will depend on the angle of attack, shown in below diagram.

However, at very high angles of attack, approaching the stall point, lift will decrease and the drag will overcome lift and thrust with an accompanying loss of speed and attitude. If the angles of attack of aerofoil almost vertical to the relative wind, the only force would be backward; that is, all dynamic drag and no lift.

Pressure-induced drag refer to left generated across the wing, the pressure below an airplane wing is higher than the pressure above it. As a result, the air at the high pressure side (bottom surface) tends to flow outwards to the wind tip. Therefore, there is a constant tendency of air to flow from bottom to top. The air flow over the top surface of a wing tends to move in towards the fuselage and off the trailing edge.

Since the airplane is constantly moving the air is forced up at the wing tips. This causes a spiral or vortex which trails behind each wingtip whenever lift is being produced. These vortices increase drag, because of the turbulence produced, and constitute induced drag.

When an aerofoil is moved through the air, as the velocity of the airflow increased at the upper surface, thereby the pressure above the aerofoil surface reduced. Simultaneously, the air pressure on the lower side surface of the aerofoil increased. Consequently, a pressure difference between the lower and upper surfaces exists in results of LIFT being produced.

The amount of lifts generated by an aerofoil depend upon:

a) The shape of the aerofoil.

b) The plane area of the aerofoil.

c) The square of the velocity.

d) The density of the air.

e) The aerofoil inclination to the airflow.

As the aircraft passes through the air it traverses a particular line of flight. The airflow passing by the surfaces of the aircraft in the opposite direction of travel is called the Relative Wind. The angle between chord line of the aerofoil and the direction of the Relative Wind is called Angle of Attack.

As increase the angle of attack of the aerofoil, the amount of lift and drag increases on the aerofoil. The angle of attack continuous increase towards 12 to 15 degrees. The air flowing over the top of the aerofoil surface begins to swirl and turbulent occur at the air stream. At this point, called the Critical Angle of Attack, where the airflow over the wing becomes so disturbed. The total lift drops suddenly, and the aerofoil enters into a stalled condition called Stalling Angle.

Below video have a good explanation how and why an airplane stalls.

Enjoy it!

Generally a flyable aeroplane model is similar to real aircraft. A small aircraft wing will look like the cross-section of the figure below. The shape of the wing is called an AEROFOIL. An aerofoil is a important lift generating device. An aerofoil function is to produce a controllable net aerodynamic force by its motion through the air.

When the aircraft move forward, the air flow over the wing surface to gets a useful reaction to generate lift. The airstream flow around the wing behavior accords with Bernoulli’s Law. The aircraft wings, horizontal tail surfaces (tail plane), vertical tails surfaces (tail fin), propellers and other parts of the control surface of the airplane are shaped as aerofoil.

Below figure, the forward part of an aerofoil is rounded is called the leading edge. The aft part is narrow is tapered is called the trailing edge. An imaginary straight line joins from the extreme of the leading edge to the trialling edges is called the chord line.

The general shapes of the aerofoil section showed below diagram. Those aerofoil shapes used on the aircrafts have been tested and designed for particular purposes.

Flat plates are commonly used in model aircrafts as tail fin, tailplane and control surfaces. The flat plate hold at a small inclined angle to the directional of the air flow, it will generate an aerodynamic force – lift and drag. Some of the low speed aircraft do use basically flat plates in their tailplane surfaces too.

A video about aerodynamics…………