The Nature Of Subsonic Airflow Engineering Essay


The nature of subsonic airflow over aerodynamic sections and over the aircraft at large must be considered, including the forces that result from such airflow and the effect these forces have on the aircraft, during steady flight and during manoeuvres.

Although there are various kinds of pressure, pilots are mainly concerned with atmospheric pressure. It is one of the basic factors in weather changes, helps to lift an aircraft, and actuates some of the important flight instruments. The pressure of the atmosphere varies with time and location. Due to the changing atmospheric pressure, a standard reference was developed. The International Civil Aviation Organization (ICAO) has established this as a worldwide standard, and it is often referred to as International Standard Atmosphere (ISA) or ICAO Standard Atmosphere.

Large modern passenger aircraft can weigh in excess of five hundred thousand kilogram's when they fly with a full fuel and passenger load, yet this combined mass is lifted into the air with apparent ease. Modern jet fighter aircraft can exceed the speed of sound and are very manoeuvrable. Thrust, drag, lift, and weight are forces that act upon all aircraft in flight. Understanding how these forces work and knowing how to control them with the use of power and flight controls are essential to flight.

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The international standard atmosphere is an atmospheric model of how the pressure, temperature, density and viscosity of the earth's atmosphere change over a wide range of altitudes.

ISA model divides the atmosphere into layers.

Atmospheric Layers

Figure 1 source=,r:1,s:0,i:72


Troposphere contains about 80% of atmospheres mass 99 % of it's water vapour and aerosol. The temperature of the troposphere generally decreases as the altitude increases. The reason for the temperature difference is that the absorption of the sun's energy occurs at the top of the atmosphere cooling the Earth, this process maintaining the overall heat balance of the Earth.


Stratosphere is the second important layer of the atmosphere. It is separated from troposphere by tropopause. It takes about 12 to 50km of the atmosphere. The temperature increases as the altitude increase. At the top of the stratosphere the thin air may attain temperature close to 0c. this is happening because of the absorption of UV radiation from the sun by the ozone layer. Such a temperature profile creates very stable atmospheric conditions and the stratospheric lacks the air turbulence that is so prevalent in the troposphere. Stratosphere is completely free of clouds and any other forms of weather. This layer is very good for the flights to fly as it is above stormy weather and has strong, steady and horizontal winds. Stratosphere is separated from the mesosphere by the stratopause.


This layer is the third highest layer of the atmosphere. This layer takes 50 to 80km above the surface of the Earth. It is separated from the stratosphere by stratopause and from the thermosphere by mesopause. Temperature drops when the altitude increases to about -100. Mesosphere is the coldest of all the layers as it is colder than Antarctica. This layer can freeze water vapour into ice clouds so that when the sunlight hits them you can see it after sunset. It is also the layer where the meteors burns up while entering the Earth's atmosphere.


Thermosphere is the outer layer of the atmosphere. Mesopause separates mesosphere from thermosphere. In this layer the temperature rise continually to well beyond 1000 The few molecules that are in this layer receives an extraordinary amount of energy from the sun therefore warms up the layer making it hotter. Air temperature however is the measure of the kinetic energy of air molecules, not of the total energy stored by the air so the air is so thin within the thermosphere, such temperature values is not comparable to other layers. Although the temperature is very high we would feel very cold because the total energy of only a few air molecules residing there wouldn't be enough to transfer any heat to our skin.

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After thermosphere it's Ionosphere. This area is full of ionized air extending from 80km above the Earth's surface altitudes of 600km and more. Technically Inosphere is not layer.In this region/area the sun's energy breaks molecules and atoms of air as the energy is so strong and hot leaving ions and free floating electrons. Ionisation of the air molecules is produced by UV radiation, other radiation from sun and cosmic rays. Ionosphere is the region where aurora appears.


Source figure 2=,r:6,s:0,i:87&tx=89&ty=37&biw=1366&bih=667

Laminar flow

Turbulent flow

Laminar flow is good for aircrafts as there is less drag and much easier to create lift. It is a very smooth and uninterrupted flow of air over the contour of the wings and other parts of an aircraft. Laminar is found at the front of the streamlined body. An air foil is designed for minimum drag and uninterrupted flow of the boundary layer is called a laminar air foil. The pattern of the flow involves of layers. Particles in each layer do not interfere with other particles in the other layer which makes it smooth flowing layers. There is no difference in velocity between the layers.

Boundary layers are thinner at the leading edge of the aircraft wing and thicker towards the trailing edge, such boundary has laminar flow in the leading portion and turbulent flow at the trailing portion.

There is more drag than laminar. In this flow the direction and velocity changes continuously. Particles move opposite to other particles causing collision which makes turbulence. The trust need to be more counteract the flow of turbulence.

Reynolds number is dimensionless quantity associated with the smoothness of flow of air/fluid. At low velocity the flow of a fluid/gas is laminar; the fluid/gas in the layers of laminar flow gives rise to viscosity. As the gas flows more rapidly, it reaches a velocity known as critical velocity. This is when the motion changes from laminar to turbulent. Viscosity is the resistance of a fluid to flow. The coefficient of viscosity of gases increases with increasing temperature.



Bernoulli's principle

As the velocity of the gas increases the pressure exerted by that fluid decreases. The aeroplane gets part of the lift from Bernoulli's principles. This principle says that increased air velocity produces. When the Bernoulli's principle is applied the fluid has these qualities

Fluid flows more smoothly

Fluid flows without any swirls(eddies)

Fluid flows everywhere through the pipe

Fluid has same density everywhere

As the fluid passes through a narrow or wide pipe, the velocity and pressure of the fluid vary. As the fluid flows through a narrow pipe the flow quickly. This principle says that fluid flows more quickly through the narrow section, the pressure actually decreases than increasing.

Air passes faster over the top of the cambered wing and results in lower pressure. The top of the wing is curved, the air that passes over the top of the wing moves faster because it travels a greater distance in the same amount of time as compared to the air that passes underneath the wing. Lift is created because the air under the wing is slower and exerts higher air pressure so the difference in the pressure creates the lift.

Venturi principle

An inverse association of gas pressure, velocity of flow, and restriction of passage. This principle states that the pressure drop distal to a restriction can nearly be restored to the pre restriction pressure if there is a dilation of the passage immediately distal to the stenos is, with an angle of divergence not exceeding 15 degrees. In venture tube the area decreases the velocity increases and the pressure decreases, and vice versa as well. So the difference in pressure creates lift.

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Total drag

Total drag is the sum of all of the aerodynamic forces which act parallel to, and opposite to, the direction of flight also it is the total resistance to the motion of the aircraft through the is the sum of other drags acting on the aeroplane which are parasite drag and induced drag.

Induced drag

is the drag created by the vortices at the tip of an aircraft's wing. Induced drag is the drag due to lift. The high pressure underneath the wing causes the airflow at the tips of the wings to curl around from bottom to top in circular ms in a trailing vortex. Induced drag increases in direct proportion to increases in the angle of attack. the circular motion creates a change in the angle of attack near the wing tip which causes an increase in drag. The greater the angle of attack up to the critical angle, the greater the amount of lift developed and the greater the induced drag.

parasite drag

the parasite drag of a airplane in the cruise configuration primarily of the skin friction, roughness and pressure drag of the major components. There is usually some additional parasite drag due to such things as fuselage upsweep, control surface gaps, base areas and other extraneous items. Since most of the elements that make up the total parasite drag are dependent on Reynolds number and since some are dependent on mach number, it is necessary to specify the conditions under which the parasite drag is to be evaluated. In the method of these notes, the conditions selected are the mach number and the Reynolds number corresponding to the flight condition of interest. This drag comprises skin, form drag and interference drag. Skin friction drag is a friction force between an object and the air through which it is moving produce skin friction drag. Form drag is when the airflow actually separates from the surface, eddies are formed and the streamline flow is disturbed. The turbulent wake so formed increase drag this is form drag. Interference drag is caused by flow interference at the wing and other such junctions. This interference leads to the modification of boundary layers and creates a greater pressure difference between the for and after area on the surface concerns. This in turn leads to greater total drag. Fairing or additional fillets are used to streamline these intersections and decrease interference drag.


Source figure 3=,r:3,s:0,i:78&tx=168&ty=69&biw=1366&bih=667

Profile drag

Induced drag

Is the drag incurred from frictional resistance of the blades passing through the air. It doesn't change significantly with an angle of attack of the airfoil section, but increase moderately as airspeed increases.

Is the drag incurred as a result of production of lift. Higher angles of attack which produced more lift also produce increased induced drag. In rotary wing aircraft induced drag decreased aircraft airspeed. The induced drag is the portion of the total aerodynamic force which is oriented in the direction opposing the movement of the airfoil.


Force diagram for an aircraft steady turning


Vector force diagram