HNC Aeronautical EngineeringAircraft Aerodynamics

Assignment 1Learning Outcomes: 1 and 2

Question 1: a (Outcome 1, Performance Criteria: a)

Describe the chemical composition of atmospheric air and explain how the earth’s atmosphere is divided into layers (50 words).

Answer:The chemical composition of atmospheric air is 78% nitrogen, 21% oxygen, 0.9% argon, 0.05% carbon dioxide and trace elements of other gases. The atmosphere is made up of four gaseous layers, the layer closest to the earth is the troposphere, then the stratosphere, then the ionosphere and finally the exosphere.

Question 1: b (Outcome 1, Performance Criteria: b)

Explain why we have an International Standard Atmosphere, and how it is used (50 words).

Answer:Atmospheric conditions vary from area to area and are continually changing, even pressure and density do not decrease uniformly. For this reason ISA is an assumed set figures of pressure and density used for comparing aircraft and engine performances and for calibrating aircraft instruments.

Question 2: a (Outcome 1, Performance Criteria: d)

Use the section of ISA tables provided to estimate the density of air in ISA conditions at 4000 metres altitude (1 line).

Answer At 4000 metres the air density will be 0.813kg/m³.

Question 2: b (Outcome 1, Performance Criteria: d)

Estimate valves for temperature and pressure at 4000 metres altitude, and calculate a valve for density using the Equation of State (Gas constant for air - 2.827)(6 lines)

Answer

My estimated temperature at 4000m is 262.2KMy estimated pressure at 4000m is 612MbP=RT P= RT 612 =2.872x262.2=0.813kg/m³

Question 2: c (Outcome 1, Performance Criteria: d)

Compare your answer for 2a with your calculation for 2b (6 lines)

Answer

My answer for 2a was 0.813kg/m³ and my answer from my calculations in 2b was 0.813kg/m³ reinforcing that the density at 4000m is 0.813kg/m³.

Question 3: a (Outcome 1, Performance Criteria: c)

Give the ISA assumed values of the following parameter (40 words):

Temperature.Lapse rate.Pressure.Density.

Answer

The ISA assumed value for temperature is 288.16K.The ISA assumed value for lapse rate is 1.98°C per 1000ft or 0.65°C per 100

metres.The ISA assumed value for pressure is 1013.25Mb.The ISA assumed value for density is 1.2250Kg/m³.

Question 3: b (Outcome 1, Performance Criteria: c)

Explain the variation of the following parameters as altitude increases (50 words):

TemperaturePressureDensityViscosity

Answer

As air raises a drop in pressure allows it to expand. This expansion allows the air to cool meaning that at greater altitude the temperature will be lower.

The gravitational effect decreases as altitude increases meaning the atmospheric pressure also decreases with altitude.

Air density depends upon the atmospheric pressure, temperature and humidity however with an increase in altitude the reduction of atmospheric pressure is the dominant factor and so air density decreases with increased altitude.

Viscosity will be greater at higher altitude as there will be less viscous adhesion to the surface of the earth then at lower altitudes.

Question 4 (Outcome 2, Performance Criteria: a)

Give a definition for each of the following (25 words each):

Indicated Airspeed.Calibrated Airspeed.Equivalent Airspeed.True Airspeed.

Answer

Indicated Airspeed is a rough indication of the airspeed taking into account manufacturing tolerances and wear and tear in use which are acceptable if within limits.

Calibrated Airspeed is the airspeed of the aircraft taking into account that pitot/static sensors may not measure the true pressure values. This is done using a correction card.

Equivalent Airspeed is the results from wind tunnel experiments which use a standard atmosphere based measure of airspeed. This is done by applying a correction factor.

True airspeed is calculated using calibrated airspeed, outside air temperature, and height (pressure). However this is only true at the time the calculation was made.

Question 5 (Outcome 2, Performance Criteria: a)

Draw a flow diagram to show how Indicated Airspeed is converted into True Airspeed, showing all corrections and procedures.

Answer

Question 6: a (Outcome 2, Performance Criteria: b)

Sketch a typical aerofoil section at an angle of attack of approximately 8°.Show by the use of streamlines,-laminar flow, turbulent flow, transition point and separation point.Explain the effect on the airflow, and in particular the separation point if the angle of attack is increased to 12° (25 words).

Answer

As the airflow encounters the aerofoil at 12° the air will flow quicker on the under surface of the aerofoil then on the upper surface. This will cause the separation point to be closer to the leading edge then in lower angles of attack.

Question 6: b (Outcome 2, Performance Criteria: b)

How does the smoothness of an aerofoil surface lift and drag (40 words)?

Answer

The smoother an aerofoil the less surface friction will cause drag and so generate more lift. If the aerofoil is smooth the separation point will be further back from the leading edge because the velocity and therefore kinetic energy in the boundary layer will not be reduced.

Question 7 (Outcome 2, Performance Criteria: c)

What is Reynolds Number and what effect does it have on the calculated performance of an aerofoil (60 words)?

Answer

Reynolds Number is a measure of the ratio between the viscous forces and the inertial forces of fluids. When calculating the performance of an aerofoil Reynolds Number indicates what speeds and pressures the scale model aircraft is subjected to to ensure structural security in a actual size aircraft. Because of this every wind tunnel test has a Reynolds Number which is published with the test results.