Pressure Distribution over an Aerofoil

Pressure Distribution

Summary

A laboratory session was carried out in which an aerofoil was put in a low speed wind tunnel, pressure readings were taken from above and below the aerofoil using a manometer, and the purpose of this whole lab is to see if experimental data gained will correlate with theoretical results. The way in which this will be carried out is by calculating the Centre of Pressure (Cp) and plotting it against each angle of attack, the Lift and Drag coefficient need to be calculated and then plotted against angle of attack and the final graphs will be compared to theoretical results.

Introduction

The purpose of this technical report is to see how airflow affects the aerofoil and to examine how pressure distribution affects the lift and drag at different angles of attack.

The experiment was carried out within a aerodynamics laboratory, an aerofoil was put within a low speed, closed circuit wind tunnel, pressure readings were taken from above and below the aerofoil using pressure tapping's, the device connected to the pressure tapping's was a multi tube manometer and this was used to read off the different pressures around the aerofoil. According to Bernoulli's theorem the top part of the aerofoil has lower pressure than the lower section. The aerofoil was moved to different angles to show how changing the angle of attack affects the pressure readings which directly affect how much lift and drag the aerofoil can produce, the highest angle of attack was chosen so we get to see the aerofoil stall.

This experiment will demonstrate if our experimental readings recorded will look similar to the theoretical reading within aerodynamic books.

Background Theory

Aerodynamic Theories

There is a reason an aircraft can produce lift and that is because of the shape of its wings within aerodynamics there are two theories that explain how an aircraft achieves lift and these two theories are:

Bernoulli's Theory

Newtonian Theory

Bernoulli's Theory

Bernoulli's theory, also known as the longer path explanation, states that an increase in speed in a fluid results in a decrease in pressure. In this case air is the fluid and it is said the air flowing over the top of an aerofoil is moving faster and travelling further than the air flowing past the bottom of the aerofoil. This simply means that the air passing the top of the aerofoil has less pressure acting on the wing than the air at the bottom; in turn the higher pressure at the bottom pushes the aerofoil upwards, which gives us lift. This explanation is not entirely correct in that planes have the capability to fly upside down and some wings are symmetric so the distance travelled by the fluid is equal.

Newtonian Theory

The second way lift is generated over an aerofoil is known as the Newtonian theory. Newton's first and third law states that "A body at rest will remain at rest and a body in motion will continue in a straight line motion unless subjected to an external applied force" and "Every action has an equal and opposite reaction". Related to the third law Sir Isaac Newton states that air molecules behave like individual pellets striking the bottom of the wing and deflect in a downwards motion transferring some of their momentum which in turn nudges the wing upward. This explanation is also not entirely complete in that the top section of the wing has been excluded.

Ideal Lift and Drag coefficient

The ideal increase in pressure in this case is a high angle of attack results in an increase in pressure, this is until the aerofoil reaches the stalling point, this is where the angle of attack is so high enough lift isn't produced.

Graphs Theoretical

Lift and Drag Coefficient against Angle of attack

The maximum lift coefficient occurs at the critical stall angle, which also has a high drag coefficient. The minimum drag coefficient occurs at zero degree angle of attack, where the lift coefficient also is at zero.

Experimental Setup

The experiment was carried out on the Friday 14th February, in which a 27-tube manometer was used to record the pressure readings recorded by the different pressure tapping's place on different sections of the aerofoil.

Once the wind tunnel was started and raised to the desired speed the following were recorded:

* Tunnel temperature at each angle of attack

* Projection Manometer at each angle of attack

* Tunnel Static at each angle of attack

* Atmospheric Datum at each angle of attack

* Pressure at each angle of attack

The 27 manometer readings were recorded and then the aerofoil was subjected to a different angle of attack and the same procedure continued until all pressure readings for each angle of attack were recorded. During the experiment different pupils would go up to the manometer and read off the pressure readings, there are 27 pressure readings for each angle of attack.

The reason the aerofoil was subjected to different angles of attack was to see how it reacted to the airflow, the purpose of the pressure readings recorded is to know where on the aerofoil lift and drag is being generated, at different angles of attack it is going to be clear the location of the forces.

Calculations

1. Firstly find the sum of x/c and y/c

The first thing that was done to carry out the calculations was finding the change in x/c and y/c; this was done by subtracting each x/c and y/c value by the one above.

2. The Cx and Cy values need to be calculated

According to the equation above Cp*change in y/c or x/c is equivalent to Cy and Cx, so now each