2. BASIC AERODYNAMICS
- How can an aeroplane fly?
You’ve probably done it on a hot summer day, resting your arm in the open window of a driving car to feel the cool air outside. When you do this, you will experience the following:
- If you hold your arm level from elbow to your fingertips into the airflow you will notice it is being pushed backwards.
- When you bend your fingertips upwards you will feel your arm is being pushed backwards even more, but you will also feel a strong force moving your arm upwards.
- Conversely, if you make a downwards-pointing angle, you will notice a strong force backwards and a force downwards.
An aeroplane can only fly or travel forwards if it has speed.
If the wing would be a completely flat board and this board would be placed parallel into the airflow, the wing would only have a frictional resistance.
When, like our dangling elbow out of the car window, the board is at an angle to the airflow (the angle of attack) the air flowing past it is deflected downwards, creating a counterforce upwards and backwards called Total Reaction (TR). This TR force can be divided into two forces: a backwards force called Drag (D) and an upwards force called Lift (L).
A plane with completely flat wings would have a lot of drag and this is why, in the early days of aviation, the curved shape of the wing was copied from birds. A bird shape wing has a very low resistance (or drag) and because of its curved design it generates an upwards force. This is exactly what is needed to fly.
In summary: by deflecting the airflow downwards we generate an upwards force called lift. The wing shape itself also generates lift using its curvature. Placing the wing in an airflow will deflect the air up in front of the wing and down at the trailing edge. The air on the top surface of the wing needs to travel a longer distance than the air below the wing which is following a flatter surface. Therefore the air on the upper surface needs to travel faster, covering a larger distance in more or less the same amount of time. In faster moving air the pressure drops. Therefore the faster moving air on top of the wing creates a lower air pressure than there is below the wing. This pressure difference makes the wing want to move upwards, adding to the Lift.
ANGLE OF ATTACK AND DRAG
If you increase the angle of attack, the drag will increase. (Remember the stronger force backwards on your arm.) Reducing the angle of attack reduces the drag.
ANGLE OF ATTACK AND LIFT
Increasing the angle of attack does not have the same effect on lift and drag. At low angles of attack the effects of increasing or decreasing the angle of attack are more or less the same for lift and drag. When we increase the angle of attack the Lift will increase.
However, we cannot increase the angle of attack endlessly; at an angle of about 15° the air can no longer follow the surface on top of the wing and it starts to swirl, leaving the wing’s surface. This separation of air from the wing surface causes a strong decrease in lift and an increase in drag. This is called the critical angle of attack.
In summary: while increasing the angle of attack, both the lift and drag increase. But when reaching the critical angle of attack, the lift strongly decreases and the drag increases.
BALANCE BETWEEN THE LIFT AND WEIGHT OF A GLIDER
In practice we say that the lift must be the same as the weight of the aircraft. For a better understanding of the theory we should say that TR (Total Reaction) equals Weight (W).
In the above illustration Lift is shown perpendicular to the airflow and Weight is pointing vertically downwards. The Drag is parallel to the airflow. Compared to Weight, TR is the resultant force of Lift and Drag. When flying straight with the wings level, Lift is about the same as TR and in this case you could say that Lift equals Weight.
ANGLE OF ATTACK AND WEIGHT
The amount of Lift depends on the airspeed and the angle of attack. When flying straight and level in a heavy two-seater glider at a normal constant airspeed, your angle of attack will be around 7°. And what happens to your angle of attack if you fly the same two-seater glider solo at the same airspeed? If Weight decreases, the Total Reaction (TR) will decrease, therefore you need less Lift. To fly at the same airspeed with a lower weight, your angle of attack will be smaller to balance Lift and Weight.
ANGLE OF ATTACK AND AIRSPEED
Let us suppose you are flying with your instructor in a two-seater glider and you need to reduce your airspeed. What happens to your angle of attack? The higher the airspeed the more Lift will be generated on a wing. So if you lower the airspeed the wing is producing less lift and you need to increase the angle of attack to generate the required amount of Lift to balance the Weight.
The principles of flight are based on basic aerodynamic laws. The same laws apply to a 1931 Slingsby made of pipes, wood and linen as on a modern high-tech glider like the Discus 2 built out of glass and carbon fibres.