Many years ago, I was lucky enough to work in “Launch Pad” in the science museum. There were lots of wonderful activities and the following blog posts are about the activities. Launch Pad has been replaced by the equally wonderful “Wonderlab”
This exhibit consisted of a model aeroplane in front of a wind generator. The aeroplane could be made to fly by altering the wind sped and the angle of the aeroplane. There were two different wing shapes to try out.
What did it show?
There were several different experiments to try on this exhibit
1) Angle of attack. Using the dial on the exhibit the wind speed was set to 20 knots and kept constant.
The knot is a unit of speed equal to one nautical mile per hour, exactly 1.852 km/h (approximately 0.514 m/s). The ISO standard symbol for the knot is kn.
Starting from a level position the nose of the aircraft was gently tilted upwards until it took off. Once it was in the air the tilt (angle of attack) was changed.
Things that were investigated included whether the aircraft could fly at a flatter angle than it needed to take off and what happened if the nose of the aircraft was tipped down or up too much.
2) Wind speed. Stage 1, above, identified the angle necessary for the aeroplane to take off. In this stage the speed of the wind was changed starting as low as possible and then gradually increased whilst identifying how much wind was necessary for the plane to take off. Another thing that was investigated involved identifying the amount of wind necessary to keep the aeroplane flying level and seeing if it was more or less than the take-off speed.
3) Wing shape. There were two different types of wing to try in the experiments. When looked sideways one was an oblong in shape and the other curved. They had different cross-sections. The shape of the curved wing was called an aerofoil.
The three experiments outlined above showed how the angle of attack, wind speed (equivalent to the speed of an aircraft in real life) and cross-section of the wing are important for flight and showing the differences between take-off and level flight.
How do heavy aeroplanes stay in the air when gravity should pull them down again? The answer is a force called lift, which can make the plane rise when its value is greater than the plane’s weight, the force that pulls the plane down.
Lift is generated by air flowing over the wings of an aircraft. A useful way to show airflow is to draw streamlines. These are a little like contour lines on a map. When they are closer together, the airflow is faster.
The image above shows streamlines over a wing with an aerofoil cross-section in an airflow. You can see that the lines are closer together above the wing and further apart below.
Smoke is used show up the streamlines
An aerofoil at the right angle to an airflow causes air to travel faster over the top than underneath. When air flows faster, it has a lower pressure. So, with the exhibit aerofoil, this means that the air above the wing has a lower pressure than the air below. The wing has to rise to equalise the pressures, and this is the force called lift.
Lift does not depend only upon the shape of the wing, but also on the amount of air flowing past it. In normal flight, this is the airspeed of the plane. In the launchpad exhibit, it was the speed at which air was blown over the model. The more air that flows over a wing, the more lift is generated.
The wind speed on the flight test exhibit could be altered by turning the silver knob. The gauge above it shows how fast the wind was flowing. More wind was needed to take off, than to fly level. This was because, for the plane to climb, the lift must be greater than the weight, whereas for level flight, the two must be the same.
When not enough air is flowing over the wings at low speeds, the aeroplane will stall, and drop rapidly. This is a problem when landing aircraft, where the descent needs to be controlled and not too fast for safety. Also, a fast landing needs a very long runway.
To solve this problem, aeroplanes have flaps on their wings, which are lowered when the plane comes into land. The flaps act as brakes, to slow the plane down, but also increase the lift of the wings, so that the plane does not stall at lower speeds, and can land safely on a smaller runway.
How does lift work?
All flight with the exception of rocket flight depends on air for lift and propulsion. That is why only rockets can go into space.
The Earth is covered in air and we are surrounded by it. We can’t see it but we can feel it when it is windy.
Air gets thinner with increasing altitude which is why mountaineers, who climb very high mountains, need breathing equipment to stop them becoming ill (although your body can be trained to withstand thinner air).
Lydia Bradey (right) and Roxanne Vogel on the summit of Mount Everest,
Mount Everest is Earth’s highest mountain above sea level, located in the Mahalangur Himal sub-range of the Himalayas.
Out in space there is no air.
All flight except rocket flight requires air for both lift and transportation
Plane and airship
Helicopter and hot air balloon
Kite and bird
2) The lift of a wing depends on:
(a) Aerofoil shape. The greater the top camber the greater the lift.
In aeronautics and aeronautical engineering, camber is the asymmetry between the two acting surfaces of an aerofoil, with the top surface of a wing (or correspondingly the front surface of a propeller blade) commonly being more convex (positive camber). An aerofoil that is not cambered is called a symmetric aerofoil,
Larger cambers increase drag and lower top speeds of the aircraft so flaps are used to increase the camber at times when greater lift is needed, such as when the aircraft is landing.
b) Angle of attack. Greater angles produce more lift, but only up to the stalling angle (about 15o for an average aeroplane).
In fluid dynamics, angle of attack is the angle between a reference line on a body (often the chord line of an aerofoil) and the vector representing the relative motion between the body and the fluid through which it is moving. Angle of attack is the angle between the body’s reference line and the oncoming flow. This article focuses on the most common application, the angle of attack of a wing or aerofoil moving through air.
As an object moves through a fluid such as air, the velocity of the fluid varies around the surface of the object. The variation of velocity produces a variation of pressure on the surface of the object. The pressure times the surface area around the body determines the aerodynamic force on the object (F = PA). We can consider this force to act through the average location of the pressure on the surface of the object. We call the average location of the pressure variation the centre of pressure in the same way that we call the average location of the weight of an object the centre of gravity. In general, the pressure distribution around the object also imparts a moment (usually the product of force x perpendicular distance between the force and the centre of gravity), on the object. If a flying aerofoil is not controlled in some way it will tumble as it moves through the air.
Changing the angle of attack changes the pressure distribution and therefore the aerodynamic force, the location of the centre of pressure and the moment also change.
So, the smaller the angle of attack the lower the lift. If the angle increases so does the amount of lift but stalling occurs if the angle becomes too large.
In fluid dynamics, a stall is a reduction in the lift coefficient generated by a foil as angle of attack increases. This occurs when the critical angle of attack of the foil is exceeded.
Stalling is caused by flow separation which, in turn, is caused by the air flowing against a rising pressure.
Whenever there is relative movement between a fluid and a solid surface, whether externally round a body, or internally in an enclosed passage, a boundary layer exists with viscous forces present in the layer of fluid close to the surface.
Flow separation or boundary layer separation is the detachment of the boundary layer from the surface.
In order to prevent the airflow from breaking away from the wing’s surface at higher angles, slots are often fitted to the wings.
Leading edge slots are ducts or passages in the leading edge of a wing that allow high pressure air from the bottom of the wing to flow to the top of the wing. This ducted air flows over the top of the wing at a high velocity and helps keep the boundary layer air from becoming turbulent and separating from the wing. Slots are often placed on the part of the wing ahead of the ailerons, so during a wing stall, the inboard part of the wing stalls first and the ailerons remain effective.
Boeing 727 flight controls.
c) Air density. Where the air is denser, usually at lower altitudes, more air can flow over the wings at any given speed and the lift is greater. Where the air density is lower, usually at greater altitudes, there is less air to provide lift.
d) Wing area. The greater the wing area the greater the lift.
However, wing aspect ratio is more important
The ratio of the length of wings to their width is called aspect ratio. A high aspect ratio indicates long, narrow wings. A low aspect ratio indicates short, wide wings.
Aspect ratio = wing length (m)/wing width (m)
Generally, high aspect ratio wings give slightly more lift and enable sustained, endurance flight, while low aspect ratio wings are best for swift manoeuvrability.
Hugh aspect ratio wings give more stability, less drag and cause less fuel to be used.
Low aspect ratio wings give greater manoeuvrability.
Aspects ratios and wing loading are combined for different flying capabilities. For example, high aspect ratio combined with low wing loading is used for slow flight such as gliding or soaring.
e) Velocity. The higher the velocity the greater the lift. Faster planes attain much greater lift than slower ones.
The four forces of flight
In lift and weight, we have met two of the forces involved in flight. Lift is the force that makes the aeroplane rise and weight is the force that pulls it down. It follows, then, that there might be forces to move the plane backwards and forwards or faster and slower, and indeed there are.
Thrust is the forward force provided by the engine of the aircraft. This is resisted by the drag, a force that holds the plane back.
Level flight at constant speed
For a plane to fly level lift and weight forces must be equal in size but act in opposite directions. If lift were greater than weight the plane would rise and if it were less then the plane would descend.
For a plane to maintain a constant speed, the thrust force and the drag force must be equal in size but act in opposite directions. If thrust were greater than the drag the plane would speed up and if it were less the plane would slow down,
Drag, like lift, is caused by air flowing over the aircraft. It is a result of air resistance, which increases with increasing speed and can cause the plane to waste fuel.
In general, if the airflow over the aeroplane breaks away from the surface or becomes even, then swirls and eddies of turbulent air form, wasting energy and spoiling the effect of the aerofoils.
There are a number of causes of drag
1) Form drag
This is the resistance to movement caused by air having to flow over an uneven shape. It can be reduced by making the shape of the aeroplane as streamlined as possible, that is, the streamlines of air should not be too disturbed by passing over the plane. That is why aircraft have smoothly curving outlines, and why many have the undercarriage retract after take-off.
2) Skin friction
This is when air sticks to the surfaces of the plane. Try rubbing your hand over a rough surface, like a carpet, and then over a smooth one, like a table. Your hand moves much more easily across the smooth surface. Air is the same; it flows better over a smooth surface. Skin friction, then, can be reduced by making the aircraft as smooth and polished as possible.
Frictional drag and form drag are different. Frictional drag is due to the normal forces that result when a fluid flows over a surface. When the surface is rough there tends to be more frictional drag than when the surface is smooth. Form drag, on the other hand, is due to the normal forces that result when a fluid flows over an object creating areas of high and low pressure. Form drag is calculated by taking the integral of the pressure on the surface times the slope of the object over the entire surface area of the object. In general, streamlined bodies have smaller form drag than bluff bodies.
3) Induced drag.
This is caused by air spiralling over the ends of the wings and forming trailing vortices: twisting swirls of air that travel back from the wing tips. This wastes some of the engine’s energy. Aeroplanes are often swept back to reduce this form of drag (smaller vortices).
Angle of attack
The model plane in flight test flew best when it was tipped back at a slight angle. When it was tipped forward and if it was tipped back too much it fell. As mentioned earlier the amount by which an aircraft is tipped forward or back in relation to the airflow is called the angle of attack.
Up to a certain point, the greater the angle of attack, the greater the lift. However, at a critical point, the airflow over the wing starts to break up. It no longer flows smoothly over the wing but becomes turbulent, breaking up into swirls and eddies. This increases the drag and decreases the lift at the same time, so that the aeroplane stalls. Unlike the stall that happens at low speeds, this occurs at any speed once the critical angle, called the stalling angle is reached.
The stalling angle varies from plane to plane, but is usually around 15o. The angle at which the plane stalls can be increased by having slots in the wing, which can keep the airflow close to the wing.
A319 slats during and after landing
Slats are aerodynamic surfaces on the leading edge of the wings of fixed-wing aircraft which, when deployed, allow the wing to operate at a higher angle of attack. A higher coefficient of lift is produced as a result of angle of attack and speed, so by deploying slats an aircraft can fly at slower speeds, or take off and land in shorter distances. They are usually used while landing or performing manoeuvres which take the aircraft close to the stall, but are usually retracted in normal flight to minimize drag. They decrease stall speed.
Slats are one of several high-lift devices used on airliners, such as flap systems running along the trailing edge of the wing.
How do aeroplanes move along?
Like all objects that can be moved we can apply three simple laws that were first suggested by Sir Isaac Newton.
Sir Isaac Newton PRS (25 December 1642 – 20 March 1726/27) was an English mathematician, physicist, astronomer, theologian, and author (described in his own day as a “natural philosopher”) who is widely recognised as one of the most influential scientists of all time and as a key figure in the scientific revolution. His book Philosophiæ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy), first published in 1687, laid the foundations of classical mechanics.
Law one states that an object will remain at rest or if already moving, move at a constant velocity if all forces acting on it are balanced. For an aeroplane, in simplified terms, this means it will maintain a constant velocity at a constant altitude if thrust equals drag and lift equal weight.
Law two states that the net unbalanced force acting on object is equal to its mass multiplied by its acceleration. If the mass is constant then the unbalanced force is proportional to acceleration (or deceleration if the force is acting against the motion). For an aeroplane, in simplified terms, this means that if the thrust becomes larger than the drag then the plane’s velocity will increase. If the drag becomes larger than the thrust the plane’s velocity will decrease. If the lift becomes larger than the weight the plane will move upwards and if the lift becomes less that the weight the plane will move downwards.
Law three is often written as “To every action there is an equal and opposite direction” but is often misunderstood. It is better to write is as “If body A exerts a force on body B then body B exerts an equal but opposite acting force on body A”. In other words, Newton’s third law says there must be pair of forces (known as Newton pairs) involved but it is important for you to realise that thrust and drag are not a Newton pair and neither is lift and weight. In other words, if something wants to move forwards, it must push something backwards. In a bicycle for instance, the parts of the wheels touching the ground are going backwards (because the ground itself can’t go backwards), so that the bike moves forwards. The force of the tyre on the ground is equal to the force of the ground on the tyre (but acts in the opposite direction).
For Newton’s third law the forces must be of the same type, be the same size, act in the opposite direction to each other and act on another object, not itself.
A plane doesn’t use wheels when it is in the air, so it must push something else backwards. This something is the air all around it. In an airplane, the propeller moves and pushes back collected air; consequently, the air provides the thrust and pushes the propeller (and thus the airplane) in the opposite direction—forward. More thrust is given if more air is pushed back, or if the air is pushed back faster, or both.
The lift force also needs a newton pair. In this case the air under the plane “feels” a force pushing it down and the lift is the reaction as the air “pushes” up on the plane.
Now you might be asking yourself about drag and weight. Well drag is simply the force of air acting on the plane pushing it backwards as the plane is pushing on the air forwards. So, air is needed to push the plane forwards but it is, unfortunately, the source of the forces trying to stop it moving forwards.
The Newton pair for weight often surprises my GCSE students (KS4 from 14 to 16 years of age). Weight is, in this case, the pull of the plane by the Earth and the Newton pair is the pull of the Earth by the plane.
It should be noted that we are not aware of the Earth moving upwards to meet the ball falling downwards because the effects of the two forces are different due to the objects having radically different masses.
Some aircraft use propellers. These are like rotating aerofoils. As they are spun by the engine, each blade pushes air backwards to make it the plane go forwards.
The rotors on helicopters act in a similar way, but they are used both for lift and thrust. When the helicopter is tipped forwards, it moves forwards. When the helicopter is not tilted at all, it just goes upwards.
Other aircraft use jets. The most simple form of jet is the Ramjet.
This collects air at the front as the aircraft move forwards. The air is heated and then squeezed past a narrow “throat”, and both of these things speed up the air. The airflow through the “throat” speeds up in the same way as the airflow over an aerofoil. The air then comes out of the back faster than it went in at the front, giving thrust.
But … a ramjet can only collect air if the aeroplane is already moving, so it cannot be used as the only means of propulsion.
A turbojet is a form of jet engine that has a turbine inside. This drives a compressor that pulls air into the intake so that it can be heated up and pushed past the “throat”. In this way, the aircraft can travel at the low speeds that are important for a ramjet.
A turbofan is a bit like a turbojet with a propeller on the front. The fan drives air into the compressor, and also around the engine. This not only cools the engine, but the otherwise wasted heat warms up the air and makes more thrust. Turbofan engines are also quieter.
The Soyuz TMA-9 spacecraft launches from the Baikonur Cosmodrome, Site 1/5 in Kazakhstan
Rockets do not need air to fly. This is because, instead of pushing air back, they push out gases made from burning the fuel that they carry inside themselves. The obvious disadvantage here is that the rocket not only has to lift itself off the ground, but also large quantities of fuel. Because of this, rockets are not practical for long flights, as they would need to carry more fuel than they could lift off with. Also, rocket fuel is much more expensive than air.
Why was flight test part of launch pad?
Flight plays a large and important part in modern everyday life. The holidays you take, communication with other people, the television you watch and even the food you eat all owe a lot to air transport. Yet people have only been able to fly for a relatively short time in history. The exhibit hoped to explain some things about how aeroplanes fly and to encourage the visitors to find out more about flight.
Of course, one special type of wing or aircraft will not solve all problems relating to drag, lift, controllability etc. Aircraft design is a matter of compromising one advantage with another, depending on how the aircraft is going to be used.
If you visit the Science Museum visit the flight gallery on level 3 where you can compare the different shapes of old and new aeroplanes. Now you know something about aerodynamics, imagine what each aircraft was good at, and if you see some unusual features, try to guess why they are there.