Flight Plan

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As an explainer I didn’t just work on Launch Pad in the Science Museum. I also did stints in other galleries, such as Flight. You can see the “whirling arm” behind me

https://www.sciencemuseum.org.uk/see-and-do/flight

During my time there, the Flight gallery had some interactive activities collectively known as “Flight Plan”. The aim of the activities was to explain to visitors the science behind flight and aeroplanes.

Lift and drag

Exhibits on this topic:

1) Whirling arm

This investigated the lift and drag forces on a flat plate, recreating George Cayley’s historic experiment of the early 19th century.

https://www.aerosociety.com/media/4862/sir-george-cayley-the-invention-of-the-aeroplane-near-scarborough-at-the-time-of-trafalgar.pdf

https://en.wikipedia.org/wiki/George_Cayley

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Sir George Cayley, 6th Baronet (27 December 1773 – 15 December 1857) was an English engineer, inventor, and aviator. He is one of the most important people in the history of aeronautics. Many consider him to be the first true scientific aerial investigator and the first person to understand the underlying principles and forces of flight.

2) Mist Tunnel

A wind tunnel with trails of oil mist showed air flows over an aerofoil. The flow was changed by altering the angle of attack of the aerofoil, or by deploying a slat or flap.

3) Wing lift

The lift force generated by air flowing over a wing shape drew balls up a tube

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The whirling arm

We all know that what goes up usually comes down. What gets an aeroplane into the air and keeps it there?

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Action and reaction

Newton’s laws can help explain how the lift force works.

https://en.wikipedia.org/wiki/Isaac_Newton

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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.

According to his second law if you want to move a stationary object you need to apply a force to it. You also need a force if you want to change the direction or speed of a moving object. For example, you need to apply a force to a shopping trolley to get it moving. You also need to apply a force to change its direction, speed it up, slow it down or stop it moving. Force is an example of a vector in that it has size and direction and can be classed simply as a push, pull or twist.

According to his third law if you apply a force to an object (the action force) the object applies a force back to you in the opposite direction. It can be summarised as: “If body A exerts a force on body B then body B exerts an equal but opposite acting force on body A”. You apply an action force when you push the shopping trolley forwards you feel the reaction force of the trolley on your hands

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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.

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Lift and drag

It isn’t only solids that can exert reaction forces. Liquids and gases can too. When a flat plate is moved through a stream of air at a small angle of attack (as in the whirling arm exhibit) the plate exerts an action force on air, deflecting it downwards. In return, the air exerts a reaction force on the plate. This reaction gives lift and drag (a force which opposed the forward movement on the plate). You experience a similar reaction when you hold your hand out of a moving car window.

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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.

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https://www.grc.nasa.gov/www/k-12/airplane/ac.html

You could also feel the drag force as it was transmitted through the winding handle of the whirling arm.

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If the angle of attack is increased too much the smooth flow of air is broken up causing eddying (turbulence). This increases drag and decreases lift.

Any shape which moves through air experiences some lift can be referred to as an aerofoil. A flat plate is not a very efficient aerofoil because, even at a relatively low angle of attack, too much drag is generated. For this reason, aeroplanes have wings with curved sections. For the same angle of attack, a curved aerofoil generates more lift and less drag than a flat plate because air flows more smoothly over the curved aerofoil.

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The Mist Tunnel exhibit showed lines of oil mist moving smoothly around an aerofoil. It also showed them being deflected downwards by the aerofoil. The downward airflow exerted a reaction force on the aerofoil, lifting it.

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Pressure and speed around an aerofoil

The air above a wing has a lower pressure than the air below. This difference in pressure acts on the wing to give lift. The Wing Lift exhibit showed this difference by lifting balls up the tube. The pressure difference above and below a wing is associated with a difference in air speed: the air moves faster over the top of the wing than below it.

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In the 18th century the Swiss scientist, Daniel Bernoulli, described how the pressure of the air in an airstream decreases as the airspeed increases. The exhibit explanations largely ignored this because there is no satisfactory, non-mathematical explanation for it.

https://en.wikipedia.org/wiki/Daniel_Bernoulli

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Daniel Bernoulli FRS (8 February [O.S. 29 January] 1700 – 17 March 1782) was a Swiss mathematician and physicist and was one of the many prominent mathematicians in the Bernoulli family from Basel. He is particularly remembered for his applications of mathematics to mechanics, especially fluid mechanics, and for his pioneering work in probability and statistics. His name is commemorated in the Bernoulli’s principle, a particular example of the conservation of energy, which describes the mathematics of the mechanism underlying the operation of two important technologies of the 20th century: the carburetor and the airplane wing.

https://en.wikipedia.org/wiki/Bernoulli’s_principle

In fluid dynamics, Bernoulli’s principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in static pressure or a decrease in the fluid’s potential energy.

Things that affect lift

1) Angle of attack

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The greater the angle of attack the more lift – but only up to the stalling angle (about 15 degrees for an average wing). At higher angles the air no longer passes smoothly over the wing but breaks up into swirls and eddies (turbulence). This was shown in the Mist Tunnel exhibit. Turbulence acts against lift. If the plane stalls it will fall.

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2) Wing shape

The cross-section of an aeroplane wing (i.e. the aerofoil) is curved, particularly on top. The curve of the aerofoil is called the camber, in general, more camber means more lift.

https://en.wikipedia.org/wiki/Camber_(aerodynamics)

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,

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3) Wing area

A larger wing area gives greater lift. This is because more air is pushed down as the aeroplane flies.

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4) Air speed

This is the speed of the aeroplane through the air. Higher airspeeds give more lift, so faster planes have greater lift than slower ones.

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5) Air density

Air is made up of molecules. Its density is related to how many molecules are in a given space. The more molecules in a given space, the denser the air. At lower altitudes, where air is denser, more air molecules can flow over the wings at any given speed, therefore the lift is greater. Every aeroplane has a ceiling above which it cannot fly because the air is too thin to provide sufficient lift. The size and shape of the wing determines what the ceiling will be.

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Aeroplanes designed to fly at high altitudes usually have large wings or fly very fast.

Things that affect drag

Drag, like lift, is caused by the effect of air flowing over the aeroplane. If the airflow breaks away from its surface, swirls and eddies of turbulent air form, increasing drag and wasting fuel. There are a number of causes of drag:

1) Form drag

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This is the resistance to movement by having to push air out of the way. It can be reduced by making the shape of the aeroplane as streamlined as possible. That is why aircraft have smoothly curving outlines, an why the undercarriage is usually retracted after take-off. In the Mist Tunnel you could see the difference streamlining makes to the air flow.

2) Skin friction

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This is due to air sticking to the surfaces of the aeroplane. Skin friction can be reduced by making the plane as smoot hand polished as possible.

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). These could be seen in the Mist Tunnel exhibit. This drag is reduced by having long narrow wings, so that the vortices are as far apart as possible.

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Drag is not always a bad thing in an aircraft. For example, When, flaps are lowered to maintain lift during landing they also increase drag, which slows down the aeroplane for landing.

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Level flight at constant speed

An aircraft is subject to four forces of flight.

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We have met three of them earlier, lift, weight and drag. The fourth force is thrust. This is the force that moves the plane forwards and will be discussed more fully in the section on “propulsion”. Thrust and drag oppose each other, as do lift and weight. Although it is important to note that each of these pairs are not Newton pairs.

Newton’s third law says there must be a pair of forces (known as Newton pairs) involved but thrust and drag are not a Newton pair and neither is lift and weight. A Newton pair must be the same type of force. Thrust and drag are different types of forces and lift and weight are different types of forces.

For an aeroplane to fly level, lift and weight must be equal. If the lift is greater than the weight then the plane rises. If the weight is greater then, the plane descends

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Once an aeroplane is moving in level flight, then it will continue to move at a constant speed, so long as the thrust and drag are equal. If the thrust is greater than the drag the plane will accelerate (speed up), and if it is less then the plane will decelerate (slow down)

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Landing and take-off

How can aeroplanes take-off and land on as short a runway as possible? Landing on a short runway requires a low landing speed. However, the plane must compensate for the loss of lift caused by a low approach speed or it will drop too rapidly. It can gain the lift required in three ways.

1) Increasing the angle of attack. However, this will cause stalling if too steep. To prevent the plane from stalling at higher angles a slot can be opened between a slat and the main part of the wing. Air flows both over the slat and through the slot; this keeps the flow smoother at the angle of attack. You could see the difference a slat makes in the Mist Tunnel exhibit.

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A319 slats during and after landing

https://en.wikipedia.org/wiki/Leading-edge_slat

2) Increasing camber. The wing camber can be increased by lowering flaps on the rear of the wing.

https://www.youtube.com/watch?v=67HUcVOAY_0

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Plane with flaps down

3) Increasing the wing area. This can be increased by moving the flaps rearwards as well as downwards.

Flaps are also used to increase lift during take-off to give a lower take-off speed and a shorter take-off run.

In the flight gallery

If you visit the flight gallery (or any museum with flight exhibits) why not see how the wings of aeroplanes have changed in shape over the years.

Can you find any information about George Cayley? Why is his work considered to be very important in the development of light?

Look for features on early aeroplanes which would have caused drag. Describe how the shapes of aeroplanes have changed to reduce drag.

Control

Exhibits on this topic:

1) Control tunnel

This was a wind tunnel that showed how the controls give roll, pitch and yaw.

2) Cessna Aircraft

This was a real aircraft in which you could sit and operate the elevator, rudder and aileron controls, and carry out a pre-flight check of controls and instruments

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Cessna F.150F

https://en.wikipedia.org/wiki/Cessna_150

The Cessna 150 is a two-seat tricycle gear general aviation airplane that was designed for flight training, touring and personal use.

Aircraft have to move up and down and left and right. How do aircraft change direction in flight?

An aircraft needs to move in all three dimensions. The only other vehicle to do this is a submarine.

https://en.wikipedia.org/wiki/Submarine

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The only way to control a flying object is to push against the air through which it is flying, and this is what the control surfaces on the aircraft do.

You could control three aircraft in the control tunnel exhibit.

There is a control stick on each aircraft. You could see what happens to the control surfaces on the planes’ wings or tails and how this affected the planes’ flight.

Pitch is the movement whereby the nose of the aeroplane rises or falls. A pilot will pitch up to climb and pitch down to dive. This movement is controlled by the elevators on the tail. Moving both the elevators down results in a pitch down, and both up result in a pitch up.

https://howthingsfly.si.edu/ask-an-explainer/what-are-types-movement-aircraft

https://www.slideshare.net/swapniljani4/flight-control-surfaces-69835118

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A plane’s tail showing the elevators and rudder

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Yaw is the movement to the left or right, while keeping the aeroplane flat. This is controlled by the rudder on the tail. Moving the rudder to the left results in a left turn, and moving it right results in a right turn.

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Roll is the movement by which a wing rises or falls. This movement is controlled by the ailerons on the wings. One aileron is moved up and one down to roll. The wing with the aileron up drops and wing with the aileron down rises.

https://www.liberaldictionary.com/aileron/ailerons-an-aviation-services-co/

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These manoeuvres are not usually performed in isolation, but in a combination. For example, a turn is generally made by banking – a combination of roll and yaw.

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A plane banking to its left. θ is the banking angle

Some planes such as aircraft with big delta wings, do not have separate tails. Concorde was an aeroplane like this. In such planes, the elevators and ailerons are combined into control surfaces called elevons. The elevons lie on the outer part of the rear of each wing.

https://en.wikipedia.org/wiki/Elevon

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https://www.liveabout.com/rc-airplane-parts-and-controls-2862953

https://www.flight-mechanic.com/wp-content/uploads/2017/07/1-57.jpg

How control surfaces work

Deploying a control surface changes the camber, or curve, of the part of the tail or wing where it is. This changes the amount of lift by deflecting more air. So, for example, if the elevators are deployed down, the air is deflected downwards, producing an upward force. This makes the tail rise which, in turn, causes the nose of the aeroplane to dip down.

In the flight gallery

If you visit the flight gallery (or any museum with flight exhibits) look at control surface on the wings and tails of aircraft.

Try to find at least one example of an elevator, rudder, aileron and elevon.

Some of the oldest aircraft have no ailerons. Try and find an example of one of these. How were they controlled?

Propulsion

Exhibits present when I worked there:

1) Pedal flight

This involved pedalling a propeller to feel how much power was needed to fly a human powered aircraft

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2) Water rocket

https://en.wikipedia.org/wiki/Water_rocket

A working rocket powered by water. It is quite easy to make one

A water rocket is a type of model rocket using water as its reaction mass. The water is forced out by a pressurized gas, typically compressed air. Like all rocket engines, it operates on the principle of Newton’s third law of motion. Water rocket hobbyists typically use one or more plastic soft drink bottle as the rocket’s pressure vessel. A variety of designs are possible including multi-stage rockets. Water rockets are also custom-built from composite materials to achieve world record altitudes.

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https://www.science-sparks.com/making-a-bottle-rocket/

https://www.youtube.com/watch?v=i_W3VkJkZF0

https://www.youtube.com/watch?v=xav_7AkUgGw

or buy one

https://www.amazon.co.uk/4M-4385-Water-Rocket/dp/B004X4WDH2/ref=asc_df_B004X4WDH2/?tag=googshopuk-21&linkCode=df0&hvadid=310876852693&hvpos=&hvnetw=g&hvrand=1566515257533184291&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=1007097&hvtargid=pla-564033057962&psc=1

https://www.storkz.com/relationshipware-700220319066.html?currency=GBP&country=GB&gclid=CjwKCAjwzIH7BRAbEiwAoDxxTjhNb68qYh02eD0V_WzOEAmEucfqPQTmRvtCfr4rhqN79IhQf1gSNBoC4VkQAvD_BwE

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I made one once, to demonstrate to a year 9 class, and it ended up on the hall roof during a public exam.

3) Harrier flight

https://en.wikipedia.org/wiki/Harrier_Jump_Jet

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The Harrier, informally referred to as the Harrier Jump Jet, is a family of jet-powered attack aircraft capable of vertical/short takeoff and landing operations (V/STOL). Named after a bird of prey,

https://www.youtube.com/watch?v=qXSG0n-mv2I

A model Harrier vertical take-off aircraft that could be hovered and flown on a circle by vectoring the jets and controlling the throttle.

Of you can’t buy one there is always simulations

https://www.justflight.com/product/harrier-jump-jet-download-wilco

The force which moves a powered aircraft along is called the thrust. Recalling Newton’s third law again, if an object is to move forwards, it must push something else back,

An aeroplane has only air all around it to push against when it is flying. Aircraft collect air and push it backwards to give them thrust. Thrust increases in proportion to the amount, and/or speed of air being pushed back.

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Piston engines, propellers and rotors

The piston engine was the first practical power source for aircraft. Most light aircraft are still powered by internal combustion piston engines in which the energy stored in the hydrocarbon fuel is transferred into the kinetic energy of the plane. Burning fuel in a piston engine produces sufficient kinetic energy to drive a propeller or “airscrew”. This, in turn, generates thrust by creating a rearward airflow. The propeller is like a set of rotating wings, each blade angled so that it pushes air backwards as it rotates.

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The equal and opposite reaction of all this backward thrust is the forward movement of the aeroplane.

The section on lift and drag showed how a moving wing acts on air, reflecting it downwards to give a reaction on the wing, this reaction we call lift. In a similar way, the propeller blade acts on the air to deflect it backwards to give a reaction on the propeller. We call this reaction force thrust.

The rotor on a helicopter acts in a similar way, but it is used to give both lift and thrust depending on the angle at which the rotor is set.

The pedal flight exhibit showed how the power from the engine (the pedaller) can be converted into airflow by a propeller.

Jet propulsion

A jet engine does not have a propeller: the engine itself collects the air. Energy is put into the system by burning fuel in the air and the air is thrown out of the back at a higher speed. It is this process, of accelerating the air, which pushes the aircraft forwards.

There are a number of variations on this idea:

1) The Ramjet

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https://en.wikipedia.org/wiki/Ramjet

As with all other jets, air is drawn into the engine and accelerated without the need for a separate propeller. However, a ramjet can only collect air if the aeroplane is already moving at high speed and so cannot be used as the sole means of propulsion.

2) Turbojet

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https://en.wikipedia.org/wiki/Turbojet

This engine is fitted with a turbine which drives a compressor, pulling air into the intake. The air is then heated up and accelerated rearwards. Turbojets allow aircraft to move at lower speeds than with the ramjet, but is still most efficient at high speeds.

3) Turbofan

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https://en.wikipedia.org/wiki/Turbofan

A turbofan engine is rather like a turbojet engine with a propeller fitted to the front. The fan drives air not only into the compressor, but also around the engine. These engines are quieter than turbojets and more efficient at lower speeds. Most large passenger jet aircraft use turbofan engines.

4) Rocket engines

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Rocket engines work in a quite different way to that of jet engines. The fuel is not burned in air from the atmosphere, but in an oxidant carried by the rocket. The oxidant is a substance which provides the oxygen for burning the rocket’s fuel, just as atmospheric air provides oxygen for the burning of fuels in jet engines. Sometimes the oxidant carried by the rocket is pure liquid oxygen. As the rocket does not need air in order to fly, it can be used to power craft which go into space. Rocket engines also have a very high jet speed and so are very good for high speed flight. The obvious disadvantage of rocket powered flight is that it has to carry large quantities of oxidant as well as fuel. The weight restrictions this imposes means that rockets are not a practical way of powering aircraft for long periods of time at the moment.

In the flight gallery

If you visit the flight gallery (or any museum with flight exhibits) look at the aircraft. Do they use propellers, jet engines of rocket propulsion? Try to find at least one example of each.

If the cut-away engine is still present in the Science Museum try to identify the compressor, turbine and combustion chamber.

Helicopters

Exhibits on this topic:

1) Helicopter Hover

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This was a model helicopter that could be hovered and flown in a circle by tilting the rotor and controlling the throttle.

2) String – Pull rotor

This was a helicopter that could be aimed and launched by pulling on a string. It was based on an ancient toy.

3) Helicopter game

This was a computer simulation of a model helicopter flight.

4) Tail rotor

This was an investigation into the action of the tail rotor in controlling yaw and torque reaction.

5) Rotor head

This was a real helicopter part to explore

How does an aircraft fly without wings? How do helicopters rise and change direction?

If you look closely at a helicopter rotor you will notice that the blades are tilted at a small angle. This is known as the angle of attack.

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The action of the rotor, as it slices through air, pushing it downwards, produces a reaction: a lifting force which makes the helicopter rise. The amount of lift generated by the rotor increases as the speed of rotation increases. This action/reaction is similar to that of a propeller, which was described in the propulsion section.

When this lift is greater than the weight of the helicopter, the helicopter will rise. If the lift is less than the weight of the helicopter, the helicopter will fall, and if the list and weight are equal, the helicopter remains at the same height – this is hovering.

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When the rotor is tilted forwards, the air is pushed back as well as down. The downwards element generates lift, as before, and the backwards element generates a forward force, thrust. Thrust increases as the rotor is tilted forwards – but at the expense of lift. The helicopter arm exhibit clearly showed that when the same amount of power is being used, a helicopter with its rotor tilted forwards does not rise as high as a helicopter with a horizontal rotor.

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https://www.youtube.com/watch?v=09bvuYRKwwc

https://www.youtube.com/watch?v=w5vC6JGEFcw

https://www.facebook.com/868b74e2-1df8-459d-a468-dbc3756e50e0

The above videos show the power of the down draft of the helicopter rotor blades.

If the helicopter top exhibit is still present ‘feel’ the down draft of the rotor blades on your hand.

Helicopter rotors do not just tilt forwards, they can also tilt sideways or even backwards. The pilot controls the tilt of the rotor to make the helicopter fly in any direction. I used to know a BA pilot and she said flying helicopters was far more difficult than flying commercial planes.

The tail rotor

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The action of the rotor, as it pushes back on the helicopter, produces a powerful reaction which, if left unchecked, would turn the helicopter in the opposite direction. Another, smaller, rotor is placed at the end of the tail to counteract the reaction to the main rotor. A further use for the tail rotor is in controlling the direction in which the helicopter is flying. The pilot can alter course by adjusting the amount of push generated by the tail rotor. This operation is controlled by the rudder pedals.

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http://www.pilotfriend.com/training/flight_training/rotary/helis.htm

Another way of combating the reaction of the main rotor is to build the helicopter with counter-rotating rotors. This arrangement, which has two rotors turning in opposing directions is complicated but very stable. The rotors are placed one above the other or at either end of the helicopter.

https://en.wikipedia.org/wiki/Boeing_CH-47_Chinook

The controls of the helicopter

A helicopter pilot controls the craft by means of two hand-operated stick controls, which are used in conjunction with a pedal control.

The cyclic pitch control is operated by the pilot’s right hand. This tilts the main rotor forwards, backwards or sideways to control the direction of the flight.

The pilot’s left hand operates the collective pitch control, which changes the angle of attack of the rotor blades, and the throttle, which changes the speed at which the rotor spins. Both of these controls affect the amount of lift generated by the rotor.

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Foot pedals control a ‘rudder’ which moves the tail from side to side by spinning the tail rotor faster or slower.

In the rotor head exhibit you could see how the cyclic pitch and collective pitch controls altered the angle of the rotor blades.

In the flight gallery

If you visit the flight gallery (or any museum with flight exhibits) look at any section on helicopters

See if you can find a counter-rotating propeller.

How is the Cierva Autogiro gyroplane different from a helicopter?

https://en.wikipedia.org/wiki/Cierva_Autogiro_Company

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Lighter than air

Exhibits on this topic:

1) Hot air balloon

There may still be a working hot air balloon present

2) Radio controlled airship

There was a radio-controlled airship model which was demonstrated periodically by the gallery explainer (but not by me)

3) Balloon design

Find a computer game that allows you to design your own hot air balloons

4) Balloon history

This was a question and answer game about balloons through the ages.

https://en.wikipedia.org/wiki/Hot_air_balloon

https://www.explainthatstuff.com/how-hot-air-balloons-work.html

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The first people to fly used a hot air balloon. How does lighter-than-air flight work and what are the problems?

On September the 19th, 1783, the Montgolfier brothers sent up the first balloon containing living things. Made of paper, it was over 17m high, 12m in diameter and filled with hot air. The first aviators on this historic first flight was a sheep, a cockerel and a duck.

https://en.wikipedia.org/wiki/Montgolfier_brothers

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Joseph-Michel Montgolfier (26 August 1740 – 26 June 1810) and Jacques-Étienne Montgolfier (6 January 1745 – 2 August 1799) were paper manufacturers from Annonay, in Ardèche, France best known as inventors of the Montgolfière-style hot air balloon, globe aérostatique. They launched the first piloted ascent, carrying Jacques-Étienne. Joseph-Michel also invented the self-acting hydraulic ram (1796), Jacques-Étienne founded the first paper-making vocational school and the brothers invented a process to manufacture transparent paper.

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First public demonstration in Annonay, 4 June 1783

Later that same year, on November the 21st, Jean Pilatre de Rozier and the Marquis d’Arlandes became the first humans to fly in a hot air balloon, floating across country for 9km

(Below left) https://en.wikipedia.org/wiki/Jean-Fran%C3%A7ois_Pil%C3%A2tre_de_Rozier

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Jean-François Pilâtre de Rozier (30 March 1754 – 15 June 1785) was a French chemistry and physics teacher, and one of the first pioneers of aviation.

(Above right) https://en.wikipedia.org/wiki/Fran%C3%A7ois_Laurent_d%27Arlandes

François Laurent le Vieux d’Arlandes (1742 – 1 May 1809) was a French marquis, soldier and a pioneer of hot air ballooning.

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First untethered voyage by Pilâtre de Rozier and d’Arlandes, November 21, 1783. Illustration from the late 19th Century.

The principles behind lighter-than-air flight differ from those of heavier-than-air flight (light and heavy in this context actually refers to density, which is the product of mass divided by volume). Lift, in lighter-than-air flight, is gained by making the total weight of the craft less than that of the air it displaces. There are two ways of achieving this.

1) Using hot air

As air is heated its molecules move faster and, on average further apart from each other. Hot air, having fewer molecules for a given volume, has a lower density than cold air, just as anything with a lower density than water, such as cork, rises up and floats in water, a pocket of hot air rises, and will float in cold air.

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A hot air balloon is an open-ended envelope filled with air, which is heated by a burner. When the balloon starts to fall, the burner is turned on to heat to heat up the air inside the balloon. Due to the expansion of the air, some air molecules will escape from the balloon. This decreases the density of the remaining air inside the balloon (fewer molecules – and less mass – in the same space) and the balloon rises.

The working hot air balloon exhibit showed how heating the air in a balloon makes it rise. It could be found in flight lab gallery.

https://www.balloonsoverbritain.co.uk/how-the-balloon-works

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2) Light gases

Shortly after the first hot air balloon flight, a scientist called. J.A.C. Charles flew a different type of craft. This used a closed envelope filled with hydrogen which, being the lightest gas, provided the most lift.

https://en.wikipedia.org/wiki/Jacques_Charles

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Jacques Alexandre César Charles (November 12, 1746 – April 7, 1823) was a French inventor, scientist, mathematician, and balloonist.

A major drawback with the use of hydrogen for lighter-then-air flight is its volatility. The fact that it can explode makes it dangerous to use. Helium, the second lightest gas, is now used for lighter-then-air flight. In addition to being more expensive than hydrogen, it is rarer and offers less lift for the same size envelope. It is however an inert gas and as such is safe from fire and explosion.

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Contemporary illustration of the first flight by Prof. Jacques Charles with Nicolas-Louis Robert, December 1, 1783. Viewed from the Place de la Concorde to the Tuileries Palace (destroyed in 1871)

Throwing out ballast

The balloon can also be made to rise by dropping ballast. This is usually in the form of bags of sand which are carried on the basket. Dropping ballast makes the balloon lighter and thereby less dense.

Airships

Balloons are an unreliable form of transport as they will only travel as fast, and in the same direction as, the wind. In 1852, Henri Giffard attached a small steam engine to a long hydrogen balloon and flew 27km at a speed 0f 8km per hour. The first practical airship, the Labaudy, was built in 1902 and with this development began the “Age of the Airship”.

https://en.wikipedia.org/wiki/Henri_Giffard

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Baptiste Jules Henri Jacques Giffard (8 February 1825 – 14 April 1882) was a French engineer. In 1852 he invented the steam injector and the powered Giffard dirigible airship.

https://en.wikipedia.org/wiki/Giffard_dirigible

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Henri Giffard’s steam-powered dirigible of 1852

https://en.wikipedia.org/wiki/Patrie_(airship)

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The Patrie leaving her hangar at Verdun for the final time, 29 November 1907.

The Lebaudy Patrie was a semi-rigid airship built for the French army in Moisson, France, by sugar producers Lebaudy Frères. Designed by Henri Julliot, Lebaudy’s chief engineer, the Patrie was completed in November 1906 and handed over to the military the following month. The Patrie bears the distinction of being the first airship ordered for military service by the French army.

https://en.wikipedia.org/wiki/Lebaudy_Fr%C3%A8res

Paul and Pierre Lebaudy were the owners of a sugar refinery who, with the assistance of their engineer Henri Julliot as designer, built semi-rigid airships which saw service with the French army, the Russian army and the Austrian army.

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An artist’s impression of the first Lebaudy airship.

Airships used a huge sausage-shaped envelope of lighter-than-air gas to lift themselves, the passengers and the freight up into the sky. Steering was achieved with enormous rudders and these craft could fly long distances at speeds of up to 120kph. The crew and passengers were accommodated in a cabin which was slung under the envelope.

https://en.wikipedia.org/wiki/LZ_127_Graf_Zeppelin

LZ 127 Graf Zeppelin (Deutsches Luftschiff Zeppelin 127) was a German passenger-carrying, hydrogen-filled rigid airship that flew from 1928 to 1937. It offered the first commercial transatlantic passenger flight service.

https://www.wondersofworldaviation.com/german_airships.html

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Built in 1928, the Graf Zeppelin is now in retirement, having flown more than 16,000 hours, covered 1,060,000 miles and carried 13,000 passengers. The Graf Zeppelin made 148 transoceanic crossings, and was at one time used on a regular service across the Atlantic.

The enormous bulk of airships made them difficult to manoeuvre, particularly near the ground, and aeroplanes proved to be faster and easier to operate. A series of disasters, most notably those of the R101 and the Hindenberg (both of which crashed, causing the hydrogen in their envelopes to catch fire), brought an end to the “Age of the Airship”.

https://en.wikipedia.org/wiki/R101

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R.101 was one of a pair of British rigid airships completed in 1929 as part of a British government programme to develop civil airships capable of service on long-distance routes within the British Empire.

The crash of R.101 effectively ended British airship development, and was one of the worst airship accidents of the 1930s.

https://en.wikipedia.org/wiki/LZ_129_Hindenburg

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LZ 129 Hindenburg (Luftschiff Zeppelin #129; Registration: D-LZ 129) was a large German commercial passenger-carrying rigid airship, the lead ship of the Hindenburg class, the longest class of flying machine and the largest airship by envelope volume.

https://en.wikipedia.org/wiki/Hindenburg_disaster

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The Hindenburg disaster occurred on May 6, 1937, in Manchester Township, New Jersey, United States. The German passenger airship LZ 129 Hindenburg caught fire and was destroyed during its attempt to dock with its mooring mast at Naval Air Station Lakehurst. There were 35 fatalities (13 passengers and 22 crewmen) from the 97 people on board (36 passengers and 61 crewmen), and an additional fatality on the ground.

Airships are still occasionally used for advertising, but are now filled with helium for reasons of safety.

In the flight gallery

If you visit the flight gallery (or any museum with flight exhibits) find a display about the first balloon flights. Identify the two types of balloon used.

Look for balloon baskets. What are they made of?

Find a high-altitude balloon capsule. How is it different from balloon baskets?

Find a display of airship models, Is the Graf Zeppelin amongst them?

Structures

Exhibits on this topic

1) Fuselage stress

This looked at the effects of pressure induced stress on a variety of window shapes

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2) Struts and strings

The idea here was to use wings, struts and bracing wire to make a strong, lightweight and airworthy biplane

3) Stressed-skin

The idea here was to turn a floppy structure into a rigid, monoplane wing by making a thin skin take the stresses.

Aircraft must be light and yet strong. What designs and materials have been used to solve this problem over the years?

Bracing wire and struts

image

Most of the early successful aeroplane designs favoured the biplane, with one wing stacked above another. This enabled the designers to create a box-like construction consisting of the wings, a pair of wooden struts to keep them apart, and bracing wires to hold everything together.

https://en.wikipedia.org/wiki/Biplane

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First World War Sopwith Camel biplane

This was not only strong, but light. The fuselage, like the wings, was normally constructed using the bracing wire technique.

Strength through triangles

The strength in this design lies in the way the bracing wires are used in conjunction with the struts to take advantage of the inherent of triangular based structures. In this instance, the triangles are formed by the struts, the wings, the bracing wires and the sides of the fuselage

When the lift force acts on the upper wing, the wire is put into tension and the wing and fuselage side are put into compression. When the lift force acts on the lower wing, the wire is put into tension and both wings, the strut and the fuselage are put into compression.

image

Can you figure out why?

The wire mentioned in this example is known as the flying wire. The other wire is called a landing wire because it resists downward forces on the wings which arise mainly when the undercarriage hits the ground when landing.

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You could test the strength and rigidity of the biplane wing by experimenting with the ‘struts and strings’ exhibit that used to be in the flight gallery (it might still be there).

Stressed skin

The wires and struts of a biplane wing cause a lot of drag. The need for faster, more efficient aeroplanes led to the use of single (monoplane) wings in which a major part of the strength came from the skin of the wing acting in tension and a spar running the length of the wing onto which are built ribs to give shape to the aerofoil.

image

When the lift force acts on the wing, the lower skin is put into tension and the spar is put into compression. Aluminium alloys are used to give the necessary strength, rigidity, and light weight to the wing. Most large aeroplanes have been built like this since the 1930’s.

image

You could test the strength of this design by investigating the “stressed-skin” exhibit that used to be in the flight gallery (it might still be there).

The fuselage is constructed on the same fashion, Using Longerons instead of spars and with bulkheads to give shape to the fuselage.

https://en.wikipedia.org/wiki/Longeron

In engineering, a longeron and stringer is the load-bearing component of a framework.

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Pressurised flight

The demands of aviation dictated that the aeroplane should be able to fly faster, further and higher. Fuselages became pressurised in order to cope with high altitude flight, and with this development came a completely new set of problems.

The stresses placed upon aircraft by the repeated expansion and contraction of the fuselage during pressurised flight led, in some cases, to fuselage failure due to metal fatigue. One of the most spectacular of these failures occurred with the de Havilland comet 1A, the first passenger jet airliner. A design fault around the forward observation window led to the mid-air break up of two of these aircraft.

https://en.wikipedia.org/wiki/De_Havilland_Comet

image

The fault lay in the fact that the window was square, and so encouraged the concentration of stress at each corner during pressurised flight.

You could see how this was overcome in later aircraft by looking at the photo-elastic fuselage exhibit that used to be in the flight gallery (it might still be there).

In the flight gallery

If you visit the flight gallery (or any museum with flight exhibits) see if you can identify ‘triangles’ in aircraft construction.

Look for examples of stressed-skin construction, and if you can, compare the construction of the Hurricane and the Spitfire planes.

https://en.wikipedia.org/wiki/Hawker_Hurricane

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The Hawker Hurricane is a British single-seat fighter aircraft of the 1930s–40s that was designed and predominantly built by Hawker Aircraft Ltd. for service with the Royal Air Force (RAF). It was overshadowed in the public consciousness by the Supermarine Spitfire’s role during the Battle of Britain in 1940, but the Hurricane inflicted 60 per cent of the losses sustained by the Luftwaffe in the engagement, and fought in all the major theatres of the Second World War.

https://en.wikipedia.org/wiki/Supermarine_Spitfire

image

The Supermarine Spitfire is a British single-seat fighter aircraft that was used by the Royal Air Force and other Allied countries before, during, and after World War II. Many variants of the Spitfire were built, using several wing configurations, and it was produced in greater numbers than any other British aircraft. It was also the only British fighter produced continuously throughout the war. The Spitfire continues to be popular among enthusiasts; nearly 60 remain airworthy, and many more are static exhibits in aviation museums throughout the world.

Glossary

Aerodynamics – the science or study of the forces acting on an aircraft in motion

Aerofoil – the cross-section shape of a wing taken at right angles to the wingspan: also known as the wing section or rib section

Angle of attack – the angle at which the wing section strikes the airstream

Angle of incidence – angle of the wing in relation to an arbitrary line fore and aft ln the fuselage.

Aileron – control surface on an aircraft wing, used to induce the rolling action needed for a banking turn, (see Bank).

Air – a gas which envelopes the Earth and is made up of tiny particles called molecules.

Airflow – direction in which air moves as a result of contact with a propeller or jet engine.

Airship – powered, steerable lighter-than-air craft.

Altitude – position relative to, and above, the Earth.

Aluminium – a lightweight metal commonly used in the construction of aircraft.

Autogiro – an aeroplane that flies with the aid of rotating wings, set above the fuselage.

Aviator – the pilot of an aircraft.

Ballast a relatively dense substance which can be thrown out to reduce load.

Balloon – a nonporous. flexible bag, inflated with a 1ighter-than-air gas.

Bank – a turn made in flight with one wing tip lower than the other.

Biplane – an aircraft with two wings fixed at different levels.

Bracing wire – strong, flexible supporting cable used in aircraft construction.

Bulkhead – a former within the fuselage used as internal support for longerons, sheet sides, stringers and so on.

Camber – the curvature of the wing or horizontal tail, from the leading edge to the trailing edge.

Ceiling – maximum height at which an aircraft may fly, above which air becomes too thin to provide lift.

Chord – the width of a w1ng or tailplane from front (leading edge) to back (trailing edge).

Cockpit -the space in an aircraft where the pilot sits.

Compressor – a device used for increasing the pressure of a gas or vapour.

Control surfaces – those parts of an aircraft which can be moved in order to affect changes in the attitude of an aircraft.

Counter-rotation – the action of a rotor moving simultaneously in the opposite direction to that of another rotor.

Drag – a force acting on an aeroplane, resisting its forward movement through the air.

Delta – a triangular shape. (Fourth letter of the Greek alphabet).

Elevatorthe hinged control section of the horizontal tailplane.

Elevon – a combined elevator and aileron.

Flaps – hinged surfaces attached to the trailing edge of the wing to increase drag and lift.

Former – see Bulkhead.

Fuselage – the body of an aeroplane.

Glider – an engineless aeroplane which sustains flight by exploiting gently rising air currents.

Heavier-than-air – anything which weighs more than the air it displaces.

Helicopter – an aircraft that can rise or descend vertically, by means of large overhead power-driven rotor or rotors.

Helium – the second lightest gas, valuable for lighter-than-air flight because it is non-reactive and non-flammable.

Hovering – a state in which the forces of lift and weight are exerting an equal influence upon a body.

Hydrogen – the lightest gas. Highly flammable.

Inert – Chemically inactive, having no reactive qualities.

Landing gear – see Undercarriage.

Leading edge – the front or entering edge of a wing or tail.

Lift – An upwards force which acts to raise an aircraft off the ground

Lighter-than-air – anything which weighs less than the air it displaces.

Longerons -the main for-and-aft strips in a fuselage.

Metal fatigue – breakdown of materials, due to stress.

Monocoque – a form of fuselage construction with rounded exterior and very little internal structure in which the skin carries virtually all stresses.

Photo-elastic – (Photoelasticity) an experimental technique allowing the visual observation of stress and strain.

https://en.wikipedia.org/wiki/Photoelasticity

Pitch – the movement whereby the nose of an aeroplane rises or falls.

Pressurised – maintaining normal atmospheric pressure ln a chamber subjected to high or low external pressures.

Propeller – an airscrew that bores its way through the air, pushing or pulling the aeroplane.

Resistance – air drag, or the opposition of the air to being displaced by the forward movement of the aeroplane.

Rib section – the cross-section shape of a wing, from leading to trailing edge.

Rotor – an assembly of blades designed as aerofoils that are attached to a helicopter or similar aircraft and rapidly rotated to provide both lift and thrust.

Rudder – the moving part of the tail surface of an aeroplane which controls movement to the left or right.

Slipstream – the column of air pushed rearward by a rotating propeller; it always moves faster than the aeroplane itself.

Slat – a device employed on a wing to prevent an aeroplane from stalling when flying at low speed with a steep angle of attack.

Slot – an opening created by the movement of the slat (see above) away from the wing.

Spars – spanwise load-carrying members of a wing or tail.

Stability – the tendency of the aeroplane to return to level flight, after having been disturbed by an upsetting force.

Stall – the complete loss of lift resulting from too steep an angle of attack, (greater than 15°).

Streamlined – the shape of the exposed contours of an aeroplane for the 1east possible air drag; usually rounded in front, pointed at rear.

Stress – a force, acting across an area of solid material, which resists movement induced by external forces.

Stringer – light, lengthwise fuselage strips intended more to give the desired shape than to add strength.

Strut – an upright, diagonal or transverse support.

Tail – the surfaces at the rear of a conventional aeroplane fuselage.

Tail rotor – small propeller type arrangement of blades used in helicopter construction to counter the reaction to the main rotor and control direction of flight.

Thrust – the propulsive force developed by a driven propeller

Torque – the reactive force generated by a revolving propeller that tends to rotate the aircraft in a direction counter to the direction of propeller rotation.

Trailing edge – the rear edge of a wing or tail surface.

Trailing vortices – a form of induced drag created by the action of wing tips passing through air.

Turbine – a fluid acceleration machine for generating rotary mechanical power from the energy in a stream of fluid.

Turbulence – motion of air in which speeds and pressures fluctuate in a random manner.

Undercarriage – the wheel and strut assembly that supports an aeroplane at rest on the ground and during take-off and landing.

V.T.O.L. – vertical take-off or landing.

Weight – gravitational force with which the Earth attracts a body.

Wing – the principle supporting surface of an aeroplane.

Wing section – See aerofoil.

Wing skin – load bearing metal sheet.

Yaw – the movement by which an aeroplane moves to the left or right whilst maintaining level flight.

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