# Taking Off and Flying

People have always dreamt of flight. Flying means being free, it is the ultimate dream, to 'fly like a bird'.

Throughout history, myths and legends of humans flying were told, from magic carpets to flight powered by 'happy thoughts'. The storytellers' imagination was always turning to lifting off the ground to show power and freedom, creating many imaginary creatures, like Dragons and Fairies, which are able to fly.

The main difficulty in flying, apart from the danger of losing your luggage, lies in the constant pull of gravity that is being generated by the Earth's mass. This makes it pretty well impossible for man to fly without mechanical help (other than straight down!).

Other animals are better suited to the task and have natural flight mechanisms. Mankind has often tried to imitate these animals and make flying machines, and to some extent succeeded. We're still working on new forms of defeating gravity and taking off, but those are all still problematic in both theory and practice. One of these ways is 'gravity shielding' which involves rotating freezing disks of superconducting material - kind of creepy really.

### Lift-off vs Take-off

Some may say, why take-off and not lift-off? Well, here's the difference:

• 'Lift-off' is the instance of the initial movement of a rocket or similarly propelled craft.

• 'Take-off' is the act of rising in flight.

All aircraft, including rockets, take-off. Lift-off is usually a take-off that is vertical.

### Take-off Methods

There are many ways to detach from the ground. Some involve aerodynamics, and some simpler areas of physics.

#### Buoyancy

Buoyancy is a force that a fluid or gas exerts on an object less dense than itself. This force will be in an opposite direction to the gravitational pull in effect.

In order to lift off by this method it is necessary to somehow make the average density1 of the thing taking off lower than that of the air at ground level2. The aircraft will then float until it settles at the height at which the density of the air is the same as the average density of the aircraft - ie when the mass of the air displaced by the craft is equal to the mass of the craft itself.

The density of the aircraft is its average density as a whole, meaning that the metal/wood/human parts of it will increase its density relatively to the air. So, in order to decrease the overall average density, a lot of material that is lighter than the air surrounding the aircraft must be used.

There are some methods for achieving this. One is having air in a large balloon, and warming it in order to make it less dense. These are often referred to as 'Hot Air Balloons'. Another method, the airship or 'Zeppelin', is a balloon filled with cells of gas (typically hydrogen or helium). These gases are less dense than the air, and cause the airship to go up.

Horizontal movement in the hot air balloon is controlled only by setting the level of the balloon to one that has a wind going your way, while an airship has propellers and flaps to aid its navigation.

#### Fixed Wings

These are the long things with flaps and a rounded finish at the sides of the aircraft.

Fixed wings (referred to as wings in the rest of this section) work on aerodynamic principles of pressure, making the air under the wings press up so that they can take off the ground.

The basic principal is as follows:

• Air pressure is divided to two kinds: Static Pressure (Ps) and Dynamic Pressure (Pd).

• Now, the total pressure (P), the sum of Ps and Pd, stays constant3.

• Now, Ps is the kind of pressure we know well, it's the kind of pressure that makes the punch painful. Pd, however, changes with speed4.

• Now, as an object moves faster through the air, the dynamic pressure rises, and so, in order to keep the total pressure constant, the static pressure drops.

In order to see how this is used to fly, we need to look at a profile of the wing. It looks a little like a flattened droplet on its side, where the upper curve is thicker than the lower. In theory, if 2 particles of air meet the front end of a wing at the same time, they will also reach the other side together, even if one travels above the wing and one travels below5. This is lucky for us, because we can easily see that the air that goes over the wing has a longer way to go than below and so it has to go faster. If the air above goes faster, then the dynamic pressure above is higher. The total pressure stays constant, however, so the static pressure on the top of the wing drops. This is then lower than the static pressure underneath the wing, which tries to equalise the pressure, pushing the wings upwards and gaining 'lift', getting us off the ground!

#### Rotating Wings

Rotating wings, fixed vertically are used widely as propulsion in aircraft that use fixed wings, pushing them along. However, mount them horizontally and you have a helicopter.

Rotating wings work much in same way as fixed wings do, just that the profile is symmetrical. This may seem confusing, because it means that the air goes in the same speed onto and below the wing. Helicopter pilots can, however, control the wing's angle relative to the airflow6 and by changing the angle of all of the rotor's wings by the same amount, the aircraft has lift. The lever that usually controls this in helicopters is called the collective. Other helicopter controls are the pedals and cyclic (the thrust in a helicopter - this is usually set to maximum, so the throttle is a less dominant control).

Changing the angles of the rotor wings separately, in a different, more complex way, causes the lift to be 'tilted', meaning that some of the lift is used to move the aircraft forward, backward, or to the side. This is usually controlled using the stick.

Helicopters are quite complex, and some Researchers think them to be the most elegant aircraft in existence.

#### Lift-off

Rockets and rocket engines use the most powerful, yet the most inefficient way to take off the ground: brute force. In short, fuel is burnt and the release of exhaust pushes the rocket-powered aircraft7 skywards. The aircraft is stabilised and controlled using rudders and flaps, and sometimes even real wings.

Because of their power, rockets are used when there's a need for fast acceleration (eg missiles, ejection seats, UAVs (Unmanned Aerial Vehicles8, etc) or when there's the need for a lot of lift power for a short instance (like launching to outer space).

#### Projectiles

Projectiles are a lot like rockets as far as initial launch is concerned. However, they have no energy added after they have first lifted off.

Throughout history, projectiles were used for many purposes, and came in a variety of forms, from arrows and spears, through men shooting themselves out of cannon, all the way to ballistic missiles. Even jumping is a small personal form of take-off.

#### Birds

There would probably be a lot less flight going on if it wasn't for animals that had the ability. Most birds can fly, and they fly in much the same way as fixed wing9 aircraft, only their wings aren't fixed: birds flap their wings. This gives them the ability to take off without jets and manoeuvre much more skillfully than we can manage.

Birds take off by flapping their wings. The flapping movement replaces the need for the bird to move fast, as the movement of the wings is what really counts. Birds make use of the gliding technique to save in wing flapping energy. The larger the bird, the higher percentage of its flight time will be spent gliding. Small wings can be flapped rapidly, but large wings that carry large bodies want to stay as stationary as possible, and gliding allows that.

#### Insects

Insects fly in a unique way unlike everything described until now. Actually, insects fly very much like fish swim. From bee to dragonfly, all insects just use very strong muscles to move thin wings rapidly and thrust the air in a way that causes a reaction and thrusts the insect in the opposite direction.

Going up and down like this, the insect thrusts up by pushing the wings down, but to minimise the reaction while getting the wings up again. This is done by pushing down with the wings, then rotating the wings so the thin lip hits the air when picking the wing up again.

#### Bats

Bats aren't insects, nor are they birds; they are in facts mammals (like humans). However, their take-off and flight technique is very similar to that of the insects. The main difference is the mechanism that moves the wings. Because bats have a skeleton rather than an exoskeleton, their wings can and must be more flexible, and the muscles that move the wings work differently.

#### Ornithopters

'Ornies' are aircraft that try to resemble the animal's way of flight to another level by moving their wings in a manner that resembles a bird's or insect's flap. 'Ornies' are hard to fly and aren't very practical. Not many working Ornithopters were built, but a man strapping a lot of feathers to his arms, jumping off a cliff and flapping his hands was probably the first and saddest attempt at human flight.

One of the most well-known and interesting references to Ornithopters is in the book Dune by Frank Herbert, where 'ornies' were described as the main form of flight on the planet Dune.

#### Unaided Lift-off

Some people believe that they are able to levitate without the use of anything other the their 'mystical ability'. This is done either by meditation, or magic. However, understandably, most people are sceptical of these claims.

### Why Take Off?

Well, any take-off will probably be the first step of an attempt to fly. Why fly then? At first, it was just the sheer challenge, the conquest of yet another realm by the developing creativity and ingenuity of the human brain. As it turned out, there are quite a few practical reasons to fly.

#### Travel

People fly in order to get quickly from one place to the other, over geographical boundaries. Either for the sake of tourism, or for more practical reasons, like relocation and business.

#### Military

Being higher and faster are advantages of flight that are being used for military purposes, either for espionage, communication, attack, evacuation or transport. This is one of the more unpleasant aspects of flight.

#### Gaining Height

Sometimes, standing on the tip of your toes won't get you high enough. On the more extreme of these cases, you might consider flying as an alternative. Some very good examples are:

• Displaying an add off the back of a plane.
• Taking a photo of a large area.
• Exploding colourfully. These craft should be unmanned (in theory).
• Effectively relaying communication.
• Crop spraying

#### Escaping Earth

Even if you travel a long way by foot, you'll still be on the Earth. In order to go into outer space, you'll need to take off and keep doing it until you escape Earth's gravitational pull. As for today, this is mainly done by rockets.

#### Art and Skill

Flight for its own sake is not uncommon. Aerobatic shows are very popular and are only one example for just flying and getting better at it. Artistic photography from the air can get you some pretty mind-boggling results, and there are many more examples.

Even in nature, It's common to see artistic flying combinations performed by male birds just to demonstrate their flying prowess to impress female birds. There is also the synchronous flying of males and females in complicated manoeuvres during mating and courtship.

1Mass per unit of volume.2 Air has different densities at different heights.3Actually it differs when the height is changed, but the relevance and consequences of this won't be discussed in this entry.4According to the formula Pd = o*V2/2 where o is a constant representing the air density.5This is not completely accurate in practice, but the difference doesn't really lower lift, it just causes turbulence and all kinds of other problems which won't be discussed here.6this is called the 'attack angle'.7Which usually is just a rocket, but there are other rocket-boosted crafts like missiles and special airplanes.8UAVs will rarely be rocket powered, but the take-off systems often are.9Actually it's the other way around. Man imitated the bird's wing structure when he started making aircraft.