Takeoff
Created | Updated Jan 28, 2002
The Challenge
People always dream of flight. Flying means being free, it is the ultimate dream, to "Fly like a bird". The main difficulty in flying lies in the constant pull of Gravity that is being generated by Earth's mass. As for today, in order for a man to fly, a big engine must be attached to him and a complicated set of wings and gear too.
Other animals are easier on the task and have organic flight mechanisms. Mankind has often tried to imitate these animals and make flying machines, and to some extent succeeded.
Throughout history, myths and legends of humans flying were told, from magic carpets through "Happy thoughts" powered flight, the storytellers' imagination was always turning to lifting off the ground to show power and freedom. Many imaginary creatures, Like Dragons and Fairies, are able to fly.
Humans are already working on new forms of aborting 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 super conducting material, kind of creepy really.
Liftoff Vs. Takeoff
Some may say, why takeoff and not liftoff? Well, here's the difference:
Liftoff is the instance of the initial movement of a Rocket or similarly propelled craft. Takeoff is the act of rising in flight. All aircrafts including rockets takeoff. Liftoff is usually a takeoff that is vertical.
What Takes off
Certain stuff is expected to successfully takeoff. A bird or a plane are obvious options. Also a helicopter, an Airship and a hot air balloon. All of these take off by using the air in one way or the other to push themselves upwards.
Takeoff Methods
There are many ways to detach from the ground. Some involve aerodynamics, and some simpler areas of physics.
Buoyancy1:
In order to lift off by this method it is necessary to somehow make the average density2 of the aircraft lower than that of the air at ground level3. The aircraft will then float until it settles at the height at which the air has the same density as the aircraft.
The density of the aircraft is it's avrage density as a whole, meaning that the metal/wood/human parts of it, along with alot of the rest will increase it's density relatively to the air. So, in order to decrease the overall avarage density, a lot of material that is lighter than the air sorrounding the aircraft must be used.
There are some methods to achieve this. One is having air in a large balloon, and warming it in order to make it less dense. This is often referred to as a "Hot Air Balloon".
Another method, The Airship4, 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 in it's navigation.
Fixed Wings:
They are the long things with flaps and a rounded finish at the sides of the aircraft. You're used to see them attached to Airplanes and get hit by lightning in the movies.
The 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 air, being as gullible as it is, is easy to trick into doing so, if you know how it thinks.
The basic principal is that the air pressure is divided to 2 kinds:
1. Static Pressure (Ps)
2. Dynamic Pressure (Pd)
And As It goes, it seems that the total pressure (P) is constant5 and equals the sum of Ps and Pd.
P = Ps + Pd6
Ps is the kind of pressure we know well, it's the kind of pressure that makes the punch painful.
Pd changes with speed, it can be calculated by:
Pd = o*V2/2
o is of no concern to us at the moment. It is a constant representing the air density.
So, as an object moves faster through the air, the dynamic pressure raises, and so, 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 flat drop, just that 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 below7. 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 (longer to go over the thick side than the thin side).
If the air above goes faster, then the Pd above is higher (oV2/2 - means it's actually much higher, because the "to the power of 2" part). Pd being higher, and having P supposed to stay constant, now Ps on the top of the wing drops. In turn, this means that Ps below the wing is now high relatively to above the wing, which pushes the wings up (this force is called "lift").
All these confusing consequences makes for how most planes leave the ground.
Rotating Wings:
Rotating wings are used widely as propulsion in aircrafts that use fixed wings. This section is NOT about those aircrafts, it is about what you expect to find on top of Helicopters.
Rotating wings work much in same way as fixed wings do, just that the profile is symmetric. This may seem confusing, because it means that the air goes in the same speed on to and below the wing. Here the main difference lies, helicopter8 pilots can control the wing's angle9 and by changing the angle of all of the rotor's wings in the same amount, the chopper has lift. The lever that usually controls this in helicopters is called "Collective", other hellicopter controls are the pedals and cyclic (the thrust in a hellicopter 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, to the sides or actually to any direction. This is usually maneuvered using the stick.
Helicopters are quite complex, and some researchers think them to be the most elegant aircraft in existence.
Liftoff:
Rockets and rocket engines use the most powerful, yet the most "wasty" way to take off the ground, brute force. For more information follow the above link.In short, fuel is burnt and the release of exhaust pushes the rocket powered aircraft10. The aircraft is stabilized using rudders and flaps, and sometimes even real wings(of the sort discussed earlier).
Because of it's power, rockets are used when there's need for fast acceleration (e.g. Missiles, Ejection seat, UAV(Unmanned Aerial Vehicle) Takeoff11...) or when there's need for a lot of lift power for a short instance (Like launching to outer space).
Projectiles:
Projectiles are a lot like rockets, main difference is that they have no energy added after the first instance of liftoff. Throughout history projectiles were used for many purposes, and came inf a veriaty of forms. From arrows and spears, through a man shooting himself out of a cannon, and all the way to ballistic missiles. Actually, jumping, is a small personal form of takeoff12
Some plants use projectiles as a way to spread seeds. The seed container dries up and snaps, spreading the seeds or leaving them for the wind to lift and carry away.
Gliding:
Gliding isn't actually a takeoff method. The takeoff is done some other way (like jumping off a plane, running off a cliff or being boosted by some engine or vehicle). Once in the air, thermal turbulences are used to gain height.
First Wing-Type Glider Flying Machine was created in 1804 by Sir George Clayley in England. Gliding usually uses the "fixed wings" type aerodynamics, but comes in other forms as well. Parachuting is a kind of takeoff too, just one that a lot of times it is done downwards13
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 wings14, just that their wings aren't fixed. Birds flap their wings. This gives them the ability to takeoff without jets, and to make great maneuvers.
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 it's flight time it will glide. Small wings can be flapped rapidly, but large wings that carry large bodies want to stay as stationery as possible, and gliding allows that.
Insects:
Insects fly in a way much unlike what was described till 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 wants to thrust up by pushing the wings down, but to minimize the reaction while getting the wings up again. This is done by pushing down with the wings, and 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). Their way of takeoff and flight is very much similiar to that of the insects. Main differences is the mechanism that moves the wings. Because bats have a skeleton rather than an exoskelaton, their wings can and must be more flexible and the muscles that move the wings work differently.Ornithopters:
Ornies are aircrafts that try to resemble the animal's way of flight to another level by moving the wings in a manner that resembles the bird or insect's flap. Ornies are hard to fly and aren't very practical. Not many working ornithopters were built, but a man jumping off a cliff and flapping his hands that have a lot of feathers attached was probably the first and saddest attempt for human flight. An Orni may be based on either bird or insect flight method.
One of the most 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 Liftoff:
Some people are belived15 to be able to levitate without the use of anything then the levitator's16 own tallent. This is done either by meditation, or magic. There is a lot of scepticism about these methods (Especially on the latter).
Why Takeoff
Well, any takeoff will probably be the first step of an attempt to fly. Why fly then? At first, it was just the sheer challange, 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 boundries. 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 unpleasent aspects of flight.
Gaining height:
Sometimes, standing on the tip of your toes won't get you heigh enough. On the more extreme of these cases, you might consider flying as an alternative. Some very good examples are:
1. Displaying an add off the back of a plane.
2. Taking a photo of a large area.
3. Exploading colorfully. These craft should be unmanned (in theory).
4. Effectively relaying communication.
5. Spreading toxins over a large area17.
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 the sake of flight 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. Or you also have the synchronous flying of males and females in complicated maneuvers during mating and courtship.
