Basics of aviation...
Basic of flight
Before looking at crazy concepts such as invisble planes (something I can just never see taking off ;);)) we must look at how aircraft fly. To do that we need to understand some basic principles of aviation. Weight is the force acting on mass due to gravity, in other words: it is the force with which something is being 'pushed' towards a centre of gravity. Every object on Earth has a mass, meaning every object on Earth has a wheight.
To counteract weight, we have a force called lift, which in aviation is what keeps a plane in the air. To achieve this, aircraft use wings whose aerofoil generates lift.
In a similar way to drag, lift, can only have effect on a fluid (liquid or gas). Whether the object is 'still' (known as stationary) and the fluid is moving, or whether the fluid is stationary and the object is moving, isnt of importance. The main factor that contributes to lift is the relative difference in the speeds of the fluid and object as well as the aerofoil.
To achieve lift, we require more pressure under the wing than over the wing. This is known as Bernoulli's Principle. Now the question is, how do we achieve this? As we know, as air speeds up, its pressure drops. Therefore we need the air travelling above the wing to be going faster than the air beneath. This is where the aerofoil comes into place. The aerofoil is the way the wing is shaped and tilted to achieve this.
When moving air flows over a sudden increase in wing angle, the trajectory it travels through narrows, causing the flow to speed up.
There are 2 main different types of aerofoil: Symmetrical and Asymmetrical
Symmetrical aerofoils -
They have identical upper and lower surface hence the name. Under several angles of attack (the angle at which the aircraft flies), the travel of air is relatively constant, earning symmetrical aerofoils the best lift/drag ratios for engine blades. On the other hand, they generate less lift than asymmetrical aerofoils and have undesirable stall characteristics.
Asymmetrical aerofoils -
They have a wide range of upper and lower surface shapes. They consistently have great lift/drag ratios and generate much more lift than symmetrical designs. They also have more desirable stall characteristics
Main forces acting on aircraft.
To achieve flight, we need to make use of four basic aerodynamic forces: lift and weight (which we've already mentioned), thrust and drag. They are the 4 things holding a plane in the air so to speak, each pushing in a different direction. Let's examine the force we havent covered yet and its counterforce: thrust and drag.
Thrust is the aerodynamic force that propells an aircraft forwards. Drag is the opposing aerodynamic force that counteracts this, being the 'friction' of sorts that resist the thrust (movement) of an object in a fluid. The amount of drag is dependent upon some factors such as the speed of the object (slower the less drag) the size of the surface area affected (why cyclists in races hunch up, becoming smaller and hence experiencing less drag) and the density of the air.
For efficient flight to take place, the thrust generated by the aircraft must exceed of be equal to the drag. Should the drag exceed the thrust the aircraft will slow down (which controlled can be used in landing or uncontrolled can result in a stall and crash). The opposite is true for thrust.
Aerial Navigation: Wings, Slats, Flaps and Spoilers (Wont spoil anything dont worry ;))
Now that we understand the basic elements of flight and the ways in which an aircraft uses them to fly, the next step is to consider how engineers allow pilots to control the airplane using these forces.
First, we must take a look at the angle of attack, in other words, the angle in which a wing is positioned to oncoming air. The greater the angle of attack - that is, the higher the elevation of the front on the wing compared to the back - the greater the lift. The smaller the angle - that is, the less elevation the front of the wing has compared to the back of it - the less lift. This is why, rather interestingly, its easier for a plane to climb that to cruise at a constant altitude, as the angle attack tends to provide more lift. To achieve zero lift and cruise in a straight line it must present a negative angle of attack ( i.e. have more depression in the front than at the back of the wing). This wing positioning also generates more drag, meaning the aircraft must generate more thrust to stay in the skies.
As most wings are designed to provide an appropriate amount of lift while the plane is cruising, pilots tend to alter the shape of the aerofoil through flaps and slats so as to be able to reduce speeds during landing etc. (slats are used to increase lift and flaps to decrease lift). Another mechanism used to slow a plane down are spoilers. They tend to appear in the form of hinged plates on the top half of wings that are deployed to reduce airspeed and descend.
Glossary - bold words (for this part only)
Weight - It is the force with which a mass is being 'pushed' towards a centre of gravity
Lift - Lift is an aerodynamic force that 'pushes' up keeping things in flight
Drag - Drag is the 'friction' of sorts that resist the thrust (movement) of an object in a fluid
Thrust - Thrust is known as the aerodynamic force that propells an aircraft forwards
Fluid - A gas or liquid i.e. a substance that can flow
Stationary - When an object is 'still'
Aerofoil - The aerofoil is the way the wing is shaped and tilted to achieve lift
Angle of attack - The angle in which a wing is positioned to oncoming air
Elevation - The angle (positive) between the horizontal line of sight and the object
Depression - The angle (negative) between the horizontal line of sight and the object
Flaps - Mechanism on the wing that causes drag
Slats - Mechanism on the wing that causes lift
Spoilers - Hinged plates on the top half of wings used to reduce airspeed and altitude
Control Surfaces: Stabilizers, Ailerons, Rudders and Elevators
So as we mentioned earlier, pilots must have a way of controlling an aircraft - briefly touching on flaps and slats. However there are many more control surfaces, of which we’ll be covering the main types. To begin, situated at the tail of a plane, you can find two types of small wings, the horizontal and vertical stabiliser. The horizontal stabiliser is what prevents pitching of the aircraft nose. The vertical stabiliser has a similar function, but instead focuses on preventing the yawing of the plane’s nose.
On the rear of the horizontal stabilisers there are some slat/flap-like structures called elevators, responsible for the pilot being able to control the pitch as well as changing the angle of attack (remember?). This enables the plane to ascend or descend in the skies. While the vertical stabiliser may also feature a similar structure, it is known as the rudder, not elevator. They rudder is responsible for yawing the plane left or right through altering the stabiliser's aerofoil.
Ailerons are next! They are flaps located close to the extremities of the wings. Through managing these, the pilot is able to make one wing generate more lift than the other or vice versa making one droop and one to rise. This is done through making them work in opposition, i.e. when one aileron deflects upward the other deflects downward. By the way, remember the spoilers from the last section? They can also be used to roll an aircraft when deployed on solely one side.
Now we have a handful of the multitude of controls a pilot has at his disposal to control a plane, we can move on...
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