How do aircraft fly?
Before we can answer this question properly, you need to know two things: which particular aircraft are we thinking of, and what do we mean for flying. Let's address both things briefly.
On the one hand, there are different types of aircraft, such as balloons and airplanes. Aircraft are so classified according to the different ways of flight they use. Thus, it is not possible to answer a generic question such as "How do aircraft fly?"; it is however possible to answer a question such as "How does a particular type of aircraft fly?". You can check the page What is an aircraft? to know more about different types of aircraft.
On the second hand, the word flying also conveys different meanings in aviation. This page, thus, will be dedicated to introduce what flying means.
To fly (tr.v.)
According to the Merriam-Webster Online Dictionary (20091), to fly is to move controllably through the air or before the wind or through outer space.
In aviation, vehicles that fly are those such as airplanes, helicopters, balloons and gliders. Therefore, asking for how aircraft fly is really asking for three different but related movements1:
- How aircraft move up and down (i.e. how aircraft achieve lift).
- How aircraft move forward or onward (i.e. how are aircraft propelled).
- How aircraft move sideways (i.e. how are aircraft steered).
How do aircraft achieve lift?
In theory, when we talk about lifting or achieving lift in aviation, we are not really referring to moving an aircraft up or down in the air. We are rather talking about how aircraft become and/or remain airborne. In practice, however, both concepts are pretty much related and can be discussed as if they were the same2.
Aircraft achieve lift and, thus, become airborne either by creating a differential pressure in two different parts of the air around them or by pure thrust power. Aircraft that use pure thrust power fly by ejecting a high energy output that propels the aircraft in the opposite direction (e.g. a rocket). Aircraft that create a differential air pressure to achieve lifth do so in two ways: either they use a container which weights less than the surrounding air (aerostatic lift) or they use wings that allow the air above the wing to be lighter than the air below the wing (aerodynamic lift).
Aerostatic lift
Aerostats (such as balloons and blimps) are lighter-than-air aircraft. They lift because they use lighter-than-air gas to create a low pressure environment within an enclosed bag. The overall weight of this bag, of the gas it contains and of any attachment to it is lighter than the air pressure outside the gas bag. Thus, the lighter air tend to move upwards taking the aircraft and any attached load with it. It will stop lifting only when the weight of the aircraft equals that of the atmospheric pressure surrounding it. Because aerostats use gas that tend to move upwards, the lift is pretty much vertical in orientation (especially if there is no wind). Aerostasts, thus, use aerostatic lift for flying3.
| (Video embedded from YouTube on 20 May 2009) | (Video embedded from YouTube on 20 May 2009) |
Aerodynamic lift
Aerodynes (such as airplanes and helicopters) are heavier-than-air aircraft and lift because their wings cut the air in such a manner that air passing over the wings is sped up in relation to the air that moves below the wings. This speeding up creates a differential pressure above the wings (it makes the air on top lighter) in comparison with the air below the wings (which, thus, become heavier). The higher pressure from below thus push the wing upwards and lift the aircraft. Aerodynes with fixed wings (such as airplanes) needs constant forward movement to cut through the air, which is why airplanes lift on a slope fashion. Aerodines with rotary wings on top (such as helicopters) cut the air in a screwing fashion and, thus, lift vertically. Aerodynes, thus, use aerodynamic lift for flying.
| (Video embedded from YouTube on 21 May 2009) | (Video embedded from YouTube on 20 May 2009) |
Power lift
Other vehicles use pure thrust for lifting purposes4. Some aerodynes, however, may use thrust for taking-off and landing vertically in confined spaces but will use aerodynamic lift otherwise. This type of aircraft uses power lift for flying.
| (Video embedded from YouTube on 20 May 2009) | (Video embedded from YouTube on 20 May 2009) |
How are aircraft propelled?
For now, when we talk about propulsion be are going to refer to how aircraft are either pushed or pulled in the air so that they move forward (like airplanes) or onward (like helicopters). Aircraft are propelled either by using internal power or by relying on atmospheric conditions. The latter are considered unpowered aircraft (such as kites). The former are considered powered aircraft (such as airplanes).
Wind
Wind is an atmospheric element that some unpowered aircraft use for propulsion. For example, balloons depend on prevalent winds to move across the sky.
| (Video embedded from YouTube on 21 May 2009) |
Lift
Air itself, although not a propulsion element per se, is used by some unpowered aircraft, such as gliders and kites, or by some powered aircraft, such as airplanes, to move forward while descending. They "fall with style" by way of controlling the lift on their wings (or, said otherwise, they glide).
| (Video embedded from YouTube on 21 May 2009) |
Propellers
Propellers are used by some powered aircraft (e.g. propeller airplanes) to produce lift. Propellers are vertically-mounted blades that cut into the air in front pulling the aircraft forward. Propellers move the airplane forward for the wings to produce lift or for gaining speed.
| (Video embedded from YouTube on 21 May 2009) |
Rotors
Rotors are used by other powered aircraft (e.g. helicopters) to produce both lift and propulsion. Rotors work similarly than propellers but they are mounted in a horizontal position. Therefore, they pull the aircraft vertically. Unlike propellers, however, rotors are able to lift the rotorcraft, control the direction of movement, and gain speed.
| (Video embedded from YouTube on 21 May 2009) |
Jet engines
Jet engines are used by some powered aircraft (e.g. jet airplanes) to also produce lift. Jet engines compress air and release it through a narrower ending, creating an opposite reaction that push the aircraft forward. Jet engines move the airplane forward for the wings to produce lift or for gaining speed.
| (Video embedded from YouTube on 21 May 2009) |
How are aircraft steered in the air?
Steering is used here to refer to controlling the aircraft direction rather than their propulsion forward or onward. Thus, it refers to how aircraft move right or left, change direction, etc.
Wind
Wind, again, is an atmospheric element that some unpowered aircraft use for steering. For example, balloons depend on wind to move across the sky. If the pilot wants to change the direction of travel, for example, he can lift or lower the balloon to a different layer of the atmosphere where it is possible to find wind flowing in the required direction. It is not a precise navigation but it is possible.
| (Video embedded from YouTube on 21 May 2009) |
Control surfaces on the aircraft
Aircraft control surfaces are used in most powered aircraft for steering. The rudder, placed at the tail of airplanes, for example, steer the aircraft right or left, especially while on the ground. Ailerons, on the other hand, are placed on the wings of airplanes to create opposite lift in both wings at the same time so that the aircraft veers right or left.
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| Ailerons steer the aircraft to the right or to the left (roll). | Elevators steer the aircraft up or down in order to change the wings' angle of attack (pitch). | The rudder can also be used to steer the aircraft to the right or to the left, especially on the ground (yaw). |
| (Pictures embedded from Wikipedia on 21 May 2009) | ||
Rotors
Tilting the rotor blades are used by rotorcrafts for steering the aircraft in the desired direction5.
| (Video embedded from YouTube on 21 May 2009) |
Want to know more?
- AviationKnowledge - What is an aircraft
- This AviationKnowledge page offers further information about aircraft classifications according to different categories, such as according to lift and according to propulsion.
