Paper Example on Lift Force: How Planes Fly

Introduction

Lift force, according to Fidkowski (2018), associates mostly with fixed wings on aircraft. While lift is generated by streamlined bodies shown in the video, for instance, propellers, racing, cars, and sail boats Fidkowski (2018), observes that lift force generation is not unique to aircraft alone. However, the video reveals aircraft as ubiquitous phenomenon given that people fly a lot. As such, heavier than air flight has a certain mystique. For example, the question that has been raised by both engineers and non-engineers is how a 10 ton of thrust keep 150 tons of aircraft in the air.

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According to Fidkowski (2018), most conceptions about lift force is that it opposes weight. Therefore, Fidkowski (2018) observes in the video lecture that lift can be achieved in any direction in regard to gravity. Fidkowski (2018), therefore, views lift in regard to the direction of the flow as opposed to the direction of the gravity. This views are underpinned by various performance carried by aircraft, for instance, cruising, climbing, and descending. Fidkowski (2018) explains this based on wing cross-section. There are various ways to explain lift force.

Simplified Explanations of Lift Force As aforementioned, there are various ways to explain the concept of lift. According to Fidkowski (2018), some of the concepts are complicated while some are mathematically inclined and the author invalidates some explanations that have been put forward. Some of the explanations discussed in in the video are Mathematical theories of lift such as the Newton's laws of motion, Bernoulli's principle, Navier-Stokes (NS) equations, Inviscid-flow equations, Circulation and Kutta-Joukowski and other alternative explanation such as Coanda effect and the equal transit-time" theory of lift.

The Navier-Stokes (NS) Equations

The concept, according to Fidkowski (2018) describes lift in terms of physical principle of conservation laws for example the Newtonian law that pertains action of viscosity fluid. With mathematical equation can be used to predict lift in any given airspace with accuracy. However, this concept is marred with some flaws. This is true given that that this principle does not automatically convey intuition. According to Fidkowski (2018), The Navier-Stokes is presented by the challenge that fluid dynamics are not always intuitive. Therefore, the scholar seeks an explanation that non-engineers can understand and the one that leaves everyone satisfied.

Equal Transit Time Theory

According to this theory, air moves faster over tops services because it has a larger distance to cover compared to air moving under the airfoil. However, the presenter in the video refer to this as a fallacy. For example, the concept incorrectly assumes that air travelling towards the foil must rejoin at the trailing edge. This being the case, then air along the upper surface travels faster. This is coined together with Bernoulli's principle which provides that air under the wings moves slower than the upper wings. With high pressure, the wing is pushed up resulting to a lift. Thus, the equal transit time theory and the Bernoulli's principle are fallacies. This is true given that there is no physical principle that need equal transit time. In fact, air moving over the top of the airfoil is faster than what the theory predicts. Also, based on Fidkowski's (2018) explanation, this violates the Newton's law of motion. This is true given that the theory describes force on the wing that lacks opposite force. Also, this theory does not explain how aircraft can fly upside or how a flat plat can generate lift. In reality, as Fidkowski (2018) puts it, fluid elements do not meet at the tailing edge. A point to note at this point is that there is nothing wrong with the Bernoulli's equation but there no need to apply it if the theory cannot explain the velocity difference. Thus, this assertion is referred to as equal transit-time fallacy.

Particle Kinetic Theory

In this theory, air molecules deflect off the underside of the airfoil. The molecules are deflected downwards and during the process, they acquire what Fidkowski (2018) refers to as negative vertical momentum. In this theory, lift is generated by exerting a downward force on air as it is moves past the trailing edge. However, based on the Newton's third law, air must exert an equal and upward force on the airfoil, and is referred to as the lift. This theory is invalidated by the fact that molecules that do not interact with the airfoil are unaffected. Thus, forgets the fact that molecules interact with each other as fundamental description of a fluid.

Venture Theory

The theory explain lift by asserting that the upper constriction causes an increase in velocity, a phenomenon attributed to conservation of mass. The explanation held by this theory is hinged on the fact that higher velocity on top has lower pressure as prescribed by the Bernoulli's equation. As such, pressure differences between top and down surfaces result to lift. However, the theory does not explain how the aircraft can fly upside down or how a plat can generate a lift. The Bernoulli's equation is not wrong but is not applicable if cannot explain the velocity difference.

Flow Turning

Based on the flow turning concept, lift is a reaction attributed to its turning of the flow downloads. This results to what is referred to as vertical momentum balance. For example, fluid receives downward momentum at some rate that is influenced by the amount of turning and affected. This result to upward momentum that causes lift.

The Concept of Streamline Curvature

Pressure is said to play a role in lift force. For example, a streamline curves due to pressure gradient as shown below. For example, higher pressure above the streamline relative to lower pressure below pushes down on the fluid. The curving streamlines explain pressure differences both top and down surfaces. This results to lift force.

The Kutta Condition

This concept prescribes that all fluid, including air have some sort of viscosity. Viscous fluids tend to separate and this is especially true when making tight turns. This is true for the trailing edge. As shown in the figure below, the trailing edge has very tight turn. According to Kutta condition, physical flow cannot turn a corner. In the potential flow solution, fluid has to turn the sharp trailing edge corner. Thus, the Kutta condition correctly explains physical potential solution. Trailing edge is a critical aspect in the flow process. Sometimes there are no flow problems on other turns. However, trailing edge present most difficult turns. For example, viscous fluids cannot pass through solid walls. For instance, small viscosity results to an effect of localized flow on the boundaries layers besides the walls. In these layers, the viscosity increasingly drops to zero and at the same time, the flow in the outside layers behaves as if it inviscid. This s illustrated below.

Fidkowski (2018) explains that there is adverse pressure gradient on the upper surfaces. However, the boundary layer does not separate at once during the first point of adverse pressure. Instead, the boundary becomes increasingly weak as the flow moves to the trailing edge. In the long run, the separation takes place a little bit before the trailing edge. However, if there is leading edge turn is too tight and the angle of attack too large separation takes place. This is, however, not typical because the boundary layer is well prepared with full velocity after a favorable pressure gradient.

Conclusion

In summary, the potential flow and Kutta condition results to flow turning. Inviscid flow always follows the curvature of the airfoil. With a closed body, there will be two stagnation points, for instance at the leading edge where the fluid hits the body and at the trailing edge where fluid come off the body. This is result to viscous effect in what is referred to as inviscid model. This happens at the trailing edge thus, setting the Kutta condition.

Thus, lift force in aircraft is achieved by turning incoming flow downwards. The common fallacies in this discussion include the equal transit time, particle Kinetics, and the venturi effect. The Bernoulli's equation is perfect but is tangled within the explanations provided by the fallacies. Therefore, flow turning provide a perfect explanation of lift force. For example, the inviscid theory provides an intuition on how fluid flow over curvature objects. The viscous effects correct the inviscid theory. Key to the viscous analysis of lift is the boundary layer separation.

Reference

Krzysztof Fidkowski (2018). Lift Force: How Planes Fly. [video] Available at: https://www.youtube.com/watch?v=aa2kBZAoXg0 [Accessed 7 Apr. 2018].