Layman’s Perspective of Aviation

pmaitra

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Layman’s Perspective of Aviation

This thread is to discuss the fundamentals of aviation. Anyone who has formal education in aerospace engineering or fluid dynamics will be equipped to go beyond the fundamentals. The purpose of this thread is to discourage assumptions about aviation prior to verification of the assumptions. Assumptions are acceptable as long as they are not insisted upon as “facts.”

Participants are encouraged to provide sources supporting their claims.
 

pmaitra

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Directional Terms

The diagram below represents the common terms used for aircraft, along with boats and ships.

Slide3.PNG


The region ahead of the fuselage, or before the fuselage, is called the FORE. The region behind the fuselage, of after the fuselage, is called the AFT. If one were facing the FORE, the left side would be the PORT side and the right side would be the STARBOARD side.

Most sailors were right handed, so the steering oar was placed over or through the right side of the stern. Sailors began calling the right side the steering side, which soon became "starboard" by combining two Old English words: stéor (meaning "steer") and bord (meaning "the side of a boat").

As the size of boats grew, so did the steering oar, making it much easier to tie a boat up to a dock on the side opposite the oar. This side became known as larboard, or "the loading side." Over time, larboard—too easily confused with starboard—was replaced with port.
Source: https://oceanservice.noaa.gov/facts/port-starboard.html
 

pmaitra

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Forces on an Aircraft


The diagram below represents the forces that act on an aircraft.

Slide5.PNG


Some fundamental points are:

Gravity is a force whereby the aircraft and the earth pull each other. This is countered by the lift which tried to pull the aircraft away from the earth.

Thrust is the force that moves the aircraft forward and drag is the force that tries to pull the aircraft backwards.

Both lift and drag increase with the square of the velocity of the aircraft. Doubling the velocity will quadruple the lift and drag.


What causes aeroplanes to fly?

The community of aviation enthusiasts are divided on this. This debate has been raging for a long time between those that are in the Daniel Bernoulli camp and those that are in the Isaac Newton camp. Interestingly, neither of the two personas even contradicted each other.


Those in the Bernoulli camp believe that lift is generate because air flows at a faster rate over the top surface of the aerofoil than the air flowing along the bottom surface, and that they take the same time in moving from the leading edge to the trailing edge of the aerofoil, thus causing a pressure differential to produce lift. This is known as the Equal Transit Theory. This is a classic example of forming an opinion without actually getting experimental data, doing the calculation, and verifying them with established equations of physics. A theory must be tested for its validity.

Slide6.PNG


Myth-Busting

The myth is: The lift is produced solely because the air travels a longer path on the upper side of the wing than the lower side. Allegedly the Bernoulli effect this causes is strong enough to lift the airplane. Usually this is the only explanation given for the lifting force.
Source: http://warp.povusers.org/grrr/airfoilmyth.html

The truth is, the lift generated is much more than the calculated pressure differential. Therefore, there has to be another force that counters the gravity to lift the aeroplane. This is best explained by Newton’s action-reaction hypothesis, so far, tested to be true. Does that mean Bernoulli’s equations are wrong? Absolutely not. His equations are totally correct, but they do not apply to aerofoils.

Also there are three other explanations of lift: the circulation-based explanation, the flow-turning or streamline-curvature explanation, and the 3D vortex-shedding explanation. These three appear in advanced textbooks, where they form the basis of the mathematics used by aircraft designers. They rely on Bernoulli's equation. The misleading "popular" or "airfoil-shape" explanation commonly appears in children's science books, magazine articles, and in pilot's textbooks. On the other hand, the public rarely if ever encounters explanations based upon circulation, upon vortex shedding, or upon Newton's Laws.
Source: http://amasci.com/wing/airfoil.html

The excerpt below is a pre-emptive assault on petulant people who do not read properly and accuse another person of saying something which the other person did not say.

Why are you prejudiced against the Bernoulli-based theory? Bernoulli's equation is perfectly correct.
Huh? Read my stuff again. Please tell me where I attack Bernoulli. Instead I only attack the "popular theory," also called the Equal Transit-Time explanation.
. . .
On the other hand, yes, Bernoulli can't be used, since real wings function by injecting energy and momentum into the air. Bernoulli doesn't cover that. Instead we need Euler's equations, of which Bernoulli is a subset. We also need fluid simulation, since most instances of Euler (e.g. vortex-shedding) will have only numeric (computer) solutions.
Source: http://amasci.com/wing/airfoil.html

Another interesting material from NASA: https://www.grc.nasa.gov/www/k-12/airplane/bernnew.html

 

pmaitra

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Types of Aerofoils

Before going into the details, it needs to be mentioned, with emphasis, that it is possible to fly with absolutely flat wings or aerofoils. There are various types of aerofoils. Some basic types are listed below, with a special mention of symmetrical wings, usually very thin, that are used in delta-wing aircraft for high-speed and high-altitude operation.

Slide7.PNG


A symmetrical aerofoil will not generate lift unless there is some Angle of Attack. On the other hand, asymmetrical/flat-bottom aerofoils will provide some lift to cause a slight increase in the Angle of Attack, after which, the Angle of Attack will impact lift as it does with a symmetrical aerofoil with some non-zero Angle of Attack.

Myth-Busting

Angle of Attack is not the angle the wing makes with the horizontal plane. Angle of Attack is the angle the wing makes with the general flow of wind against the wing. To better clarify, the angle the wing makes with the horizontal plane is termed as the Absolute Angle of Attack. If only Angle of Attack is mentioned, it means the angle with which the wind hits the wing, even if the aircraft is pointed upwards or downwards.

The tail is generally not used for turning. Control surfaces on the wings are used for turning.
 

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Roll Stability

An aircraft has Roll Stability if it regains its neutral roll after being made to roll in either direction. In other words, if an aircraft is tilted to one side by some turbulence (or any other reason), it will slowly regain its neutral untilted position.

Slide8.PNG


This is done by several ways. One way is to have a dihedral, where the wings are angled slightly upwards. If the wings are angled slightly downwards, it is called the anhedral. In this case, if an aircraft is tilted to one side, it will continue to tilt further. This is usually avoided in most aircraft except in those that need to be highly manoeuverable and are typically computer assisted. If there is no dihedral or anhedral, the aircraft is neutral.
 

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Pitch Stability

This section is best read multiple times to get a better understanding. Concepts are presented in a series but laws of physics work at the same time.

And aircraft had pitch stability if it flies horizontal. It is tends to go nose up, or nose down, it does not have pitch stability.

Aircraft have three centres. Centre of Mass, Centre of Thrust, and Centre of Lift. Most aircraft have the Centre of Mass slightly ahead of the Centre of Lift. We might think that this will cause the aircraft to go nose down, and the Centre of Mass should be moved behind the Centre of Lift. However, if that happen, the aircraft will continue to go nose up, stall, and then crash.

The diagram below shows how and aircraft flies.

Slide9.PNG


Take off: An aircraft gathers speed, and the horizontal stabilizers create negative lift. The rear landing gear is usually placed close to the Centre of Mass and not at the end. This helps the aircraft pivot around the rear landing gear and go nose-up. This increases the angle of attack, and the aircraft takes off.

In flight: The wings provide positive lift and the horizontal stabilizers create negative lift. These two balance each other out. This prevents the aircraft from going nose down, aka negative pitch.

Rare Scenarios:

Some aircraft, such as the F-16, have the Centre of Mass behind the Centre of Lift. In this case, both the wings and the horizontal stabilizers provide positive lift. This is a fuel efficient configuration, because, both the lifts are positive, and there is no component of the wing’s lift being negated by the negative lift of the horizontal stabilizer.

The stabilizers are usually placed behind the Centre of Mass, i.e. at the AFT. This is because if there is any change in the course of the aircraft due to turbulence, the stabilizers being behind the Centre of Mass will try to correct and bring the aircraft back to its original course. If the stabilizers are placed ahead of Centre of Mass, i.e. at the FORE, any change in course will be further aggravated because the stabilizers will tend to increase the disturbance of the course of the aircraft. Some modern European aircraft, such as, but not limited to, Eurofighter Typhoon, Dassault Rafale, Saab JAS 39 Gripen, have vertical stabilizers at the FORE. Many consider this to be a fetish and a bad design. Both the US and USSR tried this arrangement but eventually chose to discard it and follow the convention of having stabilizers at the AFT.

The diagram below shows how an aircraft is kept on its course.

Slide10.PNG

 

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Wing Aspect Ratio

Aspect Ratio of a wing is calculated as wing length divided by the wing width.

Slide11.PNG


Low Aspect Ratio Wings have better manoeuverability. However, the wings being wider, there is more induced drag. More the induced drag, less will be the fuel efficiency. Eventually, this will compromise the endurance of the aircraft. In other words, they will not be able to loiter for a long time, or they will have limited range. Such wings are suitable for aircraft designed for acrobatics and dog-fights.

High Aspect Ratio Wings have poor manoeuverability. However, the wings being narrower, have low induced drag. This makes such design fuel efficient. Therefore, such designs are good for aircraft that need to stay up in the air for a longer duration. Such wings offer extended range and longer loiter time. Such wings are suitable for aircraft designed for close air support.
 

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T-Tail versus Conventional-Tail

There are a lot of different ways the tail of the aircraft can be designed. There are some with two vertical stabilizers (Sukhoi-27, F/A-18, Antonov-225) and three vertical stabilizers (Lockheed Constellation), V-Tail, Y-Tail, etc. Out of the many, two are compared, the T-Tail and the Conventional-Tail, referred for convenience as C-Tail. Both are depicted in the diagram below.

Slide12.PNG


The T-Tail had the horizontal stabilizers on top of the vertical stabilizer whereas the C-Tail is mounted on the AFT of the fuselage at the bottom of the vertical stabilizers.

The T-Tail usually had a clean airflow while the C-Tail usually lies in the wake of the wings.

The T-Tail stay further away from the engine exhaust while the C-Tail lies in the wake of the engine exhaust.

The T-Tail has the vertical stabilizer endplated at the bottom by the fuselage and the horizontal stabilizers at the top while the C-Tail is only endplated at the bottom. Therefore, the T-Tail can be made smaller, because, a smaller vertical stabilizer of a T-Tail can efficiently do the job of a relatively larger vertical stabilizer of a C-Tail. This makes the T-Tail lighter. However, the T-Tail needs control mechanism for the horizontal stabilizers built into the vertical stabilizers which makes the T-Tail heavier than the C-Tail. Eventually, a comparison of individual pairs is necessary to determine which one is lighter or heavier.

If the tail is heavy, it will experience effects of increased torque and result in increased flutter.

The T-Tail is more efficient because it has less interference drag and less wave drag. Küchemann body (Tupolev-154) can be used to improve its efficiency.

The T-Tail has horizontal stabilizers mounted on top of the vertical stabilizers which increases the leverage of the horizontal stabilizers, and therefore, they can be built smaller and lighter compared to the horizontal stabilizers in a C-Tail.

The T-Tail can cause a downward pitch moment when in side-slip.

The T-Tail has often been blamed to be the cause for stalling. The T-Tail makes it difficult to recover from a deep stall but it does not cause a deep stall. When the aircraft has a very high Angle of Attack, the wake of the wings envelopes the horizontal stabilizers of a T-Tail causing it to lose control. A similar situation in a C-Tail would ensure that the tail received undisturbed airflow and thus it would have control. It is to be noted that when the aircraft is in a level flight, loss of control in the horizontal stabilizers is a lesser concern than when in a stall like situation. In other words, when the aircraft enters a stall, a stall recovery is attempted. In such a situation, a C-Tail is likely to have more control than a T-Tail. See image at the bottom.

When landing or take-off, the T-Tail experiences more ground effect at the wings but negligible ground effect at the tail. This causes the aircraft to pitch up relatively easily. During landing, this tendency to make the aircraft pitch up makes it easier to land. Many pilots refer to this as the aircraft landing itself.

Slide13.PNG
 

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pmaitra

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Engine Placement

Engine placement has been a matter of design decision. They can be mounted in a myriad of ways.

Slide14.PNG


Mounted under the wings: Most airliners have the engines mounted under the wings. This allows the mass of the engines to be closer to the centre of the aircraft. Engines under the wings also allow for less bending of the wings because the thrust is between the wing-tip and wing-root. Another benefit is ease of maintenance of the engines. One problem with this arrangement is there is excessive noise from the mid to the aft of the aircraft.

Mounted above the wings: Aircraft that need to be closer to the ground, so as to facilitate boarding/deplaning without external stairs, might have the engines on top of the wings. This also allows for the intakes to be above the wing, thus preventing debris from the runway or airstrip entering the engines. One problem with this arrangement is there is excessive noise from the mid to the aft of the aircraft. VFW/Fokker-614 has this arrangement.

Mounted at the wing-root: This arrangement ensures that the thrust is very close to the centre-line of the aircraft. Therefore, if one engine malfunctions or loses thrust, the imbalance in thrust is minimal, and it is easy to maintain the direction without suffering excessive yaw-movement. The problem with this arrangement is that if there is any malfunction in the engine, it might damage the wing and the consequences can be deadly. Close Air Support aircraft Cessna A-37 and Sukhoi-25, and civil airliners De Havilland Comet and Tupolev-104 have this configuration.

Mounted aft of the fuselage: This is the best arrangement for a noiseless cabin. This also allows the engines to be close to the centre-line. However, the weight distribution is not even. Such aircraft typically have a T-tail. Most Soviet airliners, such as, Tupolev-134, Tupolev-154, Ilyushin-62, and some others, such as, Embraer ERJ 145, Vickers VC10, have this configuration. Close Air Support aircraft Fairchild Republic A-10 also has this configuration, where the intake is above the wings and the exhaust goes over the horizontal stabilizer and between the two vertical stabilizers, thus reducing infra-red signature to potential heat-seeking surface-to-air missiles. This also prevents debris from entering the intake as Close Support Aircraft often have to operate from less than pristine airfields.

Mounted fore of the fuselage: This is an exceptional case as seen with the Lun-class Ekranoplan. The engines are mounted at the fore of the fuselage so as to prevent water from entering the engines.

Mounted above the fuselage: This is a custom modification, as done with the Fairchild C-119 Flying Boxcar by Hindustan Aeronautics Limited so as to allow enough thrust for operations at Daulat Beg Oldie. Similar modification have been done elsewhere.

Mounted under the fuselage: This is a rare configuration because the exhaust can damage the aft portion of the fuselage, especially since many aircraft are made of aluminium alloys. One example is the Yakovlev-15.

The McDonnell Douglas MD-11 has two engines under the wings and one engine aft of the fuselage but mounted on top in the root of the vertical stabilizer.

Sources:
http://www.aeronewstv.com/en/lifest...engine-location-below-or-above-the-wings.html
https://www.usatoday.com/story/travel/columnist/cox/2015/04/05/jet-engines/25244779/
https://www.usatoday.com/story/travel/columnist/cox/2015/04/05/jet-engines/25244779/
 

pmaitra

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Vortex

A vortex (plural vortices) is a swirling mass of fluid. In case of aerodynamics, it is air. It may be visualized as a tornado. The vortex size grows larger at its distance from the wing increases.

The image below shows how vortices are formed.
Slide15.PNG


As the aircraft moves forward, the wings push air down. This causes a low pressure region above the wing and a high pressure region below the wing. However, since wings do not have an infinite length, the air at the wing-tips try to move from the high pressure region below the wing up towards the low pressure region above the wing. As the aircraft moves forward, the visualization relative to the wing would look like a helix that grows larger with increase in distance from the wing.

The image below shows Nature’s equivalent of the winglet. This was observed by Otto Lilienthal.
Slide16.PNG


The image below shows how the vortices are formed in two different aircraft with different aspect ratios. The wing profile as seen from the port side shows the flow of air as would be seen in a wind tunnel with colorized fumes at the wing-tips.
Slide17.PNG


Vortices cause induced drag which reduced the efficiency of the aircraft. One way to reduce the vortex size is to have a high aspect ratio. Another way is to ensure that the wing-tips are narrow, which entails having wing-sweep, either or both at the leading edge or/and the trailing edge of the wing.

Yet another way is to use winglets so that the air moving from high pressure to low pressure region has an obstruction. The usefulness of winglets is debatable. Their benefits are prominent in long distance flights, however, they also provide the aircraft manufacturers to charge a premium, while increasing the weight of the aircraft.

So, generally speaking vortices are bad.

Or are they? Is there any possibility of vortices having some benefits?

This shall be explored later, but first, it is important to understand two concepts: (1) Boundary Layer, and (2) Flow Separation.

Sources:
https://www.sciencelearn.org.nz/resources/302-wing-aspect-ratio
https://www.onera.fr/sites/default/files/ressources_documentaires/cours-exposes-conf/onera-3d-separation-jean-delery-2011-2.pdf
https://www.grc.nasa.gov/www/k-12/airplane/winglets.html
https://www.airspacemag.com/flight-today/how-things-work-winglets-2468375/
http://www.boldmethod.com/learn-to-fly/aerodynamics/winglets-and-wingtip-vortices/
 

pmaitra

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Boundary Layer and Flow Separation

When a body moves through a fluid, in this case, air, the air molecules move with the same velocity as the body in the opposite direction, if the reference were the body. However, the air molecules that are close to the surface of the body, do not move very fast. This is due to the stickiness, or viscosity, of the air.

Aerodynamic forces depend in a complex way on the viscosity of the fluid. As the fluid moves past the object, the molecules right next to the surface stick to the surface. The molecules just above the surface are slowed down in their collisions with the molecules sticking to the surface. These molecules in turn slow down the flow just above them. The farther one moves away from the surface, the fewer the collisions affected by the object surface. This creates a thin layer of fluid near the surface in which the velocity changes from zero at the surface to the free stream value away from the surface. Engineers call this layer the boundary layer because it occurs on the boundary of the fluid.
Source: https://www.grc.nasa.gov/www/BGH/boundlay.html

Many aircraft that have intakes located next to the surface of the aircraft has a plate separating the intake from the surface. This is called the Boundary Layer Splitter. The image below depicts the Boundary Layer and the Boundary Layer Splitter.
Slide18.PNG


Similarly, the boundary layer also exists along the surface of the aerofoil, aka, the wing. When the angle of attack is increased a little bit, the lift is increased. The angle of attack can be increased further to increase the lift, however, at one point, it reaches the maximum lift, beyond which occurs flow separation. This is the point when the aircraft begins to lose lift and begins to stall. The diagram below depicts this scenario.
Slide19.PNG


Sources:
https://www.grc.nasa.gov/www/BGH/boundlay.html
https://www.wired.com/2011/06/air-france-flight-447/
 

pmaitra

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Benefits of Vortex

Vortices are not always a malefic. They can be used to provide benefits to aircraft. There are two ways this can be achieved: (1) to delay/prevent stall at high angles of attack using Vortex Generators, and (2) to create lift called Vortex Lift.

Vortex Generator: This is a small bit of metal (or any other material used for making aircraft) that is attached to the surface of the aerofoil, i.e., the wing. These are small and flat, almost like a flat plate. These look like they are placed so that their plane is perpendicular to the air-flow. However, they usually have a small angle with the air-flow. This creates a vortex that travels over the wing to (and beyond) the trailing edge of the wing. This would look like a tornado along the surface of the wing. This injects energy into the boundary layer, thus preventing or delaying the reverse flow that happens during a stall. Thus, the air-flow remain attached to the wing. The diagram below gives a contrast with a wing at some angle of attack with Vortex Generators and the flow attached which would otherwise have stalled at this angle.
Slide20.PNG


Vortex Lift: Vortex can also be used to create lift. This requires some angle of attack. Typically, this happen when the angle of attack is significantly more.

The aerodynamic characteristics of thin sharp-edge delta wings are of interest for supersonic aircraft and have been the subject of theoretical and experimental studies for many years in both the subsonic and supersonic speed ranges. Of particular interest at subsonic speeds has been the formation and influence of the leading-edge separation vortex that occurs on wings having sharp, highly swept leading edges. In general, this vortex flow results in an increase in lift associated with the upper-surface pressures induced by the vortex and an increase in drag resulting from the loss of leading-edge suction. Although, in general, it is desirable to avoid the formation of the separation vortex because of the high drag, it is sometimes considered as a means of counteracting, to some extent, the adverse effect of the low lift-curve slope of delta wings with regard to the landing attitude.
Source: https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19670003842.pdf

The diagram below shows the vortices formed at the leading edge, and how the vortex lift increases in a non-linear manner with increase in angle of attack.
Slide21.PNG


It is important to note that the vortex lift occurs in the direction of vortex, and the vortices are formed on the upper surface of the delta-wing (or highly swept wing) when there is a high angle of attack, i.e., if the aircraft is moving in such a way that oncoming air is hitting the lower surface of the delta-wing (or highly swept wing). Quite naturally, it follows that the vortices would form on the lower surface of the delta-wing (or highly swept wing) if the aircraft had a negative angle of attack, i.e., if it were moving forward with the nose pitched downwards. Thereby causing a negative vortex lift and reduction of overall lift.

The diagram below shows the vortices formed in two delta-wing SST aircraft.
Slide22.PNG


Sources:
http://www.nar-associates.com/technical-flying/vortex/Vgs_stall_wide_screen.pdf
http://www.univair.com/categories/miscellaneous/micro-vortex-generators.html
http://www.vortex-generators.com/installation-of-vortex-generators.pdf
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19670003842.pdf
https://history.nasa.gov/SP-367/chapt6.htm
 

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