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Questions tagged [fluid-mechanics]

Fluid mechanics is the branch of physics which involves the study of fluids (liquids, gases, and plasmas) and the forces on them. Fluid mechanics can be divided into fluid statics, the study of fluids at rest; and fluid dynamics, the study of the effect of forces on fluid motion.

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How do wings generate lift?

Just the basic question that every aviation enthusiast must be curious about: exactly how does a wing generate lift?
Jae Carr's user avatar
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24 votes
2 answers
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What is the reason for changing the speed reference (IAS or Mach number) with altitude?

Inspired by that question: How is the airspeed-Mach number transition handled in modern airliners? When pressure and density decrease IAS also decreases. When temperature and pressure/density ...
mins's user avatar
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6 votes
3 answers
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What is the relation between drag and weight?

Given an aircraft type and fixing its wing configuration (flaps, slats) and fixing its speed and altitude and assuming it flies horizontally, what is the relationship between the aerodynamic drag and ...
polaris12246's user avatar
11 votes
4 answers
4k views

Why do we use dimensionless expressions in flight mechanics and aerodynamics?

Why do we use non-dimensional expressions in flight mechanics and aerodynamics? We could as well directly calculate forces and moments; would that not be more relevant to the specific problem?
SAMER BABIKIR's user avatar
8 votes
1 answer
13k views

What are the mass flow rate and exhaust velocity for a CF6 or GE90 turbofan?

For a typical turbofan jet engine (two examples given in the title), what is the exhaust velocity and mass flow rate of air at sea level and cruising altitude (~ FL350)? Also, does the specific ...
flextempers's user avatar
6 votes
2 answers
5k views

Why does the diffuser section generate thrust in a jet engine?

I am studying the thrust distribution of jet engines. But I'm now a bit confused. In Rolls-Royce's "The Jet Engine" book, http://aeromodelbasic.blogspot.com/2012/05/thrust-distribution-distribution-...
Jono's user avatar
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3 votes
2 answers
3k views

How is Reynolds Number derived and used?

Reynolds number is the ratio between inertial forces and viscous forces according to wikipedia. However, I still do not understand its derivation process and how to, for example, calculate the ...
Harry Yang's user avatar
3 votes
3 answers
972 views

Why don't we place the engine in the front of the wings?

I learnt in class today that fast-moving air over the wing of the plane, which causes a difference in pressure on the airfoil due to something called Bernoulli's principle. If this is so why cant the ...
Linkin's user avatar
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1 vote
3 answers
792 views

Why is the stagnation point on the upper surface before the start of circulation?

In many textbooks, such as Anderson (Fundamentals of Aerodynamics), Bertin (Aerodynamics for Engineers), and Houghton (Aerodynamics for Engineering Students), the authors present the idea that there ...
Nick Hill's user avatar
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1 vote
1 answer
76 views

Why does the angle of a shock change when the flow leaves the influence area of an object?

In this (great) answer, it says: "far enough" from the Concorde's nose, the path followed by the airflow is not affected by its presence and it's just a straight line. Close to the nose we ...
Wyatt's user avatar
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0 votes
1 answer
148 views

Why are bow shocks angled slightly downward at the leading edges of wings?

Why are bow shocks angled slightly downward on airfoils? In this picture if you look closely at the very bottom airfoil diagram, you'll see the "subsonic airflow" part is more below the ...
Wyatt's user avatar
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-1 votes
1 answer
284 views

Why does density decrease most above Mach 0.3, and not as much below Mach 0.3?

Why does density decrease more above Mach 0.3, and not as much below Mach 0.3? From what I've heard, it isn't a linear relationship but why is that? Asked differently, what makes it a non linear ...
Wyatt's user avatar
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13 votes
3 answers
1k views

What are the limitations of the aerodynamics modelling used in flight simulators?

I've asked around, and it seems all flight training programs use a combination of flight simulators and aircraft flights to train pilots. This works well for most pilots who fly airliners which don't ...
user11377's user avatar
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7 votes
2 answers
13k views

How do you calculate the lift coefficient of an airfoil at zero angle of attack?

Thin airfoil theory gives $C = C_o + 2\pi\alpha$, where $C_o$ is the lift coefficient at $\alpha = 0$. However, I couldn't find any equation to calculate what $C_o$ is which must be some function of ...
Aviator S's user avatar
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6 votes
2 answers
3k views

How do diffusers turn velocity into pressure?

There are many resources that give explanations for the behavior of nozzles and diffusers. But at the crux point they all say, "Velocity goes down and pressure goes up." As if that doesn't warrant any ...
BoddTaxter's user avatar
6 votes
2 answers
734 views

Is the air in a jet engine supercritical?

Nitrogen has a critical point of 126.2 K and 3.4 MPa (34 bar). Air contains 75 % of nitrogen For example, nitrogen has a critical point of 126.2 K (−147 °C) and 3.4 MPa (34 bar). source : https://...
ralf htp's user avatar
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5 votes
2 answers
604 views

How is the lift generated by a number of out-of-plane staggered wings calculated?

I want to calculate the lift generated by a number of out-of-plane staggered wings but I don't think a valid approach is to simply use superposition and add the sum of the lift of all the wing's in ...
Jonny's user avatar
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5 votes
2 answers
4k views

Does the Reynolds number on a smaller model need to match the one of the original aircraft?

1.Airbus 380-800 : The overall length of it , is 72.72 m and its cruise speed is 903 km/h. Based on the mean chord of wing , its Reynolds number in cruise phase is around 75 millions. Now , a 1/18 ...
alireza abbassi's user avatar
5 votes
2 answers
2k views

Is there a downwash equation?

I need to find an equation for the following problem: Suppose a full-size glider passes 10 feet over my head at high speed. No doubt I will feel the downward air pressure (downwash) caused by the ...
user32016's user avatar
4 votes
4 answers
470 views

Will two counter rotating vortices cancel each other?

Suppose that an aircraft flies with its wing through a tip vortex of another aircraft which flew in the opposite direction. Suppose that the shed wing tip vortex of both aircraft are exactly the same ...
lWindy's user avatar
  • 301
4 votes
1 answer
4k views

What is boundary layer tripping?

I came across the term "Boundary layer tripping" but I can't really underastand it. After searching on internet I cannot find any video or an image explaining that concept. Could you please explain ...
izri_zimba's user avatar
4 votes
2 answers
1k views

How do aircraft engine manufacturers achieve a higher bypass ratio while still meeting the thrust requirements for a given aircraft?

I understand that a higher bypass ratio leads to greater efficiency as less air moves through the core, and therefore less fuel is burned. However, how is the thrust requirement for the same aircraft ...
flextempers's user avatar
3 votes
1 answer
140 views

Why do shocks travel further down the wing as the Mach number increases?

Inspired by this question, why do shocks move further back on a wing as the Mach number increases? Normal shocks on the surfaces of wings form when the air molecules of the pressure recovery area on ...
Wyatt's user avatar
  • 2,676
3 votes
1 answer
411 views

What determines the angle at which a shock will form?

What determines the angle that a shock will form? If you look at a lambda shock, you’ll see it isn’t straight up, but with an angle (both the front and rear part of the lambda shock). The second ...
Wyatt's user avatar
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3 votes
3 answers
4k views

How do a helicopter's cyclic and collective transfer commands to the swash plate?

I understand the function of the cyclic and collective, and I am specifically wondering if anyone has a good image/video showing the mechanism of how the flight control "mixer" works! I can't seem to ...
Gerry's user avatar
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2 votes
1 answer
62 views

How does a thickening boundary layer create form drag?

Imagine a normal wing with flow over it, and to simplify things it's at 0 AoA. Drag is being made mainly because of form drag and skin friction drag. I'm not sure on this, but I don't think there ...
Wyatt's user avatar
  • 2,676
1 vote
2 answers
206 views

Why do boundary layers become more turbulent as they flow over a surface?

Why do boundary layers become more turbulent as they flow over a surface? This question originated from this one. What makes the boundary become more turbulent as it flows over a surface/wing? My ...
Wyatt's user avatar
  • 2,676
1 vote
1 answer
61 views

What way does induced downwash make a wing roll?

Say you have a wing, no sweep or anything to simplify things. One side of it, let's just say the left, magically doesn't have a tip vortex, therefore no induced downwash from the tip vortex. Would the ...
Wyatt's user avatar
  • 2,676
1 vote
3 answers
103 views

Why don't shockwaves expand rearwards when they expand out?

Why don't shockwaves expand rearward when they expand out? In this for some reason really big picture, you can see the shocks expanding out past the actual plane. Why don't they also expand rearward? ...
Wyatt's user avatar
  • 2,676
1 vote
2 answers
143 views

Why will all air slow down by the same amount in an adverse pressure gradient?

Boundary layer separation is caused by air slowing down to zero and reversing before reaching the end of the body. This is due to friction (viscous effects) + adverse pressure gradient. The formation ...
Frank's user avatar
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0 votes
0 answers
117 views

Why doesn't the Coanda effect also pull air from inside the jet / streamline?

As explained nicely in this answer, the Coanda effect will make a streamline of air draw from the surrounding air outside of the streamline. This picture shows it nicely : So my question is what ...
Wyatt's user avatar
  • 2,676
0 votes
2 answers
124 views

Why is a thicker boundary layer more turbulent?

Why is a thicker boundary layer more turbulent? I’m mainly referring to the effect of the varying speed over the boundary layer, and why it makes it more turbulent. I know it has something to do with ...
Wyatt's user avatar
  • 2,676
0 votes
1 answer
126 views

Why do the shocks on the upper and lower surfaces of a wing stop at the trailing edge? [duplicate]

Why do the shocks on the upper and lower surfaces of a wing stop at the trailing edge? When going Mach 0.82, the shocks on the upper (and lower) surface don't go the whole chord length of the wing. (...
Wyatt's user avatar
  • 2,676
0 votes
1 answer
485 views

Relationship Between Aircraft Thrust and Jet Exhaust Velocity? [closed]

What is the relationship between aircraft thrust and jet exhaust velocity & thrust and mass flow. Actually, my question is how to reduce or decrease jet noise without decreasing the thrust ?
Selvakumar's user avatar
-1 votes
1 answer
63 views

Can a shock in certain circumstances be at a lesser angle than the Mach cone?

(Excuse the drawing quality) What will happen if a shockwave has an angle less than that of the Mach cone? In this picture, the black line represents the Mach cone. The orange line represents the ...
Wyatt's user avatar
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