# Tag Info

47

To get to the bottom of it, it might help to look at lift at a molecular level: Every air molecule is in a dynamic equilibrium between inertial, pressure and viscous effects: Inertial means that the mass of the particle wants to travel on as before and needs force to be convinced otherwise. Pressure means that air particles oscillate all the time and bounce ...

36

Short answer: by exerting a downward force on the air around them. Long answer: Some outreach people at NASA's Glenn Research Center have written up a very good multi-page explanation, dealing individually with each contributing effect, as well as some discussion of why explanations you might have heard at school don't work. Since the navigation there is a ...

30

After passing through the shock wave, the air still has to evade the body. If the shock-wave touched the body, the air would have to escape through infinitely thin space at infinitely high pressure and it obviously can't do that. So behind the shockwave there is a boundary layer (called shock layer) in which the air is moving with the body and flows, under ...

22

This is a broad question, hence a broad answer 😃 As high eng CFD now stands, it already is correct enough for, say, 95% of use cases. I am not aware of much utility in modelling complex turbulent flow to a high level of accuracy. CFD will proceed to become more and more accurate, but 100% accuracy will never be achieved, not at least if we are to assume ...

21

Aircraft are limited by both air speed (VMO, affects loads on the structures) and Mach (MMO, formation of shock waves resulting in buffet). At low altitudes, the speed of sound is high so an aircraft is most limited by indicated airspeed (IAS). At higher altitudes, the speed of sound is lower so the aircraft will be limited by Mach number. Aircraft ...

18

In a climb: The air density decreases. That means for a given IAS, the TAS becomes faster. The local speed of sound decreases due to the decreasing temperature. That means it takes a slower TAS to get to any given Mach number the higher the plane climbs. So as a plane climbs at a constant IAS, the plane will be fast approaching its limiting Mach number (...

16

Mind you, not all aircraft need one, especially slower piston ones. As you get closer to higher speeds, where compressiblity becomes significant, and you are approaching the speed of sound, the air starts to behave differently aerodynamicaly. Also: remember how the air is a little bit accelerated over the wing because of the shape of the wing? Well, if ...

14

Here is a dimensionful equation: $$L = \frac{\rho}{2}\cdot v^2\cdot \frac{2\cdot\pi\cdot b^2}{1+\sqrt{1+\left(\frac{b^2}{2\cdot S}\right)^2}}\cdot\alpha$$ Note that all ingredients are physical, measurable values. Now here is the same thing again, now in dimensionless form: $$c_L = \frac{2\cdot\pi\cdot AR}{1+\sqrt{1+\frac{AR^2}{4}}}\cdot\alpha$$ The ...

14

HOW AN AIRPLANE GENERATES LIFT There are usually two popular fields of thought (excluding the debunked equal time theory) behind why an airplane flies; some think it is caused by an application of Newton's 3rd law, and others think it is caused by a pressure difference on the top and bottom of the wing. Basically both the "Newtonian" explanation and the "...

12

There are two aspects to simulation error: the aerodynamic model error and less commonly known, integration algorithm numerical error. Aerodynamic models tend to break down in the transonic and post stall regime of flight for two reasons: lack of data due to safety or cost, and poor predictability created by non-linear response in turbulent flow/shockwave ...

12

Size matters here. Big aircraft have more inertia and take much longer to respond, but can equal out small-scale turbulence better. Your drone idea for training will not be representative of the big airplane at all. Turbulence can be modeled quite faithfully - you just need to fly once through rough air, collect all the data and replay it in the simulator. ...

12

In addition to the Antonov 72/74's, Boeing worked on the YC-14 concept as a replacement for the C-130's. Also, many seaplanes have to place the engines as far up as possible, to keep the engines and propellers out of seaspray. This has the secondary effect of accelerating airflow over the wing and increasing its effectiveness. The engine wash is also used ...

11

Laminar flow is easy. While there is a complete set of differential equations that describes any fluid flow, there is a wealth of simplifications and assumptions that you can use on laminar flow. This means that X-Plane doesn't need to model all the air around the wing, but can do the calculations based on wing profile and local velocities. Everything is ...

11

100% correct, no. That's because CFD (and a great many other computer applications) work on numerical approximations. There's either no exact solution* known, or the exact solution isn't computable. So the application uses numerical approximations, typically on a grid or mesh. To get a better approximation, you either make the grid smaller, or use a ...

10

In case of an infinestimally thin body in supersonic flow, the disturbances created propogate as Mach waves. However, as the the thickness of the body increases, the flow has to physically turn away from the object after the shock wave. For example, if a wedge at an angle $\theta$ is placed in supersonic flow, the fluid has to turn by that angle after ...

9

They can be, as on the An-72/74, which has the engines mounted above and forward of the wings in part to increase lift, utilizing the Coandă effect. One downside is that this makes access for maintenance more difficult, which is a large factor in operation costs. https://en.wikipedia.org/wiki/Antonov_An-72 https://en.wikipedia.org/wiki/Coand%C4%83_effect

7

The simplest answer that I know that is that is still accurate is that for any object to move through the air, some force must push the air in front of it out of the way (gravity, engines, momentum etc doesn't matter). If more of the air is pushed downwards then upwards (by for example, wings) then the difference is called lift.

7

Perhaps, at the root of your question, there is a misunderstanding of the nature of a shock wave. A shock wave is a surface, an ensemble of points, where the properties of the gas change discontinuously (it is not the only surface with this characteristic, but the other class, that of contact discontinuities is absolutely unstable, and quickly disappears). ...

7

Warning N°1: I'll avoid energy conservation, but cannot set aside mass conservation obviously. Warning N°2: Both speed and pressure settle in a way that abides the laws of physics. It's not like one settles first, then the second follows. Don't let my wording fool you. Warning N°3: I won't consider temperature in addition to speed, density and pressure. It's ...

7

Early turbojets were so inefficient that adding a fan was considered but not implemented because that would had made the engines even more sluggish and narrowed the operating limits even more. If you compare the Jumo 004 with the EJ200, you will find that both are of similar size, have 8 compressor stages, but vary widely in their compression ratio and ...

7

In wind tunnel testing you need to match several similarity parameters; the most well known are the Reynolds number and the Mach number. Since the Reynolds number depends on flow speed, viscosity and physical size while the Mach number also depends on flow speed, it looks like both can only be matched when the physical size of the model remains unchanged. ...

7

First, the Millennium Challenge asks about the existence and uniqueness of a solution to the Navier-Stokes (NS) equations given any compatible initial/boundary conditions. This question is of significant interest from an applied mathematics perspective, but it doesn't* help with actually obtaining the numerical solution and, therefore, is of limited interest ...

6

While we wait for a probably better answer, here's my tentative. You state that you understand the concept of constant mass flow. Let's for a moment write the mass flow properly: $$\dot{m} = \rho A v$$ on the left we have the mass flow $\dot{m}$, on the right we have the density $\rho$, the area $A$, and the velocity $v$. If we want $\dot{m}$ to be ...

6

Wings generate lift pushing air downwards. As a kid I used to stick my hand out of the open car window and tilt it - there is an upward force. A flat plate does this. So aircraft wings could be flat plates, but unfortunately flat plates create a lot of drag as soon as they create lift since the flow at the upper end detaches immediately (curly spiral in ...

6

The whole setting group is called transition settings, and refers to the boundary layer transition point (from laminar to turbulent). The forced transition trip locations are exactly that, expressed as a fraction of the chord, for the top and bottom surfaces. They are akin to placing a trip strip at those locations. The Ncrit value is a measure of free flow ...

5

A propeller not affecting the velocity of the free stream air would not generate any thrust (or drag), as the propeller works by the principle of creating thrust by accelerating a large mass of air, $F = m * a$. The accelerated airflow from the propeller does generally increase the lift generated by the wing. The amount of lift increase depends on the ...

5

Here is a link to John S. Denker's web book on airfoils. This is probably the definitive explanation of how wings work. John Denker has a bunch of websites worth checking out. http://www.av8n.com/how/htm/airfoils.html Bottom line: for a 150,000 lb. aircraft to stay in the air, it must impart 150,000 lbft of momentum to the air through which it passes. You ...

5

In turbojet there is no fan. Very simply, turbofan means turbojet + fan in front connected to the LP shaft.

5

Tripping the boundary layer refers to the action of artificially transitioning a laminar boundary layer into a turbulent one. It can happen intentionally (via turbulator) or unintentionally (via imperfect aero-smoothness, such as rivets, bolts, counter-sinks). At the point of boundary layer tripping, the surface protrusion destabilizes the laminar boundary ...

5

I think I have a partial answer that I have found in a paper [1]: The following is a quote from the introduction: It is too narrow a view to use vanishing surface shearing stress or flow reversal as the criterion for separation. Only in steady two-dimensional flow do these conditions usually accompany separation. In unsteady two-dimensional flow the surface ...

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