# Tag Info

41

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 ...

32

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 ...

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 (...

15

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 ...

13

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 ...

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

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 ...

11

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. ...

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

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 "...

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

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. ...

6

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'...

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

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.

6

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 ...

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

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 ...

5

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

4

According to The GE90 - An Introduction, the GE90 has a mass flow rate of 1,350 kg/s at take-off and 576 kg/s at cruise (at 10.668 km = FL350). The CF6 has a mass flow rate of 591 kg/s at take-off. Exhaust velocity isn't generally quoted, perhaps because it only bears a loose relation to the performance characteristics of the engine. I suppose if you wanted ...

4

Then you get a fully laminar boundary layer without transition. This can happen at very low Reynolds numbers, especially if no steep positive pressure gradients are encountered. A steep pressure rise can lead to separation, and if the boundary layer does not change to a turbulent one, this separation can be final, with no re-attachment of the boundary layer....

4

The direction of travel is controlled by a directional control valve for a double acting cylinder. The red line is high pressure from hydraulic pump. when the high pressure fluid is entering the right chamber and push the actuator piston to left. The low pressure fluid in left chamber (blue line) will leave the actuator and return to reservoir. If we ...

4

break down my understanding of the forces that specifically oppose gravity. There are no separate forces that oppose specific other forces. There is simply a set of forces and they all add up and the sum divided by the inertial mass of the plane equals acceleration. Gravity always points down, so to prevent downward acceleration, the sum of the other ...

4

Make a list of the derivatives you want to determine. All of them will be derivations of some force or moment with respect to some parameter or control setting. Now set up your CFD such that you get lift, side force and drag as well as pitching, rolling and yawing moment relative to the center of gravity. Next, make those forces and moments dimensionless. ...

3

Don't know what you mean by real temperature. You will certainly experience a change of static temperature in an adiabatic flow if the velocity changes. Stagnation enthalpy describes the total energy contained in the fluid which comprises inner (thermal) energy, kinetic energy (and potential energy which is often neglected). Stagnation enthalpy is constant ...

3

The vortex is rotating. This by itself should suffice to explain the expansion at its core. The mechanism is less Bernoulli (which applies to straight flows) but centrifugal force. Compare it to the decreasing height at the center of the bathtub vortex when water flows out the drain and starts to rotate. The height is proportional to the pressure in the ...

3

It depends on the vertical distance between the wings. Munk's theorem only covers the effect of their horizontal distance (in flow direction) and says it doesn't matter (in inviscid flow). Your statement is correct if this distance is infinitely high (and the atmosphere has constant density over altitude). Only then there is no interference between the ...

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