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1

of course, but to do so would represent a catastrophic design error. Far more likely from the standpoint of the history of flight is when an airplane is designed to use a certain engine to get it off the ground, and then the engine does not put out the intended power that the aircraft designers were counting on- so the aircraft cannot carry its intended ...

0

Here is the way in which you should interpret the different symbols: $\widehat{\mathbf{i}}$ is the unit vector in the direction of relative motion (of the object relative to the mass of air). $\mathbf{n}$ is the unit outer normal to the spherical surface. $\widehat{\mathbf{t}}$ is the director vector of the tangential part of the traction vector, whose ...

1

Ideally, the flywheel would run in a vacuum. In air, its surface should be as smooth as possible. With aircraft surfaces you have a stagnating point where air "hits" the aircraft straight on and then a flow path with a clear end at the trailing edge. The length of this flow path is used for calculating the Reynolds number, a similarity number which ...

2

Wind vane will always point to the direction from which the wind is blowing. I.E., if wind is blowing from East to West, then the arrow of the wind vane points to 'East'. The pointed end of the arrow offers least resistance to wind. Therefore, the arrow achieves the state of equilibrium by pointing itself against the wind (Direction from which the wind ...

7

When we model aerodynamic forces like this, one of the very useful tools is a "center of pressure." The center of pressure is a point where you can pretend all of the aerodynamic forces are exerted on that point and get the correct behavior. It can move as the angle of attack (AoA) changes, meaning the angle between the vane and the wind, but at any ...

7

While it's possible to analyze this in terms of least energy, it's not very straightforward. That's because it's not the weather vane's energy that is being minimized, but the wind's. The wind does work on the weather vane, which then causes friction, converting the energy to heat. Suppose the weather vane starts out perpendicular to the wind and rotating. ...

13

When the wind blows perfectly parallel to the wind vane's long axis, there is no rotational force on the wind vane. When the wind direction is not parallel to the long axis of the wind vane it will exert a turning force on the vane until the wind vane is parallel to the wind direction. The force of the wind on any part of the wind vane depends on its (...

72

The vane has to be designed so that it has a preference to point in the right direction. In the example that you included, this is implemented by the flag at the back providing a broader cross section than the arrow head and also by the rooster standing slightly to the back half of the arrow. You are correct that if the vane became perfectly anti-aligned to ...

0

The wings of a bird are flapping, but the wings of an airplane are fixed and cannot be flapping. So birds are not afraid of stall, and even use stall (differential pressure resistance) to generate more force to reduce speed. Note that the bird will flap its wings in the direction of speed when landing.

0

Please take a flying lesson, and please read this completely through. Also, see whatever videos you can, such as by King Flight Schools, Rod Machado, Flight Chops, and many others. Since I don't expect you to actually educate yourself on the subject, I will throw in a few points that others might read if you don't... Drag consists of two parts: 1) ...

8

It is done to reduce noise. Googling I came upon this experiment . Six ceiling fan blade models with different types of trailing edge serrations were created and were simulated using commercial software in the present studies in order to investigate the effectiveness of various trailing edge serrations in reducing fan blade trailing edge noise. They were ...

0

What you propose is an adaptable aircraft which continuously reconfigures itself according to the actual mass and, therefore, lift requirement. Something like this actually exists as a glider: The Akaflieg Stuttgart fs29 (picture source). fs29 multiple exposure with wing span changing between 13.3 and 19 m. This makes sense, because by reducing wing area ...

0

A nice idea, but it actually sorts itself out in flight. As fuel is burned and weight is reduced the aircraft trim is changed to reduce both the lift and the drag of the main wing. The drag reduction reduces the fuel burn.

-1

The adiabatic relation for the ideal gas has the assumption that all of the energy is internal $$U=C_VT,$$ while your example clearly has collective kinetic energy, and then Bernoulli's law should apply instead $$\frac{1}{2}\rho v^2+c_pT=\text{const}.$$

1

Running is certainly unrealistic, crawling might be possible if there is something to hold on to. This being Physics SE, we should run some numbers. Below you see the drag force of a typical human being in different postures. This is from Sighard Hoerner's book "Fluid Dynamic Drag", page 3-14 Running would be best approximated by the standing human; let's ...

3

If the train is moving at constant velocity, in order for the the man to walk on top of it in the direction of travel he would need to exert a force overcome the force of air resistance. On the other hand, he could easily walk opposite the direction of travel of the train since the force of the air at his back will cause him to accelerate. Bottom line is ...

3

That's FULL thrust, used when taking off (climbing the hill, so to speak). It is much larger than drag. Typical drag in a jet transport is more like 4% of weight, not 30%.

6

The thrust is used to keep the plane at a constant speed $v$ in horizontal direction. It compensates the friction, which would slow the plane down. However, the lift is due to Bernoulli's equation $p + \frac{\rho}{2}v^2 + \rho g h = const$ which for constant height becomes $p + \frac{\rho}{2}v^2 = const$. Thus the thrust is given by F = A \cdot \Delta p = ...

1

When a parcel of air, along it's streamline in a flow, curves due to the presence of an obstacle, it must develop an acceleration toward the center of the curvature, because $F=ma$. It's that simple. Not because of gravity. There is pressure around the the parcel. A change in flow speed requires a pressure difference, and same thing about a change in flow ...

0

If you want a simple intuitive way to understand it, try this. You know how you can squeeze a wet watermelon seed and make it shoot out from between your fingers? It's based on wedge action. Now let it be a sailboat with a centerboard, so the boat can only go in one direction. The sail is at an angle to the centerboard, and that angle forms a wedge, like ...

1

The blackbird, initially imagined by aerospace engineer Rick Cavallaro, was co-developed by the aeronautics departments of an university, the San Jose state university, and supervised and recognized by the North American Land Sailing Association (NALSA). Furthermore the calculations of MIT professor Dr. Mark Drela (as well as of a research group of the ...

2

Yes this does work, but the blades work opposite of, say a windmill which is turned by the wind. Initially the wind blows on the blades, which act more like a sail, to begin pushing the craft forward. This causes the wheels to rotate, the wheels are connected to the blades to turn them to push air back against the wind. this creates a higher pressure on the ...

0

I see so many people trying to get this and still making some mistakes. Mike Dunlavey and Paul Townsend have a lot correct, but still miss something very important that puts this all completely to rest. First: Bernoulli’s Principle and Equation are two completely different things Please stop conflating them. You need to understand that Bernoulli’s ...

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