# Tag Info

9

Because where they come close together the air in between circulates in such a way as to join them in a single path. Floris is right, but maybe this picture helps.

11

Intuitive start of an answer: If you have counter rotating vortices they have zero net angular momentum (to first order). If they merged they would have to have no motion -> where did the energy go. In between the two axes of rotation the fluid moves in the same direction and has no mechanism for dissipation. By contrast for two vortices with the same ...

2

Mark's answer is fantastic and steps through much of the physical reasoning involved wonderfully. But annoyingly, in this particular case the region of interest is pretty much right where both air resistance starts to become nontrivial and important and the details of the velocity of the ball off of the club also has non trivial behavior, so I decided to ...

0

My guess is, this is related to low pressure on your mouth (and much less noticeably - high pressure on your chest) which makes it harder for you to inhale. -When you lower your head, you prevent your mouth from being drained of air by making it into sort of a bubble free from the airflow. This could be related to a RollerCoaster - Have you ever tried not ...

1

I think you are misunderstanding the article. I also think that the name 'Flying Saucer' is a bit misleading. I think referring to it with the actual project name: 'Low Density Supersonic Decelerator' (LDSD) fits better. The concept of this project is that a spacecraft uses a inflatable saucer-shaped balloon to increase its reference area (the surface area ...

2

This is an optimal control problem, so I will use the rules of optimal control. First, we represent the state space equations. Also we take the total mass as a state and amount of fuel burnt as the input control. So we have: \begin{cases} \tag{1} \dot{x}_1=x_2 \\ \dot{x}_2=\frac{\eta \theta-k(x_1)x_2^2}{x_3}-g \\ \dot{x}_3=-\theta \end{cases} with these ...

0

I thought this question was interesting and I didn't want to do any proper work this afternoon so I made a simple model to find out what would happen. My matlab code is at the end of the question. So far I've tested three cases and considered changing the initial thrust and adding a linear increase in the thrust for each case. The thrust is given as a ...

0

The spinning of the wheel is a cconsecuence of the assymetry in the air stream it receives. This rotation obviously reduces the drag experienced by the car. To explain it let us consider, instead the bicycle wheel, a wheel with blades like a windmill. It is obvious that the car travels more quickly when this wheel is free to rotate.

2

This phenomenon is probably related to the cold shock response, a set of physiological changes that come about in response to rapid temperature change, such as that experienced by a human whose face is immersed in a cold fluid. The response is accompanied by respiratory changes, including an initial gasp (see here). It is related to the dive reflex. This ...

0

Consider helicopters, which are simply aircraft whose wings go in a circle. Then consider those flying platforms consisting of a fan pointed down. The only real difference is whether the wings are big and slow versus small and fast. Lift consists of the momentum (per second) of air directed downward. Momentum is $mv$. That air has kinetic energy ...

0

The lift of a wing is proportional to the square of the air velocity passing over it. If you have a wing it is thus very easy to get a lot of lift just by increasing your speed. In this way, rather than using engine force to lift the aircraft directly, you use the engine force to push you in the direction you want to go as fast as possible. This means you ...

17

Let's look at the relationship between momentum and energy. As you know, for a mass $m$ kinetic energy is $\frac12mv^2$ and momentum is $mv$ - in other words energy is $\frac{p^2}{2m}$ Now to counter the force of gravity we need to transfer momentum to the air: $F\Delta t = \Delta(mv)$ The same momentum can be achieved with a large mass, low velocity as ...

2

I've wondered about this a bit before. I think it's good to hugely simplify things to think about it. Incidentally, I'm absolutely not a source of authority here. I'm just thinking it through from what seems apparent to me. A falling plate You've said specifically that you don't want to think about how wings work, so let's not. Forget about the wing. ...

0

There are planes that tilt the engine down to take off those are called VTOL (vertical Take Off and Landing). These are usually military fighter craft. The reason why they aren't used for general aviation is multifold: First every regulation aircraft must be able to touch down safely after losing an engine, if the engine is what provides the lift then the ...

0

You can also consider how a glider can achieve much greater distances than a rocket, whilst expending no energy at all! Ah, no. One expends energy in the launch phase - an aerotow or winch launch being typical - and gains energy in slope, wave, dynamic or thermal lift and then expends it in overcoming drag. Energy management is one of the most ...

4

Why is an airplane better than a rocket? Because the plane grips the medium. For a rocket to remain at a fixed altitude, it must continually thrust upwards in order to counteract the fall caused by gravity. It does this by pushing down a large amount of air and fuel each second. An airplane can create this same upward force to counteract gravity, but in ...

0

How about this explanation (not that detailed, but simple): The reason is that, if force of lift and force of gravity cancels each other out exactly, then staying airborne requires zero energy. Basically, energy and force are different things. A helium balloon can stay in the air indefinitely, without needing energy to do so, and without the air supplying ...

0

It seems that the width of the blade is missing. If we compare it with the 'traditional formula' for drag generated by an airfoil (assumed with $\sin \phi=1$), we have: $$F=C_D \cdot 0.5 \rho V^2 S$$ see Wikipedia Drag Coefficient. Where $C_D$ denotes the airfoils capacity to generate drag. This value does not show up in the formala given for the place, ...

6

The key point is that wings allow you to "tilt the engine" much more efficiently than actually tilting it. Tilting an engine converts the power only at 1-to-1 ratio, but wings do it better - a Boeing 747 has a lift/drag ratio of 17 at cruise speed, the wing is generating 17 times more lift than the applied engine power.

2

I think You have Your intuitions a bit scrambled. Engine and wings do not have the same function. If You need to make a parallel then wings have the same function of wheels on a car: they provide an efficent way to remain above ground by pushing down on some medium. Wheels push on ground and have no problem keeping your car above it even when the car is ...

4

The important quantity in determining the effectiveness of a wing is its lift to drag ratio. It turns out that the key contributer to a large lift to drag ratio is a large wing span ($b$ in the below equation). As such the large wings on the aircraft can be far more efficient at generating maximum lift with minimal drag that the smaller "wings" of the ...

1

Wings have a much larger surface area than engines, so they are better at preventing a plane's gravitational potential energy from being converted into downward kinetic energy. A plane with gliders instead of wings and engines pointed downward at an angle should be just as energy efficient as a regular plane. The glider might be less practical because the ...

5

It is merely a long comment but hopefully it gets your intuition on the right track. I try to detail the physical part of the reason why having fixed wings is a good thing: You basically ask, why are airplanes more efficient (hence people still produce them despite their less nice maneuverability) well you probably noticed helicopter rotors work pretty ...

0

Use an incense stick to test. You will find that the only turbulence is from the heated air (smoke) rising from the stick, or from moving the stick around. If you can cool the smoke to room temperature without heating the area of interest, you are likely to get layers unless the room or outdoor area has no temperature gradient with height. Where is gets ...

1

Theoretically, the radial distribution should be symmetric. However, in jets in general it is well know that there will be a transition from purely laminar (i.e. symmetry preserving) to purely turbulent (breaks the symmetry). Have a look at this video. The presence of the plasma induces the turbulence at much earlier in space compared to when it is absent. ...

Top 50 recent answers are included