100 votes
Accepted

How can a butterfly dodge the windshield of a fast moving car?

While @Nick gave a good answer (“air flows up and around the car”), that answer by itself would mean no bugs ever hit the windshield - and we know that is false. So what’s the difference between a bug ...
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  • 116k
91 votes
Accepted

Why don't helicopters use reaction wheels to counter the main rotor?

You're talking about a device (in helicopters the tail fan imparting horizontal thrust) that counteracts the torque imparted on the main rotor (and therefore on the helicopter) by the surrounding air ...
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75 votes

Why does a weather vane arrow point in the direction of the wind?

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 ...
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  • 4,761
58 votes
Accepted

Why are aerodynamic / streamlined shapes always stumpy at the front?

You are correct if your boat will only travel in a straight line. In real life the motion of the boat will often have a yaw angle, so that it is moving slightly "sideways" relative to the ...
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  • 9,842
56 votes
Accepted

Why do archery arrows tilt downwards in their descent?

The same reason objects which are heavier on one side tend to fall with the heavy side down: the tip of the arrow is denser than the rest of the arrow. The center of gravity is offset from its ...
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47 votes

Why do archery arrows tilt downwards in their descent?

Air. Conservation of angular momentum does infact dictate that whatever rotation it starts with it should end with, provided nothing else acts on it. Air allows its forward momentum to act on it. ...
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39 votes

Slipstreaming - is there a penalty?

The opposite seems to be true. There was, or maybe still is, a company that was looking into designing a system that allowed two semi trucks to "link" to each other (basically the truck in ...
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  • 1,170
36 votes

Slipstreaming - is there a penalty?

For cycling, there doesn't seem to be a penalty, but even a small advantage to the rider in front of a group. Computational fluid dynamics simulations and wind tunnel testing have shown air resistance ...
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  • 461
34 votes
Accepted

What does a wing do that an engine can't?

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 ...
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  • 116k
34 votes

Why is an airfoil shaped like a teardrop?

The airfoil shape is optimized for two features a maximal lift coefficient $c_L$, to get a big lift force a minimal drag coefficient $c_D$, to get a small drag force Maximizing the lift coefficient $...
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31 votes
Accepted

Why do all fighter jets and aerobatic airplanes have flat wings?

The "equal-time fallacy" is alive and kicking, as shown in your question and the other answers. Look, here is the best explanation I've seen about how wings work and airplanes fly: https://...
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30 votes
Accepted

What is going on in front of and behind a fan?

There is a YouTube video that visualizes the air flow around a propeller for various configurations. I caught a screen shot of a moment that more or less shows what is going on: As you can see, this ...
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  • 116k
30 votes

Why are aerodynamic / streamlined shapes always stumpy at the front?

Any speculation about what shape might be best is meaningless without specifying the flow conditions. For the keel on a boat, the main one is the Reynolds Number, a parameter that is proportional to ...
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  • 2,010
30 votes

How can a butterfly dodge the windshield of a fast moving car?

You're right that the butterfly wasn't skilfully avoiding the car. It didn't need to because it was carried over your car by the air flowing around the car. If you look at the image below of the ...
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  • 1,388
29 votes
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Why don't humans burn up while parachuting, whereas rockets do on reentry?

Re-entry velocity from LEO is $~7,800 \frac m s$, from lunar space it is as high as $~11,000 \frac m s$ [1]. Different books give the terminal velocity of a skydiver as about $56 \frac m s$ or $75 \...
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  • 403
27 votes

If an astronaut got stuck in the middle of a space station, can he propel himself towards a wall by blowing air out of his mouth?

Just a naive and very approximate calculation: If you neglect friction, google that human lung capacity is around $6\text{l}$, air density around $1\text{kg m}^{-3}$, take the inhalation time $6\text{...
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  • 5,566
24 votes

Why does fluid pressure decrease as fluid velocity increases according to Bernoulli's principle?

I just can't wrap my head around why pressure decreases as velocity increases This is a classic misunderstanding of Bernoulli's equation. What Bernoulli's equation actually says is that the velocity ...
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  • 53.4k
23 votes
Accepted

Why isn't jumping from a high altitude fatal?

It's not the falling that's fatal, it's the deceleration at the end that kills you. Something like water or concrete does this on a sub-meter distance (which requires extremely high forces). On the ...
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  • 1,277
23 votes

Would it be possible to reenter the atmosphere without a heat shield using a glider design?

Being in orbit isn't about going very high, it's about going sideways very fast. In order to get to orbit, about 80% of the energy is used in achieving orbital speed, which only 20% is used in getting ...
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23 votes

Slipstreaming - is there a penalty?

To a certain extent, this depends on the aerodynamic design of the vehicles. But for a simple "boxy" shape, both the lead and trail vehicles could gain an advantage. The rearmost vehicle ...
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  • 34.3k
22 votes

Why do all fighter jets and aerobatic airplanes have flat wings?

Is there any particular reason(s)? There are actually two reasons. Fighter jets are designed to fly at supersonic speed. Airfoil camber (that is the proper name you were looking for) helps at ...
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  • 2,396
19 votes

How does pressure of a fluid change with area, according to the continuity equation and Bernoulli's equation?

I disagree with the most voted answer, by CAGT. He says "This area is completely different to the one above", but this means nothing. The equation $p = {F \over A}$ mentioned by the author does hold, ...
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18 votes
Accepted

Does it make sense to open one window all the way when the other window is much smaller?

Each window represents a restriction to the air flow. The greater the pressure difference across the aperture, the greater the flow. An electrical analogy: each window is a resistor. The current ...
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  • 116k
18 votes
Accepted

How much more efficient is a road bike than a mountain bike?

I often wondered about these things - then I came up with a simple experiment that works for me because I have a simple bike computer (thing with a magnetic pickup on the spokes that updates my speed ...
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  • 116k
18 votes
Accepted

How do eagles fly slowly for a long time?

How does they move slowly in air, without falling down? One possibility is soaring using a ridge lift - typically a situation when the wind is approx. perpendicular to a mountain ridge. The air is ...
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  • 1,945
18 votes

Would it be possible to reenter the atmosphere without a heat shield using a glider design?

The key insight here is that you have a lot of energy, and all that energy has to be turned into heat. There's no other way to lose it. Your goal is 0 m/s at 0 km height. It doesn't really matter what ...
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  • 5,441
17 votes

Why don't humans burn up while parachuting, whereas rockets do on reentry?

The distances and speeds involved are materially different. On the scale of a parachute dive, the atmospheric density doesn't change much (and is relatively high). A parachutist quickly reaches a ...
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  • 42.8k
16 votes
Accepted

What shape has the highest drag coefficient?

According to Sighard Hoerner's Fluid Dynamic Drag, this would be the half-sphere with the open side exposed to the wind. Its drag coefficient is 1.42. A rod with a hemispherical cross section will ...
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  • 2,396
15 votes
Accepted

Why aren't airplanes like golf balls?

This is a very good question! Drag due to viscous effects can be broken down into 2 components: $$D = D_f + D_p$$ where $D$ is the total drag due to viscous effects, $D_f$ is the drag due to skin ...
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