# Tag Info

115

A short summary of the paper mentioned in another answer and another good site. Basically planes fly because they push enough air downwards and receive an upwards lift thanks to Newton's third law. They do so in a variety of manners, but the most significant contributions are: The angle of attack of the wings, which uses drag to push the air down. This ...

71

From Stick and Rudder by Wolfgang Langewiesche, page 9, published 1944: The main fact of all heavier-than-air flight is this: the wing keeps the airplane up by pushing the air down. It shoves the air down with its bottom surface, and it pulls the air down with its top surface; the latter action is the more important. But the really ...

67

This answer is nothing more than a variation of Sklivv's answer. I simply wish to discuss some quantitative ideas following from Sklivv's answer and discuss what I understand (from an aerospace engineering friend) to be a common conceptual mistake - that the application of "mere surface effects" and "application of Bernoulli's principle" is wrong. These "...

28

Since you asked for an explanation appropriate to an non-specialized audience, maybe this will do: "A Physical Description of Flight; Revisited" by David Anderson & Scott Eberhardt. It is a revision of the earlier "A Physical Description of Flight" (HTML version).

27

Don't forget that the aeroplane will be moving forward, so it's not relying on a vacuum filling ahead of the propellor to supply the latter with air. Now I daresay there are good engineering reasons why propellors are not efficient and even impracticable for supersonic flight, but I don't think there is a fundamental physics theoretical reason ruling them ...

26

Upside-down or right side up, flight works the same way. As you stated, the wing deflects air downward. When inverted, the pilot simply controls the the pitch of the aircraft to keep the nose up, thus giving the wings sufficient angle of attack to deflect air downwards. Most airplanes are designed with some positive angle of attack "built-in," meaning that ...

22

Aircraft rely on lift generated by interacting with the atmosphere and on using atmospheric oxygen to burn with fuel they carry. Orbits aren't stable until you are high enough that there isn't enough atmosphere to interact with, and long before that the oxygen content drops too low to be useful. So, to get to a stable orbit, you will need rockets ...

21

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 ...

18

There are lots of questions here that I will try to answer, hopefully I'll get to them all... Creature Comforts It's hard to "just fly higher" when you consider passenger planes. Supersonic military aircraft like the SR-71 do fly ridiculously high. It's service ceiling is 85,000 feet! But, it has the advantage that it doesn't need to keep anybody but the ...

17

Not physically, but practically there are (currently) better alternatives. The limiting issue with propellers is similar to the limiting issue with helicopters: propellers work like wing sections in that they must accelerate flow to work; when you're near the speed of sound, this means you are going to cause shocks to form, and this issue is particularly ...

15

This problem originated with passengers using electronics (they call them PED's - portable electronic devices) during flight. While all consumer electronics have to be qualified by a regulatory body (FCC, etc.) to prove they do not emit harmful interference, this doesn't mean they emit no interference especially to high gain sensitive navigation equipment. ...

15

You have the general idea right, but the following statement is subtly wrong The pressurized fuel/air mixture is ignited and this increases the pressure inside the combustion chamber even more Unlike in a piston engine, the ignition of the fuel air mixture in a turbine engine increases the mixture's volume while pressure stays relatively constant. ...

14

Idealizing the plane's wheels as frictionless, the thrust from the propeller accelerates the plane through the air regardless of the treadmill. The thrust comes from the prop, and the wheels, being frictionless, do not hold the plane back in any way. If the treadmill is too short, the plane just runs of the end of it and then continues rolling towards take ...

14

There are quite a lot of reasons for this, but it's a complicated design environment, and that's why it's not always the case. Seals and cooling The inside of a plane's wings is the same fluid as the air around it, but the inside of a boat's hull is a different phase than the water outside. Basically you can never have a rotating shaft over a pressure ...

13

Colin's answer is right. Let me see if I can clarify a little bit. First, forget that old Bernoulli explanation. It's not wrong, but it confuses everybody. If you create a simple symmetrical teardrop-shaped airfoil, and place it in a wind stream, then the air will flow past it, and it you turn it at an angle to the wind, it will deflect the wind stream, ...

13

During the flight, you need to get up to use the restroom. There's one 10 rows in front of you, and another 10 rows behind you. Does it take longer to walk to the one that's moving away from you at 600 mph than the one that's moving towards you at 600 mph? No, because you're moving at 600 mph right along with it -- in the ground-based frame of reference. ...

13

In your own question you recognize that the Bernoulli equation is the wrong thing to apply to this situation, because obviously there are dissipative losses involved. My preferred way of looking at this is recognizing there is a lift to drag ratio that exists as a metric for aircraft. This can be 4:1 or 25:1 depending on the plane. Regardless, provided ...

13

The primary factor that determines the ability of an aircraft to takeoff is having a speed exceeding that of the liftoff speed: that is the minimum (air) speed of the aircraft to generate sufficient lift by its wings to counteract the gravitational pull from the earth. Large passenger planes at takeoff often change the wing configuration (lowers flaps etc) ...

13

There's no problem with the Bernoulli effect, only with the way it's understood and explained. It's usually explained with mistakes, like the need for asymmetrical airfoil and equal flow time above and below, and without mentioning the need to deflect the direction of airflow. Here's the best light-math explanation I've seen. Also study this section that ...

11

No. A helicopter that "stays stationary" does so in relation to the atmosphere around it and the atmosphere pretty much follows the ground underneath it. The atmosphere does not stand still while the earth rotates. If it did, we would experience constant winds on the order of 1000 km/h. That would not be pleasant.

10

This is a cost to benefit question and can only be answered by a guess in a physics board. There is a new generation of small compact reactors that could be used for powering apartment buildings The new reactor, which is only 20 feet by 6 feet, Seems compact enough, so it is not size but weight that is important, since this weight has to be lifted. ...

9

For what it's worth, even though a rocket starts its flight going straight up, once it has traveled through most of the atmosphere it soon starts to change its direction so that it spends most of its flight accelerating in the "around the earth" direction (i.e. basically horizontal). Also, to reach orbit a vehicle either has to reach a high enough speed, or ...

9

The speed of an aircraft during the flight is ideally kept constant relatively to the air surrounding the aircraft. However, in the moderate zone - between 30 and 60 degrees of latitude on both hemispheres - the dominant winds are called westerlies http://en.wikipedia.org/wiki/Westerlies for a good reason. They blow from the West to the East. So it's ...

9

Without going into the excellent and detailed mechanics explaining reaction lift that others have provided for this answer, I just want to say that contrary to popular belief/high school physics textbooks, airplanes do not fly solely on account of Bernoulli's principle. According to Walter Lewin's excellent "For the Love of Physics": "Bernoulli's principle ...

9

Rody and Mike almost got it right. :) Most aircraft are designed with swept wings. That is the primary mechanism that gives the roll effect to an airplane that may only receive a yaw input. if you look at this picture: You can see that both wings have a backwards sweep to them. Now, if you introduce a yaw to the aircraft, one wing will extend out ...

8

In general yes, you may have supersonic flow in some areas. But prop is subsonic, and should work fine till 0.9mach (~300 m/s) if it have optimal shape for that speed (some planes even had variable prop's angle of attack to get this for all speeds). So, in your case you must have excessive noise due to turbulent air flow due to non-optimal prop shape. When ...

8

Simplify. Suppose the air is still - no wind. Suppose the wheels are truly frictionless - like greased skids. (After all, that's why they have ball bearings.) The aircraft starts from a standing position, and it accelerates to rotation airspeed, about 100 km/h. It does so by thrusting against the air, not against the surface it is standing on. As it ...

8

Mythbusters did the experiment in episode 10. This blog summarises the results. The experiment was really to see if firing a bullet through the skin of the plane would cause the whole plane to burst like a balloon, and they conclusively proved that this wasn't the case. However the effects of losing a window or of major damage to the fuselage were indeed ...

7

Nuclear power is the only way to make submarines work underneath the water for long distances without coming up because of the oxygen that all other types require. And batteries for electric engines are too heavy. Historically a submarine had to go up to just below sea level to get air through the snorkel for the engines that charge the batteries. Then ...

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