Hot answers tagged aircraft
45
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 ...
27
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 ...
24
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).
15
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 ...
14
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 ...
11
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 ...
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
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 ...
8
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, ...
6
This is known as Ground Effect. Not to be confused with flaring, which is a technique used by pilots to gain lift by increasing the angle of attack as airspeed decreases.
Technicality, you can flare an aircraft at any altitude. The higher the altitude, the faster the airspeed of which you can flare an aircraft before stalling due to air thinning as ...
6
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 ...
6
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 ...
6
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 ...
6
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. ...
5
Ground effect is caused by the increased pressure under the wing because the vortex at the end of the wing which normally just twists behind the wingtips for long distances runs into the ground itself thereby increasing the pressure under the wing as a whole.
You can think of why the vortex forms if you think of the end of the wing as deflecting air ...
5
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. ...
5
You've seen biplanes.
Almost anything can fly if it has enough area, an angle of attack, and is nose-heavy.
I've seen a model airplane in the shape of Snoopy's doghouse!
It flew just fine.
If you're wondering why nose-heavy it's this.
Look at a normal plane with a main wing and a tail. It's nose heavy.
The main wing holds the weight by lifting up.
The tail ...
5
Most of the lift comes from the main wing, and in fact the tail lifts down, so the main wing also has to support that.
(That's for a stability reason.)
The lift of a wing is roughly proportional to two things:
angle of attack, and
airspeed squared
so, the slower an airplane is flying, the more it raises the nose.
You will notice this the next time you ...
5
You're correct, the ice block will not turn automatically. It will require a torque. In aviation this is basically what is called coordinating a turn. With an airplane, if the pilot does not provide the necessary coordinating torque via rudder/elevator inputs, the torque will be generated automatically via the weathervane effect, which tends to align the ...
5
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 ...
5
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 a snorcel for the engines that charge the batteries.
Then it ...
5
There's quite a few factors at play here, and granted, it's been a while since my last flight dynamics class :)
After a few iterations with the other answerers below, here is a (still not yet complete) list of reasons the effect will occur:
While yawing, the left and right wings will have slightly different speeds. This difference in wing speed causes ...
5
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 ...
4
The lead bird does gain something from the V - it's the same principle as the spoiler on the back of a car.
The vortices from the wings of the bird would create a low pressure region immediately behind it, which in simple terms sucks the bird back. The following bird prevents this vortex by splitting the upper and lower air flows with it's wings and so on - ...
4
One thing in your argument is that more lift, means a higher speed. This may not be what airliners do. Airplanes (at long flights) choosse their cruising altitude based on their weight. Higher weight means lower altitude. I think this should be included in the incremental cost calculation of additional piece of luggage.
First, simple Google hit: ...
4
Lift is roughly proportional to angle of attack, and to speed squared. As a pilot, you instinctively balance these two.
ADDED: Like if you suddenly drop a heavy weight, making the plane lighter, its lift isn't any less, so it starts to accelerate upward (climb). You notice this and either push the nose down with the trim wheel (lessen the angle of attack, ...
4
Considering the tag "homework" I know the solution that was expected. Bernoulli law:
$$\frac{\rho v_{under}^2}{2} + p_{under} = \frac{\rho v_{over}^2}{2} + p_{over}$$
$v_{over}$ and $v_{under}$ are the air flow speeds over and under the wing respectively, $p$ is pressure, $\rho$ is the air's density. The desired lift force should be equal to the plane's ...
4
I will make it an answer instead of a comment.
My guess is that the convergence is an optical illusion. This plane is flying at a level where the relative humidity is small.This means that the trail evaporates, it will evaporate faster from outside (the trail itself is humid) and finally what is left is merged the dissolution giving the impression of ...
4
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 ...
3
There is no simple equation for how a paper airplane flies like there is for a simple projectile because the airplane can interact with the air in complicated ways.
The physics of a paper airplane is described by Newton's laws of motion. These laws apply to both the airplane and the air it travels through. The plane is acted on by a constant gravitational ...
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