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#291
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The Impossibility of Flying Heavy Aircraft Without Training
On Sat, 4 Mar 2006 10:14:36 -0700, "Matt Barrow"
wrote: "Dallas" wrote in message ink.net... "cjcampbell" Actually, he is not. Not in the US, anyway. There is no one by the name of Sagadevan currently holding a pilot certificate of any kind in the US Here he is: http://www.warpaintofthegods.com/wp/about.cfm WOW!! What a fruticake!! Too bad that, or those, mind altering experiences didn't give him a grasp of rationality and reality. Roger Halstead (K8RI & ARRL life member) (N833R, S# CD-2 Worlds oldest Debonair) www.rogerhalstead.com |
#292
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lift, wings, and Bernuolli
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#293
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lift, wings, and Bernuolli
So in Larry Niven's Ringworld, when people swim through the ring of
atmosphere to the next Integral Tree, do they set it rotating ever so slightly in the opposite direction? (Livin' in a fantasy world since nineteen-sixty-mumblemumble....) Alan Baker wrote: No. It is balanced by the downflow eventually transferring its momentum back to the earth. |
#294
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lift, wings, and Bernuolli
But it is also experiencing a constant change in momentum in the
vertical direction. That's what a force is: a change in momentum over time. No. The airplane is in level flight at constant speed. I'm with fredfighter here, but it is primarily a semantic argument having to do with frame of reference. Looking from an accelerated frame of reference (a standstill in the earth's gravitational field) one gets one answer. Looking from an unaccelerated frame of reference (freefall), one gets a different answer. To reconcile them, it is important to treat =everything= from the same frame of reference, and make appropriate adjustments. Jose -- Money: what you need when you run out of brains. for Email, make the obvious change in the address. |
#295
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lift, wings, and Bernuolli
It does that
because there are fewer air molecules transfering momentum to it from above, that there are from below. But it does not do that via a coherent stream of air. I guess I should have appended this to the previous response... This is correct. However, a coherent stream of air is not necessary for this: The movement of the plane towards the earth is transferred to movement of the air towards the earth, which it does until it eventually transfers its momentum back *to* the earth, leaving the system with the same relative momentum with which it began. to also be correct. A coherent stream of air is not required, nor is it what I am proposing. A fluid can transmit force without flow in the conventional sense. That is the basis for hydraulics. We are not really talking about "flow in the conventional sense", we are talking about microscopic collisions. Flow may be involved (as in the flow that causes upwash upflight) but it needn't be (as in the case of the microscopic dribbler). The downflow observed from the wing initiates above the wing and flows down behind the wing after the wing has passed. It is not the air that suppors the wing. Well, the only air that supports the wing is are the molecules that impact it from below. They not only support the wing, they also fight against the molecules impacting from above. They win, because there are more of them. There are more of them because of downflow and the collisions it causes. Well then if the downflow is NOT balanced by upflow why doesn't the upper atmosphere run out of air? Because the wing is not of infinite weight. The upper atmosphere in fact =is= deprived of air while the airplane is in flight... that air is squeezed down below the wing, increasing the pressure on the surface of the earth, in an amount exactly equal to the weight of the airplane (divided by the area of the earth). If a sufficient (huge!) number of aircraft took to the air, the upper atmosphere would become measurably thinner. Maybe we should get a grant to do this experiment using general aviation aircraft - for the good of Science and the benefit of GA pilots. Jose -- Money: what you need when you run out of brains. for Email, make the obvious change in the address. |
#296
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lift, wings, and Bernuolli
So in Larry Niven's Ringworld, when people swim through the ring of atmosphere to the next Integral Tree, do they set it rotating ever so slightly in the opposite direction?
Yep. Jose -- Money: what you need when you run out of brains. for Email, make the obvious change in the address. |
#297
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lift, wings, and Bernuolli
Matt Whiting wrote: wrote: A fluid can transmit force without flow in the conventional sense. That is the basis for hydraulics. I guess it depends on what you mean by "conventional sense." Nothing can transmit a force without some deflection. Some molecules in the fluid have to move in order to generate a force and typically movement of a fluid, however minute, is flow. One does not generally refer to compression as flow. A fluid can transmit force without molecules flowing from the point of origin to the point of application. -- FF |
#298
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lift, wings, and Bernuolli
Jose wrote: It does that because there are fewer air molecules transfering momentum to it from above, that there are from below. But it does not do that via a coherent stream of air. I guess I should have appended this to the previous response... This is correct. However, a coherent stream of air is not necessary for this: The movement of the plane towards the earth is transferred to movement of the air towards the earth, which it does until it eventually transfers its momentum back *to* the earth, leaving the system with the same relative momentum with which it began. to also be correct. A coherent stream of air is not required, nor is it what I am proposing. I inferred coherent flow from 'downwash'. Some persons, perhpas not yourself, pointed to disturbances on the surface by low flying aricraft as evidence of downwash. That sounds like coherent flow. A fluid can transmit force without flow in the conventional sense. That is the basis for hydraulics. We are not really talking about "flow in the conventional sense", we are talking about microscopic collisions. Flow may be involved (as in the flow that causes upwash upflight) but it needn't be (as in the case of the microscopic dribbler). When we are discussing the microscopic transmission of momenta between air molecules whic is the basis for presure, yes. Is that what you mean by 'downwash' or downflow, as opposed to something that involves a flow of mass? The downflow observed from the wing initiates above the wing and flows down behind the wing after the wing has passed. It is not the air that suppors the wing. Well, the only air that supports the wing is are the molecules that impact it from below. They not only support the wing, they also fight against the molecules impacting from above. They win, because there are more of them. There are more of them because of downflow and the collisions it causes. Then it doesn't matter which way the air above the wing flows. If the air flows sideways, you still have lift. It doesn't have to flow down. Well then if the downflow is NOT balanced by upflow why doesn't the upper atmosphere run out of air? Because the wing is not of infinite weight. The upper atmosphere in fact =is= deprived of air while the airplane is in flight... that air is squeezed down below the wing, increasing the pressure on the surface of the earth, in an amount exactly equal to the weight of the airplane (divided by the area of the earth). I think that the downflow dispaces other air which flows up to replace it--conserving momentum and mass. -- FF |
#299
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lift, wings, and Bernuolli
I inferred coherent flow from 'downwash'.
That coherent flow is not necessary does not mean that coherent flow does not exist. My point is that the downwash does not have to be directly from the wing to the earth. It can be very indirect - in a multiple collision scenario, the existance of new momentum somewhere imples the existance of opposte new momentum elsewhere, mediated by collisions which may or may not be "coherent", however you wish to define it. Momentum is conserved. Always. When we are discussing the microscopic transmission of momenta between air molecules whic is the basis for presure, yes. Is that what you mean by 'downwash' or downflow, as opposed to something that involves a flow of mass? There is downwash, involving a "coherent" acceleration of mass downwards. Due to an increase in microscopic collisions below (and a scarcity of them above), there is an incoherent transfer of momentum (called pressure) to the surrounding air (and ultimately to the earth). This leads to a condition described as "low pressure above, high pressure beneath", or equivalently described as "less momentum transferred via collisions above, more momentum transferred via collisions below", which supports the wing, propping it up again and again as it tries to succumb to gravity. We call this lift. There are some neat bulk equations which help quantify this, which come embodied in a concept which is useful for understanding this in some contexts. However, an equivalent (newtonian) concept is more useful for understanding in other contexts, and explains a few things that are not addressed by the B word. Then it doesn't matter which way the air above the wing flows. If the air flows sideways, you still have lift. No, at least not directly. If there is less momentum transferred from above than from below, you have lift. This comes from lower pressure above and higher pressure below. How you get that is ultmately Newtonian, not magical. Once Newton has his say, Bernoulli can reformulate it in a useful bulk form. Consider a flying saucer, composed solely of two disks with no appreciable space between them. The one below does not spin, the one above spins rapidly. Should there be lift? Why? Does it matter if the top disk is rough or smooth? The upper atmosphere in fact =is= deprived of air while the airplane is in flight... that air is squeezed down below the wing, increasing the pressure on the surface of the earth, in an amount exactly equal to the weight of the airplane (divided by the area of the earth). I think that the downflow dispaces other air which flows up to replace it--conserving momentum and mass. What happens to the downward momentum of the downflowing air when this happens? The displaced air, flowing upwards, has acquired upward momentum - where did that come from? (and so far, conservation of mass has not been an issue) Jose -- Money: what you need when you run out of brains. for Email, make the obvious change in the address. |
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