How Aeroplanes Fly and Propellers Pull
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Originally Posted by Mugi
Nice link. Thanks, HWD. Convincing but again (and I'm playing devil's advocate here) hints at more magic and mumbo jumbo.
Now I have to believe that because the air in a vortex I left behind in my wake meets the ground, I get a magic drag reduction, therefore lift boost
But I left that air way behind - it's 30m gone, history etc...
Now I have to believe that because the air in a vortex I left behind in my wake meets the ground, I get a magic drag reduction, therefore lift boost
But I left that air way behind - it's 30m gone, history etc...
Put a lid on a vessel so that the lid is slightly smaller than the vessel.
The lid could sink into the vessel, because the lid is not actually stuck to the walls - there is a gap all around. But the gap is narrow everywhere. Therefore the air in the vessel is compressed until it supports the weight of the lid and will only slowly escape through the gap. It does not take much power to pump the air back under the lid and keep the lid suspended.
Now, imagine that the wing is in a long and narrow ditch, the wingtips nearly touching the banks. The air can escape in front and behind of the wing - but not to the sides. So, you have to do quite some work to fly along the ditch.
But the moment you leave the ditch, air gets the possibility to escape around the wingtips, too. Which means that you have to do somewhat more work to replace the air cushion beneath you.
Is this a meaningful explanation of ground effect?
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High Wing Drifter,
At the very high angles of attack the delta and swept wings seem to be able to achieve, there must be some kind of blanking effect that must reduce the pressure of the air on top of the wing to some degree.
You are missing another flow pattern which is relevant (especially to deltas) .. leading edge vortex which provides a substantial part of the lift and accounts for the high alpha. Plenty of references around - for instance. A very common real world flow visualisation in high humidity conditions as in the Concord shot in the previous link. The phenomenon is important in the flight mechanics of many living entities (for instance).
At the very high angles of attack the delta and swept wings seem to be able to achieve, there must be some kind of blanking effect that must reduce the pressure of the air on top of the wing to some degree.
You are missing another flow pattern which is relevant (especially to deltas) .. leading edge vortex which provides a substantial part of the lift and accounts for the high alpha. Plenty of references around - for instance. A very common real world flow visualisation in high humidity conditions as in the Concord shot in the previous link. The phenomenon is important in the flight mechanics of many living entities (for instance).
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Cause and consequence
All these discussions are really a cause and consequence discussion.
Take this example.
Two fellas walking down the street, one keeps snapping his fingers. When asked why, he says, it's to keep the tigers away. But there are no tigers here? He answers, yes, you see, it works.
This IS really what it is all about. So many theories can explain this, and for some reason it is thought by many that one has to choose one of them only.
Cause and consequence is often a chain of consequences. Without mentioning too many theorems, laws, postulates and clever scholars, my natural explanation is this. The only one law I have to fall back to, is Newtons Third.
We generate thrust by accelerating air opposite the way we want to fly.
Speed picks up, until it balances out drag - as hitting the air molecules takes our momentum away. The energy does not dissapear, it is just transferred to these molecules we hit. No atmosphere -> no drag, but also no lift.
As speed picks up, air is deflected downwards by the wing. Any wing will do, as long as there is 1. Enough thrust 2. Stability/control. I would say even a tank can fly, given enough thrust and stability/control surfaces.
From generating thrust to obtaining the lift, so may therories/postulates/laws can be used, like the Bernoulli law causes the pressure difference, which in turn causes downwash. Or "the molecules are forced down" (like waterskis, mentioned above). How much, and why ... it matters mainly to aerodynamic engineers when the design and build aircraft, so they can predict the characteristics before they even test the first aircraft.
So even if I believe that Newtons Third, there are events that must happen before we get that far. It is not a religious you-have-to-pick-one-law-only thing!
Another similar cause and consequence discussion is the one about ground effect. I have seen about 5 different explanations to it. The one I thing best describes it (though they are probably all true), is that the wake turbulence is reduced by the ground stopping the turning motion at the wingtips, and when less molecules are deflected the wrong way (up instead of down), less molecules must be accelerated down. Knowing that, you can call it an air cushion, tilting the lift vector, reducing the downwash, air that cannot escape as easy etc.
Take this example.
Two fellas walking down the street, one keeps snapping his fingers. When asked why, he says, it's to keep the tigers away. But there are no tigers here? He answers, yes, you see, it works.
This IS really what it is all about. So many theories can explain this, and for some reason it is thought by many that one has to choose one of them only.
Cause and consequence is often a chain of consequences. Without mentioning too many theorems, laws, postulates and clever scholars, my natural explanation is this. The only one law I have to fall back to, is Newtons Third.
We generate thrust by accelerating air opposite the way we want to fly.
Speed picks up, until it balances out drag - as hitting the air molecules takes our momentum away. The energy does not dissapear, it is just transferred to these molecules we hit. No atmosphere -> no drag, but also no lift.
As speed picks up, air is deflected downwards by the wing. Any wing will do, as long as there is 1. Enough thrust 2. Stability/control. I would say even a tank can fly, given enough thrust and stability/control surfaces.
From generating thrust to obtaining the lift, so may therories/postulates/laws can be used, like the Bernoulli law causes the pressure difference, which in turn causes downwash. Or "the molecules are forced down" (like waterskis, mentioned above). How much, and why ... it matters mainly to aerodynamic engineers when the design and build aircraft, so they can predict the characteristics before they even test the first aircraft.
So even if I believe that Newtons Third, there are events that must happen before we get that far. It is not a religious you-have-to-pick-one-law-only thing!
Another similar cause and consequence discussion is the one about ground effect. I have seen about 5 different explanations to it. The one I thing best describes it (though they are probably all true), is that the wake turbulence is reduced by the ground stopping the turning motion at the wingtips, and when less molecules are deflected the wrong way (up instead of down), less molecules must be accelerated down. Knowing that, you can call it an air cushion, tilting the lift vector, reducing the downwash, air that cannot escape as easy etc.
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The question is "why planes fly".
There are only a few things to understand:
(1) There are no pulling forces in explaining how a plane flies.
Unfortunately much of the models and mathematics developed came before the atomic theory of matter was finally accepted (thanks to Einstein) and developed. As a crude approximation fluid equations seemed to answer most questions and deliver reasonable results.
This has continued to this date. Aerodynamics is a series of approximations and cludges. Mixtures of different mathematical models (which sort of work .. mostly) and empirical results (such as certain aerofoils are better than others for a particular flight regime).
However misconceptions continue to this day .. such as:
(a) The Coanda effect. This is a symptom of something else that is more fundamental, not a thing in itself. It has no contribution to flight whatsoever (despite what NASA says).
(b) Low pressure "pulls" something up (see NASA site for this explanation ... they should know better). Air does not suck. It has no attactive force whatsoever (well I suppose there is a little gravity or some interactions between oxygen atoms and the metal of the wing .. sod all of anything).
(2) You can explain everything by understanding 2 things:
(a) Air is full of molecules, each with mass and speed and there are a lot of them.
(b) Each molecule is moving, but at different speeds and different directions.
Therefore, air exert a force in all directions, up, down, sideways, at an angle.
A wing (or barn door) when stationary has all the forces balanced. Gravity pulling down, air pressure pushing up, air pressure pushing down. What we call pressure is simply the sum of all the forces of all the air molecules bouncing off the wing. Straight foward Newton.
When we start moving and the wing (or barn door) is angled up, we start bouncing off the air molecules, pushing a lot of them down. The net transfer creates a force upwards (lift).
We also push some molecules forward (remember they are coming in from all directions and at different speed, i.e. different momentums). This is what we call induced drag. Because the wing is angled we can't help pushing some of them forward rather than down, thus wasting energy.
What happens above the wing is a little more subtle. The air is pushing down at all times by molecules (M for short) hitting the wing. But remember, not all air M travel at the same speed (it actually follows a normal distribution). The average speed of an air molecule (at 0C and sea level is about 1600 km/hour. However some are far faster, some are slower, some are virtually stationary.
The wing is angled, as it moves forward the molecules have further to travel to hit the top of it (a simple diagram will show this, basically the surface moves away from it) and hence bounce off it and exert a force downwards. The fastest ones will hit quickly (which is why there is still air there) the slowest will take longer, so overall there is less air there and hence less momentum transfer (=force) at the beginning of the upper part of the wing. At sub sonic speeds and a reasonable sized wing most of the molecules will eventually hit the wing, exerting a force downwards (some will miss it entirely).
So we have a pushing upwards from below, but less of a push downwards from above (particularly at the beginning of the wing).
Sum up the forces .. and a plane flies if you go fast enough.
This explains why the majority of the lift comes from the beginning of the wing (or barn door), there are more net upward forces (upwards + less downwards).
Next post(s):
- Why there are wingtip vortices, why wingtips have less lift, why vortices have nothing to do with loss of lift or drag.
- Why temperature is important to describe why there is less l/d drag issues at height (= why can we go faster the higher we go).
- Why does air flows (= sum total movements of air M) seem to curve.
There are only a few things to understand:
(1) There are no pulling forces in explaining how a plane flies.
Unfortunately much of the models and mathematics developed came before the atomic theory of matter was finally accepted (thanks to Einstein) and developed. As a crude approximation fluid equations seemed to answer most questions and deliver reasonable results.
This has continued to this date. Aerodynamics is a series of approximations and cludges. Mixtures of different mathematical models (which sort of work .. mostly) and empirical results (such as certain aerofoils are better than others for a particular flight regime).
However misconceptions continue to this day .. such as:
(a) The Coanda effect. This is a symptom of something else that is more fundamental, not a thing in itself. It has no contribution to flight whatsoever (despite what NASA says).
(b) Low pressure "pulls" something up (see NASA site for this explanation ... they should know better). Air does not suck. It has no attactive force whatsoever (well I suppose there is a little gravity or some interactions between oxygen atoms and the metal of the wing .. sod all of anything).
(2) You can explain everything by understanding 2 things:
(a) Air is full of molecules, each with mass and speed and there are a lot of them.
(b) Each molecule is moving, but at different speeds and different directions.
Therefore, air exert a force in all directions, up, down, sideways, at an angle.
A wing (or barn door) when stationary has all the forces balanced. Gravity pulling down, air pressure pushing up, air pressure pushing down. What we call pressure is simply the sum of all the forces of all the air molecules bouncing off the wing. Straight foward Newton.
When we start moving and the wing (or barn door) is angled up, we start bouncing off the air molecules, pushing a lot of them down. The net transfer creates a force upwards (lift).
We also push some molecules forward (remember they are coming in from all directions and at different speed, i.e. different momentums). This is what we call induced drag. Because the wing is angled we can't help pushing some of them forward rather than down, thus wasting energy.
What happens above the wing is a little more subtle. The air is pushing down at all times by molecules (M for short) hitting the wing. But remember, not all air M travel at the same speed (it actually follows a normal distribution). The average speed of an air molecule (at 0C and sea level is about 1600 km/hour. However some are far faster, some are slower, some are virtually stationary.
The wing is angled, as it moves forward the molecules have further to travel to hit the top of it (a simple diagram will show this, basically the surface moves away from it) and hence bounce off it and exert a force downwards. The fastest ones will hit quickly (which is why there is still air there) the slowest will take longer, so overall there is less air there and hence less momentum transfer (=force) at the beginning of the upper part of the wing. At sub sonic speeds and a reasonable sized wing most of the molecules will eventually hit the wing, exerting a force downwards (some will miss it entirely).
So we have a pushing upwards from below, but less of a push downwards from above (particularly at the beginning of the wing).
Sum up the forces .. and a plane flies if you go fast enough.
This explains why the majority of the lift comes from the beginning of the wing (or barn door), there are more net upward forces (upwards + less downwards).
Next post(s):
- Why there are wingtip vortices, why wingtips have less lift, why vortices have nothing to do with loss of lift or drag.
- Why temperature is important to describe why there is less l/d drag issues at height (= why can we go faster the higher we go).
- Why does air flows (= sum total movements of air M) seem to curve.
Originally Posted by MustangFlyer
(b) Low pressure "pulls" something up (see NASA site for this explanation ... they should know better). Air does not suck. It has no attactive force whatsoever (well I suppose there is a little gravity or some interactions between oxygen atoms and the metal of the wing .. sod all of anything).
