# How many physical mechanisms for flight are there
**URL:** https://boards.straightdope.com/t/how-many-physical-mechanisms-for-flight-are-there/763030
**Category:** Factual Questions
**Created:** [August 16, 2016, 11:59pm UTC](https://boards.straightdope.com/t/how-many-physical-mechanisms-for-flight-are-there/763030 "2016-08-16T23:59:54Z")
**Posts on this page:** 20
**Page:** 2
### Author:  [@scr4](https://boards.straightdope.com/u/scr4)
#### Post date: [August 17, 2016, 4:20am UTC](https://boards.straightdope.com/t/how-many-physical-mechanisms-for-flight-are-there/763030/21 "2016-08-17T04:20:54Z")
I would categorize all those into 3:
1. Buoyancy (lighter-than-air craft, includes balloons, airships, hot-air balloons)
2. Pushing down against air. Which includes all airplanes, helicopters, birds, insects, etc.
3. Throwing something downwards - something stored onboard, not air. Just rockets, I guess. (Includes bottle rockets.)
I could be persuaded to add a 4th category, which is to be lightweight and be blown around by wind. Dandelion seeds, ballooning spiders, etc.
Solar sails use photon pressure, which is so weak that its effects can only be measured in vacuum. So it won’t work in the atmosphere.
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### Author:  [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)
#### Post date: [August 17, 2016, 4:35am UTC](https://boards.straightdope.com/t/how-many-physical-mechanisms-for-flight-are-there/763030/22 "2016-08-17T04:35:07Z")
> [@Wesley\_Clark](#):
>
> I considered 1 and 7 to be different because they are using different mechanisms to cause a gas to rise. One is using lighter than air elements like hydrogen and helium, while the other is using temperature differences to cause a gas to rise.
>
> So both cause a gas to rise, but the mechanism is totally different. If the world suddenly runs out of gaseous hydrogen and helium tomorrow, hot air balloons will still work but zeppelins will not.
I think it’s the same effect: an object being less dense than the air around it so it rises. It’s just two different ways of achieving that effect.
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### Author:  [@Little\_Nemo](https://boards.straightdope.com/u/Little_Nemo)
#### Post date: [August 17, 2016, 4:36am UTC](https://boards.straightdope.com/t/how-many-physical-mechanisms-for-flight-are-there/763030/23 "2016-08-17T04:36:13Z")
> [@scr4](#):
>
> 1. Pushing down against air. Which includes all airplanes, helicopters, birds, insects, etc.
I don’t think this is an accurate description of how an airplane works.
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### Author:  [@Folly](https://boards.straightdope.com/u/Folly)
#### Post date: [August 17, 2016, 4:38am UTC](https://boards.straightdope.com/t/how-many-physical-mechanisms-for-flight-are-there/763030/24 "2016-08-17T04:38:48Z")
> [@Richard\_Pearse](#):
>
> By “top” I mean in the direction the lift is being generated.
Me too.
> [@Richard\_Pearse](#):
>
> Turn a wing upside down and give it sufficient angle of attack to counteract any asymmetry in its shape (many aerobatic airfoils are symmetrical anyway) and it will create lift as well as have the air moving over what used to be the bottom of the wing faster than the air moving over what used to be the top.
I’m having trouble picturing this. Lets say it is not a symmetrical wing and you are just supplying sufficient power to stay in the air. The air over the top\* is still faster than the air under the bottom?
\*the bottom in ordinary flight.
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### Author:  [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)
#### Post date: [August 17, 2016, 5:03am UTC](https://boards.straightdope.com/t/how-many-physical-mechanisms-for-flight-are-there/763030/25 "2016-08-17T05:03:22Z")
> [@Richard\_Pearse](#):
>
> I’d be interested if you can show me some examples of a wing creating lift but not having the top air moving faster.
A stalled wing still produces lift, and the flow on the top is not necessarily moving more quickly–it may even be moving opposite to the flow on the bottom! Massively overpowered RC planes don’t care, as the Newtonian action on the bottom surface is enough to keep them flying.
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### Author:  [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)
#### Post date: [August 17, 2016, 5:12am UTC](https://boards.straightdope.com/t/how-many-physical-mechanisms-for-flight-are-there/763030/26 "2016-08-17T05:12:26Z")
> [@Folly](#):
>
> Me too.
>
> I’m having trouble picturing this. Lets say it is not a symmetrical wing and you are just supplying sufficient power to stay in the air. The air over the top\* is still faster than the air under the bottom?
>
> \*the bottom in ordinary flight.
Angle of attack is more powerful, generally, than wing shape.
Take a flat plate and hold it at a positive angle to the airflow and it will turn the flow downwards and create some lift. It will work the same upside down or right way up as it is symmetrical.
Shape the plate so it is curved over the top and flat or has a lesser curve at the bottom and it will create more lift for the same positive angle the original flat plate had. It is working more efficiently and you could get the same lift as before with less angle and therefore less drag.
Turn this new curved plate upside down and hold it at the same positive angle and you will get less lift or possibly none or zero lift. Increase the angle though and you can get the lift back. In this case the angle of attack is working against the wing shape, but angle of attack is king and for a normal wing shape you can get enough angle of attack to still make the wing work.
In the above example the air over the top (what used to be the bottom) is moving faster than the air over the bottom. You are having to use a large angle of attack to achieve this but it is still happening.
Make the wing curved but symmetrical and it will work the same upside down as it does right way up but will not be as efficient as the asymmetrical wing in either case.
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### Author:  [@Richard\_Pearse](https://boards.straightdope.com/u/Richard_Pearse)
#### Post date: [August 17, 2016, 5:17am UTC](https://boards.straightdope.com/t/how-many-physical-mechanisms-for-flight-are-there/763030/27 "2016-08-17T05:17:48Z")
> [@Dr.Strangelove](#):
>
> A stalled wing still produces lift, and the flow on the top is not necessarily moving more quickly–it may even be moving opposite to the flow on the bottom! Massively overpowered RC planes don’t care, as the Newtonian action on the bottom surface is enough to keep them flying.
Yes fair point. I’d have thought that the small amount of Newtonian action was not enough and that the additional lift was actually coming from the engine thrust though?
What about for wings in their normal angle of attack range?
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### Author:  [@Dr.Strangelove](https://boards.straightdope.com/u/Dr.Strangelove)
#### Post date: [August 17, 2016, 5:33am UTC](https://boards.straightdope.com/t/how-many-physical-mechanisms-for-flight-are-there/763030/28 "2016-08-17T05:33:26Z")
> [@Richard\_Pearse](#):
>
> I’d have thought that the small amount of Newtonian action was not enough and that the additional lift was actually coming from the engine thrust though?
A reasonable quesetion. Some RC planes can definitely just hover on pure engine thrust. But a plane flying with, say, a 45 degree AoA must have a reasonable contribution from the wing. The upforce from engine thrust is at the front of the plane; for the body to be at an angle requires a force further back, both from the wing and tail. This demands some vector diagrams :).
> [@Richard\_Pearse](#):
>
> What about for wings in their normal angle of attack range?
I’d posit that what you said is accurate for pure laminar flow. Also, I’d agree with the overall message that there’s no way to cleanly separate the different effects–Bernoulli is just a special case of Newton.
What’s certainly true is that if you covered the entire wing with pressure sensors, you could compute the exact amount of lift by integrating the vector contribution of each one. The question then is the best explanation for the varying pressure. I suspect that for laminar flow, the Bernoulli equation is exact and hence an adequate explanation. Newton of course always works but isn’t necessarily as intuitive.
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### Author:  [@Mangetout](https://boards.straightdope.com/u/Mangetout)
#### Post date: [August 17, 2016, 6:52am UTC](https://boards.straightdope.com/t/how-many-physical-mechanisms-for-flight-are-there/763030/29 "2016-08-17T06:52:01Z")
Some very small creatures climb through the air exploiting its viscosity. Still an action/reaction thing, so technically similar to both jets and wings.
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### Author:  [@Leo\_Bloom](https://boards.straightdope.com/u/Leo_Bloom)
#### Post date: [August 17, 2016, 12:59pm UTC](https://boards.straightdope.com/t/how-many-physical-mechanisms-for-flight-are-there/763030/30 "2016-08-17T12:59:08Z")
> [@Richard\_Pearse](#):
>
> Not unrelated at all. And not “deflected”. The air is accelerated downward. The air above and below the wing is turned (accelerated). An inescapable side affect is that the wing across the top is moving faster and the lower air is moving slower, or perhaps an inescapable side affect of the top air moving faster and the lower air moving slower is that the air is turned.
>
> I’d be interested if you can show me some examples of a wing creating lift but not having the top air moving faster.
>
> To **markn+** , Bernoulli is not a “small part of the lift”, it is one way of measuring ALL of the lift. Measuring acceleration of air mass is another way of measuring ALL of the lift. It is not a bit of this and a bit of that, they are just different ways of measuring the same effect.
>
> I suggest you both educate yourselves here [https://www.grc.nasa.gov/www/k-12/airplane/bernnew.html](https://www.grc.nasa.gov/www/k-12/airplane/bernnew.html)
>
> A key quote:
>
> My bolding.
Nice site/cite.
Their whole “what is lift” section, complete with a mess of “incorrect theories,” begins here: [https://www.grc.nasa.gov/www/k-12/BGA/Monroe/lift\_theories\_act.htm](https://www.grc.nasa.gov/www/k-12/BGA/Monroe/lift_theories_act.htm)
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [August 17, 2016, 1:04pm UTC](https://boards.straightdope.com/t/how-many-physical-mechanisms-for-flight-are-there/763030/31 "2016-08-17T13:04:28Z")
The Bernoulli effect is real, but it doesn’t mean what most people think it means. Bernoulli’s equation is just a way of re-writing Newton’s Second Law that’s easier to use with fluids (note that this is a definition of “easier” which includes calculus). The lift of an airplane is 100% due to air being accelerated downwards (call it “pushed” or “deflected” or whatever), as described by Newton’s laws and the Bernoulli equation.
What most people mean when they refer to the Bernoulli effect is actually called the Coanda Effect. Which is also real, and which is often relevant to some degree for wings, but is not usually a big deal.
As for wings where the air is not moving faster over the top surface, I can do one better than that: You can make a wing that doesn’t even have a top surface at all. Picture a wind tunnel with a hole cut in its top. Fit a large-cross-section piston into that hole. Cut the end of the piston at a bevel, such that the downwind side extends further into the wind tunnel than the upwind side. Blow air through the tunnel, and there will be a lift force on that piston pushing it out of the tunnel. It’ll probably be horribly inefficient, at least until you get up to supersonic speeds, but it’ll work, and produce nonzero lift at any speed.
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### Author:  [@TriPolar](https://boards.straightdope.com/u/TriPolar)
#### Post date: [August 17, 2016, 1:33pm UTC](https://boards.straightdope.com/t/how-many-physical-mechanisms-for-flight-are-there/763030/32 "2016-08-17T13:33:43Z")
> [@Chronos](#):
>
> As for wings where the air is not moving faster over the top surface, I can do one better than that: You can make a wing that doesn’t even have a top surface at all. Picture a wind tunnel with a hole cut in its top. Fit a large-cross-section piston into that hole. Cut the end of the piston at a bevel, such that the downwind side extends further into the wind tunnel than the upwind side. Blow air through the tunnel, and there will be a lift force on that piston pushing it out of the tunnel. It’ll probably be horribly inefficient, at least until you get up to supersonic speeds, but it’ll work, and produce nonzero lift at any speed.
A plane like that was actually built. The engine nacelles and propellers sat in U shaped channels in the middle of each wing. It was able to lift vertically off the ground with some forward motion. It was much more of the traditional concept of Bernoulli’s principle creating low pressure above the wing.
[Here is a picture of what may be the first such plane.](http://www.aerofiles.com/WRCuster.jpg)Actually flew.
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### Author:  [@Chronos](https://boards.straightdope.com/u/Chronos)
#### Post date: [August 17, 2016, 2:34pm UTC](https://boards.straightdope.com/t/how-many-physical-mechanisms-for-flight-are-there/763030/33 "2016-08-17T14:34:15Z")
A fascinating design, to be sure, but I’m not sure how that relates to the design I described.
Here’s an ASCII diagram of what I’m envisioning:
```auto
~~~~~~~~~
| |
| |
---------------+ +--------------
| |
| |
\ W |
\ I |
--> \ N |
--> \ G |
--> \ |
Wind \ |
\|
--------------------------------------
Wind tunnel
```
(that’s a very high angle of attack, of course, but such are the limitations of backslashes)
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### Author:  [@TriPolar](https://boards.straightdope.com/u/TriPolar)
#### Post date: [August 17, 2016, 2:43pm UTC](https://boards.straightdope.com/t/how-many-physical-mechanisms-for-flight-are-there/763030/34 "2016-08-17T14:43:58Z")
Ok, I see what you mean. There are bottom surface optimized airfoils but what you describe sounds like the high angle of attack flight of supersonic planes at high altitude where the wing is just deflecting air downwards.
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### Author:  [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)
#### Post date: [August 17, 2016, 2:48pm UTC](https://boards.straightdope.com/t/how-many-physical-mechanisms-for-flight-are-there/763030/35 "2016-08-17T14:48:17Z")
> [@Dr.Strangelove](#):
>
> A reasonable quesetion. Some RC planes can definitely just hover on pure engine thrust.
In that situation the propeller blades themselves serve as the wings, in that they are airfoil shapes deflecting air to produce a reaction force. No different than a helicopter or RC drone.
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### Author:  [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)
#### Post date: [August 17, 2016, 3:33pm UTC](https://boards.straightdope.com/t/how-many-physical-mechanisms-for-flight-are-there/763030/36 "2016-08-17T15:33:30Z")
Differences in fluid densities … density is proportional to pressure … thus all the OP’s examples (except #6) can be explained by fluid motion to equalize it’s density. The airfoil “cutting” through the air creates higher density below (higher pressure) than above so the air pushes up on the airfoil. The hot air inside a balloon is less dense than outside (equal pressure) creating buoyancy.
As far as solar sails, I was not aware this has been demonstrated to actually work.
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### Author:  [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)
#### Post date: [August 17, 2016, 3:41pm UTC](https://boards.straightdope.com/t/how-many-physical-mechanisms-for-flight-are-there/763030/37 "2016-08-17T15:41:51Z")
I have wondered if it were possible to create some kind of very light weight solar wings that simply absorbed heat on the top side that would concentrate with in the hollow wing itself and somehow use that heated air for thrust. I doubt you get get much perofrmance out of it and it might be more of a solar assisted glider. Watching the turbines of roof vents spin makes me think there might be some recoverable energy there.
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### Author:  [@Machine\_Elf](https://boards.straightdope.com/u/Machine_Elf)
#### Post date: [August 17, 2016, 3:52pm UTC](https://boards.straightdope.com/t/how-many-physical-mechanisms-for-flight-are-there/763030/38 "2016-08-17T15:52:44Z")
> [@watchwolf49](#):
>
> Differences in fluid densities … density is proportional to pressure … thus all the OP’s examples (except #6) can be explained by fluid motion to equalize it’s density. The airfoil “cutting” through the air creates higher density below (higher pressure) than above so the air pushes up on the airfoil.
Except lift-generating shapes also work in water and other incompressible fluids. See [hydrofoils](https://en.wikipedia.org/wiki/Hydrofoil) and [diving planes.](https://en.wikipedia.org/wiki/Diving_plane) And [marine propellers.](https://en.wikipedia.org/wiki/Propeller)
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### Author:  [@watchwolf49](https://boards.straightdope.com/u/watchwolf49)
#### Post date: [August 17, 2016, 4:47pm UTC](https://boards.straightdope.com/t/how-many-physical-mechanisms-for-flight-are-there/763030/39 "2016-08-17T16:47:51Z")
> [@Machine\_Elf](#):
>
> Except lift-generating shapes also work in water and other incompressible fluids. See [hydrofoils](https://en.wikipedia.org/wiki/Hydrofoil) and [diving planes.](https://en.wikipedia.org/wiki/Diving_plane) And [marine propellers.](https://en.wikipedia.org/wiki/Propeller)
Water is compressible, it just takes one hell of a force to do so. That same force is returned when we decompress. Certainly water obeys Bernoulli’s principle, so that can be used to describe 'foils behavior. I’m using density as the causative agent because hot air balloons have equal pressure inside and out, it’s only density that is different.
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### Author:  [@drewder](https://boards.straightdope.com/u/drewder)
#### Post date: [August 17, 2016, 4:57pm UTC](https://boards.straightdope.com/t/how-many-physical-mechanisms-for-flight-are-there/763030/40 "2016-08-17T16:57:58Z")
> [@watchwolf49](#):
>
> Water is compressible, it just takes one hell of a force to do so. That same force is returned when we decompress. Certainly water obeys Bernoulli’s principle, so that can be used to describe 'foils behavior. I’m using density as the causative agent because hot air balloons have equal pressure inside and out, it’s only density that is different.
Yes, if water didn’t compress it wouldn’t transmit sound waves.
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