Random facts about planes..

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pushthebutton

1,096 posts

181 months

Thursday 20th April 2017
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AVV EM said:
Due to the shape of a wing, the air going over it is slowed down, slower than the air passing under it. This produces a pressure differential between the top and bottom of the wing. The top surface being of a lower pressure than the underside therefor the wing is sucked upwards by the lower pressure. Hence why its the top of a wing that produces lift, not the bottom.
The NASA link is correct (obvs), but the Bernoulli explanation above relates to the air going over the wing accelerating and not slowing down.

CanAm

9,115 posts

271 months

Thursday 20th April 2017
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The actual "suction" on the wing is the equivalent pressure to a baby sucking on a straw. Of course you'd need an awful lot of babies on straws to cover the mainplane of a 747.

Ginetta G15 Girl

3,220 posts

183 months

Thursday 20th April 2017
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Nah, it's all about Bernoullis.

The engines produce Bernoullis and it is these that magically make the a/c fly.

Of course, in the case of engineless a/c such as gliders, what you have to do is pump the fuselage full of Bernoullis such that the glider will fly. Given that the glider is not sealed the Bernoullis gradually leak out so the glider returns to earth.










At least, that's how we always explained the miracle of flight to the Pongoes...wink

dvs_dave

8,581 posts

224 months

Thursday 20th April 2017
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AVV EM said:
Due to the shape of a wing, the air going over it is slowed down, slower than the air passing under it. This produces a pressure differential between the top and bottom of the wing. The top surface being of a lower pressure than the underside therefor the wing is sucked upwards by the lower pressure. Hence why its the top of a wing that produces lift, not the bottom.
You've got that the wrong way round. Faster moving air is lower pressure air. Due to the aerofoil shape of the wing section, the air that flows over the wing has to travel faster than the air below it, as it has a greater distance to travel. That's what creates the pressure difference between the lower and upper surfaces of the wing, and hence lift.

amongst other forms, "lift" can be created by "skipping" across the surface of a higher density fluid like a stone over water (space re-entry vehicles), and through forced flow direction changes. Basically forcing a fluid to move in a different direction will create an equal and opposite force. E.g a fan, or a crude non aerofoil section wing, or very simply something being blown by the wind

anonymous-user

53 months

Thursday 20th April 2017
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dvs_dave said:
You've got that the wrong way round. Faster moving air is lower pressure air. Due to the aerofoil shape of the wing section, the air that flows over the wing has to travel faster than the air below it, as it has a greater distance to travel. That's what creates the pressure difference between the lower and upper surfaces of the wing, and hence lift.
Woops, my bad,

Ayahuasca

27,427 posts

278 months

Thursday 20th April 2017
quotequote all
AVV EM said:
dvs_dave said:
You've got that the wrong way round. Faster moving air is lower pressure air. Due to the aerofoil shape of the wing section, the air that flows over the wing has to travel faster than the air below it, as it has a greater distance to travel. That's what creates the pressure difference between the lower and upper surfaces of the wing, and hence lift.
Woops, my bad,
Nope, that is a load of Bernoullis.

To be fair, that is kind of how how most people thought wings worked until comparatively recently.

But it is not.

If it were, an aircraft would not be able to fly inverted.

Brother D

3,698 posts

175 months

Thursday 20th April 2017
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Ayahuasca said:
AVV EM said:
dvs_dave said:
You've got that the wrong way round. Faster moving air is lower pressure air. Due to the aerofoil shape of the wing section, the air that flows over the wing has to travel faster than the air below it, as it has a greater distance to travel. That's what creates the pressure difference between the lower and upper surfaces of the wing, and hence lift.
Woops, my bad,
Nope, that is a load of Bernoullis.

To be fair, that is kind of how how most people thought wings worked until comparatively recently.

But it is not.

If it were, an aircraft would not be able to fly inverted.
Nor would paper planes or wooden planes with flat sheet balsa wings... I'm not sure how bernoullis has such a prevalence over newtonian physics as the primary 'reason' for lift.

Eric Mc

121,779 posts

264 months

Thursday 20th April 2017
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There's this old trick -




thebraketester

14,192 posts

137 months

Friday 21st April 2017
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Eric Mc said:
There's this old trick -



Is it sad that I recognise who that guy is from his woodworking YouTube channel? getmecoat

Brother D

3,698 posts

175 months

Friday 21st April 2017
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And coincidentally on topic.

http://www.telegraph.co.uk/news/science/science-ne...

Eric - Re the blowing over the top of paper making it rise. You honestly think there is any comparison regarding the difference in velocity between the slightly longer path over curved surface, vs 100x the difference in speed by blowing across a flat surface over the stationary air below?

CanAm

9,115 posts

271 months

Friday 21st April 2017
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And if you look at some very early airfoil sections, the bottom surface actually ran parallel with the upper surface (like a very thick yacht sail) so no difference in distance travelled by the airstream....and they still worked.

Eric Mc

121,779 posts

264 months

Friday 21st April 2017
quotequote all
Brother D said:
And coincidentally on topic.

http://www.telegraph.co.uk/news/science/science-ne...

Eric - Re the blowing over the top of paper making it rise. You honestly think there is any comparison regarding the difference in velocity between the slightly longer path over curved surface, vs 100x the difference in speed by blowing across a flat surface over the stationary air below?
I'm not stating or claiming anything - just that such little experiments are often used to demonstrate the principle of how differing pressures on either side of a sheet cause it to move towards the faster flowing airflow.

I've no idea how relevant they are to how an actual aircraft wing (or any other shape that can be used to create lift) actually works.

gregs656

10,818 posts

180 months

Friday 21st April 2017
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I found the NASA pages a bit awkward to navigate, but did find this which may be of interest; titled 'How Airplanes Fly': http://www.allstar.fiu.edu/aero/airflylvl3.htm

edit: there is an updated of the article here: http://www.allstar.fiu.edu/aero/Flightrevisited.pd...

Edited by gregs656 on Friday 21st April 09:48

Atomic12C

Original Poster:

5,180 posts

216 months

Friday 21st April 2017
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thebraketester said:
100% true.

Same as magnets. No one know how they work either.
Magnets work due to special relativity.
https://www.youtube.com/watch?v=1TKSfAkWWN0

Again, nice theory, but then outside of the world of maths, every reason for why something works or not is a "theory".
So if you accept the theory then one does not have to deal with black & white 'fact' wink
You only have to worry about a superseding theory that matches prediction and measurement more accurately.

Alias218

1,485 posts

161 months

Friday 21st April 2017
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It is indeed because of Bernoulli's Principle. Simply put, there is a relationship between pressure, velocity and fluid density.

Taking fluid density (rho p) to be constant (in this instance air) then an increase in velocity results in a decrease in pressure. On an airfoil, the upper surface is longer than the bottom surface, resulting in two different velocities (due to the greater distance the body of air has to travel relative to the underside of the airfoil) and ultimately a pressure differential is formed as per Bernoulli's equation.

As a consequence, the pressure is lower above the wing than that below this creating lift. Turn a wing upside down and you have a spoiler! Same principle, but tipped arse over tit. It's also how carburettors work.

Other factors are in play like boundary layers and laminar/turbulent airflow but that's the bones of it, although I've probably left out the nuances of it. nerd

Personally, it's my favourite principle owing to its simplicity and far reaching applications!

Dr Jekyll

23,820 posts

260 months

Friday 21st April 2017
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Alias218 said:
It is indeed because of Bernoulli's Principle. Simply put, there is a relationship between pressure, velocity and fluid density.

Taking fluid density (rho p) to be constant (in this instance air) then an increase in velocity results in a decrease in pressure. On an airfoil, the upper surface is longer than the bottom surface, resulting in two different velocities (due to the greater distance the body of air has to travel relative to the underside of the airfoil) and ultimately a pressure differential is formed as per Bernoulli's equation.

As a consequence, the pressure is lower above the wing than that below this creating lift. Turn a wing upside down and you have a spoiler! Same principle, but tipped arse over tit. It's also how carburettors work.

Other factors are in play like boundary layers and laminar/turbulent airflow but that's the bones of it, although I've probably left out the nuances of it. nerd

Personally, it's my favourite principle owing to its simplicity and far reaching applications!
Except that aircraft can fly upside down. The Bernoulli principle is a minor effect and some wings have upper surfaces the same length as the lower.

Stick a hand out of the window of a moving car palm down. Twist your wrist so the airflow hits the palm and you have lift, twist it the other way and it's pushed down, no need for Bernoulli.

A better example of Bernoulli in action is toilet bowls.

Alias218

1,485 posts

161 months

Friday 21st April 2017
quotequote all
Dr Jekyll said:
Alias218 said:
It is indeed because of Bernoulli's Principle. Simply put, there is a relationship between pressure, velocity and fluid density.

Taking fluid density (rho p) to be constant (in this instance air) then an increase in velocity results in a decrease in pressure. On an airfoil, the upper surface is longer than the bottom surface, resulting in two different velocities (due to the greater distance the body of air has to travel relative to the underside of the airfoil) and ultimately a pressure differential is formed as per Bernoulli's equation.

As a consequence, the pressure is lower above the wing than that below this creating lift. Turn a wing upside down and you have a spoiler! Same principle, but tipped arse over tit. It's also how carburettors work.

Other factors are in play like boundary layers and laminar/turbulent airflow but that's the bones of it, although I've probably left out the nuances of it. nerd

Personally, it's my favourite principle owing to its simplicity and far reaching applications!
Except that aircraft can fly upside down. The Bernoulli principle is a minor effect and some wings have upper surfaces the same length as the lower.

Stick a hand out of the window of a moving car palm down. Twist your wrist so the airflow hits the palm and you have lift, twist it the other way and it's pushed down, no need for Bernoulli.

A better example of Bernoulli in action is toilet bowls.
Angle of attack and the vectoring of air also plays a significant role. To say it was entirely Bernoulli's was a little over-enthusiastic.

LimaDelta

6,507 posts

217 months

Friday 21st April 2017
quotequote all
Alias218 said:
It is indeed because of Bernoulli's Principle. Simply put, there is a relationship between pressure, velocity and fluid density.

Taking fluid density (rho p) to be constant (in this instance air) then an increase in velocity results in a decrease in pressure. On an airfoil, the upper surface is longer than the bottom surface, resulting in two different velocities (due to the greater distance the body of air has to travel relative to the underside of the airfoil) and ultimately a pressure differential is formed as per Bernoulli's equation.

As a consequence, the pressure is lower above the wing than that below this creating lift. Turn a wing upside down and you have a spoiler! Same principle, but tipped arse over tit. It's also how carburettors work.

Other factors are in play like boundary layers and laminar/turbulent airflow but that's the bones of it, although I've probably left out the nuances of it. nerd

Personally, it's my favourite principle owing to its simplicity and far reaching applications!
Take a look at an aerofoil cross section for a fast jet, or a simple foam glider and report back. Amazed at the number of posters on here who still believe this.

Alias218

1,485 posts

161 months

Friday 21st April 2017
quotequote all
LimaDelta said:
Alias218 said:
It is indeed because of Bernoulli's Principle. Simply put, there is a relationship between pressure, velocity and fluid density.

Taking fluid density (rho p) to be constant (in this instance air) then an increase in velocity results in a decrease in pressure. On an airfoil, the upper surface is longer than the bottom surface, resulting in two different velocities (due to the greater distance the body of air has to travel relative to the underside of the airfoil) and ultimately a pressure differential is formed as per Bernoulli's equation.

As a consequence, the pressure is lower above the wing than that below this creating lift. Turn a wing upside down and you have a spoiler! Same principle, but tipped arse over tit. It's also how carburettors work.

Other factors are in play like boundary layers and laminar/turbulent airflow but that's the bones of it, although I've probably left out the nuances of it. nerd

Personally, it's my favourite principle owing to its simplicity and far reaching applications!
Take a look at an aerofoil cross section for a fast jet, or a simple foam glider and report back. Amazed at the number of posters on here who still believe this.
I was calling on what I learnt during my BEng, which unfortunately didn't go into great detail regarding aeronautics. The example I remember being taught involved a 'classic' (if you will) airfoil. As stated already, it was a mistake of mine to overstate Bernoulli's.

Ayahuasca

27,427 posts

278 months

Friday 21st April 2017
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There are, at any given moment, over 1,000 B737s in the air.