Lift

[fly_tornado]

mystery solved

 

 

 

[Guest pookemon]

This still doesn't answer the question as to WHY this happens. It's just a correction of a "common" belief which is actually wrong.

 

The airflow increases it's speed above the aerofoil because of the venturi effect. The same occurs above hills/mountains. This causes the increase in speed and the drop in pressure results from Bernoulli's principle (inverse correlation between speed and pressure). Below the aerofoil in this example the pressure would increase because of the skin/form friction and the drop in the velocity of the air.

 

My 2c...

 

 

[Ultralights]

so, nothing really new here, air over the top is accelerated, looses pressure, only change from the conventional taught theory is it doesn't reach the trailing edge at the same time, but in fact earlier than air flowing under the wing. or is it the air flowing under is slowed more than initially believed?

 

 

[winsor68]

I think we over complicated things at some stage here...

 

I guess all the pressure is actually below the wing rather than above... (or below its direction of lift)

 

 

[Guest ozzie]

No Mystery. I'ts Magic

 

 

[kaz3g]

This still doesn't answer the question as to WHY this happens. It's just a correction of a "common" belief which is actually wrong.The airflow increases it's speed above the aerofoil because of the venturi effect. The same occurs above hills/mountains. This causes the increase in speed and the drop in pressure results from Bernoulli's principle (inverse correlation between speed and pressure). Below the aerofoil in this example the pressure would increase because of the skin/form friction and the drop in the velocity of the air.

 

My 2c...

Personally, I think:

 

• Newton arrived at the correct answer when he postulated the third law of dynamics; and
   
   
  
• Venturis work in a closed system
   
   
  

 

 

 

If the system above a hill is explained by Bernoulli's theorem then why is it that the windspeed closest to the ground - and subject to the greatest change in direction and presumably therefore distance travelled - is also generally lower than the windspeed higher above the hill? Bernoulli doesn't take into account friction or the fact that a symmetrical airfoil also produces lift (even flat ones like the tailplane on a J3).

 

See http://www.grc.nasa.gov/WWW/k-12/airplane/wrong1.html

 

NASA's explanation is here http://www.grc.nasa.gov/WWW/k-12/airplane/bernnew.html

 

But it's always an interesting discussion and I think the PPL/CPL exams still require reliance on Bernoulli?

 

kaz

 

 

[Ultralights]

If the system above a hill is explained by Bernoulli's theorem then why is it that the windspeed closest to the ground - and subject to the greatest change in direction and presumably therefore distance travelled - is also generally lower than the windspeed higher above the hill? Bernoulli doesn't take into account friction or the fact that a symmetrical airfoil also produces lift (even flat ones like the tailplane on a J3)

i think this might have something to do with Coriolis forces and surface friction.

 

 

[Guest pookemon]

i think this might have something to do with Coriolis forces and surface friction.

The Coriolis effect is the motion of a fluid affected by the rotation of the earth - Eg. water down the toilet/drain, weather formations (Cyclone/Hurricane).

 

The Venturi effect does occur above land features. The surface friction is what causes the wind gradient (even on a flat piece of ground the wind will be stronger further from the ground than near the surface). With a hill the atmospheric pressure forms the other side of the venturi. A symmetrical aerofoil creates lift only at a +ve AoA - this increases the friction on the under surface and it also increases the upper surface length.

 

 

[Yenn]

the change of speed which is supposed to be what causes a wing to lift is only part of the equation. It is my belief that the majority of lift is caused by the downward deflection of air under the wing. There is an equal force acting upwards, to the amount of air deflected down. The venturi scenario doesn't seem to me to be enough to give the lift required. Just work out the change in speed and the change in pressure will be in proportion and opposite to it. Not enough in my opinion.

 

 

[Powerin]

My understanding is that it is a combination of processes (including Bernoulli) that produces lift. But the net effect of these processes is that air is turned downward and that is what causes lift. Low pressure above an aerofoil certainly occurs, but in itself it is not responsible for lift but contributes to changing the direction of the air.

 

I always like to think of a propeller, which is a moving aerofoil with an angle of attack. It's obvious to me that the net effect of that moving aerofoil is to throw huge amounts of air backwards which in turn produces a forward force (every action produces an opposite and equal reaction). No one can tell me that an aircraft is pulled forward purely by an area of low pressure in front of the prop. It is certainly there as evidenced by a prop sucking up debris in front of it, but it is only part of the process that results in a large rearward movement of air.

 

Wings work the same way as far as I can see (but vertically).

 

 

[kaz3g]

My understanding is that it is a combination of processes (including Bernoulli) that produces lift. But the net effect of these processes is that air is turned downward and that is what causes lift. Low pressure above an aerofoil certainly occurs, but in itself it is not responsible for lift but contributes to changing the direction of the air.I always like to think of a propeller, which is a moving aerofoil with an angle of attack. It's obvious to me that the net effect of that moving aerofoil is to throw huge amounts of air backwards which in turn produces a forward force (every action produces an opposite and equal reaction). No one can tell me that an aircraft is pulled forward purely by an area of low pressure in front of the prop. It is certainly there as evidenced by a prop sucking up debris in front of it, but it is only part of the process that results in a large rearward movement of air.

 

Wings work the same way as far as I can see (but vertically).

And I think that is what NASA is now saying, too, Powerin. I prefer Bernoulli's law to "venturi" because I still think a venturi needs to be in a closed system and I also think it is the lesser contributor aerofoil lift as compared to Newton's law for the very reason you have given (think helicopter downwash).

 

But it's a great topic for discussion and both sides have strong support from their respective adherents. I'm waiting for DJP to set us all straight :-)

 

kaz

 

 

[Powerin]

(think helicopter downwash).

Exactly. The downward movement of air is what gives the lift (via Newton), but there are several things that cause it. And yes, I can't see how a venturi can exist in an open (ie above a wing) system.

 

I'm waiting for DJP to set us all straight :-)

...and he almost certainly will

 

 

[Guest Maj Millard]

The coriolis effect is important here . It is what make the air on top of the wing 'stick' or flow down towards the Trailing edge, as opposed to just continuing to go straight back. Fog coming over mountains does the same and flows down the mountain instead of flowing straight off it.

 

 

[Powerin]

The coriolis effect is important here . It is what make the air on top of the wing 'stick' or flow down towards the Trailing edge, as opposed to just continuing to go straight back. Fog coming over mountains does the same and flows down the mountain instead of flowing straight off it.

I don't think that is Coriolis Maj. If memory serves that is the force caused by circular/twisting motions such as that caused by the Earth turning. Airflow sticking to a surface has to do with viscosity and boundary layers (laminar flow puts in an appearance too I think). But yes, it is very important in lift. When the boundary layer separates from the upper surface of a wing (ie...the air stops sticking to the wing) you lose the majority of your lift...which of course is exactly what happens in a stall.

Edit- you can see the same effect if you hold the back of a spoon against a stream of water from a tap. The water sticks to the spoon and curves around it.

 

Edit 2: here's a

 

https://www.youtube.com/watch?v=zrwlpHE7P8Q. It has a heap of tufts of wool/twine stuck to the upper surface of the wing to show the airflow. When the wing is flying normally the tufts are dead straight. When the wing begins to stall (and the flow separates) the tufts are all over the place showing the turbulent airflow that occurs in a stall.

 

 

[facthunter]

The wing works by moving air. If it didn't move the air there would be no lift (or drag), which are a reaction to what it's doing. A flow of air will not bend/curve unless the pressure on the inside of the curve is less than the pressure on the outside.

 

The beauty of that video is that it dispels the widely accepted fallacious view that the particles flow faster over the top surface of the wing to get back together at the trailing edge. Nev

 

 

[turboplanner]

......the spoon on proon.......

 

 

[Guernsey]

This all raises an interesting question (here I go again ) If a bald man was travelling very fast on a motorbike would his head create more lift and downward draft than a man with short tufted hair. I know he should be wearing a crash helmet but this is purely ' Highpothetticall '.

 

Alan.

 

 

[facthunter]

Dimpled golf balls have better boundary control and therefore less drag than smooth ones. Nev

 

 

[Guernsey]

Dimpled golf balls have better boundary control and therefore less drag than smooth ones. Nev

So if our bald pilot had dimples on his head......????????

 

Alan.

 

 

[winsor68]

How about this for complicating things???

 

 

http://www.fanwing.com/index.htm

 

I had a foam kite that employed a fan wing principle when I was a kid... it worked perfectly... Have never seen another...

 

Video---->http://www.fanwing.com/videos/1stflight.mpg

 

 

[facthunter]

See a doctor, but you can work it out Alan. Nev

 

 

[winsor68]

Ah... What a cool name... The Magnus Effect.

 

http://en.wikipedia.org/wiki/Magnus_effect

 

 

[winsor68]

Planned LSA experimental for 2013...

 

 

 

[old man emu]

I don't think that is Coriolis Maj. If memory serves that is the force caused by circular/twisting motions such as that caused by the Earth turning.

You are correct there, Powerin. That is indeed the Coriolis Effect, and it is what makes Low Pressure areas generate clockwise winds in the Southern Hemisphere, but anti-clockwise in the Northern Hemisphere.

 

The effect Maj was trying to name is the Coanda Effect which describes why a jet of air will seem to adhere closely to the surface it is flowing over. But yes, it is very important in lift. When the boundary layer separates from the upper surface of a wing (ie...the air stops sticking to the wing) you lose the majority of your lift...which of course is exactly what happens in a stall. It is the basis for an anti-stall device called the Blown Flap developed by the English in the 1950s.

 

In a conventional blown flap, a small amount of the compressed air produced by the jet engine is "bled" off at the compressor stage and piped to channels running along the rear of the wing. There, it is forced through slots in the wing flaps of the aircraft when the flaps reach certain angles. Injecting high energy air into the boundary layer produces an increase in the stalling angle of attack and maximum lift coefficient by delaying boundary layer separation from the airfoil. Boundary layer control by mass injecting (blowing) prevents boundary layer separation by supplying additional energy to the particles of fluid which are being retarded in the boundary layer. Therefore injecting a high velocity air mass into the air stream essentially tangent to the wall surface of the airfoil reverses the boundary layer friction deceleration thus the boundary layer separation is delayed.

 

In meteorology, the Coandă effect theory has also been applied to some air streams flowing out of mountain ranges such as the Carpathian Mountains and Transylvanian Alps, where effects on agriculture and vegetation have been noted.

 

You can see the same effect if you hold the back of a spoon against a stream of water from a tap. The water sticks to the spoon and curves around it.

No, that's incorrect. The Coanda Effect only occurs within similar fluids eg gas/gas or liquid/liquid. It does not happen with dissimilar fluids eg gas/liquid.

 

To get back to the original argument, Newton and Bernoulli do not contradict each other. Explanations which are based on Newton's and on Bernoulli's principles are completely compatible. Air-deflection and Newton's Laws explain 100% of the lifting force. Air velocity and Bernoulli's equation also explains 100% of the lift. For the most part they're just two different ways of simplifying a single complicated subject.

 

For one side of the Newtonian (Angle of Attack) -v- Path Length (Airfoil Shape argument, check out http://www.amasci.com/wing/airfoil.html

 

http://www.av8n.com/how/htm/airfoils.html has good explanaton obtained from a review if smoke trails over an aerofoil in a wind tunnel.

 

Old Man Emu

 

 

[Yenn]

From memory the fanwing concept was tried on boats in the 1930's. I don't know what happened but it didn't catch on, so was obviosly not such a great improvement over sails.

 

Spelling highpathetical.