How can aeroplanes fly upside down?

[Progenitor A]

We are told aeroplanes lift because of the geometry of the wing - the Venturi effect- causes air to move more quickly over the top surface reducing the pressure in comparison to the less-curved lower surface

And so it does

But if the wing is turned upside down the air will flow faster over the lower surface causing a downward pressure, causing the aeroplane to descend.

But aeroplanes happily fly upside down (if their bodies can take the strain).

Is there some other lift mecanism in operation greater than the venturi effect?

[principled]

Naymissus
A good question and one that introduces us to quite a heated discussion on exactly how lift is produced. The links below are very interesting. They answer your question, but also show how the debate about how much Bernoulli and how much Newton there is in lift continues on the web!
P
wiki.answers.com/Q/How_do_airplanes_fly
amasci.com/wing/airfoil.html

[speakertoanimals]

I think it is the fault of the simplistic explanation for lift in terms of Bernoulli effect, which then gets stumped if you have either symmetric aerofoils, or are flying upside down.

Short answer is - there are actually TWO important things going on -- the shape, and the angle of attack. Hence you can get lift from symmetric aerofoils, and you can get lift in the appropriate direction from inverted asymmetric aerofoils as well.

Nevertheless, the Bernoulli effect (which is after all rather counter-intuitive, with faster meaning lower pressure), does lead to the idea that lift can be generated in the first place, even if it isn't the whole story.

[Progenitor A]

Oct 26, 2010 13:30:31 GMT 1 principled said:

Naymissus
A good question and one that introduces us to quite a heated discussion on exactly how lift is produced. The links below are very interesting. They answer your question, but also show how the debate about how much Bernoulli and how much Newton there is in lift continues on the web!
P
wiki.answers.com/Q/How_do_airplanes_fly
amasci.com/wing/airfoil.html

To my mind (I am open to correction by experts such as Carnyx in this field), it is simply a question of action and reaction. Consider a flat board being towed under water making an angle of 45 degrees. The water pusing against the inclined board will create an upward thrust and the board will rise in the water for as long as there is laminar flow. Same with air on an inclined wing being thrust through the air. There will be an upward reaction causing the wing to rise.

[carnyx]

@nm

To answer your question, no at fairly slow speeds. The upper suction effect exceeds the underneath pressure effect in the order of 3 to 4. And given a symmetrical aerofoil, a positive or negative angle of attack (up to about +/- 12 degrees or so) will give you the same, symmetrical, result

e.g a nose-up angle of attack will give you 3x suction 'lift' in the nose-up direction, and a nose down angle of attack will give you 3x suction 'lift' in the nose down direction ..

I.e at slow airspeeds, Bernoulli beats Newton by around 3 for a symmetrical aerofoil .. and this hold true for say weathervanes, and sails.

What is interesting is to look at the pressure pattern around a typical house roof, in a moderate wind. Apart from a very small area of positive pressure around the windward gutter area, the roof is experiencing a negative. suction pressure. So, roof fixings really need to be anchors to withstand this pull, in order to hold the roof on. Also, the roof example shows that at very low airspeeds, the the aerofoil shape is not that critical.

(BTW Years ago, I was looking at the potential for very large-span curved-roof buildings to act as a static aerofoil, and to exploit the considerable pressure-difference over such a huge area, to provide an occasional source of power to offload the aircon system of such buildings.)

PS; On take-off, a jumbo-jet is effectively being sucked off (!)

[Progenitor A]

Oct 26, 2010 17:31:23 GMT 1 carnyx said:

@nm

To answer your question, no at fairly slow speeds. The upper suction effect exceeds the underneath pressure effect in the order of 3 to 4. And given a symmetrical aerofoil, a positive or negative angle of attack (up to about +/- 12 degrees or so) will give you the same, symmetrical, result

e.g a nose-up angle of attack will give you 3x suction 'lift' in the nose-up direction, and a nose down angle of attack will give you 3x suction 'lift' in the nose down direction ..

I.e at slow airspeeds, Bernoulli beats Newton by around 3 for a symmetrical aerofoil .. and this hold true for say weathervanes, and sails.

What is interesting is to look at the pressure pattern around a typical house roof, in a moderate wind. Apart from a very small area of positive pressure around the windward gutter area, the roof is experiencing a negative. suction pressure. So, roof fixings really need to be anchors to withstand this pull, in order to hold the roof on. Also, the roof example shows that at very low airspeeds, the the aerofoil shape is not that critical.

(BTW Years ago, I was looking at the potential for very large-span curved-roof buildings to act as a static aerofoil, and to exploit the considerable pressure-difference over such a huge area, to provide an occasional source of power to offload the aircon system of such buildings.)

PS; On take-off, a jumbo-jet is effectively being sucked off (!)

At what point Carnyx, does the upward thrust of air hitting the wing exceed the upward (or more pertinently the downward) thrust due to the Bernoulli effect?

[carnyx]

I guess it is just and after the angle of 'stall', which is where the airflow around the camber breaks down into turbulence and the 'lift due to suction' drops rapidly. (The stall angle is about 14 degrees or so for a simple aerofoil) At the same time, the 'lift due to pressure' increases with the cosine of the angle of attack.

(In the extreme, at 90 degrees .... Newton is circa 90%, and Bernoulli is about 10% )