[Just thinking (OP)]

Does an aeroplane engine push or pull, if it pushes it would need to compress the air, yet if it pulls the outflow is inconcequential. A propellor, relies on pushing. The gaseous nature  of air means it is far more likely to fill the void in a non directional manner than being ejected in a non directional manner, especially if the power unit is travelling forward.

If you stand in front of a propeller you will be sucked in and stand at the back of a propeller you will be blown away. So we have suck and push. We could say it's the two sides of one coin.

[Bored chemist]

Do you know what "the induction stroke" means?

Yes, it is the intake stroke just before the compression stroke then the firing stroke, and last the exhaust stroke. providing that we are referring to a four stroke engine.

And, in what way do you think t resembles a gun firing?

[alancalverd]

I think I have found BC's problem. The convention in fluid dynamic analyses is to set the ambient pressure at zero, so we can talk about positive and negative pressures and forces relative to ambient.

If you measure the pressures around an aerofoil at a positive angle of attack, you find  the pressure above to be negative and below to be positive.

With a complex aerofoil like Cessna's NACA2412 the upper surface force is about 3 times the lower surface force in the region of normal flight, say  3° < α < 10°. This makes it very difficult to fly inverted (even if the plane was stressed to do so) as the wing does its best to pull you downwards until it is very close to the stall angle (α > -15°). 

A simple symmetric aerofoil generates the same lift either way up (definition of "symmetric"). The only essential difference in a practical airplane is the angle of incidence (the angle between the wing chord and the longitudinal axis of the fuselage)  which is close to zero for an aerobatic monoplane like the Extra,  and the offset between the AOI of the wing and the stabiliser (can be zero for an "all-flying tail" or a canard). Problem with setting both AOIs to zero and using two symmetric aerofoils is the reduced pitch stability, which is why you might want to show off in an Extra, but take your family on holiday in a Cessna.

[Bored chemist]

I think I have found BC's problem.

Yes, my "problem" is that I remember what I was taught at school.
There is no such thing as a negative pressure; Molecules hitting a surface push on it; they can not pull.
You can measure differences in pressure from any datum you choose.
But pressures are, in fact, positive.

So...

If you measure the pressures around an aerofoil at a positive angle of attack, you find  the pressure above to be negative and below to be positive.

No.
These pressures will all be positive; some more so than others.

[Just thinking (OP)]

And, in what way do you think t resembles a gun firing?[/quote                                                                                                                                                                                                           The

And, in what way do you think t resembles a gun firing?

The intake stroke of the engine is the minus presser the outside presser is the positive presser flip the coin and in the gun, the firing is the positive presser and the outside presser is the minus presser same thing in reverse. Internal vs external.

[alancalverd]

There is no such thing as a negative pressure;

There is no such thing as negative money, but lots of people and businesses have overdrafts, or commitments greater than their current asset values. The concept of negative numbers is very useful.

The attraction of "zero ambient pressure" fluid dynamics is that you can use the same analysis over a range of ambient pressures. Not hugely important (but still convenient) with liquids but for practical aviation purposes lift = ½ρAC_lv² where v is true air speed. Since indicated air speed u  is inversely dependent on air density ρ, the lift generated by a subsonic wing in normal (non-stalled) flight is thus simply related to the indicated air speed at any altitude, making life a lot easier when calculating required and limiting bank angles.

Apart from French, Latin, rugby and cricket, most of what I learned in school turned out to have little practical use. A bit more German might have been handy.

[Just thinking (OP)]

I think that the dynamics of the aerofoil are rather different when the wing is inverted and the attacking angle is greater. The curved surface of the aerofoil is corrupted in inverted flight and acts as a flat surface and the bottom of the wing now at the top has the effect of lift due to the extreme incline of the angle of attack. So in this mode the wing generates lift but in a very poor way.

[Bored chemist]

The concept of negative numbers is very useful.

I know a mathematician who really didn't like negative numbers.
He would stop at nothing to avoid them.

However, all pressures are still positive and a vacuum can't suck.

[Just thinking (OP)]

However, all pressures are still positive and a vacuum can't suck.

That makes sense high presser has authority over the presser that is lower.

[alancalverd]

The curved surface of the aerofoil is corrupted in inverted flight and acts as a flat surface

Corruption is a (frequently fatal) problem with Rogallo wings in negative g, but the shape of a rigid airfoil shouldn't change when it's inverted. And again you are only considering asymmetric airfoils - a symmetric wing (e.g. NACA 0010) works exactly the same either way up. I think "rigid sail" boats use a symmetric foil so they can tack into wind.

And if you want to get really complicated, try https://www.scienceabc.com/eyeopeners/can-helicopter-fly-upside.html

[Just thinking (OP)]

Corruption is a (frequently fatal) problem with Rogallo wings in negative g, but the shape of a rigid airfoil shouldn't change when it's inverted. And again you are only considering asymmetric airfoils - a symmetric wing (e.g. NACA 0010) works exactly the same either way up. I think "rigid sail" boats use a symmetric foil so they can tack into wind.

And if you want to get really complicated, try https://www.scienceabc.com/eyeopeners/can-helicopter-fly-upside.html

In normal flite the airfoil works as intended but if we over rotate the upper surface will lose lift so when inverted the upper surface now at the bottom is over rotated and loses lift so becomes a surface that gains a positive presser.

[alancalverd]

Still depends on what was "intended". A Cessna 172 is designed to get in and out of short fields and cruise sedately at 120 mph, "right way up", so has highly asymmetric airfoils. A fighter may need to fly inverted from time to time (but I have my doubts about the Harrier or the F35)  and an aerobatic plane should be reasonably able to sustain flight at any roll angle - see the Wortmann FX71 airfoil used by the Extra 300. 

[Just thinking (OP)]

Still depends on what was "intended". A Cessna 172 is designed to get in and out of short fields and cruise sdately qat

That may well be true but if we look at a simple example let's say a plank of wood 8x1 if we place that in a wind tunnel and including the surface to the airflow it will generate lift and that is with no aerofoil at all. The braking airflow over the top will generate lift and the bottom surface will pick up the airflow and react to the positive pressure that it is being exposed to.

[alancalverd]

Anything that deflects the wind is an airfoil, but some are better than others. If you sharpen the leading edge of your plank, you won't be far from an F104 wing!

[Bored chemist]

Anything that deflects the wind is an airfoil, but some are better than others. If you sharpen the leading edge of your plank, you won't be far from an F104 wing!

I forget the details, but there is a quote which says something like "A tea-tray will fly if you put a jet engine on it."
It looks like they checked.

[Just thinking (OP)]

Anything that deflects the wind is an airfoil, but some are better than others. If you sharpen the leading edge of your plank, you won't be far from an F104 wing!

Bingo, see that's the basic fundamentals of any wing only the increased aerofoil is for lower speed but still can work when pushed in the inverted with the increased angle of attack.

[Bored chemist]

Planes actually fly by pushing air down.

Anything that deflects the wind is an airfoil,

Snap!

[Just thinking (OP)]

When water skiing the ski is riding on the water with pressure between the two surfaces. Go to slow and sink.

[alancalverd]

Planes actually fly by pushing air down.

Anything that deflects the wind is an airfoil,

Snap!

The upper surface does not push anything down, but contributes 60 - 70% of the lift.

[Bored chemist]

The upper surface does not push anything down, but contributes 60 - 70% of the lift.

Not really.
It may well provide 70% of the pressure difference.
There isn't a mechanism for the air molecules to pull on it  and lift it.