Flying Inverted in 737

[Paul_Smith 92]

I dont see how stuff like flying inverted can be discussed without a common understanding of basic aerodynamics. If you dont know how the wing actually works, then how are you going to fly it? We see Sara's reasoned, and commonly repeated objections above, despite the fact that they dont make sense and can be simply disproved. You say inverted flight will behave one way, she will claim, based on her beliefs, that it will behave another. On the forum, it does not matter. In the air, you would live, she would die.

 

Please dont think I am picking on Sara. These beliefs are widely held and commonly taught and still appear in print. 

[Great Ozzie 101]

 

I dont see how stuff like flying inverted can be discussed without a common understanding of basic aerodynamics.

 

Paul,

 

You should know I have the greatest respect for you.

/brown-nosing

 

I was providing my reasoning for not getting into it.  I did not mean to imply (or have you infer) it was not a good thing to discuss. Sorry that wasn't clear. Btw, I did do a "Buckwheat surprise face" when I first read that statement - the one you quoted - I just shook my head and and kept goin'. (Good enough backtracking?    )

 

No, I don't think you are picking on Sara (lol the dying part she might have trouble with).

[Sara Louise Capon 21]

No Fear,
I don't feel picked on at all,
I do feel however that it would be interesting to go inverted,
And yes would have to push the nose down (up inverted) to get any lift out of the wings to invert the Aerofoil to work up again instead of pulling down to the ground,
Wing dependant of course, Concorde being a different kettle of fish altogether for instance,
different wing geometry, but still an Aerofoil.
I would not be holding it for longer than it would take to go, yes I'm inverted,
I think If i did, depsite all the myriad of systems alarms I would be expecting to hear,
I would be more concerned about trying to recover before the engines failed altogether due to lack of lubrication and cooling,
Hopefully the nose would come down first anyway, and manage to level the wings.

Interesting to know if any real pilots have tried this, or intend to now try this in a real simulator? lol,
and if so, can we have a video of it?

Not all of what i said above was incorrect lol.
But bits, yes,

 

http://www.cam.ac.uk/research/news/how-wings-really-work

[Great Ozzie 101]

Interesting to know if any real pilots have tried this

 

Inverted flight? Only in a DHC-1 Chipmunk... and then not for very long.  While the airframe is stout enough, it (unmodified) does not have an inverted fuel / oil system.  While 4-5 pos. g's weren't much of a problem for me when I took that Unusual Attitude course, -1g was a bit uncomfortable... I always felt I would fall out the canopy... also had a hard time reaching the rudder pedals when pulling neg g.

 

 

 

But bits, yes,

 

No biggy... it happens... I am guilty of it here (avsim) on things where memory failed me.  And it isn't as if many aviation texts aren't guilty of this.  Fair to blame them.

 

 

 

Hopefully the nose would come down first anyway, and manage to level the wings.

 

Better if you can roll out... way less g and loss of altitude.

[Sara Louise Capon 21]

I meant more in a simulator for actually trying it, preferably a passenger plane,

much less likely to lose job and lives

[z06z33 781]

Any plane can fly inverted or do a roll the aero dynamics don't change the wing still works the same way. The only thing you have to keep in mind not to over g and oil and fuel starvation. For example mine and my dad's Yak-52 has inverted oil and fuel systems which keeps the engine lubricated and running but I doubt such a system exists on a 737. In that case keep the g's positve and you wont 't have an issue which means you can't push forward on the yoke. The best way to do a roll is to pitch up to 30 degrees and unload the back pressure do the roll and recover wings level on the horizen also remember to not pull back on the stick while doing the roll as rolling g's cause alot of stress. I belive a Southwest 737 was seen doing rolls a few years back so it can be done without a crash.

[kevinh 409]

I am really surprised that no one has corrected you on this yet. Lift has nothing to do with the airflow over the top of the wing. Basic physics 101, "For every action, there is an equal and opposite reaction", lift is the reaction to the wing pushing air downwards. If you want a really good primer on aerodynamics, all the way through to acrobatics, then I highly recommend http://www.flybetter.com.au/index.html

Paul,

 

You can explain lift EITHER by the pressure difference between the upper and lower surface (which is to do with air flowing faster over the upper surface than the lower. Air pressure is demonstrably lower on the upper surface than the lower and the lower static pressure can be computed using Bernoulli's equation if you know the total pressure and local velocity.)  OR you can explain it in Newtonian terms, air being deflected downwards. BOTH do a reasonable job at explaining things and predicting lift.

 

However the angle of attack of the wing alone does not account for all the air deflection that takes place. The aerofoil shape and its camber create more air deflection than a simple flat plate would.  A rotating cylinder produces lift in an airflow.  No obvious angle of attack to deflect the air, yet air is indeed deflected.  What deflects it in this case is the circulation of air around the cylinder created by its rotation.

 

To say airflow over the top of the wings has nothing to do with it is ridiculous.  If that was the case extending a spoiler would have no effect.  I'm not sure you actually meant to say that, but unfortunately that is what you wrote.  What is however wrong is to explain the velocity difference (and hence the pressure difference) by saying the path over the top surface must be longer than the path over the bottom surface to create this effect. You real disagreement was (correctly) with paulyg123, who had mentioned the path length idea. Sara didn't use that incorrect reasoning.

 

So there's nothing wrong with what Sara said.  The aerofoil shape is important, vital even.  That's why a flat plate tends not to fly as efficiently as an aerofoil.

[Paul_Smith 92]

 

Not all of what i said above was incorrect lol.
But bits, yes,



http://www.cam.ac.uk...ngs-really-work

The bits you got spot on were about eductors and ejectors (though I had to look up what an eductor was).

 

However, from your link

“What actually causes lift is introducing a shape into the airflow, which curves the streamlines and introduces pressure changes – lower pressure on the upper surface and higher pressure on the lower surface,” clarified Babinsky, from the Department of Engineering. “This is why a flat surface like a sail is able to cause lift – here the distance on each side is the same but it is slightly curved when it is rigged and so it acts as an aerofoil. In other words, it’s the curvature that creates lift, not the distance.”

 

Aaaaggghhh! No! It is not! Lift is the vertical reaction to the displacement of the air, regardless of what shape is doing the displacing. Why is this so difficult to grasp  that even a professor of engineering gets it wrong or gets misquoted?

 

Simple experiment... Jump out of a plane! You will fall at about 120 mph. Adopt a divers pose and you can reach maybe 200mph. Same weight, same thrust (gravity), different surface area presented to the air.  Open your parachute and you will slow to (hopefully) about 5mph. Anybody want to talk about the speed difference being related to how far the air has to travel in one direction around the body compared to another? Or how about the pressure difference above and below the aerofoil? Still No? But the only difference between falling out of the plane and being inside it, is that the plane has an engine providing thrust in addition to gravity.

 

The mere fact that something is getting in the way of the airflow means that air is pushed out of the way and Newton's third law says that the air pushes back. The thrust (engine or gravity) tries to speed the body up. The air that is pushed out of the way to make room, pushes back, slowing the body down. When the forces match, the speed will be steady. It really doesn't matter if the body is the skydiver, or the plane you jumped from.

 

I am sorry for getting so excited about this stuff, but I am a physicist, not an aerodynamicist. That means I like simple things that make sense and that I can prove to myself. As a child, I stuck my hand out of a car window and played with the effects of different shapes in the airflow. I still find it fun.

[Paul_Smith 92]

To say airflow over the top of the wings has nothing to do with it is ridiculous. If that was the case extending a spoiler would have no effect.

I am sorry I gave that impression, and you are correct, it was the suggestion of different path lengths giving rise to mystical thrust that I was objecting to. However, in the case of a spoiler, you can successfully treat it as a separate system where you are introducing a deflector into the path of an airflow, that generates lift and drag in proportion to the direction and quantity of air displaced. The primary advantage of the aerofoil shape over a flat plate is that it allows for much simpler prediction and management of the balance point of the resulting thrust vector as moves through a much smaller range within the deflectors critical angle of attack. The secondary advantage being the management of parasitic drag induced by turbulent airflow.    

 

And thanks for bringing up rotating cylinder's. They gave me nightmares for months, though I did solve them eventually. I think it had something to do with parasitic drag being unevenly distributed resulting in effective profiles not matching physical profiles, but that was years ago.

[kevinh 409]

I am sorry I gave that impression, and you are correct, it was the suggestion of different path lengths giving rise to mystical thrust that I was objecting to. However, in the case of a spoiler, you can successfully treat it as a separate system where you are introducing a deflector into the path of an airflow, that generates lift and drag in proportion to the direction and quantity of air displaced. The primary advantage of the aerofoil shape over a flat plate is that it allows for much simpler prediction and management of the balance point of the resulting thrust vector as moves through a much smaller range within the deflectors critical angle of attack. The secondary advantage being the management of parasitic drag induced by turbulent airflow.    

You're wrong Paul, the shape is crucial to how lift is created.  It really isn't just about reducing drag and managing the balance point.

 

Please read my earlier post again.  Lift can be explained EITHER by pressure difference OR by Newtonian mechanics.  Both arrive at roughly the same answer.  Newtonian is not the only correct explanation. There really is a static pressure difference across the aerofoil which when summed accounts for all the lift produced, without considering momentum change.  It all depends on where you draw your boundary conditions.  The example of the lifting spinning cylinder shows geometric angle of attack is not the complete answer.  A non-symmetrical aerofoil will produce lift at zero angle of attack.

 

You may be a physicist, but I studied aerodynamics at University.  I haven't learned this from wiki or using some secondary school simplistic explanation.

 

 

Simple experiment... Jump out of a plane! You will fall at about 120 mph. Adopt a divers pose and you can reach maybe 200mph. Same weight, same thrust (gravity), different surface area presented to the air.  Open your parachute and you will slow to (hopefully) about 5mph. Anybody want to talk about the speed difference being related to how far the air has to travel in one direction around the body compared to another? Or how about the pressure difference above and below the aerofoil? Still No? But the only difference between falling out of the plane and being inside it, is that the plane has an engine providing thrust in addition to gravity.

This is not relevant to a debate about what causes lift.  You are talking about drag here, albeit drag in the vertical direction.  Drag is a force opposing the motion of a body through a fluid.  Lift is a force produced at right angles to the direction of motion through a fluid.

[Sara Louise Capon 21]

Tee hee,

Nice to open up a can of worms,

I'm an engineer, not the aviation type though.

I do work with a lot of ejector's and eductor's for various purposes though,Wether they be Air, Steam, or Water / oil driven, the principle is the same in them.

Also an orifice as a restriction, with a differential pressure gauge either side of it to enable flow measurement,

you create a pressure differential from one side to the other.

Funnily enough, the flow of fluid over the orifice looks exactly like a venturi, or if you could freeze it, and do a cross section,

you get the Aerofoil shape.

[Paul_Smith 92]

Mae culpa - maybe I should be the one jumping from the plane, it just seemed like such a cool analogy 

 

However, sorry Kevin to question your knowledge in this area, but I think you will find that the Newtonian equations always give the correct results but are much harder to calculate exactly, while the pressure differences, which are easily measured, can in fact be explained by the Newtonian air deflection, and so are the consequence of lift, not the cause of it.

[kevinh 409]

Mae culpa - maybe I should be the one jumping from the plane, it just seemed like such a cool analogy 

 

However, sorry Kevin to question your knowledge in this area, but I think you will find that the Newtonian equations always give the correct results but are much harder to calculate exactly, while the pressure differences, which are easily measured, can in fact be explained by the Newtonian air deflection, and so are the consequence of lift, not the cause of it.

The two explanations of lift force (pressure difference and momentum change) are two side of the same coin.  Both are the causes of lift, it's just a question of where you draw the system boundary.  Streamlines flowing over the wing really are closer together than those under the wing, air flowing over the wing really does flow faster than that flowing under it.

 

Exactly the same can be said of explanations of jet engine thrust.  You can understand this from the pressure differences between inlet and exhaust, or from the momentum change of the air between inlet and exhaust.  Both approaches have their uses.

 

In the days before CFD, people didn't design aerofoils by computing momentum changes, they computed and measured pressure distribution around the shape and tailored that to get the effects they wanted. Supercritical wing sections are a good example of how careful shaping of the aerofoil section can produce specific advantages.

 

The air deflection would not happen in the way it does were it not for the aerofoil shape, which you said was not important.  A simple flat plate produces some lift, an aerofoil shape produces a lot more.