Debunking Lift Theories Still Taught

[completeaerogeek]

There is no contradiction between Newton and Bernoulli. Here's how it works.....The fundamental equations of fluid dynamics are the Navier-Stokes equations, which have been known for 170 years. These were derived by applying Newton's laws to fluid flow, and even if you are good at partial differential vector equations, (which I'm not), you can't solve them.

Put in a lot of simplifying assumptions, like how the fluid is incompressible and frictionless and in steady streamline flow and you finally get to Bernoulli, via the Euler equations.

 

Another way of arriving at Bernoulli is through applying conservation of energy, which is inherent in the assumption of frictionless fluid. This is how we derived Bernoulli in high-school back in the good old days. So Bernoulli is really just a statement of the law of conservation of energy.

 

Bernoulli helps explain the pressure distribution around a shaped object, but not a thin plate, in the streamline flow-field of an incompressible and frictionless fluid.

 

There is a good article in the May 2010 magazine by Daffyd Llewellyn where he explains ( without a single differential equation ) how Newton, Bernoulli, and vortex theory are all parts of the same story.

 

I agree that its wrong to say that Bernoulli is sufficient in itself to explain lift. But its also wrong to say Bernoulli is nonsense.

 

.... Bruce

Hi Bruce- who was saying Bernoulli is nonsense? Bernoulli's equation is derived from Newton's 2nd law. It has some problems as it assumes air is inviscid but it can be used by aerodynamicists and people who understand it's proper use. The problem is when people don't understand it they make up stories to explain how it works and that is where we get equal Transit Time and venturi theory

 

Life would be much easier for pilots if we stuck to Newton and turning flow. It is a physical principle that anyone can understand..

 

 

[completeaerogeek]

Fundamentally, I am agreeing with you.A practical understanding of what is happening is not difficult, and helped immensely by videos like the one at the start of the thread. Also videos of tuft testing wings as they stall - widely available now on Youtube - help immensely in visualizing flow and what happens when the wing stalls.

 

However I wouldn't say Bernoulli, Newton etc. are practical aerodynamics - they are theory. And anytime you have to include assumptions you are acknowledging complexity, because the assumption basically says "this bit is too complex to include, but it doesn't make a significant difference in this situation so we can ignore it." It introduces complexity because you have to understand exactly when the assumption holds, and when it does make a difference and you can no longer ignore it e.g. assumptions that ignore compressibility.

 

Newton and Bernoulli and all these theories of lift tell us nothing useful about the cause of a stall. Fluid dynamics can probably predict a stall based on viscosity and pressure gradient or something (I don't know) but we learn nothing about that as a pilot.

 

What we need to know is that at some AOA the flow can no longer follow the shape of the wing, and there is a large reduction in the amount of lift. This is based on AOA, which in turn is based on speed and load factor. It can also be influenced by contaminants on the surface, depending on the airfoil. This is what I meant when I said "why the wing stalls and what happens". This is practical aerodynamics, and I agree that it is not complex.

____________________

 

Hi Aro, If we use flow turning as the explanation, it is easy to understand that as the wing approaches the stall the upper surface flow cannot make such a rapid angular change and breaks away. I I use teh example of a jet boat or car taking a sharp bend and spinning out. The physics is very similar.

 

Also the reason why this is useful conceptually, is that the same illustration works when we are increasing load factor and therefor stall speed. Most people have seen bow waves from a boat so they have a visual conceptual reference.

 

I think it is important to keep mathematics away because many pilots do not have the skills or knowledge to understand them-and it is not necessary.

 

The physical properties of their aircraft are determined at the design and flight test stage.. The fact that fluid flow is consistent in water and air means that most of us have a ready frame of reference.

 

 

[completeaerogeek]

Agree - I fell into the trap of arguing about his comments such as "Bernoulli is irrelevant" and just focusing on the top surface, however I did recommend that people buy the book which goes into much more detail than is practical here, including formulae and much more detailed explanations. And it also covers wings for supersonic aircraft with enough support and calculations to make your eyes spin.

 

Sprint car wings are just a high camber wing at a high angle of attack, upside down so they produce downforce rather than lift. Totally different and unrelated to the wing on an airliner.

Agree - Sprintcar wings have nothing to do with an airliner, and I only introduced it because of this quote from the geek: "Well SC wings were a real change maker. Curved on the bottom and flat on top. Messes with some people's heads!"

 

That's what he said, and a sprint car wing is curved on the bottom and flat on the top, so the resulting force is down, not up.

 

_____________-

Oh Turbo, sprint car wings and aircraft wings use exactly the same laws of physics and fluid flow principles. The aerodynamic forces create resultant forces in exactly the same way.

 

The only variation is the direction of the resultant force...

 

 

[completeaerogeek]

Agree - I fell into the trap of arguing about his comments such as "Bernoulli is irrelevant" and just focusing on the top surface, however I did recommend that people buy the book which goes into much more detail than is practical here, including formulae and much more detailed explanations. And it also covers wings for supersonic aircraft with enough support and calculations to make your eyes spin.

 

Sprint car wings are just a high camber wing at a high angle of attack, upside down so they produce downforce rather than lift. Totally different and unrelated to the wing on an airliner.

Agree - Sprintcar wings have nothing to do with an airliner, and I only introduced it because of this quote from the geek: "Well SC wings were a real change maker. Curved on the bottom and flat on top. Messes with some people's heads!"

 

That's what he said, and a sprint car wing is curved on the bottom and flat on the top, so the resulting force is down, not up.

 

_____

Ahhh Turbo- at no point did I say that Bernoulli was incorrect. As I have repeatedly said, it is derived from Newton's second Law. However, you can explain lift accurately and effectively without ever hearing of Bernoulli.

 

What I have said is that Newton provides a much clearer explanation and does not require mathematical interpretation. Also people do no understand what Bernoulli is saying, as you clearly do not, and so make up stories like Equal Transit Time and Venturi Theory.

 

 

[completeaerogeek]

If you understood Aerodynamics for Naval Aviators better you probably wouldn't be arguing with completeaerogeek. They did mean all the lift, not just the top part of the aerofoil. Bernoulli's equation explains the pressure above and below the wing (as long as the assumptions in the theory are valid) - it also tells us that as the flow slows down, the pressure increases. And lift is the difference in pressure between the top and bottom of the wing. It doesn't even matter if the pressure above increases, as long as the pressure below increases more.Sprint car wings are just a high camber wing at a high angle of attack, upside down so they produce downforce rather than lift. Totally different and unrelated to the wing on an airliner.

Hey again Aro - the 'lift is pressure thing' while true in principle can be misleading. AOA which creates angular change creates the pressure differences. NO AOA (or camber) no lift.

 

The pressure diffs are a result of turning flow caused primarily by AOA.

 

That's why a turning flow explanation is easier to understand.

 

 

[Bruce Tuncks]

Here's an example of where a knowledge of Bernoulli helped me.

 

There was a whiff of exhaust smell one day when flying. Bernoulli says that the flow in the fuselage will go from a hole in the rear forwards to exit at the doors.

 

Why? Because the flow velocity is higher at the doors and therefore the pressure is less.

 

Sure enough, on removing the sub-fin, I found that the sealing tape blocking the cable-hole had come off. Exhaust gases were mixed in with the flow at the fuselage rear and were entering the sub-fin and then into the fuselage to be sucked forward and exit at the imperfectly-sealing doors.

 

There was another aircraft at Gawler where the builder had designed and put in an "exit vent" at the fuselage rear, and I couldn't make him believe that he had made an inlet vent.

 

Air flowing forward is counter-intuitive, even inside a fuselage.

 

But yes, Complete, if you must dumb down the physics and only tell part of the story, the reaction to pushing air down is a better description of lift than Bernoulli.

 

You would be in good company, the universities now give out masters degrees for what used to be school stuff.

 

Personally, I reckon the vortex stuff explains things like wake danger and it should be taught, even though I personally find it difficult.

 

 

[Dafydd Llewellyn]

Dead right, Bruce; telling a convenient half of the story - in any technical issue - is not satisfactory; the students who hear that crap become the next generation of instructors, and so the myth becomes gospel.

 

We have had a whole series of those - one notable one was the spiel given out when tubeless tyres were first introduced: Run cooler, more blow-out resistant etc. I had all four of a set of Dunlop B5s - their first generation of tubeless, still cross-ply - blow bubbles on me, (and one massive blowout) on one trip - at night - from Wagga to Sydney. They were disastrous, back then. The real reason was that they could be put onto rims by machine, whereas tube type tyres could not; my father-in-law worked at GM Pagewood at the time, and took me through the production line; one car was coming off the line every forty seconds or so. It took about 20 men with rubber hammers to put five tube-type tyres on their rims and inflate them, in 40 seconds, and they were working flat-out. Tubeless tyres allowed that to be done by two men, working easily. It was wholly and solely a production economy reason - but the public never got told that.

 

Similarly, we got a great spiel about airborne lead (!) when lead-free petrol came in; the real reason was to allow catalytic converters to deal with the oxides of nitrogen that cause brown smog - but we got the bit about brain damage to babies, not the real reason.

 

That's only two instances; it's happening all the time. I detest it.

 

 

[metalman]

So is the horizontal stabiliser using a downforce ( due to a negative angle of attack) or is it being sucked down, because having a movable surface at the trailing edge makes Newtons law more relevant

 

 

[Dafydd Llewellyn]

The condition for longitudinal stability means that the more forward "lifting" surface - the wing in a conventional layout, or the foreplane, in a canard - must always carry a greater upward load per unit of its planform area, than the more rear surface. It does not necessarily mean that the tailplane mustalways produce a downforce; tho with a highly-cambered wing, it generally work out that way, especially at forward CG, especially if the aircraft has conventional wing flaps. Use of negative flap in cruise reduces the tailplane loads and helps performance a bit for that reason. Some aircraft - for example, the Fokker F-27 - use an inverted airfoil section on their tailplane, because it predominantly works to generate a downforce.

 

The mechanism whereby the tailplane develops either upward or downward force, is exactly the same as the mechanism by which a wing does it; and it will have its own system of vortices accordingly.

 

The fact that a tailplane operates inside the downwash field of the wing, simple alters its zero-lift angle, and the rate of change of the wing downwash as the wing angle of attack changes, means the tailplane has a reduced effective lift-curve slope - which means it has to be larger than if it were not affected by the wing downwash.

 

The presence of a separate elevator is simply a means of simultaneously altering the tailplane incidence as well as its camber.

 

 

[completeaerogeek]

Here's an example of where a knowledge of Bernoulli helped me.There was a whiff of exhaust smell one day when flying. Bernoulli says that the flow in the fuselage will go from a hole in the rear forwards to exit at the doors.

Why? Because the flow velocity is higher at the doors and therefore the pressure is less.

 

Sure enough, on removing the sub-fin, I found that the sealing tape blocking the cable-hole had come off. Exhaust gases were mixed in with the flow at the fuselage rear and were entering the sub-fin and then into the fuselage to be sucked forward and exit at the imperfectly-sealing doors.

 

There was another aircraft at Gawler where the builder had designed and put in an "exit vent" at the fuselage rear, and I couldn't make him believe that he had made an inlet vent.

 

Air flowing forward is counter-intuitive, even inside a fuselage.

 

But yes, Complete, if you must dumb down the physics and only tell part of the story, the reaction to pushing air down is a better description of lift than Bernoulli.

 

You would be in good company, the universities now give out masters degrees for what used to be school stuff.

 

Personally, I reckon the vortex stuff explains things like wake danger and it should be taught, even though I personally find it difficult.

G'day Bruce,

 

"Why? Because the flow velocity is higher at the doors and therefore the pressure is less."

 

 

See this is where not understating the specifics of Benoulli can lead you astray.

 

Please remember that the plane is moving, the air is not. What you are talking about is relative flow speed and that is not really an explanation using Bernoulli.

 

Bernoulli is simply expanding on Newton's 2nd law. F=Ma. Bernoulli applies in a single streamline not at two different unrelated locations.

 

What you are describing is a displacement disturbance. A lot of these misinterpretations would disappear if we imagine the aircraft submerged in water which behaves the same as air.

 

Imagine your aircraft immersed in water like in a shipping test tank and being moved along. Where would the water come in and why?

 

Areas exposed to direct flow will 'scoop' the water in and the water will continue moving until it finds an exit where the dynamic pressure is less.

 

As for your comments about Master's Degrees, I would respectfully suggest you either have never been to university or went to a very dodgy one. On my MSc course I had military and civilian test pilots, a professor from Cranfield University and former chief pilots from major airlines as my course supervisors.

 

It was by no means easy.

 

Your statement is unfair, inaccurate and silly.

 

Australia ranks very highly in the world in University education and in my years lecturing I was very tough on the knowledge level of would be pilots.

 

Please restrain yourself to statements supported by facts not opinions.

 

 

[completeaerogeek]

So is the horizontal stabiliser using a downforce ( due to a negative angle of attack) or is it being sucked down, because having a movable surface at the trailing edge makes Newtons law more relevant

A horizontal stabiliser is an aerofoil that uses a design that turns flow upwards.

 

It does not 'suck' anything. It creates a resultant force on the aircraft by bending the airflow upwards instead of downwards. The amount of force can be varied by moving the stab through a range that either increases or decreases the angular momentum change of the air, no different from pitching an aircraft up and down..

 

Simples...

 

 

[completeaerogeek]

The condition for longitudinal stability means that the more forward "lifting" surface - the wing in a conventional layout, or the foreplane, in a canard - must always carry a greater upward load per unit of its planform area, than the more rear surface. It does not necessarily mean that the tailplane mustalways produce a downforce; tho with a highly-cambered wing, it generally work out that way, especially at forward CG, especially if the aircraft has conventional wing flaps. Use of negative flap in cruise reduces the tailplane loads and helps performance a bit for that reason. Some aircraft - for example, the Fokker F-27 - use an inverted airfoil section on their tailplane, because it predominantly works to generate a downforce.The mechanism whereby the tailplane develops either upward or downward force, is exactly the same as the mechanism by which a wing does it; and it will have its own system of vortices accordingly.

 

The fact that a tailplane operates inside the downwash field of the wing, simple alters its zero-lift angle, and the rate of change of the wing downwash as the wing angle of attack changes, means the tailplane has a reduced effective lift-curve slope - which means it has to be larger than if it were not affected by the wing downwash.

 

The presence of a separate elevator is simply a means of simultaneously altering the tailplane incidence as well as its camber.

Hi Daffyd,

 

What it a downwash field? I thought the whole downwash thing was dead and buried? If the resultant flow vector of the air at the trailing edge is downward (there is no 'washing, the air is exiting the trailing edge at a lower 'relative speed' than the wing due to form drag and induced drag) there is no way for the disturbed air to impinge on the stabiliser except at very high AOA.

 

(The is the reason for deep stall in T tailed aircraft)

 

Also most non canard aircraft that I am aware of have the 'inverted aerofoil' tail as a requirement of dynamic stability but the primary reason for it is that the CofG is forward of the C/L and the stab provides a balancing force to maintain longitudinal stability. It is trimmed to allow for drag minimisation as CofG and C/L changes due to fuel use, airspeed changes, flap changes and other changes in flight.

 

 

[completeaerogeek]

So is the horizontal stabiliser using a downforce ( due to a negative angle of attack) or is it being sucked down, because having a movable surface at the trailing edge makes Newtons law more relevant

Newton's laws are never NOT relevant. They govern motion in every thing we do. Remember it is not a competition. Bernoulli is derived from Newton's 2nd Law.

 

 

[completeaerogeek]

He answered a question no one asked and then expects everyone to fall over themselves praising him at the foot of his plinth.Now excuse me, I'm late for a witch trial, anyone got a spare duck?

No but I have some wood for a weight comparison.

 

Who asked the question no-one was asking? Isn't understanding how your aircraft works a good thing?

 

 

[AVOCET]

Quote"... "If any body ever tells you anything about an aeroplane which is so bloody complicated you carn't understand it , take it from me - its all balls ". - R J Mitchell .

 

Mike

 

 

[completeaerogeek]

Wouldn't start a victory dance just yet, he may have just grown tired of trying to educate the flat earth society

Oh no, I will not give up trying to educate people. Aviation is an applied science not a storytelling venture. Mr Turbo has been materially incorrect I every statement he has made... It just shows how stubborn people can be when their false beliefs are challenged. The key to aviation safety is learning. Always learning. The days of 'I am the Captain don't argue" are long gone because they left the countryside strewn with bodies and burning aircraft.

 

Many years ago, on my aircrew course I was taught that challenge and response checklists must be given correctly. If one word of the response was correct it was repeated so that the understanding was clear. We cannot have aviation sitting 'thumb in bum - mind in neutral' and just accepting what they are told.

 

it just leads to tragedy.

 

 

[completeaerogeek]

Quote"... "If any body ever tells you anything about an aeroplane which is so bloody complicated you carn't understand it , take it from me - its all balls ". - R J Mitchell .Mike

Largely true. (just not so much in the small details of aerodynamics)

 

Lift is conceptually simple. Anyone who has stuck their hand out of a car window has experienced it. It can be explained simply to a six year old who has never heard of Bernoulli but apparently not to some amateur pilots who have...

 

 

[Dafydd Llewellyn]

Hi Daffyd,What it a downwash field? I thought the whole downwash thing was dead and buried? If the resultant flow vector of the air at the trailing edge is downward (there is no 'washing, the air is exiting the trailing edge at a lower 'relative speed' than the wing due to form drag and induced drag) there is no way for the disturbed air to impinge on the stabiliser except at very high AOA.

 

(The is the reason for deep stall in T tailed aircraft)

 

Also most non canard aircraft that I am aware of have the 'inverted aerofoil' tail as a requirement of dynamic stability but the primary reason for it is that the CofG is forward of the C/L and the stab provides a balancing force to maintain longitudinal stability. It is trimmed to allow for drag minimisation as CofG and C/L changes due to fuel use, airspeed changes, flap changes and other changes in flight.

I suggest you look at the Royal Aero Society data (now ESDU data) on the subject of the downwash field caused by the wing vortex system. Call it a flow vector if you will; I'm not going to get into an argument with you over semantics. I'm not aware that the Biot-Savart "law" has been repealed. I'm not talking about the disturbed air in the wing wake - though that can have significant effects on longitudinal stability. However the positioning of the horizontal tail is a more critical design consideration than is commonly recognised.

 

 

[completeaerogeek]

Hi Daffyd, no intent to argue here just to make sure I understand you. Lots of people refer to' downwash' as a reason for lift but one the air has transited the wing it has no further effect.

 

I am very aware of the positioning of the tailplane as a a design factor hence my reference to Deep Stall.

 

I will look into the issues you have mentioned. I am always keen to learn something new. Thanks for your feedback.

 

 

[Bruce Tuncks]

Hi Complete, all motion is relative. If the wind was blowing at 60 knots and the Jabiru was flying at 60 knots into the wind then there would not be the slightest difference in what I described, even though some silly people might think that because the Jabiru was now stationary with respect to the ground there was a big difference.

 

And Bernoulli does indeed apply to two different points along a streamline, and a streamline which passes the Jabiru doors and then passes the tail fits this perfectly. I don't know what you mean by "displacement disturbance" and how you relate this to pressure differences, please explain with some calculations.

 

On the subject of university degrees, I know only too well how dumbing down has taken over in some places, having fought a rearguard action for years from within. And yet there are high standards still out there in some places as you say. I reckon we need a national public examination authority to set and mark university exams. I personally knew lecturers who passed students (who they should have failed) because they saw that this was in their best career interests.

 

And, Complete, I am wondering if you deserve a pass grade after your comments about relative motion and Bernoulli and displacement disturbances.

 

 

[Dafydd Llewellyn]

Hi Daffyd, no intent to argue here just to make sure I understand you. Lots of people refer to' downwash' as a reason for lift but one the air has transited the wing it has no further effect.I am very aware of the positioning of the tailplane as a a design factor hence my reference to Deep Stall.

 

I will look into the issues you have mentioned. I am always keen to learn something new. Thanks for your feedback.

You may find this helpful http://www.tongji.edu.cn/~zyjin/AerodynamicsChapter5.pdf - especially when he gets to explaining the application of the Biot-Savart law to the "horseshoe" vortex system.

 

 

[completeaerogeek]

This is what I said: "Supercritical wings were introduced originally to help break the sound barrier and are irrelevant to RA and GA flying and irrelevant to supersonic flying, and seem to have been used just as distraction by the geek."This was the Wikipedia paragraph I quoted from:

 

"Research aircraft of the 1950s and '60s found it difficult to break the sound barrier, or even reach Mach 0.9, with conventional airfoils. Supersonic airflow over the upper surface of the traditional airfoil induced excessive wave drag and a form of stability loss called Mach tuck. Due to the airfoil shape used, supercritical wings experience these problems less severely and at much higher speeds, thus allowing the wing to maintain high performance at speeds closer to Mach 1."

 

Since despite, explanations you've been unable to comprehend that this comment was solely used due to your assertion that a supercritical wing has a flat top and a curved bottom, I'll leave you to work through this one yourself.

 

I don't now how you do it! You have misread this again! It did not say that supercritical wings provided the solution to supersonic flight. They were not invented until years later.. They are two separate statements. it says that SC aerofoils do not suffer this as much it did not say they were the cure to M1.0. Tha's what you get for using Wiki... educated people do not. We use the references and actually read them.

 

Don't be sad, I only quoted a passage verbatim from the book and recommended people buy it. How that translates into understanding/not understanding I don't know.

 

Yes, I'm sorry Tomahawk

 

Let's not get off the subject, you were talking about supercritical aerofoil sections, so you can bite your tongue on this one.

 

You were corrected on this one by another poster but appear to have overlooked it.

 

I didn't say you did although you certainly didn't emphasis that in the NASA material you supply to cadets NASA says it is right. What you did say is that Bernoulli is irrelevant and that's BS.

 

Did I say that Bernoulli is irrelevant? If you choose to see that as irrelevant so be it.

 

The fact is that you don't need it at all to understand how wings work unless you are an aerodynamicist and then only for very specific uses. Newtonian explanations are much clearer and not open to misinterpretation.

 

No that's not my assertion - you said "A SC wing (that most pilots will spend most of their careers sitting on top of)"

 

I pointed out you were talking to the wrong audience on this site where most of the people fly for recreation and only three or four are involved in airline flying. I also gave you a link to a site where professional pilots, instructors and students wo will go on to fly for airlines hang out.

 

And I showed you that a Tomahawk uses an aerofoil with SC characteristics.

 

It is amusing that virtually all your triumphant winning bullet points appear to have flaws; what's not so amusing is that this two dimensional garbage could be coming out the end of one of our Universities.

You mean all the times I proved you wrong? And I assume you do not have a tertiary education. Your insult to Universities is not supported by any facts. Just your own ignorant hubris.

 

 

 

Your understanding of how things work is so poor that you say that 'sometimes a Sprint car wing acts like a flat plate and sometimes like an aerofoil.'

 

 

 

A flat plate inclined to airflow IS and aerofoil.

 

 

 

 

 

 

 

 

 

[completeaerogeek]

Hi Complete, all motion is relative. If the wind was blowing at 60 knots and the Jabiru was flying at 60 knots into the wind then there would not be the slightest difference in what I described, even though some silly people might think that because the Jabiru was now stationary with respect to the ground there was a big difference.And Bernoulli does indeed apply to two different points along a streamline, and a streamline which passes the Jabiru doors and then passes the tail fits this perfectly. I don't know what you mean by "displacement disturbance" and how you relate this to pressure differences, please explain with some calculations.

 

On the subject of university degrees, I know only too well how dumbing down has taken over in some places, having fought a rearguard action for years from within. And yet there are high standards still out there in some places as you say. I reckon we need a national public examination authority to set and mark university exams. I personally knew lecturers who passed students (who they should have failed) because they saw that this was in their best career interests.

 

And, Complete, I am wondering if you deserve a pass grade after your comments about relative motion and Bernoulli and displacement disturbances.

_________________________________

Please read my statement again and let let me know specifically where it is incorrect. I am happy to learn.

 

Bernoulli does apply along two different places of a streamline. That particular streamline, not somewhere else in the flow field.

 

Bernoulli is explaining F=Ma.

 

So what I would like to know is:

 

To what force is the static air in the hidden part of your tail section subject?

 

By conventional Bernoulli explanations the higher relative speed of the flow past the empennage would mean a lower pressure.

 

Would not the static air inside the aircraft push the fumes out not in?

 

When you crack the window of your car does the air go out or in?

 

It goes in because it has been displaced by the solid form of the car just like a boat displaces water.

 

If the 'faster air' outside, moving past your window was at a lower pressure as you suggest, the air would go out not in would it not?

 

Let me know about that passing grade.

 

In mathematics, as I have said earlier the perspective does not matter but conceptually it does vert much. Relative velocity as a concept is skewed by imagining the air moving as it implies that the air has kinetic energy at rest which it does not.

 

Pilots only need a conceptual understanding of lift and Newton does that without any complications.

 

Can you supply a wind tunnel photo of a Jab and we will see here the streamlines go. Again, imaging the aircraft immersed in water and see what you think the cause is.

 

 

[completeaerogeek]

Hi Complete, all motion is relative. If the wind was blowing at 60 knots and the Jabiru was flying at 60 knots into the wind then there would not be the slightest difference in what I described, even though some silly people might think that because the Jabiru was now stationary with respect to the ground there was a big difference.And Bernoulli does indeed apply to two different points along a streamline, and a streamline which passes the Jabiru doors and then passes the tail fits this perfectly. I don't know what you mean by "displacement disturbance" and how you relate this to pressure differences, please explain with some calculations.

 

On the subject of university degrees, I know only too well how dumbing down has taken over in some places, having fought a rearguard action for years from within. And yet there are high standards still out there in some places as you say. I reckon we need a national public examination authority to set and mark university exams. I personally knew lecturers who passed students (who they should have failed) because they saw that this was in their best career interests.

 

And, Complete, I am wondering if you deserve a pass grade after your comments about relative motion and Bernoulli and displacement disturbances.

Hi Bruce,

 

My apologies about the Uni bit I have sticky key problems. That was meant for Turbo and his nasty little insults.

 

I agree with you about the National Assessment idea but the variation in lecturers is a problem world wide. I lived and worked in the US for 9 years a lot of that for a major carrier and I was often astounded at the lack of knowledge of college graduates I dealt with.

 

Still the level of education here is far better than many places. OECD shows us 13th in the world where the US/UK are around 35th.

 

 

[completeaerogeek]

I suggest you look at the Royal Aero Society data (now ESDU data) on the subject of the downwash field caused by the wing vortex system. Call it a flow vector if you will; I'm not going to get into an argument with you over semantics. I'm not aware that the Biot-Savart "law" has been repealed. I'm not talking about the disturbed air in the wing wake - though that can have significant effects on longitudinal stability. However the positioning of the horizontal tail is a more critical design consideration than is commonly recognised.

Hi Daffyd,

I have looked up and read several references on this. Biot-Savart is associated with Prandtl's lifting line theory and to do with circulation theory and unbound vortices.

 

A couple of problems I can see:

 

The Kutta condition ceases with vortex breakaway at relatively low speeds.

 

Winglets and swept tips significantly reduce the strength of wing tip vortices and unbound vortices have no influence on the wing.

 

Downwash velocity of wingtip vortices is does not appear to be relevant.

 

In any case this is far too complex for a pilot. Again this can be left to aerodynamicists to argue over.

 

The basic Newtonian explanation is clear and correct in terms of lift due to turning flow which was my original point.

 

I hope that clears up my question.