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Theory on lift

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Theory on lift

Old 18th Aug 2012, 18:56
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Hi Pugilistic animus !

Karman of course ! and Libchaber, Lorenz, Feigenbaum, and so many great great great physicians, mathematicians....

RIP taboos, and Boltzmann, Navier or Stokes unsolvable equations.

Welcome to all the real BIRDS who know all that from the nest... I am jaleous !

The AF447 discussions shows that it is essential to open the door to future.... Really !
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Old 19th Aug 2012, 10:41
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roulishollandais it's not really that there's 'no solution' per se, it's just that there's no closed solution to the NS formulations...a complete examination of the equations of motion and thermodynamics, with repect to fluids yields two very important non-dimensional quantities...Mach Number and Reynold's number--- that is exactly where ALL of our problems arise...

Aerodynamics, is for the most part an experimental art, and it is an art, as so many technical field's really are...

Mad(flt)Scientist has a habit of revealing some of the secrets of design on PPRuNe...for serious students of the field I believe his posts are definitely worth a read...

of course we must all remember two very important thing to progress with a design... for better or worse, new ideas are always dangerous and that nature laugh at complex mathematics...laughs!!!


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Old 19th Aug 2012, 14:12
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Aerodynamics, is for the most part an experimental art, and it is an art, as so many technical field's really are...
Isn't that the truth.

It always makes me smile when people get adamant about Newton in discussing where lift comes from. They seem to totally ignore the fact that wind tunnels have been around for quite a while and in those it is easy to measure the actual pressure at the wing surface on both the top and the underside.

I was brought up to believe that to be correct a theory had to fit in with the facts. Hey ho.
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Old 19th Aug 2012, 14:26
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Wind tunnels are very useful for making measurements but it is important to acknowledge that they are physically bounded so that airflow is always horizontal at the upper and lower boundaries. In free air the flow differs. Traditional aerodynamic theory is based on dimensional analysis and simple algebra, but with the power of modern computer modelling it would be possible to run large-scale elemental simulations based entirely on Newtonian mechanics and thermodynamic behaviour of each element.

Last edited by mike-wsm; 19th Aug 2012 at 14:35.
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Old 19th Aug 2012, 17:36
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Ah the Professor and Mr. Farley, great company indeed!

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Old 19th Aug 2012, 19:47
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Humbling, eh, PA? Only a few blokes could make me doubt Newton...
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Old 19th Aug 2012, 19:54
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Originally Posted by Pugilistic Animus
complex mathematics
... "Math pures" complexity start as soon at ... zero ! 1 is not simpler, etc. . Heappily we have "mathematiques appliquées" and "calculs astronomiques" to do it simplier ! And Libchaber two little turbulence are no more complex at all !


Originally Posted by Pugilistic Animus
roulishollandais it's not really that there's 'no solution' per se, it's just that there's no closed solution to the NS formulations...a complete examination of the equations of motion and thermodynamics, with repect to fluids yields two very important non-dimensional quantities...Mach Number and Reynold's number--- that is exactly where ALL of our problems arise...
I did it a little short whith NS ! as I am a little lazy with words (in my "fractured English" as Organfreak said it to me... ) : "no analytical solutions" of course (I knew you knew!). And computer solutions don't get much to see before us (distance and time). And what with cavitation..

Originally Posted by Pugilistic Animus
new ideas are always dangerous
Marcel Dassault had that art to be very prudent with "new ideas".

Originally Posted by mike-wsm
Wind tunnels are very useful for making measurements but it is important to acknowledge that they are physically bounded so that airflow is always horizontal at the upper and lower boundaries. In free air the flow differs
Henri GIRAUD was a geant data base of airflow in free air when he died in nov.1999. I asked to the INRIA of Grenoble if they could work with him, it would have been yes, but as I asked to "Monsieur GIRAUD" he said me "No, it is too late". The civil aviation denied Henri GIRAUD importance during his whole life.


Shall visit Mado with your recommandation !

Last edited by roulishollandais; 19th Aug 2012 at 19:56. Reason: has => had (Marcel Dassault)
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Old 20th Aug 2012, 07:43
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Windtunnels just like flow dynamic computer codes are alwys just a model of reality, and none gives exact results when compared to free flight measurement. Many phenomena have been discovered in free flight and could not be confirmed in the wind tunnel or by calculation. It is hard to produce a real free air stream in a wind tunnel, there always is some turbulence created by the fan or by the walls of the air intake, there is always some slight speed gradient close to the walls, there is always some restriction in the air flow if you add walls. Vice versa any computer code can just predict what has been part of the model from where formulas have been derived.

A recent example is that a turbulent boundary layer has been found to become laminar again under certain conditions, while mathematics tells you that this is impossible, and wind tunnels can not reproduce this.

Any theory is only good until a better one comes along. Any wind tunnel is only as good as the criteria it was designed for. Any measurement method (pressure by piezo sensors, speed by LDA, drag by wake analysis...) only works if you exactly know what phenomena to measure. There is not a lot of static flow in a steady flight, there is a lot of small amplitude high frequency variations in the parameters required to fully describe certain phenomena.

It looks like we understand aerodynamics for some hundred years, but in fact we are still learning something new every year.
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Old 21st Aug 2012, 20:39
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agreed
Let us try to get better teachers to young pilots
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Old 22nd Aug 2012, 12:06
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Let us try to get better teachers to young pilots
A good point, however what they teach needs careful consideration.

Being up in the air some way above the ground with an aircraft strapped to your back is essentially a practical problem not a theoretical one.

In my view pilots need to be taught the factors that affect lift not the theory of lift. They may well be interested in the theory – and why not for goodness sake – but not at the expense of thoroughly understanding the factors that determine the lift available when they are flying.

The lift equation contains all the factors but only one term in that equation (the lift coefficient) is available for the pilot to vary in order to adjust the lift during the normal course of flying. It is the lift coefficient that is controlled by pulling and pushing on the stick (or wheel) as that varies the Angle of Attack on which the value of the lift coefficient depends.

Only Angle of Attack not speed makes a wing stall. Believing in that and living your life by it is vital to junior (and senior) pilots . The theory of how lift happens will not help you when turning finals. Only a proper appreciation of how to control lift will keep you alive.
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Old 23rd Aug 2012, 20:40
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The pilots of AF447 seemed to be ignorant of AoA.
Airbus considered that showing the AoA on the display was not useful.
Certification administrations accepted such equipped aircraft to be used for passengers public transport, not matter of AoA.
Air France buyed that aircraft without AoA display.
Pilot unions (SNPL SNPNAC SPAF UNAC, etc.) did never any strike to get AoA.
In other Countries it was the same.
ICAO did no global recommandation.
AESA accepted that mix of AoA ignorance.
WHY?
Many pilots knew enough physik before to get pilot (math sup or mor fo someone.
BUT : At the first page of any aerodynamics book for airline pilot you see the figure of a wing with airstreams thiner when passing over the wing : Well! And then it is written that these airstreams are independant (Bolzmann theory of gas). Question:How does the third airstream knows the wing is there to get thinner?
The pilot trainee decides then to forget definitively the logic and the physic laws!!! And in flying lessons it is said that good pilots are "stupid"...
NO NEED OF AOA!
Some aerobatics flights would SHOW him he is wrong. But it is the time for "NEW METHOD" (Thank you Monsieur DEBIESSE, who initiated that method in the French DGAC)
NO NEED OF AOA SENSORS!
Only a good theoric teaching can bring back confidence to physic reality.
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Old 23rd Aug 2012, 21:04
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mount a bubble in the cockpit...its as easy as that.

Old school AoA angle and inertial speed..

on a side note, if the pilot has an iPad, use the level bubble app.

there are many concepts in the works that use these types of concepts...

Last edited by FlightPathOBN; 23rd Aug 2012 at 21:11.
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Old 23rd Aug 2012, 21:14
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Just to make a quick point...regarding Rn Mn

we simply use Rn as a rough guide depending on what types of forces are dominant-to predict flow separation point and when laminar becomes turbulent---in reality it's not so simple as there are usually mixed flows...plus the interaction with shockwaves etc

At slow speeds and low altitudes and 'conservative' airfoils rather simplistic treaments can be used...as one goes higher and faster it gets progressively harder; but they try to get it as good as possible on paper---and these complex treatments are for designers-not pilots; the wind tunnel provides a better picture and is still cheaper than lots of full scale models provided you do your tests at representative Rn and Mn [or EAS] ---but the aforementioned limitations---described by mike-wsm and Volume---also prevent a full appreciation of the flight characteristics---only through flight testing can we be sure---but even at the most esoteric level---Newton still rules---i.e wing pushes air down---air pushes wing up---that never changes---the mathematical mess is for designers---John Farley's word once again ring true...Brian Abraham's synopsis of the various formulations is very interesting just hope this post puts it into some more perspective...also there are many different treatments with different limiting conditions different acuracies/precision depending on exactly what you're doing...Von Karman corrected most of Prandtl's formulations...he was probably the most exacting one...

roulishollandais
your assesment is correct 'pilots are not supposed to be too smart'..
and don't worry about your typing I'm a native English/Spanish speaker and I can guarantee that at times mine is much worse...

Il y'a etait depuis tres longue temps lorsque j'ai parlait Francais aussi ---J'ai oublie comment tout de mon Francais...I think I said that right...sorry for the missing accent marks too lazy...

I forgot most of my Italian too...



Last edited by Pugilistic Animus; 23rd Aug 2012 at 21:44. Reason: illiteracy
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Old 24th Aug 2012, 00:11
  #94 (permalink)  
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Old school AoA angle and inertial speed..

Could be just my dotage and old timer's disease taking hold again .. but, how does that work ?

I can follow the argument for body angle (rather than AoA) under non-accelerated steady state flight conditions but no more than that.
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Old 24th Aug 2012, 00:27
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John...I was wondering the same thing!
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Old 24th Aug 2012, 00:45
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Now that I think of it I get AoA part but the inertial speed???

why can't one just use AoA in isolation? ... unless are you discussing the lift slope curve dCl/alpha you could get the TAS in order to compute Cl and perhaps through that get a lift slope curve, but I dont think that's a usual method, just guessing though...
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Old 24th Aug 2012, 02:13
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.. an inclinometer knows naught about AoA, the latter involving the windy bits as well as where the wing might be pointed ? and not much about anything if there be other than gravitational forces involved.

In the same category as the old furphy about using a pencil suspended on a bit of string to let the pilot fly in IMC without an AH etc.

Or am I just getting too old for all this heavy stuff ?
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Old 24th Aug 2012, 02:55
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Oh J_T I was on my phone not my laptop, couldn't see clearly...I thought that was an AoA indicator...now I see; you can't get any information from the above set -up...but you, as you know, can get the curve from AoA and TAS data...

Last edited by Pugilistic Animus; 24th Aug 2012 at 02:56.
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Old 24th Aug 2012, 03:06
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Pass, brother.
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Old 24th Aug 2012, 03:40
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why can't one just use AoA in isolation
You can. Naval aviators use it as the prime source on a carrier approach.
In the same category as the old furphy about using a pencil suspended on a bit of string to let the pilot fly in IMC without an AH
You not telling me you can't JT? A certain test pilot from over the water not far away writes in a book that that was his back up on a round the world venture.
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