Lift %, Upper/lower wing sections
 

How much lift is created by the upper surface and how much by the lower one?

It's not some thing you can quantify because it depends both on the particular airfoil and its angle of attack. The diagram is of a representative example of the pressure field with the arrows pointing in the direction of "lift"..


https://cimg7.ibsrv.net/gimg/pprune....ccbb74c016.png

with the arrows pointing in the direction of "lift"

In essence. Strictly, the graphic shows a representation of pressure variation via pressure tapping measurements.

I've just done a few net searches to find some interesting stuff to illustrate what is going on.

this is a basic setup video for the pressure tappings

this gives a more serious story. Unfortunately the pressure variations are shown as a graph rather than as direct pressure tapping pressures which are the basis for the picture in megan's post.

I found this video showing the manometer height variation with alpha. Not as useful as I had hoped to find but it gives you the idea.

I have always been confused about this. At flight school we were told that 2/3rds of a wing's lift came from the top surface 'sucking' the wing upwards - owing to the air speeding up over the top lowering its pressure relative to the air underneath the wing.

But when looking at a propellor in flight school and commenting that since it has a similar profile to a wing, does the propellor mostly 'suck' the aircraft along?, I was told oh no, a propellor works by pushing air behind it. And it manifestly does push air behind it.

And more recently I have seen it asserted that an aircraft flies by its wings pushing down the same weight of air as the aircraft weighs, and certainly a helicopter's blades do push air down.

So, which is it, or do wing and propellor do both?

I've never liked the explanation that wings (or blades) "push" the air - at best it's a gross simplification, at worst it's badly misleading regarding the physics of what's going on (and I have a masters in this stuff).
IMHO, the best explanation of how a wing works is something like what megan posted. The physics of lift (and drag) can be explained by integrating those pressures over the entirety of the wing - or any other surface moving through the air. That's part of the beauty of it - it can be used on anything moving through the air (or even water) - integrate the pressures over the entire surface of the object, and you'll get lift, drag, as well as any rotational forces being exerted on the body.

These links give some good explanations of the many factors contributing to lift rather than just the Bernoulli considerations:

How Airplanes Fly: A Physical Description of Lift

https://www.cam.ac.uk/research/news/...gs-really-work

One of the best descriptions of how a wing works I've heard is that it throws air at the ground (by means of the pressure field about the wing). If you think in terms of a propeller or helicopter which throws air in a particular direction you can make sense of the description. A propeller produces thrust, so does the wing, the wings thrust being equal to the aircrafts weight, all things being equal, in straight and level flight.

A plot of the pressures pertaining to my previous post. Note that negative values of cp produce arrows pointing away from the airfoil, and positive values point towards the airfoil. In all cases, the formula for cp is cp=(p−p∞)/q∞
p is the local pressure, p∞ is the pressure far away from the airfoil, and q∞ is the dynamic pressure far away from the airfoil.

The dashed line is for inviscid flow, the solid line for pressure with boundary layer effects.


https://cimg7.ibsrv.net/gimg/pprune....a010cbaccf.png

A few more thoughts.

How much lift is created by the upper surface and how much by the lower one?

It varies, as suggested by others. When I find and post a better manometer bank video showing the pressure variations with angle of attack, you will get a good intuitive picture of how the pressures (and lift) vary with wing alpha. Unfortunately, while we olde phartes grew up with wind tunnels hooked up to such banks (which were great for visualisation), these days we have you beaut electronic gadgets which measure the pressures directly. Great for engineering calculations but not so great for visualisation and intuitive understanding.

Megan's initial graphic is a direct picture of a snapshot of the manometer bank story at a particular alpha value in the wind tunnel and you will see lots of similar pictures in older texts. Keep in mind that it is talking about gauge pressure which is the difference between actual pressure and the background ambient pressure. So when we are talking about a "negative" pressure, we are talking about pressures a bit below ambient. When we are talking about "positive" pressures, we are talking about pressures a bit above ambient. His second graphic is how the first usually is shown in modern texts and is more useful for the engineering folk. The first (ie a time history of the manometer display) probably is better for the student pilot's understanding of what is going on ...

At flight school we were told that 2/3rds of a wing's lift came from the top surface 'sucking' the wing upwards - owing to the air speeding up over the top lowering its pressure relative to the air underneath the wing.

Proportions will depend on alpha and the particular wing section profile. In general, the "top" surface (ie whichever side is pointing to the heavens) will carry the greater burden on pressure delta from ambient. Better to think in terms of both surfaces providing pressure stuff and the forces which arise from those pressures. What is left when you average these out by doing some sums results in the net lift left over which is the useful bit for keeping the aircraft up, rather than going down.

It is very important to keep in mind that we can't generate forces directly in fluids - fine with solids - but doesn't work with fluids. For example, if you punch a solid brick wall, hard, it hurts real bad. The brick wall is a solid and you certainly can generate forces directly by interacting with (punching) it. However, if you try to punch a fluid, say, the water in a large bucket, or the air around you, the fluid just moves out of the way and doesn't do much anything. For fluids, to generate forces, you have to have pressure differences or gradients which then can apply forces to the bulk of the fluid - much the same level of difficulty as herding cats.

You can look at this from both sides of the table. If you see the effect of a force in a fluid, eg the fluid accelerates, turns or whatever, then there MUST be a pressure gradient somewhere there in the mix which is generating the force which, in turn, is causing that acceleration, turning, or whatever it is you are watching happen. This latter consideration is a very important part of explaining what air is doing as it meanders around an aerofoil. Because it is generally so poorly explained, most students are left confused with a thought that it is all a bit of magic going on. Not at all - for the speeds we are dealing with, it can be explained in terms of Newtonian mechanics - that discipline has stood the test of time for quite a while so it probably is pretty much OK for figuring out physical things. The only "difficulty" arises because so many people forget (or never knew) that we have to think pressure gradients, rather than directly in terms of forces, to figure out what is going on in fluids.

But when looking at a propellor in flight school
I was told oh no, a propellor works by pushing air behind it

Typical lack of understanding in the flight instructor fraternity. The propeller is much the same as a wing (hence, "fling wing" for helicopters - just ask Megan about that - apart from being a nice bloke, he has spent most of his adult life assisting helicopters to repel the earth in various rotary operations). Think more in terms that the propeller is accelerating a mass of air from one side to the other. Just like with a wing, it is not one side or the other doing the work - it is the mean, or net, of both sides each doing their bit to help. Again, Professor Newton had the basics of the story for the lower speed environment (after all, he was a Professor of Mathematics at the Univ of Cambridge).

And more recently I have seen it asserted that an aircraft flies by its wings pushing down the same weight of air as the aircraft weighs, and certainly a helicopter's blades do push air down.

That all sounds about right. Prof Newton, once again.

Tdracer, being another one of us engineers, tells the same sort of story.

Andycba's links are useful stuff for reading.

At the end of the day, Coanda really was talking about other stuff. His observation about fluid jets following surface profiles is fine and can be extended to the distributed flow over a lifting surface. But you can, probably more easily and with less appeal to black magic, explain that in terms of Newtonian mechanics, pressure gradients and the resulting forces generated on fluid bodies, allied with a consideration of boundary layer pressure gradient degradation leading to flow separation from the surface.

Bernoulli's theorem. Yep, works fine for the constraints inherent in the theorem, which are then conveniently ignored in the typical (wide of the mark) aviation student textbook. This is just a statement of conservation of energy and is very useful for figuring out the sums relating pressures and speeds. Has very little (directly) to do with lift stuff.

The engineering and physics story is that of the circulation model. This started off back in the mid-1800s, and was sorted out for wings independently by Lanchester (early 1900s in England) and Prandtl (war years in Germany). Kutta and Jukowski, again independently, fixed up a problem which caused some academic head scratching. The resulting model has been used over the past century to figure out values for lift by the aerodynamics fraternity. This model is able to be demonstrated experimentally and gives results which are so close to what is measured that the difference isn't really worth worrying about. Trailing vortices are a part of this story.

For me, I think the easiest way to explain lift is to start with the circulation model and bring in other stuff as necessary to fluff out the story as required along the way for the student .... Certainly, my theory students don't seem to have all that much difficulty getting their heads around the stuff.

From the AP3456 Manual of Flying:

My head starts to hurt if I go much beyond the basic idea that efficient lift requires both Newton and Bernoulli (with a dash of Coanda, Kutta and Jukowski, and others).

We could fly with the proverbial "barn door" for a wing, given sufficent power and speed and some AoA. All Newtonian: reaction, 3rd Law and all that. But Bernoulli camber and suction/reduced-pressure on top makes it far more efficient and effective.

The fact that catastrophic loss of lift (an AoA stall, or surface contamination stall) generally comes from disrupted flow over the top of the wing seems to me to be significant.

I'm not aware of disrupted flow on the lower surface of a wing (leaving out horizontal stabilizers) ever causing much in the way of "lift" problems. Drag, yes. Weight, sometimes. Not so much lift. But I'm willing to be educated.

Thank you John and others.

So in simple terms; the pressure changes caused by an aerofoil section as it moves through the air 'turns' the "incoming" airstream through a significant angle, which produces a reaction according to Newton's 3rd law.

The majority of the pressure changing is done by the upper surface of a wing, but the wing is not 'sucked' upwards as such,

and

Both wing and propellor are doing the same thing by the same mechanism - one to produce lift, the other to produce thrust. :ok:

That sounds fair enough. Now, if you want to do some sums and figure out loads, it gets a tad more involved. Fortunately, pilots don't need to do that, we have aerodynamicists for that sort of heavy stuff.

And we have a few of those good folk on PPRuNe to keep the rest of us on the straight and narrow paths of airflow.

NASA makes it easy for ordinary folks "to do some sums and figure out loads."
https://www.grc.nasa.gov/www/k-12/airplane/foil3.html
"FoilSim Student JS is the latest (April 2019) version of the FoilSim family of interactive simulations. The different versions of FoilSim require different levels of knowledge of aerodynamics.... This web page contains the on-line student version of the FoilSim Student JS program."
Press some buttons to play around:
- select a shape - make up an aerofoil setion with a couple of clicks
- choose an angle of attack (you can vary camber on your simple aerofoil section as well)
- see the upper and lower surface pressures

It has been a long while since I've been a working aerodynamicist - but two of my current aerobatic students are aeronautical/aerospace engineers and one of them was talking Kutta etc to me today.

er...'kay.

I've just skimmed the posts, and so will offer the following... feel free to move the response to jet blast if you want to.

we have had Bernoulli's theory rammed down our throats since time amoral, and immemorial... etc. like, forever!. The morality is that it still keeps being rammed and it just ain't so. We teach it, and we examine on it and it is a nice analogy but that is all it is.

The pressure distribution charts are valid, as are the pointy arrow thingies, but the problem is we have a massive disconfubulation (like a discombobulation but better!) as to the why. The FAA teaches that the air wants to get from A to B over the top, at the same time as the flow goes underneath from the same points. Why? Does air have a mind? Does it have evil intent? is air sentient? Add a slot, and start doing the maths on what the little air mites are gonna do next, it's enough to confuse the poor wee things. For the wing/propeller comparison, if one does, and one doesn't, then we be needin' parallel universes in the same few feet, which just ain't gonna do it. No sir, it just ain't so.

We also read we gotta have deux Kutta points to get le lift de großer Flügel, which is curious, cuz the last 35 years of experimenting on propellers was all about getting rid of the trailing edge Kutta condition, which massively reduces drag (except at low AOA) and gives the CLs a happy smile. So that seems at odds with actuality.

Next time at the sink, bath, basin or pool, grab a flat/curved/airfoil shape or your hand or tea spoon... knife or another kitchen utensil sans tines and translate it across the fluid (water/beer/milk/cappuccino... etc) at no angle of attack to the vector of movement. The fluid doesn't object, and it doesn't make determinations as to going uppity or downy, it just don't, not a bit. Now do it again, and this time do it at a slight angle to the vector of motion and guess what.......

At the point where it started to move there is a vortex formed, and it rotates in a direction as... the forward flow of the vortex is in the direction of the leading edge of the blade to its trailing edge.... the whole blade tries to move in the direction that the leading edge is relative to the trailing edge (spoons, talking about mean camber lines)
when the blade gets to the other end of the pond, stop the blade, and the point where the blade stops will generate another vortex off the leading edge... both the start and stop vortexes are opposite directions to the bound vortex that was developed by the flow around the spoon/knife, foil, cheese slice, etc. So we have just proved over coffee, (tea works well too... in fact better, as the tea leaves are pretty to watch move around... and you can tell the future by reading them too). The bound vortex has a circulation that is rearwards over the "upper" surface and forwards on the lower surface... for a blade moving right to left with a positive AOA, the flow is clockwise, and start and stop vortex are anticlockwise... (Fig Newton be praised...) Add the translation velocity to the vortex and you get the velocity profile, and then go get the maffs from sitting on Boyles. And that is yon pressure distribution. join happy dots or arrows normal to the surface and you get your answers. The nice thing is, it gives the correct answer for flows in Slats, flap gaps and for Flettners thingey, and tennis balls with spin, or Donald J's slice through the heart of the constitution.

Aerohydrodynamic lift arises from bound vortex structures, the wing is just a vortex device, as is the propeller, rotor, fan blade, compressor blade etc. Now in turbines, (termites?) when there is lots of separation going along, then the flow gets more fun, and a simple approximation comes out of impingement/reaction-flat plate geometric lift but it is still in fact vortex flow. around the blades. the wake is just that, the flow in the wake of the blade. Ion or chemical rockets are simple reaction to the impulse, but then the flow in a throat or around an expansion body is pretty interesting where there is happiness to be found in maintaining laminarity of the flow near the surface, to minimize thermal transfer to the structure and to minimize erosion.

All good fun, just like Bernoulli thought it would be.

...Of course, that also explains why fan jet engines lose efficiency at altitude when considering TSFC. It's almost like they are a FANCY FIXED PITCH PROPELLER.... what a shame that you can't change the flow around a fixed-pitch propeller... wait on, as 44 said, yes you can! And it is a lot easier than transwarp beaming...

What really puts the Bernoullis in the balus ay wok wok is playing with supercritical sections, or sonic flow, then the poor little mites get all collywobbly and confused. Vortex flow still werks gladly. kind of.
https://cimg4.ibsrv.net/gimg/pprune....469cef913.jpeg
https://cimg6.ibsrv.net/gimg/pprune....d5fdc9a51.jpeg
https://cimg7.ibsrv.net/gimg/pprune....4411ed369.jpeg
https://cimg8.ibsrv.net/gimg/pprune....af038a902.jpeg
https://cimg9.ibsrv.net/gimg/pprune....dbee3cfed.jpeg
https://cimg0.ibsrv.net/gimg/pprune....13d80b92a.jpeg
https://cimg1.ibsrv.net/gimg/pprune....01eaba4aeb.png



Maski long planti toktok!




On barn doors...

• radius of the LE alters the shape of the CL/AOA curve around peak Cl...
• Camber shifts the A-slope upwards...
• TE thickness moves the slope like camber...
• T/C alters Cd/AOA...
• %MAC for max thickness alters Cd...
• % chord for max camber alters Cm...

For helicopters, none of the above means much once the blade starts to turn, dynamic pitch changes make everythang much more interesting, there is no clean smooth line of coefficient for a rotor if the inflow is not perfectly axisymmetrical.... they all look like D,s P's and lopsided and inverted T's instead of pretty clean lines like you get to see in I'm an Abbott and a VonWanderedoff's [1] wonder book on Fairies of wing sections, which gets us back to Bernoulli.




PS: when you get to sit over the wing sometime, on yon jet transport Boeing/Bus/Embraer... etc and you have the sun aligned with the quarter chord, either away from the top or in the opposite direction, you will see the shockwave shadow, Herr August Toepler's schlieren image like. you will note that for most wings, it's nowhere near where it should be. It sits far forward of where the flow modeling of a section would suggest it should, for conventional sections such as BAC 450, 451, 452 etc, or for supercritical such as SC(1)-0710 and similar Whitcomb style sections, [2] [3][4] [5], Eppler's 403 etc... Flying is fun.


[1] Abbott, I. H. & Von Doenhoff, A.E., (1959) "Theory of Wing Sections Including a Summary of airfoil Data", Dover Publications, 1 Jan 1959
[2] Charles D. Harris, "NASA Supercritical Airfoils", NASA Technical Paper 2969, pp. 1-76.
[3] Richard T. Whitcomb, "REVIEW OF NASA SUPERCRITICAL AIRFOILS", National Aeronautics and Space Administration, Langley Research Center Hampton, Virginia pp. 1-17.
[4] K.Harish Kumar, CH.Kiran Kumar, T.Naveen Kumar, "CFD ANALYSIS OF RAE 2822 SUPERCRITICAL AIRFOIL AT TRANSONIC MACH SPEEDS", International Journal of Research in Engineering and Technology, Volume 04 Issue 09, September 2015, pp. 256-262
[5] Sana Kauser, Mr. Kumara Swamy, Dr. MSN Guptha "Aerodynamics Analysis Of Naca Sc (2) -0714 Supercritical Airfoil Using Computational Fluid Dynamics", International Journal of Scientific Research and Engineering Studies, 02 (06), June 2015, pp. 38-41.

Hold a sheet of paper in front of your mouth; blow over it (on the top side only). The paper should rise, the pressure falls thus the paper rises. And in this instance how to you define the ‘bottom’ surface.

Similar effect blowing through a paper tube - a paper bag open at both ends. The sides move in due to air moving through it and reducing pressure - reduced with respect to ambient.

So for either the paper sheet or tube, if the ambient pressure has not changed then the surface over which the airflows faster must contribute all of the lift - reduced pressure with respect to ambient. cf JT # 8.

Re the question, the proportions of lift should to be considered and stated relative to a datum pressure (and AoA, wing section, etc).
An example would be to normalise the pressure plots in post #2 so that the lower surface is datum (zero), then all of the lift is from the top surface because the top surface ‘arrows’ have increased due to the added lower surface ‘arrows’ being reversed.

Over to the tech-science views who might wish explain otherwise.

Oh, then there is the rotating tube, Coandã effect, in still air …

And a wedge shape wing section in supersonic flow where one surface at the leading edge is parallel with the airflow …

And my simple view of the lower wing surface is so the top surface can be fixed on it to provide lift, and structural strength enabling mounting on the fuselage.

They don't teach this anymore, it's been corrected. But there are still stragglers, since it's so simple and elegant and ties everything together...

The starting and stopping vortices are opposite to each other. The starting is CCW, stopping CW (stopping same as bound vortex). So all 4 vortices surrounding the rectangle swept by the wing's motion (starting, stopping, and 2 wingtip vortices), curl so the top goes inward, and the inside goes downward, which matches the downwash where the lift occurred.

This is two different scopes of the explanation. They don't contradict, but it can be confusing since they use some of the same terms.

The first one is a fine-grained detailed view of the action around the wing, where we can see the individual area contributions above and below. How much sucks from above vs. how much pushes from below? Here it is broken down by individual non-overlapping contributions. And as we've seen from the diagrams of all the little arrows or manometer tubes, the answer is that more sucks from above.

But there is also the larger-scale view where we're not concerned with the detailed view above. And in this view, the wind/blade simply moves air downward/backward, and this movement we tend to call pushing, and it accounts for 100% of the force. (When it comes to props and rotors this is called the actuator disk model, where we're not concerned with individual blades and all the complicated things that happen between and around them... it's just a magical disk that imparts momentum, and for a certain class of problems, this abstraction is still good enough to work with.)

"The FAA teaches that the air wants to get from A to B over the top, at the same time as the flow goes underneath from the same points. "


That would explain things, just not aerodynamic things. Why would they teach that? Where do they teach that?

Thanks, vessbot I got it now, but my flight school instructor clearly hadn't all those years ago :ok: At the time, it did not seem likely to me that a wing and a propellor worked in completely different ways.

Regarding the air "wanting to get from A to B" and why it does this above and below; remember the air is static, and the aerofoil is moving through the static air.

So the air molecules are not trying to get anywhere, they are just sitting there, having a great time, when this hulking great wing or propellor blade suddenly whizzes past them. As it goes by, the molecules near the upper or forward surface are squeezed together more than the ones underneath/behind.

I guess this lower pressure makes the air above move downwards towards the wing, and then continue downwards over the back edge, resulting in a large mass of air being "pushed" down.

My bad, late nite. A CW rotation around a section has a CCW start vortex, and the stop vortex is CW, same. as if the section is removed from the fluid, the flow continues CW.:ugh:.

Now, hold the paper so it hangs vertically. Blow down over one side (ie same experiment but twisted 90 degress so paper hangs down,). Shouldn't the paper curl to the side you blow?

Try it! Very easy to do.

It just hangs vertically, doesn't move towards the "blown" side despite air moving over one side faster.

It just hangs vertically, doesn't move towards the "blown" side despite air moving over one side faster.

Indeed, the paper trick is a little misplaced. The driver is the curvature of the paper (think boat sails), which exists when blowing horizontally, and leads to a momentum change (force which is the lift) ... but not if the paper hangs vertically.

For those who have never heard of circulation and Prandtl (and that is 99 % or more of pilots), the first video in Vessbot's post #16 is of a 16 mm film taken of a water tank experiment by Prandtl in 1929. He, and his students, had been working on getting a flow visualisation of the circulation starting/stopping vortices which they knew, theoretically, existed. They experimented with several types of particles to achieve the reflective record - one was just small flakes of aluminium. A neat sideline in the video is that the Reynold's Number suits the formation of Karman (or von Karman) vortex streets - the oscillating vortices in the wake. To demonstrate the small, incestuous nature of aviation, von Karman was one of Prandtl's academic students ...

Providing you don't want to get too involved in the sums (David, and the link to which he referred, can do that sort of stuff for us much easier then we can) the basic circulation model concepts are very easily explained and understood. I use them for my theory class students and no-one seems to have any great problem. However, due to the poor communications in the early days of aviation (when there was not much academic interest) all the enthusiast guesses as to what was going on led to all sorts of half baked ideas which became entrenched in subsequent pilot training. Think about it .. folks still prattle on about Coanda (which is of little relevance) and Bernoulli (his stuff is fine, but used rather out of place to try and explain aspects of lift) when physicists and engineers have been using the circulation model for a CENTURY to work out flow numbers.

Is it any wonder, then, that thousands upon thousands of pilots are totally confused about lift and lifting surfaces ? As a great little tee shirt, given to me last Christmas by a grandson, has it, lift is all about magic. Different bits of the aeroplane have different contributing magical qualities .... and not a word about circulation, Prandtl or fact-based simple explanations.

Aaaah! The power of vortices. Thank you John T, Vessbot, tdracer, megan and others for a really 'adult' discussion on lift. Well informed, erudite and slayers of myths!!

Insects are rather good at it too:-
https://www.nature.com/articles/521S64a
https://arxiv.org/pdf/1803.07330.pdf

Mother Nature had it sorted millions of years ago.

Going on from the two references, it is useful to review dynamic stall in our world. A significant problem for the design of those fling wing devices so Megan probably can comment ?

Interestingly, as with insects, fish use shed vortices to aid in their swimming motions ...

"The FAA teaches that the air wants to get from A to B over the top, at the same time as the flow goes underneath from the same points. "

That sounds suspiciously like the "equal transit time" idea. Air doesn't work that way if the wing is generating lift. A pet peeve of Holger Babinsky who put together this little video using his classroom portable wind tunnel.


Another video shows a bit about how things vary with alpha

Martin did his PhD thesis in the area of dynamic stall effects and then went on to do the Boeing Vertol study on the VR-7 blade and the 1094 to try and suppress the hysteresis that arises on the coefficients on the spinning rotor. The plots of CL, Cd and CM on a dynamic pitching blade is pretty depressing when you consider how much effort is taken in trying to obtain an elegant Cm for a rotor blade, like the VR7/1094, OA series, Eppler and various other attempts to achieve a Cm that doesn't result in high pitch link and swash plate loads, and then the dynamic pitching swamps by an order of magnitude the steady-state Cm values, and nothing is steady at all. The comment previously on vortex streets comes into play, every pitching event generates a shed vortex that just messes up everything. Add on top SBLI effects near the tips and that messes up the boundary layer, and is itself unsteady too. Babinsky had some success with micro VGs and with bleed for shock-related issues, but conventional VGs massively increase drag as Martin found on the rotor that has dynamic stall conditions. Our own studies showed the same outcome, you can. suppress the vibration, you just need another engine in the UH-60... When we went off with the submerged VG, that changed, drag went down in all cases over the clean blade.

The following are Star CCM+ DES of a B737 wing section, with shock formation. The section models a slat sitting with a 1mm aft step over a chord of around 5 mtrs, so the mesh used is pretty fine, the solutions were taking around 2 months of continuous supercomputer time to get a single solution concluded.





The next is an absolute pressure plot


The following is a look down the surface over the VG, again, a 1mm aft facing step of the VG on a 5 mtr foil section for Re scaling.


The R22/44/H500/B206/OH58, Safari, UH1H, and the CMRB on the SK61/HH3 showed both improved performance but Craig W's comment on the SK61/CMRB was the removal of vibration that was apparently spooky. The EN280 had an odd outcome, it had the least benefit in required hover power, but had the largest effect in the cruise power required. Then the Chinese went to cheese head land... On the B737, the old classic went from 0.805 to 0.823 for the same flow, but the effect at lower speeds was fairly modest, but shock-related vibration was damped noticeably. For all that, the trailing edge mods are vastly more interesting than the leading edge conformal VGs. Turns out that rotors like TE mods, a lot, like propellers, and fan blades. The V22 and the MH47 remain the most interesting rotors to play with, but fan blades were without a doubt most spectacular. Apart from large changes in Cl and reduction in Cd, tabs on a fan blade reduced vibration in every case we tried, including asymmetric hemispheres of the disk. That led to the observation that lift enhancement tabs also suppress dynamic stall, which occurs any time that the intake does not have axisymmetrical flow, which is pretty much always. The JT15D-1A that we used was pretty daggy, it was beaten up and was puffing out 1800lbs ST instead of 2200 at 100%. We got 1800 lbs at 92% with tabs, and 2400 at 100%, which seemed quite fair. I left before we tried the TFE 731 or our CFM56-3 in flight, but the potential remains pretty interesting. A turbo jet like the CJ-610-8/J85 dry has an TSFC of 1.00 static/SL, and about 1.00 at M0.8/FL350. A TFE731 has 0.35 static, and that degrades to 0.7 at M0.8/FL350... the fan CL collapses, it is after all a fancy fixed pitch prop. Tabs shift Cl by around 0.4-0.6 CL and can delay drag rise considerably but are geometry dependent. In all cases, tabs reduce vibration. On the R22's 63015 section, the mid-span TE tab dropped the stall Nr from the test 82% through 77% for the LE mod only to 68% for the LE & TE mod. The vibration reduction was considerable.
Wendt's study on Gary O. Wheeler's "wishbone" shows how neat Gary's design was. J.C. Lin's parametric study on micro VGs was really neat too. They have issues on a rotor, but they are interesting on a normal wing. Gourdains stuff was neat, as was the flow modeling that was being done on compressors in the Whittle lab a few years back, but compressor effectiveness is nice top have, but the fan blade is losing most of its effectiveness in the cruise and is readily modified, even in the field.

Anyway, bottom line is...

• dynamic stall is a pain on rotors, blades, props and fans;
• anything that sticks up on a rotating blade causes separation and the flow flicks spanwise instead of chordwise and then sheds in unsteady vortex structures
• the conformal vortex generator surprisingly didn't exhibit the spanwise disruption, the wake remains attached as a sub-boundary embedded vortex series.
• fan blade tabbing, like sub-warp transporters, is actually quite straightforward to achieve, but you need a sense of humor and deep pockets to conclude the STC.

PS: don't try this at home... messing about with propellers/rotors and fans have inherent warning flags.

https://cimg4.ibsrv.net/gimg/pprune....7f4f113c69.png
https://cimg0.ibsrv.net/gimg/pprune....6deb3cae7a.jpg

https://cimg2.ibsrv.net/gimg/pprune....c72b26b021.jpg
https://cimg1.ibsrv.net/gimg/pprune....ffaa15d636.png

https://cimg0.ibsrv.net/gimg/pprune....415d87ba48.jpg

https://cimg6.ibsrv.net/gimg/pprune....78c76ae4c.jpeg




Babinsky, H. (2011). Shock Boundary Layer Interaction Flow Control with Micro Vortex Generators (Issue May).
Babinsky, H. (Cambridge U. (2008). Understanding Micro-Ramp Control for Shock Boundary Layer Interactions (Vol. 298).
Holden, H. A., & Babinsky, H. (2007). Effect of Microvortex Generators On Separated Normal Shock/ Boundary Layer Interactions. Journal of Aircraft, 44(1), 170–174.
Lin John C. (1999). Control of Turbulent Boundary-Layer Separation using Micro-Vortx Generators. 30th AIAA Fluid Dynamics Conference.
Lin, J. C. (NASA). (2002). Review of research on low-profile vortex generators to control boundary-layer separation. Progress in Aerospace Sciences, 38(4–5), 389–420.
Lin, J., Robinson, S., McGhee, R., & Valarezo, W. (1994). Separation control on high-lift airfoils via micro-vortex generators. Journal of Aircraft, 31(6).
Lin, J. C. (NASA La. R. C., Howard, F. G., & Selby, G. V. (Old D. U. (1990). Control of Turbulent Separated Flow Over a Rearward-Facing Ramp Using Longitudinal grooves. Journal of Aircraft, 27(3), 283–285.
Casper, J., Lin, J. C., & Yao, C. S. (2003). Effect of sub-boundary layer vortex generators on incident turbulence. 33rd Fluid Dynamics Conference.
Yao, C., Lin, J., & Allan, B. (2002). Flow-field measurement of device-induced embedded streamwise vortex on a flat plate. NASA STI/Recon Technical Report …, June, 16.
Martin, P., Wilson, J., Berry, J., & Wong, T. (2008). Passive Control of Compressible Dynamic Stall. 26th AIAA Applied Aerodynamics Conference, 1–33.
Titchener, N., & Babinsky, H. (2013). Shock Wave/Boundary-Layer Interaction Control Using a Combination of Vortex Generators and Bleed. AIAA Journal, 51(5), 1221–1233.
Storms, B. L., & Ross, J. C. (1995). Experimental Study of Lift-Enhancing Tabs in a Two-Element Airfoil. Journal of Aircraft, 32(5), 1072–1078.
Ross, J. C., Storms, B. L., & Carrannanto, P. G. (1995). Lift-Enhancing tabs on Multielement Airfoils. Journal of Aircraft, 32(3), 649–655.
Carrannanto, P. G., Storms, B. L., Ross, J. C., & Cummings, R. M. (n.d.). Navier — Stokes analysis of lift-enhancing tabs on multi-element airfoils. Aerospace.
Storms, Bruce L (Sterling Software, Moffett Field, C. 94035), & Jang, Cory S (California Polytechnic State University, San Luis Obispo, C. 93407 E. (1993). Lift enhancement of an airfoil using a Gurney flap and vortex generators. Journal of Aircraft, 31(3), 542–547.
Cavanaugh, M. A., Tech, V., & Mason, W. H. (n.d.). Wind Tunnel Test of Gurney Flaps and T-Strips on an NACA 23012 Wing. Test, 1–18.
Singh, M. K., Dhanalakshmi, K., & Chakrabartty, S. K. (n.d.). Navier-Stokes Analysis of Airfoils with Gurney Flap. Dimension Contemporary German Arts And Letters, 1–6.
Hage, W., Meyer, R., & Schatz, M. (n.d.). Comparison of experimental and numerical work on three dimensional trailing edge modifications on airfoils. Cfd4aircraft.Com, 1–11.Catalano, F. M., Ceròn, E. D., & Greco, P. C. (n.d.). TRAILING EDGE TREATMENT TO ENHANCE HIGH LIFT SYSTEM PERFORMANCE. Icas.Org, 1–10.
Wendt, B. J., & Hingst, W. R. (n.d.). Measurement and Modeling of Flow Structure in the Wake of a Low Profile “Wishbone” Vortex Generator.
Fernández, U., Réthoré, P., & Sørensen, N. N. (n.d.). Comparison of four different models of vortex generators. 1.
Gourdain, N., & Leboeuf, F. (n.d.). Unsteady Simulation of an Axial Compressor Stage with Passive Control Strategies. Cerfacs.Fr.

Some interesting stuff there. I never cease to be amazed at what PPRuNe manages to pick up in the way of expertise.

I guess fdr has let Megan off the hook re dynamic stall problems in the rotary world.

The utilisation of the lift phenomena can show up in unexpected places. There is not a helicopter pilot in the world who doesn't under stand what the phrase "running out of pedal" means. For the incognisant it is when the pilot has demanded a level of power from the main rotor that the tail rotor is unable to counter, so the helo begins an uncontrollable yaw. The pilots of UH-1C gunships flown in Vietnam didn't have the power to sustain a prolonged hover when loaded for bear. To get from the revetment to the runway involved a series of jumps. Pilot would pull into the hover and start hover taxiing, the rotor would begin to bleed RPM, when reaching the point that you ran out of pedal, chop the throttle, thus regaining pedal control, perform a hovering autorotation, wind throttle back on after landing, rinse and repeat. On reaching the runway a running take off would be made, at times the two back seat crew would run along side to ease the load and jump back on board when the aircraft reached translational lift. An enterprising individual some where realised that the rotor wash passing over the tail boom generated lift in a horizontal direction and could be tailored to aid the tail rotor. Strakes fitted to the side of the tail boom trip the airflow into turbulence, effectively stalling that side of the tail boom. Now standard fit on some types.

PS: Just after we left the UH-1C was upgraded to UH-1M standard by simply replacing T53-11 engine with the T53-13, bliss, you could hover.


https://cimg2.ibsrv.net/gimg/pprune....9847a9adaf.png

Now, how do we keep this thread running forever, just getting better and better ?

Good stuff Megan.

While I was doing the slat trick on the B737, I got asked to help a UH-1H/T700 that was doing some interesting work over water, at low speed, and with one very large mass appended at the end of the sling. The guys were having some excitement when operating below ETL, at high power, lots of barf bags needed, and expensive splashes were issues... lots of torque, same ol' TR system... I had previously done some testing on a B206BIII and an OH58 with different blades... and they had been able to come and see what we were doing on wings and props. The UH1 with the big donk gets lots of available torque, but the TR is standard. We placed a mod on the MRB LE and that worked well, but they wanted as much TR authority as could be gained. While i have modded TRBs before, I expected that they would get enough out of 2 foam rubber strips on the port side of the boom, at the 7 and 10 positions, and a couple of rows of foam rubber VGs on the RHS. The 3 remaining options, tabbing the MRB, putting CVGs on the TRB and tabbing the TRB were not needed.

The trick with tabbing a propeller, rotor or fan blade is doing that and not killing yourself at the same time. Most of the outcome is benign, remarkably so, but I recall being taught that adding mass to a TE could define bad days, and having half a dozen torsion and bending modes happening seemed like a bit of an issue to put anything in there that would alter the moment of inertia of the area with some of the highest strains... seemed like a good way to cause a super quick failure of the skins, or to cause the bonding to fail instantly, or to have the tab blow apart, or all 3 options. Upside was that fluids don't care what a device is made of, just if it will cause the desired flow modification, and for most tabs, that's a set of 3 transverse vortices and a couple of stagnation points. 30+ years on props gave some confidence on the rotor to try that out, and golly, if it isn't quite an interesting outcome.

While rotors suck as far as dynamic stall effects go, they really do, they are also rather sensitive to the addition of any disruption to the boundary layer, which just doesn't seem fair; there is no silver lining. Nevertheless, things can be improved, although lordy, the Enstrom and its 0015 section is darn hard to make happier in the hover without getting serious. Still, a rotor head that permits you to take your hands off the cyclic and fold them has some drawcards. Don't try that on an R-22...

anyway,

for some background, for a certain Brand X rotor, a vanilla 63105 club, the following are recorded data outcomes... It is consistent with the 2 D RANS CFD analysis done well before and published in AIAA J. Aircraft, and is also consistent with an EN done in early '90s (IIRC) looking at the BO-105 MR in autorotation, and certain modifications to same; using Wayne Johnsons CAMRAD II code.

Putting a tape on the LE increases the stall Nr of the blade, and takes more ergs to hover. Representative stall clean at 82%, 68 units of torque, add tape to the last 3 ft of the blade, and the stall Nr goes to 85%, and torque to 72u.
Place a conformal VG tape (a weird variant of a zig-zag tape although doing a different resultant flow...) on the same location, and the stall drops to 77%, and torque goes to 65u... add 3 feet of tab mid-span, and the stall Ne goes to 68%, and (seriously) the torque drops to... 34u. that is thirty-four, relative to a NACA clean section, 68... Look closely at the effect of an LET and it can be seen that the outcome is geometry dependent, a matter that caused major delays with development of LETs anywhere on planes, but in the right spot, it is possible to both get massive changes to drag at the same time as lift gets a large boost, of around 0.4 to 0.6 CL. Works on props, works on rotors, and it worked on the static test of the fan blade. LETs can impact the trim condition for auto rotation, and that was, without doubt, the most concerning flight that I ever flew, and that did include a parachute and a means to remove blades and not heads from the helo.... In comparison, playing with props and fans is much less exciting.

To prove the point on the effectiveness of the LET being actual, and not observer bias apart from the instrumentation of the performance and engine instruments, and straining the rotor root and pitch links, the flight controls were measured and recorded as well s displayed in real-time. The "standard" NACA clean rotor gets to stall Nr about the time the pedal hits the stop, For the conventional HS8663/8671 linear blade tape, a stall occurs as LTE has commenced, gets interesting. For the conformal VG tape, around 25% pedal remained at stall. for the tabbed blade, the pedal required was less than 50%, and turns in the anti-torque direct had essentially no constraints, other than ear plugs for the blaring underspeed (underspend?) warnings. From idle, the collective could be raised fully up, the throttle increased, and the rotor would accelerate. (don't do that at home, certainly not with anything that uses TT straps...) increasing the Nr to 68% resulted in liftoff, hover and some weird and uncomfortable flight. Rotors should be at normal Nr at all times, but from a standpoint of weird science, not sure that any blade at full collective will accelerate from idle without a tab. It suggested that the reduction in torque demands with a tabbed blade are quite real. They are repeatable and are consistent with published works with AIAA and on NASA NTRS. Just happens that tabs can be done.

The V-22 and the MH-47 are the poster children of systems in need of a bit of tabbing... look at the video of the U.S.S. Green Bay bingle... But, as much fun as rotors are, the most interesting application, other than ships props, C-130 props etc, are fan blades... I bought a jet just to do that, and circumstances intervened, but darn if that isn't more interesting than the effect on a rotor.

Just to be clear, fan blades are fancy fixed pitch props, they are a little more, once the overall effect of intake geometry has occurred. Thereafter, the TSFC collapses as TAS changes. The TSFC for a pure jet more or less is constant, nearly so, whereas the TSFC of the fanjet degrades by a factor of at least 2 to M0.8/FL350 from SL/static. That is where the parametric effects of tabs gets interesting. If anyone has a spare CFM56, TF-30, JT9, RB211, GE90... (or TFE731... mine are spoken for which is a necessary nuisance)

End of the day gotta go stop a large cat from snoring. Maine coons are an acquired taste :}

Aerodynamic cat plan form:

https://cimg0.ibsrv.net/gimg/pprune....86f4e1810.jpeg

Coanda effect has relevance indeed ; it effect shows in its purest form the Newton law, thanks to
which the lift is generated. In fact , even without any pressure difference between upper/lower side, the curved shape generates lift,
thanks to imparting a deviation to the fluid flow - try with a spoon under tap water flow, where pressure is the same
on the upper and lower side of the spoon (it is the athmospheric pressure, both for the water side and the air side ) .

Obviously , for Coanda effect to take place, fluid shall not be inviscid,
otherwise it will not remain attached to the "upper" side, and consequently will not be forced to deviate from its initial direction.

When I tought basic aerodynamic to pilots, I started with water skis ; there, the lift is generated purely by deviating
a mass of water, creating an opposite force which lifted the ski. Independently from the difference in static pressure above and below the ski
(which is nil, since above the ski there is the static sea level atmospheric pressure , and below there is the static sea level water pressure,
which is equal to the atmospheric pressure !), lift is created just by deviating, the incoming water flow, which obviously creates a dynamic pressure.

The beauty of the wing profile is that , thanks to specific pressure distributions obtaine throug a peculiar shape, it is able to deviate the incoming mass flow with much less drag
than a water ski, even at a zero angle of attack, wsomething that the water ski is not able to do.

(a) Coanda effect has relevance indeed .... even without any pressure difference between upper/lower side, the curved shape generates lift

I don't think so. Else, how do you explain Jonkster's last comment in Post #21 ? If you don't have a pressure delta, you don't have a force. You really don't need to appeal to Coanda, the effect is explained with reasonable ease by Newtonian thoughts.

While you can impose a force directly on a solid due to its ability to withstand shear stresses, that is not feasible with fluids, either liquid or gaseous. In the case of a fluid, there must be a pressure gradient or delta over a region for a force to be developed. With a moving fluid turning a corner, there is a transverse pressure gradient which does the honours - hence the usual graphics associated with flow over an aerofoil.

Can you offer any objective evidence (say, pressure tapping data) to support your theses that even without any pressure difference between upper/lower side, the curved shape generates lift and try with a spoon under tap water flow, where pressure is the same on the upper and lower side of the spoon (it is the athmospheric pressure, both for the water side and the air side ) .

(b) lift is created just by deviating, the incoming water flow, which obviously creates a dynamic pressure. But this is not very relevant to what an aerofoil might be doing ?

Playing with lift. Talking to a delta wing fighter driver who had a penchant for low flying related how if you had the aircraft sitting on the ground cushion you could push the stick forward and the nose would lower, but that's all that happened. Amount/degree of 'push' not elaborated upon. The only explanation I could come up with later when giving it some thought was the increased camber by the elevons being deflected down counter balanced the "loss" of lift due to the reduced angle of attack, so the status quo remained. Not a chap inclined to stretch things for a good story.

My father held a B.S. in Mechanical Engineering from the Georgia Institute of Technology and a M.Sc. in Marine Architecture from the United States Naval Academy. Dad was the lead designer of the Essex Class aircraft carrier hull; he was also a true gentleman with a superlative sense of humo(u)r and the heart and patience to be a wonderful parent.

I had lengthy conversations with him regarding the possibility of hydrofoil sailing vessels. He claimed that such a ship was an impossibility - I felt that the design was practicable. I lost my father in 1975, but wish for myriad reasons that he had been with us when the 75-foot beauty shown below made its debut in 2012. Had he seen it, Dad would have clapped me on the back and said "You were right all along, son. Let's go get a beer!"

I wonder how lift production in a hydrofoil "wing" traversing an incompressible fluid (water) varies from that of an aircraft wing encountering compressible airflow. Extra credit for an explanation of that beautiful sail/wing....

- Ed

https://cimg3.ibsrv.net/gimg/pprune....54ced8fe9a.jpg

Playing with lift

An interesting thought, I shall have to ponder it a while. Or, perhaps enlist the expertise of a couple of our aerodynamicists on the matter. Ah, looking back, again, I see Dave has been here already, so I have sent an email off to Clive to see if he might have some thoughts to offer for the other end of his professional speed spectrum.

He claimed that such a ship was an impossibility .... I wonder how lift production in a hydrofoil "wing" traversing an incompressible fluid (water) varies from that of an aircraft wing encountering compressible airflow.

What a lovely shot. A bit above my little RL24 trailer sailer toy class, I'm afraid.

For lower speeds, we can treat air as incompressible with little error. While I have never done any work on hydrofoils, I don't see that they would be doing anything much different to a wing, so far as flow and forces are concerned. Some area, some camber, and a good dose of speed and away we go ?

Indeed, much work is done in water tanks ... the earlier link to Prandtl's 16mm movies relate to work done in a water tank. Air (at a suitably low speed) and water ... same, same ?

Far higher Reynold's number for water at similar speed; also, look at the relative areas for carrying the opposing forces.

Ah, once again, I get pulled up for being a bit brief in a story. Consider my wrist to have been slapped.

For those who might be wondering a bit at MechEngr's comments, it may be worth having a play with the following links

Reynolds number and dynamic similarity of fluid flows – Flow Illustrator

Similitude - Wikipedia

If it all gets a bit too confusing, don't worry about it. Just accept that the boffins can play with toys to get useful measurements in ways which don't always make immediate sense to the casual observer.

I recall a tale as an undergrad student - a bevy of politicians was being given a Cook's Tour of a water model of a local beach structure in the Mech Eng lab which had been set up to investigate flow patterns to figure out a way to reduce some erosion problems. One of the pollies was aghast - the model was nothing like the dimensions and depths of the real deal so, quite clearly, the whole thing was a bit of nonsense. Likewise, I can recall a Senate Enquiry with which I had some involvement. A couple of the worthy Senators just couldn't get their heads around what we were talking about in respect of some flutter stuff with a particular aeroplane's empennage - Dave will recall that one, I'm sure.

Sometimes, it gets a tad difficult to get folks to understand the protocols used when playing with similitude and flow modelling.

No slap intended - just a couple of neurons waking up to recall the Fluid Dynamics class and recalling seeing dye flow tests on small scale models for supersonic designs operating at low speed and high angle of attack in water tunnels to visualize vortex flow.

The one that people get wrong all the time is thinking that rubber is compressible; it is stiffer in compression than steel is. However, it's more elastic and can, if allowed, flex enough to get out of the way. Confine it and the difference becomes really clear. People think air is compressible because they can use an air pump and compress it - but that's only because it is confined. Let it have freedom to get out of the way and compressing it is really difficult.

it is stiffer in compression than steel is

I learn something new every day ..

Let it have freedom to get out of the way and compressing it is really difficult.

A useful tale which highlights the confusion in many minds regarding the need for a pressure gradient /delta to generate forces in fluids. It is very hard to belt a fluid into submission, in the manner of a lump of hard stuff, when it just slips and slides out of the way.

Thank you for your answers, john tullamarine and MechEngr! I found the site hyperlinked below to be interesting. It includes some brief but breath-taking video of hydrofoil vessels under full sail. Interesting that the foil itself is a blade two meters in l.o.a. and a width of 0.7 meters. I continue to search for a cross-sectional representation. Astounding to me that these beasts can make in excess of 60 knots!

America's Cup Hydrofoils

- Ed

John,
I wasn't going to post anything as it has all been said, but since you ask:
Concorde, like many supersonic aircraft, has wings swept so that the flow normal to the LE is subsonic. That being so, the usual explanation fits, that is to say:-

An airfoil passing through a block of static air changes the state of that block in that some of it is left with a substantially downwards velocity

Newton tells us that to change something's state there must be a force applied

In this case that force is the mass flow affected per second times the incremental velocity

Newton can tell us nothing about the affected mass or the velocity, so is useless for calculating lift

Newton also tells us that every action (force) has an equal and opposite reaction

That reaction is the force we call lift acting on the airfoil

That’s Newton’s contribution

The magnitude of the force can most conveniently be calculated using the circulation hypothesis; Lift equals fluid density times circulation times velocity.

Circulation is effectively the strength of the vortex flows around the airfoil

Vortices can only be generated in a viscous fluid, and on a finite wing form a closed loop.

That loop is formed by vortices rotating around a spanwise axis (bound vortices), trailing (wingtip) vortices and way downstream, a starting vortex.

So much for lift generation

Force can only be applied to an object immersed in a fluid by changes in pressure of the fluid in contact with the body.

The distribution of those pressures and forces is best calculated using Bernouilli’s theorem

That covers Bernoulli

That's it really