# Stalling behaviors of foils based on aspect ratio or chord?

Discussion in 'Hydrodynamics and Aerodynamics' started by dustman, Jan 7, 2023.

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### DCockeySenior Member

When a flow is referred to as "incompressible" it does not literally mean that the density is absolutely constant. Instead it means that the changes in the density and temperature are small enough that they can be ignored. For the flow around a foil the pressure can be determined using Bernoulli's equation outside of boundary layers, areas of separation and similar. Then the corresponding density and temperature can be found using the isentropic gas equations. For other flow situations the pressure and temperature can be measured experimentally and the corresponding density calculated, or numerical modeling can be used.

Also note that water is compressible. Change the pressure of water and the density will increase or decrease. But the change will be small enough that it can almost always be ignored. If water was not compressible the speed of sound in water would be the speed of light.

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### BlueBell. . . . .

I'm starting to wonder the same thing.

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### dustmanSenior Member

Here is a recap of your contribution.

"All foils have "issues with stalling".

"Aerodynamics.
There are entire books written on it.

"You may want to reread them.
Otherwise, I can't help you any further.
Foils stall for many reasons, simply avoid them.
Best of luck in your endeavours."

You'll be happy to know that through the contributions of the other responders as well as further reading and analyzing airfoil behaviors with this tool Airfoil comparison http://airfoiltools.com/compare/index#polars, my question has mostly been answered and I have gained a lot more insights.

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### jehardimanSenior Member

<sigh>

Dustman, et al...

1) You need to embrace the following concept...."All models are wrong....Some models are useful" (George E P Box, inferred from "Science and statistics", Journal of the American Statistical Association, 1976). Just because you don't understand or need a particular model formulation does not mean that it is wrong or useless, it just means that it is not the tool you are looking for. This is what I meant when I said that every engineer need to come to his own terms with what is actually happening in the TBL (Turbulent Boundary Layer). As Heinlein (a USNA trained engineer) wrote "one man's theology is another man's belly laugh" (from Time Enough for Love, 1973).

2) DCockey is absolutely correct, there is no known incompressible material,...solid or fluid. Without the impossible "perfect" incompressibility no real material can be "perfectly" rigid or "perfectly" transmissible to forces. That does not mean that for most hydrodynamic cases on the earth you can't ignore compressibility, you can.... but for Black Hole quantum physics....eh... see 1) above.

3) Reynolds Numbers are not about size, shape, absolute forces, or velocities. Reynolds Numbers are about streamlines. If two different shapes, in two different fluids, at two different velocities have the same Reynolds Number;...then the ratio of viscous pressure forces from the fluid moving over the surface to fluid density causes the same amount of mathematical curl in the viscous dynamic pressure field; i.e. the geometry of the streamlines (i.e. lines of constant pressure and velocity) from the surface are the same proportion at that point. Reynolds numbers have little to do with the actual lift or drag.

4) The Reynolds Number changes as the fluid flows along the body. There is a reason it is called RANS (Reynolds Averaged Navier-Stokes) and 1) applies again. This change in Reynolds Number is important in the development of the TBL. The mixing that happens over hundreds of feet in atmospheric conditions happens over millimeters in a hydrofoil. Conversely, with 3) above; if two geometrically proportional shapes, in two different fluids, at two different velocities, have the same exact final Reynolds number, then the streamlines are proportionally exactly the same relative to the surface (much like there is only a single catenary curve shape for similar mass per length and gravity).

I could go on, but for a lot of this it can only be fully comprehended after getting down and doing the maths yourself...which per 1) above...are not always exactly correct. The path to enlightenment is not normally an immediate illumination, but rather stumbling into a lot of hard walls.

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### baeckmoHydrodynamics

You have got some decent info on the subject (and some crap.....) in the thread above, but there is more to the "issues" that should be noted. By now, you may have realized that the stall phenomenon comes with different definitions. It can be described as a point, where lift is no longer increasing with increasing AoA, or it can mean a more or less sudden loss of lift. The answer to your original question is that the aspect ratio is the dominating factor.

With increasing AR, the foil flow tends towards a 2D-condition, while the low AR foil goes towards a 3D flow condition. The consequence is that when the 2D foil reaches its lifting limit, the whole span stalls at the same instant, and as a result of the changed pressure distribution, the lift center is moving along the chord.

The flow past the low AR foil is three-dimensional from the first increase in AoA (ie start of lift), and continuously increasing with AoA, resulting in a more gradual change. The flow detachment sequence is governed mainly by the geometry, or aspect ratio, but with a strong influence from the foil thickness distribution along the chord, longitudinal sweep, planform shape and surface roughness.

So, it is not only a question of getting maximum lift or minimum drag (the high AR foil having the highest L/D ratio for similar profiles), but just as important that the foil has stable and controllable characteristics; it must not stall momentarily, but give due warning, and when the lift is changing, it should not change the wing moment suddenly. I think this is really what the "issues" in the litterature are about in the real world.

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### SkyakSenior Member

You didn't answer my question about interpolation or first principles. Limited math is far more suited to interpolation than first principles, but you need to start with a design that is proven successful in the conditions you will encounter.
If I was to list math prerequisites for first principle small sailboat design, it would be;
-algebra
-trigonometry
Your use case is pretty demanding. Making a small sailboat that will carry you safely through the Bahamas and across the gulf stream is a serious undertaking. You might even encounter objections when you try to check out or file a float plan with USCG. There is a formal process called "failure modes effects analysis" FMEA I suggest you use. I suggest you start a new thread dedicated to refining your SOR -statement of requirements. I will have much to say there. If it gets slow or negative, you can have design competitions or debates.

I wasn't suggesting you tolerate inefficiency, I was saying do the math. The word "efficiency" is slapped on anything divided by something else -REGARDLESS OF RELIVANCY.

Two thumbs up for your "must have windward sailing capability". Don't ever settle for less. Sailing the Bahamas you will run into the situation of being in a cove that is no longer sheltered due to shifting wind and you will need to sail up wind through a cut. Alternately you might need to sail upwind through a cut to get into a sheltered cove.

No, you don't calculate displacement length ratio differently for catamarans. Multihulls have more performance options but weight and waterline are the same. Wave making resistance will be significant at least, likely performance defining.

Broaching in waves is complex. I should cover it in a separate post. I also think you need kick up foils for the Bahamas.

more later

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### dustmanSenior Member

It seems like you may be partially responding to Wet Feet's comments?

As far as reynolds number, as I was playing with the airfoil tool and looking at the difference between low to high thickness ratio foils, naca 0012 through 0021, i couldn't help but come to the conclusion that a thicker foil delays separation to a larger angle of attack, the lift builds more slowly and falls off at a larger angle of AoA, also that thicker foils perform significantly better at lower reynolds numbers. At higher reynolds numbers all the foils seem to perform similarly with the exception of 0012, which has greater lift and better lift/drag up to about 20 degrees AoA. Also, a longer chord length equals a higher reynolds number, which means to me that, given equal area, a lower aspect foil would operate better at lower wind or water speed, at least in relation to its thickness and shape. But then there is the affect of the lower aspect on the overall lift/drag...

At this point in my life I'm not going to take the time to relearn higher math, or go too deeply into fluid dynamics. I aim to learn enough to build an effective, relatively efficient and safe boat. General concepts and basic math are enough, no matter how complicated people make it sound. Humans have been building successful and safe sailing vessels since well before algebra and fluid dynamics were a thing. Points taken though.

I've stumbled into a lot of hard walls, mostly in the form of people. (Not aimed at you)

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### dustmanSenior Member

Thank you for the straightforward answer addressing the root of my question! That's what I'm looking for, trends.

Yes, I read something saying the same thing about the stalling characteristics of foils of infinite span on an aviation aerodynamics forum. I also read that an elliptical lift distribution can contribute to more sudden stalling. Not sure if that's true, but it seemed to make sense the way it was explained. I plan to use a rectangular planform for ease of construction and easier implementation of junk sheeting, but they also seem to have more docile characteristics. Would you agree with this? Thicker foils also seem to be more docile.

Designing a sailing vessel is quite a balancing act.

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### dustmanSenior Member

Is this the form of interpolation you were speaking of? "In the mathematical field of numerical analysis, interpolation is a type of estimation, a method of constructing (finding) new data points based on the range of a discrete set of known data points." I've basically looked at a lot of different boats and analyzed their characteristics based on their use case. It wasn't long before I realized that every design has it's own limitations. Not sure what else to say on that.
I can do most of the math well enough. I won't be able to precisely calculate every little thing, and I don't think that's necessary. I'm just building a cheap, effective and safe boat that will get me over to and around the bahamas. That can be done on general principles and fairly basic math, at least that's my assertion. If I can calculate the stability, balance, and structural requirements I've done the import things. I think it would be hard to mess up the boards, rudders and sails enough to be unsafe or poorly performing. It will have a very high length/disp and very low windage, so I've mostly won the performance battle right their. I'm here trying to learn enough to optimize all the foils, within reason. It seems by doing so it will lead to a whole range of benefits, and add to the safety and ease of use of the boat, and construction costs for that matter.
I'd say the use case is pretty benign compared to the open ocean. I've looked at the wind and waves in the bahamas pretty much every other day for the last 2 years and probably spent over a hundred hours on google earth looking at the bahamas and the rest of my route.
And I dare say, weather forecasting is pretty darn good these days. The boat will literally be unsinkable by nature of the materials. By the time I make the gulf stream crossing I will have been sailing the boat for almost 2 months already and will have a good feel for it. As far as the actual building of the boat, I have been building and fixing things for a living for more than 20 years, I am confident in that aspect of this process. Forgive me for sounding nonchalant, but boat building is not just the domain of high level engineers and craftsmen. Blah blah, the point is that I have done my research on many factors and I'm not going to stop doing it until I cast off.

My statement of requirements is simple: Get me to and around the Bahamas; keep me alive. Amenities are of little concern. This will be a pure sailing vessel, not something to sit in sipping champagne with my pinky sticking out. This is intended solely for the exploration of nature. Honestly I don't want to start that thread because it will just be a big debate that will go off the rails. I will look into the failure modes effects analysis as you suggest, thank you.

Are you suggesting that some kind of bureaucracy exists that would keep me from being able sail my self built boat to the Bahamas?!

I will certainly do enough math to have a good approximation. Efficiency to me is a more holistic term. Too many people slapping too many terms on things to make the all important dollar, or to sound fancy. I'm pretty fatigued by wading through all that stuff to get at the truth. Thank goodness for the forums and other resources that still exist, or that can still be found.
Thank you for highlighting this. I've read too many accounts and watched too many videos of people struggling to go upwind to accept that. Seems quite important, especially for the Bahamas.

That reminds me I need to put some more time into analyzing the the apparent wind factor as it relates to stability.
Are you sure? Aside from wave interaction between the hulls I would imagine they would be calculated individually, with each hull carrying half the load. Though things will change with a large heeling force.
I would be very interested in hearing your input on broaching. Yes, the kick up foils seem a must for what I'll be doing. I racked my brain figuring out a good solution for that, I think I found it.

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### SkyakSenior Member

This sentence is dead wrong. Open ocean sailing is benign and predictable (pack enough supplies, pick rout to go with flow, stay afloat). The
Bahamas are a million places with greater than a 50% chance of complete loss, punctuated with gem locations to lure you out, all powered by an immense flow of warm water. The only reason to think you will be safe is that you will not go to many unsafe places and very carefully evaluate your travel plans against the weather. The real measure of your boat design will be the conditions it can safely navigate and the comfort it will provide while waiting for those conditions to occur. I am not sure what you see in satellite data, but in the Bahamas, or any archipelago, the depth, currents, winds, waves are all very local. A two foot swell in the seaway (where numbers are reported) can mean 6ft breakers at the entrance to a lagoon.

We are not here for sailor stories, but my limited experience was as crew delivering a 47ft sailboat to Puerto Rico. We didn't check in but the island we anchored by to stretch legs was 300yards from what the chart indicated. We anchored in 30ft of water near a cliff for its wind shadow so we could string a new main halyard. That's an eddy and wind shadow for 47ft long by 65ft tall boat. Turn the wind 60 degrees and that location becomes a boat crusher. We spent a week waiting in DR for a weather window to cross the Mona, actually 5 days, a break attempting a cross, 4 more days repairing and waiting. In the time since my trip the Dominican Republic no longer allows foreign boats to anchor in its undeveloped locations.
There are always boats that need crew and going upwind through the Bahamas is sufficiently unpleasant that there isn't competition. You will learn allot.

Displacement length ratio is not used much on multihulls because they have the power to exceed hull speed but you don't divide the displacement. At max performance, the catamaran displacement is all on one hull.

I am not up to date on regulations but yes, the US and the Bahamas have laws and regulations you must follow. You should check them and be aware how foul their prisons are.

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### The QSenior Member

An interesting set of opinions, as i'm busy constructing a new rudder myself.
Luckily I don't have to consider open sea sailing. Just shallow waters, and serious amounts of weed.
16ft mini keelboat.. max draft for our waters 3ft.

So my choice for the under hull rudder was NACA 0012,
A slope of 45 degrees for the leading edge,
Aspect ratio, 1:2, actually almost 1ft wide by 2ft deep. Except for the tip of course.

One odd effect we get is one side of my main sailing waters has hard vertical quay headings. So when the wind is in the right direction, you can balance the non compression of the water, between hull / keel, and the quay sides, against the wind pressure, sailing along the river banks at seemingly impossible angles just 6 inches from the bank. All goes a bit pear shape when you reach a dyke / slipway and the balancing force disappears, you have to be quick to react...

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### dustmanSenior Member

In my view, a lot of the dangers in the Bahamas come down to the type of boat you are sailing and decision making based on forecasting and observation. In the open ocean there is nothing to do but ride it out. I am well aware of the dangers of shallow water and breaking waves. I am no stranger to reading the water, having rafted a number of rivers with significant whitewater.
My primary consideration is to have as little to break or fail as possible(meaning keeping everything simple as possible and robust), followed by redundancy, and stability. The hulls will be filled completely with high density foam.
Length to displacement ratio in my understanding is a pretty direct indicator of wave making resistance. This boat will never come close to flying a hull.

I tried to look at regulations for the boat itself. Didn't see anything about it on the Bahamian website. The US coast guard seems to exempt sailing vessels from most requirements. A lot of the requirement have to do with inboard engines and fuel storage, ventilation, etc.

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### dustmanSenior Member

You haven't noticed any issues with its performance? What made you settle on the naca 0012?

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### DogCavalrySoy Soylent Green: I can't believe it's not people

Fantastic! Sometimes this forum is less than a good use of people's time. Like threads about whether or not commonly observed phenomena are real (like circulation to take a completely random example). Other threads are like this one. I wish I'd read this before reading Aerohydrodynamics of sailing, by the Blessed CA Marchaj. It wiuld have helped.

NACA 0012 is section that has a very good balance of characteristics without being disproportionately optimized for one attribute at the expense of everything else. As a result folks have been using it, and the NACA 00 series in general ( well really all the NACA foils) for 90 years.

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### DogCavalrySoy Soylent Green: I can't believe it's not people

@jehardiman et al, anyone who quotes Heinlein for any reason can have a meal on me.

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