# 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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### jehardimanSenior Member

It is literally chaos theory.....every engineer needs to come to their own understanding about what is going on in the TBL based on their own needs.

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### wet feetSenior Member

This topic has long fascinated me,I just wish I had more understanding of it.I have read and even understood some of the aerodynamic references and failed to understand others.They describe a parallel universe to the operating conditions of a rudder and the reason is the fluid in which rudders operate.My reason for arriving at this conclusion is that the medium in which rudders operate is not only a lot denser than the medium that aircraft operate in,it is also incompressible.While the density difference can be substituted for a higher figure in the calculations,the compressibility is where I believe the problem lies.At the kind of angle where a rudder is stalling,the low pressure side can't stretch the water that has separated and in my experience ventilation occurs as air is drawn down.I don't believe I have experienced cavitation as the forces would be more violent.I have no idea if a calculation exists in which it is possible to determine how far down the rudder foil the ventilation will travel as the flow of the water will draw the air stream aft.The closest aerodynamic situation would probably be at the interface of a seaplane float and the water at take off speed as this is probably the only time that a flying surface is at the interface of two distinctly different mediums.It is this mixture that I believe limits the application of pure aerodynamic theory.A fully immersed rudder,beneath a hull,has the hull acting as an endplate and seems to function with enough extra efficiency that a smaller area can be used.

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### DCockeySenior Member

wet feet is correct that ventilation can be an issue with rudders. Ventilation is not due to a lack of "compressibility" of the water. It is due to the presence of the free surface dividing fluids of different densities.

As the angle of attack of the rudder increase the pressure on one side of the rudder becomes lower. The low pressure on one side can pull the free surface down so that the surface is partially covered by air. The pressure of that air is atmospheric which limits the amount of lift the rudder can generate. The air also causes the flow to separate on that side of the rudder. Reduced pressure pulls the free surface down introducing air at atmospheric pressure. The air then causes the separation. Separation prior to ventilation does not cause subsequent ventilation. When the flow separates the pressure downstream of the onset of separation is reduced. That is what causes the classic reduction of lift at high angles of attack.

Ventilation can be calculated, but the calculation is more complicated than algebraic formulas. I believe there are some criteria for the onset of ventilation based on the low pressure "peak" on the airfoil surface and depth below the surface.

That depends on what you mean by "pure aerodynamic theory". The applicability of the simple algebraic equations such as the effect of aspect ratio on lift are limited. However the same field equations apply as long as the presence of the free surface is accounted for.

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### wet feetSenior Member

I'm totally happy to accept that aerodynamic theory works in air.What I have difficulty with is the notion that it works in precisely the same way in water,because the compressibility of a gas will allow it's volume to expand or contract.I don't believe the same can happen in water,which is partly behind the reality of ventilation;the water can't expand to fill the low pressure zone that may have been created and the resultant suction draws down air.

The stalling that we occasionally witness with a high aspect ratio keel or centreboard is a very different thing and I have only ever witnessed it once.It was in trapezing dinghies and we were making good gains on a boat a short distance ahead,which then headed up to defend his position.The outcome was a slide to leeward of about twice the beam of the boat in a couple of seconds,while the sails remained full and apparently drawing well.

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### BlueBell. . . . .

You may want to reread them.
Foils stall for many reasons, simply avoid them.
Best of luck in your endeavours.

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### SkyakSenior Member

My advice is to give up thinking about the microscopic level until you are PHD candidate doing a thesis in boundary layer development. Fluid dynamics for inviscid flow at the scale you are interested in is derived entirely mathematically. The intuition you get from thinking of particles will mislead you.

"In reference to my original question, a longer chord length would raise the reynolds number, and since a higher reynolds number delays separation, a lower aspect ratio and thus greater chord length would mean you could maintain lift at a lower boat speed or greater angle of attack, relative to a higher aspect ratio foil of the same area but smaller chord? But has lower overall lift due its lower efficiency for a given area."

This quote from your post #11 indicates you see what I was trying to tell you but still think there is some secret alternative to compromise.

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### SkyakSenior Member

You see the formulas for 2D lift and 3D lift. Now instead of looking for alternatives in particle physics, move on to the free body diagram of the boat in operation. You ask questions like you are designing by first principles, but you seem to cling to proportions as if there are some relevant designs you are interpolating from. Are you interpolating?
"Efficiency" is a completely ambiguous term best left to salesmen. You are right to question its relevance.

For sailing, shallow water is a game changer. Waves slow and steepen. If the wind is from offshore the waves break and just getting to sailable depth presents a challenge/hazard. If the wind is from shore your design and sailing skill must be sufficient to get back upwind to shore.

I still don't know enough about your requirements, but the AR decision is a moot point. Draft limits the span of your keel and rudder. Span is not a variable and the cord will have to make up for the force lost. Shallow water also pulls wave resistance to lower speeds. You will have to correct your "hull speed" resistance calculation for the slower wave speed.

There is another failure mode more sinister than stall -at high speeds like surfing steep waves sailboats can broach, and shorter low aspect rudders are more susceptible.

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### SkyakSenior Member

Compressibility of air is not relevant to sail operation. Compressibility of air becomes an issue when flow gets near supersonic, not before. Boats have more issue with cavitation (phase change of water at low pressure). Your trap dinghy experience sounds more like ventilation and recovery.

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### dustmanSenior Member

You can help by confirming or denying my hypothesis and then educating me on the reasoning. Otherwise I'm not sure why you bothered to respond to this thread. And part of the reason I'm here is so I don't need luck when I build this thing and go sailing, which I am going to do, no matter how much people knock me down.

Be well.

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### dustmanSenior Member

Yes, so incredibly interesting. Can't wait to put it to use.

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### dustmanSenior Member

This kind of thinking has served me greatly in gaining an understanding of many things and how to apply the fundamental principles in my work. And it is just how I think, my best learning tool. As far as mathematics, I took one college algebra semester almost 20 years ago, so I won't be doing a lot of complicated equations or relearning algebra to complete this project, so I'll be leaning on modest math, general principles, and trends, which is more than good enough for this project. I'll probably be getting mathematical assistance for the structural aspect.
I understand that I have to make compromises, but I want to fully understand the magnitude of these consequences, and minimize them where possible. I can already build a decent boat, but I want to build a really good one.

So my interpretations of everyone's input were correct, more or less?
I have looked at everything from old heavy full keel monohulls to modern foiling racing catamarans, hobies, tornadoes, but the design philosophy that appeals most to me are the Wharrams. If that's what you are asking. But, that's not anything like what I will be building due to the nature of the sailing I want to do, at least in my first iteration.
Forgive me for disagreeing, I think efficiency is quite relevant for a number of reasons. What I am questioning is the wisdom of large compromises in the pursuit of a small gain in efficiency. Like is the difference between an aspect ratio of 2 or 4 for the keels and rudders really worth the compromise, in draft etc.
Yes, I understand all that. I'll be sailing almost entirely in sheltered waters or where I can sail to sheltered waters in a relatively short period. The riskiest part of my whole journey will be from south Miami to Bimini, and maybe Georgetown to Conception Island. And the concept of my own mortality scares the crap out of me, so I won't be taking big risks. Referring to the efficiency issue, to me efficiency means windward ability, it means the ability get out of harms way, it means more stability(being able to drive the boat at reasonable speed with less sail and weight up high). And I am taking many steps in the design process to make it simple to handle, easy to reef, and structurally sound, with ample backups.
So at 4:1 my keels would have a draft of 30", rudders 21", which doesn't seem so bad. But I want to explore pretty much every nook and cranny of the Bahamas with the exception of the far northern and far southern islands, including the interior of Andros Island. At 2:1 the draft would be 21" for the keels, 15.5" for rudders. I guess an extra 9" could make a difference in this application. I'm trying to come up with a good design to make the keel and rudders be able to kick up on impact without significant damage, which could afford me some extra draft. If I went with lower aspect foils how much extra area should I add in your opinion? Looking into lift to drag ratios it seems that it gets pretty bad below 2:1, and I wonder about the wisdom of having rudders too shallow because of ventilation and possible emersion in certain conditions.

I assume I will have very little wave making resistance with a displacement to length of 16.7(assuming you calculate catamaran hulls using half the total displacement), so I'm imagining the shallow water will make little difference in my case. (Edit: d/l of 27.9)

Are saying this because they lose lift somehow or simply emerge?

Thank you for being patient and helping me work through this. It is appreciated. I think you all will be pleased at the outcome in the end.

By the way, I found a bit of a gold mine here: Airfoil Tools http://airfoiltools.com/index

Last edited: Jan 10, 2023
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### dustmanSenior Member

That's very comforting

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### wet feetSenior Member

I have no issue with sails and compressibility,my point was that foils operate in a medium that doesn't permit compression,or it's negative opposite,Consequently I don't expect phenomena that occur in air to be precisely matched by the behaviour of foils in an element that doesn't permit any amount of elasticity.

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### DCockeySenior Member

Your expectation is completely understandable. I had the same before studying fluid mechanics and aerodynamics. But in the real world at boat speeds the compressibility of air has essentially zero effect on the flow. The effects of compressibility of air only become significant at speeds of 200 mph or so in atmospheric conditions. And significant qualitive differences require a speed of 450 mph or higher where local velocities approach the speed of sound. This behavior is not intuitive but is reality.

For boundary layer thickness, transition from laminar to turbulent and location of separation (if any) to match between air and water (at atmospheric conditions) a foil in water needs to be moving at about 8 times the speed of the same size foil in air. Or the foil in water needs to be 8 times the size of a similar shape foil in air. This difference is due to the difference in the ratio of dynamic viscosity / density, known as the kinematic viscosity. It has nothing to do with compressibility.

If the speed of the foil in water is high enough that cavitation (boiling) occurs the flow in water will be different than the flow in air.

Also it should be noted that at extremely high speeds the flow of water can be compressible with shock waves. But real world situations where such speeds occur are rare.

rxcomposite likes this.

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### wet feetSenior Member

I'm having some difficulty understanding that in relation to both Bernoulli's theorems and Boyle's Law.At least as far as flow in air is concerned.

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