Foils for a mini 6.50, new project

[Desingfoil]

Hello to everyone, I'm new in this forum.

I have a project in mind that will see light in June of this year.
It's about putting foils in a Mini 6.50

I have almost everything fitted, sponsor, money, place, geometry of the foil, internal structure ... but ... I'm a little lost in the subject of which profile to choose. To determine which profile is the most appropriate, I will use the Altair Software. But I would like to have some opinions on which profiles are the most suitable for this purpose.

Features:
Ship weight: 800Kg
Estimated speed of operation: Start at 10 knots to have an optimal lift at 15 knots.

The first project will be to put some foils on the sides of the boat, but not on the rudders. Later on, if I see that this part is resolved, I will implement the foils on the rudders.
For the lateral foils I have thought about a 500 Kg lift.

I found this database: UIUC Airfoil Data Site https://m-selig.ae.illinois.edu/ads/coord_database.html

But I see that they are quite old profiles, I suppose that there are currently more modern profiles.

-Any idea of what profile you could use?
-Any contribution will be welcome.

I was hanging information, you can also follow the project in my Instagram account: a.cortadella

Greetings and thank you very much in advance!

 

[Doug Lord]

You might consider consulting with this guy and/or his designer:
Foiling Mini 6.5--Verdier Design https://www.boatdesign.net/threads/foiling-mini-6-5-verdier-design.61621/

 

[tspeer]

The precise section shape is one of the last things you need to design. All decent-performing sections have approximately the same slope to their lift vs angle of attack curve, and if you assume the boundary layer is fully turbulent, the minimum profile drag depends mainly on thickness and is very insensitive to section shape. That means you can do the bulk of your design work by just assuming a symmetrical section (any symmetrical section of the desired thickness) and getting on with the most important aspects of the design - sizing the span and area, and shaping the rondure.

Once you have those fairly well defined, you can go about determining the requirements for the section shape at different stations along the span. Vortex lattice or even lifting line theory will give you the local lift coefficients to use in the section design. A panel code will show the difference between 2D and 3D pressure distributions in areas with significant interference effects, like the elbows of L foils or strut-foil junctions. You can replace the assumed symmetrical sections with cambered sections by aligning the zero lift line of the cambered section with the chord of the symmetrical section.

The section requirements will include the minimum physical thickness needed, the speed range, the range of lift coefficients, and any changes to the profile pressure distribution due to 3D effects. The best section choice is one that is specifically designed to your requirements using a tool like Xfoil. You might choose a NACA 6-series section as a starting point, but the section you will end up using will be considerably different. You can use the difference between 2D and 3D pressure distributions to generate biased design pressure distributions for inverse design. By subtracting the 3D-2D pressure difference from the desired pressure distribution, you can come up with a 2D shape that produces the desired pressure distribution when it is inserted into the 3D context. For example, in the elbow of an L foil, this will result in a flattening of the contour on the inside surface and bulging out of the contour on the outside surface. The section shape will look like a pregnant guppy, but it will still have a similar pressure distribution to the more conventional shapes well away from the junction area.

Sections that are designed for a high incipient cavitation speed tend to be nearly symmetric in the front half of the section and to produce all their lift at high speed using the aft portion of the section. This causes a large torsion load on the foil that needs to be factored into the structure as you get to the detailed design stage.

So don't worry about what the section shape should be, just yet. The UIUC database is definitely the go-to location for choosing an initial starting point. But at this stage, a program like AVL is where you ought to be spending your time.

 

[Erwan]

For example, in the elbow of an L foil, this will result in a flattening of the contour on the inside surface and bulging out of the contour on the outside surface. The section shape will look like a pregnant guppy, but it will still have a similar pressure distribution to the more conventional shapes well away from the junction area.

Thank you for this explanation,
Is it to address the 2 overlaping pressure distributions at the elbow:
The vertical one + the horizontal one?

Best Regards

 

[tspeer]

Yes. Think of the pressures at a given section spreading in three dimensions, as though the section were glowing. A 2D section is equivalent to a straight 3D shape that extends to infinity. A given section gets an equal contribution from panels on either side of it. Although there's an influence from the sides, most of the influence extends out away from the surface.

On the outside of the elbow, the pressure can spread out and there's less influence from either side of a given section. The flow behavior there is more akin to an axisymmetric shape. On the inside of the elbow, the two panels are shining towards each other and you get a (metaphorical) hot spot. The superposition of the high velocities raises the velocity on the inside of the elbow to higher speeds than for a flat panel.

Although the velocity at a given point is influenced by the entire configuration, it is most strongly influenced by the curvature of the surface at that point. A highly convex curvature will have low pressure/high velocity, while a concave curvature will have high pressure/low velocity. So on the inside of the elbow where the velocity is higher than desired, the curvature needs to be flattened or even made negative. On the outside of the elbow where the velocity is lower than allowed by the design requirements, the curvature can be increased by bulging the contour outward.

 

[Erwan]

Thank you very much Mr Speer for this explanation.
It is a long time I had this intuition, I am very happy for the confirmation.

I was guessing that with basic wing sections, the elbow's low pressure side was likely to separate much sooner than the strut or than the foil.

Thanks again

Erwan

 

[Desingfoil]

Thank you very much for your responses. Fast and quality!

I will take into account all the details expressed.

From what I understand, to counteract the differences in speeds that are created between the inside / outside of the convex curves. It is necessary to make a flatter and symmetrical profile on the inside and a more profile of the asymmetric type on the outside. I'm right?

In reference to the attack angles, for the information I have read, I see that very low angles are used, around 2 degrees, right?

How is the area calculated? length of the foil by the profile rope?

Assuming that the length of foil that works inside the water is 200cm and that I want to create a force of 500Kg at a speed of 15 knots, what length of rope do you think is adequate to start working with the simulation? About 40cm?

On the other hand, and just out of curiosity, has something kind of been invented as a way out of the profile that is flexible? so that at low speeds it has more curvature and that at more speed it flattens out? like the wings of planes that at low speed increase their curvature.

Forgive my English
Thank you so much.

 

[tspeer]

...From what I understand, to counteract the differences in speeds that are created between the inside / outside of the convex curves. It is necessary to make a flatter and symmetrical profile on the inside and a more profile of the asymmetric type on the outside. I'm right?

The interference between the wing and fuselage on an airliner presents a similar issue. The air has to accelerate as it flows out and around the fuselage. To compensate for this, the airfoil shape for the wing root is distorted. Here is the Boeing 707 wing as an example. The swaybacked appearance of the root section is not unlike the section in the middle of a foil elbow. The midspan airfoil is more along the lines of what one would expect for a transonic transport or a section with a high incipient cavitation speed.
Boeing 707 root:

Boeing 707 mid span:

 

[tspeer]

...In reference to the attack angles, for the information I have read, I see that very low angles are used, around 2 degrees, right?

This depends on your airfoil shape. It is more convenient to talk about the lift coefficient. Each section has a different zero lift angle of attack, so different shapes will have different angles of attack for the same lift coefficient. If you keep their zero lift lines aligned, then you can swap out section shapes without changing the lift.

At takeoff, the lift coefficient might be on the order of 0.8 to 1.0 and the angle of attack on the order of 8 deg. At maximum speed the lift coefficient might be on the order of 0.1 to 0.15 and the angle of attack zero or even slightly negative, depending on the camber.

How is the area calculated? length of the foil by the profile rope?

Assuming that the length of foil that works inside the water is 200cm and that I want to create a force of 500Kg at a speed of 15 knots, what length of rope do you think is adequate to start working with the simulation? About 40cm?

The reference area can be calculated any way you want, depending on how you set up your lift/drag bookkeeping. One common approach is to project the area on horizontal and vertical planes. If you want to base it on the arc length of the rondure, that would be fine, too. The choice you make depends on what is most convenient for the purpose. The important thing is to clearly establish what approach you are using, and then stay consistent with that approach.

A program like AVL will let you define the geometry and calculate the lift. You can put in whatever value you want for the reference area and calculate coefficients based on that.

On the other hand, and just out of curiosity, has something kind of been invented as a way out of the profile that is flexible? so that at low speeds it has more curvature and that at more speed it flattens out? like the wings of planes that at low speed increase their curvature.

Yes.

 

[Desingfoil]

Thank you very much tspeer, for the explanations and the graphic examples.
I continue with my research ...