hydrofoil foils

i got to design the foils for my project. "hydrofoil" but i have no idea of where to start?
can someone guide me? help appreciated! thanks.

 

Here's a start:
http://en.wikipedia.org/wiki/Hydrofoil
http://web.mit.edu/2.972/www/reports/hydrofoil/hydrofoil.html
http://www.foils.org/

 

Go to this site:
http://www.mh-aerotools.de/airfoils/javafoil.htm

Read the information and then run the Applet.

You will need to set the parameters for water on the Options page. Density is 1000kg/Cu.m and viscosity is .0000013 will be OK. They change somewhat depending on the water type and temperature. One very important parameter is the aspect ratio. The foil efficiency improves as the aspect ratio goes up. You can approximate to the width/chord.

You also need to know the Reynolds number for your operating conditions. Google Reynolds number. It is a function of the speed, the chord length, viscosity and density.

A good foil to start with is the modified NACA 4 digit as shown on the attached screen dump.

To get results go to the Polar page and Analyse. This gives all the foil coefficients at a selectable range of Re# and Angles of Attack. The lift and drag forces are determined using:
Force = Density /2 * Coefficient * Velocity^2 * Foil Plan Area.

Hope this is useful.

Rick W.

 

foils

There is good info under "Foiler Design" in the "Sailboats" forum of boatdesign particularly if your project is a sailboat. Good Luck!

 

thanks for helping me start off. i'm doing a human-powered hydrofoil. i read up that i have to calculate drag. but how do i calculate the total drag?

 

The very best design will have a lift to drag ratio of around 40:1. So to calculate the drag you need to determine the total weight and divide it by 40.

For example if you can build a frame and floats that weigh 100kg, including the pilot, then the drag will be around 2.5kgf, say 25 Newtons.

You need to know what you are doing to get an L/D ratio of 40. Getting ratios around 20 are not too hard.

So overall weight is a critical parameter. Basically you want to build a water glider.

If you want to see what is possible the Google Decavitator. This link shows something lower tech but it worked:
http://www.me.dal.ca/~dp_04_5/

Rick W.

 

Ah, you strike right to the heart of the problem! Think of it as a business proposition. Performance = profit; drag = cost; power = income. How would you attack the problem if you asked, "How do I calculate the total cost?" There are two broad methods - you look at other similar things to see how much they cost, or you start to add up all the various losses of different kinds (material costs, labor, facilities, tooling, taxes, etc) to get the total. In reality you do both. You use the first method to get an initial guess to see if the project is remotely affordable, and then you do the second to refine your estimate. Good bookkeeping is essential.

It is the same with hydrofoils. Start with the "income". You know you have something like a quarter of a horsepower to work with from each peddler. You know what kind of speed you are trying to achieve. In the steady state thrust horsepower equals drag times velocity, so if you multiply the power available by some efficiency factor for the drive train and propeller and divide by the speed, you get the total drag as a system-level requirement. Rick Willoughby has suggested another method based on weight and assumed lift/drag ratio. Either way, this is your drag budget.

Now you have to set up your bookkeeping. There are many sources of drag, and there are different ways of accounting for them - the choice is largely a matter of convenience based on what data you have available to you. This figure shows the start of a drag breakdown for an aircraft - a hydrofoil will be similar:


Picture the craft from the fish's eye view - everything you see, every detail you can zoom in on, will have a drag associated with it. And the same for the bird's eye view. Each drag will have its own peculiarities as to how it varies with size, speed, fluid density, and relationship to the rest of the configuration. Just write down each physical component in a spreadsheet, label the columns for parasite drag, induced drag, wave drag, etc., and start tracking down numbers for each one's contribution to the various categories.

For a displacement-type hull, Froude divided the drag into skin friction and residual drag, and the residual drag is further divided into wave drag and form drag. Wave drag is divided into contributions from the transverse waves, divergent waves, and interference between mulitple hulls. Michlet is an excellent CFD tool for calculating hull drag.

For a hydrofoil, parasite drag consists of all the contributions that increase with the square of the speed, while induced drag depends on the lift (squared) and decreases with the square of the speed.

Within the parasite drag, skin friction and pressure (form) drag are typically combined together as profile drag. Profile drag is proportional to the planform area or the wetted area, depending which one you choose as your reference area. Parasite drag also includes contributions like junction drag (due to the effect of interference between strut and foil on the boundary layer in the junction between the two) and spray drag. These depend on the thickness and chord, rather than the area. The variation of profile drag with angle of attack could be accounted for as part of the parasite drag or included in the induced drag. Parasite drag also includes excrescence drag - the losses due to miscellaneous pieces of hardware like exposed drive shafts or sensors or cavities.

Induced drag is inversely proportional to the square of the span, but doesn't depend on the area. Wave drag for a hydrofoil can be calculated separately or included in the induced drag using biplane theory (infinite Froude number approximation). Induced drag also includes interference between the hydrofoils.

Trim drag, the variation in drag due to control deflection from the non-deflected baseline, may have elements of profile drag and induced drag (due changing span loads or interference with other surfaces). You might not include trim drag at all in your drag bookkeeping system - it could be accounted for by variation in the induced drag and other parasite drag terms.

You will have aerodynamic drag due to the boat topsides and crew. This will be a parasite drag, too, but of course the density of air is very different so the aerodynamic drag is best carried as a completely separate contribution to the total drag from the hydrodynamic drag.

You may choose to add up the drags in nondimensional coefficient form and then multiply by dynamic pressure and a reference area. Or you may calculate the drag in N for each item and add them up in dimensional form. This has the advantage that each part can have its own reference dimensions (area, length) that are most convenient.

An intermediate approach that I rather like is to multiply the drag coefficient by the reference area for a given component to get its drag area. The drag area can be handled much like a nondimensional coefficient, except that it has the units of area. You add up all the drag areas to get the total drag area (often called equivalent flat plate area) and then multiply by the dynamic pressure to get the total drag. This is especially convenient if the physical areas are changing constantly, as with a surface piercing foil, making it awkward to have a fixed reference area.

A hugely important resource for this is Hoerner's "Fluid Dynamic Drag." I guarantee you'll find a copy in your engineering school library, or from used book shops on the web. Hoerner will give you a decent first guess for just about any form of drag you can imagine. Then it's a matter of deciding if that first guess is adequate, or if you have to do more analysis and testing to get a better number.

Another excellent resource is the collection of technical papers available on three CD's from the International Hydrofoil Society.

Many books on aircraft design are also applicable to hydrofoils, such as Dan Raymer's "Aircraft Design - A Conceptual Approach" (AIAA).

 

thanks guys for all the help you gave me! =)
but it still stuck with lots of doubts.

is the lift generated by the front or rear wing?
how does the drag play a part in the lift?
does the size of the front wing and rear wing caculated using the same formulae?

i did some caculation by using the life/drag graph to determine my angle of attack. i pick 3 degree as it is the most efficient angle. next i set my v = 10 m/s and using densityof 1025 kg/m^3. and by setting Lift = Weight(100kg)
i use the formulae L = cL * 1/2 * density * v^2 * S(area)
i got the area as 0.013677.
and by using the formula of aspect ratio.
i got the chord for the wing as about 17cm.

am i going in the wrong direction?

 

You need to show more of your workings.

What formula did you used for aspect ratio?

What is the foil span?

What CL did you use and where did you get it from?

The best approach is a large foil near the centre of gravity and a smaller foil to give control. Both foils can provide lift.ck W.

 

Human powered foils

You may be interested in the web site

http://www.foilkayak.com/

where they seem to have a few good ideas

 

Whether majority of the lift is provided by the front or rear foil depends on the design. In some designs, the rear foil could actually produce a downward force.

By definition, lift and drag operate at right angles to each other - lift is defined to be perpendicular to the free-stream velocity and drag is defined to be parallel to the freestream velocity. However, lift and drag change their orientation with respect to the craft's body axis system as it changes its angle of attack.

The loading of the foils must first meet the requirements for trim. The lift must equal the weight, the thrust must equal the drag, the side force must be zero for a human-powered craft, and all of the yawing, pitching, and rolling moments must sum to zero.

The foil configuration must also provide adequate stability. For static stability, the change in lift with an increase in height must be negative, the change in pitching moment for an increase in angle of attack must be negative (bow down), and the change in pitching moment for an increase in height should be negative, and the change in pitching moment for an increase in speed should be positive. These considerations lead to some constraints on the foil loading and the position of the center of gravity relative to the foils. The forward foil should carry a greater load per unit area than the aft foil, for example, and the change in lift per unit area with a change in height should be greater for the forward foil than the aft foil.

You have the general idea correct. You need to check your units. The mass of the boat is 100 kg, but the weight of the boat is 980 N. A chord of 0.17 m for an area of 0.0137 m^2 implies an aspect ratio les than 0.5 - you will never generate enough power to fly with a low aspect ratio like that. I can't square the lift coefficient with your choice of 3 degrees angle of attack (presumably measured from the zero lift line).

Three degrees is not necessarily the most efficient angle of attack. It will depend on your foil configuration. Do not be concerned about the angle of attack at this stage. Work with the lift coefficient instead. You only need to know the angle of attack to set the incidence angle of the foil. The craft will change its pitch attitude to adjust the angle of attack.

Work with span and area separately, and let the aspect ratio be determined from them. You will have structural stiffness and strength constraints, but from a pure performance point of view, the larger the span the better.

What is critical is setting the takeoff speed in relationship to the maximum speed. A fully submerged hydrofoil has a trimmed power required curve that looks like the Nike swoosh. You can shape that curve by your choice of span and area. The drag at maximum speed will correspond to the maximum power available. The speed for minimum power to fly will be less than that. The takeoff speed could correspond to the minimum power speed, or it might be lower yet. The higher the takeoff speed, the smaller the foils and the higher the maximum speed is likely to be.

You need to derive the power required as a function of span and area. Below takeoff speed you will have hull drag, too. As you accelerate to takeoff, you have both the drag of the hull and the drag of the foils. After takeoff, the drag of the hull drops away, and the total drag will be less than for the hull alone without the foils - that's the whole point of flying in the first place. But until that happens, the combined hull and foil drag will result in a hump in the drag curve (or power required curve), and the peak of that hump has to be less than your power available or you will never get to takeoff speed.

Your foil size will be a compromise between the demands of low-speed drag for takeoff and high-speed drag for performance.

Once you've sized the over-all foil area, you can distribute the area between the front and rear foils to satisfy trim and stability requirements.

 

I too want to design some foils for my 44 foot 14 ton power cat 500HP in total. max speed today is 23KTS. It is a semi displacement design by Crowther built by Seawind of Australia. It has round bilge and It has surface drive

I have posted before about my thoughts on trying to make a foil that spans the hulls . I had even suggested using a discarded helecopter rotor spanning the 2 hulls. The problem with this is attachment of the foil to the hull and support in the centre.
As I am not sure that this, or any of this will work satisfactory it must be removable untill it has proven itself. Also all attachments will have to be glued only,-- no access is a available to bolt anything.

I have seen the Hysucat foil and of course loads of money cures every thing. This turns out to be a very expensive project, working out where it shoud be fitted ,pre manufacturing the foil and then what, post it to me in Malaysia?

Ok here it is. My thoughts are to make some small wing foils say ( I have posted this idea too but this is the latest and different) a sheet of 1 inch ply bonded together to give the thickness required to make a naca 3308, or similar.
At 4 foot wing span x 2 foot chord x 4 of them would be epoxied to the hulls. I thought that this would take away the sensative positioning that the Hysucat system requires.

So lets say a 8x4 sheet of ply cut into 2 foot bits along its 8 foot length doulbed in thickness and shaped to naca 3308 and epoxied to the hull.

8 sq foot per wing of naca 3308!!! x4 = 32 sq foot.

It is my thoughts that this 4 wing method would balance the lift better and be more stable, I have no mathamatical equations to argue this just a gut feeling. Obviously turning the boat would produce a slowing effect of one hull and allow that to sink slightly allowing a banking effect.

My thought are also that the single hull spanning type would allow pitching in swells where as the single 4 wing would be better?

I am not a mathamatician be any stretch of the word, Pretty dumb at maths, well the type that are requied for this stuff.

What kind of lift would I need and would the wings I have in mind achieve it?