The wings

Operation

The front wing of the foil takes all your weight (and any pull resistance from the kite) while the rear wing is only a stabilizer to help you keep the angle of attack stable.

The rear wing should have either no lift, or a little bit of lift force downwards causing the nose of the board to want to pitch upwards. If the rear wing has a lot of lift upwards, then your center of lift of the foil will move back, causing your center of mass to have to move backwards as well and making the foil less stable. The most important thing about the relative angles of the wings is that the foil will stay pitched the same no matter what speed you are doing. It must not require you to balance on the back of the board for slow speeds, and then have to move forward as you speed up. Ideally you want to stay centered on the board for the full speed range. 

With only the front wing making lift, the rear wing would be unnecessary if we could control the foil angle. But controlling the pitch angle with only one small wing is probably too difficult, although I would guess not impossible. Normal aerodynamic (or hydrodynamic) profiles are unstable in pitch, making them want to pitch away from the straight and level angle. Hang gliders overcome this problem with techniques such as reflex to make the whole wing pitch stable. This makes the wing return to its normal angle of attach after the pitch was changed. I think a hydrofoil need much more aggressive pitch stabilization which is why we need a small wing about 70cm behind the main wing. This adds a lot of pitch stability, but for anyone learning it probably feels like it does not add nearly enough.

Size

The front wing is normally about 600 square centimetres (cm2). Faster foils will have a less area and will need more speed before they can hold your weight up. Larger foils will normally be easier to learn on since you can stay up at a slower speed. The size of the wing makes a very big difference on how fast a foil will be, and how much glide feeling it will have. I have found that larger wings on the same low drag keel and fuselage can make a foil feel very slow and almost make it feel stuck. But getting used to a fast foil takes time and I definitely would suggest anyone learning to ride a foilboard to start on a slower, larger wing.

The rear wing is normally about 2.5 to 3 times smaller than the front wing. The further the wings are apart, the smaller the rear wing can be.

Outline

The shape or outline of the wings are normally delta shape. A delta shape acts like a tapering wing that has a little bit of sweep back. Sweep back is normally used to give a wing yaw stability. When the wing yaws slightly to the one side, say left of the direction of travel, there will be more surface area and hence drag on the right hand side thus forcing the wing to face directly forward again. This is why hang gliders are delta shaped and it would be impossible to control yaw on a hang glider without a delta type wing sweep back. But on a hydrofoil, the rear wing is so far behind the front wing that any bit of drag there will create a lot more yaw stability than the wing shape can hope to achieve, unless the wingspan is very large compared to the fuselage length. A much more efficient way to add yaw stability (if needed) would be to put a small vertical fin/rudder near the rear wing and this will do much more for yaw stability than a delta wing shape I believe.

Some foils are more of an elliptical shape. This shape minimizes induced drag, so makes the foil as efficient as possible for a given aspect ratio. The drawback of this shape is that the stall happens across the whole wing at once, which is why aircraft do not have elliptical wings anymore. But for a hydrofoil, I have not found elliptical wings to cause sudden stalls any more than other wing shapes I tested. We can change our angle of attach and add pull from the kite quickly to get out of a stall, and even if we stall we just hit the board on top of the water and take off again. I have found that the stiffness of the keel and fuselage is a far more important factor in determining whether a foil would cause unforeseen stalls than the wing outline shape.

Wings that gradually get narrower towards the sides seems to be standard for hydrofoils. This makes sense in that the loading on the wing decreases from the middle towards the sides and the wing can be narrower towards the sides. (If the profile is kept constant, the wing thickness should become narrower as the length/chord of the wing gets narrower). A narrower wing tip also means less mass at the outsides of the wing, decreasing its moment of inertial (making it more maneuverable) and less likely to vibrate.

Another consideration in wing shape is that there are no sharp corners so that it is at least a bit safer when you do fall off. But a foil is dangerous already, so this is probably not really important.

Aspect Ratio

At the moment the aspect ratio separates racing foils from free riding easy to ride foils. Higher aspect ratio wings are the long skinny wings and they have lower induced drag than lower aspect (shorter fat) wings. Racing foils will have high aspect ratio wings, because it gives them a better lift to drag ratio, enabling better upwind performance.

A wider wing will also add more moment of inertia, which will probably make the foilboard more roll stable. This is probably a good thing if you want to go fast, but maybe not so good if you want to make sharp turns. 

Going for lower aspect wings has advantages that are suited to most foil riders. A lower aspect wing will be stronger and less likely to vibrate than a similarly constructed high aspect wing. It will also have a wider range of angles of attack before stalling, allowing a wider speed range which is very useful for riding slowly. Since a foil is often ridden with quite a bit of a roll angle to resist the pull of the kite, having shorter wings also allows the fuselage to ride closer to the water surface (or ground if you ride too shallow!) before one of the wing tips break the surface of the water. This should give you a little bit more room for error.

Anhedral

Anhedral is the curve of the wing  downwards. Dihedral is a curve in the opposite direction (upwards) and is normally found on aircraft such as gliders to give them roll stability. With dihedral, if the wing rolls left, there will be more wing area on the left, causing more lift on the left and causing the wing to want to roll back upright. One would think that this is what a hydrofoil should have- roll stability to help keep it upright. Yet, if you look at hydrofoils, almost all of them have anhedral making them even more unstable!

This seemingly contradiction have made me think quite a bit as to why? The amount of roll stability that dihedral would give is quite small compared to the length of the keel with most of the mass so far above. Dihedral would probably not add any noticeable roll stability, and neither would anhedral take away any noticeable roll stability. I used to think that is was mainly for the handling dynamics of roll/yaw interaction, but after making a couple of different wings I think the main reasons for anhedral are:

  • To allow more room before the wingtips break the water surface when riding upwind (with the board riding at an angle)
  • and to add vertical wing area to stabilize the yaw axis

I had a foilboard which had a keel of 800mm and a flat wing of close to 600mm. It was very irritating when the wing tip exited the water. It was not much fun to ride in chop conditions. If the wing could have had a bit of anhedral so that the wing tip is 100mm lower, it would have given the same controlling range as having a 100mm longer keel. The shorter a keel can be, the less stiffness it needs so using anhedral as a way of increasing one's control range is a smart idea. This does not mean that flat wings are much inferior, since a shorter flat wing with a longer keel can also give a very nice control range to the rider.

When the foil becomes more streamlined, some vertical wing components are required to give the foil yaw stability and help it stay gliding in a straight line. Using anhedral, especially a sharp curve near the tip of the main wing effectively adds a vertical component of the wing near the front of the foil. But more importantly I think low drag foils also need a vertical wing component near the rear, which can either be incorporated onto the rear wing or consist of a vertical fin in the rear.

Vertical Fins

On slower foils, especially ones that are not very streamlined, a vertical fin does not seem to be needed. But as the foils get more streamlined, some vertical wing component, either incorporated in the wings, or as a separate vertical fin, makes a big difference to the handling of a foilboard.

Most foils seem to add large vertical winglets to the rear wing to add a vertical wing compoenent at the rear of the foil to give it yaw stability. With flatter wing, a small vertical fin near the rear of the foil can give the same effect.

A second vertical fin further forward is normally not needed, since the keel acts like a massive vertical fin. But most foils have quite a lot of downward curve on the tips of the main wing (or simply a lot of anhedral) which would act as a vertical fin near the front of the wing. Others have a fin as part of the keel structure near the front like the previous MHL Lift, which then doubles up as a structural element and also protects the wing from taking impacts if the rider hits the sand when coming in.


 

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