Wingboat Design

Hi Jimboat,

If I understand you correctly -- you are suggesting that the hydrodynamic and aerodynamic center of lift of the marine craft are two separate forces which need to be as close as possible because if they are not they could conflict with one another and cause some stability problems during flight or high speed water operations.

If that is you point -- it is well taken. It is my guess that the aerodynamic lift of the overhead wing during flight would overpower most hydrodynamic lift or drag of the payload boat.

Flare-craft boats as shown in the attachment work as good as they do because their aerodynamic wake vortex forces near the water surface can't fall far and attach themselves to the hydrodynamic forces during flight -- resulting in less drag and better performance than a fast boat "stuck to the water". My theory is that the higher it flies the more the aerodynamic wake is allowed to form and cause the drag that brings the Flare-craft back to earth.

 

Sailing Wingboat

Hi all, Some of you may be interested in a new concept in sailing craft which combines sailing with wingboat principles (WIG). A few years back, I was discussing this concept with Richard Jenkins (project co-ordinator of the UK Windjet Team - who is seeking to raise the WWSSR above 50 knots).

The Wingboat design originally proposed by MasterBlaster was a reverse three-point hull design (two points aft, and one point forward). As I suggested earlier in this thread, IMO, this is not a good configuration for hi speed boats (or WIG's). A number of people have tried the reverse 3-point design in outright WWSR attempts. These designs failed miserably, and many have died utilizing this configuration. Among those most notably were John Cobb and Lee Taylor.

The original (land & ice sailing) designs of the Windjet also employed the reverse 3-point configuration. The configuration seems to work well on flat hard surfaces and in the wind tunnel, but as I suggested to Richard, as far as operating on a fluid surface it does not work well, and it would be better to change the configuration to a proper water based 3-point design, that is 2 points forward and one point aft. I also theorized that in order to break the legendary 50 knot barrier for sail craft that the hull of the boat would have to raise clear of the water surface, and the best way I could see of doing that would be to combine WIG attributes with a sailing craft. The most important attributes being to add anhedral (drooping) ram aerofoils for lift and a T-Tail stabilizer for longitudinal stability. The Windjet Team have finally published their long awaited 'secret' design and lo & behold they have incorporated the above design features.

Can a sailing craft operate on WIG principles? Many said, "NO!" Here's a link to some of my design thoughts on this question a few years ago ...

http://foxxaero.homestead.com/indtri_flydtch.html

On the new Windjet website the following claims are made ...

"Planing on its stepped hulls, it accelerates smoothly (but rapidly!)
up to 30+ knots. Above 40 knots, it can support and control itself
aerodynamically. In theory the craft becomes clear of the water at
top speed apart from the keel foil which remains submerged in order
to carry the side forces." http://www.windjetproject.com/

Well, have a look at the new Windjet 'Sailing Aerofoil', and let me hear your thoughts on this new 'Wingboat' sailing design. I was somewhat hesitant to add this 'sailing' concept to the powerboat section, but this has turned out to be a thread discussing WIG and 'Wingboat' designs, and in my opinion the craft definately fits these classifications.

 

Hydrodynamic vs. Aerodynamic

Duane - There are some aircraft designed to plane on the water at low speed, and in ground-effect. These are not really designed to take advantage of both aerodynamic and hydrodynamic forces simultaneously. Tunnel hulls are designed to use both - but aero and hydro must be balanced throughout the operating velocity range - this is what makes it tricky. The combined center of lift (both hydro and aero forces) should be close to the aerodynamic center to minimize instability. This is the trick, since the "balance" of aero/hydro changes for a tunnel hull throughout the operating velocity range. There are ways to analyze and to achieve close to dynamic balance in operation.

As for Flarecraft "boats" - these were really a copy of the old Fischer Airfish WIG aircraft. As I understand it, this craft was not really intended to operate well as a boat, but rather as an WIG aircraft. The tough part about 'balancing' wake vortex forces and lift/drag force enhancement through WIG effects is very difficult - and often inherently unstable. The Flarecraft was not particularly successful, I don't think, due to problems with the crafts stability (the craft even crashed). The 'changing' balance of hydro/aero dynamic forces was apparently experienced in the Flarecraft design due to location of fuel tanks relative to CofG causing different problem at various fuel levels.

/Jimboat

p.s. - the WindJet design is certainly an interesting one! Very ingenious!

 

There has been a lot of discussion about the simultaneous application of both aerodynamic (WIG) and hydrodynamic forces and their interaction on balance etc. This is the condition in Wig craft on the way to becoming completely airborne,but it is the final condition for many types of high performance racing boats. I have wondered for a while about some questions relative to this condition. There can be considerable air pressure under the foil. Does this pressure depress the water's surface under the foil?
This pressure can reach the dynamic pressure in local areas and remain at a high average pressure elsewhere. It seems to me that depression is inevitable. Now, how does the depression affect the induced drag and the friction drag on the foil?
My intuition tells me the induced drag is unaffected but there can be a considerable reduction in friction drag because the wetted area should be made smaller. Does any one have any insight into this? Measured data or theory? If air lift through WIG reduces planning friction drag above and beyond simply "unloading" the planning surfaces, it would be a very interesting phenomenom.

 

See http://www.cyberiad.net/library/pdf/tl01.pdf for optimum distribution of air pressure for minimum wave drag of the resulting depression.

However, I suspect for the wingloading of a reasonably efficient WIG, the depression of the water would be minimal, and the induced drag far more influenced by the air wake.

Another factor to consider is a WIG would be operating at high Froude number based on wing chord. So while the pressure under the wing may cause a depression in the water surface, it may not occur under the wing itself. At any given location on the water, the WIG's passage is essentially an impulsive disturbance that imparts a vertical velocity to the surface. But by the time the surface bottoms out and rebounds back up, the WIG is long gone. The result would be a wave train with a long wavelength left behind the WIG.

 

TSPEER
Thanks for reminding me one more time that Froude knew what he was doing. I believe you are correct that pressure is an impulse disturbance and the waters inertia, delays the actual depression to well behind the boat at Froude numbers at racing speeds. Too bad; there is no reduction in wetted area from this effect and hence no benifit in friction drag.
When the trailing edge of the foil is touching the waters surface,(or slighty below) the
lift coeficient will be 1 for an infinite aspect ratio. With a reasonable aspect ratio and end plates(usually sponsons), .5 might be achievable. At 100 feet per second, the dynamic pressure in air is 12 psf. Reduced to 6 psf by the finite foil end leakage. This is enough to depress the water(staticaly) by about an inch.Is this large or small ? It depends on ones perspective.
As far as drag is concerned, there is no difference that I know of between induced drag due to lift from air or planning, if the angles of attack of the respective surfaces are the same. In practice the chord angle for the air foil is usually a little bigger than
the planning angle and thus air drag from this source is also bigger.

 

Low Aspect Ratio Wings in Ground Effect

With low aspect ratio aerofoils in ground effect, such as a tunnel boat, we see CLa= .1, and CLw=0.003 at 100fps. AR of 0.3 is representative of typical power catamaran/tunnel boat. CDa = 0.04 typically. Hydrodynamic lift/drag also participate, and can be calc'd separately. All vary quite alot, of course, with height above surface (h/c), AR, velocity.

 

Tracie Barber, WIG aerodynamicist from Austrilia, recently wrote in the other forum that the depression of the water surface from GE (in the speed ranges typical of WIGs and fast boats) is insignificant, but that the depression from the wingtip vortices can be significant.

Luc De Keyser

 

jimboat and lucdekeyser,
Thanks for your interesting comments on my post. The boat that I have designed and built has an actual AR of 1.4 and an apparent AR of 2.0 (due to end plate effect of the demihulls) I calclate the CLa seperately for the upper and lower surfaces. The lower surface CL(the one important to the effect we are discussing) is about .6 . This factor of 6 differance,shows the great impact of AR on CL. In addition, since the loss in lift is due to air flow around the tip, I would think the production of vortices is greater (per unit lift) in low AR foils. A few years ago, before starting design, I did a tradeoff to help decide the foil configuration. Starting with two slim demihulls, connected at the transom by a foil of a fixed span and small chord, I calculated the air lift. I then increased the chord forward in steps calculating the lift at each step. Obviously the area is growing and the AR is dropping. The lift rapidly reaches a point where it is gowing less than the added area's structural weight. I concluded that most boats (hydroplanes and tunnels) have a lot of useless foil area(worse than useless if it adds weight)
The CDa you mention tells me that the angle of attack of the foil is about 2.5 degrees.
I am using a slightly higher value ,3.0 degrees.

 

Our research would show that with AR=1.4, with end plates (tunnel hull design), a CLa = 0.4 to 0.5; can't see getting as high as 0.6 unless h/c is very (impractically) small.

 

I thought we were talking about the depression of water under a WIG, not around a hull. So there's no wetted area to change.

The fact that there's no depression at the location of the wing doesn't mean there's no effect on induced drag. The same argument holds for a hydrofoil operating near the surface, for example. But the induced drag is increased up to 100% for the shallow hydrofoil compared to one that is deeply submerged. Even though the surface is flat.

The difference comes from the boundary condition that's assumed for the surface. At low speeds (approaching zero Froude number), the surface doesn't deform significantly because the change in head due to gravity is high compared to the dynamic pressure. The surface is essentially rigid and the boundary condition is all the vertical induced velocity components have to sum to zero.

At high speeds (approaching infinite Froude number), the surface is flat because, although it's had a velocity imparted to it, it hasn't had time to deform yet. The boundary condition this time is constant pressure at the surface, which requires that the horizontal components have to sum to zero.

These sums include the representation of the foil and its mirror image needed to make the surface a plane of symmetry throughout the fluid volume.

In the case of a WIG, I've not tried to work out what the effect of the surface velocity might be. Given the density of water compared to air, I think the zero Froude number approximation is usually assumed.

I don't understand where this comes from at all. You are evidently assuming the pressure under the wing is stagnation pressure because the trailing edge is sealed. CL=1 in this condition assumes the pressure on the upper surface of the wing is the same as ambient pressure. But the pressure on the upper surface of the wing can be considerably less than ambient. So there's no reason why the lift coefficient has to be limited to 1 in ground effect, even for a finite wing.

Once you get very close to the surface, you can get nonlinear effects that are more complicated than the usual considerations of induced drag, etc. predicted by the linear aerodynamics of potential flow. For example, the downwash and deceleration of the air ahead of the wing can separate the oncoming flow from the surface and roll up into a horseshoe vortex.

I don't think angle of attack has anything to do with it, per se. The induced drag from planing is twice that of a fully submerged hydrofoil of the same wetted planform (see http://naca.larc.nasa.gov/reports/1958/naca-tn-4168/naca-tn-4168.pdf, especially Fig's 15 and 16, http://naca.larc.nasa.gov/reports/1955/naca-report-1232/naca-report-1232.pdf, and http://naca.larc.nasa.gov/reports/1958/naca-report-1355/naca-report-1355.pdf). The induced drag depends on the lift, but the lift could come from either the angle of attack of the planing body or the shape of its surface - a hooked trailing edge, for example.

Clearly the area of the wing is very different from the area of the planing surface. Varying the area of either will change the amount of lift due to a change in angle of attack for the modified surface. So I don't see any linkage between the angle of attack of the wing and the planing surface, other than that they are part of the same structure.

 

Universal Hovercraft...

Hello...

Sorry if this has already been posted - check out the video on this thing....

See http://www.hovercraft.com/ ....

Cheers....

SH.

 

In the class of boats we race, the planning element and the air lift element are one and the same.IE the bottom of the foil near the trailing edge is used for planning. The foil I am presently using is a Naca 4415.The boat uses water propulsion with a specified outboard engine.This defines the horsepower and the lower unit geometry.Within the confines of this configuration there is only a very limited vertical height in which the propulsion can operate.Obviously no thrust accurs when the prop is out of the water.In fact, above the point where the shaft is near the waters surface, efficiency drops rapidly,but even worse cooling water pick up stops.Too deep is also bad because of increases in lower unit drag.The goal here is the same as any racing boat: minimum drag and no loss of engine thrust.
I call the vertical space between the foil trailing edge and the water surface the "slot"(same as "h" in your terminology.) When the boat is at or near top speed, the slot can be permitted to vary only a few inches, from negative 1 inch to perhaps 2 inches. At a slot of 0,or greater, the boat is supported entirely by wig air lift.With h/c at these small values the derivitive of lift vs height is large and the foil tends to "lock- in" to this height. Negative values of slot mean the foil is water planning., deriving lift from this source as well.The question that I am alluding to,is "what fraction of the boats weight should be air borne and what part should be planning borne to minimize drag?"
Now, I didn't say that there was no lift from the upper surface of the foil.I merely said I calculate it seperately from the lower surface lift. The foil I use has considerable uppersurface lift. I believe I am using the traditional definition of inducd drag,that is the horizontal component of the normal force. This is the lifting force times the tangent of the angle of attack. In the case of conventional air foils, this is the angle of the chord , while for planning it is the angle of the planning surface. I was referring to the fact that this source of drag remains the same whether the foil is producing air lift or water lift. perhaps I am wrong but I see little advantage induced drag wise by tranferring planning lift to air lift. Indeed ther is a reduction in friction drag but you get most of this reduction at 60% air lift. Total air lift is not much less drag.

 

...the question re: "what fraction of the boats weight should be air borne and what part should be planning borne to minimize drag?" is indeed, the crux of the matter. This is a difficult solution to create algebraically. I have developed software to generate solutions for the power catamaran, tunnel hull and modified vee bottom configurations, making the analysis easier (although complex).

Conclusion...there is no 'single' answer to the question.

 

jimboat--- We are in total agreement as to "crux of the matter". It is as you say, a complex issue. I have done my studies through closed form analysis.I have no "software program". I am not knocking computer simulations but I personaly find that
if you can get a realistic handle on the analysis ( even if it is a little crude) there is more intuitiion for what's going on.
I see the complexity coming from two sources.Namely, how planning friction drag varies with aspect ratio of the wetted area and angle of attack of the planning surface and differences in the angles of attack between the planning surface and the airfoil.I think I intuitively understand whats going on for a limited set of parameters. Friction drag is particularly troublesome for me. I believe my analysis is really good only for high aspect ratio planning wetted areas.
When you get into the higher percentages of air lift, another problem rears it's head, pitch stability. This is a serious saftey issue with many boats and must be dealt with along with minimization of drag.
What percentage of the weight is typically borne by air lift in tunnel boats?