Power and drag during take-off
Most WIG boats are water based or at least amphibious and although there are some examples and proposals for land based WIG craft, they are very few. Generally a WIG boat will not have water contact at cruise speed, so there is only water contact during take-off and landing.
Seaworthiness of WIG boats is often expressed as a certain maximum wave height for take-off, cruise and landing. Some specifications state that the maximum wave height at take-off and landing are the same and relatively low and that higher waves can be cruised over. From a safety point of view this is not good practice, since a WIG boat must always be prepared to land in case of an engine failure or other emergency, even when cruising over the highest intended waves. Therefore seaworthiness of WIG boats should be defined by (at least) the take-off wave height and the cruise/landing wave height, where the latter will always be higher.
The importance of these definitions lies in the fact that the installed power is determined by the maximum wave height at take-off, whereas the structural strength of the hull is determined by the maximum landing (=cruise) wave height, where the highest hydrodynamic loads occur. Note that the latter is only true if the take-off and landing speed are similar.
History has shown that for WIG boats the drag in the take-off run is much higher than the drag at cruise speed. This means that the engines must be sized for take-off and only run on very low power in cruise, sometimes as low as 30-40%. Although this is an illustration of the efficiency of flying in ground effect, it is not very desirable because of the weight and cost penalty. The extra power cannot be used for increasing the cruise speed, since WIG boats, by their nature, have a very limited speed range. For most WIG boats there is a maximum safe speed above which they become unstable. This makes it even more undesirable to overpower a WIG boat, since this will enable the captain to accelerate to unsafe speeds.
It may be clear that the power mismatch can be solved by minimising take-off drag. The drag during take-off consists of several contributions, the most significant of which are the hydrodynamic contributions due to viscous and wave pattern drag. The viscous drag is caused by friction between the wetted surface of the hull and the water. Wave pattern drag is the energy that is lost due to formation of a wave pattern on the water surface.
In front of an object moving through the water there is a bow wave. This wave becomes higher as the speed increases and lower as the displacement decreases. As a result the wave drag has a maximum somewhere before take-off. This maximum is sometimes referred to as the hump drag and the associated speed as the hump speed. For many WIG boats the hump drag determines the installed power.
Power and drag during take-off
Hump drag is illustrated in the simplified picture above where the drag is shown as a function of the speed for two different WIG boats, one with a higher hump drag than the other. The necessary installed thrust to overcome hump is also given for each of the craft.
The theoretical maximum speed that these craft can reach is the point where the thrust and drag lines meet, as indicated in the graph. This is not necessarily the maximum speed in practice, since this theoretical maximum speed may be beyond the safe (stable) speed range of the craft. Therefore installed thrust is determined by take-off requirements alone and WIG 2 is obviously a better design than WIG 1.
Many regular seaplanes have a sophisticated wing and flap design for creating as much lift as possible at take-off in order to reduce the take-off speed and thus the hydrodynamic loads and drag. Furthermore a seaplane can rotate so that the angle of attack at take-off is much higher than that in cruise flight. This way the lift coefficient at take-off may be 10 times higher than in cruise flight. A take-off lift coefficient of 2 to 3 is not exceptional for a seaplane, but a WIG boat cannot take full advantage of flaps. Practically this limits the lift coefficient of a plain WIG boat to a little over one, which assumes total pressure recovery under the wing (Cp=1) and a relatively small contribution of the top of the wing. The easiest way to minimise the take-off speed is to design for a low wing loading, but this severely limits the maximum speed.
The take-off power is determined by the take-off drag, so the drag must be minimised. Since drag increases with the speed squared the take-off speed should be minimised, but for a given aerodynamic configuration and weight the minimum airborne speed is fixed, so the drag can only be decreased by optimising the way the hull generates its lift or introducing other lift sources. The two main ways, other than aerodynamics, to carry loads are hydrodynamic lift and aerostatic lift. Hydrodynamic lift can be generated by the hull, a hydroski or a hydrofoil and aerostatic lift by air injection (PAR) or a static air cushion.
Some of these solutions have an additional advantage in the fact that they alleviate hydrodynamic loads on the hull at take-off and landing. A pneumaticlly damped hydroski can serve this purpose, but also flaps are often damped with pneumatic cylinders in order to decrease the chance of damage. Some of the more recent Russian craft like the Volga-2 and the Amphistar have inflatable cushions under the hull and endplates. These cushions are powered by a separate fan and not only alleviate loads but also ensure good sealing of the endplates in wavy conditions.
Hull design is often overlooked by designers as a source for improving take-off performance, since they are focused on aerodynamic design. Many features of speedboats and seaplanes can be very helpful to increase hydrodynamic L/D. Some of those features are steps, chines and ventilation. Steps help to decrease wetted area and prevent the hull from "sticking" to the water. Chines can be very helpful in suppressing spray and thus spray drag. Friction drag may be reduced by forcing air into the step or even through small holes in the hull bottom, this is called ventilation or air lubrication.
It may be clear that hydrodynamic hull design is not a simple task, it is a specialism in itself and a WIG designer should pay much attention to it. Some existing WIG boats are aerodynamically very sophisticated, but hydrodynamically very poorly designed.
Details of the sophisticated design of the Orlyonok hull
A hydroski is not used very often, however it can be very helpful for providing hydrodynamic lift. One of the few water based jet fighters, the Sea Dart, used a retractable hydroski for take-off and landing. A disadvantage of the hydroski for take-off is its poor L/D ratio, therefore it is only effective for landing. The Orlyonok uses a pneumatically damped hydroski for this purpose. It slows the craft down and alleviates the hydrodynamic loads. It is not known whether the ski is used at take-off too.
Orlyonok with hydroski extended
The Sea Dart, a sea-based jet fighter with a hydroski
Hydrofoils may be used to lift the hull out of the water before the aerodynamic lift can carry its full weight. Hydrofoils have much better L/D ratio than hydroskis and are therefore much more effective at take-off. They can be arranged in several ways, for example in a tandem arrangement or just as a single foil amidships. A lot of different layouts are possible, for example a V, an inverted V or just a plain straight hydrofoil.
Potentially the hydrofoil is very effective for WIG boats, but not many WIG boats have been fitted with them. This may be due to the experience with the X-114h. The X-114H was a test craft for using hydrofoils for decreasing the take-off distance. Therefore it was fitted with three sets of V shaped hydrofoils, one on each float and one at the aftmost point of the trailing edge. The craft indeed lifted out of the water at a much lower speed, but the foilborne distance was very long. Later one concluded that this may have been caused by the hydrofoil at the back which lifted the trailing edge out of the water so that there was a leakage of air which decreased the pressure under the wing. The test did indicate however that the maximum weight could be increased by as much as 15%.
X-114h with hydrofoils under its floats
Even worse for the reputation of hydrofoils as a take-off aid for WIG boats was the accident that happened with the X-114H. Its hydrofoils were not retractable and therefore extended some distance below the craft in cruise. At take-off and landing speed the angle of incidence of the foils was (of course) positive, but at maximum speed the angle of incidence of the foils was negative. At one such test run for maximum speed the pilot went too low and a hydrofoil touched a wave, due to the negative angle the downward lift force on the foil immediatedly crashed the craft.
The only example of well designed and effective use of hydrofoils is the VT-01. This craft has retractable inverted V hydrofoils located amidships. These hydrofoils demonstrated a take-off distance reduction from 1000 to 400 metres. A beneficial side-effect of the hydrofoils is the damping of vertical motions of the craft at speeds just before take-off, which makes it much more comfortable for the passengers.
Power Augmentation (PAR) or air injection is the principle of a jet or propeller in front of the wing that blows under the wing at take-off. The cavity under the wing is bounded by endplates and flaps, so that the air is trapped under the wing. This way the full weight of the WIG boat can even be lifted at zero forward speed. Hydrodynamic friction drag is therefore theoretically eliminated and consequently the hump drag is reduced. Almost all Russian WIG boats employ this principle and it is very effective, although not very efficient. This may be a reason for not using this for commercial WIG boats. The thrust that is required to lift a craft out of the water is enormous, especially because of the leakage and the pressure loss in the jets. The KM illustrates this inefficiency by using 8 large turbojets for PAR which can be shut off in cruise flight. Recent examples of Russian ekranoplans, such as the Volga-2 and the Amphistar show that using propellers a more efficient PAR system may be possible.
Orlyonok with PAR engines engaged
A very recent development is the use of a static air cushion for take-off, similar to SES and hovercraft. Although it may be argued that PAR is also a static air cushion, there is a significant difference. A hovercraft or SES-like static air cushion is sealed all around and air is injected somewhere in the cavity under the wing. The amount of air and the pressure of the air are much lower than with PAR. The Hoverwing uses air from the propeller that is captured by a door in the engine pylon to power up the cushion. Some other designs propose a very low power auxillary fan for this purpose.
Principle of Hoverwing technology: in hover mode the green skirts go down and the blue door behind the propeller opens up, so that air is pushed through the blue channels into the cavity under the body
