A Comparison of Different Hovercraft Lift, Thrust and Transmission Systems\
Four Variations Compared:
These are systems where the air handling devices (fans or propellers) are in no way connected together. No mechanical linkage and no aerodynamic linkage, each with separate power (engines) sources. Each system can be operated completely without regard for the other.
These systems are partially integrated because they share a common power source that can be shared between them in a variable manner according to demand but other than that they operate separately to each other and are aerodynamically independent. The mechanical drive systems are organised so that the drive ratios are adjustable thereby allowing speed variation and the application of more or less power to either the lift or thrust systems as required. The power engine can change speed and the operating speed of either or both systems can optionally remain constant or vary as desired and controlled by the hovercraft operator.
These systems are mechanically integrated because they share a common power source and the drive ratios to each are fixed and non-variable. Output speed changes from the power source (engine) will increase the rotational speed of both the lift fans and the thrust propellers in the same ratio. Other than that, they are aerodynamically separate. Increase in engine speed will give more thrust and more lift simultaneously and vice versa.
These systems typically have the fans (or propellers) providing both lift and thrust. There may be one or more fans (or propellers) but the common factor is that some air passing through the system will be used for thrust and some will be diverted for lifting the hovercraft. Variable ratio drive systems are pointless because the power source (engine) speed can be changed to alter the fan or propeller speed. Increase in engine speed will give more thrust and more lift simultaneously and vice versa.
AirLift Hovercraft design and supply hovercraft with the different lift and thrust systems types 1,2 and 4 listed above. Our small HoverFlyer 560 design is of the aerodynamically and mechanically separate type 1. Our RIVAC 680Ri design is available in two versions, the separate lift version (SL) type 2 and the integrated lift (IL) version type 4. All of our larger designs employ the aerodynamically separate but mechanically integrated variable ratio system type 2. We do not employ the aerodynamically separate but mechanically integrated fixed ratio system type 3 in our larger designs because we believe that it is not appropriate to design operating compromises into large high value commercial hovercrafts.
Because we offer most system types, we feel that we are in an impartial position and able to make the following technical comparisons.
It cannot be said that any one system is always the best or always the worst, it is a matter of realising the advantages and constraints of each system and looking at them in context with the type of work that you hope to do with the hovercraft which incorporates them. If you have a low budget and are looking for a low first cost hovercraft with little regard for operating efficiency or not needing exacting control then the aerodynamically integrated system is the one to choose. On the other hand you might be more interested in a small hovercraft with great controllability and good operating efficiency so you would select a hovercraft with separate lift and thrust system. If you are a commercial operator and are more interested in long term operating efficiency and economies as well as the utmost reliability and initial purchase price is not such a consideration then you would choose a aerodynamically separated but mechanically integrated variable ratio drive system hovercraft with an industrial diesel engine. It is a matter of choosing the right hovercraft for the right job and that is why we at AirLift Hovercraft offer the different types to all of our customers.
Different System Requirements
Air pressure can take the forms of dynamic pressure (like the force you feel when the wind blows on your face) and static pressure (like pressure in a gas bottle). The hovercraft thrust system concerns itself mainly with dynamic air pressure and the hovercraft lift system is more concerned with static air pressure.
Thrust System Requirements
In the case of the thrust system, it is the acceleration of a mass of air that produces the thrust. Newtonian physics state that every action has an equal and opposite reaction and this is exactly what causes the thrust, accelerating air out the back of the hovercraft causes a thrust reaction propelling the hovercraft forward. Now this can be achieved different ways, for example a small area high velocity jet of air might be seen to be equivalent to a large area low velocity jet of air with the same power input. However there are many other factors such as frictional losses in the intake, the propeller, the outlet, the rudders and within the air itself. Without going into detailed analysis, these can be expressed, as efficiencies at each stage but suffice to say for this discussion that most losses are proportional to the air velocity squared. That means in simplistic terms that if you double the air velocity you will double the thrust but you will get four times the losses! In other words, you can get much more thrust per Kilowatt (or horsepower) of engine power input with a system that passes a larger quantity of air at a relatively low velocity compared with a system (same power input) that passes a smaller quantity of air at a higher velocity.
Therefore the factors to consider mostly when designing the thrust system is air mass flow and reduction of losses. This leads to large area thrust devices (propellers and ducts) with very clean through-flow characteristics such as streamlined ducts, nice fairing on belt drives, struts and other obstructions and minimal surface areas. The propeller itself will typically have between two and five blades, a small hub diameter and an aerofoil without camber such as ‘Clark Y’ or similar. In the case of the ducted propeller, the pitch (blade angle) will typically be coarse to minimise tip speed and reduce operating noise. This can be achieved because the propeller is mainly concerned with volume flow and not too much concerned with static pressure rise.
Lift System Requirements
In the case of the hovercraft lift system, it is the air pressure that acts upon the underside of the hovercraft that provides the lifting force to make the hovercraft hover. Also, because the skirt does not form a perfect seal around the hovercraft perimeter there is leakage of the air and that must be balanced by constant air input from the lift fan.
Hovercraft lift systems usually have a rather ‘dirty’ airflow characteristic so most of the dynamic pressure of air moving is lost as it passes around bends and obstructions in the ducting system. The amount of lost dynamic air pressure can be minimised by designing a lift system that operates at low velocity. That leaves static air pressure as the over-riding consideration combined with getting enough air into the lift system to allow for skirt leakage.
Therefore the factors to consider mostly when designing the lift air input system are the pressure characteristics of the fan and the volume flow capabilities of it at the required pressure. This is a completely different design criterion than what is needed for an efficient thrust system and the design of the fan results in a completely different device to that which would be chosen for maximum thrust efficiency. Comparing an axial flow lift fan with an efficient propeller you would note different blade aerofoil sections, considerably more camber in the aerofoil, more blades, larger hub diameter and different operating tip speeds. Also you would note that the blades run at a finer pitch and the tip speed needs to be relatively high to achieve pressure generation compared with what would be needed to achieve the required volume flow. To use a lift fan for thrust is to accept the considerable aerodynamic losses that will result.
Additional and equally important factors found in a well-designed lift system will be it’s flexibility in operation because it will have a favourable pressure volume curve, it will be without nasty stall characteristics and it will be as quiet as possible. Additionally it will be easily controlled so more air can be delivered to cross porous-terrain or accommodate torn skirt fingers and it can be slowed down to save power (fuel and noise) when the hovercraft is operating on water with a well-maintained skirt system. The best result is only possible if the lift system is designed for lift alone without regard for the provision of thrust air.
Most importantly, a good lift system must be easily controlled by the hovercraft operator to accommodate a variety of operating conditions and it should do the job quietly and efficiently.
|AERODYNAMICALLY SEPARATE||AERODYNAMICALLY INTEGRATED|
|Separate design criteria, easy to get the best results for each system.||Compromised design criteria, hard to get the correct balance between system requirements. Thrust requirements are usually compromised by minimum lift requirements.|
|Ability to have a very flexible operating characteristic in the lift system making driving the hovercraft easier.||Operation is compromised. Usually over-ridden by minimum lift requirements so if too much thrust coincides then it has to be ‘lost’ by aerodynamic spoilers or skirt dragging, both of which result in lost power and possibly lost rubber from the skirt.|
|Aerodynamic efficiencies can both be high||Aerodynamic efficiencies are compromised; a balance is needed which can be more or less in favour of either system depending upon the design criteria. Either way it is very difficult to achieve good efficiencies in both systems while satisfying the different the requirements|
|Less noise possible through targeted aerodynamic design.||Much noisier. The fan design usually operates according to the hovercraft lift requirements, which results in a very poor thrust performance. This compounds into needing more fan speed and thus much more noise generation.|
|Placement of lift and thrust systems can be separated to improve the lift system efficiency. It is preferred to introduce most of the lift air near the front of the cushion system for good dynamic response when operating at high speed over rough water. Additionally the lift fans can be arranged to blow directly into the lift system with less bends or ducting and thereby suffer less losses.||Placement of both systems is usually at the rear of the hovercraft, which is not conductive to good lift system ducting efficiency since most of the lift air input to the skirt system is preferred at the front of the hovercraft. Usually the air is travelling at high velocity in a rearwards direction and it needs to be turned into a downward or forward direction to get to the skirt system. This results in considerable pressure losses from the bends and additional ducting required to redirect the air into a favourable position within the skirt system.|
|Because of the above, smaller engines are required.Performance can be achieved with less installed power that means smaller, lighter and more economical engines. This has a compound effect because less weight also needs less power so the final result is quite noticeable.||Larger engines required.More installed power required to achieve the same hovercraft payload result. Heavier engines reduce payload capability and require more fuel, which further reduces payload (or duration). It is a negative compound effect.|
Fan characteristics (axial flow).Typically large number of blades, large hub diameter (30% to 50% of overall diameter), large amount of camber in the aerofoil section, tip speeds governed by pressure rise characteristics, pitch governed by flow requirements, static efficiency range (well designed) between 40% and 65%. Mixed flow and radial flow lift fans are not compared here but their static efficiencies are typically much higher at around 74% to 83%.
|Fan characteristics.Similar characteristics applied to lift/thrust fan but efficiency will typically be lower due to design compromises and additional ducting losses.|
|Propeller characteristics.Typically with less blades, usually 3 to 5 blades but can range from 2 blades to 8 blades. The aerofoil is usually without camber and the leading edges are more radiused to accommodate more variable inlet flow conditions. In the case of ducted propellers, the pitch is usually coarser and the tip speeds lower to minimise noise. The hub diameter is smaller at around 15% to 30% of overall diameter. With good design thrust efficiency and noise can be quite low.||Propeller characteristics.Looks more like a lift fan but the hub diameter may be smaller. Efficiency in thrust application is terrible. Usually very noisy.|
|MECHANICALLY SEPARATED||MECHANICALLY VARIABLE DRIVE RATIO||MECHANICALLY FIXED DRIVE RATIO|
|Two independent power sources needed.||Can share power sources.||Can share power sources.|
|Very simple transmission systems possible because of direct coupling possibilities.||More complicated transmission systems required. Usually employing hydrostatic systems to the lift fan drive and mechanical systems to the thrust system. Sometimes variable mechanical systems are employed involving clutches and different drive ratios or gearboxes but this is even more complicated and generally not as reliable as a well designed hydrostatic system.||Transmission systems are usually mechanical and not too complicated but duplicated seperately for lift and thrust.|
|Very easy to control separately. Lift system can be operated without even starting the thrust engine. Also full thrust can be achieved while the hovercraft is floating.||With good design, separate control can be nearly as good as a mechanically separate system. Full lift is obtainable with zero thrust for easy craft handling. During take off, increased thrust is available for by using some power normally allocated to the lift system.While cruising down wind, additional power may be directed to the lift system while keeping the thrust system at a controllable level. This system is ideally suited to middle size hovercraft with diesel engines and gives a big increase in overall hovercraft performance levels by allowing the most advantageous application of the limited power available from the diesel engine.||Not able to be controlled separately. If you need more lift then you also get more thrust, even if it is not wanted. Also if you need more thrust, for example when taking off or runnning into head wind then you automatically get more lift which may be a waste of power needed for thrust in difficult conditions.
It is not possible to change the ratios so thrust destroying tactics are needed such as aerodynamic spoilers to waste power or by trim changing to implement skirt dragging (to loose rubber), either of which are wasteful and can give the driver difficult control problems.
|Placement of lift and thrust systems can be separated to improve the hovercraft overall layout.||Placement of lift and thrust systems can be separated to improve the hovercraft overall layout. With hydrostatic transmission there is no restriction to positioning of the lift fans so equal advantage is obtainable as in mechanically seperate systems.||Placement of lift and thrust systems can be separated to improve the hovercraft overall layout. However this feature will be more restricted because of the need to align mechanical drive systems more carefully.|
|Placement of lift and thrust systems can be separated to improve the hovercraft balance. It is preferable to get the variable payload as close as possible to the lift system centre of pressure so that payload variations will have minimum effect on the hovercraft operating. A balanced hovercraft is more controllable and requires less power for thrust.||Placement of lift and thrust systems can be separated to improve the hovercraft balance. With hydrostatic drives, the flexibility of lift system placement is such that the fans can be placed for best lift airflow and best weight balance without compromise of having to provide protestion for or allow for access to servicing a lift engine.||Placement of lift and thrust systems can be separated to improve the hovercraft balance. With the rearward placement of the lift and thrust systems overall hovercraft balance is harder to control as the machinery weight is concentrated at the rear thus forcing the variable payload area well forward to balance it. This results in a hovercraft design that is overly sensitive to load placement.|
|Very good control available to the driver.||Very good control available to the driver.||Not so good control available to the driver. However this system is still much better than the aerodynamically integrated system.|
|Efficient use of installed power results in smaller engines required.||Efficient use of installed power results in smaller engines required.||Not so efficient use of installed power results in larger engines required or a reduction in available performance.|
|Two engines to maintain c.f. one engine. However maintenance is usually very simple.||One engine to maintain plus a hydrostatic system to maintain. With good design, the hydrostatic is very low maintenance and can outlast several engines. Less maintenance than two engines.||One engine to maintain. Sometimes additional belt drives to maintain depending upon design.|
|Easiest system to design to achieve good operating efficiencies.||Not so easy system to design but able to achieve the best overall operating efficiency.||Not so easy system to design. Good operating efficiencies are only achievable at one operating point, away from that operating point there are considerable losses.|
|Most expensive system in terms of buying original engines.||More expensive in small hovercraft, less expensive in middle size hovercraft.||Least expensive system to build.|
|Medium to low expense system to maintain.||Low expense system to maintain.||Low expense system to maintain.|
|Low operating cost.||Lowest operating cost.||High operating costs due to power wastage under operating conditions other than ideal.|