Comparisons of Systems

Overview
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.