4wings.com Hovercraft development 47751 Michigan ave., Port Isabel - 78578 - Texas, US Tel.: (956)943 5150


Hovercraft development
47751 Michigan ave. , Port Isabel, TX, 78578
Tel:(956) 943-5150

The "alternative drive" or  Hydraulic system  - "Fluid power " !
a Basic introduction...

This file is provide free for education purposes with the assumption that the reader is fully aware of the risks involved. The author assumes no responsibility or liability for its use or consequences of its use by others. 

This script  shall give you a basic idea of a simple hydraulic system ( also known as fluid power system ) for single engine hovercraft. The hydraulic system is used for separate lift fan operation. 
It is not a working copy to set up any system and shall provide you only with the basic idea and some tips in the layout - please consult a hydraulic specialist for detailed calculations and project specific layout.
You will find some  required calculations and formulas  in the linked pdf files. 

The basics : 
A hydraulic system consist of a hydraulic pump and motor. 
The pump is attached to the main engine. The shaft of pump is connected to the engine shaft via reduction or belt drive. 
With each revolution of your main engine, the hydraulic pump is brought into pumping motion.
It is important to take care of the pump max. rated RPM which in most cases, will require a reduction drive. Hold in mind that you will need adequate RPM for your lift unit before you reach high RPM on your thrust output shaft. As well as max. thrust shaft RPM should not exceed max. rated pump RPM.
The pump is connected via hydraulic fluid line to the hydraulic  motor.  This is your high pressure line. For further reference please use :  Eaton Pressure-Flow Compensated Piston Pumps all credits to the original source.
The motor most likely will be attached to the fan shaft directly or in the shortest possible way.  As the hydraulic pump builds up pressure with each rotation, the high pressure line transfers this pressure to the intake side of the motor which will start turning in a nearly equal manner ( minus loss of friction in the line,...). For further reference please see :  Eaton Medium Duty Piston Motor - all credits to the original source. The low pressure line is attached to the motor's exit port and it will transfer the hydraulic fluid back to the pump as seen in layout 1.
Alternative supplier are :
 CROSS Manufacturing, Inc.
 KNF Neuberger, Inc. (USA)
 Parker Mobile Hydraulics

basic layout of fluid power system for hovercraft
This is the most simple set up of fluid power in a closed system. Kind of  stripped down basics which will work on very low hp application and with generally no major control over your system. 
To set up your system you need to know the max. HP and RPM on your motor side - once this is known you can work yourself to the pump side.
I hope you could still follow so we can start to complicate this quite simple system a bit.
Lets start with the motor - what else is there to take care of besides low weight and long lifetime for a reasonable price?.... Most motors won't be sitting on the shelf and have a 8 - 10 week delivery time - so it is wise to get the right unit in the first place. Make sure you know the shaft turning direction, location of the motor and most appropriate ( high pressure and low pressure ) port direction so you can reduce 90 degree elbow on the high pressure side. Make sure your drain port point's up to drain air from your hydraulic system. Last but not least motor attachment is important.

Once we have this selected it is wise to think about the most important medium in the system and that is your hydraulic oil ( try to use a good non foaming oil ) . With the use of pump and motor you will get small amounts of abrasion particles in your hydraulic oil - these tiny buggers surf your oil and get pumped in equal manner through your system as the oil it is in - so the use of a filter unit will increase the lifetime of your hydraulic pump and motor and allow these buggers to get out of the system. The filter unit will provide a certain amount of resistance to your hydraulic oil flow. 
For further reference please see : Low, Medium & High Pressure Filters from Eaton Hydraulics - all credits to the original source.
Your  filter unit should be on the low pressure side - as seen in Layout 2 - and is most likely close to the pump. Make the filter unit easy accessible to change filter cartridges on a regular basis. As your filter clogs up over time, resistance will increase in your system.
Other aspects you want to know about your hydraulic oil is  hydraulic oil pressure, hydraulic oil  flow rate  and the temperature of your hydraulic oil.
In general - the more you know about your hydraulic oil the easier it is to avoid expensive components failure. 
Alternative supplier : 

basic layout of fluid power system for hovercraft

So we covered motor aspect, oil and some means to hold it clean ... now lets look at the 
hydraulic hoses ... somehow we need to get the hydraulic oil from point A ( pump ) to point B ( motor ) and again back through a few obstacles. Hydraulic hoses need to have adequate cross section to reduce friction in the line ( friction = resistance = heat build up in your oil) For further reference please see :  Velocity and Pressure drop in pipes and especially at the section : FLOW / VELOCITY NOMOGRAM - as well as :  SAE Hydraulic hose specifications (SAE J517) . Select the appropriate cross-section from your motor's maximum required GPM. and PSI range. Try to lay the high pressure line as straight as possible from pump to motor. Try to avoid 90 degree elbows in your high pressure line. Now you have the theoretical location of your hydraulic line - so check if the hydraulic hose will be able to flex around all corners it is laid - some high pressure lines have quite big minimum radius. You can bend them but do not underestimate this and try to take care of it before hand. A Hovercraft specific problem in the layout of your hydraulic system is the balance of your craft ( CoG ). As your hydraulic pump will be attached by some means to your main engine, it is most likely off center. Your motor on the other hand will be either on Centerline or off centerline - all are fixed weights which won't change during operation - the location of the hydraulic lines allows you to compensate your balance to a certain extent - low pressure lines are lighter and more flexible than the high pressure lines. Last but not least the common sense suggestions - don't run your  hydraulic lines under your fuel tank since hydraulic oil warms up quite considerable and it is not the safest way to fly with hot fuel ...but who knows some might enjoy an extra boost.

So far we have a system which has no leaks. But over the service life of your system, this might change. As well as for hovercraft applications you will run on high RPM = high GPM. = relatively high PSI level which will break down your hydraulic oil ... - the means to increase the lifetime of your pump and hydraulic oil  is a hydraulic oil reservoir tank. This tank holds an amount of hydraulic oil and is vented to the atmosphere. Unfortunately in a weight sensitive Hovercraft this tank is dead weight and not appealing et all. Also it is a fact that custom made tanks are not cheap.  A good holding tank needs to have certain features. The return line fitting which is attached to your hydraulic hose, an internal extension which  should extend in the tank to the bottom of the tank to reduce foam build up. Internal thread on the oil pick up side to attach a filter cat ridge, as well as external threaded fitting  to attach your hydraulic line feeding your pump. As seen in figure Layout 3.  The tank has to be located above the pump to allow a gravity feed of hydraulic oil to the hydraulic pump. An  inspection hatch big enough to pass your hand through ( a bit bigger would be handy so you don't loose your skin if you clean your tank ). A vent on its highest point . And if you want to go fancy, a thread to attach an oil level / temperature sensor would make it safer. So you see we ask for a lot from the poor builder of these tanks.

basic layout for fluid power system for hovercraft

You can use an alternative with a basic factory tank which has inline filter on the pick up side. But this will reduce the life span of the tank. You  must also discharge all the build ups from the tank in an environmentally safe manner.
The sender unit in your tank is more or less nothing more than an electric switch .
You have to determine first hand if the switch shall interrupt the circuit ( as it would be useful if you run your ignition system over the switch ) or close the circuit ( which is the case for a dashboard emergency light or sound which should be activated when your level goes low ) .
The appealing version is that in case the oil level drops ( the system has a leak ) the sender unit would stop the main engine and damage could be reduced to a minimum ... or you can run a red light to your dash board ...... glue your eyes to it and hope you react quicker as your pump is pumping the left over hydraulic oil out the leak..
Alternative supplier :
 Temposonics - 

Even if our basic system got now quite complicated and fancy - we still have no means of actual control of the system = any difference in the  RPM on the hydraulic  pump shaft equals to equivalent RPM change on the hydraulic motor shaft. The only means of control in this aspect is a 
Valve in the high pressure line which regulates the fluid flow. For further reference please use Vickers valves file - all credits to the originator.
The valve  - as seen in Layout 4 -should be the only component between pump and motor in the high pressure side of the system. All the valve does is to reduce or open the fluid flow to the hydraulic motor but it cannot increase the flow beyond the pump capacity according to it's input shaft RPM. The excess hydraulic fluid will be returned into the holding tank. This process will heat up your hydraulic oil considerably and more or less you burn unnecessary HP into heat. An alternative is to use a pressure flow compensated pump with a load sensing line as described in :    Eaton Pressure-Flow Compensated Piston Pumps pages 25 and  down  - which will adjust fluid output according to system requirements. But this variation exceeds the basic layout of this file and might just complicate it unnecessarily.
Alternative supplier :
 Marwin Valve 
 Kepner Products Company 

basic layout for fluid power system for hovercraft

As you have seen in the description of most components - they all have the nasty habit of changing a small amount of energy into heat -for further reference please see :   COOLING AND HEATING - from friction in the lines to resistance in the filter units - your hydraulic oil will heat up considerably if you have the system running on high  RPM. This heat needs to be anticipated to the environment via an adequate heat exchanger which will hold your hydraulic oil temperature as low as possible. For further reference see :  Heat Exchangers for small systems or:   Heat exchanger for bigger systems The heat exchanger will be located in the low pressure side of your system and either you have to use a heat exchanger with relief valve as seen in Layout 5 - in case the unit clogs up - or two units in parallel. All heat exchangers need direct airflow = airflow through the unit and not over the unit which is something we do not really like to provide in Hovercraft where we try our best to produce the highest mass flow of air we can achieve with our HP.

basic layout for fluid power systems for hovercraft

  This is a basic explanation of hydraulic systems , in case it created  a desire to get a more in depth knowledge  , please download the following file : "Hydraulic systems in aircraft"
Distributor for hydraulic components :
Small HP ratings :
 Northern Tool 
General aspect / high Hp ratings :
 Motion Industries 
Unfortunately their web site is as useless as their service and staff is useful - so just look up your nearest Service center and get "hands on" with them.

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