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Hydraulics

Posted: Wed Oct 21, 2015 2:45 pm
by Ryan921204
Can somebody explain me what are the differences between open and closed system? What are their basic principle?

RE: Hydraulics

Posted: Thu Oct 22, 2015 10:07 am
by ZaphodHarkonnen
My understanding is that in a closed system when the hydraulic fluid has been used it gets pumped back into the reservoir and repressurised so it can be reused. This requires extra pumps and heavier piping as the system is closed and the working fluid reused.

In an open system once the fluid is used it's dumped overboard or collected in an unpressurised container. The advantage of this is you don't need heavy pumps or all your piping to be thick enough to handle the pressure as you use it once. The downside of it is you have a very limited amount of time to use the fluid as the amount you have at the start is all you've got.

Generally you'll want closed systems even though they are heavier and more complicated because it gives you the ability to keep going. However if you've got a niche use where weight is a real important and you know you only need the system for x amount of time, then an open system can be useful. The best example of that is SpaceX's Falcon 9 rocket where the booster attempts a soft landing. The guide fins use an open hydraulic system to save weight. And one of the first attempts shows why not having enough is a risk as ran out of pressurised fluid and crashed.

RE: Hydraulics

Posted: Fri Oct 23, 2015 3:39 am
by Dalmd88
Open systems are found on most small aircraft and your car brake system actually. The reservoir is vented to the atmosphere while in a closed system it is not and has a head pressure. The main advantage of the closed system is it works at higher altitudes. An open system would leak out at high altitude. That is why most airliners have closed systems.

RE: Hydraulics

Posted: Sat Oct 24, 2015 8:46 am
by HAWK21M
Open System is when the Fluid post usage is not returned back to the system reservoir unlike a Closed system where the fluid never leaves the Aircraft.

Anyone having some practical examples of an open system & if there are any commercial jets utilizing them, though I would think not.

RE: Hydraulics

Posted: Mon Oct 26, 2015 8:12 pm
by boeingfixer
Quoting HAWK21M (Reply 3):
Open System is when the Fluid post usage is not returned back to the system reservoir unlike a Closed system where the fluid never leaves the Aircraft.

Anyone having some practical examples of an open system & if there are any commercial jets utilizing them, though I would think not.



An open hydraulic system VS a closed system refers primarily to the pump, work required of the system and the reservoir. Both systems, which are better known as Open Circuit or Closed Circuit, retain their fluid in the reservoir after use.

As Dalmd88 stated the Open circuit is generally of simpler design using constant output pumps which usually have a regulator, to maintain a constant pressure, or valving to return the unused portion of fluid to the tank when there is little or no demand from the system. The fluid reservoirs can be quite large and are also vented to atmosphere. A classic aircraft with an Open Circuit design would be the C-47/DC-3.

Closed Circuit systems can get very complex and will have constant pressure variable displacement pumps which, as the name implies, vary the fluid output of the pump based on system demand. These systems will usually have a pressurized reservoir of lesser capacity than an open circuit. A good example of a pure closed circuit design would be a generator CSD (Constant Speed Drive) or an IDG (Integrated Drive Generator).

There's a lot of good material on the web regarding both of these circuits but it mostly covers industrial applications. A simplified explanation can be found at the following link.

http://en.wikipedia.org/wiki/Hydraul...machinery#Open_and_closed_circuits

Cheers,

John

[Edited 2015-10-26 13:13:45]

RE: Hydraulics

Posted: Tue Oct 27, 2015 5:41 am
by KC135Hydraulics
Concur with Boeing fixer, he has posted the most correct explanation.

RE: Hydraulics

Posted: Sun Nov 01, 2015 8:47 pm
by wingscrubber
Oh dear, a few mistaken interpretations above, BoeingFixer's got it right.  

Easiest way to think of it is this;
Open centre systems create constant flow*, variable pressure. (*At fixed pump RPM)
Closed centre systems create variable flow, constant pressure**. (**Providing flow demand doesn't exceed pump supply at given pump RPM)

The principal difference is how the pump and system react to load, an open centre system is typically going to have a 'dumb' constant displacement vane pump or gear pump, it's cranking out the same volume of fluid with each revolution of the shaft, with the goal of reaching the pressure setting of the relief valve, at which point the relief valve cracks and the fluid is escaping back to tank. Flow through the relief valve has to equal the flow from the pump unless the system is moving something/doing work.

The power steering on your car is open-centre, likely as is the hydraulics on the garbage truck which picks up your rubbish, also many small backhoes/diggers, farm tractors etc. These systems are typically of simple utility, and temporary duty. Open-centre systems make a lot of heat, as the fluid squeaking through the relief valve is basically lost work, so the fluid gets hot - they need heat exchangers/radiators if they're going to run continuously.

For aircraft, open-centre systems are best for basic, temporary duty consumers, like landing gear, brakes, steering, flaps, etc. B-17 is another example btw. Probably some smaller GA aircraft with retractable gear, like a C172RG have constant displacement pumps. (Because they only have one function)



Closed centre systems are usually for systems which must run continuously, powering higher complexity multiple consumers, so more complex backhoes or off-highway equipment, locomotives with hydrostatic transmissions, large-scale robotics etc.

For aircraft, closed-centre systems are inately better suited to flight controls, and supplying multiple consumers which may demand concurrent flow supply, instantaneously. Besides flight controls this might include, flaps, spoilers, thrust reversers, landing gear, steering, brakes, cargo doors, gun drives (i.e. gattling guns) and so-on.

http://www.apt-usa.com/assets/images/Hawe_Hydraulics/V30D.JPG

Closed centre systems have no relief valve pressure control, they instead have a variable swash plate pump with a pressure compensator which controls the displacement of the pump. The compensator is mechanically spring loaded to change the stroke of the pump depending on the flow demand within the system.

Just to further confuse, there are also systems with variable-pressure compensators, a couple of good aircraft examples would be the Piaggio Avanti, which runs at 1000psi except when gear is extending or retracting when it switches to 3000psi, by use of a dual-mode compensator. Or the F-18F, which has a 3000psi system, but increases to 4000psi when supersonic the same way.

Just to be clear, no hydraulic system is intentionally designed to dump fluid overboard after use to my knowledge!
The only caveats to that would be, burst discs in components which are designed to fail in case of a fault or over-pressure, components expected to leak such as pump shaft seals, or fueldraulics, where aircraft kerosene fuel is used to do work before it is burnt in the engine, i.e. F35.

[Edited 2015-11-01 13:08:43]

RE: Hydraulics

Posted: Mon Nov 02, 2015 7:12 pm
by KC135Hydraulics
Quoting wingscrubber (Reply 6):
Just to be clear, no hydraulic system is intentionally designed to dump fluid overboard after use to my knowledge!
The only caveats to that would be, burst discs in components which are designed to fail in case of a fault or over-pressure, components expected to leak such as pump shaft seals, or fueldraulics, where aircraft kerosene fuel is used to do work before it is burnt in the engine, i.e. F35.

This is most certainly correct. You typically only have overboard drains for things such as shaft seals on pumps/motors, overpressure relief valves for things such as brakes or bleed air reservoir pressurization systems, at least on the aircraft I have worked on. It does no good to consume and discard hydraulic fluid overboard.

RE: Hydraulics

Posted: Wed Nov 11, 2015 9:39 pm
by rwessel
Quoting wingscrubber (Reply 6):
Just to be clear, no hydraulic system is intentionally designed to dump fluid overboard after use to my knowledge!

More than a few liquid-fueled rocket engines use fuel as the hydraulic working fluid for the gimbal actuators (and anything else that needs hydraulics - on large engine the main propellant valves are often hydraulic). Usually it's just a tap off the fuel turbopump (which is usually producing pressures* in the right range anyway), and the used hydraulic fluid is commonly just dumped overboard (although not exclusively - the S-V's F-1s ran the "used" fuel back into the turbopump inlets). With some engines you see interesting hybrid sources - the F-1's for example, ran on ground hydraulic power until the turbopumps produced enough pressure to run the system.

The Falcon-9 mentioned upthread is interesting, since they're changing the design from using a separate hydraulic fluid in a use-once-and-dump system (and where they ran out on one of their landing tests), to using fuel as the working fluid (and if you run out of that during a landing the loss of hydraulics will be a minor issue at best).

But yes, open vs. closed in common applications has nothing to do with using the working fluid only once.


*On the SSME, for example, the turbopumps are dual stage, and the output of the high pressure fuel turbopump is on the order of 6000psi. Most designs are rather lower than that, but fuel pressures in excess of 1000psi are pretty typical.

RE: Hydraulics

Posted: Fri Nov 13, 2015 5:54 am
by Apprentice
Quoting KC135Hydraulics (Reply 7):



Hi:
The very old mechs in the forum may explain that a Pneumatic System is, by definition, an Hydraulic System too.
https://en.m.wikipedia.org/wiki/Pneumatics.
In a/c like F-27, hydraulic Fluid was instead compressed air, wich was dump overboard thus reducing return lines weight.
There are several GA a/c which uses also Pneumatic Sys, in this case, a Vacum System, mostly as a driving power for instruments like Gyros. Air is also dumped after been used.

Rgds

RE: Hydraulics

Posted: Fri Nov 13, 2015 1:15 pm
by Starlionblue
Quoting Apprentice (Reply 9):
The very old mechs in the forum may explain that a Pneumatic System is, by definition, an Hydraulic System too.

Pneumatics and hydraulics are not the same, although their use has sometimes been the same.

They are very different in a fundamental way. Pneumatics use gases. Hydraulics use fluids. (From the greek words for "breath" and "water".) Gases are compressible. Fluids are not.

Most large airliners have both hydraulic and pneumatic systems, with hydraulics used for things like brakes, high-lift devices, landing gear and control surfaces, and pneumatics used for anti-ice and pressurization.

[Edited 2015-11-13 05:17:06]

RE: Hydraulics

Posted: Fri Nov 13, 2015 2:34 pm
by Apprentice
Quoting Starlionblue (Reply 10):

Hi, that is the point: Air is a Fluid. (not a liquid). This is one basic principle of aerodynamic, that You need, to understand physics of air flow around a wing.


http://www.ucmp.berkeley.edu/vertebrates/flight/physics.html
" First we have to recognize that air is a fluid, just like water. It is not a liquid, like water, but is a called a fluid because the force needed to deform it depends on how fast it is deformed, not on how much it is deformed"

Rgds

RE: Hydraulics

Posted: Sat Nov 14, 2015 2:14 am
by Starlionblue
Quoting Apprentice (Reply 11):

Quoting Starlionblue (Reply 10):

Hi, that is the point: Air is a Fluid. (not a liquid). This is one basic principle of aerodynamic, that You need, to understand physics of air flow around a wing.


http://www.ucmp.berkeley.edu/vertebrates/flight/physics.html
" First we have to recognize that air is a fluid, just like water. It is not a liquid, like water, but is a called a fluid because the force needed to deform it depends on how fast it is deformed, not on how much it is deformed"

You are right on that. I was using unclear terms. Pneumatics use gases. Hydraulics use liquids.They are both fluids.

However there is a crucial difference. Gases are compressible and liquids are not.

The typical airliner applications are quite different. Hydraulics are used for "heavy lifting" that pneumatics are not suited to providing. Hydraulics are more efficient (I assume due to incompressibility of the fluid) but require heavier and more expensive hardware.