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Extra300
Topic Author
Posts: 87
Joined: Wed Sep 21, 2011 8:04 pm

Hydraulic Circuits: Where To Find An Overview?

Fri Aug 08, 2014 10:40 am

Hi

As an aviation enthusiast and hydraulic engineer it´s funny and almost embarising how little I know about hydraulics in commercial airliners.

-What type of pumps and valves are used?
-How is redundancy solved?
-How is cavitation isseus due to low cabin pressure solved?
-What type of oil?
-Are ackumulators used to cope with flow peaks etc.
-Where in the A/C are the hydraulic eguipment located

My questons are many, and I´m thankful for any response. A link or reference to some litterature about this subjuect would be great.

Thanks
 
roseflyer
Posts: 9602
Joined: Fri Feb 13, 2004 9:34 am

RE: Hydraulic Circuits: Where To Find An Overview?

Fri Aug 08, 2014 12:01 pm

Quoting Extra300 (Thread starter):

-What type of pumps and valves are used?

Variable displacement continuous pressure pumps. Eaton and Parker are the most common pump suppliers.

Quoting Extra300 (Thread starter):
-How is redundancy solved?

Usually independent systems (sometimes 2 or 4) which have no interconnection other than a power transfer unit. No fluid should go between the systems, but occasionally there are ways to manipulate it so that some fluid transfers through an actuator. Flight control surfaces are triple redundant and can be controlled by any one of the system.

Quoting Extra300 (Thread starter):
-How is cavitation isseus due to low cabin pressure solved?

The reservoirs are pressurized. Either through pneumatic pressure from the air conditioning system or through a reservoir with a piston maintaining head pressure referred to as a boot strap reservoir.

Quoting Extra300 (Thread starter):
-What type of oil?

The fluid goes by the trade name skydrol or Hyjet 5.

Quoting Extra300 (Thread starter):
-Are ackumulators used to cope with flow peaks etc.

Sometimes there are accumulators to cope with flow peaks near very sensitive hydraulic equipment like flight controls power control units. They are not always used. Also an accumulator usually provides reserve pressure for the brakes in case a hydraulic system is lost. This also allows the parking brake to remain pressurized when the pumps are off.

Quoting Extra300 (Thread starter):
-Where in the A/C are the hydraulic eguipment located

Usually the wheel well. Sometimes the struts to the engine or in front or behind the wheel well. Sometimes equipment is located in the air conditioning pack bays.
If you have never designed an airplane part before, let the real designers do the work!
 
Extra300
Topic Author
Posts: 87
Joined: Wed Sep 21, 2011 8:04 pm

RE: Hydraulic Circuits: Where To Find An Overview?

Fri Aug 08, 2014 3:18 pm

Thanks a lot for your reply!

Do you know anyting about the reservoir design? I find it hard that conventional tanks found in land mobile equipment is suitable for aircraft. Some sort of pressurized tank, rubber bladder etc?
 
roseflyer
Posts: 9602
Joined: Fri Feb 13, 2004 9:34 am

RE: Hydraulic Circuits: Where To Find An Overview?

Fri Aug 08, 2014 5:50 pm

Quoting Extra300 (Reply 2):


Do you know anyting about the reservoir design? I find it hard that conventional tanks found in land mobile equipment is suitable for aircraft. Some sort of pressurized tank, rubber bladder etc?
Hydraulic Reservoir Pressurisation (by A/c train Apr 9 2001 in Tech Ops)

http://www.youtube.com/watch?v=7RAbiYpN_v4
If you have never designed an airplane part before, let the real designers do the work!
 
wingscrubber
Posts: 823
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RE: Hydraulic Circuits: Where To Find An Overview?

Sun Aug 17, 2014 11:45 pm

Ah my favourite topic   Nice answers from roseflyer but I'll join in and elaborate too...

Quoting Extra300 (Thread starter):
-What type of pumps and valves are used?


Variable displacement pressure compensated in-line pumps are the norm, as most aircraft hydraulic systems are closed centre systems (Constant pressure, variable flow). Vane pumps and open centre systems (constant flow, variable pressure) like the sort you'd normally find in power steering or maybe garbage trucks are generally not used.

As a general rule if the hydraulic system is a 100% duty cycle system (i.e. switched on all the time in flight) then engine driven pumps and/or AC motorpumps are used. If the system is temporary duty (i.e. switched on only for take-off and landing) then DC motorpumps can be used. DC motorpumps can be lighter but are usually brushed motors so they wear out, and can overheat, if used continuously. Fluid-cooled AC motorpumps do the job where they're required to run continuously. Some aircraft (Boeing 767) have bleed-air driven pumps instead of electric.

Quoting Extra300 (Thread starter):
-How is redundancy solved?


Varies depending on the size of the aircraft and flight control architecture; if the aircraft is small, has powered flight controls but has manual reversion (i.e. not fly-by-wire) then two hydraulic systems might do the trick. (MD-80, Citation X)
If the aircraft does not have powered flight controls and the system only powers landing gear/brakes/steering then one system might do. (Beech 1900, Learjet 45)
If however the aircraft is fly-by-wire, with no manual reversion, you need at least three hydraulic systems to cover your various failure modes. (Airbus A320, Bombardier CRJ)

Older, larger airframes such as L1011 & 747 justified 4 hydraulic systems but A380 only has two! A380 gets away with this because it has local back-up hydraulic powerpacks for critical flight controls.

Quoting Extra300 (Thread starter):
-How is cavitation isseus due to low cabin pressure solved?


Cavitation is not strictly related to cabin pressure... cavitation is induced by low suction pressure supply to the hydraulic pumps, it has to remain at least above atmospheric pressure - about 15psi (you want to push the fluid into the pump, not let it suck the fluid), but ideally around 50psi to stay healthy. If suction line pressure dips below atmospheric pressure and you induce cavitation, the collapsing micro-vacuum pockets can damage the pump.

Suction line pressure is addressed with reservoir pressure, and there are different types of reservoirs. Small powerpacks might have spring-loaded reservoirs, but otherwise they are air pressurised or 'bootstrap' self-pressurised reservoirs.

The cabin pressure association would only be because cabin pressure is maintained by the service air/ac system, which is pressurised with bleed air from the engines. Air pressurised reservoirs use the same bleed air, but have their own dedicated line. If you lost bleed air, in principle you also lose your reservoir pressure supply - but if the reservoir is sized properly, the pressure within the reservoir will be maintained by the existing trapped air until the fluid level drops, i.e. to cover fluid delta needed for unbalanced-volume actuators, like landing gear or spoilers maybe.

Quoting Extra300 (Thread starter):
--What type of oil?


There are generally two categories used on transport category aircraft - phosphate esters and red oils.
The phosphate esters are the skydrols and hyjets - of which there are various versions. 500B, Type 4 and Type 5 - type 5 being the latest and greatest. Practically all the Airbuses, Boeings, Bombardiers, Embraers and Gulfstreams are skydrol.

The red oils are MIL-5606, 87257 and 83282, predominantly used on military aircraft but also smaller corporate aircraft. There are pros and cons to both types but I won't go into that here.

Chevron M2V is worth a mention too - this was the fluid specific to Concorde.

Quoting Extra300 (Thread starter):
--Are ackumulators used to cope with flow peaks etc.

Yes - accumulators are like hydraulic springs, they can be used to absorb transient flow peaks/pressure spikes or compensate for flow shortfalls. There are various types - air pressurised rubber bladders, pistons or metal bellows.
You might use an accumulator to help something deploy very rapidly, like spoilers or thrust reversers, or you might use it to hold continuous pressure on a parking brake for instance, but brakes and steering accumulators are very common.

Quoting Extra300 (Thread starter):
--Where in the A/C are the hydraulic eguipment located

Engine driven pumps are usually in the nacelle, and reservoirs/motorpumps/PTUs/filters are often clustered together in the wheel well on something like an A320/737, but will be nestled somewhere in the 'hell hole' tailcone area between the engines on a rear engined aircraft.

Some good resources;
http://www.daerospace.com/HydraulicSystems/index.php
http://www.ieeems.org/Meetings/prese...-IEEE_Hyd_Systems_Presentation.ppt

Smartcockpit has hydraulic system descriptions for many aircraft too.
Feel free to PM me if you want to know more specifics  
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KC135Hydraulics
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RE: Hydraulic Circuits: Where To Find An Overview?

Mon Aug 18, 2014 3:31 am

Quoting wingscrubber (Reply 4):
The red oils are MIL-5606, 87257 and 83282, predominantly used on military aircraft but also smaller corporate aircraft. There are pros and cons to both types but I won't go into that here.

Pros: It doesn't sting you when you get it on your skin, and it's very easy to see when you have something leaking!
MSgt, USAF
KC-135R / C-17A Pneudraulic Systems Mechanic Supervisor
 
737tdi
Posts: 1116
Joined: Sun Sep 02, 2007 10:05 am

RE: Hydraulic Circuits: Where To Find An Overview?

Mon Aug 18, 2014 6:20 am

Something not mentioned so far, unless I missed it. I will speak only to the modern? 737. I worked McD prior to WN.

There are 3 systems on the Boeing 737. A system, B system and Standby. A and B do have congruity through a couple of valves/power transfer units. Also through brake valves and thrust reversers. You can physically transfer fluid between A and B. It takes a slightly complicated bit of operations of systems but it is done on a regular basis. All aircraft have little idiosyncrasies that have to be learned by the flight crew and mechanics and transferring fluid from one system to the other is one of these. I keep a little card in my badge holder so I don't have to keep this info. in my little brain??? If you really want to know the schematics go to this site. http://www.b737.org.uk/. He has built a very nice site on discovering the 737 and alas hydraulics systems of it.

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