Hydraulical/Electrical Systems
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Shortround6
Major General
depends on who you talk to......the electrical system salesman or the hydraulic system salesman 
Both systems evolved through time. Higher voltage electrical systems were lighter than lower voltage systems. High pressure hydraulic systems are lighter than low pressure systems. Which system was lighter swapped back and forth over time.
Finding hydraulic leaks may be easier than finding electrical shorts or broken/bad connections. Fixing the leak may be more difficult. Wires should be easier to run than hydraulic lines.
Both systems evolved through time. Higher voltage electrical systems were lighter than lower voltage systems. High pressure hydraulic systems are lighter than low pressure systems. Which system was lighter swapped back and forth over time.
Finding hydraulic leaks may be easier than finding electrical shorts or broken/bad connections. Fixing the leak may be more difficult. Wires should be easier to run than hydraulic lines.
FLYBOYJ
"THE GREAT GAZOO"
Anything electrical is going to weigh less. Anything hydraulic is going to be able to move heavier stuff but weigh more, possibly quicker if a high pressure system is used. With hydraulics you're carying around several gallons of a fluid in the aircraft. Electrical you have fire risks...
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Shortround6
Major General
Hydraulics were actually the older system. A Hydraulic system used a lot less copper than an electric system.
michaelmaltby
Colonel
Bell's P-39 was a pioneer platform for "electrical" systems.
While my own personal experience with rubber tired earthmovers "electrical vs hydraulic (Letourneau-Westinghouse vs Euclid) convinced me that "electronics" were far more sensitive to lack of maintenance in the field than hydraulics, the Russians -- the major client for the P-39 in tough field conditions -- seemingly never complained about the electronics. The electric pitch controlled propeller was modified to the more common hydraulic pitch control rather early, IIRC.
MM
While my own personal experience with rubber tired earthmovers "electrical vs hydraulic (Letourneau-Westinghouse vs Euclid) convinced me that "electronics" were far more sensitive to lack of maintenance in the field than hydraulics, the Russians -- the major client for the P-39 in tough field conditions -- seemingly never complained about the electronics. The electric pitch controlled propeller was modified to the more common hydraulic pitch control rather early, IIRC.
MM
davebender
1st Lieutenant
WWII era hydraulic turret control was more precise. Probably more of an issue for tanks then aircraft.
Electrical systems were generally safer, as there was less fire hazard. OTOH, they were usually slower in motion, ie. the propeller pitch change on German fighters were noted to be somewhat slow.
One of the salient advantages of an electric system was that it ran on batteries, while hydraulics require the engine running. On both tanks and aircraft this had some distinct disadvantages. Most importantlz, an aircraft that suffered an engine failure or hit could not feather the propeller and decrease drag in the very moment it was required the most!
Same for the landing gear (though emergency lowering often existed for this reason) and other systems (flaps, control surface servos etc). I am not sure if breaks were hydraulic.. Its like loosing servo steering or breaking on a car if the engine has some sort of proplem and stops.
On tanks the need to operate the engine to have hydraulic power turret traverse meant disadvantage in ambush/hidden positions, since tank engines running give away your presence/position. In contrast those tanks with electric traverse (T-34, Pz IV and iirc Sherman for example) could operate the turret with ease. Hydraulics also have flammable fluids which is not a good thing if the high pressure system is holed or leaks. For this reason Israeli Merkavas for example retain electric turret traverse, even if its slower. Electrics rely on batteries, which may have troubles in winter.
Despite all that, hydraulics are often preferred because they can be more easily and accurately controlled by pressure and they offer much more power. I am not an engineer but I would believe a hydraulic system is also much more cheaper to make, being a simply device.
One of the salient advantages of an electric system was that it ran on batteries, while hydraulics require the engine running. On both tanks and aircraft this had some distinct disadvantages. Most importantlz, an aircraft that suffered an engine failure or hit could not feather the propeller and decrease drag in the very moment it was required the most!
Same for the landing gear (though emergency lowering often existed for this reason) and other systems (flaps, control surface servos etc). I am not sure if breaks were hydraulic.. Its like loosing servo steering or breaking on a car if the engine has some sort of proplem and stops.
On tanks the need to operate the engine to have hydraulic power turret traverse meant disadvantage in ambush/hidden positions, since tank engines running give away your presence/position. In contrast those tanks with electric traverse (T-34, Pz IV and iirc Sherman for example) could operate the turret with ease. Hydraulics also have flammable fluids which is not a good thing if the high pressure system is holed or leaks. For this reason Israeli Merkavas for example retain electric turret traverse, even if its slower. Electrics rely on batteries, which may have troubles in winter.
Despite all that, hydraulics are often preferred because they can be more easily and accurately controlled by pressure and they offer much more power. I am not an engineer but I would believe a hydraulic system is also much more cheaper to make, being a simply device.
davebender
1st Lieutenant
Normally there is a manual back up. However it's rather slow.
Shortround6
Major General
Some hydraulic systems had an "accumulator" or pressure tank that could supply a limited amount "power" for a very limited amount of time. But it cost weight/volume.
While brakes may have been hydraulic they were usually on their own circuit and not tied into the retract, flap, etc system.
Hydraulic controlled props almost always used the engine oil and not Hydraulic fluid. Some Hydraulic props used a separate oil supply from the engine but it was not tied into the " Hydraulic" system of the airplane.
Hydraulic systems may only leak when being used, depends where the leak is and valving. If the control valve is closed ( neither opening or closing) Any leak between the control valve and the actuating cylinder will only leak out the fluid in the line between the valve and the cylinder, not the entire system. A leak between the pump and the control valve will drain the system. We blew a line on the traverse system on a 100ft platform truck. Quite a mess. You could lower and retract but moving left or right greatly increased the "HAZMAT" spill
While brakes may have been hydraulic they were usually on their own circuit and not tied into the retract, flap, etc system.
Hydraulic controlled props almost always used the engine oil and not Hydraulic fluid. Some Hydraulic props used a separate oil supply from the engine but it was not tied into the " Hydraulic" system of the airplane.
Hydraulic systems may only leak when being used, depends where the leak is and valving. If the control valve is closed ( neither opening or closing) Any leak between the control valve and the actuating cylinder will only leak out the fluid in the line between the valve and the cylinder, not the entire system. A leak between the pump and the control valve will drain the system. We blew a line on the traverse system on a 100ft platform truck. Quite a mess. You could lower and retract but moving left or right greatly increased the "HAZMAT" spill
Yes I am aware. The situation I had in mind is a tank in a well concealed position, with engines shut off not to give away the position or presence (tank engines are easily heard several hundreds of meters away, in fact I believe modern armies even synchronize engine startup between the tanks in the platoon, probably to conceal their numbers). If that tank laying in ambush spots a moving tank column and wants to engage it, the gunner may find it less than optimal to frantically crank that manual traverse while trying to aim...!
FLYBOYJ
"THE GREAT GAZOO"
More like 90% of them. I've seen very few aircraft that don't have an an accumulator.
FLYBOYJ
"THE GREAT GAZOO"
I think you got that backwards. Unless your hydraulic fluid was flammable (5606 red oil was not) electrical systems post the greatest fire risk.
FLYBOYJ
"THE GREAT GAZOO"
Yes - some hydraulic fluids, although petroleum or mineral based are manufactured with an extremely high flash point to the point where they are considered inflammable. The most common in the US in WW2 is Mil-H-5606, I believe it has a flash point of over 270F. I've seen lit matches tossed into a pan of 5606 and they just go out.
There are more modern hydraulic fluids developed, Skydrol for one, which is absolutely inflammable but has some corrosive properties, it's really nasty if you get it on your skin.
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Shortround6
Major General
Part of the problem with "flammable" hydraulic fluid (like a lot of flammable liquids) is the state it is in and the temperature. Trying to light spilled (low pressure/leaked) hydraulic fluid is going to be an exercise in frustration. Igniting high pressure "leaking" hydraulic fluid that is forming a well atomized mist is going to be a lot easier, especially if you have exposed ignition sources around, like glowing chunks of metal from a tank penetration.
A bit like trying to light a 6in log with a cigarette lighter or fine sawdust in the air.
A bit like trying to light a 6in log with a cigarette lighter or fine sawdust in the air.
FLYBOYJ
"THE GREAT GAZOO"
Part of the problem with "flammable" hydraulic fluid (like a lot of flammable liquids) is the state it is in and the temperature. Trying to light spilled (low pressure/leaked) hydraulic fluid is going to be an exercise in frustration. Igniting high pressure "leaking" hydraulic fluid that is forming a well atomized mist is going to be a lot easier, especially if you have exposed ignition sources around, like glowing chunks of metal from a tank penetration.
A bit like trying to light a 6in log with a cigarette lighter or fine sawdust in the air.
True to a point but remember, a HP system has to be under pressue for what you described to happen, like when the landing gear of a large aircraft is being raised or lowered. An accumulator will always retain some line pressue. More than likely if a fire source is igniting atomized hydraulic fluid you're probably alread dead.
