Explain hydraulics

[TSK06]

This is kind of related to RCGroups and kind of not - mostly not I would guess.

Anyway, what I want to know is how hydraulics work. I read and watch (simple?) programs about airplane crashes (like air emergency, some seconds from disaster and many others) but I never exactly got what hydraulics are. I understand that the hydraulic system on an airplane allows control surfaces to be moved and that if the hydraulic fluid (which is what exactly?) leaks then it becomes impossible to move the control surfaces. I do not understand how the hydraulic system works, and how the fluid helps move things (makes 'em slippery, I mean I really don't know :confused: ). I guess what I need is an explanation of this: lets say the pilot wants to turn a jet (like a 747) to the right by using the rudder(I think that is what would be used, right?), once he moves the appropriate thing (which is what?) in the cockpit, how does that get transfered to the rudder? That should help me understand what hydraulics are and how they work, though I have seen that this same way of moving parts is used on heavy equipment.

Sorry for the rantish explanation, couldn't find the words to express myself better.

[MarkusN]

Think of hydraulics as motors that can develop tremendous force in a small space.

You have a pump that you can stow away anywhere where there is space. This pressurizes the hydraulic fluid (which is kind of an oil that is designed so it can transmit high pressure without being compressed). This pressurized oil moves through thin tubes to the points where it is needed at the control surfaces.
At the control surfaces you have so called actuators; cylinders that convert the pressure in the oil into logitudinal movements, which in turn operate the control surfaces. Since space is limited and forces required are high, the oil must be under very high pressure.
Which actuator moves where is determined by valves that steer the oil either on the front or back side of the pistons in the actuators. Those valves are controlled by the controls operated by the pilot (control column or stick, and rudder pedals.)


It is not so much different from electric drives where you have power source (the generator), lines transporting the electric current, and motors converting electric current to force.
The pump takes the role of the generator, the hydraulic fluid corresponds to current, the pressure to voltage, tubes to wires, and actuators to motors.

Thus hydraulics are a means of transporting and converting energy/power. With the advantage that in high pressure systems you are able to do this in very small space.

Concerning your question on leakage: When the hydraulic system is damaged, it loses pressure. Nothing remains to move the control surface. Kind of like a short in an electric mains system.

And, yes, hydraulics are also used in heavy machinery, such as hoes and bulldozers. You pobably have seen the shiny pushrods of the cylinders actuating movements on these. Think of actuators in airplanes as such cylinders, only much smaller. Here the advantage is that you can drive a complex and powerful system that can perform many movements with one big diesel engine driving the hydraulic pump.

[JetPlaneFlyer]

http://en.wikipedia.org/wiki/Hydraulic

:)

[TSK06]

Thanks for that great explanation. Can you point me to a link that explains in semi-detail the actuator works, the kind used on jet airplanes?

So as I understand it, the actuators "convert" (not literally) the fluid into movement?

[JetPlaneFlyer]

So as I understand it, the actuators "convert" (not literally) the fluid into movement?

They just 'convert' movement of a fluid into movement of a piston... The fluid under pressure flows into a cylinder and forces the piston outward, simple at that.

[biber]

It's quite similar to a pushrod, only that the rod is fluid 'sliding' in tubes.
Push it on one end and its other end will push, let's say, a control surface.
Though the Fluid definately won't work without the tube, while a rod can do without,
the fluid in a tube has the great advantage of maximum bendiness.
You can route it around any number of corners and radii and it will still work perfectly,
wich obviously isn't quite as easy to do with rods...

biber

[ghoti]

In fluid mechanics there are two types of fluid: compressable and incompressable. The latter are oil, liquid water and are used to move actuaters. The compressable ones, air, steam, etc. can store a lot of energy under pressure and make good bombs when the container ruptures. Incompressable fluids don't store energy like this. Thus all conpressed gas tanks (such as SCUBA bottles) must be tested periodically for safety (they do rust, etc.) and this is done with water or some other incompressable fluid. A tank rupture during such pressure testing is then harmless. Does this help you understand? Bill

[MarkusN]

This (http://www.pirate4x4.com/tech/billavista/PR-Hydro_Steering/index2.html) expains the principle of hydraulic control in steering of an all-terrain vehicle. It has some schematics making things clearer. Cylinder/Piston are no
t expained in detail though, but the schematic drawing is kind of self-explanatory.

Biber: Flexible pushrod! Great analogy!

[BMatthews]

Have you ever seen motorcycle brakes on the newer bikes? They don't have cables. Instead they use oil in the lines to conduct the force from the pump up where the brake lever is down to the caliper where the pistons inside the caliper respond to the pumping pressure caused by the hand in order to clamp the pads to the rotor.

As mentioned above it's like wire in tube pushrods but in this case the pushrod inside the tube is a liquid. The term "hydrualics" is just a label used to describe a liquid being used to conduct pressure to a different location. Much like electricity uses wires to move "pressure" (voltage) from a compressor or pressure source (battery) to a device that can convert that pressure to work (like a motor or solenoid or relay or light).

[downunder]

I would have thought the simplest example would have been a car's brakes. The pedal pushes a piston that forces oil through the lines to the brake piston in the caliper. Get a leak and the oil just squirts out before it can apply pressure to the brakes and you crash.

[TSK06]

Thanks for all your replies, I understand how it works now.