Hydraulic Systems Flashcards
What is a hydraulic motor?
A hydraulic motor is a relatively small unit - compared to electric motors - that convert hydraulic power back into mechanical power.
They are normally rotary impeller units - basically “pumps run backward - which convert hydraulic pressure back into rotary output to turn shafts operating things like flaps or landing gear.
What is the basic principle that explains how hydraulic systems work?
Hydraulic systems are based on the principle that fluids are flexible but not compressible.
Assume you have a long tube full of fluid. If force is applied to the fluid at one end of the tube, that force is transmitted, with virtually no loss, to the other end.
By controlling the sizes of the master and slave brake cylinders and pistons, you can establish proportional mechanical advantage.
That’s why you can stop an airplane weighing thousands of pounds using just the pressure applied with your feet.
What is a hydraulic pump?
A hydraulic pump pressurizes fluid to power a hydraulic system.
They are generally rotary pumps and are geared directly off the aircraft engines or driven electrically.
Hydraulic pumps convert the rotary motion of the power source such as the engine to hydraulic pressure and flow. That pressure and flow is then delivered to mechanical systems around the aircraft via hydraulic lines. Pumps may be designed for continuous use or for periodic operation.
While brake systems on smaller turbine aircraft may be manually powered, other aircraft hydraulic systems are commonly pressurized by hydraulic pumps to around 3000 psi (pounds per square inch).
What is a hydraulic valve?
Valves direct the flow of hydraulic fluid to where it’s needed.
For example, in the case of hydraulic landing gear, valves can direct hydraulic flow/pressure to one side of a hydraulic landing gear motor for retraction or to the other side to reverse the motor’s direction for extension.
Remember that the valve needs to get the message somehow from the cockpit gear handle in order to properly direct the power. This actuation requires power to change the position of the valve.
A check valve is a type of valve that only allows flow in one direction such as from the hydraulic pump to the rest of the hydraulic system.
What is a hydraulic fuse?
A hydraulic fuse is a safety component designed to prevent a catastrophic loss of hydraulic pressure. The term “fuse” is an analogy to the more familiar electrical fuses that serve similar functions in electrical circuits.
Hydraulic fuses are installed at strategic locations throughout an aircraft hydraulic system and are designed to detect leak-producing failures such as a failed fitting or hydraulic line break. If a leak develops in the hydraulic lines or subsystems, a hydraulic fuse prevents excessive fluid loss while still permitting operation of remaining hydraulic system components.
Typical hydraulic-fuse designs incorporate a spring-controlled mechanism that normally allows a certain volume of fluid flow per minute to pass through the fuse. If the fluid passing through the fuse becomes excessive, as in a rapid flow toward a hydraulic-line break, the fuse closes to shut off the flow, thus isolating the area with the damaged line.
What does a hydraulic reservoir do?
Hydraulic reservoirs store hydraulic fluid at low pressure. They keep enough hydraulic fluid to operate the aircraft systems as well as some extra in case there’s a leak.
What does a hydraulic accumulator do?
Hydraulic accumulators store hydraulic pressure in order to provide backup for key operations in case there’s a pump failure.
They normally consist of a sealed pressure container with a diaphragm or piston.
They compress nitrogen or “dry air” (and sometimes a spring) to store energy for a short application of power in the event of a pump failure. Accumulators may also be used to control power fluctuations or surges in the system, much like a battery in an electrical system.
What do backup hydraulic systems do?
Backup hydraulic systems provide pressure to operate important systems, such as flight controls or landing gear, when the primary source of hydraulic pressure has failed.
Hydraulic accumulators store pressure as backup power sources. However, they store only enough pressure for limited or one time use.
Smaller turbine-powered aircraft usually come equipped with a hydraulic hand pump, which is used as a backup method to provide hydraulic pressure to extend the landing gear when the primary pump fails.
Most hydraulic hand pumps are either single- acting pumps (fluid moves during power stroke but not during return stroke) or double-acting pumps (fluid moves during both strokes).
Many hand pumps require more than
100 strokes to extend the landing gear, which means emergency extension takes a lot of physical work and a good deal of time. So don’t attempt manual gear extension on final approach - climb to level flight at safe altitude first.
In larger turbine aircraft that use engine-driven pumps as their primary source of hydraulic pressure, electrically driven hydraulic standby pumps are used to supplement the primary pumps and serve as emergency backup.
What is an air turbine motor?
An air turbine motor is an air turbine hydraulic pump. They use compressed air from the pneumatic system that comes from an engine, auxiliary power unit / APU, or ground air source to spin an air turbine geared to a hydraulic pump.
What is a ram air turbine / RAT?
Ram air turbines / RAT produce emergency in-flight hydraulic pressure. They work by extending an air turbine into the outside air.
The turbine is spun by ram air pressure, which drives a hydraulic pump and provides emergency hydraulic pressure.
RATs are normally found on aircraft that have hydraulically actuated flight controls and no mechanical linkage between cockpit controls and flight control surfaces.
In case of a hydraulic system failure, the RAT supplies necessary hydraulic pressure to actuate the flight controls. In order to produce the required hydraulic pressure to do this, the aircraft must be flown at a high enough airspeed to generate enough air pressure to turn the air turbine.
What are the advantages of using hydraulic power on an aircraft?
Hydraulic systems are really powerful and relatively lightweight.
They can be drawn from engine power instead of taxing the electrical system. (Electric motors draw tremendous current when used for heavy-duty, intermittent operations. That’s why all the cockpit lights dim when you use electrically powered landing gear in a small aircraft.)
Hydraulic power is more reliable and less maintenance intensive for heavy-duty applications than traditional electromechanical drivetrains.
Since fewer and smaller electric motors are needed, aircraft electrical systems can be lighter and last longer too.
Finally, an engine-driven hydraulic system provides another power source for redundancy on critical systems.
