Hydraulic energy storage
By Chris Grosenick
right) Accumulators provide backup power
for brakes, landing gear, emergency applications,
and APU starting. The average pneumatic charge
in an accumulator is 1,000- to 2,000-psi pressure.
While accumulators are closely identified with hydraulic systems, they find applications in other aircraft systems too. They come in a number of capacity sizes, and depending on the system application, are either charged with a gas or use mechanical force to store energy in the form of pressurized fluid. Pneumatically pressurized accumulators are used primarily in hydraulic systems, and mechanical accumulators are used in various applications like fuel for APU starting and grease for stab trim lubrication systems. Accumulator construction varies and has evolved over the years, with the cylindrical shape being the most predominant.
Principles of operation
Accumulators are simple devices that are constructed of a piston, a cylindrical sleeve, and two end caps. The piston is free to move through the entire length of the cylinder sleeve, similar to a rod-less piston in a hydraulic actuator. Pressure from the aircraft hydraulic system enters the fluid side and forces the piston toward the pneumatic end of the cylinder. As the piston is forced away from the fluid end, it compresses the trapped gas on the pneumatic side. When the pressures equalize, the piston stops moving and the accumulator is now storing a predetermined amount of pressurized fluid. A check valve from the pressure supply, and selector/shut-off valves keep the pressurized fluid trapped until it is needed to perform work. The main physical principles at work here are the theoretical incompressibility of one fluid (hydraulic oil) and the highly compressible nature of another fluid (nitrogen or air).
Most hydraulic accumulators are cylindrical with a pneumatic and fluid side separated by an internal free-floating piston. Depending on the available real estate inside an aircraft, the pneumatic side may use a tubing run to locate the pressure gauge and servicing valve. Older accumulator styles were spherical with bladder type diaphragms to separate the pneumatics from the hydraulics. Some accumulators are constructed using a spring instead of pneumatic pressure to provide the force to move the piston. This type is used mainly in low pressure applications such as APU fuel systems, grease dispensing mechanisms, and pump suction-side hydraulics.
Another type of accumulator is the self-displacing variety. This accumulator has three chambers with two piston heads attached together by a common rod. This type of accumulator is used in hydraulic systems where reservoir volume is small or speed of operation is important. Fighter planes and helicopters have this configuration due to tight spaces and small hydraulic system volumes. The latest technology in accumulators is the helium charged bellows type, which is being used in advanced generation fighters. This accumulator is maintenance-free, and requires replacement if the pneumatic charge leaks out, or becomes otherwise unserviceable. Accumulator capacities range from 500 cubic inches (C-5, self-displacing) to 50 cubic inches (many aircraft applications), and hydraulic system design determines what capacity is required.
Accumulators provide backup power for brakes, landing gear, emergency applications, and APU starting. They are also used as system dampers, absorbing pressure spikes in hydraulic systems with large volumetric output piston pumps. In the dampening role, the accumulator is plumbed into the pressure tubing downstream from the pump(s), and the capacity for this function is usually 100 cubic inches. Accumulators in the 10- to 25-cubic-inch capacity range are used as local dampers, and depending on system requirements, subsystems like flight controls or landing gear may need protection from pressure spikes or plumbing design-induced flow characteristics. Large aircraft have at least one brake accumulator, and some have up to four.
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