Hydraulic Systems and Hydromechanics
Pascal and the basics
Hydraulic systems transmit force by pressure acting on area: F = p × A. Because liquids are effectively incompressible, pressure is uniform throughout a connected static system, so a small piston moving a long distance can move a large piston a short distance with the same work. Volume flow rate Q = A × v gives actuator speed; pressure gives force. A common exam trap is mixing these: increasing pump pressure does not increase actuator speed unless flow increases.
System architecture
Typical transport aircraft use two or three independent systems (often designated green, blue, yellow or 1, 2, 3) so that loss of one does not remove all actuation. Power comes from engine-driven pumps, with electric pumps and a ram air turbine (RAT) or power transfer unit as backup. Reservoirs are pressurised to prevent cavitation at pump inlet and to feed the pump at altitude; a pressurised reservoir also provides a positive head. Accumulators store energy for peak demands and emergency operation, damp pressure pulses, and absorb thermal expansion.
Valves and control
Check valves allow one-way flow; relief valves protect against overpressure; priority valves shed non-essential consumers (e.g. landing gear) when pressure falls so that essential services (brakes, flight controls) keep pressure. Selector valves direct flow to actuators; servo valves meter flow in proportion to electrical or mechanical input.
Fluid and contamination
Skydrol-type phosphate ester fluid is fire resistant but attacks many paints and seals; mineral fluid is used on smaller aircraft and the two must never be mixed. Contamination by water or particles causes erosion and jamming; filters and sampling are the defence.
Worked example
A 2 cm² piston under 200 bar carries 200 × 10⁵ Pa × 2 × 10⁻⁴ m² = 4000 N. Double the area at the same pressure and force doubles while speed halves for the same flow.
Traps
Assuming pressure drops along a pipe in a static system (it does not; it drops only when flow is present), and forgetting that accumulator precharge is nitrogen, not hydraulic fluid.