Hydraulic systems

[Panther91]

1. Does a Cessna 172 have a hydraulic system and for what it uses it?
2. Does it use it for moving ailerons, elevators and rudder, or not?
3. Which aircraft do have hydraulic systems for moving the control surfaces? Maybe fly by wire aircraft?
4. If Cessna 172 don't use a hydraulic system for moving the control surfaces how does the autopilot move the control surfaces?

[DaveSims]

1.  The only "hydraulic system" a 172 has is its brakes.  Most light airplanes don't have any more than that.

2.  Most light planes uses a system of cables and pulleys to move control surfaces.

3.  You have to get into large aircraft to find true hydraulic control systems.  Fly by wire systems sometimes just use servos or control the hydraulics by computer.

4.  The autopilot uses an electronic to servo to control the cables and the control surfaces.

[beaky]

[quote]
3. Which aircraft do have hydraulic systems for moving the control surfaces? Maybe fly by wire aircraft?

That would be a very long list...  :D But no, it doesn't have to be "fly by wire"... "fly by wire" aircraft, in fact, often use electric motors rather than hydraulic actuators to move surfaces... saves weight and problems with fluid leaks, etc.

As for the 172, it's all done with cables and pulleys. Many similar aircraft use pushrods, but it's the same concept: direct mechanical linkage between the controls and the surfaces.

An autopilot in such an aircraft uses little electric motors which are linked to the rods or cables. Elevator control, though, is usually done by a motor turning the trim wheel, as far as I know... and if the plane in question has rudder and/or aileron trim, the same rule applies... if those trim tabs are already moved by small electric motors, it's much simpler to rig the autopilot so it simply overrides than manual switch or pot, sending its own commands to the motors.