DC-8-71F Flight Control System

[durbin]

I am currently writing a report on the DC-8 flight control system for university. I have searched google for the answers and numerous books but have come up short on the following questions:
1. I know that the ailerons are power operated, but how is the actuator connected to the cockpit controls, cables, rods, electric?
2. I know that the rudder is also power operated but once again how is it connected to the cockpit controls?
3. How are the flaps connected to cockpit controls?
4. Is everything that is power operated connected to only a single hydraullic system or several?
Any other additional information that may be pertinent is also welcomed. In advance thank you for any replies.
Matt Durbin

[ferrydude]

The DC in DC-8 stand for Direct Cable. The actuators are connected by cable to the cockpit controls.

There is one main hyd system divided into two manifolds, left and right.
Spoilers, Standby Rudder and the main cargo door each have their own independent hyd acutation systems.

[sniper9652]

The DC in DC-8 stand for Direct Cable. The actuators are connected by cable to the cockpit controls.

There is one main hyd system divided into two manifolds, left and right.
Spoilers, Standby Rudder and the main cargo door each have their own independent hyd acutation systems.

And here was I thinking DC stood for 'Douglas Commercial' :E O'h well!

[homebuilt]

And here was I thinking DC stood for 'Douglas Commercial' :E O'h well!

Yes, don't be affraid! Must have been a joke from the guy who answered! DC stands really for Douglas Commercial.;)

About the DC8 flight controls, as a former DC8 pilot I'm going to try to gather some of my reminds: the DC8 has only one hydraulic system, which is parted in two halves, that become automatically isolated from each other in case of a pressure drop. The half that remains powered energizes some important stuff like braking system, flap and gear extension, etc...

OK, but what about the flight controls which is the subject of your topic?

Well, the DC8's flight control have a dual way to operate:
-primary way, with hydraulic actuators like almost every airliner.
-and back-up: in case of an hydraulic pressure drop as I told higher the hydraulic system parts in two halves, the hydraulic actuators of the flight controls are matching the part of the hydraulic system that is no more powered and thus what happens? Loss of control? No, no, no!

The flight controls on this airplane are linked to cables and rods coming from the cockpit via a reversion system that automatically switches from normal to back-up (and the opposite in case of pressure increase):

-Primary way: direct link from cables and rods to hydraulic actuators,

-Back-up: the reversion system switches the cables and rods to servo-tabs that aerodynamically power the flight controls. Each flight control (each aileron, each elevator and the rudder) owns one of these servo-tabs and handling the airplane with them is not a matter. I remember of a loss of hydraulic pressure that occured 2 hours before arrival, at FL390...We carried out the procedure, disengaging the autopilot, shutting down the remains of hydraulics (the goal beeing to keep a little fluid to try flap extension at arrival - If you can't it will imply a 0 flap landing with a Vref increased by 70 kts - and normal gear extension + normal breaking and nose gear steering). I was then at the controls of what would have been a turbofan powered DC3!:ok:
(Well, it was one of my smoothest landing at the controls of this wonderful airplane!).

Hope this helps, sorry for maybe having had a little "thick" explaination!:O

Dom

[ferrydude]

Quite surprised that anyone ever having crewed any Douglas product would not be familiar with the "direct cable" joke. Heard that one in me first ground school on the 8. Cables are quite a Douglas design hallmark. Oh, Diesel-8 and Douglasaurus are jokes too!:rolleyes:

[homebuilt]

Quite surprised that anyone ever having crewed any Douglas product would not be familiar with the "direct cable" joke. Heard that one in me first ground school on the 8. Cables are quite a Douglas design hallmark. Oh, Diesel-8 and Douglasaurus are jokes too!:rolleyes:

Sorry Ferrydude! Indeed I didn't know this, although I've been logging 1700 hours on the DC8 between 1995 and 2000!;) BTW it's true I'm on the RH side of the Pond, so that should be an explaination of my ignorance!:}

But whatever these jokes, let's be sure a guy asking for technical data about a system he's not aware of (and here about the wonderful DC8) should understand what we're all saying!:ok:

All the best,

Dominique, stinkin' cheese eater from France, the country that gives lessons to all the Universe and its surrounding....:suspect:

[FR8TDOGii]

Hope this clarifies things.

AILERONS:
The DC8 has two (inboard, outboard) ailerons on each wing. The outboard ailerons are connected to the inboard aileron by a torque (slip clutch) linkage, and at low airspeeds and airloads, follow inboard aileron movement. To prevent over-controlling at higher airspeeds, airloads "freeze" the outboard aileron in the faired position, and all lateral control is accomplished with inboard aileron movement only.

The inboard ailerons have aerodynamic boost (control) tabs, which are directly connected to the control wheel through a manual reversion mechanism. When hydraulic pressure is available to the aileron, this tab is locked mechanically, and hydraulic pressure is routed through control valves to move the entire aileron. If hydraulic power is lost, the spring-loaded reversion mechanism allows the tab to operate by direct mechanical linkage from the pilot’s control wheel. When the wheel is moved, the tab is deflected and the aileron is "flown" to a new position to achieve the desired roll rate.

RUDDER:
The DC8 rudder is normally powered hydraulically by the left aircraft hydraulic manifold. A standby rudder pump is provided as an alternate hydraulic source. When powered hydraulically, the entire rudder is moved in response to pilot inputs. Should hydraulic power be unavailable, a rudder boost (control) tab is provided to "fly" the rudder to the required position.

The manual reversion mechanism determines whether the rudder pedal input will be mechanically linked to the hydraulic actuator, or the aerodynamic boost tab . Its operation is identical to that of the aileron. If hydraulic power is available, it is held in a POWER ON mode.

A rudder load limiter restricts the rudder to one third hydraulic pressure and travel to prevent over controlling in high speed (flaps up) flight.

ELEVATOR:
The elevator has no hydraulic input. It is strictly mechanical.
Two types of aerodynamic boost tabs are provided. The elevator control tabs, moved by the pilots, and the geared tabs, moved by the stabilizer when it is trimmed.

The elevator and control tabs are directly connected to the pilot's yoke. Initial movement of the yoke will reposition the aero_dynamic control tabs. When tab limits are reached, further yoke movement will result in limited movement of the entire elevator.

The geared tabs are designed to keep the elevator faired with the stabilizer to allow full control authority of the elevator as the stabilizer leading edge moves throughout the stabilizer trim setting range.

[WingOffLight]

I have the AOM and study guide if you need some fancy pictures :ok: I already have some as PDF files

Great topic to choose btw. Im partial to the ol'bird myself.