Held together with cables?

[sv51macross]

My dad and I are at odds here. He stands that he heard about (as he recalls) one of the F-teen fighters designed to be held together internally with calbes to allow give and flex in the airframe. I think this is rather preposterous, but neither of us is sure. Can anyone shed light on this?

[pigsty]

I'd be astonished if it were true. I don't think you can stress an airframe to 9g or more with wires. All modern aircraft are monocoque structures and rely on the strength of the skin panels and the underlying frames to hold them together. You want some flexibility, but not too much - that invites flutter, distortion of the airflow, all sorts of unpleasantness. The materials used have enough flexibility in them to absorb flight loads or the weight of stores without over-stressing them.

I wonder, might there be some confusion with fly-by-wire? (Which, in case anyone's wondering, is operating the control surfaces by electrical signals to the motors, rather than by mechanical links or hydraulic pressure.)

[Eagle21]

The F-15 flight controls are operated by cable, backed up by what is essentially a fly-by-wire system. However, there are no cables holding any of the components together, only nuts, bolts and salants for the composite stuff.

JMC

[Jennings]

Perhaps he's thinking about the Curtiss Jenny? :)

J

[dylan]

definately not held together by cables, especially not for flexibility.....BUT maybe the person who was talking to him was refering to lockwire, as seen here. EVERY fastener has to be "safetied" ie. held in a way where it cannot come loose during flight. the lockwire is twisted in such a way that the bolt is bieng pulled tight.

I wonder, might there be some confusion with fly-by-wire? (Which, in case anyone's wondering, is operating the control surfaces by electrical signals to the motors, rather than by mechanical links or hydraulic pressure.)

most fly by wire sytems in modern aircraft use hydraulic power to operate the control surface. the hydraulic valve is controlled by the computer.

Edited April 15, 2009 by dylan

[Ham]

I know the F-15 is not held together by cables. The flight controls are not operated by cables, either. The F-15 has a dual flight control system. A standard hydro-mechanical system operates the flight controls. In other words, the stick and rudder pedals are connected to push rods that operate hydraulic valves. Hydraulic actuators move the flight controls. There is no direct mechanical link. The F-15 also has a fly by wire system called the Automatic Control Augmentation System (at least it was in the 1980s when I flew them). This system uses computers and servos to move the flight controls.

On a different subject, the F-4D did have a direct link mechanically actuated rudder as a backup. The rudder was actuated primarily by the hydraulic system but the backup was mechanical.

[sv51macross]

Alright guys, thanks for the help clarification.

[Reddog]

No control cables in the F-14 and F-18 either, all hydrualic actuators and connecting rods.

Reddog

[Eagle21]

I know the F-15 is not held together by cables. The flight controls are not operated by cables, either. The F-15 has a dual flight control system. A standard hydro-mechanical system operates the flight controls. In other words, the stick and rudder pedals are connected to push rods that operate hydraulic valves. Hydraulic actuators move the flight controls. There is no direct mechanical link. The F-15 also has a fly by wire system called the Automatic Control Augmentation System (at least it was in the 1980s when I flew them). This system uses computers and servos to move the flight controls.

On a different subject, the F-4D did have a direct link mechanically actuated rudder as a backup. The rudder was actuated primarily by the hydraulic system but the backup was mechanical.

From the flight control mixer assembly (aft of the cockpit on the left side of the airframe, just forward of the speedbrake-I think it is panel 60) down the wingroots to just about where the flaps are for the ailerons, and farther aft into the tail section for the stabs are the flight control cables. Between the stick and the beginning of those cables and then from the cables to the surfaces is a series of flight control rods. The rudder cables run from the rudder breakout assembly under panel 15 to the actuator at the control surface. They are actually more like a metal tape than a braided cable (like a steel tape measure). From the rudder pedal to the breakout is a series of control rods. There is a direct mechanical link between the stick in the cockpit and the actuator at the control surface, and between the rudder pedals and the actuators.

The CAS system uses LVDT's on the rudder and stabilator to move those flight controls if the mechanical linkages are broken or get bound up, you get about 50-75 percent of the normal movement of the surface.

There is more involved in the whole flight control system, I was just trying to simplify to explain that there were cables in the system

JMC

[jbrundt]

My dad and I are at odds here. He stands that he heard about (as he recalls) one of the F-teen fighters designed to be held together internally with calbes to allow give and flex in the airframe. I think this is rather preposterous, but neither of us is sure. Can anyone shed light on this?

Patently untrue.

I know because I work where they are built and have worked on them for the past 20 years.

Rivets and other fasteners hold the structure together. You would be surprised how much flex there is in any given airframe. hell, a B-52 or 747 wings flex up and down almost 50' from takeoff roll to flight condition.

Jeff

[LanceB]

No control cables in the F-14 and F-18 either, all hydraulic actuators and connecting rods.

Sorry, but I very strongly doubt that. Boeing lists Loos & Co. Inc.'s Wire Rope Division as a supplier for "wire rope flight control cable assemblies" on the Superhornet, and as I can recall seeing rigging diagrams for the Legacy Hornets when I was in the Corps back in the 80s I am 99.9% positive there were cables connecting the pedals and stick in the cockpit to the hydraulic actuators at some point in the chain. I can also find references to F-14s having cables. Unless an airplane is 100% "fly by wire", there is a mechanical connection between the pilot's stick and pedals and the hydraulic actuators that move the control surfaces, and that connection is very likely going to involve cables. Rods are great if you have a perfectly straight line and room to spare, but they don't handle curves and they are bulkier than cables, both of which can be negatives when you are dealing with airframes.

Remember, hydraulic actuators and any actuating rods between them and the flight control surface they are hitched to do not <i>replace</i> flight control cables, the actuators are there to boost the pilot's inputs. No man can move ailerons, rudders and elevators of a supersonic or high-subsonic aircraft, even a small one. The control forces are too great. The pilot moves his stick, which pulls on cables or pushes on rods which then move the switch on the hydraulic actuator so as to use hydraulic pressure to move the control surface. In fly-by-wire those cables and rods are replaced by wires which transmit stick and pedal movements to a computer which interprets them against what it is trying to do to keep the plane flying in a straight line and then sends signals down wires to a switch on the hydraulic actuator which in turn moves the control surface.

[GreyGhost]

Just look at all the flex in the wings of Blue Jet #5 in >>> This <<< picture ...

Gregg

[Reddog]

Sorry, but I very strongly doubt that. Boeing lists Loos & Co. Inc.'s Wire Rope Division as a supplier for "wire rope flight control cable assemblies" on the Superhornet, and as I can recall seeing rigging diagrams for the Legacy Hornets when I was in the Corps back in the 80s I am 99.9% positive there were cables connecting the pedals and stick in the cockpit to the hydraulic actuators at some point in the chain. I can also find references to F-14s having cables. Unless an airplane is 100% "fly by wire", there is a mechanical connection between the pilot's stick and pedals and the hydraulic actuators that move the control surfaces, and that connection is very likely going to involve cables. Rods are great if you have a perfectly straight line and room to spare, but they don't handle curves and they are bulkier than cables, both of which can be negatives when you are dealing with airframes.

Remember, hydraulic actuators and any actuating rods between them and the flight control surface they are hitched to do not <i>replace</i> flight control cables, the actuators are there to boost the pilot's inputs. No man can move ailerons, rudders and elevators of a supersonic or high-subsonic aircraft, even a small one. The control forces are too great. The pilot moves his stick, which pulls on cables or pushes on rods which then move the switch on the hydraulic actuator so as to use hydraulic pressure to move the control surface. In fly-by-wire those cables and rods are replaced by wires which transmit stick and pedal movements to a computer which interprets them against what it is trying to do to keep the plane flying in a straight line and then sends signals down wires to a switch on the hydraulic actuator which in turn moves the control surface.

Lance,

Having worked on the flight control system of the F-14 I can tell you that there are no cables connecting them. I was responsible for QA'ing them and have seen every part of the flight control system.

Some times, large wire bundles are called cables and that maybe is what is being reference.

The only controlling cable there is on the F-14 is the main landing gear door timing cable located at the top of the main landing gear wheel wells, it's a thin cable frunning from the front to aft end of the roof on the inboard side.

The F-14 used connecting rods and hdrualic actuators and there were NO cables used the flight control system.

As for the F-18, there may have been one or two small cables but I never saw them all the times we had the birds opened up.

V/R

Reddog :D

Edited May 8, 2009 by Reddog

[LanceB]

OK, Reddog, I stand corrected. Obviously I never worked on the F-14, but our Harriers sure had a bunch of flight control cables (had to check them on dailies), and when I went on to I-level we had to deal with Hornets and from almost 20-years-past memories there were cables listed in the rigging diagrams when we were checking out the pubs on different things. Don't recall exactly where, we didn't do line checks and our only work with cables would have been to make them and hand them off the the squadron for installation, but we never got a MAF to do that. I do recall seeing our NDI guys checking Hornet control rods from time to time, though.

[ALF18]

OK, Reddog, I stand corrected. Obviously I never worked on the F-14, but our Harriers sure had a bunch of flight control cables (had to check them on dailies), and when I went on to I-level we had to deal with Hornets and from almost 20-years-past memories there were cables listed in the rigging diagrams when we were checking out the pubs on different things. Don't recall exactly where, we didn't do line checks and our only work with cables would have been to make them and hand them off the the squadron for installation, but we never got a MAF to do that. I do recall seeing our NDI guys checking Hornet control rods from time to time, though.

For the F-18A/B/C/D, here is what may have caused this confusion:

Flight control (all surfaces) - pure fly by wire. Two FCCs (Flight Control Computers), with 2 channels each, giving 4 total channels of fly by wire control to actuators. Some surfaces have two channels (leading edge flaps, ailerons), and some have four channels (trailing edge flaps, horizontal stabilators).

There is one exception, though, and that is the mechanical backup mode. If there is a total failure of the two types of fly by wire to the horizontal stabilators, those stabilators can operate in MECH (mechanical) mode. There are actual control cables hooking up the control stick (only, not the rudder pedals) to the horizontal stabilators. This mode operates with no electrical power, strictly cables and hydraulic power to the actuators.

The horizontal stabilators are the only control surfaces in the Hornet that have mechanical cables that back up the fly by wire. It is possible to have only the horizontal stabs in MECH, but the rest of the controls still working in some kind of fly by wire mode, or to have only the stabs in MECH mode (with no other surface functional at all).

MECH is intended as an emergency mode to get a heavily battle-damaged Hornet back into friendly territory (or close to the carrier for the Naval ones), so that the pilot can eject and be recovered by friendlies. It is possible to control the aircraft with only the stabilators operating differentially, but without the other control surfaces working in fly by wire mode, it is difficult to control.

I've flown MECH mode in simulators - it is very sporting to try and land on a 10,000 foot runway without crashing. Add a challenging crosswind or gusty winds, and it becomes almost a death act to try this. Some of my friends have experienced MECH mode reversions in flight, and it was quite a roller coaster ride!

When starting up a Hornet, MECH mode is checked. With the first engine turning and providing hydraulic pressure (engine turning with air, not yet lit off and started up), the pilot moves the control stick and checks horizontal stab differential and symmettrical movement, then he advances the throttle to idle and starts the first engine.

The F-16 has no mechanical backup at all, so no cables from control stick/rudders to any control actuators.

ALF