Mechanical Back-up for Pilot Control

[wolram]

All very good, but would it not be an idea to have a mechanical back up,
that if all else fails the pilot had some control, the mechanical system could control servo tabs that drive the main control surfaces, surly not very expensive or heavy.

 

[chroot]

The control surfaces on a 777 are large and are subjected to enormous aerodynamic forces -- a pilot would lack the physical, muscular strength to move them anyway. Thus, you'd need, at a minimum, some kind of hydraulic system, which is really no more reliable than fly-by-wire -- and perhaps less so.

- Warren

 

[Cyrus]

The control surfaces on a 777 are large and are subjected to enormous aerodynamic forces -- a pilot would lack the physical, muscular strength to move them anyway. Thus, you'd need, at a minimum, some kind of hydraulic system, which is really no more reliable than fly-by-wire -- and perhaps less so.

- Warren

I wonder if the 707 was fly by wire or not (actually according to wiki it used Servo tab assist). You can always gain mechanical advange using a system of pulleys and counterweights on the control surfaces.

 

[chroot]

I wonder if the 707 was fly by wire or not (actually according to wiki it used Servo tab assist). You can always gain mechanical advange using a system of pulleys and counterweights on the control surfaces.

Except a system of pulleys and counterweights would have significant weight and not offer any real reliability advantage over any other system.

- Warren

 

[Cyrus]

Except a system of pulleys and counterweights would have significant weight and not offer any real reliability advantage over any other system.

- Warren

In addition, if you notice now most airliners tend to use 'mixing' where the ailerons act as flaperons giving you a larger flap along the entire wing span.

I agree that fly by wire is much more robust; however, I do not immediately see any physical limitations to using a conventional system in theory, even if the airplane is large.

 

[wolram]

In addition, if you notice now most airliners tend to use 'mixing' where the ailerons act as flaperons giving you a larger flap along the entire wing span.

I agree that fly by wire is much more robust; however, I do not immediately see any physical limitations to using a conventional system in theory, even if the airplane is large.

Cyrus, did you and Chroot miss the idea of using servo tabs to force the main controlling surface one way or the other? may be this would not give 100% control due to extreme forces, but would it not at least make the aircraft controllable enough to manoover?

 

[chroot]

At 800 knots, when pushing an enormous control surface around in a second or less, I doubt that servo tabs would really still be controllable with human muscles. They'd have to be extremely small, which would mean it would take them a long time to move the control surfaces.

- Warren

 

[Cyrus]

I am no expert, perhaps Rainman can chime in here, but that sounds very very reasonable Chroot.

 

[LURCH]

All passenger planes have redundant systems. I just think that the fly-by-wire system is by far the most reliable means of controlling the aircraft. So, if you're really trying to build-in safety, the best thing to do is build the aircraft with two fly-by-wire controll systems.

If one system fails, you switch to the other and it is very likely to work. More likely than a system of pullies or even hydraulics.

 

[FredGarvin]

In military aircraft, the F-16 has two separate flight control systems and the F-15 has 3.

For civialian, the 777 has a triple redundant system in all components of the flight control system (hydraulics and computers).

 

[Danger]

I would not feel the least bit comfortable flying an electronic control system without some sort of back-up. If that back-up is another electronic system with an independent power supply, I'd be okay with it. The odds are that if I encounter an EMP strong enough to blow both systems, neither I nor the plane will be in any condition to remain airborne anyhow.

 

[LURCH]

Since millitary airfcraft have now been mentioned, I should point out that, unlike civillian aircraft, a backup flight-controll system in a modern fighter jet has to be a fly-by-wire system. AFAIK, all modern jet fighters are built aerodynamically unstable. Trying to fly them without a computer's assistance would be like throwing a dart, feathers-first; no way a human could controll it. Loosing the fly-by-wire system in an F-16 (or later model) and swithing to direct controls would be suicide.

 

[Danger]

But not as suicidal as losing it without a back-up.

 

[russ_watters]

Regardless, the point is a good one: when you start to lose your fly-by-wire in a modern fighter, you reach for the ejection handle immediately. Some of those planes are so unstable, you have only fractions of a second before the g-forces make ejection impossible (then rip apart the plane).

The F-16 is unstable in both roll and pitch - the F-177 in roll, pitch, and yaw. In such a plane, direct human control would be pointless.

 

[NoTime]

Until the last few years all commercial aircraft had manual control systems with power assist.
Even a small single engine plane uses servo tab assist on some control surfaces.

Experience with military craft had a lot to do with the changeover to fly by wire.
Instabilities apply to all aircraft and as planes became larger human reaction times became a problem.

 

[LURCH]

Until the last few years all
Experience with military craft had a lot to do with the changeover to fly by wire.
Instabilities apply to all aircraft and as planes became larger human reaction times became a problem.

Could you expound on this a bit? Are you saying that all aircraft are unstable? And what role does the size of the aircraft play in the reaction time needed?

 

[NoTime]

Having a pilots license, I know, with unqualified certancy, that its easy to cause an aircraft to behave like a rock.

For any mass, once it starts moving the wrong way, it takes time and energy to stop it. By the time a person notices the movement there may not be enough energy and time left to prevent it from moving into the fly like a rock region. Bigger objects take more time and energy.
You can only apply energy so fast without physically breaking something.