Stability Wars: Why Did Relaxed Lose?
 

Speed stabilty (Boeing); vector stability (Airbus); relaxed stability (MD).
If relaxed is the most natural form of stability, why is it that Airbus chose vector stability, Boeing speed stability and not MD's relaxed stability which never became a trend let alone norm.

Relaxed stabilized Makes most Pilots really unrelaxed
 

Dont compare fly by wire with conventional aircrafts. The beloved and hated MD11 has so far MAC that with 32% it's almost indifferent. With longer fuselage and tailstrike tendency towards DC10 it needs LSAS to help the pilot. Automatic nose lowering, pitch rate dampening etc were necessary to keep it within requirements. Still it has a crappy accident rate and everybody who flies it knows it can bite if you don't pay attention. Relaxed stability is nothing else than indifferent stability due rear CG to meet fuel economy promised to early customers

Warmkiter, its not "Sheeps", its "Sheep", likewise its "Aircraft", not "Aircrafts!"Sorry, but if I had a nickle for every CV which came to my desk with this on it I would be a rich man, none of them were hired by the way!

"People in glass houses...."

Winnerhofer, you attempt to compare differing (incomparable) aspects and seek judgement of an ideal solution.
Relaxed static stability in achievable in conventional aircraft; incorrect loading, aft cg will create a sensitive aircraft. In most situations this would be flyable but increasingly require above average skills and moderation of manoeuvres up to a point of loss of control
Certification requirements for cg and handling characteristics provide an adequate safety margin; but where modern aircraft are designed to be operate with adverse (relaxed) stability for economic reasons, then the safety margin for flight handling has to be provided by other means.
This is achieved with computed flight control laws and systems (ideally FBW) negating the need for enhanced skill or limiting manoeuvres.

The choice of ‘speed’, ‘vector’, or ‘attitude’ as the primary parameter in the flight control computation depends on many parameters; aircraft type, role, degree of relaxation, etc. There may be no one best solution.

The use of FBW (computational control) enables other protections to be replaced – stick force per ‘g’ for structural protection, speed (alpha) limit for low speed, similar for high speed, the need to trim, and perhaps the need for ‘feel’.
It could be questioned if the implementations of these other aspects are ideal, particularly if the skills in their use and mechanisation are not fully understood.
How might we train in a Cessna 172 with ‘vector’ control and no trim?
Would such an aircraft be comparable to a modern airliner?

gums will be here shortly, but "relaxed" stability implies lots of maneuverability and pitch authority which isn't needed in transport category planes. AF 447 might have hit tail first with it.

Clunckdriver wrote

Hahaha, Clunckdriver, tell us in which company is your desk, so that I make certain that I will never ever apply.
:yuk:

Safetypee: Why not?

Imagine a planet with a civilization so highly developed that they never needed to fly, because they had star trek like teletransportation, but one day they decide to make planes just to have fun.

They could use a fbw system like that of Airbus, and that would be the conventional way. Airplanes have to be stable, and trimmable. Airbii are trimmable by releasing the sidestick. Other airplanes use other methods.

Airbii are intuitive to fly, easy to fly. If that is what you are destined to fly, why not have initial training in a Cessna with a flight path stable fbw?

Misnomers
 

By T.S.
This is bad terminology.
Vector stability and speed stability are control laws.
Relaxed stability is an aerodynamic property.
You can have relaxed stability with speed stability (e.g. F-16) or with vector stability (e.g. A340).
Relaxed stability is just a measure of how far the CG is from the CP.
It has nothing to do with which pitch control law you use.

Yes, that is the idea I had. You can extend the CG aft limit by means of some form of stability augmentation. Wether mechanical or electronic.

Micro ‘why not’ … indeed why not.
I suspect that one view would terminate with cost effectiveness; others debate the extent and nature of training. As for the type of control system in a training aircraft the differences might only be similar to those between FD formats and the nuances of implementation (FDs use a range of different control laws).

One (military) argument was that irrespective of future aircraft characteristics, the handling qualities / characteristics for all types could be identical with the use of ‘FBW’ and thus eliminate the need for extensive differences training. This of course falls down when considering different roles – fighter vs transport (would they use different control parameters), and again cost effectiveness, particularly when considering how much of military training is situational and decision oriented – an extensive range of experiences and mental skills.
Perhaps current civil operations should take note; ‘FBW’ can reduce the need for a range of manual skills, thus reducing training duration, differences and cost, but this could be at the expense of experience and mental skill. Neither type of control law will replace those.
A Cessna with FMS, a full Flight Guidance system, ECAM, in a realistic operating environment … a pointless debate.

If comparisons are to be made, look at the implementation (training) and use of these systems. Consider operators’ and individual attitudes and assumptions.
Modern aircraft are easy to fly, to be enjoyed, but to operate they require greater, different standards of professionalism, particularly at the extremes of normal conditions.
These are the aspects which could be equated and judged, but there is no such thing as an ideal or best practice, as these are always in the eye of the beholder.

Terminology
 

Yeah, Galaxy, we gotta define terms and laws.

The F-16 was not statically stable below 0.95 mach. That'a aerodynamic "stability". So the plane would not go to a trimmed AoA if you let off the stick. In fact the tail actually produced "up" lift in most flight conditions, so we had an extra bit of lift , and much reduced trim drag cruising. At very high AoA, the leading edge of our stabilators were full up ( trying to keep AoA under the limit).

We had a gee command for pitch like the 'bus, and the result of the control laws was neutral speed stability. Neither jet trimmed to an AoA in normal laws. But it looks like the 'bus has speed stability in Direct Law, from FCOM's I got from the folks here. And it apparently has enough tailplane to maintain a very high AoA and pretty good directional control as we have seen at least once.

BTW, you can have a FBW system AND static stability in fighters, too - the Hornet is a good example.

I don't think you will ever see "relaxed static stability" implemented on a commercial airliner.

Because it helps you to understand what the technology is assisting in the event that one day it might fail to operate as designed.

Relaxed Static Stability on Commercial Transports
 

The FARs dictate pitch handling qualities including certain levels of pitch stability as experienced by the flight deck crew when flying manually. For airplanes without augmentation in the form of feedback control laws that increase stability, the FARs translate into fore/aft limits on the CG range. Moving the CG further aft would reduce stability and in the context of this discussion thread constitute "relaxed static stability". The motivation for doing so is the resultant improved fuel economy as the horizontail tail carries less down load (or if you move CG far enough aft, actual positive lift).

A key reason for Boeing to introduce FBW on the 777 was to enable relaxed static stability by allowing the cg to be moved further aft than would have been possible without the C*U pitch control law augmentation. Augmentation allows tuning of the response characteristics independent of the basic, open-loop configuration stability deriviatives. Pitch damping was also added to the backup, reversionary modes to yield adequate handling characteristics for failure conditions. This basic control concept served as the baseline for 787 configuration definition.

All future Boeing commerical airplanes will include relaxed static stability as a core feature.

Interesting!
So if I have read you correctly the 777/787 handling with aft CG is inadequate without stability augmentation. This goes further than all the AI aircraft I have been involved with from A310 to A340 where the aircraft has to be satisfactorily flyable without any augmentation even with aft CG - i.e. direct law for A320 onwards. Several PPRuNers have posted to the effect that this is so.

Yeah, OG.

I cannot imagine a certification nowadays for a commercial jet that does not have positive static stability once in the so-called "direct law". You know, the one that relies upon the natural aero of the jet to seek a trimmed AoA.

Our problem, and this shows up in last paragraphs of the AF447 report, is the Airbus has neutral "speed" stability until in "direct" law. And then the basic design is primary and you have something close to what all the planes since the mid-fifties have had with hydraulic flight control systems.

Boeing Direct Mode not Unaugmented
 

One thing to be careful with is terminology regarding control system modes. The Boeing 777 and 787 have three basic modes for manual control: Normal, Secondary, and Direct. All three involve pitch stability augmentation.

Normal Mode is provided unless failures result in insufficient equipment availability to support the full-up suite of functionality. Most pilots will only ever experience this mode as the other two are for rather remote failure conditions.

Secondary and Direct Modes are activated in the event of failures that preclude Normal Mode. These two reversionary modes share the same level of augmentation and thus the same handling qualities. Secondary Mode is activated if sensor input data required to support Normal Mode are no longer available. Direct Mode is activated if control system computational resources required to support Normal or Secondary Mode are no longer available.

Both Secondary and Direct Modes involve pitch rate feedback to improve pitch stability. Because these are reversionary modes and Normal Mode has sufficient availability, the handling qualities provided by Secondary and Direct Modes are permitted to be degraded. These response characteristics would not be sufficient for certification for every day use.

It turns out that the limitation as to how far the stability can be relaxed is the acceptability of the reversionary mode handling qualities, not what the full-up Normal Mode system can provide. Making it handle well when all of the resources are available is one thing, having it handle acceptably when the stuff has hit the fan is something else.

Another point of critical interest, presuming OG's observation to be correct, relates to system reliability. That is, the level of confidence that the operator/crew can have that the crew won't be left with a degraded mode where the LSS is grossly inadequate ?

While an aircraft can be so flown (even if it be statically unstable - providing that the pilot knows the tricks of the trade) the workload, as LSS reduces, becomes progressively intolerable and prolonged flight associated with recovery becomes impossible.

@FCeng84


It is not easy, apart from the use of Secondary rather than Alternate, to see any substantive difference between the two sets of logic. In both cases progressive loss of system resources (computers, sensors) is accompanied by a loss of stability augmentation.


Again if I have read you correctly, the main differences seem to be that in the Boeing design the first step after a failure is to drop to a level of stability augmentation that is pitch damping only whereas the Airbus design retains the essentials of the C* function, albeit sometimes with compromise gains. The next step to "Direct" law has no change of augmentation in the B777/787 but no augmentation at all on the AI designs.


Your comment that the allowable level of relaxed stability depends on the "bottom level" of handling qualities available and JT's comment that this is linked to reliability are both spot on. This implies that Boeing have convinced themselves and the FAA that the probability of losing all pitch damping is less than 10^-9 per flight hour.


It may well be that in the long gap between the appearance of the A320 and the B777 computer technology (and sensor technology - do not forget the sensors!) had improved to the point where it was possible to construct a mathematical case to prove that this was so, but realistically, when talking of "proof" at that level of probability one is as much in the realms of faith, engineering judgement and faith in one's engineering judgement as in the arithmetic.


Ther is nothing wrong with that, but we should recognise that it is so.

Boeing Augmentation Approach
 

The Boeing FBW Normal Mode has an availability of 10^-7 per flight hour. The system can endure a number of failures of redundant equipment and still provide the full-up, Normal Mode control augmentation.

You are correct that the Direct Mode is better than 10^-9 per flight hour. Very much simplified augmentation is provided by the Actuator Control Electronics (ACE) LRUs. The separate LRUs that host the Normal Mode functionality are bypassed in Direct Mode. The four ACEs are brick walled from each other. In the case of the pitch axis there are four separate pitch rate sensors that feed each of the ACES and elevator actuator control is partitioned among the ACEs. Flight deck column position is measured by separate sensors that feed the respective ACEs. In Direct Mode, two ACEs respond to the left column and drive the left elevator while the other two ACEs respond to the right column and drive the right elevator. The columns are mechanically linked through a breakout element that allows one to move free of the other if sufficient differential force is applied.