spiral instability
 

wat is spiral instability???

It means that if the aeroplane is disturbed in roll, and the pilot does nothing about it, the aircraft's bank angle will continue to increase (rather than tending to roll towards wings level).

Generally regarded as a bad thing in training and IMC-use manually flown aeroplanes; only a mild nuisance in a VMC only aeroplane, and probably not noticed in something normally flown through an A/P. The degree of instability is also relevant - most commonly you'd measure it as time to double bank angle from a small (e.g. 15°) disturbance - if it takes 20 seconds you'd probably regard it as effectively neutral and not worry much, if it takes 3 seconds in a training aeroplane I'd regard that as unsat(HD) verging on unacceptable depending upon the precise aircraft role.

G

It is caused by an aircraft being more directionally stable than laterally.. For example a lateral disturbance causes the plane to bank, the nose will then tend to follow the direction of the bank which in turns increases the bank (as the aircraft is not as stable laterally). If nothing is done, the aircraft will end up in a steep spiral dive.
The opposite effect (when the aircraft is more laterally stable than directionally) is called Dutch roll..

Monsieur - don't think so.....unless you mean something else by 'longitudinal'?

I agree with India-Mike, also rather suspicious about the rather odd explanation of what causes Dutch Roll.

G

An aircraft with positive spiral stability tends to roll out of a turn by itself if the controls are centred. Some light aircraft with little or no wing dihedral and a large fin tend to have strong static directional stability but are not so stable laterally. If a sideslip is introduced by turbulence – and left to their own devices – such aircraft will gradually start to bank and turn, with increasing slip and hence increasing turn rate and rapid increase in height loss. The condition is spiral instability and the process is spiral divergence which, if allowed to continue and given sufficient height, will turn into a high speed spiral dive, which often occurs when a pilot without an instrument flight rating strays into thick cloud. Neutral spiral stability is the usual aim of the designer.

It is evident that directional stability and lateral stability are coupled and to produce a balanced turn, i.e. with no slip or skid, the aileron, rudder and elevator control movements and pressures must be balanced and co-ordinated.

Spiral instability.

With regards to large jet transport aircraft, can be induced by too large of a vertical stabilizer, as was found out by Boeing many yeras ago when they were trying to figure out a way around DP Davies' insistance that the B707 be improved, otherwise it would not be allowed onto the British civil register.
A 39 inch ventral fin was the first action, then the ventral fin was discarded and the vertical fin extended, whereupon spiral instability was found.
Enter, stage left, the hydraulic guys.
Improved rudder power system design solved the problem to a large extent, and the taller vertical fin could remain
However, if the yaw damper failed in flight with these early airplanes, best you should find a place to land...right now, least you found yourself in moderate/severe turbulence, and the very unstable dutch roll tendancies of the airplane became apparent at higher altitudes...uncontrolable alternating fifty degree plus banks will get your attention in a heartbeat.:ooh:

altitude being an important word there, air density playing a big role in these stability modes.

Quite true,
However, there were other problems with these very early B707's that might not be appreciated by younger folks.
The yaw damper (parallel type) had to be switched off for takeoff and landing, and an approach in a very gusty crosswind could become a rockin'-an-rollin' affair, and it was not a pleasant experience.
The series type yaw damper fitted to later aircraft solved this problem to a large degree.

I'm curious - why? Was it some concern about reliability and the thing going haywire at a critical stage of flight, or something else?

G

My understanding is that it was too disconcerting and distracting for the pilots to feel the rudder pedals moving under control of the (parallel) yaw damper at take off and landing, so it was only switched on for cruise once the pilots had their feet up. One advantage of a Serial Yaw Damper is that they are transparent to the pilot.

You understanding is rather inadequate, richatom.
The parallel yaw damper was switched off for takeoff and landing due to excessive rudder pedal forces necessary with it ON and an engine inoperative...an outboard engine especially.

ON at 1000 feet agl in the climb, OFF by 1000 feet agl, during the approach.

411A, an old 707 would make an ideal 'training jet' I would say:ok:

at least it would impart some appreciation for all of these new relatively docile toys of today

I never claimed to have personal experience of parallel yaw dampers - just quoting what I have read in 1960s era books by authoritative test pilots (eg DP Davies) to answer Genghis Khan's question which has remained unanswered until now.

What has it got to do with an outboard engine failure? My understanding of parallel yaw dampers is that they faithfully transmitted to the pedals the actual rudder deflection required to dampen dutch roll - which was fine during the cruise where the period of dutch roll was typically resonant with pilot reaction time leading to pilot induced oscillation, but was somewhat disconcerting during other flight regimes because it would be out of phase with what the pilot sensed. However, in the case of an outboard engine failure at take off, both pilot and yaw damper would typically have the same pedal input - so there would be no conflict. The yaw damper was thus turned off during take off and landing in case it lead to unneccesary feeback to the pilot.

And why would the pedal input on outboard engine failure be any higher with yaw damper in operation than without yaw damper? I don't understand your point at all.

But as I say, I am only quoting from memory from books I have read and I have never flown a plane with parallel yaw dampers so I am sure you may like to explain in detail your point.

Indeed it would..so much so that I'm afraid that the younger pilots of today would likely throw in the towel in desperation...what a hoot it would be to watch 'em (try) to cope....:}

Was the 707 the only aircraft with a parallel yaw damper?

G

Maybe you could try to answer my question instead of just posturing and bragging? Maybe I might then believe you that you really are a super-hero pilot.:D

Yaw dampers are rate/rate devices that apply rudder deflection in response to a rate of change of yaw, so would not apply any rudder deflection once the aircraft was established in its N-1 yawed or wing-down attitude. The yaw damper probably would apply some deflection at initial engine failure as it would react more quickly than the pilot - but both pilot and yaw damper would apply deflection in the same direction.

I think what you mean by claiming that "pedal forces would be excessive in the event of an outboard engine failure" is that the parallel yaw damper could prevent pilot obtaining maximum pedal deflection to maintain VMCG or VMCA. But I'm just guessing and I await your expert response.

To the best of my knowledge, the early DC-8's had one also.

With the early type of parallel yaw damper, that is correct.

From what I remember of the B707-436 circa late seventies (which had a parallel yaw damper installed) the rudders were effectively `locked` with the yaw damper engaged. Mild dutch roll on approach was a well known feature of that marque and usually countered with short jabs of aileron. Base training on the aircraft included dutch roll exercises at altitude with the yaw damper off - and it was spectacularily divergent!

For those younger posters who may not have had the rather `dudious` pleasures of coping with those somewhat deficient early aircraft please refrain from trying `to shoot the messenger` when the `oldies` reminisce!

I have no doubt that these aircraft had their foibles but suggesting that today's pilots would not be able to fly them is as silly as suggesting that yesterday's pilots would not be able to fly today's aircraft.