Positive static spiral stability???

[ShockWave]

I've been trying to make sense of my AB330 manual, ( Yeh! I know, it's been a slow week)
and they talk about using "positive static spiral stability" to roll the wings level or to 33 deg of bank depending on the stuation.

Does anyone know what positive static spiral stability is and how it works? :rolleyes:

[Genghis the Engineer]

Yes.

Spiral stability is the tendency of an aircraft to increase or reduce bank angle once disturbed.

So, an aircraft with positive spiral stability tends to roll wings level from, say, a 30° bank. If negative, it increases the angle of bank, and if neutral it stays where you put it.

In training aircraft positive spiral stability is generally regarded as a good thing. In an Airbus, I'd suggest it's probably not too important so long as it's not actually negative. In something like a Pitts, you'd want it neutral or negative.

The standard flight test technique is to roll about 15° with the rudder (keep the ailerons central), release the controls and start a stopwatch. The time to half or double bank angle is recorded in the FTR - if it takes more than 10 seconds to do either, consider it neutral.

G

[Zeke]

Above 33 deg it will roll wings level to 33 deg (positive), 33 deg or less it keeps the AoB that you have commanded (neutral).

Z

[411A]

For those that can remenber back far enough, Boeing increased the vertical stab height on later models of the 707 for increased stability.

[Genghis the Engineer]

A bigger vertical stabiliser will increase directional stability. If you alter the ratio of lateral to directional stability you alter the spiral stability. In this case, increasing the fin area whilst doing nothing to the dihedral will DECREASE the spiral stability - that is reduce it's tendency to roll wings level when disturbed.

G

[fantom]

G.
hugely enjoyed the wing drop thing.
now enjoying this one. me only humble footsoldier pilot type but learning a lot.
wish I had the grey matter for tp.
with 14000hrs, is it too late?
keep up the info! :eek:

[ShockWave]

Thanks for the info "G".
I can atleast make some assumptions now, regarding what it is doing.

On the AB330, in an overspeed, overbanked condition positive static spiral stabillity is used to level the wings. In an alpha or high angle of attack, low speed, overbanked condition the same positive stabillity is used to reduce the bank angle to 33 deg. (which is the normal mode, max allowable bank).
Is there an aerodynamic reason for the difference( eg. lower speed, higher AOA so less stabillity??), or is it just another fly by wire mystery?

[John Farley]

ShockWave

A thought. Attempts at providing the sort of spiral stability effects of which you speak by natural aerodynamic means (ie the shape of the aeroplane) while not impossible would very likely compromise other very much more important things like cruise efficiency and low speed high lift capability. If I were part of a FBW design team I would be pressing for the (highly desirable) characteristics that you speak of to be provided artificially by the system. Do you think this is what is in fact going on and the manual is merely describing these abilities that the FBW system provides by using stability terminology?

[Tinstaafl]

I think the general trend is to reduce aerodynamic stability, and use artificial augmentation instead eg with FBW, to reduce drag/increase efficiency.

[Genghis the Engineer]

JF makes a very good point, particularly in light of Shockwave's more recent comment.

The overspeed / overbank condition (which I trust you haven't tried with Pax on board) is probably what I'd know as a spiral dive. If I were dealing with a "conventional" aeroplane, I'd associate high spiral stability with a tendency to lock into this mode - this killed a fair number of WW1 fighter pilots before it was understood. So, what you're saying is inconsistent if there were no AFCS in the loop. I think JF is probably right, the AFCS is probably doing most of the work. If I were Chief Designer on the A320 I think I'd be content to make most of my apparent (i.e., as seen from the cockpit) stability artificially. I suspect that whatever the spiral stability really does, you have a system designed to display apparent moderate spiral stability in small perturbations and low spiral stability in a spiral dive.

Now if only I could work out how to do that with reversible controls...

Freelunch, be careful about using the word stability. It is a very complex subject to wrap up in one word. Spiral stability is a function of the RATIO of lateral to directional stability, both of which you need. The efficiency drive in the Airbus is pretty much all about longitudinal static stability, which is a rather different beast and nearly, but not-quite, unrelated.

G

[ 24 November 2001: Message edited by: Genghis the Engineer ]

[Keith.Williams.]

The two fundamental questions posed by Shockwave appear to be:

a. What is spiral stability?
b. Why does the AB330 roll back to wings level in an over speed - over bank, but only back to 33 degrees when in high alpha-over bank.

Although I am neither an aircraft designer nor an AB330 expert, I think that both questions can probably be answered from first principles. (But I could of course be wrong)

FIRST QUESTION
In order to answer this question we need to examine lateral stability, directional stability and the interaction between the two.

The term lateral stability refers to the manner in which an aircraft responds to (rolling) disturbances about its longitudinal axis. When banked an aircraft will sideslip and it is the response to this sideslip which determines lateral stability. If it rolls away from the sideslip it is positively laterally stable and will tend to level itself. If it rolls into sideslip it is negatively laterally stable, and any bank will be self increasing. If it neither rolls into nor out of sideslip it is neutrally stable and will neither return to nor diverge from the wings level condition following a disturbance.

The term directional stability refers to the manner in which an aircraft responds to (yawing) disturbances about its normal axis. Following a disturbance in yaw an aircraft sideslips in the opposite direction. If it then yaws into the sideslip it is positively directionally stable and will tend to correct disturbances in yaw. If it yaws away from the sideslip it is negatively directionally stable and disturbances in yaw will be self increasing. If it neither yaws into nor out of the sideslip it is neutrally directionally stable and will neither return to nor diverge from its original heading following a disturbance.

The term spiral stability refers to the combined effects of lateral and directional stability. Following a disturbance in yaw or roll an aircraft will sideslip. If its directional stability is stronger than its lateral stability it will yaw into the sideslip more than it rolls away from it. This will decrease the airspeed over one wing and increases that over the other, producing a dissymmetry of lift, rolling the aircraft towards the sideslip. This will cause further yawing and rolling to that side, such that the aircraft enters a wide spiral dive. This condition is termed spiral instability.

If lateral stability is stronger than directional stability then the aircraft will roll away from the sideslip more than it yaws into it. If it then rolls beyond wings level it will sideslip in the opposite direction. The process will then be repeated, with the aircraft rapidly banking from side to side in the phenomenon of Dutch Roll.

Most commercial jet aircraft possess dihedral, sweepback and large fins. Sweepback and large fins both increase directional stability, whilst all three factors increase lateral stability. Such aircraft are generally prone to Dutch Roll which suggests that they possess a fair degree of positive spiral stability.

SECOND QUESTION
If an aircraft is spirally stable it will tend to roll away from a bank rather than yawing into it. This means that continuous aileron deflection is required to maintain any given bank angle. If at any given bank angle, the ailerons are returned to neutral, the aircraft will tend to roll wings level. It is probably this effect that is referred to in the AB330 manual. In effect the protection system is reducing aileron deflection in order to permit the inherent spiral stability of the aircraft to roll it away from the bank. It is of course possible that the AB330 employs artificially induced spiral stability, in which case the ailerons would need to be deflected to correct the roll.

The question of why the system takes the aircraft back to wings level in an over speed -over bank condition, but only back to within limits (33 degrees) in a low speed, high alpha-over bank situation is probably safety related. The high alpha-over bank is most likely to occur when manoeuvring at relatively low altitude. In this situation an automatic (slight) reduction of bank angle would increase turn radius only slightly, whereas a wings level correction would stop the turn completely. If the purpose of the turn was to avoid a collision, its automatic cancellation might have disastrous consequences.

In the over speed -over bank condition the aircraft is subject to overloading due to two factors. These are high dynamic pressure due to excessive airspeed and high load factor due to excessive bank angle. Although the load factor at the quoted 33 degrees would be less than half of the JAR 25 limiting value(1.2 compared with 2.5), the combined effects of dynamic pressure and load factor might approach critical values. If the situation was caused by an upset during high altitude cruise, the reduction in Mcrit caused by the high load factor would make the need for corrective action even more pressing. Under these circumstance rolling to wings level would immediately reduce load factor and increase Mcrit, enabling increased alpha and/or spoilers to be used to reduce speed.

[bookworm]

I'm with you up to about:

If lateral stability is stronger than directional stability then the aircraft will roll away from the sideslip more than it yaws into it. If it then rolls beyond wings level it will sideslip in the opposite direction. The process will then be repeated, with the aircraft rapidly banking from side to side in the phenomenon of Dutch Roll.

Most commercial jet aircraft possess dihedral, sweepback and large fins. Sweepback and large fins both increase directional stability, whilst all three factors increase lateral stability. Such aircraft are generally prone to Dutch Roll which suggests that they possess a fair degree of positive spiral stability.


It doesn't quite work like that. The combination of roll, yaw and sideslip get intertwined to produce three mixed modes.

In fact one is a heavily damped mode almost entirely in yaw called "roll damping". If the aircraft is rolling and you put the ailerons neutral, it stops rolling (gradually). And that's about all there is to that one.

Another is the "Dutch roll mode. That is typically stable but underdamped, so if you induce a Dutch roll the aircraft tends to wallow in the mode for a few oscillations which then disappear. I presume unstable Dutch roll modes are a Bad Thing, as it requires an interesting combination of feet and hands to get the aircraft back to neutral. Perhaps JF has played with them in that Bassett they had rigged up for stability demos at the ETPS?

The last is the "spiral" mode, which is usually unstable as the name implies (otherwise you'd call it the "wing rock mode" wouldn't you? :)). The instability means that with no control input bank gradually increases into a spiral dive. I think that it's possible to make an aircraft stable in this mode in principle, but you pay a high price in terms of handling quality, and it generally causes more trouble than it's worth.

The tendency to Dutch roll does not necessarily imply a positive stability in the spiral mode, or vice versa.

[ 24 November 2001: Message edited by: bookworm ]

[Genghis the Engineer]

Bookworm is correct.

Mathematically, lateral stability is the partial derivative of rolling moment with respect to sideslip. Directional stability is similarly the partial derivative of yawing moment with sideslip.

The spiral mode (aerodynamically at-least) is largely down the ratio of the two. If lat / dir is greater than 1.0, then the spiral stability should be positive.

Dutch Roll excitement is down to the lateral and directional damping. The damping, and the actual stability values are not closely related.

The ratio of the DR is roughly the same as the ratio of the stabilities. So, if the wingtip describes a circle during DR, expect roughly neutral spiral-stab. Alternatively, if the wingtip describes a flat oval with it's axis parallel to the horizon, expect it to be spirally divergent, because this indicates that directional stability is greater than lateral.

G

[Keith.Williams.]

In attempting to answer questions I think it is usually best to apply the following principles:

a. Match the complexity of the answer to that of the question.
b. Limit the range of discussion to that strictly necessary.

In this case the original question "what is spiral stability" suggested a need for a relatively low level of complexity. Applying the above principles in composing such an answer I deliberately avoided discussion of dynamic stability.


BOOKWORM
QUOTE. "It doesn't quite work like that. The combination of roll, yaw and sideslip get intertwined to produce three mixed modes.

In fact one is a heavily damped mode almost entirely in yaw called "roll damping" if the aircraft is rolling and you put the ailerons neutral, it stops rolling (gradually). And that's about all there is to that one."

COMMENT. The term dynamic stability describes the subsequent motion of an aircraft following a disturbance. If it oscillates, but the oscillations gradually subside then it is dynamically stable. If it oscillates but the oscillations increase in amplitude it is dynamically unstable. If it oscillates such that the oscillations neither subside nor increase then it is neutrally dynamically stable.

Roll damping is the principle factor in providing dynamic stability in roll. Although you state that roll damping is "almost entirely in yaw", your example actually illustrates its effect in roll. (Strangely) although a similar effect is the main factor in providing directional dynamic stability, this isn't (often) called yaw damping.

QUOTE. "Another is the Dutch roll mode. This is typically stable but underdamped, so if you induce Dutch roll the aircraft tends to wallow in the mode for a few oscillations which then disappear."

You are correct (in normal circumstance) but this is not always the case. If for example you switch off the yaw damper of a modern swept wing jet cruising at high altitude, you would probably experience a far more unstable and potentially destructive Dutch roll. The yaw damper prevents Dutch roll by artificially increasing directional stability so that it is stronger than lateral stability.

QUOTE. "The spiral mode, which is usually unstable as the name implies (otherwise you'd call it wing rock wouldn't you.) The instability means that with no control input bank gradually increases into a spiral dive. I think that it is possible to make an aircraft stable in this mode in principle, but you pay a high price in terms of handling, and it generally causes more trouble than its worth.

COMMENT. I agree. The spiral dive is quite predictable and easy to correct manually so it isn't worth putting in too much effort to eliminate it. But the original question indicates that the AB330 is spirally stable. Whether this effect is being generated aerodynamically or artificially matters little in attempting to answer the original questions (Which were, what is spiral stability and why does the AB330 go wings level at high speed and only to within the 33 degree limits at high alpha). If we accept that Shockwave has quoted the manual accurately and that the manual itself is correct, then we must accept the (effective) spiral stability of the AB330 as a given fact. I must confess that I went beyond the original questions in examining how this aircraft might actually use its spiral stability to achieve the specified effects.

QUOTE. "The tendency to Dutch roll does not necessarily imply a positive stability in the spiral mode, or vice versa."

COMMENT. I do not agree. Dutch roll requires that an aircraft repeatedly rolls from one side to the other. By your own definition a spirally unstable aircraft will not return to wings level, but will yaw towards the low wing. This will increase the roll which will in turn increase the yaw. It is a fundamental requirement for Dutch roll that the (bank reducing) rolling tendency due to lateral stability is stronger than the (bank increasing) rolling tendency due to directional stability.


GENGHIS
Returning to my comments above regarding giving a simple answer to a simple question, any answers involving term such as "partial derivative of" is probably aiming a bit too high in this case.

QUOTE. " Dutch roll (aerodynamic at-least) is largely down to the ratio of the two (lateral and dynamic stability). If lat/dir is greater than 1.0, then spiral stability should be positive.

COMMENT. I agree. In fact this is exactly what I said in my previous post!

QUOTE. " Dutch roll excitement is down to the lateral and directional damping. The damping, and the actual stability values are not closely related."

COMMENT. I agree to the extent that an aircraft that is heavily damped in roll and yaw will not exhibit significant Dutch roll. The fact remains however that unless a sideslipping aircraft rolls away from the sideslip it cannot exhibit any Dutch roll.

QUOTE. "The ratio of DR is roughly the same as the ratio of stabilities. So if the wingtip describes a circle during DR, expect roughly neutral spiral stability. Alternatively, if the wingtip describes a flat oval with its axis parallel to the horizon, expect it to be spirally divergent, because this indicates that directional stability is greater than lateral. "

COMMENT. I agree. This again reinforces my assertion that Dutch roll is caused by lateral stability being greater than directional.


SO WHERE ARE WE NOW?
Well, I think we have defined spiral stability pretty well, so we can probably move on to the second question of "why does the AB330 go wings level in an over speed / over bank, but only back to within the 33 degree limits in a high alpha / over bank. I freely admit that the suggestion in my previous post is little more than conjecture. It is of course based upon the assumption that the difference was intentionally created, rather than simply a fact of the aerodynamics or electronics. The suggestion by Shockwave that it might be due to "lower speed, higher AOA so less stability" is I think incorrect. Aircraft tend to become less (dynamically) stable as speed increases, due to a reduction in roll damping.

[John Farley]

Amazing the stuff we find to talk about on a wet Sunday afternoon

Keith

Well you have tidied up a fair bit of theory there!

In point of fact the practice of checking the spiral stability of an aircraft in the sky can present quite a good gotcha to young lads (and lasses).

At one stage I used to take Kinston Aero Eng Uni students up three at a time in a PA44 to show them some of the practical reality of flight test issues. The spiral stab bit used to go like this

Q: “OK – how do we see if this thing is spirally stable or not?”
A: “Trim it out S&L, roll on 30 deg of bank, let go the controls and observe”

So that’s what we did. 30 secs after abandoning the controls and approaching Vne at 70 deg of bank, the view of the crew was that it was unstable in the spiral mode.

Recover, explain flight test results not worth a damn unless they are repeatable.

Repeat test. Same result. Remark “Hey, tell you what, lets try that one more time”

Repeat test. This time it rolls wings level.

So what is going on?

Any comments Keith?

Regards

[bookworm]

Roll damping is the principle factor in providing dynamic stability in roll. Although you state that roll damping is "almost entirely in yaw", your example actually illustrates its effect in roll.

Don't read what I wrote, read what I meant! :) Thank you, yes, "almost entirely in roll"

You are correct [that Dutch roll mode is usually stable] (in normal circumstance) but this is not always the case. If for example you switch off the yaw damper of a modern swept wing jet cruising at high altitude, you would probably experience a far more unstable and potentially destructive Dutch roll. The yaw damper prevents Dutch roll by artificially increasing directional stability so that it is stronger than lateral stability.

It certainly improves stability but I don't think it can flip the sign. FAR 23.181 requires:

"Any combined lateral-directional oscillations ("Dutch roll") occurring between the stalling speed and the maximum allowable speed appropriate to the configuration of the airplane must be damped to 1/10 amplitude in 7 cycles"

FAR 23.672 requires the aircraft to be stable after a failure of "the stability augmentation system...trim, stability, and stall characteristics are not impaired below a level needed to permit continued safe flight and landing".

I'd be surprised if there's anything certified that's actually unstable in Dutch roll even if the yaw damper fails. But it might be stomach churning and concentrate the mind somewhat!

The crux of my disagreement with what you previously wrote was in:

QUOTE. "The tendency to Dutch roll does not necessarily imply a positive stability in the spiral mode, or vice versa."

COMMENT. I do not agree. Dutch roll requires that an aircraft repeatedly rolls from one side to the other. By your own definition a spirally unstable aircraft will not return to wings level, but will yaw towards the low wing. This will increase the roll which will in turn increase the yaw. It is a fundamental requirement for Dutch roll that the (bank reducing) rolling tendency due to lateral stability is stronger than the (bank increasing) rolling tendency due to directional stability.


You may be reading more into "tendency to Dutch roll" than I intended. All I meant was that the mode is lightly damped, not unstable. Surely it's unthinkable that the Airbus without artificial stability input has an unstable Dutch roll mode.

McCormick's Aerodynamics, Aeronautics and Flight Mechanics has a nice section on lateral stability, including an example of a Cherokee after an impulsive rudder displacement. For 10 seconds it wallows around in Dutch roll, but over longer timescales the unstable spiral mode takes over.

But putting aside the interpretation, I don't see anything in the maths that prohibits a stable Dutch roll and stable spiral mode. Though I salute your quest for simplicity, I think you're in danger of oversimplifying. These are coupled oscillations.

As Genghis said (my brackets []),

Mathematically, lateral stability is the partial derivative of rolling moment with respect to sideslip [Lb]. Directional stability is similarly the partial derivative of yawing moment with sideslip. [Nb]

I think that there are a couple of other derivatives you have to bring in to think about the spiral stability. They are the rolling moment with yaw rate [Lr] and the yawing moment with yaw rate [Nr]. According to McCormick, the constant term in the characteristic equation is proportional to:

Nb*Lr - Lb*Nr

(that feels a bit more symmetrical than Nb -Lb or Nb/Lb, which feel like apples and oranges)

If that constant term changes sign, so does one of the roots. That will doubtless be the spiral mode. But I don't see that Dutch Roll stability/instability (i.e. the sign of the root) is affected directly.

[To take a wild guess at John Farley's puzzle, did you start off at higher speed in the stable case which reduces Lr?]

[Genghis the Engineer]

Quote:

"a. Match the complexity of the answer to that of the question.
b. Limit the range of discussion to that strictly necessary. "

I think you're digging a small hole for yourself KW, until your first post on this thread everything else was short and to the point.

Personally, I've no problem with long technical discussions, but having started the trend I wouldn't recommend you start suggesting it is a bad thing.

I have a suspicion that I know the answer to JF's question, but intend standing back and seeing what others say first, it's far more fun.

G


"I apologise for sending you a such a long letter, I did not have time to be brief", George Bernard Shaw.

[Keith.Williams.]

JOHN,
You haven't provided much information on which to base an analysis of your puzzle but I can suggest two possible answers.

The first is speed related. Aerodynamic damping varies with speed so if the experiments were conducted at different speeds this might cause the spiral stability to change from being unstable to stable.

My second suggestion is C of G related, and is based upon the (possible) clue that the aircraft became stable in the final test. Aft movement of the C of G reduces longitudinal and directional stability but has little if any effect on lateral. If in the first two tests the C of G was sufficiently forward to give strong directional stability, this might produce the spiral instability effect you observed. If by the time of the third test, the burning of fuel had moved the C of G far enough aft, it might have been enough to make the aircraft spirally stable.

BOOKWORM
The main area of our disagreement appears to relate to whether or not an aircraft exhibiting Dutch role can be spirally stable. In my first post I did not actually say that an aircraft must be spirally unstable. What I actually said was that if it rolled away from a sideslip more than it yawed into it then it would exhibit Dutch roll. And if it yawed into a sideslip more than it rolled away from it then it would be spirally unstable. I must confess however that in my second post I made the mistake of disagreeing with your statement that " the tendency to Dutch roll does not necessarily imply a positive stability in the spiral mode, or vice versa." I think this is case of "you should have read what I actually said in my first post and what I meant to say in my second".

Perhaps we can agree on the following:

If lateral static stability is stronger than directional static stability the dominate mode is likely to be Dutch roll. This might however be accompanied by weaker spirally stable or unstable motion. If directional static stability is stronger than lateral static stability, then the dominant mode is likely to be spiral instability. This might however be accompanied by a weaker Dutch roll.

GENGHIS
I did not suggest that your arguments were too long, but rather that they were perhaps aimed too high. This suggestion is based upon an assessment of SHOCKWAVES profile and the type of questions he has asked. These led me to conclude that he has probably gained his ATPL quite recently and is now attempting to master the technicalities of his first commercial aircraft. In view of this assessment, I based my first post on the level of knowledge that he would have required to pass his ATPL exams. My post was intended to assist him in applying his existing knowledge to solve the problem for himself. If my assessment was correct, then your references to partial derivatives would have added little to his understanding. He could of course have done a Phd in aerodynamics and just be testing (and probably laughing at) us all.

JOHN, BOOKWORM AND GENGHIS
Now what about SHOCKWAVE'S second question?

[ShockWave]

Thanks guys for all the info: long,short,simple and technically challenging.
My profile is probably well out of date, just like my indepth aerodynamic knowledge.
I believe my ATPL is around 12 years old and my cpl 18 yrs, my first swept wing wide body commercial jet was back in 1989.
What does all that mean? not a lot really when your trying to understand a new aircraft
type. :D
I don't have the time at the moment to fully understand some of the principles involved but I do appreciate the thoughts and now have a much better practical understanding of what might be going on.
The simulator backs up what the manual suggests so I believe the aircraft should do likewise if you were ever in the unfortunate position to see it.
The return to wings level and 33 deg when the stick is released in both cases is very precise, which leads me to think that the primary (or secondary)flight control computers are at least partially responsible for the recovery or maintaining it.
I think that the last time a non AB test pilot switched off the flight control computers in flight at altitude, it took around 18000ft before they managed to recover control!

note: without the computers the side stick is just something else to hang on to!

I don't think I'm brave enough to even try letting the simulator recover from an overbanked spiral condition with the computers switched off to test this theory further.

cheers.

[bookworm]

We've almost converged Keith!

If lateral static stability is stronger than directional static stability the dominate mode is likely to be Dutch roll. This might however be accompanied by weaker spirally stable or unstable motion. If directional static stability is stronger than lateral static stability, then the dominant mode is likely to be spiral instability. This might however be accompanied by a weaker Dutch roll.

Let me try to summarise what I believe to be the case without using the words strong/weak or dominant.

1) The Dutch roll mode is always stable in certified aircraft and is damped with a characteristic time (e.g. time to halve) of a few seconds.

2) The spiral mode can be stable or unstable, and its characteristic time (e.g. time to double if unstable) is much longer than that of the Dutch roll.

3) The coupling coefficients work so that the further along the axis from instability to stability is the spiral mode, the less strongly damped is the Dutch roll mode.

Does that make sense?

[John Farley]

Hi Chaps

Very interested to read ShockWaves last post. If the sim (and prob therefore the aircraft) goes to 33 deg on a recovery and stops there, then it sure is thanks to the FBW rather than the aerodynamics. Aerodynamics could well make it recover but they would not stop at 33 deg.

As to the PA44 story, I think it is a very important lesson for aero eng studes. For the first two entries (and they were all entered the same way) I made sure that my last aileron input before releasing the controls was in to the turn and on the third my last input was out of the turn. Because of control circuit friction releasing the controls does not quite centre the ailerons. Thus the manoeuvres were not the result of just the spiral stability.

My bottom lines for these studes re spiral stability (since we seem well into the subject here) were

One. If you ask pilots whether the aircraft they fly are spirally stable they usually will look blank (which I cite as evidence that spiral stability is not a very important characteristic for pilots compared to many other handling related issues)

Two. If you ask pilots to check with a test on their next flight (put into turn, release and observe) they may well come up with the wrong answer

Three. If you are an FTE and need the answer from your pilot you must give him some sort of indicator that will enable him to check the ailerons are neutral for the test.

As to Keith’s point about ShockWaves second question how about:

In non FBW aircraft spiral stability works because the aerodynamic design of the aircraft is arranged so that (in a controls free turn) any disturbance that results in an increase in bank angle also results in a small sideslip angle being generated which slightly increases the angle of attack of the lower wing (and also reduces the angle of attack of the upper wing) thanks to something called the dihedral effect.

End of broadcast.

Good here init

Regards

[Genghis the Engineer]

Current ETPS teaching is to trim the a/c straight and level, clamp the stick in the centre, and then to roll the aircraft with the rudder for a spiral stab test. When that approach came in I don't know, other than it was before the mid 90s.

What JF's trick on his students does, is emphasise that the results from pure academic tests are not always truly representative of a real aeroplane in service.

G

[John Farley]

Very interesting Genghis. Needs the rudder to be put back to centre though!..........

Seriously, very small control deflections can make a very big difference to the outcome if (as is common) the spiral mode is nearly neutral

A practical mans report on the PA44 might also say that since the aileron friction is enough to change the sign of the result, then for all practical purposes the aircraft can be considered neutrally spirally stable

[Genghis the Engineer]

Concur. G.

[Keith.Williams.]

SHOCKWAVE,
Sorry, no offence intended. Your profile just states plot and your pprune registration date of 1999 suggests a more recent arrival. This (incorrect)first impresion was then reinforced by the question "what is spiral stability".

The majority of the people who have taken their JAR ATPL POF exam over the past couple of years would probably ask that question, quite simply because the emphasis tends to be on longitudinal , lateral and directional, with only a brief mention of spiral. About 95% of those people would immediately tilt upon hearing terms like partial derivative.

BOOKWORM,
That sounds close enough to me. (and this string is getting a bit too long anyway)

JOHN,
Although your statement regarding dihedral is entirely true,it isn't actually an answer to Shockwave;s second question. This was "In an overspeed/overbank condition positive static spiral stability is used to level the wings. In an alpha or high angle of attack, low speed, overbank condition, the same positive stability is used to reduce bank to 33 degrees(which is the normal max allowable bank).

But again this string is getting very long!!!

[ShockWave]

Keith: No offence taken. Thanks for the info, once again. The date of joining pprune changes when your email changes eg. when you change countries and employers, which can happen often in this industry. :cool: