Why is the PA28 designed to be spirally unstable?
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Why is the PA28 designed to be spirally unstable?
The thread is the question.
This Q. appeared in my homework.....
"Why is the PA28 designed to be spirally unstable?"
My immediate answer is....."I didn't know it was".
Any thoughts?
This Q. appeared in my homework.....
"Why is the PA28 designed to be spirally unstable?"
My immediate answer is....."I didn't know it was".
Any thoughts?
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I guess the question is designed to discover if you understand the connection between directional stability and spiral instability:
The PA28 is designed to be directionally stable and, ergo, is spirally unstable.
Given the type of manoeuvring a PA28 is expected to experience this is the right end to pitch the trade-off between the two.
The PA28 is designed to be directionally stable and, ergo, is spirally unstable.
Given the type of manoeuvring a PA28 is expected to experience this is the right end to pitch the trade-off between the two.
I didn't know it was, and I fly one. Then again, I've never specifically looked to check.
Standard test for spiral stability is to trim level, bank to about 15° with the rudder, centralise and start a stopwatch. If bank angle doubles within 8 seconds it is unstable, if it halves within 8 seconds it is stable, if it takes more than 8 seconds to do either, standard test-flying practice would decide that it is neutral.
Spiral stability is down to a the ratio of lateral to directional stability. As a general rule of thumb if lateral stability is greater than directional the aircraft should be spirally stable, if directional is greater it should be negative, if they're about the same it should be about neutral.
Now, assuming that the statement of spiral instability is correct the reason is almost certainly to make the aircraft spin-resistant. As a rule of thumb, a divergent spiral mode implies a great reluctance to spin, and a strongly positive spiral mode can often imply a spin-prone aeroplane. I emphasis that this is not an absolute, but I'd put a fiver on it being the answer you're looking for.
G
Standard test for spiral stability is to trim level, bank to about 15° with the rudder, centralise and start a stopwatch. If bank angle doubles within 8 seconds it is unstable, if it halves within 8 seconds it is stable, if it takes more than 8 seconds to do either, standard test-flying practice would decide that it is neutral.
Spiral stability is down to a the ratio of lateral to directional stability. As a general rule of thumb if lateral stability is greater than directional the aircraft should be spirally stable, if directional is greater it should be negative, if they're about the same it should be about neutral.
Now, assuming that the statement of spiral instability is correct the reason is almost certainly to make the aircraft spin-resistant. As a rule of thumb, a divergent spiral mode implies a great reluctance to spin, and a strongly positive spiral mode can often imply a spin-prone aeroplane. I emphasis that this is not an absolute, but I'd put a fiver on it being the answer you're looking for.
G
That feels like a pretty broad "neutral" brush, Genghis. Barnes McCormick uses the PA-28 for his worked examples on stability in Aerodynamics, Aeronautics and Flight Machanics. He calculates a doubling time of somewhere between 20 and 30 seconds, IIRC.
But it definitely diverges, even though the frequency of the mode is within the band you consider to be neutral. The question really is "why didn't they change the sign of the mode, so it's stable?"
My impression, and I'm not a test pilot like Genghis, was that making the sprial mode stable made the dutch roll mode almost intolerable. Are there any aircaft that are designed to be stable in the spiral mode, even if they have a "halving time" of more than 8 seconds?
But it definitely diverges, even though the frequency of the mode is within the band you consider to be neutral. The question really is "why didn't they change the sign of the mode, so it's stable?"
My impression, and I'm not a test pilot like Genghis, was that making the sprial mode stable made the dutch roll mode almost intolerable. Are there any aircaft that are designed to be stable in the spiral mode, even if they have a "halving time" of more than 8 seconds?
In the air, anything beyond about 8 seconds is so weak that it can't reasonably be discerned from the external effect of turbulence, slight backlash in the roll circuit, slight mis-trimming, etc. So, that brush, whilst broad, is valid from a piloting viewpoint. (Well that's what they taught us as ETPS anyway, and it has served me pretty well as a rule since). 20-30 seconds to diverge, to me is as near neutral as makes no difference, and in practice will probably be overwhelmed by engine torque (but then the PA28 has a rudder trimmer to solve that).
The Bulldog is spirally stable, and very happy to spin. I am reasonably certain that the high wing Cessna's generally are too, but haven't flown one for a while so wouldn't wish to swear to it. The Easy Raider is spirally stable, but that is known to have slightly weak directional stability - it's also incidentally very hard to spin. The X'Air is neutrally stable with strong lateral and reasonable directional stability, and just able to spin.
Mild spiral stability is generally a good thing in a trainer, since (a) it gives students a get-out-of-jail card if things go mildly wrong in early training, and (b) it allows you something that can be spun. Excessive spiral stability (suffered by many WW1 aircraft) can cause an aircraft to lock into an unrecoverable spiral dive - which is arguably worse than a spin.
The relation to DR is not quite (in my opinion) as BW states. The DR ratio, like the spiral mode, is a function of the lateral to directional stabilities. Whether DR is a problem is far more a function of damping than it is of ratio; the ratio becomes most significant in DR if it is badly damped, because it helps you decide in which axis to try and increase the available damping.
G
The Bulldog is spirally stable, and very happy to spin. I am reasonably certain that the high wing Cessna's generally are too, but haven't flown one for a while so wouldn't wish to swear to it. The Easy Raider is spirally stable, but that is known to have slightly weak directional stability - it's also incidentally very hard to spin. The X'Air is neutrally stable with strong lateral and reasonable directional stability, and just able to spin.
Mild spiral stability is generally a good thing in a trainer, since (a) it gives students a get-out-of-jail card if things go mildly wrong in early training, and (b) it allows you something that can be spun. Excessive spiral stability (suffered by many WW1 aircraft) can cause an aircraft to lock into an unrecoverable spiral dive - which is arguably worse than a spin.
The relation to DR is not quite (in my opinion) as BW states. The DR ratio, like the spiral mode, is a function of the lateral to directional stabilities. Whether DR is a problem is far more a function of damping than it is of ratio; the ratio becomes most significant in DR if it is badly damped, because it helps you decide in which axis to try and increase the available damping.
G
In the air, anything beyond about 8 seconds is so weak that it can't reasonably be discerned from...
I've never come across the idea that Cessna's high-wing singles are stable, nor the Bulldog. I've not flown the latter, but I don't remember the former types to be stable, and they seem regularly to take pilots to their graves in VFR-into-IMC type accidents.
I don't think any of them (or any other modern type) are strongly stable, which means that in IMC, with a pilot in the loop, and the turbulence that is common in IMC an unstable total system is entirely possible - even with a stable aircraft.
G
G
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Genghis,
Your first post in this string states that:
Standard test for spiral stability is to trim level, bank to about 15° with the rudder, centralise and start a stopwatch. If bank angle doubles within 8 seconds it is unstable, if it halves within 8 seconds it is stable.........
Your second post states that:
Excessive spiral stability (suffered by many WW1 aircraft) can cause an aircraft to lock into an unrecoverable spiral dive....
These statements appear (to me at least) to be contradictory. Is it not the case that an excessive DIRECTIONALLY stable aircraft is likley to be SPIRALLY UNSTABLE, which means that it has a tenedncy to enter a spiral dive?
Your first post in this string states that:
Standard test for spiral stability is to trim level, bank to about 15° with the rudder, centralise and start a stopwatch. If bank angle doubles within 8 seconds it is unstable, if it halves within 8 seconds it is stable.........
Your second post states that:
Excessive spiral stability (suffered by many WW1 aircraft) can cause an aircraft to lock into an unrecoverable spiral dive....
These statements appear (to me at least) to be contradictory. Is it not the case that an excessive DIRECTIONALLY stable aircraft is likley to be SPIRALLY UNSTABLE, which means that it has a tenedncy to enter a spiral dive?
Almost any aircraft is capable of entering a spiral dive (many do, for example, upon recovery from a spin). My understanding is that excessive spiral stability (which implies high lateral stability, or weak directional stability) defines difficulty in recovering from a spiral dive, rather than necessarily a tendency to enter one.
I don't think it's correct that spiral instability implies a tendency to enter a spiral dive, the co-incidence of terminology there is unfortunate. It happens that both spiral stability (defined as the tendency to return to wings level when disturbed, or not), the tendency to spin, the DR ratio, and the tendency to recover from a spiral dive, are all functions of the ratio between lat-dir stabilities. This makes them interdependent.
G
I don't think it's correct that spiral instability implies a tendency to enter a spiral dive, the co-incidence of terminology there is unfortunate. It happens that both spiral stability (defined as the tendency to return to wings level when disturbed, or not), the tendency to spin, the DR ratio, and the tendency to recover from a spiral dive, are all functions of the ratio between lat-dir stabilities. This makes them interdependent.
G
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Genghis,
I agree with much of what you have said, including the fact that spiral stability is determied by the relative magnitudes of lateral and directional stability.
Whan an aircraft is subjected to sideslip airflow its directional stability tends to yaw it into the sideslip, while its lateral stability tends to roll it away from the sideslip. The overall effect is (largely) determined by the relative magnitudes of these two modes of stability.
An aircraft in which the lateral stability is stronger than the directional stability will tend to roll away from sideslip to a greater extent than it yaws into it. Such an aircraft is not inherently liable to enter a descending spiral but (if damping is sufficiently weak) might exhibit dutch roll.
On the other hand, an aircraft in which the directional stability is stronger than the lateral stability, will yaw into the sideslip to a greater extent than it rolls away from it. This yawing into the sidelsip is likely to cause further roll into the sidslip. This repeated yaw - roll- yaw- roll action into sideslip will then result in a spiral dive.
Your comment regarding the WW1 aircraft implied (to me at least) that the spiral dive was caused by the excessively strong spiral stability. Many pprune readers are studying for their ATPL exams and I would hate any of them to go away with the idea that spiral stability leads to spiral dives. It does not. An aircraft that exhibits a tendency to enter spiral dives is in fact spirally unstable.
One of the things that students are required to know for the POF exam is that excessively strong lateral stability (in comparison to directional stability) gives a tendency to dutch roll, while excessively strong directional stability (in comparison to lateral stability) gives a tendency to spiral instability and spiral dives.
In answer to the original question, I suspect that this aircraft is designed to be slightly spirally unstable, because this is easier to deal with than dutch roll.
I agree with much of what you have said, including the fact that spiral stability is determied by the relative magnitudes of lateral and directional stability.
Whan an aircraft is subjected to sideslip airflow its directional stability tends to yaw it into the sideslip, while its lateral stability tends to roll it away from the sideslip. The overall effect is (largely) determined by the relative magnitudes of these two modes of stability.
An aircraft in which the lateral stability is stronger than the directional stability will tend to roll away from sideslip to a greater extent than it yaws into it. Such an aircraft is not inherently liable to enter a descending spiral but (if damping is sufficiently weak) might exhibit dutch roll.
On the other hand, an aircraft in which the directional stability is stronger than the lateral stability, will yaw into the sideslip to a greater extent than it rolls away from it. This yawing into the sidelsip is likely to cause further roll into the sidslip. This repeated yaw - roll- yaw- roll action into sideslip will then result in a spiral dive.
Your comment regarding the WW1 aircraft implied (to me at least) that the spiral dive was caused by the excessively strong spiral stability. Many pprune readers are studying for their ATPL exams and I would hate any of them to go away with the idea that spiral stability leads to spiral dives. It does not. An aircraft that exhibits a tendency to enter spiral dives is in fact spirally unstable.
One of the things that students are required to know for the POF exam is that excessively strong lateral stability (in comparison to directional stability) gives a tendency to dutch roll, while excessively strong directional stability (in comparison to lateral stability) gives a tendency to spiral instability and spiral dives.
In answer to the original question, I suspect that this aircraft is designed to be slightly spirally unstable, because this is easier to deal with than dutch roll.
Last edited by Keith.Williams.; 8th Dec 2002 at 09:31.
