GA Aircraft Long Period Pitch Osc.
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GA Aircraft Long Period Pitch Osc.
Only a short question; I've noticed when flying a couple of the more stable light aircraft (e.g. Cessna 182) there's a very small but noticeable long period pitch oscillation when stick-free. I seem to recall it was around the 30 second mark, seeing a variation of a couple of knots or so.
Any ideas as to what might be behind it? The loading was more aft than usual on the most notable occurrence, perhaps the stick free static margin reached a fairly low to neutral value?
Any ideas as to what might be behind it? The loading was more aft than usual on the most notable occurrence, perhaps the stick free static margin reached a fairly low to neutral value?
Pretty much all aeroplanes (including little aeroplanes) will phugoid to some extent, although since it's invariably a nuisance mode, you'd hope that it'll be well damped.
Classical theory, with a bit of crunching, should give you a phugoid period in seconds of about 0.85 of the trimmed speed in knots. In practice this is hardly ever the case and the period is usually rather faster - as you've observed.
I'm not sure I've ever seen a rigorous study of what causes the phugoid in light aeroplanes, but in my experience it's usually elasticity and inertia in the pitch control circuit. In particular long and reasonably thin elevator / stabilator cables can usually give quite a marked and lightly damped phugoid.
G
Classical theory, with a bit of crunching, should give you a phugoid period in seconds of about 0.85 of the trimmed speed in knots. In practice this is hardly ever the case and the period is usually rather faster - as you've observed.
I'm not sure I've ever seen a rigorous study of what causes the phugoid in light aeroplanes, but in my experience it's usually elasticity and inertia in the pitch control circuit. In particular long and reasonably thin elevator / stabilator cables can usually give quite a marked and lightly damped phugoid.
G
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Cheers for the reply. Just about to start the Dynamics module here at Uni (2nd year AeroEng), hopefully will get a bit more of an insight into the mechanics behind it.
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Glasgow Flying Club have two Tomahawks - one has a dutch roll in cruise that if you try to sort just becomes worse. The other doesn't. Both Tommy 1's but one is 70lbs heavier than the other and has bigger wheels.
I never could figure them out, but the dutch roller really annoyed me.....
I never could figure them out, but the dutch roller really annoyed me.....
Glasgow Flying Club have two Tomahawks - one has a dutch roll in cruise that if you try to sort just becomes worse. The other doesn't. Both Tommy 1's but one is 70lbs heavier than the other and has bigger wheels.
I never could figure them out, but the dutch roller really annoyed me.....
I never could figure them out, but the dutch roller really annoyed me.....
G
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Phugoid
Phugoid is a classical flight dynamic in longitudinal axis. The other is the short period.
Its been a while I've done one for real, but the phugoid can be excited by trimming the AC at a speed (IAS) and noting the pitch attitude/AoA. Next rapidly bleed off about 10KIAS and then rapidly re-establish original pitch attitude/AoA . The above should excite the phugoid.
Control linkage play should be irrelevant to the phugoid period, I think, but I wouldn't bet a pint on that without more research.
Its been a while I've done one for real, but the phugoid can be excited by trimming the AC at a speed (IAS) and noting the pitch attitude/AoA. Next rapidly bleed off about 10KIAS and then rapidly re-establish original pitch attitude/AoA . The above should excite the phugoid.
Control linkage play should be irrelevant to the phugoid period, I think, but I wouldn't bet a pint on that without more research.
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Re the last. Not so sure. Since a Phugoid involves a series of slight bunts and pulls, if there is freeplay in the controls it's possible that the 'g' loads on mass-unbalanced portions of the control system may cause them to drift around in their backlash. That might cause pitch inputs during the manoeuvre, either exaggerating or damping the underlying mode, and changing the frequency too. I've seen a non-sinusoidal Phugoid, which we attributed to the changing pitching moments as we wandered up and down the Mach range, as a consequence of which we saw pronounced variations in alpha, something not in the classical Phugoid. If those PM changes had been due to control movements instead, it would have looked similar I suspect.
For example, if the elevator had a bit of backlash, and was mass-unbalanced to be heavy aftwards, during the bunts you'd have the trailing edge tend to drift very slightly upwards, tending to pull against the bunt, which would tend to damp the motion. Correspondingly, during the pulls the elevator would tend a bit t/e down, reducing the pulls. If the balance were other way around, I think there'd be a destabilising tendency.
For example, if the elevator had a bit of backlash, and was mass-unbalanced to be heavy aftwards, during the bunts you'd have the trailing edge tend to drift very slightly upwards, tending to pull against the bunt, which would tend to damp the motion. Correspondingly, during the pulls the elevator would tend a bit t/e down, reducing the pulls. If the balance were other way around, I think there'd be a destabilising tendency.
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I don't know if this adds much, but an early ultralite, the American Eagle, had a pronounced phugoid at cruise. For those who never risked their lives in one, they had a swept tube-and-fabric wing with vertical "rudders" at its tips for directional control, and a canard that flew in trail except in the landing flare. The 25 Hp engine with belt-driven prop was immediately behind the pilot; both were under the wing. There was no tail. Pitch control was solely via engine power, and directional control, such as it was, was via the wingtip "rudders", in quotes because they operated like drag devices, very much like flying a wingtip through a treetop. Please don't enquire how I know this.
The phugoid period was approx 5 seconds, and only went away with power off descent, or a full power climb. Very disconcerting, but you got used to it.
These aircraft managed to fold their wings all too readily, in a time before ballistic parachutes were available. I doubt if any are still flying. Had I known how weak the wing was, I might have stayed on the ground.
The phugoid period was approx 5 seconds, and only went away with power off descent, or a full power climb. Very disconcerting, but you got used to it.
These aircraft managed to fold their wings all too readily, in a time before ballistic parachutes were available. I doubt if any are still flying. Had I known how weak the wing was, I might have stayed on the ground.
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As an old and now not so bold TP I recently had the opportunity to assess the characteristics of a C172 on a long cruise. The pitch control had about a half pound breakout which made it a bitch to dampen out its natural phugoid complicated by some indefinable free float of elevator between the breakout stiction which seemed to be sourced where the control went through bushes in or near the instrument panel. This all meant that here was an aircraft that tried to fly in trim with a terrible mix of stick free and stick fixed stability.
But not as bad as the RAF's Comet 2C which had 11 pnds breakout on elevator into spring feel. Ugh. Was forced to fly it in pitch using a big convenient trim wheel. Yes - there was a strong recommendation to convert it to Q feel and minimise breakout.
But not as bad as the RAF's Comet 2C which had 11 pnds breakout on elevator into spring feel. Ugh. Was forced to fly it in pitch using a big convenient trim wheel. Yes - there was a strong recommendation to convert it to Q feel and minimise breakout.
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Genghis
cc Mad Scientist
Where did you get that formula from for the natural period of a phugoid?
"phugoid period in seconds of about 0.85 of the trimmed speed in knots."
cc Mad Scientist
Where did you get that formula from for the natural period of a phugoid?
"phugoid period in seconds of about 0.85 of the trimmed speed in knots."
Last edited by Genghis the Engineer; 3rd Feb 2008 at 10:10. Reason: Finger trouble!
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Have some vague recollection that in the days of astro nav they used to apply corrections to the shots taken which were aircraft specific due to the natural phugoid motion about all axis. Or have I lost it all - finally. 
Milt,
I derived it years ago from the equations in Donald McClean's "Automatic Flight Control Systems". (Prof. McClean was one of my flight mechanics teachers when I was a student, although I think he's been retired almost a decade now).
If it's important PM me and I'll dig my copy out and copy/post the relevant pages for you (since I think it's been out of print for about a dozen years).
G
N.B. Apologies for accidentally editing your post - finger trouble on my part. Hopefully I've put it back the way you wrote it.
I derived it years ago from the equations in Donald McClean's "Automatic Flight Control Systems". (Prof. McClean was one of my flight mechanics teachers when I was a student, although I think he's been retired almost a decade now).
If it's important PM me and I'll dig my copy out and copy/post the relevant pages for you (since I think it's been out of print for about a dozen years).
G
N.B. Apologies for accidentally editing your post - finger trouble on my part. Hopefully I've put it back the way you wrote it.
These aircraft managed to fold their wings all too readily, in a time before ballistic parachutes were available. I doubt if any are still flying. Had I known how weak the wing was, I might have stayed on the ground.
G
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Only a short question; I've noticed when flying a couple of the more stable light aircraft (e.g. Cessna 182) there's a very small but noticeable long period pitch oscillation when stick-free. I seem to recall it was around the 30 second mark, seeing a variation of a couple of knots or so.
Most longitudinally stable aircraft do that to some extent. It is generally assumed that angle of attack and forward speed are approximately constant, and that there is a slow exchange of kinetic and potential energy around a constant lift (ie product of lift coefficient and square of velocity). Using this assumption and a bit of basic oscillation mechanics, you can model the period of the phugoid as PIE*SQRT(2xV/g).
That's assuming that there is no damping of the oscillation. Sometimes the oscillation can be divergent which can be quite interesting, but normally it is convergent and of such long period that the pilot is able to control it easily.
It is caused by small changes in tailplane incidence causing the centre of lift to move. With rearward CG the moment between lift centre is greater so the oscillation is more pronounced - as you have noticed.
I haven't heard of it being due to slack in the control runs. I have, however, experienced marked pitch oscillations on some aircraft while under autopilot due to a small amount of play in the elevator control runs causing the ap to be very slightly dephased from the elevator. However, this is not a true aerodynamic phugoid and could best be described as a pilot induced oscillation.
Most longitudinally stable aircraft do that to some extent. It is generally assumed that angle of attack and forward speed are approximately constant, and that there is a slow exchange of kinetic and potential energy around a constant lift (ie product of lift coefficient and square of velocity). Using this assumption and a bit of basic oscillation mechanics, you can model the period of the phugoid as PIE*SQRT(2xV/g).
That's assuming that there is no damping of the oscillation. Sometimes the oscillation can be divergent which can be quite interesting, but normally it is convergent and of such long period that the pilot is able to control it easily.
It is caused by small changes in tailplane incidence causing the centre of lift to move. With rearward CG the moment between lift centre is greater so the oscillation is more pronounced - as you have noticed.
I haven't heard of it being due to slack in the control runs. I have, however, experienced marked pitch oscillations on some aircraft while under autopilot due to a small amount of play in the elevator control runs causing the ap to be very slightly dephased from the elevator. However, this is not a true aerodynamic phugoid and could best be described as a pilot induced oscillation.
