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#181
02-22-2012, 04:15 PM
 brett s Gold Supporter Join Date: May 2004 Location: Roswell, GA Posts: 2,307

One thing similar for way of comparison in a single rotor helicopter is what tail rotor thrust does if you unload a teetering rotor ship. It'll start to roll...

Far less thrust involved, things happen much slower & can be recovered from but can still kill you if mishandled - like trying to correct the roll by adding lateral cyclic before loading the rotor, mast bumping is almost always fatal.
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#182
02-22-2012, 04:33 PM
 thomasant Member Join Date: Mar 2011 Location: Katy, Texas Posts: 342
Very Confusing.

Quote:
 Originally Posted by fara Dear GyroGreg, I am not extremely versed with gyroplane being new to this world but I am versed in general with aerodynamics. Perhaps you meant to use different words because it is in fact quite possible to have Positive Dynamic Stability while NOT having Positive Static Stability In describing technical concepts we should try and stick with the same language to understand things faster. I am sure that Magni is stable and damped nicely but you probably want to choose different words there to describe the concepts as they did indeed confuse me at first. In fact many times designers will design an aircraft with low or even negative static stability while keeping very good and positive dynamic stability to improve maneuverability of certain experimental aircraft.
I am confused with this statement. Positive static stability implies that the object or system will converge or return to equilibrium. If it is negative static stability, it will continue to diverge. So how can it be dynamically stable if it cannot return to equilibrium? I also understand that in order to be positively dynamically stable, the oscillations will overshoot the equilibrium point and reverse, but the amplitude of the oscillations will decrease till the oscillations are dampened completely and equilibrium is again restored.

Negative static stability can be incorporated into the design feature, but probably in one axis, but I cannot see how it can also be dynamically stable in the process. Also as an example, it could be statically and dynamically unstable in the yaw axis, but statically and dynamically stable in the roll and pitch axes. Could anyone verify the above please?
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Sport Gyroplane
AGI

Last edited by thomasant; 02-22-2012 at 04:48 PM.
#183
02-22-2012, 08:38 PM
 fara Gold Member Join Date: Oct 2011 Location: Tampa, FL Posts: 743

Quote:
 Originally Posted by L_Butler Fara, I applaud your desire to standardize on terminology. It is a notorious problem. However we disagree on terms. It is not possible to have dynamic stability if you don't have static stability. Without outside forces a statically unstable object will diverge and cannot be brought back to equilibrium. Dynamic stability is meaningless. Bicycles are statically unstable. They are not dynamically stable, they are controllable. They can be returned to their original position by the driver exerting forces that change the static forces to return to equilibrium. However, this is control not dynamic stability. Bicycles are only "stable" when the driver is included in the system to control and direct the inputs. In aircraft, it is desirable to have a craft that is stable unto itself without the interaction of the pilot. Many unstable aircraft can be flown, I have been told that the Wright Bros aircraft was unstable. But it was controllable. Greg and I have had a few discussions about why the Magni appears to have several characteristics that would want to make the craft unstable, yet it seems to react in a stable manner. I don't understand the reason and I would love to get a rational explanation. Doug also suggested that it is statically stable, but I would like a simple force diagram to show what is happening. My lesson in the Magni was pleasant but I didn't have enough experience at the time to feel the difference between stability and apple sauce. Thanks, Larry
Dear Larry, I am not sure about that. How do you simply proclaim that because I know otherwise? If you have any source beyond these discussions, please let me know. It is quite possible I am missing something.

Positive static stability is the initial response of the aircraft from a disturbance. As soon as the disturbance force is removed, the aircraft to be called to have positive static stability should at least in part show a return towards original position.
If the aircraft is disturbed and the disturbance force is quickly removed and it (the aircraft) continues further towards the direction of its disturbance but then stops at a certain amplitude and then returns back towards its original position and then say perhaps overshoots it and then stops and returns back, overshoots again with decreasing amplitude till it settles ... well this aircraft has negative static stability slightly but positive dynamic stability.

Try and read this and follow it through. I assure you such aircraft exist. Dynamic stability is the aircraft's response overall over time. This should be covered properly in all ground school 101 but either we as pilots forget it or its not covered in as much detail as to cover these finer points.

I just searched the web for some easy link enabled references instead of giving you some flight dynamics or aerodynamics for engineers (good book btw) book reference that you may not have in hand. Here are some:

"Occasionally, the initial tendency is different or opposite from the overall tendency, so distinction must be made between the two. Dynamic stability is the overall tendency that the airplane displays after its equilibrium is disturbed."
Source: From Dynamic Stability section of http://www.free-online-private-pilot...ronautics.html

"An airplane that has positive dynamic stability does not automatically have positive static stability. The designers may have elected to build in, for example, negative static stability and positive dynamic stability in order to achieve their objective in maneuverability. In other words, negative and positive dynamic and static stability may be incorporated in any combination in any particular design of airplane."
Source: http://www.allstar.fiu.edu/aero/axes33.htm

"Dynamic Stability
Static stability has been deﬁned as the initial tendency to return to equilibrium that the aircraft displays after being disturbed from its trimmed condition. Occasionally, the initial tendency is different or opposite from the overall tendency, so a distinction must be made between the two. Dynamic stability refers to the aircraft response over time
when disturbed from a given AOA, slip, or bank"

Source: Pilot's Handbook Of Aeronautical Knowledge (PHAK) by FAA, Chapter 4
http://www.faa.gov/library/manuals/a...apter%2004.pdf

I hope this suffices.

I am sure there are gyroplanes out there that are neutral or even slightly negative on static stability in some axis but are dynamically stable in the same otherwise. In fact, I will check and do some careful test observations on the one I am learning on right now (Dominator). I believe it may have this characteristic but I was not doing a proper test pilot like observation so I may be wrong about that.

Designers would design slight negative or neutral static stability condition in flight dynamics of their aircraft while keeping its positive dynamic stability intact to give the aircraft a more sporty feel, more maneuverable while keeping it controllable.

Last edited by fara; 02-22-2012 at 08:49 PM.
#184
02-23-2012, 06:02 AM
 thomasant Member Join Date: Mar 2011 Location: Katy, Texas Posts: 342

IMHO, I still do not see how a statically unstable system (negative static stability) can return to trim if it is diverging from the trimmed condition, unless acted upon by some external force. Can anyone explain this please?

I will not bring in dynamic stability at this point.
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AGI
#185
02-23-2012, 06:53 AM
 fara Gold Member Join Date: Oct 2011 Location: Tampa, FL Posts: 743

Well, now we are getting into engineering and differential equations that are beyond the scope here. But a good Flight Dynamics book would be a good read.
Many RV-8 airplanes for instance, have a reversed force gradient on the stick when flown at aft CG limit in slow flight regimen. Simply not large enough tail surface to completely balance out the aircraft positively in that combination of factors but designer left out larger tail surfaces to compromise "feel" in other areas. One can put positive camber in elevator there or trailing edge trim tab bent downward to alleviate this a bit. The airplane overall exhibits positive dynamic longitudinal stability.

Please note that this does not generally hold true over the whole flight range of an aircraft. It could be simply at one spot or one small range. Slight negative static stability fixes itself because of dynamics. As the attitude of the aircraft diverges, things do not remain the same. They can and should get out of the range of this small negative stability and then dynamically can balance themselves positively. If this doesn't happen then it is a badly negotiated compromise by the designer. Its not a very desirable aircraft to fly. Though a good number of fighter jets are like that and need computers to fly them as humans by nature are not very good at measuring distances and rather judge the divergence amplitude by force feel

Last edited by fara; 02-23-2012 at 07:25 AM.
#186
02-23-2012, 10:07 AM
 L_Butler N41423 Join Date: Nov 2003 Location: Dillinger's Station, PA About 10 miles south of Allentown Posts: 239
Stability

Hello Fara,
I guess it depends on your sources. My textbook on statics and dynamics has a slightly different definition. It is summarized in the graph below. I don't work in the field of aerodynamics so I'm not surprised when I hear different terminology for terms that I use in my work.

Mark's Standard Handbook for Mechanical Engineers states clearly that static stability is necessary to obtain dynamic stability, but static stability does not ensure dynamic stability.

It is unfortunate when different fields use different terminology that conflicts. Having a discussion of the phenomenon is extremely difficult.

Thanks,
Larry
Attached Thumbnails

#187
02-23-2012, 10:17 AM
 Doug Riley Platinum Member Join Date: Jan 2004 Posts: 5,388

A-men, Larry! We end up "talking about talking" instead of getting anything done. Meta-talk.

In terms of the diagram, I think the Magni's static reaction to some types of disturbance could be graphed using the red line that is there, but extending it to the right (over time) so that its slope reverses and it proceeds "downhill" back to equilibrium after some interval. Obviously, that represents the first half of a pitch oscillation, so it's very important that there be enough damping to kill the oscillations promptly. I understand that there is.

The classic static-dynamic terminology does not include a name for this type of "rubber band" behavior. Possibly aircraft test pilots have a name for it.
#188
02-23-2012, 11:18 AM
 gyrogreg Senior Member Join Date: Dec 2003 Location: Ste. Genevieve, Missouri, USA Posts: 967
Confused?

I think some of these discussions are having issues in how STATIC stability is determined to declare that an aircraft is statically stable.

If you use only the STATIC sum of moments that add up to the wing or Rotor Lift Vector being forward of the CG, some people, and many on this forum, seem to declare that aircraft to be statically unstable. If there were no dynamic damping utilized, that aircraft WILL very likely exhibit static instability - a divergent response to a disturbance (buntover in gyro parlance).

Paper determined: If you use strictly the sum of static moments to determine the physical location of the CG relative to the wing or rotor lift vector, some might declare that aircraft statically unstable. (The paper determination is more incomplete and misleading when it does not, or cannot easily, include all static moments on the airframe such as the airspeed induced drag line or the pitching moment from a slanted windscreen.)

Flight Test determined: If you flight test this aircraft and find that it tends to return to its original static trimmed state, and/or it actually oscillates about that original static trimmed state, this same aircraft would be determined, by flight testing, to be statically stable.

Flight testing is the proper way to determine static stability, and the only way to determine dynamic stability. If you employ a good dynamic pitch damper, such as a large HS on a long tail, then the flight characteristics may not be divergent for even the paper determined statically unstable aircraft; but instead may actually be convergent with the aircraft oscillating about its static steady state condition. Those oscillations may indeed be decaying (damping – Positive DYNAMIC stability), constant amplitude (Neutral DYNAMIC stability), or growing (Negatively DYNAMICALLY stable). The strength of the dynamic damping determines the DYNAMIC stability and how quickly the oscillations may damp out (positive dynamic stability) or grow to extremes (negative dynamic stability). The difference in the two determination methods above is that, for the second, DYNAMIC damping (and the sum of all the moments and parts) is included in that determination.

For my determination of static stability I use the Flight Test determined method – the paper determined method does not reflect all the elements both static and Dynamic, involved. But, if you use only the paper determined method, using only static elements that do not consider dynamic elements, one might find the RTV forward of the CG and then argue that that aircraft is statically unstable, but still dynamically stable. When a system actually displays static stability characteristics, such as in a flight test, it is statically stable and therefore capable of oscillations and possibly be positively dynamically stable.

But, this may not even be the pitfall that Fara has fallen into in this RV explanation. The condition that is described for the RV is simply a lack of static trim ability of the smaller tail, not really a reflection of negative static stability:

Fara states that that the tail surface is simply not large enough to “balance out the aircraft”. I do not believe that he is saying this aircraft has an actual CG aft of the wing lift vector. He seems to be saying that, at slow speed the HS is not large enough to raise the nose (forward CG) to prevent increasing airspeed. The size of the HS in a fixed wing has nothing to do with the position of the CG. So, if the CG is forward of the wing lift vector at higher airspeeds, then it is still forward of the wing lift vector at slower airspeeds – CG does not change with airspeed or power on a FW – still statically stable as determined on paper! This argument is simply a static sum of balances argument, not an argument that it is statically unstable. That RV is not statically unstable! If the HS is not large enough at slow airspeed to hold the nose up, the aircraft simply noses down and gains airspeed until the tail forces are large enough to “balance” the forward CG at a new “trimmed” airspeed. This response is not an indication of static instability – the CG is still forward of the wing lift vector.

In Fara’s description of the RV, there is no consideration of the DYNAMIC effect of the tail – only the STATIC “balancing” effect of the tail. Since this RV is actually still statically stable, it would also show oscillations about the new “trimmed” condition and would probably still exhibit dynamic stability (damped oscillations) due to the dynamic damping provided by even the smallish HS – on a long tail.

Fara, I mean no disrespect, but I believe that you are still confusing STATIC elements with DYNAMIC elements. Your arguments above seem to be stuck in or confused with static elements. The tail of the RV, even though it might be a bit undersized to maintain a minimum trimmed airspeed, still will provide dynamic pitch damping. “Balance” is a STATIC term. “DYNAMIC damping” doesn’t balance anything – it only provides a force or moment in the opposite direction to movement, and only to movement (i.e. the HS rising as the nose drops.)

For the record:
• A statically unstable system cannot be dynamically stable. A statically unstable system will simply diverge, and never attempt to return to its static trimmed condition or oscillate about it. For a statically unstable system, the term dynamic stability does not even apply – it never oscillates, just simply diverges. There are no oscillations to damp – reducing oscillations, damped oscillations, are the definition of dynamic stability. If it does not even oscillate, dynamic stability does not apply. BUT, “DYNAMIC DAMPING” does still apply – the amount of damping determines how quickly the statically unstable system diverges when disturbed from its “trimmed” condition. (For instance the difference between a rapid buntover, or a slowly diverging dive into the ground.)

• A statically stable system may be dynamically stable, but that depends on adequate dynamic damping. A statically stable system will likely oscillate around its static trimmed condition. With enough dynamic damping, those oscillations will reduce in amplitude (dynamic stability). Ideally, the dynamic damping will be enough to reduce those oscillations to zero quickly – so as to not require pilot intervention to stop the oscillations. When oscillations are not even detectable, that means the oscillations die in less than one cycle – “critically” damped. This level of dynamic damping is important for very quick oscillation tendencies in order to not excite the pilot into over control at those quick AOA oscillation rates – possibly exciting PIO.

Thanks, Greg
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#189
02-23-2012, 11:30 AM
 gyrogreg Senior Member Join Date: Dec 2003 Location: Ste. Genevieve, Missouri, USA Posts: 967

Quote:
 Originally Posted by Doug Riley In terms of the diagram, I think the Magni's static reaction to some types of disturbance could be graphed using the red line that is there, but extending it to the right (over time) so that its slope reverses and it proceeds "downhill" back to equilibrium after some interval. Obviously, that represents the first half of a pitch oscillation, so it's very important that there be enough damping to kill the oscillations promptly. I understand that there is.
Doug, you really need to take a ride in the Magni again. There is no divergent reaction to a disturbance, commanded or un-commanded. The green line is closer to the Magni reaction - and the Auto Gyro, the ELA, the Xenon - all with long and large tails. The Magni oscillations have about 13 second period and decay in about 1-1/2 oscillations - at all airspeed and power combinations. This is the case for both fixed stick and free stick.

I'm not sure where this description of an unstable reaction that becomes a stable reaction comes from? From all flight testing done by many and by professionals, it displays strong static AOA, Airspeed and G-Load static stability and highly damped dynamic stability - immediately. I would sure like to fly with you again - at Mentone? I promise we will put it through all these flight tests - maybe take about 15 minutes! It's that easy!

- Greg
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#190
02-23-2012, 01:23 PM
 Jazzenjohn Gold Member Join Date: Oct 2004 Location: Franklin Mich. Posts: 2,256

I believe there are statically unstable systems that are dynamically stable. An example might be the Segway. Quite unstable statically but stable dynamically. A system that has more than one influence on it's stability can be unstable statically if the dynamic component of it's stability overcomes or becomes the dominant force in a dynamic situation.

This is the way I've always understood it. If a gyro were at the apex of a zoom climb and pushed over, the dominant force would be the engine. If it had 300 pounds of thrust at a spot 1 foot above the CG there would be an overturning force of 300 foot pounds. Looked at instantaneously, the HS would have negligible influence if it were out of the airstream and at no airspeed, and the rotor has little or no authority because it is unloaded. Dynamically, the 300 foot pound rotational force tries to move a 12 square foot stab on an 8 foot stick through the wind quickly. There is a great resistance to that, which, hopefully, slows the forward rotation enough to allow the rotor to regain authority and a new equilibrium is then established. This seems a logical and intuitive view of the dominant forces to me.
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#191
02-23-2012, 06:22 PM
 fara Gold Member Join Date: Oct 2011 Location: Tampa, FL Posts: 743

No disrespect taken Greg. I love to learn just need references from aviation engineering documents or similar to be convinced. I have been wrong more than I care to admit .
BTW, regardless of the little hickup positive dynamic stability which is anything further than the very initial reaction to a disturbance, is much more important in real life.

Well perhaps PHAK wasn't emphatic or clear enough but then there was a .edu link as well. Well here is a Georgia Inst. Of Tech lecture notes in a Microsoft Powerpoint (viewer or PowerPoint needed)
http://tinyurl.com/7e3g4a6

Look for PowerPoint slide titled:
"Aircraft may be statically unstable, but dynamically stable"

Last edited by fara; 02-23-2012 at 06:30 PM.
#192
02-23-2012, 06:47 PM
 thomasant Member Join Date: Mar 2011 Location: Katy, Texas Posts: 342
Powerpoint slides

Hi Fara,

Thank you for the link to the PPT. Looking at slides 6 and 7 it is quite clear from the graph that the oscillations are similar. In both cases the nose first pitches up, but the fact that they both return to trim right away and then overshoot and finally dampen completely illustrates Positive static stability according to all known teachings. If it were negative, then it would be impossible for a return to trim.

I would very much doubt the validity of slide 6 because it goes against the very basic definition of static stability. IOW, the person who has compiled the PPT seems to have misinterpreted the concept of negative static stability. Just my thoughts.
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Sport Gyroplane
AGI
#193
02-24-2012, 12:58 AM
 Jean Claude Gold Member Join Date: Jan 2009 Location: Centre FRANCE Posts: 899

Yes Thomas. IMHO:

#194
02-24-2012, 05:10 AM
 thomasant Member Join Date: Mar 2011 Location: Katy, Texas Posts: 342

Hi Jean Claude,

Very nice representation. I like the way the moments have been depicted. Thank you.
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Antony Thomas
Commercial/Instrument Helicopter
Sport Gyroplane
AGI
#195
02-24-2012, 05:58 AM
 fara Gold Member Join Date: Oct 2011 Location: Tampa, FL Posts: 743

Antony,
I wish it was as simple as a professor at Georgia Tech not knowing what he was talking about in a full slide very deliberately pointed out. It has to do with the finer point of definition though I am not sure in practicality it matters because you will arrive at a stable aircraft if you have dynamic stability because that is what the test determines in practicality and that is what matters.

When a disturbance force is applied to an aircraft and it is removed, the continued movement of the aircraft past the point of removal of that vector well that's what's in question. Some aircraft will continue in the direction of the disturbance threw it towards for some time before stopping and changing momentum back towards trim.

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