Helicopter Dynamics: Gyroscopic Precession
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Helicopter stability
Hi all! New the the forum but wanted to chime in with a quick question. I've never understood why exactly the main rotor acts like a gyro in some cases but not others. On the one hand we have gyroscopic precession but on the other there doesn't seem to be any gyro stabilization. Counter intuitively, the opposite seems to be true, in that the helicopter is an extremely unstable aircraft and requires a high level of pilot control. Any input would be very welcome!
"Precession" is just a simple way of getting a student to comprehend the way the blade reacts with phase lag and advance angle. It is not exactly 90 degrees, which is what precession must have, but varies between 90 degrees and around 72 degrees, depending on the rotor system.
Stability is a big area to delve into. An aircraft must first have static stability, i.e. you displace it, and it wants to return to its original position.
Add some moving air, and you are looking at dynamic stability - will it return to its position, with a few oscillations, and settle down (dynamically stable), or will it just keep using the energy from the airflow to continually go from side to side through its original position, not decreasing or increasing (dynamically neutral stability) or does it go into a rapidly increasing oscillation through its original position until it breaks apart or crashes? (Dynamically unstable.)
Helicopters are generally dynamically unstable in pitch and roll, but have some stability in yaw due to the weathercock effect keeping the tail behind the centre of gravity.
Nothing to do with gyroscopes.
Stability is a big area to delve into. An aircraft must first have static stability, i.e. you displace it, and it wants to return to its original position.
Add some moving air, and you are looking at dynamic stability - will it return to its position, with a few oscillations, and settle down (dynamically stable), or will it just keep using the energy from the airflow to continually go from side to side through its original position, not decreasing or increasing (dynamically neutral stability) or does it go into a rapidly increasing oscillation through its original position until it breaks apart or crashes? (Dynamically unstable.)
Helicopters are generally dynamically unstable in pitch and roll, but have some stability in yaw due to the weathercock effect keeping the tail behind the centre of gravity.
Nothing to do with gyroscopes.
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Ok, that makes sense. It always seemed like "precession" was a little glossed over. So the main rotor isn't a gyro. But why? Certainly it has some angular momentum. Would a rigid rotor act like one? If you connected the blade tips with a ring so that it more closely resembled our toy gyros would that change matters? Why DOESN'T it act like one?
Connecting blade tips with a ring? That would make it act like the toy helicopters, and crash.
Allow the blades to flex, flap, lead and lag, and feather. All things that a gyroscope can't do. That stops it doing real precession. But it helps people to understand phase lag, so it keeps popping up.
Read the Nick Lappos posts, now that this is merged. Despite what Dave Jackson or the late Lu Zuckerman might say, the rotor is not a gyroscope.
Allow the blades to flex, flap, lead and lag, and feather. All things that a gyroscope can't do. That stops it doing real precession. But it helps people to understand phase lag, so it keeps popping up.
Read the Nick Lappos posts, now that this is merged. Despite what Dave Jackson or the late Lu Zuckerman might say, the rotor is not a gyroscope.
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Thanks for linking these threads. I figured this must have been tackled before. I asked for more in depth and it seems like I've got it now. I've made it through about half of the first page; please excuse me while I dunk my head in a bucket of ice water 
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The rotor is a gyroscope.
It doesn't matter whether it interacts with air or not - it's because it has an angular momentum vector along the mast axis. It doesn't matter whether the blades twist, lag or flap.
If you apply a couple normal to this axis, it will turn around the mutually perpendicular axis. This might not be a big effect compared with aerodynamic forces, or it might be.
Can you snap (or aileron) roll a helicopter?
It doesn't matter whether it interacts with air or not - it's because it has an angular momentum vector along the mast axis. It doesn't matter whether the blades twist, lag or flap.
If you apply a couple normal to this axis, it will turn around the mutually perpendicular axis. This might not be a big effect compared with aerodynamic forces, or it might be.
Can you snap (or aileron) roll a helicopter?
Can you snap (or aileron) roll a helicopter?
The gyroscope effect is stuff-all. The way the blade reacts is simply Newtonian. Apply a force to the blade, you get acceleration, which takes time. While the blade is starting to climb (or fall) due to the aerodynamic force, it is also still turning, and by the time it reaches its highest or lowest point, it has turned approximately 90 degrees. The force in the original direction has reversed, and the blade starts to accelerate in the other direction, and away we go again.
There is no force being applied 90 degrees off the axis to create precession, the force is being applied through a full 360 degrees of travel of the blade in a sinusoidally variable amount, via the swashplate.
The force is instant, the resulting movement takes time. A high-inertia blade will react slower than a piddly little R22 blade - hence the 72 degrees of advance in that system - look up Lu Zuckerman's "missing 18 degrees" thread.
Try to spin around on the spot with a bucket in your hand, and raise your arm to shoulder level while spinning. See how far around the circle you get before the bucket is up there. Then put water in the bucket and see if the gyroscope theory works now.
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Look at the design of the bearings on the mast. Roll right, you hammer them fore and aft. Nose down, you stress them left and right. That's where the gyroscopic forces act.
A barrel roll is more gradual. How fast can you barrel roll? What sets that limit?
You can't beat the system - and gyroscopes are Newtonian objects.
An individual blade isn't a very powerful gyroscope, but the cuff still needs to deal with the orthogonal kick when moving it in pitch. The rotor disk is a more powerful gyroscope: by a factor of the number of blades.
Editted: I think I see - I had in mind a picture where a spinning fan was being swung around and pointed by a seated person, whereas it's aerodynamic forces on the blades that do all the pointing, just gently shepherded by the control movements. There will be a maximum rate at which they can reorient the disk, but it doesn't have anything to do with imposing motion from the hub.
A barrel roll is more gradual. How fast can you barrel roll? What sets that limit?
You can't beat the system - and gyroscopes are Newtonian objects.
An individual blade isn't a very powerful gyroscope, but the cuff still needs to deal with the orthogonal kick when moving it in pitch. The rotor disk is a more powerful gyroscope: by a factor of the number of blades.
Editted: I think I see - I had in mind a picture where a spinning fan was being swung around and pointed by a seated person, whereas it's aerodynamic forces on the blades that do all the pointing, just gently shepherded by the control movements. There will be a maximum rate at which they can reorient the disk, but it doesn't have anything to do with imposing motion from the hub.
Last edited by awblain; 5th Mar 2014 at 11:21. Reason: Misunderstanding
awblain, please read the first two pages of this merged thread. The concept that gyroscopic precession has anything to do with how the rotor disk responds to control inputs has been debunked. Your example of a rotor disk acting like a gyroscope only applies to a (very) rigid rotor system.
awblain, I have saved you the trouble.
This is from Nick Lappos, test pilot extraordinaire:
This is from Nick Lappos, test pilot extraordinaire:
Dave Jackson said:
I believe that a simple teetering rotor, with no delta3, exhibits a 90-degree phase lag, which is totally caused by the blade flying to position. Perhaps aerodynamic precession, but not gyroscopic precession.
Nick Sez:
Bravo Dave! You are absolutely correct. The old bugaboo about Gyroscopic Precession is quite mythological, but almost impossible to squelch, because it seems so plausible, and the real issues are so difficult to describe intuitively.
Basically, the whirling blade flaps at a natural frequency that depends on the centrifugal force to return it to its normal position. Picture the blade with a strong pair of springs, one below and one above the blade that oppose its flapping motion. If we pull the blade tip down, and let go, it will bounce a bunch of times like a diving board. Gor a helicopter, there are no springs, (an elastomeric rotor has such little spring force it changes this not a bit) but there is a strong centrifugal force that opposes a flapping motion. This force is a spring term that acts just like that diving board.
It turns out (in math that gets pretty stinky) that the blade resonates at 1 per rev, because the centrifugal spring changes its force with rpm, so it always allows the blade to resonate at its whirling rpm frequency. It also turns out that the phase relationship between the cyclic pitch (swash plate angle) and the tip path is about 90 degrees, depending on a bunch of blade properties.
Gyroscopes have nothing to do with it at all!
I just reaffirmed my understanding by re-reading a good source (for science and engineering pros):
Stepniewski & Keys - ROTARY - WING AERODYNAMICS
New York: Dover Pub.
I believe that a simple teetering rotor, with no delta3, exhibits a 90-degree phase lag, which is totally caused by the blade flying to position. Perhaps aerodynamic precession, but not gyroscopic precession.
Nick Sez:
Bravo Dave! You are absolutely correct. The old bugaboo about Gyroscopic Precession is quite mythological, but almost impossible to squelch, because it seems so plausible, and the real issues are so difficult to describe intuitively.
Basically, the whirling blade flaps at a natural frequency that depends on the centrifugal force to return it to its normal position. Picture the blade with a strong pair of springs, one below and one above the blade that oppose its flapping motion. If we pull the blade tip down, and let go, it will bounce a bunch of times like a diving board. Gor a helicopter, there are no springs, (an elastomeric rotor has such little spring force it changes this not a bit) but there is a strong centrifugal force that opposes a flapping motion. This force is a spring term that acts just like that diving board.
It turns out (in math that gets pretty stinky) that the blade resonates at 1 per rev, because the centrifugal spring changes its force with rpm, so it always allows the blade to resonate at its whirling rpm frequency. It also turns out that the phase relationship between the cyclic pitch (swash plate angle) and the tip path is about 90 degrees, depending on a bunch of blade properties.
Gyroscopes have nothing to do with it at all!
I just reaffirmed my understanding by re-reading a good source (for science and engineering pros):
Stepniewski & Keys - ROTARY - WING AERODYNAMICS
New York: Dover Pub.
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I strongly disagree. I think the disagreement is partly semantic, and this is about how each blade is pushed or pulled as the control input is added. However, I suggest that the dynamics of the whole aircraft is strongly affected by the angular momentum of the disk.
Whether the blade flaps up and down has little relevance for where the force is applied to the bearings as you try to twist it or lift it over periods longer than the rotation rate.
The designers make the cuffs and mast tough enough to cope, but those gyroscopic forces exist and must be accommodated. If the designers do a good job, then you won't feel their effects, but they are there.
It doesn't matter if the rotor's attached with rigid steel plates or cables. I agree that aerodynamic forces are more important, but you can't discount that gyroscopic effects are also at work and are dealt with by the design.
To avoid the confusing issue of advancing and receding blades, come to a hover. What forces act on the bearings/hub/rams when you apply small cyclic inputs?
And back to flying along:
What is the maximum rate of pitch, roll or yaw?
Knock off the tail rotor - with a rigid or flexible rotor - could you roll or pitch that quickly? I suggest not, due to having to change the direction of the angular momentum of the disk in those manoeuvres, but not in yaw.
Added:
Where's the error here? I think the assumption that the disk is being moved by the helicopter, whereas it's the helicopter that's just hanging from the disk. I was focussed on the angular momentum vector of the rotor, missing that it is changed by the forces on the flying blades not from the hub/controls.
Whether the blade flaps up and down has little relevance for where the force is applied to the bearings as you try to twist it or lift it over periods longer than the rotation rate.
The designers make the cuffs and mast tough enough to cope, but those gyroscopic forces exist and must be accommodated. If the designers do a good job, then you won't feel their effects, but they are there.
It doesn't matter if the rotor's attached with rigid steel plates or cables. I agree that aerodynamic forces are more important, but you can't discount that gyroscopic effects are also at work and are dealt with by the design.
To avoid the confusing issue of advancing and receding blades, come to a hover. What forces act on the bearings/hub/rams when you apply small cyclic inputs?
And back to flying along:
What is the maximum rate of pitch, roll or yaw?
Knock off the tail rotor - with a rigid or flexible rotor - could you roll or pitch that quickly? I suggest not, due to having to change the direction of the angular momentum of the disk in those manoeuvres, but not in yaw.
Added:
Where's the error here? I think the assumption that the disk is being moved by the helicopter, whereas it's the helicopter that's just hanging from the disk. I was focussed on the angular momentum vector of the rotor, missing that it is changed by the forces on the flying blades not from the hub/controls.
Last edited by awblain; 5th Mar 2014 at 11:16. Reason: It was wrong
so, awblain, can you please explain why we bother having a swash plate and the ability to cyclically feather the blades?
If it is truly a gyroscope, then all we need is some device mounted near the transmission that applies a force to the transmission / mast and the precession effects would be enough to make the disc tilt in the direction we want, EXACTLY 90 degrees later. Ummm... what was that? Oh, you mean that the mast and the fuselage don't rigidly follow the disc??? The disc can be at an angle to its axis?? The fuselage is at a different angle from the disc?
Oh dear, sort of blows the gyroscope idea to bits, doesn't it...
Unless you know more than one of the senior test pilots of the industry, who developed the S-76, Blackhawk and many other projects, perhaps you could read what he says. Ask Nick.
If it is truly a gyroscope, then all we need is some device mounted near the transmission that applies a force to the transmission / mast and the precession effects would be enough to make the disc tilt in the direction we want, EXACTLY 90 degrees later. Ummm... what was that? Oh, you mean that the mast and the fuselage don't rigidly follow the disc??? The disc can be at an angle to its axis?? The fuselage is at a different angle from the disc?
Oh dear, sort of blows the gyroscope idea to bits, doesn't it...
Unless you know more than one of the senior test pilots of the industry, who developed the S-76, Blackhawk and many other projects, perhaps you could read what he says. Ask Nick.
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So you can relatively gently change direction, and to balance the relative lift from each side of the disk respectively.
How does the disk respond to this change of direction? As I said.
*No - not as I said - it doesn't respond to a change of direction, it is manipulated to change direction by beating up the air differently.
Now would you like to answer my questions?
I'm sure Nick is excellent at testing and designing helicopters. While he may find it useful to stop people obsessing about gyroscopes while explaining their operation, it doesn't take away from the fact that there is substantial angular momentum involved, and that a rotor reacts just like any other rotating system to couples imposed on it.
*I think this is the case if it's resting on the ground, but it doesn't reflect the situation when it's flying or almost flying - the way in which the blades are moved around by shepherding the airflow not by wrenching by the controls.
How does the disk respond to this change of direction? As I said.
*No - not as I said - it doesn't respond to a change of direction, it is manipulated to change direction by beating up the air differently.
Now would you like to answer my questions?
I'm sure Nick is excellent at testing and designing helicopters. While he may find it useful to stop people obsessing about gyroscopes while explaining their operation, it doesn't take away from the fact that there is substantial angular momentum involved, and that a rotor reacts just like any other rotating system to couples imposed on it.
*I think this is the case if it's resting on the ground, but it doesn't reflect the situation when it's flying or almost flying - the way in which the blades are moved around by shepherding the airflow not by wrenching by the controls.
Last edited by awblain; 5th Mar 2014 at 11:24. Reason: Corrections
Now would you like to answer my questions?What is the maximum rate of pitch, roll or yaw? How fast can you barrel roll? What sets that limit? Can you snap (or aileron) roll a helicopter?
Stability and manoeuvrability are at opposing ends of the scale. Helicopters are unstable and so are manoeuvrable, but we try to damp out most of that to get a smoother ride.
Nick can answer the questions on the maximum rates of pitch roll and yaw, though again for a passenger-carrying machine, these questions are a bit pointless. "Sufficient" is a pretty good answer.
Chief Bottle Washer
I'm sure Nick is excellent at testing and designing helicopters. While he may find it useful to stop people obsessing about gyroscopes while explaining their operation, it doesn't take away from the fact that there is substantial angular momentum involved, and that a rotor reacts just like any other rotating system to couples imposed on it.
Nick Lappos graduated as a Bachelor of Aerospace Engineering from the Georgia Institute of Technology in 1973. Honors include Dean's List, Who's Who in American Colleges and Universities, Tau Beta Pi, and Sigma Gamma Tau. Elected to the Academy of Distinguished Alumni of Georgia Tech in 2004.
Fellow of the American Helicopter Society as well as Frederick Feinberg Award as most outstanding pilot. Society of Experimental Test Pilots Tenhoff Award, 1988. Holds 16 U.S. patents and three FAI world speed records. Authored numerous technical papers for the American Helicopter Society, the Royal Aeronautical Society and the SAE. Written articles for magazines such as "Rotor and Wing," "Interavia," and has a regular column in "HeliOps Magazine." Appeared on several television shows on the History and Discovery channels.
US Army Vietnam veteran, flew Cobra attack helicopters for over 900 combat hours. Awarded the Bronze Star and the Republic of Vietnam’s Cross of Gallantry.
Fellow of the American Helicopter Society as well as Frederick Feinberg Award as most outstanding pilot. Society of Experimental Test Pilots Tenhoff Award, 1988. Holds 16 U.S. patents and three FAI world speed records. Authored numerous technical papers for the American Helicopter Society, the Royal Aeronautical Society and the SAE. Written articles for magazines such as "Rotor and Wing," "Interavia," and has a regular column in "HeliOps Magazine." Appeared on several television shows on the History and Discovery channels.
US Army Vietnam veteran, flew Cobra attack helicopters for over 900 combat hours. Awarded the Bronze Star and the Republic of Vietnam’s Cross of Gallantry.
The Sir Barnes Wallis Medal: Nicholas Lappos
During 40 years of work in the US aerospace industry Nicholas (Nick) Lappos has made an immense contribution, as a test pilot and as an engineer, to the development and application of advanced technologies for aircraft, particularly rotorcraft. During this time, he has accrued over 7,500 flight hours in helicopters, including over 2,500 in experimental or engineering test flying. He has contributed both as an experimental test pilot and in a variety of project engineer, project management and strategic management roles with US aerospace companies. In so doing he has been instrumental in identifying, developing and testing a wide variety of advanced technologies which, when introduced to company products, have made a major contribution to the expansion of civil and military helicopter capabilities on an international scale.
Nick joined the US Army in 1968, training as a helicopter pilot on AH-1 Hueycobra and serving in Vietnam where he was awarded the US Bronze Star and the Vietnam Cross of Gallantry.He left the Army and after graduating with a BSc in Aerospace Engineering from Georgia Institute of Technology in 1973, joined Sikorsky as a Flight Test Engineer, before being appointed as Experimental Test Pilot. In the following 27 years of flight testing he had a number of important project development roles on CH-53, UH-60 Black Hawk, and RAH-66 Comanche platforms. However his main development task was as Project Pilot for the S-76 civil helicopter; he carried out the first flight, led development and certification flying and was closely identified with this programme throughout the world.
His combination of test pilot skills and engineering training allowed him to make a substantial contribution to a number of world-leading projects. These included Sikorsky’s co-axial, rigid rotor, Advancing Blade Concept high speed aerodynamic research platform, the Shadow fly-by-wire flight control research programme and the Fantail embedded fenestron rotor research project. His ground-breaking work on the theoretical understanding of helicopter manoeuvrability and agility lead to the award of a Technical Fellowship from the American Helicopter Society.
Nick amassed 17 patents for inventions in helicopter engineering, including advanced engine and flight controls offering greater flight safety in degraded visual flight conditions and high manoeuvring states. Many of these concepts were tested in the fly-by-wire flight controls of the RAH-66 Comanche, and are now part of the standard suite of digital control techniques used in rotorcraft. In 2002 he became Programme Manager for the S-92 helicopter and under his leadership, the programme was awarded the prestigious Robert J. Collier Trophy for the most outstanding achievement in US Aeronautics.
In 2005 he moved to Gulfstream Aerospace Corporation, serving as Vice President of Government Programmes, and was responsible for the successful integration of advanced radar and sensor technology onto the Gulfstream G-5 aircraft. He then joined Bell Helicopter Textron in 2008 first as Senior VP Research Development and Rapid Prototyping and later as Chief Technology Officer. Many of the advanced features of the Bell 525 Relentless medium helicopterwere developed and proven during his tenure.
He returned to Sikorsky in 2011 as Senior Technical Fellow for Advanced Technology, identifying the advanced technologies essential to the development of new company products and capabilities. He is Chairman of the United States Vertical Lift Consortium, which is chartered to help the US Department of Defense steer the development of the next generation of rotorcraft, known as the Future Vertical Lift (FVL) initiative.
During 40 years of work in the US aerospace industry Nicholas (Nick) Lappos has made an immense contribution, as a test pilot and as an engineer, to the development and application of advanced technologies for aircraft, particularly rotorcraft. During this time, he has accrued over 7,500 flight hours in helicopters, including over 2,500 in experimental or engineering test flying. He has contributed both as an experimental test pilot and in a variety of project engineer, project management and strategic management roles with US aerospace companies. In so doing he has been instrumental in identifying, developing and testing a wide variety of advanced technologies which, when introduced to company products, have made a major contribution to the expansion of civil and military helicopter capabilities on an international scale.
Nick joined the US Army in 1968, training as a helicopter pilot on AH-1 Hueycobra and serving in Vietnam where he was awarded the US Bronze Star and the Vietnam Cross of Gallantry.He left the Army and after graduating with a BSc in Aerospace Engineering from Georgia Institute of Technology in 1973, joined Sikorsky as a Flight Test Engineer, before being appointed as Experimental Test Pilot. In the following 27 years of flight testing he had a number of important project development roles on CH-53, UH-60 Black Hawk, and RAH-66 Comanche platforms. However his main development task was as Project Pilot for the S-76 civil helicopter; he carried out the first flight, led development and certification flying and was closely identified with this programme throughout the world.
His combination of test pilot skills and engineering training allowed him to make a substantial contribution to a number of world-leading projects. These included Sikorsky’s co-axial, rigid rotor, Advancing Blade Concept high speed aerodynamic research platform, the Shadow fly-by-wire flight control research programme and the Fantail embedded fenestron rotor research project. His ground-breaking work on the theoretical understanding of helicopter manoeuvrability and agility lead to the award of a Technical Fellowship from the American Helicopter Society.
Nick amassed 17 patents for inventions in helicopter engineering, including advanced engine and flight controls offering greater flight safety in degraded visual flight conditions and high manoeuvring states. Many of these concepts were tested in the fly-by-wire flight controls of the RAH-66 Comanche, and are now part of the standard suite of digital control techniques used in rotorcraft. In 2002 he became Programme Manager for the S-92 helicopter and under his leadership, the programme was awarded the prestigious Robert J. Collier Trophy for the most outstanding achievement in US Aeronautics.
In 2005 he moved to Gulfstream Aerospace Corporation, serving as Vice President of Government Programmes, and was responsible for the successful integration of advanced radar and sensor technology onto the Gulfstream G-5 aircraft. He then joined Bell Helicopter Textron in 2008 first as Senior VP Research Development and Rapid Prototyping and later as Chief Technology Officer. Many of the advanced features of the Bell 525 Relentless medium helicopterwere developed and proven during his tenure.
He returned to Sikorsky in 2011 as Senior Technical Fellow for Advanced Technology, identifying the advanced technologies essential to the development of new company products and capabilities. He is Chairman of the United States Vertical Lift Consortium, which is chartered to help the US Department of Defense steer the development of the next generation of rotorcraft, known as the Future Vertical Lift (FVL) initiative.
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It seems like this discussion has shifted back to the topic of precession. While I'm not equipped to dissect the opposing viewpoints it does seem clear that there's more going on with this issue than many of us have been originally taught. Primacy again rearing it's ugly head as I, for one, struggle to understand this phenomenon.
I'd like to return to a comment by Ascend Charlie about maneuverability vs stability. It seems like this might be more relevant to my question about gyroscopic stability (or lack thereof.) are stability and maneuverability really mutually exclusive traits of an aircraft? Can you talk about this relationship a little more as it relates to helicopters?
Also, I'd like to humbly request that this discussion be carried forward in a more respectful tone. I think we're all trying to grasp some non trivial concepts and it's getting a little hot in here.
I'd like to return to a comment by Ascend Charlie about maneuverability vs stability. It seems like this might be more relevant to my question about gyroscopic stability (or lack thereof.) are stability and maneuverability really mutually exclusive traits of an aircraft? Can you talk about this relationship a little more as it relates to helicopters?
Also, I'd like to humbly request that this discussion be carried forward in a more respectful tone. I think we're all trying to grasp some non trivial concepts and it's getting a little hot in here.
Ahhh, Grasshopper, if you can't stand the heat, turn the engine off.
Stability in the helicopter is nothing to do with gyroscopes. It is all to do with "Does it want to quietly return to its original position, by itself, after being displaced?"
As said before, you must have static stability (for a mechanically controlled aircraft) before you can look at dynamic stability, because the energy extracted from the airflow can make the machine to weird things. (You have heard your venetian blinds going berserk in a strong wind, clattering against each other as they undergo dynamic instability.)
In the new fly-by-wire fighter aeroplanes, computers are fast enough to force a totally unstable plane to behave itself and do what the pilot tells it to do. But if the computers all fail together, the machine is uncontrollable.
Helicopters are now undergoing the same revolution, but it is fearfully expensive, so the average Joe still relies on a non-computerised, non-stabilised Jet Franger or R22. If you have progressed past GF 2 you will have seen phenomena like flapback, inflow roll and stick-fixed instability, all things that we learn to live with , and which make us far superior beings to our fixed-wing cousins.
Stability and manoeuvrability are not mutually exclusive, but they are not next-door neighbours either. A 747 is very stable, but a little heavy on the controls. A fighter plane darts into the fray but is on the edge of stability (such as a Mirage of the pre-computer era) and easily departs into a spin which may not be recoverable. The only stability in a mechanical chopper is the yaw stability in forward flight, where the tail tends to stay behind the rest of the aircraft. In the hover, it has no stability at all, and will diverge from a disturbance (wind gust, pilot input) and within 3 or 4 oscillations will crash. It is only the steely-eyed wind-swept hero in the front seat who can stop the crash. With any luck, that will be the instructor.
Stability in the helicopter is nothing to do with gyroscopes. It is all to do with "Does it want to quietly return to its original position, by itself, after being displaced?"
As said before, you must have static stability (for a mechanically controlled aircraft) before you can look at dynamic stability, because the energy extracted from the airflow can make the machine to weird things. (You have heard your venetian blinds going berserk in a strong wind, clattering against each other as they undergo dynamic instability.)
In the new fly-by-wire fighter aeroplanes, computers are fast enough to force a totally unstable plane to behave itself and do what the pilot tells it to do. But if the computers all fail together, the machine is uncontrollable.
Helicopters are now undergoing the same revolution, but it is fearfully expensive, so the average Joe still relies on a non-computerised, non-stabilised Jet Franger or R22. If you have progressed past GF 2 you will have seen phenomena like flapback, inflow roll and stick-fixed instability, all things that we learn to live with , and which make us far superior beings to our fixed-wing cousins.
Stability and manoeuvrability are not mutually exclusive, but they are not next-door neighbours either. A 747 is very stable, but a little heavy on the controls. A fighter plane darts into the fray but is on the edge of stability (such as a Mirage of the pre-computer era) and easily departs into a spin which may not be recoverable. The only stability in a mechanical chopper is the yaw stability in forward flight, where the tail tends to stay behind the rest of the aircraft. In the hover, it has no stability at all, and will diverge from a disturbance (wind gust, pilot input) and within 3 or 4 oscillations will crash. It is only the steely-eyed wind-swept hero in the front seat who can stop the crash. With any luck, that will be the instructor.
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Senior, Ascend, No disrespect or belittling intended. Apologies.
I must have been completely wrong: while there is a lot of angular momentum involved, all the couples imposed come entirely from the airflow, just spurred by the control inputs, and these are powerful enough to change that angular momentum direction then there we are.
I guess my error is in assuming it's like a chair with a bike wheel, where the reaction forces with the ground matter, whereas they don't, it's just a wheel in space with an oddly shaped hub, and all the forces come from the blades interacting with the air.
I would still say, Ascend, that that deviation from control tends to keep the blades up and the wheels down, with a faster slip away than roll, since that big angular momentum vector still needs twisting, if all by the air.
I must have been completely wrong: while there is a lot of angular momentum involved, all the couples imposed come entirely from the airflow, just spurred by the control inputs, and these are powerful enough to change that angular momentum direction then there we are.
I guess my error is in assuming it's like a chair with a bike wheel, where the reaction forces with the ground matter, whereas they don't, it's just a wheel in space with an oddly shaped hub, and all the forces come from the blades interacting with the air.
I would still say, Ascend, that that deviation from control tends to keep the blades up and the wheels down, with a faster slip away than roll, since that big angular momentum vector still needs twisting, if all by the air.
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Apologies. I really didn't get it from the descriptions before, but I think I do now. Edited messages above to, hopefully, note the errors.
If I were to say that there is a lot of angular momentum in the rotor, but that the aerodynamic forces, tweaked and directed by the control inputs, are strong enough to re-orient it, would anyone disagree?
No wonder it took so long to work out how to make helicopters work. It's much subtler than I thought when I waded in before, without thinking at the forces as well as the momentum.
If I were to say that there is a lot of angular momentum in the rotor, but that the aerodynamic forces, tweaked and directed by the control inputs, are strong enough to re-orient it, would anyone disagree?
No wonder it took so long to work out how to make helicopters work. It's much subtler than I thought when I waded in before, without thinking at the forces as well as the momentum.
AW can connect with this! If he understands it is another question all together!
