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I'm starting to comment more on helicopter stuff and less on physics stuff. I feel I understand the helicopter stuff, but I'll still try restrict my comments to areas where I have the appropriate background.
Lu said, "If gyroscopic precession plays no part in the tilting of the disc either as a single entity or as individual blades then please explain this." I guess it gets down to semantics. As best as I can tell, the definition of gyroscopic precession is limited to rotating bodies whose geometry allows the 90 degree apparent lag. Rotor systems whose axis of rotation does not pass through the flapping hinges are one example of a geometry that won't allow a 90 degree lag. Answering your question, since the definition of gyroscopic precession doesn't really apply, it is fair to say it plays no part. For a teetering rotor, it seems that gyroscopic precession is a valid description. Since the "blade flying to position" derivation can be used for a general derivation of gyroscopic precession, I think that in special cases the two theories are the same. One (fly to pos.) requiring more basics, the other (GP) accepting more terminology. Lu also said, "In the light of the above I ask why the 90-degrees if gyroscopic precession is not involved. Why couldn’t they just put the control inputs anywhere and just let the blades seek their own position." Read through my previous long post where I talk about the pendulum. The natural frequency of the blade acting as a pendulum is exactly one per revolution when the axis of rotation passes through the flapping hinge. It is less than 90 degrees when the flapping hinge is offset. helmetfire, I believe that flapping to equality applies only with translational movement of the helicopter. In a zero wind hover, there is no flapping to equality. Matthew. [ 10 August 2001: Message edited by: heedm ] |
I don't want to pretend to be any kind of expert on this subject, however the aerodynamic principles that are talked about with reference to phase lag and flapping to equality do seem fair enough to me.
Here's a shot at trying to describe what happens, for an anticlockwise when viewed from above rotor: For a helicopter in the hover in nil wind, the pitch setting on the blades could be said to be the same, and lift equal, all the way round. Let's say you want to tilt the disc forward to, funnily enough, fly forward. You move the cyclic forwards. The pitch change system acts to make the pitch of the blades vary around the disc, with the maximum setting occurring on your left, and the minimum on your right. That means that from the rear of the disc, the blades experience less lift than they had in the hover and therefore start to fly down. The minimum pitch setting occurs on your right, so that's where the blades are flying down fastest - but they haven't finished heading downwards. That doesn't happen until they get to the front of the disc, where they will again be at the original 'equilibrium' pitch setting. From there, on the left side, they will start to fly up again, so you finish up with the disc tilted forwards like you wanted. When the aircraft just starts to fly forwards, the left and right sides are producing the same amount of lift, so you are 'wings level'. As you pick up speed, the advancing side gets more airspeed and the retreating side less, so the aircraft should roll, you'd think. However, because we have flapping hinges and/or flexible blades, the advancing blade flaps upwards (decreasing it's angle of attack) and the retreating blade downwards (increasing it's angle of attack) until they are back at the original equilibrium between lift and centripetal force (or whatever the force is called that wants to throw them outwards!). They have therefore 'flapped to equality' of lift, and the machine doesn't roll. However, to achieve this, as we said, the advancing blade flaps up and the retreating blade down, with the maximum and minimum pitch angles respectively occurring at the left and right sides of the helicopter. Lift is equal laterally, but the blade is flapping, with the maximum rate of flap at the sides. As described above, however, the maximum extent of travel isn't reached until 90 degrees later; i.e. the blade would be flapping down fastest on our left, reaching its lowest point at the back, relatively speaking (bearing in mind it's already been pushed forward in the first place). This is the phenomenon of flapback, meaning we now have to push more forward cyclic than we would have otherwise to keep going forward. No gyroscopics required! That's not to say no gyroscopic forces are present, but I think aerodynamics explains things easily and adequately. That's quite enough of a post for now, my brain hurts. |
I am constantly being chastised because many of you say I am wrong because I don’t agree with you as individuals or as a group. I keep telling you that I do agree with you because I accept what you are saying as an alternate theory that obviously works for you. I use terms that you do not accept and you use terms that I am not familiar with. Granted it has been a long time since I attended a factory or military school for helicopters and maybe the teaching methods have changed but I never heard of flapping to equality until I started posting on these threads. Try to understand that POF is taught differently in the US as opposed to the POF theory that is taught in the UK, OZ and NZ. Now if RW-1 can keep his nose out of this post I will make the following suggestion. All of the theories I have expounded on relative to gyroscopic precession are contained in the Sikorsky Blue Book and I think you should contact the Factory School at Sikorsky and request a copy of the Blue Book. That is the source of my statements so if you disagree with me then do as I mentioned to Nick Lappos and tell them they are wrong and tell them why they are wrong.
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To All,
It is obvious that the discussion is going nowhere, just as I originally feared it would. My understanding remains as it always was. I have worked with many pilots of other nationalities, American pilots included and NONE of them has ever been at odds with my understanding. Rotor blades are controlled by adjusting the aerodynamic forces affecting them. They are FLOWN around the disc by pilot inputs on the cyclic. The result of them being flown around is that forces are fed back to the swashplates which result in the hull of the aircraft experiencing those forces. The way in which the feedback forces act to alter the aircraft's flightpath depends on a number of factors. Gyroscopic forces do of course exist by definition but they are NOT modified directly in order to control the helicopter (as Lu seems to prefer). An aircraft with lighter blades generally has a more responsive rotor, one with heavier blades less so because of the gyroscopic forces appertaining to each. Gyroscopic precession in the case of helicopter rotors is a result of aerodynamic effects, not the cause of them. That is why we refer to cyclic PITCH, not cyclic GYROSCOPIC PRECESSION. Lu's understanding of helicopter rotor dynamics is not the same as mine because he appears to put a different emphasis on the factors involved. I think his understanding is incomplete but I can't see him ever being prepared to accept this. There is only one set of laws of physics and it doesn't actually matter what colour the cover of the book is. I will now leave it to those of you willing to slug it out, I'm afraid. ShyT :( |
Lu SHOULD READ ShyTorque's last post, which is true in all aspects. And make the small attempt to understand what was said.
I keep telling you that I do agree with you because I accept what you are saying as an alternate theory that obviously works for you. What those have stated to you are not ALTERNATE THEORIES to anything. They are FACTS. FACTS that trash and disprove LZ's FLAWED THEORIES. Because LZ has no capability to accept that he is wrong. It isn't because of an alternate thery, what rubbish. Study the two short paragraphs above well, and quit making yet another excuse; the one now where you retreat to another manual that you believe is wrong or has some other non-LZ understood theory or fact. Again someone said it correctly: LZ's mantra: "Anytime I appeared to be wrong it was because someone told me to say the wrong thing, or the manual is saying something else, and anytime I am right, it is because I have been around since Christ flew crew changes on the Arc." The books are ALL FINE, it is your INTERPRETATION that is flawed, and thus leads to flawed output. And since you wanted to talk about my website, lets remember a key sentance on the page you want to quote so often: "precession is not a dominant force in rotary-wing aerodynamics" It ISN'T, and you must STOP treating it as the RULE governing all. All these threads have gone as predicted by me at the beginning, LZ has no change of ever learning anything here, it is always a rehash of another misinterpretation on his part, restated or reformulated to yet have another useless discussion on the topic. Just place this by your name and everyone can take a rest: I am "the" Lu Zuckerman. I am not a pilot. I am not an engineer. I am not involved, nor have experience with rotor head design/aerodynamics to be making any type of assertions to those who do on this forum. I am simply a wannabe guru. I am never wrong. My heli post's are for entertainment purposes only, and should not be used or considered in actual flight operations/planning. |
Thanks, Marc. You've made your point. You don't like Lu.
Now could we please keep this on topic, even if Marc doesn't agree with all the posts. Thanks. Matthew. |
This has nothing to do with the person. But I'll allow that in this case his personality is as flawed as the theory.
It has to do with fundamentals listed above. If it were you then the names would change, but the reality would remain the same. It is not about education, not about learning, he will rant and rave giving any possible excuse until someone agrees with the flawed theory put forth, which isn't going to happen for it just doesn't happen, and no amount of preaching will make the facts, nor the physics change to suit that flawed theory. I don't like bull****, and unfortunately with each new excuse he places to avoid the issue at hand, he reeks of it. Plain and simple. [ 10 August 2001: Message edited by: RW-1 ] |
heedm
Please forgive me for all the things I thought about you. Looks like you're right. The amount of rotor tip that will result from the creation of a moment on a teetering rotor disk was done. These calculations were done for aerodynamic 'precession' and for gyroscopic precession. The results are essentially the same. A moment of 50 pounds at 75% of rotor radius gave 0.47" and 0.44" of tipping at the 75% radius, 90 degrees later. For those that doubt, are really bored, or want to find fault, the attached page is offered; http://www.synchrolite.com/0940.html#Comparison Lu; please note that the above is for a simple teetering rotor and its 90-degree phase angle. Any delta3 or flapping hinge offset will reduce the phase lag to below 90-degrees. [ 11 August 2001: Message edited by: Dave Jackson ] |
What exactly did you think about me, Dave?
Interesting information on the unicopter site. I had a similiar discussion to this one with Doug Marker a year or two ago. Back then I knew even less about rotors than I do now. It's talking like this that brings the point home. I wouldn't say it proves I'm right, but it does support what I've said. It may be fluke that that one set of data worked out so closely. You going to Abbotsford this weekend? Matthew. |
I'm sure that someone will sort this out for me. More years ago than I care to remember I displayed the British Army Lynx for a year. From memory whenever I pitched from a steep climb, (aiming at 90 nose up, but probably more like 80 on a good day) to 90 nose down, the aircraft had a tendency to roll(right I think but I can't be sure). I always thought that this was some from of gyroscopic reaction, thoughts ?
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To: Dave Jackson
“Lu; please note that the above is for a simple teetering rotor and its 90-degree phase angle. Any delta3 or flapping hinge offset will reduce the phase lag to below 90-degrees”. This may be true relative to the calculation being applied to a simple teetering rotorhead. Although there is no offset hinge on the teetering rotorhead there is a delta 3 pitch coupling that occurs when the blade teeters. This coupling tends to restore the pitch in the blade moving forward and takes pitch out of the blade moving backwards very much like a tail rotor. Again referring to your statement about an offset hinge reducing the phase angle to a level below 90-degrees. This to may be true but in the design of almost all rotor systems the designers assume a 90-degree phase angle and they design the control system to accommodate that 90-degree phase angle. If the rotor system does no respond to direct control input and flies as if the phase angle is 88 or 92 –degrees the pilot will simply adjust his cyclic input to result in forward flight as opposed to flying slightly to the left or right. The certification of rotorcraft stipulates that it is acceptable to have a few degrees of coupling but the direction of flight must be in the same sense of control input. |
heedm
What exactly did you think about me, Dave? Only good things. You postings portray a person who can give and take a joke. Interesting information on the unicopter site. Thanks. The UniCopter concept may not work. But, until some person or calculation shows that won't, it is being pursued. You going to Abbotsford this weekend? No. Too busy thinking up postings, and also, Nick didn't say that the Comanche was going to be there. Are you going? ______________________ Lu; I agree with what you say and have reiterated it in different words The basic Bell rotorheads have a 90-degree teetering, without delta3. The Robinson and the Kaman rotorheads have teetering with delta3, but their types of delta3 differ from each other. In tail rotors, the delta3 is 45-degrees, therefore every degree of teeter is 'removed' by an identical number of degrees of opposing pitch. The only area of disagreement is the small point below and it is probably only the terminology; This coupling tends to restore the pitch in the blade moving forward and takes pitch out of the blade moving backwards very much like a tail rotor. This [teetering] coupling (ie. delta3) removes pitch from the blade that is teetering up and adds pitch to the blade that is teetering down. ________________ We are talking about a number of different 'devices' such as delta3 (wonder what happened to delta1 and delta2), flapping hinge offset, phase angle and gyroscopic precession. Each one has its own sub-divisions and complexities, even before addressing the complexities of tying them together. To separate the subjects; I would like to start a separate new thread [Helicopter Dynamics: Phase Angle]. Maybe we can jointly learn some more about this subject as well. [ 11 August 2001: Message edited by: Dave Jackson ] |
To: Dave Jackson
“This coupling tends to restore the pitch in the blade moving forward and takes pitch out of the blade moving backwards very much like a tail rotor”. This [teetering] coupling (i.e. delta3) removes pitch from the blade that is teetering up and adds pitch to the blade that is teetering down. We are both saying the same thing. The blade moving forward has pitch added to it by the pitch coupling and the blade going back has pitch subtracted from it. My comparison to the tail rotor was incorrect, as it is the opposite of the main rotor. The advancing blade because of the oncoming air loads, will flap out, and in the process will have pitch taken out of it. The retreating blade, which is moving with the relative wind and is mechanically connected to the other blade, will flap in and the delta hinge will add pitch to it, which equalizes the lift across the tail rotor disc. |
Lu, you said:
>>We are both saying the same thing. The blade moving forward has pitch added to it by the pitch coupling and the blade going back has pitch subtracted from it.<< I believe you are starting to understand cyclic feathering and flapping to equality. I note that Arm out the window posted an excellent description for you above, which raised no comment from you: are we beginning to help your understanding? |
To: Helmet Fire
“I believe you are starting to understand cyclic feathering and flapping to equality. I note that Arm out the window posted an excellent description for you above, which raised no comment from you: are we beginning to help your understanding? Please understand that I did not have a light bulb go off over my head as a result of any of the above posting. I have been aware of cyclic feathering and pitch coupling ever since 1949 when I was first introduced to helicopter maintenance. What I did not know was the term flapping to equality. As to my knowledge flapping to equality is not used as an explanation for the phenomenon in the United States and that is the root of the entire argument, which is how, POF is taught in different parts of the world. [ 12 August 2001: Message edited by: Lu Zuckerman ] |
Heedm said; ~ I wouldn't say it proves I'm right, but it does support what I've said. It may be fluke that that one set of data worked out so closely.
It might be interesting to put the algorithms for calculation by gyroscopic precession and those for calculation by aerodynamics side by side. Then look at the sources of the data for each. This will show what inputs they have in common and what inputs are unique to each. Another thought is to change the rotor's mass and see if it is possible to get the results from both methods to show a greater discrepancy. Something for a very very rainy day. Also, I just found a reference to gyroscopic precession in 'Helicopter Flight Dynamics: The Theory and Application of Flying Qualities and Simulation Modeling', 1999, by Gareth D. Padfield. _______________ It's appearing that there may not be a definitive answer. Lu ~ Your use of gyroscopic precession may well be totally acceptable. |
JTC,
Interesting point about the Lynx. Confirm it has an anticlockwise when viewed from above rotor? I'm just thinking out loud here, but there could be a couple of effects at play when you pitch forward - Firstly, there should be a reduced inflow at the front of the disc and increased at the rear due to your pitching action which would tend to increase the angle of attack at the front and reduce it at the rear. That should produce a rolling moment to the right, I think. Secondly, if gyroscopic forces are significant, you would be providing a 'downward force on the front of the gyro' which should manifest itself as a roll to the left. So I guess in balance, the aerodynamic force appears to be the winner (!) Anyone else got ideas about it? There are probably a bunch of other things influencing the motion, even assuming that you kept all the controls relatively still once the pitching was initiated. |
I remember a thread some time ago, can't remember if it was here or on Mil Pilots, about this particular Lynx flying quality.
If the search is working, might be worth a gander. P.S. - if it was a "gyroscopic" type reaction, in a push-over, would the expected result not be a roll to the left ? [ 13 August 2001: Message edited by: The Nr Fairy ] |
Yeah, that's what I said! :)
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Whoops. Teach me to skim-read, won't it !!
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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!
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"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. |
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?
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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. |
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? |
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. |
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. |
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.
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awblain, I have saved you the trouble.
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 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. |
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. |
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. |
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. |
Originally Posted by awblain
(Post 8352828)
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. 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. |
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. :) |
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. |
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. |
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. |
AW can connect with this! If he understands it is another question all together!
https://scontent-a-atl.xx.fbcdn.net/...12019144_n.jpg |
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