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Model Forum / Radio Controlled / Helicopters / August 2003Large Scale turbine Jet Ranger at 3D masters
Looking at the pictures on Alan's site (cheers Alan :) I notice the big Jet Ranger has no flybar.. and so the head looks incredibly simple. Were there any negative effects to this? I presume it's only feasible on such a large machine with a low rotor rpm since we need the dumbing-down on our 3x-faster heads.. > Looking at the pictures on Alan's site (cheers Alan :) I notice the big Jet > Ranger has no flybar.. and so the head looks incredibly simple. And so it is and so it is:) > Were there any negative effects to this? I presume it's only feasible on > such a large machine with a low rotor rpm since we need the dumbing-down on > our 3x-faster heads.. Perfectly feasible even on a Piccolo, but it's a bit easier to maintain a constant head speed when the blades have enormous amounts of inertia, and it's variations in head speed that makes flybar-less flying more problematic than flybar flying. The Jet Ranger didn't appear to have any serious pitching up tendencies (a claasic symptom of poor rotor speed control) and as all turbine models are governed, that aspect is a non issue.  SignatureBeav Please note my E-mail address is "beavis dot original at ntlworld dot com" (with the obvious changes) Beavisland now lives at www.beavisoriginal.co.uk > > Looking at the pictures on Alan's site (cheers Alan :) I notice the big > Jet [quoted text clipped - 15 lines] > claasic symptom of poor rotor speed control) and as all turbine models are > governed, that aspect is a non issue. So what would happen if I removed the flybar from my GV-1 equipped Raptor? One less thing to bend in a dirt biter ;-) John > > > Looking at the pictures on Alan's site (cheers Alan :) I notice the big > > Jet [quoted text clipped - 19 lines] > So what would happen if I removed the flybar from my GV-1 equipped Raptor? > One less thing to bend in a dirt biter ;-) If the ONLY thing you did was remove the flybar (or lock it in place), then the heli would feel dead in the air. (Lock it and see, it won't bite) You'd need to make changes to the blade weights and deflection to get back a reasonable amount of agility, but you'd also run into the flapback phenomenon which the flybar masks quite magnificently. You'll also have much less drag from the cabin top upwards and less parasitic drag from the twirly bits, so performance is increased and FFF is much more F. SignatureBeav Please note my E-mail address is "beavis dot original at ntlworld dot com" (with the obvious changes) Beavisland now lives at www.beavisoriginal.co.uk >So what would happen if I removed the flybar from my GV-1 equipped Raptor? >One less thing to bend in a dirt biter ;-) > >John Hi John, Although I'm interested in reading Beav's response to this question, I'll insert my 2 cents too. If "all" you do is remove the flybar, I think you'll find that you'll have your hands full the next time you fly. Most of us don't really comprehend or appreciate how much the flybar does for us in terms of stabilizing the main rotor blades. One way to compensate is to run heavier main blades. The added mass and inertia help offset the stability lost when the flybar is removed. You'll also find that you'll probably have to cut way back on the collective and cyclic throws. They won't be going through the bell/hiller mixers any more and that automatically increases their travel. While I'm always intriqued to watch multi-bladed models fly, that is, those with more the "2" rotor blades. Personally, I'd keep the flybar if 2 blades were all I was going to use. That's just a personal preference. Two bladed flybarless rotor systems are certainly workable. I know because I learned to hover on one. A model built here in the States back in the late 70's and early 80's, called a Horizon. Beav is familiar with it if I'm not mistaken. Fly Safe, Steve R. > You'll also find that you'll probably have to cut way back on the collective > and cyclic throws. They won't be going through the bell/hiller mixers any more > and that automatically increases their travel. Good point. On the other hand, simply locking the flybar (mentioned earlier) would have the opposite effect, i.e. reduced travel at the blade holder. Therefore it may seem at first blush that the same effect would be true if the flybar and associated mixers were removed altogether. But as you correctly point out, the opposite is actually the case. >On the other hand, simply locking the flybar (mentioned earlier) You know, that brings up a question. I have accidentally taken off with the flybar lock in place. I didn't like it, to say the least. However! Has anyone simply tried removing the flybar paddles and weights and leaving everything else in place? You'd have to increase the cyclic throws I'd imagine. On the plus side, I don't think you'd have to change your collective setup. What do you think? Fly Safe, Steve R. > >On the other hand, simply locking the flybar (mentioned earlier) > [quoted text clipped - 3 lines] > However! Has anyone simply tried removing the flybar paddles and weights and > leaving everything else in place? You'd have to increase the cyclic throws I'd > imagine. Isn't this what I said (in other words) in my response to JB?. Just removing (or locking) the bar leaves you with a dead feeling heli, so to get some agility back, OTHER things need to be changed too. (Blade weight and deflection are the two things I mentioned) On the plus side, I don't think you'd have to change your collective > setup. There is a requirment to change the collective slightly, but only because we now have substantially less drag to overcome, so the heli performs "as was" with slightly less collective. Beav >Isn't this what I said (in other words) in my response to JB?. Just removing >(or locking) the bar leaves you with a dead feeling heli, so to get some >agility back, OTHER things need to be changed too. (Blade weight and >deflection are the two things I mentioned) Hi Beav, I read what you said (quoted above) and I'm not sure we're talking about the same thing here. It's close, but not quite. You were (if I'm reading you correctly) speculating on what might need to be done to the base setup if the flybar were "removed" or "locked." My last question is not quite the same thing. I was wondering about what would happen if the flybar paddles and weights (if installed) were removed from the flybar system itself. Leave everything else in place, the flybar, it's teeter mechanism, the bell/hiller mixers, etc. All of it! The result would be a stabilizer bar instead of a flybar. Without the paddles, it would be aerodynamically blind and would only be interested in maintaining it's position 90 degrees to the rotor mast. I'm sure the cyclic would get pretty unresponsive this way and would most likely need some adjustment. Collective, as you pointed out, shouldn't need much of a change although it might need some. It might make an interesting experiment although I'm not sure I'd do this with any of the machines I've got now. FWIW! :-) Fly Safe, Steve R. > The result would be a stabilizer bar instead of a flybar. Without the paddles, > it would be aerodynamically blind and would only be interested in maintaining > it's position 90 degrees to the rotor mast. Without the paddles, the bar would not have any incentive to "maintain it's position 90 degrees from the rotor mast". What it would do is remain in a single plane until the mast rotated enough to bump the bar. The result likely would not be pretty. >Without the paddles, the bar would not have any incentive to "maintain it's >position 90 degrees >from the rotor mast". Hi Steve S, I disagree. The flybar will try to maintain a position, 90 degrees to the rotor mast. The reason for this is simple centrifugal force. There will be a time lag between the time that the system is disturbed and the time the flybar reorients itself, but it "will" attempt to reorient itself. Fly Safe, Steve R. > I disagree. The flybar will try to maintain a position, 90 degrees to the > rotor mast. The reason for this is simple centrifugal force. There will be a > time lag between the time that the system is disturbed and the time the flybar > reorients itself, but it "will" attempt to reorient itself. While that does seem logocal, it is incorrect. Gyroscopic force would hold the spinning bar in a single plane unless acted upon by an outside force. Absent paddles, there is no outside force . . until the mast bumps the bar. Consider that centrifugal force exerts a force 90 degrees to it's OWN rotation. In this case the relevant revolution is about the middle of the flybar, not the center of the mast. The mast tilt has negligible effect because it is hinged at the bar and therefore cannot exert influence. >Consider that centrifugal force exerts a force 90 degrees to it's OWN >rotation. In this case the >relevant revolution is about the middle of the flybar, not the center of the >mast. Well, exactly "where" is the middle of the flybar, if it's not in the middle of the rotor mast?? The rotor mast "is", in this case, the center of rotation for the flybar and it will try to orient 90 degrees to that point. The mast tilt >has negligible effect because it is hinged at the bar and therefore cannot >exert influence. I would agree with this IF the flybar hinge point was movable in "all" directions, like the center of the swashplate. Only thing is, it's "not!" The flybar can teeter in one plane only, relative to the rotor mast. Now, if the rotor mast is changing it's axis at the exact instant that it's in line with the teeter axis of the flybar, then for that brief instant, the flybar will maintain it's plane of rotation and everything will be happy. Only problem is, 90 degrees later, the flybar is being forced off of it's plane of rotation because it can't teeter in all directions, relative to the rotor mast. It won't like that and "will" try to reorient back to a position 90 degrees to the rotor mast. FWIW, I'm getting this from a video I own, bought it years ago along with several others when I first got into RC helicopters. I was trying to learn as much as I could about how they fly. Anyway, it's part of the "Aviation A.V. Library" and I got it from a company called: Aircraft Components, Inc. Benton Harbor, Michigan 49022 616-925-8863. The publisher is: Ferde Grofe Films 3100 Airport Ave. Santa Monica, CA. 90406 213-397-7524 The specific tape is called "Attack Copter" and includes six parts titled: 1. Evolutions of Attack Helicopters - Concepts of Deployment. It has footage of the Huey, Cobra, and Cheyenne. 2. Tactical Formation Flying and the parts I was most interested in at the time! 3. Helicopter Aerodynamics - Rotor blade actions 4. Helicopter Aerodynamics - The Stabilizer Bar (which is the basis for most of what I'm trying to talk about here) 5. Helicopter Aerodynamics - Dissymmetry of Lift 6. Helicopter Aerodynamics - Rotor Blade Angles. These are basically old army training tapes and they are pretty dry watching a lot of the time. They do have good info on some basics which didn't hurt me to learn about. I have no idea if these tapes are still available. The copies I have are over 20 years old now. Fly Safe, Steve R. > >Consider that centrifugal force exerts a force 90 degrees to it's OWN > >rotation. In this case the [quoted text clipped - 4 lines] > the rotor mast?? The rotor mast "is", in this case, the center of rotation for > the flybar and it will try to orient 90 degrees to that point. The centers may be coincident, but the cetrifugal force acting on the flybar is not effected by the mast. > The mast tilt > >has negligible effect because it is hinged at the bar and therefore cannot [quoted text clipped - 3 lines] > directions, like the center of the swashplate. Only thing is, it's "not!" The > flybar can teeter in one plane only, relative to the rotor mast. Quite a brain twister, huh? You had me convinced there for a second . . . . . :-) The flybar IS hinged in all directions . . think about it. It is a gimbaled mount that can teeter and rotate (paddle pitch). However, even if the model's bar was not rotatable, absent paddles, the bar's rotation about it's own centerline would suffice as the 'hinge' perpendicular to the teeter 'hinge'. In many cases, it is correct to think of a spinning bar or rotor as a solid 'disk', and in this case, a disk hinged in only one axis would indeed behave as you describe. However, a bar needs no second hinge becuase there is nothing (like a disk or paddles) connected to it so it can simply rotate about it's own centerline. > FWIW, I'm getting this from a video I own, bought it years ago along with > several others when I first got into RC helicopters. Sounds like my kind of video tape! > 4. Helicopter Aerodynamics - The Stabilizer Bar (which is the basis for most > of what I'm trying to talk about here) I think you'll find that the stabilizer has mechanical dampers which force it to follow the mast inclination. Never the less, I am very curious about what the tape says about our interesting debate. I have a finely honed skill called "failing to recognize the obvious" which may be at play here . . . . . . :-) >You had me convinced there for a second . . . . . :-) > >The flybar IS hinged in all directions . . think about it. It is a gimbaled >mount that can >teeter and rotate (paddle pitch). I'm not talking about the rotate part, only the teeter part. My original speculation was about what would happen if the flybar paddles were removed and everything else related to the flybar "system" were left in place. Under that circumstance, the rotate part would be irrelevant and you'd be left with only the influence of the teeter part of things. >However, even if the model's bar was not rotatable, absent paddles, the bar's >rotation about >it's own centerline would suffice as the 'hinge' perpendicular to the teeter >'hinge'. Ummm, I'm not sure I'm totally following you here. The bar's rotations about it's own center line would be irrelevant in the grand scheme of things. It wouldn't effect the positioin of the bar, relative to the rotor hub one way or the other. The system would simply try to maintain a position 90 degrees to the axis of rotation, that being the rotor mast! >In many cases, it is correct to think of a spinning bar or rotor as a solid >'disk', and in this [quoted text clipped - 3 lines] >to it so it can >simply rotate about it's own centerline. Which, as I said, in the absense of paddles, does nothing. It won't have any effect of the position of the flybar system relative to the axis of rotation, in any way. >> FWIW, I'm getting this from a video I own, bought it years ago along with >> several others when I first got into RC helicopters. > >Sounds like my kind of video tape! I've enjoyed it! :-) >> 4. Helicopter Aerodynamics - The Stabilizer Bar (which is the basis for >most [quoted text clipped - 3 lines] >to follow the mast >inclination. You're absolutely correct on that one Steve. The way it went was, they developed a two bladed rotor system and discovered that it was so twitchy as to be almost uncontrollable. At the very least, it was way too unforgiving for the new heliocopter pilots of the day to learn on. So, someone came up with the great idea of putting a stabilizer bar on the rotor hub which had mixing arms through which all cyclic commands passed. This gave the pilot a percentage of control over the main blades and the stabilizer bar a percentage of control too. Since the stabilizer bar, being a gyroscope, liked to maintain it's plane of rotation in space, it would automatically apply a corrective cyclic commands to the rotor blades when they were displaced from their original plane of rotation. Only problem was, it didn't matter if that displacement came from a gust of wind or a command from the pilot. The result was, the stabilizer bar would fight the pilots applied commands. In time, the flybar would try to reorient itself to a position 90 degrees to the axis of rotation but this took anywhere from 30 to 50 seconds which left the whole system feeling pretty unresponsive to what the pilot needed the aircraft to do. This was because it would take 30 to 50 seconds for the pilots cyclic command to make it all the way through the stabilizer bars mixers and on to the rotor blades. So what did they do? Enter those mechanical dampers you mentioned. These are simple devices, hydraulically dampened, that pull the stabilizer bar back into it's normal position of 90 degrees to the axis of rotation (rotor mast) within a time frame of about 5 seconds. This time frame is long enough to help stabilize uncommanded excursions of the rotor disk yet not so long as to significantly reduce the pilots control of things. As with so many things in helicopter design, it was a compromise they made that seemed to work at the time. Hope this makes some kind of sense, Fly Safe, Steve R. <big snip> What you guys are saying it totally over my head, but it seems pretty obvious to me that if you spin the rotor head with the flybar paddles removed, the flybar will move to, and hold itself at 90 degrees to the main shaft. > What you guys are saying it totally over my head, but it seems pretty > obvious to me that if you spin the rotor head with the flybar paddles > removed, the flybar will move to, and hold itself at 90 degrees to the main > shaft. Were that true, then it begs two questions: 1) If the bar naturally wants to follow the mast, how does it provide any stabilization at all? 2) What are those paddles for? > > What you guys are saying it totally over my head, but it seems pretty > > obvious to me that if you spin the rotor head with the flybar paddles [quoted text clipped - 4 lines] > > 1) If the bar naturally wants to follow the mast, how does it provide any stabilization at all? True... > 2) What are those paddles for? Extra weight at the tips to improve gyroscopic forces in addition to their aerodynamics. If I add similar weights to each tip of the flybar in the power drill then I'm quite sure it would act more purely as a gyroscope than it will when only a relatively lightweight flybar alone... [UK]_Nick... >> What you guys are saying it totally over my head, but it seems pretty >> obvious to me that if you spin the rotor head with the flybar paddles [quoted text clipped - 8 lines] > > 2) What are those paddles for? shall I remove blades & flybar paddles from my Sceadu and film it to stick on the web? Perhaps you're right but my instinct tells me not. My helis not good for anything else so I might as well.. > My original > speculation was about what would happen if the flybar paddles were removed and > everything else related to the flybar "system" were left in place. Exactly. I am working to keep the debate exactly on that track. > >However, even if the model's bar was not rotatable, absent paddles, the bar's > >rotation about [quoted text clipped - 5 lines] > wouldn't effect the positioin of the bar, relative to the rotor hub one way or > the other. True enough. It would be relevant if one was to consider the sweep of the bar as if it were a solid disk for the purpose of considering the hinge axis of same, but since there is no practical effect in your scenario (a plain bar), we can discount it. > The system would simply try to maintain a position 90 degrees to > the axis of rotation, that being the rotor mast! The crux of the debate. Based upon your statement, I am going to speculate that what you are missing is that the AXIS of the flybar rotation is NOT the mast centerline. The MIDDLE of the bar is ON the mast CENTERLINE, but the AXIS of rotation of the bar and the AXIS of rotation of the mast are NOT the same (except when bar and mast are perfectly perpendicular) The axis of rotation of the bar is defined by whatever plane the BAR is spinning in . . . which may or may not be perpendicular to the mast. Centrifugal force is always perpendicular to the axis of rotation of any mass . . at least so I was taught. If you accept that as fact, then you must concede that the centrifugal force exerted by the bar will attempt to pull the end of the bar exactly 90 degrees from the AXIS of rotation of the BAR itself . . not to be confused with the CENTER of rotation or mass. Another way to state this is to imagine the mast vertical and the bar tracing a perfectly horizontal plane. Incline the mast 10 degrees from vertical. Centrifugal force on the bar does NOT change unless it's axis changes. The bar (gyroscope) axis is still vertical where the mast axis is not 10 degrees from vertical. Your contention the centrifugal force will pull the bar 90 degrees from a rotation axis is absolutely correct . . . you're just looking at the wrong axis Just to confuse things a little further, centrifugal force and gyroscopic force are different animals . . . 'G' forces are centrifugal, but there is no gyroscopic effect to speak of. The solid armature of an actual gyroscope contains centrifugal force, but that force has nothing to do with the gyroscopic function. > their original plane of rotation. Only problem was, it didn't matter if that > displacement came from a gust of wind or a command from the pilot. The result > was, the stabilizer bar would fight the pilots applied commands. In time, the > flybar would try to reorient itself to a position 90 degrees to the axis of > rotation but this took anywhere from 30 to 50 seconds which left the whole > system feeling pretty unresponsive to what the pilot needed the aircraft to do. That sounds plausible. With the mast axis and bar axis out of alignment, the teeter bearings are moving and reversing direction with every revolution of the head and the resulting friction, while slight, acts as very weak 'damper' which in time would result in the shaft sort of 'floating' to a perpendicular orientation. But that would not occur because the bar was 'seeking' to be at 90 degrees because of centrifugal force altering gyroscopic force or something of that nature. > So what did they do? Enter those mechanical dampers you mentioned. These are > simple devices, hydraulically dampened, that pull the stabilizer bar back into > it's normal position of 90 degrees to the axis of rotation (rotor mast) within > a time frame of about 5 seconds. So given that the bar would take 30 to 50 seconds to settle back to perpendicular with the mast, is it fair to say that (for the purpose of this discussion) absent mechanical or aerodynamic means to force the bar to follow the mast's inclination, the mast would incline until the teeter hinge ran out of travel and (potentially destructive) mast bumping would occur? I can tell you for certain that even 5 seconds would be far too long a delay for my big old gasser which does a roll in about one second. >> The system would simply try to maintain a position 90 degrees to >> the axis of rotation, that being the rotor mast! [quoted text clipped - 4 lines] >is that the AXIS of >the flybar rotation is NOT the mast centerline. If that's true, then you're right. I'm missing it completely because at this point, I totally disagree with that statement. To try to clarify this, I'm talking about mounting the rotor hub w/flybar to the rotor mast and spin the darn thing. The flybar will then be going around in circles with the spinning rotor mast. The flybars center, the teeter point, will be centered on the rotor mast, the axis of rotation in this case. That's the ONLY center of rotation that's relevant in this conversation. As I think we've already agreed, the rotation of the flybar itself, IE: the cyclic movements that would matter if the paddles were in place, are irrelevant in this conversation. >The MIDDLE of the bar is ON the mast CENTERLINE, but the AXIS of rotation of >the bar and the >AXIS of rotation of the mast are NOT the same (except when bar and mast are >perfectly >perpendicular) Oops, I don't think so. The flybars "middle", the hinge point on which it teeters up and down, being mounted on the mast centerline, IS it's axis of rotation when it's spinning with the rotor mast. Just because the flybar is not in a plane, 90 degrees to the rotor mast at a given point in time, "does not" mean that the axis of rotation is different and that's why ti will try to reestablish a position, 90 degrees to the rotor mast. >The axis of rotation of the bar is defined by whatever plane the BAR is >spinning in . . . which >may or may not be perpendicular to the mast. I don't see how you're coming up with that. <pause to think about light that just came on! ;-) > Oh, wait a minute. Let's see if I'm getting this right. You're contention is that the axis of rotation is "always" 90 degrees to the plane or rotation. "So", if the flybar isn't 90 degrees to the rotor mast, then it's axis of rotation will not be parallel to the rotor mast and I think, you're saying that it will want to stay that way. If the flybar were totally independant of the rotor mast and hub, I'd agree with that but it's not. The flybar's teeter hinge is solidly tied to the axis of rotation of the rotor mast, via the rotor hub. It is, therefore, influenced "by" the axis of rotation of the rotor mast. If the flybar isn't 90 degrees to the rotor mast, then it's axis of rotation (as I think you're defining it) is wobbling in space relative to the axis of rotation of the mast and it will not like that. It's natural preference will be to line back up with the rotor mast and that will put the flybar back into an orientation of 90 degrees to the rotor mast. >Centrifugal force is always perpendicular to the axis of rotation of any >mass . . at least so I >was taught. If you accept that as fact, I do! then you must concede that the >centrifugal force exerted >by the bar will attempt to pull the end of the bar exactly 90 degrees from >the AXIS of rotation >of the BAR itself . . not to be confused with the CENTER of rotation or >mass. Well, because the two (flybar and rotor mast) are tied to each other, the center of rotation and the flybars axis of rotation will want to be the same, thus establishing (or at least trying to) that position, 90 degrees to the rotor mast. >Another way to state this is to imagine the mast vertical and the bar tracing >a perfectly [quoted text clipped - 3 lines] >vertical where the mast >axis is not 10 degrees from vertical. I totally agree with this IF the mast and flybar were completely independant of each other. Problem is, they're not! I refer back to what I stated above. >Your contention the centrifugal force will pull the bar 90 degrees from a >rotation axis is [quoted text clipped - 4 lines] >animals . . . 'G' forces are centrifugal, but there is no gyroscopic effect >to speak of. I'd agree with that in a free flying mass. Example, a fixed wing aircraft in a turn. The aircraft will experience centrifugal force but no gyroscopic effect. >The solid armature of an actual gyroscope contains centrifugal force, but >that force has nothing >to do with the gyroscopic function. Oh boy! <BG> "That's" an entirely different discussion. Centrifugal force may be "different" from gyroscopic function but to say they have nothing to do with each other is incorrect. Here's how I look at it. We all know the law of nature that states; "Any object, set in motion, will tend to remain in motion and it will tend to "TRAVEL IN A STRAIGHT LINE." For the purposes of what I'm about to say, it's the "travel in a straight line" part that's important. If a mass is traveling through space at 100 mph, it will maintain that 100 mph and it will (absent any gravitational forces) continue to infinity in a STRAIGHT LINE at 100 mph. Now, let's tether that mass to a fixed point so that it's now traveling "around" that point in a circle at 100 mph. The point is now the axis of rotation. The mass is also now experiencing centrifugal force because it's trying to go in a straight line but it can't, it's being restrained to that central point. As a result, the "straight line" that it's experiencing is the "plane of rotation" that it is established in and it will not want to deviate from that plane of rotation. Gyroscopic function comes into play if a force is applied that tries to change the plane of rotation. Getting back to our free flying mass in space for a moment. If we introduce a gravitational force to the equation, the mass will start to turn in the direction of that gravity. The amount of turn will be dictated by a balance of the masses forward inertia and the strength of the gravitational pull. If the inertia is stronger, the mass will see a small turn and then break free. If they balance, the mass will enter orbit of the gravitational point, and if the gravity is stronger, the mass with spiral in to impact. On our gyroscope, because the mass has forward inertia along the plane of rotation, it tries to maintain that plane of rotation. A force applied to change that plane is reacted to somewhere up to 90 degrees along the plane, in the direction of rotation. That's why a helicopters cyclic commands are applied 90 degrees "ahead" of the point where we really want the rotor system to tilt. What I'm trying to say in this long winded explination is that, while I agree that centrufugal force and gyroscopic function are seperate things, they ARE related to each other. While you can have centrifugal force without gyroscopic function, you can't have gyroscopic function without centrifugal force. FWIW, Fly Safe, Steve R. > in circles with the spinning rotor mast. The flybars center, the teeter point, > will be centered on the rotor mast, the axis of rotation in this case. That's > the ONLY center of rotation that's relevant in this conversation. I'm convinced that you (and others) are mixing "center" and "axis". Center is a point, axis is a line. There is no such thing as perpedicular to a point. PLug that into your thinking and see if you don't get a different perspective. If you have a rubber ball floating in a swimming pool and you place your finger on top and pull towards you, it's axis of rotation goes from your left to right. If you were to place your finger on top and pull sideways, the axis of rotation would go from behind you to in front of you. IN each case, the center of rotation is the same, but the axis is very different. Both centrifugal and gyroscopic forces are relative to rotation AXIS . . . i.e. a line in space, not a point. If the flybar is tettered at all, then it is out of perpendicular to the mast AXIS (centerline in our example), then it's own AXIS (perpendicular to the plane of the bar) will be different. Incidentally, it would not be wobbling, just rotating about it's own AXIS. > Oh, wait a minute. Let's see if I'm getting this right. You're contention is > that the axis of rotation is "always" 90 degrees to the plane or rotation. Yes. > "So", if the flybar isn't 90 degrees to the rotor mast, then it's axis of > rotation will not be parallel to the rotor mast and I think, you're saying that > it will want to stay that way. Provided it is spinning, yes. > If the flybar were totally independant of the > rotor mast and hub, I'd agree with that but it's not. The flybar's teeter > hinge is solidly tied to the axis of rotation of the rotor mast, via the rotor > hub. Here again, I thing you are considering a point as if it were an axis. The bar ans mast are only 'solidly' connected at a single point. They are NOT solidly connected by AXIS. The hinge alows the rotation axis of the bar abnd the mast to deviate from each other. The condition is similar to what happens in an automotive U-joint or CV joint. Both of these mechanisms transfer rotational power through a point while allowing the axis of rotation to be different on each side of that point. In fact, it may be useful to understanding the concepts if one were to mentally cut away the mast above the flybar and consider the hinge to be an actual U-joint, which is in fact what is. > rotation of the mast and it will not like that. It's natural preference will > be to line back up with the rotor mast and that will put the flybar back into > an orientation of 90 degrees to the rotor mast. Well here we come full circle. If you can let go of the natural fixation on 'point' you may see that there is no 'natural preference' for the bar to spin on the same axis as the mast so long as there is no connection between those axis. The bearing friction provides a very small connection . . at 30 to 50 seconds response time, obviously not quite enough . . . .and the physical dampers (mechanical on a real heli, aerodynamic on our models) are needed to force the bar to follow the mast. -------snip----- > Perfectly feasible even on a Piccolo, but it's a bit easier to maintain a > constant head speed when the blades have enormous amounts of inertia, and [quoted text clipped - 4 lines] > claasic symptom of poor rotor speed control) and as all turbine models are > governed, that aspect is a non issue. ------snip------ I don't understand how inconsistent head speed can affect pitching up tendencies, can anybody elaborate on this? Anyway, you say that the inertia of the flybar is enough to help keep the head speed constant? One should think that the weight of the rotorblades themselves carried a LOT more inertia than the flybar, I mean they're heavier and longer. I see how the flybar can stabilize due to it's own gyroscopic unwillingness to move when spinning and thus damping any pitching tendencies because of this effect on the blade pitch through the (is it?) hiller mixing arms (I mean; heli moves, flybar doesn't, thus heli doesn't move anyway because flybar inputs corrective cyclic in a sense). Is this a complete bollocks understanding of things? On a side note: I know some people use gyro's on the roll and elevator axises on flybarless helicopters to improve their flying characteristics. -- Lars remove ***nospam*** from email address to mail me web: http://www.turntool.com > I don't understand how inconsistent head speed can affect pitching up > tendencies, can anybody elaborate on this? Head speed has a significant effect on handling characteristics. If a heli has a tendency to pitch up, then varying head speed may accentuate or reduce that tendency making the handling inconsistent if the head speed is changing during flight. > Anyway, you say that the inertia of the flybar is enough to help keep the > head speed constant? One should think that the weight of the rotorblades > themselves carried a LOT more inertia than the flybar . . . They do . . . I think you may have misinterpreted the comment. > I see how the flybar can stabilize due to it's own > gyroscopic unwillingness to move when spinning and thus damping any pitching [quoted text clipped - 3 lines] > > Is this a complete bollocks understanding of things? You've pretty much got the idea. > On a side note: I know some people use gyro's on the roll and elevator > axises on flybarless helicopters to improve their flying characteristics. Were I a betting man, my money would be on all model heli's eventually having gyro stabilization. A few years ago I designed this: http://members.cox.net/simpson34/helipage.htm control scheme which is CCPM but with specific provision for a gyro on the elevator. Higher priorities have this heli project gathering dust at the moment, but a time is coming when a model heli without an engine governor and gyro stabilizers on cyclic will seem as outdated as a heli with no rudder gyro would seem today. Hi, Thanks for your answers, I'm trying to learn something new here. I'm still unclear about a few things, see the comments below > > I don't understand how inconsistent head speed can affect pitching up > > tendencies, can anybody elaborate on this? > > Head speed has a significant effect on handling characteristics. If a heli has a tendency to > pitch up, then varying head speed may accentuate or reduce that tendency making the handling > inconsistent if the head speed is changing during flight. Shouldn't it then have an equal tendency to pitch down? I'm not sure I understand why the head speed is the determining factor when a heli is pitching up. > > Anyway, you say that the inertia of the flybar is enough to help keep the > > head speed constant? One should think that the weight of the rotorblades > > themselves carried a LOT more inertia than the flybar . . . > > They do . . . I think you may have misinterpreted the comment. Then why are flybar-less heads more prone to irregular rotor speeds than heads with a flybar? -------snip------ Thanks Lars > Hi, > [quoted text clipped - 13 lines] > understand why the head speed is the determining factor when a heli is > pitching up. It's not the speed changes that cause the pitching tendencies, it's just that the changes make the pitching more obvious with a flybar-less rotor head. The REASON for pitch instability is usually more to do with the relationship between the positions of the centre of lift and the centre of gravity of the blades. A blade with a rearward C/G will pitch up a lot more readily than one with a forward C/G, which is one reason why we see large lumps of lead in the leading edge of rotor blades these days. (Model AND full size) > > > Anyway, you say that the inertia of the flybar is enough to help keep > the [quoted text clipped - 5 lines] > Then why are flybar-less heads more prone to irregular rotor speeds than > heads with a flybar? They're not, in fact it's the opposite IF the blades are the correct weight. Inertia must play it's part and as we all know, inertia has the effect of trying to keep moving things moving and stationary things stationary, so the heavier the blades, the less prone to speed changes they are. They slow down more slowly and they speed up more slowly which is good for flybar-less flying, but a governor and a powerful engine help too. The problems all come when there IS a speed change of the rotors and a simple examination of the rotor system as a whole will demonstrate why they pitch up as speed increases. The advancing blade picks up lift and the retreating blade loses lift, even given the fact that pitch differences exist on both blades, so if the blades speed up, the advancing blade will provide even MORE lift, but the result of the force on the advancing blade isn't seen until 90 degrees later (which places the effect perfectly for raising the nose, so up goes the nose of the machine and you've encountered pitching up from nothing more than an increase in rotor speed. SignatureBeav Please note my E-mail address is "beavis dot original at ntlworld dot com" (with the obvious changes) Beavisland now lives at www.beavisoriginal.co.uk Sorry Beav, I think I probably misinterpreted your first post. All sorted now :-) > > Hi, > > [quoted text clipped - 51 lines] > raising the nose, so up goes the nose of the machine and you've encountered > pitching up from nothing more than an increase in rotor speed. > Sorry Beav, I think I probably misinterpreted your first post. All sorted > now :-) That's fine:) SignatureBeav Please note my E-mail address is "beavis dot original at ntlworld dot com" (with the obvious changes) Beavisland now lives at www.beavisoriginal.co.uk Why try to get rid of the flybar? On a real Jet Ranger's head, there are counterweights that are affected by centrifugal force. Also, the H-1's head uses a stabilizer bar, but instead of paddles, it uses massive weights. SignatureBiggie in PA sjg1958 at hotmail dot com > Looking at the pictures on Alan's site (cheers Alan :) I notice the big Jet > Ranger has no flybar.. and so the head looks incredibly simple. > > Were there any negative effects to this? I presume it's only feasible on > such a large machine with a low rotor rpm since we need the dumbing-down on > our 3x-faster heads.. >Why try to get rid of the flybar? On a real Jet Ranger's head, there are >counterweights that are affected by centrifugal force. Also, the H-1's head >uses a stabilizer bar, but instead of paddles, it uses massive weights. No counterweights on the Jet Rangers I have flown in Josh SignatureJosh Haigh Member of the Calderdale Model Aircraft Club http://www.thehaigh.demon.co.uk (updated 02/02/2002) |
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