Rotors and control principles

[MadMuz]

After a discussion in another thread called "Flat Spins" by Mad Max, I accidently hijacked the thread Woops

[Disco]

MadMuzHey mate, you must have stayed up too late writing on the forum.

[gyroo]

Hi Mad Muz,Thanks for starting this thread, because it is a serious topic. I pounced last night because although the forum is a place for discussion, and even robust discussion, so far as the dynamics of a 2-bladed teetering rotor are concerned there really isn"t really any scope for debate. The general principles applicable to tilt spindle or tilting hub rotor systems have been well known since the late 1920"s. One of my favourite texts dealing with the subject is one that is older than I am, namely "Principles of Helicopter Engineering" by Jacob Shapiro, 1955, Magraw Hill Publications in Aeronautical Science. Shapiro dealt with tilting hub dynamics in one section. Tilting hubs were experimentally tried in helicopters the late 1940"s and the early 1950"s, but the difficulty of coping with vibration caused by power going through the tilting hubs resulted in them basically dying out. Tilting hubs work in gyroplanes because they are usually small and light machines, thereby resulting in acceptable control forces, and no power is usually being channelled through the hub.Our rotors are first and foremost a closed rotating system, where the rotor in an engineering sense is just a freely teetering or see-sawing beam.One of the really noteworthy features of a rotating system with a teetering beam is that any up or down force applied to the tips of the rotating beam at any given point during its 360 degree rotation will cause maximum deflection to occur almost exactly 90 degrees later in the rotation. This is the principle of gyroscopic precession. So, force applied at, say, 12 o"clock on a gyro rotor will result at maximum deflection at 9 o"clock (90 degrees later in the sweep or rotation).In practical terms, what this means for a gyro pilot is that if you do as Birdy says and go to your stationary gyro and push the stick either to the left or to the right and then tie it off, then walk to the tip of one rotor blade and walk it around the full 360 degrees of a circle - holding the blade tip at the same height above the ground - you will clearly see that the pitch of the blade does cycle in your hand as you walk the blade tip around. Hence, that"s why it is valid to use the expression "cyclic pitch variation" even though the rotor itself is a fixed pitch rotor.If you have tied off the control stick over to the left, and you position the rotor blades in line with the longitudinal axis of your gyro, then hold on to the front blade tip, you will see that the nose of the blade tip is noticeably pitched down, and if you look to the opposite tip way back over the tail, you will see that the blade is noticeably pitched up.Next, walk your front blade tip 90 degrees in the normal direction of rotation so that it is now at the 9 o"clock position on the left side of the gyro. You will have noticed and felt the blade pitch changing - pitching back up toward neutral as you walked it around.The final thing you need to do in this process is to use your imagination and consider what will be actually happening in flight.In flight, the rotor will be spinning about 6 times per second, meaning that a blade tip will sweep past 12 times any given point on the tip path plane per second. If the pilot is putting in a bit of a left stick input over 1 second, for instance, then there will be 12 times when the teeter bolt can impart a small incremental tilt input to the hub-bar and rotor. If you think about this for a moment, there will also be 12 times in that second when the teeter bolt won"t be able to impart any tilt to the hub-bar because the direction of the hub tilt is in line with the rotor tip-to-tip axis.Therefore, it"s again valid to talk in terms of "cyclic response" with our rotors because roughly 12 times per second the teeter bolt is imparting tiny tllt forces to the hub bar. This is the principal reason that we should all be flying smoothly and never making abrupt control stick inputs.With the left stick scenario - and using your memory of your "walk-around" experiment as the guide - you will readily be able to imagine that the forward "nose down" blade in real flight will want to sweep down to a lower point at the 9 o"clock position, while the opposing blade at the back with is pitched up will want to sweep up to a higher point at the 3 o"clock position. Gyrocopic precession causes each blade to do precisely that, and the net result is that the pilot sees the rotor disk following his left control stick tilt by tilting in the same direction - to the left.If the rotor was hypothetically replaced by a piece of round steel pipe, control stick inputs wouldn"t achieve anything - it would just spin indefinitely in its original plane until the tiny friction forces from the teeter bolt caused it to slowly tilt slightly in some direction or other. The pilot could be literally "stirring the pot" with the control stick and the piece of pipe would be basically ignoring the input.Take the hypothetical pipe off and put back your finely crafted airfoil rotor blades back on the hub bar and the game changes completely. Each blade will adopt a track through the air that is directly in response to the aerodynamic forces it is experiencing. The final remarkable aspect about our fixed-pitch teetering 2-blades rotor systems is that each of the 2 blades acts as a restraint on the other blade from doing anything too wild. In other words, the blades are completely rigid in relation to each other by being rigidly bolted to the hub bar, but simultaneously the rotor itself is free to teeter within the hub and find it"s own "plane" of rotation in direct response to the aerodynamic forces produced by the constantly changing angles of attack each blade encounters in its rotation. The tilting spindle is what induces the disk tilt, but the disk tilt is produced solely by aerodynamic forces acting on the blades. The disk tilt is not caused by any weight shift from the gyro hanging from the rotor. The pendular response of a 2-bladed rotorcraft during high energy manoeuvring is something all helicopter and gyro pilots subconsciously allow for in their control inputs, but transient pendular dynamic response in manoeuvring is sometimes confused by a small number of people as "evidence" of weight shift. This is a sad - and dangerous - misconception.

[tonydenton]

I could wait to see if some one else asks but what is flap back effect ? is that flapping.

[gyroo]

Hi Tony,Flapback is an expression in standard rotary-wing texts that is used to describe the tendency of any rotor translating horizontally through a range of increasing airspeeds to nose-up because of dysymmetry of lift. In the gyro world for some reason the expression "blowback" appears to have taken hold, but it"s the same thing.The rate at which an articulated rotor will flap back is described in the standard texts as 1 degree flapback tendency for every 19.5 knots forward airspeed.With a helicopter, to go anywhere the pilot has to tilt the disk forward to get going and then hold it tilted forward to accelerate. An outside observer watching will simply see the chopper"s rotor tilted forward at a constant angle as it flies away. Inside the cockpit, however, the pilot will be progressively feeding more and more forward stick in during the acceleration to keep the forward edge of the disk tilted down. The difference between the forward stick angle and the actual disk tilt during this manoeuvre is the flapback. In other words, the rotor doesn"t actually flap back because the flapback tendency has been cancelled out by the pilot putting more and more forward stick in. All helicopters are subject to flapback effect, even those you write about doing rolls and so forth. It"s the direct outcome of dysymmetry of lift. Pilots subconsciously compensate for it in their forward-stick control inputs.With a gyro, the principle is exactly the same. Once a gyro flies off and the pilot is holding the disk at exactly the disk angle he or she wants on a steady low speed cruise, if he or she accelerates horizontally by 19.5 knots the rotor disk will want to flap back by 1 degree. Obviously, if the pilot wants to fly straight and level and not climb, then during the horizontal acceleration he or she will simply feed in more forward stick to cancel out the flapback tendency.We all know we need to keep in quite a bit of forward stick at higher speeds to keep the disk from rising at the front. As we accelerate, and keep the disk at the same or an even lower angle to prevent climbing, the flapback becomes the angle between the rotor disk path plane and the spindle axis. In other words, the angle between the torque bar (almost always perpendicular to the spindle axis) and the rotor tip path plane represents how the flapback tendency has been accommodated for.Cheers,Mark R

[MadMuz]

I could wait to see if some one else asks but what is flap back effect ? is that flapping.

[gyroo]

Hi MadMuz,I agree with most of what you say in this post of yours. Just a few fine points1) with the Iroquois it"s true that intentional flight below .5 G is prohibited, but the worry with approaching negative g is not so much cutting the tail off because any rotor flap backwards will instantly restore positive G and simultaneously pitch the tail down quite a bit due to pendular effect. The major hazard is in fact attempted rolls to the left or to the right in a substantially reduced G situation. You will have read my several articles about unconstrained flapping of teetering rotors in reduced G conditions, and to recap the sermon it is that the reduced G also reduces the rotor thrust vector, part of which is any tilting vector the pilot is trying to introduce. So, if the rotor goes to 0.2 G, it will only have 1/5th the rolling moment. Therefore, for a given left or right cyclic stick movement where the pilot is anticipating a normal roll response, the Iroquois rotor will only have 1/5th the roll response. The soon-to-be-dead pilots then impulsively, unwisely and presumably without thinking add more stick to try and induce a normal roll response (then grossly over-controlling, in other words), and the rotor flaps well beyond normal limits in an unconstrained way to the point where "mast bumping" is experienced. This will fracture the mast and wrench the whole head and rotor off the machine.

[MadMuz]

Thanks Mark, yes, in my last line of my last post, that is what I mean by lateral inputs, evidently lateral inputs tend to remove the tail or impact the front brough of the canopy (office) due to GP.From what I have heard, but never been in a hiller, they are very susceptible to tail impacts due to their added "balance bar" making the rotors extremely stable, however the steel tube truss tail boom was a prime target once the pendulum effect of any too "hot" flying is tried......Finesse is the key in most rotary wing situations, just my opinionLast line of my previous post [especially if the pilot tries to input any lateral stick whilst unweighted]

[tonydenton]

well I"m glad we cleared all that up.

[birdy]

Bloodyell ??? :-I"m too tired and sor to post much, so I"ll just make blunt points.These are facts, not opinions.First, Tony, if it has a teetering rotor, like our gyros, or fully articulated, it"s automatic.If it"s a ridged head setup, the pilot does it, subconsciously.Mast bumping is just a helicopter way of sayn excessive flapn. ( the blades outrun the machine and bang the teeter stops).If not corrected imeadiatly, the blades will hack into the machine, no matter wot the configuration.R22s, R44s, H300s, gyros, Jetrangers........... Have all dun loops n barrel rolls.And they all archive it by keeping positive Gs on the rotor, even wen upside down.Only machines with ridged rotor systems like the Bo (wotever the rest of its name is) can survive neg G flight..5g or 0g ina teetering heli ( or gyro) isn"t instant death. The blades won"t chop anythn, solong as the 0G isn"t sustained.-G on any rotor means cyclic responce is reversed, so don"t do it deliberately, specialy on Sundays.You can loop a heli or gyro with rotors so limber they touch the ground at idle, if you have the power to do one. Blade ridgidity plays no roll in the capability to loop.Any abrupt cyclic input, on any rotor while at 0G will result in mast bumping( heli)

[birdy]

But, back to the original subject ( waiting for a fone call > )Our rotors, as is with heli rotors, fly into wotever angle the pilot commands.Even at 0G, you still have cyclic control, so there"s defiantly no weight shift.The rotor is NOT leavered over, it flys over.Both helis and gyros control the angle of the rotor cyclically.Wen one blade pitches down on its feather bearing ona heli, the other pitches up equally, through pitch links.Zactly the same Asa gyro, only it"s the hub bar that"s tilted.

[gyroo]

Hi Tony -Well put - I get your point,My dreary technical posts are hard going for most. The short answer is that we all fly our rotary wing aircraft very much by feel and instinct. All a helicopter pilot does when accelerating away is just fly close to the ground until the climb speed is reached and then start the climb. No-one thinks "OK, I have to move the stick three and a half centimetres forward".

[MadMuz]

One of the main and most prominent differences between flying rotary wing and FW, is that most older helos and our gyros are inherently stable. Most FW (sport and smallGA) is that they are much less stable, anything aerobatic in FW is usually purposely built to be quite unstable.... but controlled by the pilot. My meaning is that a FW, generally has to be made to fly straight and level with constant control adjustments.... because the machine is quite happy to roll over.... the pilot has to keep it upright. Our gyros especially (if well set up) tend to be very stable, the pilots job is to take it from its stable flight and make it do all the fun things....Where a lot of FW pilots coming to rotary wing tend to go wrong, is overcontrolling the gyro.... they are not always used to reducing stick inputs to regain stability (on calm days) as they often believe it is up to them to force it to be stable.... when doing less it will be stable anyway. I have heard of gyro instructors letting a student (FW trained) chase the stick trying to achieve a nice straight and level attitude.... when the instructor has let the student try to chase the controls to get the gyro to settle for long enough, it is not uncommon for the instructor to tell the student just to centre the stick and wait a tick..... suddenly the machine settles nicely... ;DBirdy is correct, people have looped and chandelled most helicopters and some gyros, the rigid rotors I was discussing earlier, are on the modern combat and Red Bull stunt type choppers that can do like snap rolls or climb nose up then stick over forward in a sort of tumble as opposed to a + G loop or chandelle. If these manouvers were attempted in most older helos or our gyros, it would generally end up as a short trip to the ground in a rain of confetti sized components that were once the machine

[Tim]

Rotary wing cyclic control systems introduces one of the most advanced problems of modern physics. With reams of formula involved, you need a couple of degrees and a list of initials after your name to understand it.Fortunately for us, some smart cookies of past have done the numbers and come up with the laws involved to save us the trouble.Put very simply, and in terms that relate to us, The law of conservation of angular momentum states that when no external torque acts on a spinning object, no change of angular momentum can occur, the object thus maintaining a fixed position in space.Torque-induced precession (gyroscopic precession) is the phenomenon in which the axis of a spinning object changes position when a torque is applied to it, which causes a distribution of force around the acted axis. The phenomenon is common to all rotating objects, its movement at any instant being at right angles to the direction of the torque.It is well described in previous posts how the joy stick applies cyclic pitch change to the rotors, which in turn supplies the torque on the rotor-spin axis to effect a change in the orientation of the rotor disc, at right angles to the applied force. Thus Gyroplane control is true cyclic control and is identical to that of a helicopter, it is just achieved a different way, and once understood it is deceptively simple as stated by Mark.IMHO, Both Dr Igor Regan and Dr Igor Bird are totally correct.

[birdy]

With respect Tim, it"s not your oppinion ( the IMHO thing), it"s a fact you understand fully. This is where I"m occasionally irritated on this pooter thingy.Sumone will state their oppinion.If it"s rong, sumone who knows tyres to correct them with facts.The oppinioner will argue with the facter and the oppinioners feelns will get hurt, coz the facter is pissedoff with opinions be,n stated as facts.Ol mate Mr physics made laws long before we started wasten clean air, and he don"t giva **** wot your oppinion is.Your oppinion is breakn his laws, and if your stupid enuf to argue, the penalty can be death.