Rotors and control principles

[Tim]

A slight further clarification to Marks earlier post:-"as a rotorcraft accelerates in level flight, that the pilot will be feeding in more and more forward stick to avoid climbing (and in doing so he"s subconsciously compensating for dysymmetry of lift)."The statement is spot on but it is important to understand that there is in fact no dissymmetry of lift in straight and level flight, in the manner to which we are referring. As Mark correctly stated, the tendency for dissymmetry of lift is subconsciously compensated for by the pilot, who, by applying forward stick decreases the pitch on the advancing blade and increases the pitch on the retreating blade thus keeping the lift equal on both sides. Multi blade helicopters compensate by using individual Delta 3 hinges which change pitch as they rise and fall, eliminating much of the pilot input and keeping the stick closer to centre.Given that the rotor thrust vector is perpendicular to the tip plane axis, then if there was any dissymmetry of lift, it would result in a turn or climb/descent.I"ll duck down now.

[tonydenton]

hello mark, all tongue in cheek mate . I can clearly read your passionate about the subject and i"ll never have a problem with that. not very often there is 2 at least giving important sermons that are what seemingly all saying the same thing. "well in my struggling to understand view anyway."

[MadMuz]

It is great to have all the brilliant minds come together on this thread, like I said earlier, I"m not trying to stir up poop and create a ruckus.... I am just on my endless quest to get more knowledge on our magic machines....Again, I believe weightshift has nothing to do with a gyrocopter.... gyrocopters are not, never have been and never will be a weight shift aircraft, that is very well proven. Weight is critical in gyros (but not in a weightshift meaning) as we have all heard the terrible stories of what happens if the weight is suddenly no longer there.... as in the case of people having a complete rotor separation on take off or god forbid, as has happened in flight

[gyroo]

Well with all this back slapping why don"t we have an investiture ceremony at Bond Springs -Emeritus Professor Tim McLureAssociate Professor BirdyAssociate Professor Reigs

[MadMuz]

Is that you on the far left of the bottom pic Mark? ;D

[MadMuz]

You guys know I like to get people thinking....... try this for size

[gyroo]

Could be Muz - after all, I am known for my sermons!

[gyroo]

Muz - UFOs?That"s an easy Dorothy Dixer to answer:If you wanna turn to the right and climb slightly - stick yer right arm straight out sideways pointed up about 30 degrees.If you wanna do a turn to the right without climbing or descending - stick yer right arm straight out about 15 degrees down.If you wanna turn to the right with a slight descent - stick yer right leg out to the side as far as you canOr just do a skidding yaw to the right using yer pedals; orSeeing that the Aliens have prevented you from putting in any right stick, do a quick 270 deg turn to the left!MR

[MadMuz]

Not very easy pulling the wool over your eyes is it

[birdy]

[i] So weight is a critical part of our machines..... mind you, the correct weight. By that, I mean, if a light pilot has a 503 open machine, to keep the disk loading around the 1.2 mark, he needs only around 22-23" of efficient rotor.... if he were to put a set of 29" rotors on this machine, with an eventual low disk loading, I believe the machine would be very floaty, possibly quite dangerous because the rotors would not be carrying enough rpm to have enough centrifugal force to maintain rigidity.... so coning would be very high..... all in all a scarey thought.

[Adrian S]

Guys,The concepts are clearly well understood by all contributors to this thread. Now if we could now ensure that all ASRA instructors understood this and educated thier students to the same level we would be really moving forward.Just my 10c as usual.AdrianS

[MadMuz]

[i] Changing the static loading on the disc won"t change the cone angle in S/L flight.The cone angle is the balance between two forces. Lift and centriphical.If the lift ( weight) is halved, so is the centriphical force, coz the rotor only has to produce half the lift force, it"s rpm will drop automatically to compensate, so the cone angle stays the same.It"s wen the balance is changed, as happens wen G load is changed, either more or less, the cone angle changes until the rotor catches up with the new weight.Over rotored machines are generaly only slower in cyclic responce coz of the slower rrpm.Under rotored machines are power hungery.Unless your do,n stunts, or fly in very crappy air, there"s Nuthn much dangerous bout mismatched rotors.But well matched rotors make a very slick, efficient machine.Smackn your rotors against sumthn immovable ain"t gyroscopic pressession, it"s energy dissipation.If one blade comes to a sudden stop, the other blade will, in most cases, tear the head/ mast off the machine. This is why, if possible, the pilot should try to get the rite blade to hit first, coz then the head will get tore away from your head, instead of getn rapped round it.2 things can change the coning angle, one is overloading the rotors by too much weight, or very savage turns creating a lot of positive G.... that is why some machines are perfectly smooth straight and level, but develop more than usual stick shake in rapid turns or manoeuvers, then perfectly smooth again back in s&l..... could mean the tips start to cone a bit above usual..... if a machine is not quite smooth s&l, but smooth as in a energetic turn.... the tips might be a tad lower than the teeter bolt, in the turn/manoeuvre the tips might rise to the right level for smooth. Yes, the cone angle is a direct result of 2 things, one, what is designed into the hub bar and two, the result of centrifugal force and lift.... if there is more lift than centrifugal force, the tip plane will be high due to the built in coning angle of the hub bar plus the extra height from upward bending blades thru insufficient cent. force.If a machine is really over rotored (say 29" on a 503 machine) the rotors can suffer from excessive coning.... not so much at the hub, but the rotors cone higher from about centre of the individual blades, so the blades actually curve up. It is caused by the lift generated at the outermost of the rotor disk and the fact that the disk doesn"t have the centrifugal force to give the rotors the required rigidity. I have seen this when a bloke in southern NSW wouldn"t listen and thought bigger must be better and bought huge rotors. He was 65 kg, I was over 100 and even with ballast on the tail (because we hung it for him and for me and knew what ballast was needed for me to fly it) and it was awfully over rotored even for me to fly it.... float like a flutterby, sting like shoot : He had to admit defeat and bought some 24" rotors and the machine was absolutely awesome which ever of us flew it (although I am sure I could hear this little, muffled voice from the machine saying "get off me you fat pr!ck whenever I flew it ;D)When a rotorblade hits something it cant move, that is an absolute demonstration of raw gyroscopic precession in the flesh..... that is why machines continue to roll as the blades beat themselves to a stop, yes whilst also dissipating their energy.By ufo"s, I meant the type from mars or there abouts..... not the usa (bad example) well, aliens

[mad max]

In the early days "instructors" used to call it weight shift because they didn"t fully understand. These days I attempt to explain "Tim style" as he has the ability to explain well. Some students look a bit "glazy eyed" and you know that you have to give them some time and try again.

[birdy]

2 things can change the coning angle, one is overloading the rotors by too much weight,I"ll try agin :Weight changes ( static) wont change the cone angle.Conning is determined by the manufacturer, and is set at the hub bar after calculating the effects of pitch, rrpm, blade weight and probably a zillion other things, but not the weight hangn under them.Changeing the G load only momentarily changes the CA if the pilot is too aggressive in stick inputs.If the pilot is smooth and gradual, the rotors have time to spin up to take the extra load of the turn, and this increases the centriphical force equally, so the angle don"t change.If he yanks the stick back, the disc pitches up much faster than the rrpm can increase, creating more lift coz of the sudden increase in the AOA ( G load). Now you have an imbalance between rrpm and AOA, and the cone angle increases till the rrpm catches up and flattens the disc back to the sweet spot.The 28" AK blades on the wasa cone zactly the same amount wether I"m on my own with buggerall fuel, or witha 100kg passenger and 80kg of fuel.Static weight don"t effect the cone angle, only the flight rpm and power required.Cone angle, as with rrpm, is self balancing.If, wot you say is rite Muz, bout over rotoring makeing the blades cone excessively, why don"t the blade cone up excessively wen you land?You still have near flight rrpm, but no weight at all.And no, blades hitting sumthn hard is not a demonstration of GP.If your rite blades come to a sudden stop, the left blade wants to keep go,n backwards, and drags everythn that"s attached to it with it, backwards.That"s 180*, not 90*, as is with GP.Adrian, sorry, I disagree bout the clear understanding bit .

[aussie_paul]

In the early days "instructors" used to call it weight shift because they didn"t fully understand. These days I attempt to explain "Tim style" as he has the ability to explain well. Some students look a bit "glazy eyed" and you know that you have to give them some time and try again.Sorry Max, I never called it weight shift. Aussie Paul.