How does the rotor work?

[Vance]

When I read the explanation of how a rotor works it sounds a little like perpetual motion. I know that it works, I have seen it work, I can even explain it, but I don't realy get it. The Idea of their being a driven region on the inside and lift generated on the outside seems implausible on the face of it. I have made a model and I see how the teeter helps with a tilting rotor head, but it all seems a lot like magic. I would be gratefull if I could be confused on a little higher level. I always take great pleasure in seeing how magic works. Thank you ,Vance

[Ted Eggleston]

Vance,

I too am asking about the effects along the length of the rotor.

My impression is that the rotor design generates lift but that the distance from the center of rotatation is the critical number for the RPM being generated. The velocity at any point along the rotor can be determined and as such so can the lift potential. If the lift potential being generated is forward of the COG of the chord then it must be of a driving nature. Conversely if the lifting potential is equal to or behind the COG of the rotor point then is can be considered as being driven not driving. So I offer to you the following information.

The lift equation
When an aircraft is cruising in straight and level flight, at low altitudes, the wings are set at a small angle, 2 to 5 degrees, to the line of flight. The dynamic pressure of the airflow over the wings produces an aerodynamic force with the resultant vector quantity being directed upwards and backwards. Aerodynamicists have found it convenient to divide this vector into two components, that acting backward along the flight path is the wing drag and that acting perpendicular to the flight path is the lift. (This is not quite true but we will look at it further in the aerofoils & wings module). The amount of lift, and drag, generated by the wings is dependent on:-

(a) the angle at which the wings meet the airflow or flight path
(b) the shape of the wings particularly in cross section – the aerofoil
(c) the density (i.e. mass per unit volume) of the air
(d) the velocity of the airflow
(e) the wing plan-form surface area

There is a standard formula for calculation of lift from the wings:

Lift = CL × ½rV² × S newtons

The expression ½rV² (pronounced half roe vee squared) represents the dynamic pressure of the air flow in newtons per square metre (N/m²). (Please note – if a 'Symbolic' font is not available your browser will not display the Greek letter rho, the accepted symbol for air density, and may display r or ? instead.) The dynamic pressure expression, ½rV², is very similar to the kinetic energy expression ½mV², where m = mass. Air density, of course, is mass per unit volume.

The values in the expression are:
r (the Greek letter rho) is the density of the air, item c, in kg/m³
V² is the aircraft velocity, item d, in metres per second squared
S is the wing area, item e, in square metres
CL is a dimensionless quantity – the lift coefficient – which relates mostly to item a, but also to item b.
In normal operations for our type of aircraft, and when there are no high lift devices incorporated in the wing structure, CL usually has a value between 0.1 and 1.5. It is just the ratio of the conversion of dynamic pressure into lift, by the wing, at varying angles to the flight path.

[birdy]

Gees Ted,I thought you told me you was a slaughterman,not a mathamatition.

[mrford61]

I fall into a category of people I read about once: " most gyro pilots are content with just knowing that , with enough relative airspeed , the rotors will keep on spinning".

But my un-learned brain could almost grasp the analogy of squeezing a wet watermelon seed between your thumb and finger. :o

Mark Clifford

[birdy]

Yeh,I actualy read that crap in an official airo book once too Mark.Wot stuffs that theory up is ,why do they stop when the load is removed???[the one I read said to think of the blade as a wet cake of soap between your fingers and the air pressure is your fingers squeez'n the soap.]

An easy way to imagine it is "the total of the sum of lift is forward of up"so the lift is pulling the blade round as well as up.The more load applyed to the blade,the greater the sum of lift up AND forward.

Hasn't failed this SCG yet anyway.

[Vance]

Thank You Ted, I am hoping to operate on a little lower level here. It looks to me like you are leaving out the part of how it gets powared by the wind. I see that it takes horsepower to power the rotor and create the lift. I believe that an auto gyro produces a down wash from the lift and I believe that your information helps to quantify that, but I am confused about what imparts the horsepower to the rotor. The power in and power out paths are a little counter intuitive for me. I also do better with english units.

I am trying to bring the level of discorse down to where I am able to truly understand it. I am looking for something to hold on to in my moment of terror when the rotor rpm drops dramaticaly. I believe in it, I have seen it work, but the idea of a single piece of equipment being both driven and driving challanges my thinking skills.

Thank you Birdy, I see the words, but when I try to bounce them off what I have seen and read I am still confused on a low level. I have seen it described as flow thru the driven region as up and the lift as down. This seems like very confused air to me and I know that nature is always less confused than I am. Your excelent explanation is in conflict with the Idea of driven and driving regions that NACA described in such detail in the fourties. That certainly doesn't make it wrong, I am the one who is confused. Thank You, Vance

[Doug Riley]

Vance, a gyro rotor does seem pretty far-fetched when you first consider it. It's not that bad, though, once you break it down.

Think first of a sailplane descending in a straight-line glide. Its weight pulls only straight down, yet it flies forward against a certain amount of air drag. WHAT PULLS IT FORWARD? The wing! In fact, the wing's net aerodynamic force is forward of vertical (with vertical measured w/respect to the direction of gravity).

Now, imagine the sailplane descending in a spiral instead of a straight line. Again, what pulls it forward along its corkscrew path, overcoming drag? The wing!

Next, imagine two sailplanes, bolted together wingtip-to-wingtip, fuselages facing in opposite directions. This rig will spiral down nicely, much as if the planes weren't bolted together (except that now they don't have to bank to stay in their turning paths). You now have the equivalent of a gyro rotor in a vertical descent: The rotor blades are nothing more than a pair of small gliders, bolted together.

So far, we've used gravity as the force that drags the wings through the air and gives them an angle of attack. However, a force is a force, (of course of course). Any other force that drags the wings through the air so that they have positive AOA will do just as well -- and such a substitute force has the added benefit that (unlike gravity) it can point in some direction other than straight down.

So try it. Take your two bolted-together gliders, put a swivel (a big version of the one you'd use on a fishing lure) at the point where the wingtips join. Attach this swivel by a cable to a car and tow the whole thing along. You are now dragging the bolted-together planes through the air in the same way that gravity did during the free glide. They still see a positive AOA. They still spin around and generate a net aerodynamic force that both maintains the spinning and pulls taut on the tow cable. You've just invented the gyroglider!

The rest is obvious. Shrink the two gliders down to just a pair of skinny wings and speed them up to compensate for reduced wing area. Eliminate the separate cabins (the pilots were getting dizzy anyway), and attach the cabin down below the swivel. Use trailing-edge reflex on the blades instead of a HS out behind on a boom. Keep the towline or swap it for an engine-prop unit bolted to the new (non-rotating) cabin.

The thing that remains unusual is that this rotor will work even with the air NOT approaching it from directly underneath (as it always did in the vertical spiral examples). Luckily, this setup will work even with the air approaching it nearly edgewise -- though we then need teeter hinges and we do get some new vibrations that aren't there when the rotor descends vertically. IOW, at the cost of some complexity and vibes, we can make this contraption actually carry us a distance through the air.

Amazing.

[Vance]

Thank you, Doug, that is on my level. It makes sense, but it still seems like magic.

How does this relate to the driving and driven regions that NACA spoke of.

In the model teeter rotor head that I built to understand, when it reaches a certain speed it seems like power steering when I tilt the mast and it gyroscopic tendenses seem to evaporate. When I do that with my sail planes in my head it seems to bend the wings. I would be gratefull if you would bring that concept down to my level also. Thank you, Vance

[Al_Hammer]

Autorotation Basics
If you consider the motion of the blades in 3D, they are spiraling through the air, Rotating and moving forward at the same time. It might be easier to imagine an airplane in a tight spiral glide. It has lift holding it up and it is moving itself forward and it is turning in a spiral. Somehow this seems less magical, but it is really very similar to the way autorotation works.

The rotor is divided into two major regions called driving and driven. This does not mean that air is moving up in one and down thru the other, although there are some areas where that may be happening. Both regions produce lift. The inner region sees less rotational airspeed, obviously, due to the smaller radius. Here, airspeed from rotation is still dominant over airflow coming from forward flight. A bug on the blade sees a mostly "horizontal" wind in his face.
The angle is slightly from below at the blade tips and as you move inward the angle gets more and more from below. This means that Angle of attack is increasing as you move inward. The blade angle is fixed, it is the wind that is shifting to be more from below. As that happens the lift angle also tilts forward.
The rule of thumb is that lift is always at right angles to the direction of airflow. If air is coming from below, the lift is not straight up, it is tilted forward of vertical and thus it tends to propel the blade.
As you go further out along the blade, the rotational speed is greater and the airflow is coming from a more "straight on" direction. Angle of attack is less and the lift direction is tilted back to vertical and beyond. This is called a driven region because the lift has this rearward component and it takes power that is supplied by the driven region to keep the rotor at a constant rpm. Net driving force equals net drag and the rpm reaches a constant value (which varies with load.) rpm varies as the sq. rt. of load change. double the load and the rpm increases by sq. rt of 2.

If you remove the load on the rotor(weight) then the rotor will no longer be able to thrust air down and loses its lift. Same as letting go of a kite string. Without the weight pulling down, the kite string goes slack and a slack string is obviously not lifting anything. The rotor mast can't go slack, but the lift still goes away when the load is removed (in a zero g maneuver).

[Al_Hammer]

I didn't see Doug's post when I was typing mine. Guess I said some of the same things.

Vance, the reason for the "power steering" effect that you mention is that you are not trying to tilt the mast of a rigid gyroscope ala bicycle wheel. The bike wheel is on a bearing that is rigidly mounted to the axle. On a gyro the teeter joint allows no torque to be transmitted thru it. To tilt the rotor you tilt the hub which tilts the blades about their long axis. IN other words the blades change pitch at the advancing/retreating sides if you push the stick forward. This change of pitch is only acting over a portion of the circle. When the blades get to the fore and aft position they are no longer affected by the forward stick because the teeter joint is lined up fore and aft and rocking the head does not change pitch of the blade at that position. The rotor sees a differential lift across the disc and flies to a new position. Each blade reaches the new position at a point 90 degrees from where the pitch change was made since mass does not accelerate instantaneously. This lag is called precession.

[Vance]

Thank you Al, That is definatly confusion on a higher level. I believe I am starting to get it. Am I corect in understanding you,that the change in the angle of attack is changing as you describe as a percentage of the air speed? I'm still a little fuzzy there.

I am still a little confused about driving and driven regions, it sounds to me like they are both driven.

I would apreciate it if you would also shed some light on my power steering question in post #8. Thank You, Vance

[Al_Hammer]

Vance, good questions. I think you're getting it. :)

Airspeed coming from forward flight is constant everywhere over the disc, (although it is positive on the advancing side and negative on the retreating side). Airspeed due to the rotation of the blade changes with radius. The bllade tips have a higher total airspeed, but also, the angle of attack is less there because you have a mostly in-plane wind and a fixed amount of wind coming from below the plane of rotation (the forward airspeed). At the inner portions of the rotor the in -plane wind is less of the total wind that the blade sees. total airspeed is less and lift is less, and angle of attack is greater.

The driving and driven regins both produce lift. As you move out along the span, the lift constantly is tilting more towards the rear, away from the spin axis because of the angle of attack situiation I decribed before.
any lift acting forward of the spin axis will propell the blade to go faster(driving) and any lift acting behind the the spin axis will tend to slow the blade(driven)

[Vance]

There is a good example of why I need to learn to type with more than two fingers.

Actualy, it worked out well because Doug started on a little lower level, raised my confusion to the point I could be confused by you, without abandoning my tenious grip on sanity.

Thank You again Al, In your discription are there torsional loads put into the blade root like with a swash plate, or is this the avantage of a tilting head? When I watch it go around it is very hard to see what you describe. I don't dought it though. Am I understanding you correctly, that the rotor flies to its new angle when I move the mast? Where did the precesion go? The blades, acting as a gyroscope, still moved so shouldn't there be a precesion at ninety degrees? If this is delt with aerodynamicaly doesn't it want to bend the blades? Thank you, Vance

[Vance]

ARRRRRRGGGG!!! I cant keep up and I have to go be functional for a while. When I use more than two fingers I can't see the letters on the keys, I know that there is a better way. I will be back this evening. Thank you, Vance

[rehler]

Vance, the angle of the lift component tilts forward in the "driving" region (driving the blade forward - making it spin). It is not tilted forward in the "driven" portion (outer area of the disk) and is not driving the blade forward, just providing lift (this portion of the blade therefore is driven by the inter portion of the blade).

As explained earlier, the relitive wind angle of attack changes along the blade length due to the speed of the blade changing form almost zero at the center to 400 mph at the tips. This causes the lift angle mentioned above to also change.

The question that many can not comprehend is why does the sail plane move forward while in a slight decending glide. How does the wing make the plane move forward while gravity is pulling straight down. Visualizing the sail plane riding on the air is easy to see, but the same thing on a spinning gyro rotor blade is more difficult, but it is the same thing.

[Al_Hammer]

Where did the precesion go? The blades, acting as a gyroscope, still moved so shouldn't there be a precesion at ninety degrees? If this is delt with aerodynamicaly doesn't it want to bend the blades? Thank you, Vance

The movement is 90 degrees away from the applied force. The rotor tips forward when you apply differential lift at the sides. There is no bending force on the rotor, in fact no bending moment can ever develop because of the free joint.


Here's a diagram I just whipped up to illustrate how the "power steering" effect works.(No effort required to move massive spinning rotor.)
Control with a swashplate is similar, but then you have feathering hinges to allow for collective change of pitch. If you didn't need collective on a helicopter then you could use a gyro type head.

[Victor Duarte]

a link to some simple explanations about autorotation :
http://www.copters.com/aero/autorotation.html
There is not rule to define the stall/autorotative/drag regions, depends on the blades airfoil, RPM, twist etc..

ken, about the sail plane, the motion id due to the CoG to be placed a little forward the aerodynamic center of the wing(s), but the chord of an airfoil is not the line of "zero lift", so, even if the wing chord is oriented straight, there is still a lift.
Centerd forward, the fuselage tends to pitch down, and get the sailplane in a fast horizontal motion + a slow vertical descent, this ratio is called "finesse", it can be about 60 on the best sailplanes (60 miles forward, 1 mile down), 17 - 10 on ultralights, and often under 1 on gyros. (correct me if i'm wrong)
thanks

[Al_Hammer]

Victor, I think the glide ratio of a gyro is more like 4 to 1 or even 5 to 1. A bit better than a helicopter. I didn't know it was called finesse. More like fini, as in, you're finished.
Just look down between your feet and that's where you are going.

[Victor Duarte]

Al, i'm not shure of the word "finesse".. ok i'll call it glide ratio.
Thanks for correction, i must also correct my self : i meant that a gyro is able to have a glide ratio under 1 according to some witnesses.. falling more than advancing.. is it right or impossible ?.
So what is ,on your opinion the smallest safe GR for a gyro?
thanks

[Al_Hammer]

So what is ,on your opinion the smallest safe GR for a gyro?

I don't know...what do you mean by safe? I prefer 60:1 :D

[Victor Duarte]

I don't know...what do you mean by safe? I prefer 60:1 :D
lol, and what about infinite ? :D
i was asking if there is a recommendation to keep the glide ratio over or under a given level , on landings for example, to stay in the flight envelope
thanks

EDIT : Al, nice drawing
I am not shure the teetering was made to allow pitch change in this way, ok, i speak more for helicopters..
But the teeter was more made to manage adv blade problems..
The "ease" on pitch is through the blades CoT, longitudinal.
so , i think you should swap yout drawings, theorically, the pitch input happens when the blades are aligned with you eye, and the blade disk change is only effective 90 ° after (your first drawing)...

the problem with the bensen design is that it is not a "perfect" teetering design, the gimbal being under the teeter axis while it is On the teeter axis in an helicopter hub (ex : bell47 or r22), this allow the rotor disk to hang the mast like under a chinese hat, doing so, the center of the disk is shifted, allowing more controlability..

corrections please

[Al_Hammer]

I am not shure the teetering was made to allow pitch change in this way, ok, i speak more for helicopters..
But the teeter was more made to manage adv blade problems..


Victor, I do not understand your objections to my diagram. I agree that the reason for teetering is to help with disymmetry of lift on advancing/retreating side, but the teetering hub also allows for cyclic control as I've shown. Flapping causes a pitch change by virtue of the blades vertical motion(flaps up/down) Ok? which changes relative wind. But stick input causes a direct pitch change.

The offset gimbal in the gyro adds stability in pitch, as I think you must be aware.

I'm sorry if I don't fully understand your comments.

[Victor Duarte]

Al, it wasnt really an objection, your diagram is ok, i just visualize better the gyroscopic effet another way, but drop it, no matter to objection.

Yes i agree about stability

Sorry if, sometimes my comments are a little confuse..
what i see, is that the aero behaviour of a Bensen head cant'be compared to a helicopter one and i think i will learn a lot, just go on, ..

thanks

[Al_Hammer]

Ok, Victor. :D

Vance, just for you...
How about a comparison of a gyro head with a helicopter swashplate control?
(Dominator vs Helicycle)

The pictures are marked to show what happens with left stick input this time.
The two methods achieve the same result. In the helicopter, the head does not tilt, but swashplate does. In both cases the rotor winds up tilted to left from where it was.

[Victor Duarte]

that's ok for gyroscopic effet AL, clear.
the difference still in the way an offset gimbal works vs the helico hub..
a basic question, so, why is the teeter axis positionned above the disc ?
thanks

[Al_Hammer]

so, why is the teeter axis positionned above the disc ?

Victor, are you referring to "undersling"?
www.scotiabladerunners.ca/underslinging.htm

That is to put the teeter bolt on a line connecting the cg of each blade. Coning places the cg's above the plane of spin.
It reduces coriolis effect. (No lead-lag hinges needed) Another piece of magic, for sure.

[MichaelBurton]

What if we looked at it like this. The aoa at the center is high due to the lack of rotational speed so this part of the blade is "stalled". As we move out from the center the blade is moving faster. At some point the speed of the blade is fast enugh to make the aoa small enugh to begin generating lift. As this part of the blade generates lift it pulls the rotor forward "driving" the rotor. As we move even further out on the blade we get to the point where more lift is being generated than is needed to spin the blade. This part of the blade is being "driven" forward by all but the stalled portion of the blade.

[Vance]

Thank You Doug, Al and Victor. That helps a lot! I feel reinvigorated with with a new level of confusion!

Victor, that is a puzzelment of mine also, if I am understanding you correctly. There doesn't apear to be much effort to explore different offsets for the "offset gimbal rotor head" and I would think that the pivot being close to the bearing would be a good thing. It may be one of those things where an inch is the best ofset and it is also easy to do.

It may be that moving things out of line by the distance most of the rotorheads do is not a problem, but it doesn't seem like best practice. I don't see much effort to make the distance from the pivot to the rotorhead shorter. I think that it is a little better than your drawing on most machines though. It would seem to me that at least on a theoritical model it would be best to move the rotating bearing as little out of line as possible, especialy since we are stuck with the teeter height.

In the gyroplanes I have flown it is all about pressure and it seems fairly steady (different pressures provide appropriate response rates), but it does not seem to be quite even in a side by side when turning left vs right. I asume that the springs are to address this, but it still seems like I am lifting the ship. I realize that the prop torque changes the left to right responce also, especialy in climb out.

I would be gratefull for insight on what governs the amount of offset and the height of the rotor head above the pivot. I believe I have a grasp of the need for teeter height. Thank You, Vance

[MichaelBurton]

Check this out.

http://www.pilotco.com/SpindlesandGimbals.htm

[Vance]

I was in the middle of my post when all this new stuff happened. I am trying to multi task, which acording to the FAA I should not attempt. Could they be right??

Al, My big question about the difference in how things work between a swash plate rotor head and an offset gimble rotorhead, which you so kindly provided is, Does the blade on a swash plate rotor go through a twist cycle per revolution for as long as the rotor thrust vector is not in line with the shaft? Is this twisting part of why a helicopter is so much harder on blades than an Autogiro? And then of coarse is the effect of the tilting rotor head doing efectivly the same twist?

I realize that the forces must be low because they use those itty bitty links that always made me nervious in my preflight. Looking at the pictures of the helicopter rotor head I see great big arms with itty bitty links. Over 300 cycles per minuet even if they are weak cycles, seems somewhat distructive to a shape that is not really designed for torsion.

Michael, one of the interesting differences of a helicopter rotor compared to an autogiro rotor is how it stalls. It is my understanding that drag goes up a lot when stall occurs and generaly stall happens because the angle of attack is too high, not the air speed too low.

The NACA pictures that I am so fond of show the driven region to be roughly the first third of the rotor blade in a not quite round pattern. This is where I have always gotten stuck before this series of answers.

Michael thank you for your help and I am not saying you are wrong, only counter intuitive. For me this describes many of the aspects of an autogiro and that is part of why I think that it was such a brilliant invention. Thank you, Vance

[Vance]

Thank you Michael, I have printed that out and am going to study it now. From what I have read so far it addresses some of my confusion. I read slow and think even slower, so it will take a while for me to digest it. I suspect that it will lead to more questions. Thank you, Vance

[Al_Hammer]

Vance, the offset is there for a good reason. It provides a mechanism by which the rotor can pitch forward in an updraft. This improves stick-free stability. Springs are used in conjunction with the offset pivot and the system is balanced at one thrust level.
If an updraft increases rotor thrust, the rotor can swivel forward, reducing the disc angle of attack. It also provides force feedback into the stick for stick feel.

As I recall, that article that MichaelBurton posted has some serious errors regarding the spindle head. (Doug can explain more..) Parts of it are informative, though. Also, that version seems to be missing the original diagrams. try this
http://www.autogyro.com/technic/offsetg.htm

[MichaelBurton]

Also, this version seems to be missing the original diagrams.

Al, can you see any of the diagrams? All I did was clean them up a bit.

[Al_Hammer]

Vance, just to bombard you some more;

You are correct. There will be a cyclic torsion load on the blades as long as the rotor is not parallel to the swashplate. In the gyro, this load also exists and will show up as a resistance in the stick as you make a control input( and it is very slight). The thing to remember is that the rotor does not have to precess very far on each rotation. To make a 5 degree change in tilt the rotor moves in tenths of a degree per revolution. The load on the pitch links is gone once the rotor reaches the new tilt angle. The swashplate is tilted at the new angle and the rotor and swahplate are again parallel.
Hope that is what you were looking for.

[Al_Hammer]

Michael, I can't see any images, sorry.

[Vance]

Thank you Al, That doesn't seem right. I would think that the swash plate has to continue to comand the rotor to fly at any angle that is not square with the rotor shaft. It would seem like this would cause one complete twisting cycle pre revolution as long as the rotor wasn't at right angles to the rotor shaft. You are right I am Hammered again.

I believe that I am imagining too much movement. I have watched valve gear for too long where the push rod is moving .5 inches 3,000 times per minute.

Why are the pich arms so beefy and the pitch links so spindly? Thank you, Vance

[Al_Hammer]

Vance, if the rotor and swashplate are aligned, how can there be a load on the links? The distance from pitch arm to swashplate is constant around the circle.

The fact that the shaft and the rotor are not aligned causes problems in the form of vibration, but that is a hookes joint issue, perhaps?
I suppose the pitch arms are so beefy because they are L shaped and carry the load around a corner. Also, on my Helicycle, they are aluminum.

[Vance]

Al, I am seeing the rotor wanting to return to level and the reason that it stays at the odd anglel is because the swash plate is comanding the blades to fly at this odd angle thru the links and twisting. I am sorry that I am not getting it. I may have held this incorect belief for too long to easly let go of it. Thank you for all your help, Vance

[Al_Hammer]

Vance, I'm not sure what force would be trying to return the rotor to level.
The rotor does not act like it is spring centered like a joystick, I don't think.
Move the stick in a Robbie and see if it wants to center. There are stick forces in forward flight, but not centering forces. The rotor tries to blow back form flapping and it wants to tilt left from inflow roll effects, but that's not what you're talking about.
I'm keeping an open mind, though. :)

Thanks

[Vance]

I remember trying to sneeze in a robbie and not knowing what I could let go of. The cyclic is definatly not self centering. I feel that any time the swash plate is not square to the rotor shaft it is asking the rotors to twist as they go around. Thank you Vance

[Al_Hammer]

Vance, that sneezing story is very amusing.

I'll try to convince you tomorrow that there is no cyclic "twisting" required to keep the rotor flying in a given plane.

[rehler]

Michael, Al and Vance,

Don't take that article as being completely correct.

For example his theory shown on figure 12 is all wet (wrong). You can not determine the trim spring tention as he describes.

On my rotorhead the teeter bolt is 7.38" above the pitch bolt (rather than 5" as in the diagram). The gyro load is 800# (rather than 550# in the example) and the offset is the same 1". My distance from the pitch bolt to the spring attachment is 7" and the drag is about 145#. Using his theory I would need to be pushing UP (not down) on the back of the torque tube with a 38# force, but I actually need to pull down (the normal way).

[Vance]

Thank you Ken, I missed the error. I was confused on a lower level. Thank you for the corection, Vance

[Vance]

Hi Al, I am picturing the angled swash plate as a cam that causes the pitch links to rise and fall once per revolution. I am sorry for being so pig headed on this one.

My friend in Canada works for NRC, their version of NASA and he showed me a high speed movie of a rotor blade on a helicopter going around and it had a violent whip that apeared to come from a torsional imput. I guess that is just stuck in my head.

Thank You, Vance

[Al_Hammer]

Thanks, Ken.

Vance, I stand by my answer. (Explained in a later post.)

[Vance]

Hammered again, Thank you Al. I have been struggling to come around to your way of thinking, and the immersion in thought has helped a lot.

Back to the questions, does that twisting shorten the life of helicopter blades?

Would a swash plate system shorten the life of autogiro blades?

Is there a down side to a tilting head?

Why did Pitcarin go to a shash plate head?

How much stability do you gain with an offset gimble rotor head compared to a swash plate head?

If someone made an in flight pitch adjustable rotor head for an autogiro for better spin ups, no jump take off, how many degrees of pitch adjustment would you want?

With the aboove head, would a tilting mast be better than a swash plate head?

See what happens with thought immersion!!! Always an expansion of questions!

Thank You, Vance

[Al_Hammer]

I'm sure others will jump in to help you , Vance, but here is my attempt to answer a few of your questions:

Is there a down side to a tilting head?
(from a cartercopter site)

Problems Associated with Tilting-Spindles.

Because the CarterCopter's rotor has such high inertia, it will exhibit a lag in control when responding to the tilting spindle's shift because of the gyroscopic inertia of the rotor. The higher the inertia the greater the lag. This can be countered by using delta-3 (see below for a more detailed description of delta-3) in the rotor head teeter. Delta-3 allows a high-inertia rotor to track the spindle much faster than a conventional 90 degree angled see-saw teeter hinge.

One of the reason lightweight gyros use tilting spindles rather than swash-plates, is the utter simplicity of the tilting spindle concept and mechanisms. Comparatively easy to design and manufacture.

But it has its price. The longer the tilting spindle is the harder it is to tilt it if the gyro is at the same time subject to heavy G forces. This is because the weight of the gyro especially if magnified by a dive or other G forces means the pilot is having to fight the leverage created by the length of the tilting spindle and the instantaneous weight of his gyro slung beneath the centre of lift which is effectively trying to hold the spindle straight.

The fact that helicopters are much heavier than gyros has meant that any type of tilting spindle has usually been out of the question.

Also it is uncommon for a craft to employ a tilting-spindle for cyclic-pitch control as well as to have collective pitch control (rather than a fixed pitch head) at the same time. 'Tilting spindle' is usually synonymous with 'fixed pitch'.
But the CarterCopter has both a tilting spindle and collective pich.

While the CC has taken the 'tilting-spindle' approach and although the CarterCopter is much heavier than most other like gyros - CarterCopters have succeeded in getting the tilting-spindle universal gimbal bearings to with 1/2 an inch of the centre of lift. This is because of its mono construction of the whole rotor assembly.
http://www.internetage.com/cartercopters/writeups6.htm#Answer2

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How much stability do you gain with an offset gimble rotor head compared to a swash plate head?
As much as you want, probably. A little is good. Too much offset and the rotor wants to fight any change in Angle of attack.

If someone made an in flight pitch adjustable rotor head for an autogiro for better spin ups, no jump take off, how many degrees of pitch adjustment would you want?
Probably 4 degrees is a good number, if you want a guess. :)

[Vance]

Thank you Al, that is an excelent start. I would think that the Carter Copter wouldn't be able to use a swash plate because of the twisting I have been talking about. They have very heavy, weighted blades and the torsional forces go up dramaticaly as the mass increases.

It seems to me that their information is in conflict with itself. A tilting head doesn't work well on heavy rotor, but we use a tilting head because it works best because we have heavy rotors???

So Al, your saying that the length from the hinge to the rotor is important and you can have too much offsett in high g turns that can't be delt with with springs??? This is one of my sorces of confusion. I don't see much expermentation in this area. What do you think is the minimum offsett for stability? I keeping up with .625" and less distance from the hinge to the head is good.

The 4 degrees is just where I ended up. Thank you, Vance

[Al_Hammer]

Vance, newsflash!
I should have stuck to my guns. I must now add to your confusion by saying that I think my original answer was correct regarding cyclic forces. I know this is tough to believe, but although there is twisting motion between the blades and the hub, it is what is called kinematic motion and the only load is bearing friction,(which may be appreciable, so we're both partially right) no inertial load from accelerating the mass of the blades. You are NOT forcing the blades to move relative to the plane of motion.

In the diagram below, you can see that as the blade moves 90 degrees from top view to bottom view, that the angle changes with regard to the hub as I have marked.
In the lower drawing the blade leading edge is lower than the trailing edge and there is a 10 deg angle as shown. But it is not the blade that has rotated, it is the hub that has not followed the motion of the blade.
The blades are moving in a plane and they do not change pitch with regard to that plane. If the rotor was a solid disc spinning at the same angle as the rotor, a point on the edge would move "up" and "down" just like a blade tip, but would have no motion relative to the disc. No portion of the solid disc would experience a torsional motion.
You could draw or paint a rotor blade on the disc and it would move through space exactly as the real blade. it is only because the shaft is not at the same ange as the disc that there is need for a hookes joint type of freedom between the hub and blades.

In the tilt head rotor, the shaft tilts along with the rotor, the shaft being the rotor axle.
There is no torsional load on the blades in either rotor, (other than bearing friction in the case of the swashplate rotor.) There is a torsinal load whenever the stick is moved, but only so long as it is in motion, i.e. while the disc is flying to a new plane.
It helps to take a CD and rotate it as if it were a spinning rotor.(2nd diagram) you could mount a swashplate to it and the links would move up and down when the disc was inclined, but there is obviously no feathering motion involved.

[Vance]

Hammared again, thank You Al, If you are twisting the blade 350 times per minuet I would think that the blade would have an inertial resistance to reversing direction 700 times per minuet. I guess that I am just stuck on this point. Sorry, Vance

[Al_Hammer]

No, Vance, the blades are not twisting about their own centerlines. I can't do better than the CD example. The blades are painted on the surface of the CD, so how can they feather? Yet they still appear to reverse direction on each revolution.
The only force is from bearing friction. In the Helicycle it is less than 5 lbs cyclic load on the pitch horn in flight, as I saw explained in one of the construction videos.

Thank you for helping me to deepen my own understanding by thinking about this problem.

[Victor Duarte]

Hi vance and al,
al, isnt there a little delay between the pitch imput at 12:00 and its final state at 3:00 ? the airfoil doesnt react instantly to the pitch change, and there may be a little torsion moment ( particularly with an asymetrical airfoil) while a bending moment. (assuming that on an ideal airfoil, there should be no effort on the pitch horn...)
right ? wrong ?
BTW, in semi-rigid rotors, the precesion can be evaluated to 70° .. how ?
just clearing black points for me, not digging the sh..
thanks !

[Al_Hammer]

Victor, yes, there is a delay, lets call it 90 degrees in this case. There are forces acting on the blade to get it to move, including , possibly, some small change in pitching moment.
But once the blade reaches the new position, there are no further forces needed. The blade will stay in the new plane same as a rock will stay in any plane it is spun in.

Regarding your second point, the 70 degrees...
The amount of the phase angle is usually close to 90, but depends on the ratio of aerodynamic damping to gyroscopic damping. Mass of blades, density of air, etc, all effect the angle. Also, if there is a delta-3 angle, then the phase angle can be way off 90. The R22 helicopter has an angle of 72 degrees, because it has delta-3 of 18 degrees.
delta-3 acts like a (aerodynamic)spring opposing the flapping motion of the rotor.
It limits flapping angle and has other desirable effects, such as reducing the roll with forward airspeed and the cross coupling of pitch into roll.
Frank Robinson wrote a good post on it which I can put here if you want

[Vance]

Al, I would be gratefull for any ingo you can share. I have trusted my life to Mr. Robinson, so I would be especialy interested in anything he has to say.

Al I am gratefull that you enjoy and benifit from the proscess of re-examination. Thank you, Vance

[Al_Hammer]

Ok, Robbie fans, here it is

Frank Robinson's remarks:
I have read various explanations in this forum attempting to explain the dynamic and aerodynamic characteristics of the R22 rotor system, especially the 18-degree delta-three angle designed into the R22 swashplate and rotor hub. This is a highly technical subject which can only be fully explained using very technical engineering terms. However, since there appear to be a number of misconceptions and a great deal of interest by some pilots and mechanics, the following is a physical explanation of the reasons for the 18 degree delta-three phase angle.
First, keep in mind that the 18 degrees is only in the upper rotating half of the swashplate. The lower non-rotating swashplate is aligned with the aircraft centerline and always tilts in the same direction as the cyclic stick. Many helicopter engineers have difficulty understanding how delta-three (pitch-flap coupling) affects the phase relationship between the rotor disc and the swashplate. Delta-three only affects the phasing when the rotor disc is not parallel to the swashplate and there is one-per-rev aerodynamic feathering of the blades.
For instance, feathering occurs while the rotor disc is being tilted, because an aerodynamic moment on the rotor disc is required to overcome the gyroscopic inertia of the rotor. But once the rotor disc stops tilting, the rotor disc and swashplate again become parallel and the delta-three has no effect on the phasing. Aerodynamic feathering also occurs in forward flight, because it is necessary to compensate for the difference in airspeed between the advancing and retreating blades. Otherwise the advancing blade would climb, the retreating blade would dive, and the rotor disc would tilt aft.

The R22 rotor system was designed with 18 degrees of delta-three to eliminate two minor undesirable characteristics of rotor systems having 90-degree pitch links. In a steady no-wind hover, when forward cyclic pitch is applied, the 90-degree rotor disc will end up tilted in the forward direction, but if no lateral cyclic is applied, the rotor disc will have some lateral tilt while the rotor disc is tilting forward, sometimes referred to as "wee-wa." This occurs because while the rotor disc is tilting, the forward blade has a downward velocity and the aft blade has an upward velocity. This increases the angle-of-attack of the forward blade causing it to climb, and reduces the angle-of-attack of the aft blade causing it to dive. If no lateral cyclic was applied, this would result in a rotor disc tilt to the right while the rotor plane was tilting forward. Pilots subconsciously learn to compensate for this by applying some lateral cyclic as the cyclic is being moved forward. The amount of delta-three required to eliminate "wee-wa" in the R22 rotor system was calculated to be 19 degrees.
The other undesirable characteristic in rotor systems having 90-degree pitch links is the lateral stick travel required with airspeed changes during forward flight at higher airspeeds. The ideal rotor control system would require only longitudinal stick travel to increase or decrease the airspeed. This is not possible with a 90-degree pitch link system, because the rotor coning angle causes the rotor disc to roll right as the airspeed increases. This occurs because the up-coning angle of the forward blade increases that blade's angle-of-attack with increased airspeed, while the up-coning angle of the aft blade reduces its angle-of-attack. Consequently, the forward blade then climbs while the aft blade dives, thus causing the rotor disc to roll right with increased airspeed. To compensate for this with a 90-degree pitch link rotor, the pilot must apply some left lateral cyclic as the airspeed increases. The amount of delta-three required to compensate for this effect in the R22 rotor system was calculated to be 17 degrees.
A delta three angle of 18 degrees was selected as the best compromise angle to reduce or eliminate the two undesirable characteristics described above, which would have been present in the R22 had a 90-degree pitch link design been used. Subsequent instrumented flight test data confirmed the choice of the 18-degree delta-three angle. Hopefully, this will help clarify a few of the misconceptions concerning the design of the R22. -Frank Robinson

[Victor Duarte]

Thanks Al, interesting explanation, always learning.
Vance and Al, what could you say about the bending bars on hub-bars, i see alot of rotors using them for coning, according to my understanding, it is an heresy as the teerering rotor doent use flexible blades, the flapping being allowed by the teetering tower. So, sould a bensen-like hub be rigid with a built in angle ? is are th ebending plates useful for vertical damping while pitch changes...
still in the dark...
another question, do you think the teeter pivot must be strictly rigid or could it be "elastic" like with elastomeric bearings?
thank you

[Vance]

Thank You Al, My head is spinning like the girl in the movie "The Exorcist." I am not sure where this 18 degree delta-three is. I have read about this in the past, but past it, whistling in the dark of my ignorance. Any light you could shed would be much apreciated. Thank you, Vance

[Al_Hammer]

Very funny, Vance. Maybe I'll sprinkle some holy water on you with this photo.

Notice how the pitch links are not on the centerline of the mast. As the rotor flaps or teeters, it will tend to pull pitch out of the blades. This is pitch-flap coupling, also called delta-3. You can also get the same effect by skewing the teeter bolt at an angle to the head (like on a tail rotor.) Anyway, it changes the precession rate and does all those things Frank R. talked about. The swashplate is rotated 18 degrees on the robbies as a result of this change in phase angle.
Keep that head spinning, just don't vomit on me. :D

[Vance]

Al, My head has stoped now, but it seems to be pointed backward. Thank you for the holy water. That is particularly interesting. How would you get the same effect with a tilting head, rotate the hinge 18 degrees?? This is great stuff. I sense that I am not going to sleep well tonight, at least till I get my head pointed the right way. Thank you, Vance

[Victor Duarte]

I only know 2 ways to have a d3 effect..

[Al_Hammer]

How would you get the same effect with a tilting head, rotate the hinge 18 degrees?? This is great stuff. I sense that I am not going to sleep well tonight, at least till I get my head pointed the right way. Thank you, Vance

Yes, rotate the hinge 18 degrees around the vertical axis so that teetering also causes a pitch change.

Victor, maybe 3 ways--- www.unicopter.com/0941.html :D

[Victor Duarte]

Victor, maybe 3 ways--- www.unicopter.com/0941.html :D
Oh, just right, but i admit i didnt get it at first sight and didnt went deeper into that... perhaps i missed something.

applicable to bensen-like ?

you can also imagine a skewed flexible plate, simple

[_MOL_]

AL
This is a great discussion. AL, I’m getting fond of yours and Dougs comment on things, I’m getting it at a higher level. So to speak. :rolleyes: (For me it is anyway)
Sorry if I keep it on a level of simplicity, I Danish remember? Don’t know how else to say it in English.

The talk of having a Delta-3 angled teeter system on a “small” gyroplane, isn’t that a little much you think?
I mean, do we have the “wee-wa” effect in a degree where we could benefit from this Delta-3?
Maybe we have but I can’t seem to remember if we do, or I have just learned to compensate for it without knowing it.

By having a Delta-3 hinge would that reduce some of the rotor shakes such as the H-force?

Do you know of anyone have tried to use a Delta-3 angled teeter?
(Referred to the gyro rotor system we use of cause)

Thank you

[Al_Hammer]

Brian, I appreciate your comments.
I don't think there is much need on a small gyro to compensate for wee-wa(nice term, isn't it?), but delta-3 is used on the Air&Space 18A certified gyroplane if I'm not mistaken, or maybe its the J2 I'm thinking of. It limits flapping and makes for a rotor that is "mast following", which makes for crisper feel, according to some comments Chuck Beaty made, but stability is less. Pitch-cone coupling is good on a jump takeoff gyro because it pulls out pitch as the rotor slows down, a safety feature.

delta-3 is used on a lot of the r/c model gyros. You might search the r/c forums for info. There have been attempts made to use delta-3 on experimentals.
The designer of the Mosquito ultralight helicopter tried some delta-3 on his helicopter and said he didn't notice anything different,so he took it out.

Thanks

[Victor Duarte]

Pitch-cone coupling is good on a jump takeoff gyro because it pulls out pitch as the rotor slows down, a safety feature.
Interesting Al, i didnt think in that "border" usage.. how do you think it could be useful ? delay rpm loss ? isnt it too late when the rpm decrease comes to lower the cone ?
thank you

[peter]

Hi RotorHeads
I am a new in this business. Coming from hang gliding. Have a question. Have you seen a design of a helicopter with weight shift - not a autogyro?

[Doug Riley]

There have been a few tries. Flying platforms are helicopters with the rotor on the bottom. At least the early one (Hiller) was intended to use weight-shift.

Weight-shift requires a rigid suspension between the rotor and the frame (no hinges). This, in turn, creates a raft of problems, from the problem of gyroscopic effects to the problem of dissymmetry of lift once you start moving forward.