Lead or lag ?

[Sikorsky]

I'm a little embarrassed to ask, after 15 years at the controls on 7 different types of helicopters, but now I would like to clarify the lead lag movement on a fully articulated rotor. I wouldn’t be surprised if some of you experts could help.

During forward flight:
The theory books explains, that the retreating blade flaps down and lags about the drag hinge, to compensate for dissymmetry of lift. That's make sense to me!

During flight, "this variation in the radius will cause the blade to speed up or slow down about the dragging hinge"(Coriolis effect), quote from Pooleys JAR Helicopter Manual.
This explains that retreating blade now will lead about the drag hinge, due to the reduced distance, to shaft axis, of the blades centre of gravity.

Is the retreating side leading or lagging or both? Is it so that the rotor blade starts to lag on the forward retreating part of the disc, and ends up leading on the aft part of the retreating side?

A question in Pooleys book goes like this:

20 When in normal flight, the advancing blade will:

a) Lag about its drag hinge
b) Increase its angle of attack
c) Lead about its drag hinge

The answer sheet states that a) is the right answer. The answer is correct if it you use the Coriolis explanation, but wrong if you use the dissymmetry effect.

:confused:

[the wizard of auz]

I would have thought that the coriolis effect was the prime mover of the blades leading or lagging....... distance of center of gravity to the point of rotation. It was explained to me as the effect a skater gets whilst spinning and raising and lowering their arms.... arms straight out = center of gravity further from point of rotation, spinining slows. arms lowered or raised = centre of gravity closer to point of rotation, spinning speeds up. add in the drag hinge, inertia, and all the other techy stuff = blade lead/lag. I would have thought that disymetry of lift would have been the prime mover of flapping and as a result of the flapping, coriolis effect comes into play, but leading/lagging being only dependant on coriolis effect.
bugga, its hard to put all that into words. I know what I mean, but its hard to express it. Um maybe, coriolis is a secondary effect of flapping but a primary cause of lead/lag.
Its all smoke and mirrors anyway, they dont fly because of any of that other bunk, they are just that ugly that the earth repels em. :}
bugga bugga, now I'm confused. I'm going looking for my books now.

[Lu Zuckerman]

To: SIKORSKY

The theory books explains, that the retreating blade flaps down

The only time the retreating blade flaps down is in a blowback / flapback condition or in a retreating blade stall condition.

Many years ago Sikorsky placed a camera on a rotorhead mounted on a whirl stand and one of the things they discovered was that the advancing blade led and the retreating blade lagged. Up until that time it was assumed that the opposite was true.

On a four blade fully articulated rotor system that was viewed from above the blades would be arranged like a peace sign with the forward blade and aft blade in alignment with each other and the advancing blade forward of its radial axis and the retreating blade aft of its’ radial axis. This is an example as there are other factors influencing blade position two of which are drag and inertia.

Duck for incoming (read Nick Lappos)

:rolleyes:

[Nick Lappos]

Lu has it right! Way to go, Lu.

[heedm]

The retreating/advancing effect would cause a lead lag cycle of the same freq as the rotor. The 'coriolis effect' would cause a lead lag cycle of twice the rotors freq.

One way to think of the 'coriolis effect' is to look down the mast axis at the tip path plane. If they aren't perpindicular then the tip path plane will be an oval. To conserve angular momentum, the blades must move faster wherever the oval has a minor axis and slower across a major axis. Thus, with the disc tipped forward of the mast, the 'coriolis effect' would cause leading at the front and back and lagging at the sides.

From what's been posted about real data, it seems that advancing/retreating has a bigger effect on lead/lag than coriolis.

[RotorRooter]

In support of, and in addition to Mr. heedm's comments;

Perhaps this thread is looking at two separate contributors to lead/lag.

1/ In forward flight, when the tip path plane is low at the front and high at the back, the Cyclical Coriolis Effect (Hookes Joint Effect) will cause the tips to lead when the blades are pointed forward and aft. This is due to the shorter moment arm from the mast to the center of the blade's mass.

2/ The lag on the retreating blade, which was mentioned by Mr. Zuckerman, may be due to the fact that the greatest portion of the profile drag on the retreating blade is out near the tip. On the advancing blade, the profile drag will be more evenly distributed along the span of the blade. The length of the moment arm to the center of the drag on the retreating blade will be greater than the length of the moment arm to the center of drag on the advancing side.

1 & 2 revisited/. Any lead in the forward and the aft pointing blades may require that Mr. Zuckerman's "peace sign" be rotated by 1 or 2 degrees. This rotation may result in a picture that appears to show the retreating blade as having lag.
_________________

An additional comment could be made regarding Mr. Zuckerman's "peace sign", but this is probable not the time or the place. :O

[[email protected]]

Not wanting to be too picky but I believe the term Coriolis effect is misused here and relates to a moving object being apparently deflected in it's trajectory due to the movement of the earth eg wind, artillery shells etc.

The blades lagging and leading as they flap is because of Conservation of Angular momentum - re the analogy of the skater, mass moves inboard - rate of spin increases and vice versa. The movement of the C of G of the blades towards and away from the hub is purely the effect of flapping, whether from cyclic input or flapback.

The positioning of the blades when viewed from above appearing to take their rightful position on the tilted cone of the rotor is Hookes Joint Effect

[Lu Zuckerman]

I believe and I may be wrong that we are entering into another game of semantics. Remember the big arguments that stemmed from my usage of the term “Gyroscopic precession” and I was told that it was “Aerodynamic precession”. Well in the USA they teach Corriolis Forces and in other parts of the world they call it “Hookes Joint effect". Leading and lagging are the result of blade flapping that is the result of tilting the disc.

The following reflects (under ideal conditions) a fully articulated rotor system that is on a hovering helicopter where the tip path plane is parallel to the horizon. Under this condition the rotor system is rotating about the driving axis and the driven axis which are coincident with each other.

When the pilot pushes forward cyclic (or any other direction) the disc will tilt. When it tilts the driving axis and the driven axis will deviate from each other. The center of the driving axis is coincident with the rotor mast and the driven axis will be coincident with the center point of the tip path. When this occurs the laws of conservation of angular momentum kick in and lead and lag is the result. The greater the deviation the greater the lead and lag.

In my previous post I referenced the peace sign and I should have said an inverted peace sign.

:cool:

[Winnie]

to [email protected]

I was just wondering if you are maybe confusing Coriolis effect, which is really Conservation of angular momentum, and Coriolis Force, which is where "winds" are directed to the right in the northern hemisphere?

[heedm]

crab, I don't like using the coriolis term here either. I think it adds to the confusion and contributes little to no understanding. However, I do think it is accurate. The skater bringing arms in decreases the moment of inertia and thus increases angular velocity. The blade spinning around and starting to move up and inwards is analagous to water in your sink trying to move northwards to the drain. It took a lot to convince me that coriolis was an appropriate term, but I agree with you that conservation of angular momentum is a better way of describing it.

[Sikorsky]

I think the complicated things are clear now, many thanks :D

[tu154]

I'm still a little confused. The answers posted seem to indicate that due to the inclination forward of the disc, hookes joint effect will cause the advancing blade to lead. yet the question seems to indicate that it lags. the books are good at describing how the blades behave in a hover, and in a hover with the disk just tilted forward, but are light on what happens regarding leading and lagging in forward flight.
so, in forward flight, is the question correct, does the advancing blade lag? and why?
thanks!!

[Crashondeck]

I'm just glad it all works and I only have to fly.

I trained as a naval architect and struggled to get steel to float. How Sikorsky got aluminium to fly is beyond me!

[Lu Zuckerman]

To: tu154

In a previous post I indicated that the rotor would behave as described under ideal conditions.

I don’t know if this is still true for Sikorsky rotor systems but it applied to those Sikorsky helicopters that I am most familiar with. Most Sikorsky rotor masts are (were) tilted forward by three degrees.

Many Sikorsky rotor control systems compensate for tail rotor translation so in a stable hover the disc is tilted 7 degrees to the left. This means that the helicopter fuselage will act like a pendulum and hang down three degrees tail low and seven degrees down to the right. The compensating 7-degrees is in effect 7-degrees of left cyclic so, in a stable hover there will be some leading and lagging. The advancing blade leads and the retreating blade lags. The advancing blade in this case is flying from the longitudinal centerline over the left quadrant.

One of the checks that can be performed on a Sikorsky rotor system is a check for a bad damper. This test can be performed in a hover by rotating the cyclic stick rapidly in the direction of blade rotation. This induces leading and lagging so that if there is a bad damper it will manifest itself in a lateral shuffle or beat.

Nick Lappos can be the final arbiter on this.

:cool:

[[email protected]]

Winnie - no I don't think I am confused - Conservation of angular momentum is conservation of angular momentum.
Coriolis force is the application of Coriolis effect to movement of air- the apparent deflection of a moving object to the right in the Northern Hemisphere is not conservation of angular momentum.

[heedm]

Crab, coriolis is a manifestation of conservation of angular momentum. In third year mechanics we had to derive all the formulae using Vector Calculus initially, then using simple mechanics and conservation of angular momentum. PM me if you want details.

[Dave_Jackson]

Lightweight, high-strength composite construction combined with advanced bearings should offer the ability to produce absolutely rigid rotors, in the near future. This rigidity will negate the need for coning and flapping. It will also resist the remaining sources of drag, and thereby eliminate lead lag.

http://www.pprune.org/forums/images/infopop/icons/icon14.gif Just a thought:

[Lu Zuckerman]

To: Dave_Jackson

This type of rotor already exists. It is the “Prop-Rotor on the V-22. The blades are not completely rigid but for all intents and purposes, they are. With the input of cyclic pitch the rotors respond similarly to those on a conventional rigid rotor head. However in this case there is minimal interlock with the “mast” (read prop shaft) as the entire rotor system is on an elastomeric “spring” and this “spring” acts as a constant velocity joint so the is no “Hookes Joint” effect and therefore, no lead and lag.

If the rotor system you are describing the interlock would be total and therefor the response to cyclic input would be almost instantaneous. This is both good and bad as with forward cyclic the fuselage would be aligned with the rotor with no means of trimming this tilted angle with a controllable horizontal stabilizer placing the pilots and passengers in an uncomfortable position.

With the rotor aligned with the mast there is no separation between the driving axis and the driven axis. Therefore there would be no leading and lagging and if the rotor system is properly designed with the pitch axis ahead of the driving axis the tendency towards spanwise bending would be minimized. So, the rotorhead would only be exposed to those loads caused by the centripetal / centrifugal forces and those loads associated with lift.

(EDIT) I forgot, the blades are free to flap but this is most important in the propeller mode. If a blade flaps a sensor detects the flapping and sends a signal through the flight control system, which modifies the position of the swashplate to minimize the effects of the flapping and then returns the swashplate to the rigged neutral position.

:cool:

[Dave_Jackson]

:D

Lu;

" ... with forward cyclic the fuselage would be aligned with the rotor with no means of trimming this tilted angle with a controllable horizontal stabilizer placing the pilots and passengers in an uncomfortable position."

True. But, a helicopter designed for fast forward flight (300 knots +/-) will have blades with larger chords. During fast forward flight, the rotational speed of the rotor will be reduced, so that the blades function partially like wings. Most of the forward propulsion will come from a pusher propeller.

" ... the rotorhead would only be exposed to those loads caused by the centripetal / centrifugal forces and those loads associated with lift."

As you say, the rotor will be subjected to thrust loads. It will also be subjected to centrifugal and aerodynamic forces. The centrifugal forces will be relatively small. and have a phase lag of 90-degrees. The aerodynamic forces will be very large, and will have a phase lag of 0-degrees. The resultant phase lag will probably be around 5-degrees.

[imabell]

coriolis effect; the acceleration of the blade that flaps up and deceleration of the blade that flaps down. simple aerodynamics.

hookes joint effect; the theoretical point about which the blades of a multi bladed fully articulated rotor head rotate in forward flight due to lead and lag. simple aerodynamics.

lead and lag, (hunting); the tendency for a rotor blade to hunt for the centre of pressure of the blade, forward on the advancing blade, (lead), and rearward on the retreating blade, (lag). simple aerodynamics.

lu, you show your ignorance when you say that the us calls coriolis effect what the rest of the world calls hookes joint effect.
i find it hard to believe that you make it sound all so difficult.
it's basic aerodynamics.:rolleyes: :p

[[email protected]]

HeedM,

Coriolis effect is an inertial force described by the 19th-century French engineer-mathematician Gustave-Gaspard Coriolis in 1835. Coriolis showed that, if the ordinary Newtonian laws of motion of bodies are to be used in a rotating frame of reference, an inertial force--acting to the right of the direction of body motion for counterclockwise rotation of the reference frame or to the left for clockwise rotation--must be included in the equations of motion.
The effect of the Coriolis force is an apparent deflection of the path of an object that moves within a rotating coordinate system. The object does not actually deviate from its path, but it appears to do so because of the motion of the coordinate system.

I don't think the above (which was gleaned from an internet search) can be confused with conservation of angular momentum.
The blades C of G is trying to move towards the rotor hub as the blade flaps up - the only impact of coriolis would be the apparent movement to the right as it tried to do so (counterclockwise rotation viewed from above).
If you assume (rightly or wrongly) that relative to the rotor system this produces a leading tendency of the blade as it flaps up then that is why you need to modify the effects of conservation of angular momentum with this term.
Does the blade really try to move forwards on it's hinge due to coriolis or is it just apparent movement relative to the rotor hub?
Newtons Laws governing conservation of angular momentum would argue that the blade will speed up as the C of G moves inwards but would not explain why.
Maybe you are completely correct and Coriolis effect is the driving force behind the C of A M - I dunno I'm not a graduate just a pilot so if you do know please explain.

[Lu Zuckerman]

To: imabel and Crab

coriolis effect; the acceleration of the blade that flaps up and deceleration of the blade that flaps down. simple aerodynamics.

"lead and lag, (hunting); the tendency for a rotor blade to hunt for the centre of pressure of the blade, forward on the advancing blade, (lead), and rearward on the retreating blade, (lag). simple aerodynamics".


The blade that flaps up is the retreating blade and the blade that flaps down is the advancing blade. Please note that the two statements above (quoted from your post) are diametrically opposed to each other. In the first quote (Coriolis effect) you state that the blade that flaps up accelerates and the blade that flaps down decelerates and in the second (lead and lag) you state that the advancing blade leads and the retreating blade lags which is what I have stated in my posts. Lead and lag results from the aerodynamic effect that causes the flapping but the actual movement of the blade about the lead-lag hinge is a purely mechanical effect that has nothing to do with aerodynamics.

Regarding my ignorance I have attended 18 helicopter factory schools and I have taught POF and I never heard of Hooke’s joint effect until I worked on the A-310 and had many English colleagues on the program. In the USA what you call a Hooke’s joint is called a universal joint.


:cool:

[Dave_Jackson]

heedm,

In an earlier posting, you mentioned " ... look down the mast axis at the tip path plane. If they aren't perpendicular then the tip path plane will be an oval.".

Your statement applies to both a teetering rotor and an articulated rotor; but, it raises an interesting consideration. When looking down the tip path axis; the tip path plane of a teetering rotor will be a circle, whereas the tip path plane of an articulated rotor will be an oval.

This leads one to think that the Hooke's Joint Effect may not be truly representative of articulated rotors.
_________________

imabell

You may be oversimplifying things.

"coriolis effect; ..... deceleration of the blade that flaps down. simple aerodynamics."

True; but if the blade flaps down below the position of no coning angle (normal to the mast) the mass of the blade will start moving inward towards the mast, and this will result in both blades accelerating. Incidentally, when a teetering rotor tips, the undersling will result in mass of both blades immediately moving inward, and "wanting to' accelerate.

"hookes joint effect; ... a multi bladed fully articulated rotor ..."

A Universal Joint, a Hooke's Joint and a Cardan Joint are just different names for the same device. Actually, when referring to a helicopter rotor, the teetering and the flapping hinges might be more accurately refereed to as Knuckle Joints, not Hooke's Joints. This is because the teetering, the flapping and the knuckle joint have only one hinge, whereas the Hooke's joint has two hinges. Perhaps the original use of the phrase 'Hooke's Joint' came from the Bell 47, which had two hinges in its teetering rotorhead.

I believe that when viewing the activity of a tipped teetering disk, Cyclical Coriolis achieves the same results as the, so-called, Hooke's Joint. The algorithms of the two are different, but they appear to arrive at the same answer.

_____________

RotorRooter,

Ah, forget it.
It's no fun talking to oneself. :O :O

[Lu Zuckerman]

Maybe this will help clarify this matter of what kind of joint we are discussing. On a fully articulated rotor there are two axes. The driving axis and the driven axis. Under ideal conditions in a hover these two axes are coincident with each other. When the pilot places a cyclic input (in any direction) the driven axis will deviate from the driving axis as a result of flapping and the flapping results in leading and lagging. Does the Hookes’ joint effect or the universal joint effect or the Carden joint effect cause this? The answer is no.

The characteristic of a Hookes’ joint is that it has the capability to drive off of the primary drive axis and that the resultant is that the driven shaft is not rotating at exactly the same speed as the driving shaft. However, the speed differential will be the same around the shaft rotation. The speed differential is dependent on the angular difference between the driving and driven shafts and this will continue until the Hookes’ joint locks up. However on the articulated rotorhead the speed differential is not constant as it varies around the rotational plane.

The lead and lag is the resultant of conservation of angular momentum resulting from the variation in the mass of the blade shifting relative to the driving axis during the flapping up and down.

But then again I never studied Newtonian Physics.


:confused:

[Head Turner]

Could someone explain why the effect of the earths rotation and the movement of air which is affected by that rotation (corriolis) is used to explain the lead/lag of a rotor blade during one cycle?

Main Rotor Blades are affected real or apparent by, Cconing, Lift, Drag, Density, Rotational speed, Surface area, Flapping, Pitch changes, Angles of attack, Profile (cross sectional design) and Bending. Tail Rotor Blades have addionally a Delta hinge.

My understanding is;-

When the rotors are turning at right angles to the rotor shaft and in still air, the forces on a blade are constant through one complete cycle.

Now introduce a forward cyclic command. This will cause the advancing blade to reduce pitch, reduce the A/A, reduce lift and therefore reduce drag. Does the blade velocity increase and therefore 'leads' due to the reduction in drag? Or because the blade has flapped down the radius has increased (coning angle decreased) that the blade slows down causing the blade to 'lag'?

[Winnie]

[email protected]
Yes you're right and I'm wrong, just never seen it that way before, thank you for correcting me and expanding my mind even more!
I wonder what will happen to my head the day it states that I can no longer fill it up like thi?
:)

Lu
You stated that the advancing blade flaps down, but this is contrary to all the aerodynamics texts we use in ground school, and teach...
You may be right, but I simply don't see it. I thought the advancing blade had to flap UP to reduce the AOA, thus creating less lift, to balance out the Dissymmetry of lift?

May be right may be wrong, but please explain
Thanx for listening:confused:

[Lu Zuckerman]

To: Winnie

The upward flapping of the advancing blade you are addressing is also accompanied by downward flapping of the retreating blade. This occurs at about 20-25-knots during translational lift and is referred to as flap back or blow back depending on your religious persuasion. The technical term is Transverse Flow Effect.
If Frank Robinson is your religious leader then it is called "We-Wa".

To answer another post the movement of the blade is mechanical and has nothing to do with profile drag. The advancing blade leads where drag is at its' greatest.

:cool:

[heedm]

Crab,

If you're sitting in the rotating reference frame, you observe a deflection of an object that is moved you call it Coriolis effect. Becuase you're sitting in that reference frame, you don't see that the object is rotating before it's moved, you don't see that the object is being pulled to a point that is beneath you (ie center of earth or the rotor hub) so you don't think about angular momentum.

If you're outside that reference frame in an inertial reference frame (non-rotating, non-accelerating...laws of inertia apply) then you see that the object has angular momentum prior to it moving. You also see the object has a constraint that causes it to deflect in an arc when pushed in a line. You realize that pushing the object imparts more rotation on it and thus to determine the net effect, you must conserve angular momentum and do a vector sum of the individual angular momenta.

Matthew.

[vorticey]

coriolis happends when something moves closer to the spining axis its spinning around, eg wind heading south or north from the equater (closer to the poles) advances, ice scaters arms advance when pulled in and so do blades, they all work the same.:p .

[Lu Zuckerman]

To: vorticey

coriolis happends when something moves closer to the spining axis its spinning around, eg wind heading south or north from the equater (closer to the poles) advances, ice scaters arms advance when pulled in and so do blades, they all work the same.

Are you saying that Coriolis only works when the skater moves his / her arms closer to their respective bodies causing the rotation of the skater to speed up. If that is the case what force is involved when the skater moves their arms outward causing the speed of rotation to slow down.

In most American POF texts and even in the FAA helicopter handbook they mention Coriolis force and then immediately go into conservation of angular momentum as the reason for the leading and lagging.

Here is another point to ponder. In the FAA Helicopter handbook they state that when a rotor blade flaps upward the center of mass of that blade moves closer to the axis of rotation and blade acceleration takes place. Conversely, when the blade flaps downward the center of mass moves further from the axis of rotation and blade deceleration takes place. As the blade moves past the longitudinal centerline of the helicopter it starts to climb. And, when the blade passes over the tail it starts to flap downward. The climbing blade is the retreating blade and the blade flapping downward is the advancing blade. Contrary to what is stated in the FAA handbook and many other POF texts the diving blade leads and the retreating blade lags. Try and reconcile that with the textbooks.

:confused:

[vorticey]

i seem to remember a previese thred not so different to this one.
no lu, coriolis works both ways. when clowds move toward the equater they 'slow down'. skaters slow down when arms go out.
i think of it as more of a distance thing. skaters hands dont speed up when pulled in, they just travel the same distance in the same amount of time. if the hands are pulled in half way, the hands will need to travel twice aroud the body to cover the same distance in the same time. sure the body rpm increcese by 2. (angular momentum maybe?)

naturally in forward flight the advancing blade is flapping down and retreating flapping up to get the corect disc attitude. if you let it flap back you stop!

i would imagine the blade at 0' lagging and 180' leading. does this cause a movement of the mast toward the leading/lagging side?:sad:

[Lu Zuckerman]

To: vorticey

naturally in forward flight the advancing blade is flapping down and retreating flapping up to get the corect disc attitude. if you let it flap back you stop!

Yes, but the texts say that the blades flapping up will lead and the blade flapping down will lag. The opposite is true. The blade flapping down will lead and the blade flapping up will lag. That is why I said it is a point to ponder.

:hmm: