flight force

[vorticey]

i fly a r22 and cant work out the force in foward flight that means i have to pull the cyclic right to continue strait. its not inflow roll because its to the right and its not the differance in adv. or ret. blade reactions witch would cause flap back or forward so what is it?

[Lu Zuckerman]

Here is you answer.

Here is the real story. When the Robinson R22 and R44 are rigged for cyclic control ranges the blades are rotated so that the pitch links are directly over the lateral or longitudinal axes of the helicopter. Unlike the Bell where the blades are disposed over either the longitudinal or lateral axes of the helicopter the Robinsons’ blades are offset 18-degrees ahead of the respective axes.

This means that when the cyclic is pushed forward from the neutral center the blades will have reached their maximum pitch input and with gyroscopic precession they will have their maximum response to that input 90-degrees later. That means, that the blades will tip down left of the longitudinal centerline. To counter this left tilt, the pilot must move his cyclic to the right.

To get the complete story go to the archive of the Rotorheads forum and look for the following posts http://www.pprune.org/ubb/NonCGI/frown.gifall of them will be under my name).

January 8, 2001 To Helo teacher

December 23, 2000 Certification of Robinson Helicopters

December 18, 2000 The 18 degree offset

I hope every Robbie driver reads your post as I have been advancing this premise since late last year and every on tells me I am full of it.


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The Cat

[Vfrpilotpb]

Good Morning Lu,

Vorticey, is quite right, I agree with his statement about the need to pull slightly right cyclic to stay on a straight heading, and I have read most of your past posts regarding this item, whilst some posters have argued with your theory I do believe the majority have learned quite a lot from the past threads. However not wishing to start an encyclopeadic type of answer , what would happen if Frank R removed this 18deg P/C situation and followed what you see as the correct angle, would this have some detrimental effect to the Robinson Teetering head and pitch change controls, or is it a whole new engineering problem of design and re-engineering of all the parts including the blade/horns
My Regards

[Lu Zuckerman]

To: Vfrpilotpb

In order to eliminate the 18-degree offset the rotorhead would have to be redesigned and the cone hinges eliminated. This would allow the rotorhead on the Robinson to be like that of the Bell. By eliminating the cone hinges the pitch horn could be made into a 90-degree pitch horn as opposed to one of 72-degrees.

In doing this another major problem would develop and that is severe blade bending and no way of minimizing the flapping loads resulting in blade failure. For the short period that the rotor would stay intact the Robinson would behave like a Bell and It could be rigged like a Bell.


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The Cat

[This message has been edited by Lu Zuckerman (edited 29 June 2001).]

[This message has been edited by Lu Zuckerman (edited 29 June 2001).]

[Grainger]

Since reading the original posts I have had a very close look at the rotor head and it would be very easy to set the pitch horns at 90 degrees if that is what Frank R wanted to do.

Would just need an 'S' bend in the pitch link and you could keep the ball joint in line with the coning hinge.

[Lu Zuckerman]

To: Grainger

Making an S bend in the pitch horn and ending up at the cone hinge would give you a 72-degree pith horn with an odd shape. In order to have a 90-degree pitch horn you would have to cross the cone hinge and end up at the teeter hinge like on a Bell.

This would cause severe pitch coupling so strong as to render the helicopter uncontrollable. All of this has been said in past posts and most likely will cause a lot of pain on the part of the participants on this thread if I have to repeat my comments. Please read the past posts as indicated in my response to vorticey above.



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The Cat

[Flight Safety]

I'll add this to help illustrate Lu's comments.

http://www.cherishedlife.org/images/r22head.jpg

If you look at the diagram, you'll see that the Robbie rotorhead in unique in design in that it has both a flapping hinge and a teeter hinge. No other rotorhead design has this combination. It's not possible to locate the pitch horn ball joint on both the center of axis of the flapping hinge and the center of axis of the teeter hinge at the same time.

On the Bell rotorhead, which has no flapping hinge, the pitch horn ball joint is located on the center of axis of the teeter hinge. This allows the rotorhead to teeter without coupling any uncommanded pitch changes through the pitch horn/pitch rod arrangement into the rotor blades, which keeps the rotorhead stable during teetering angle changes.

On fully articulated rotorheads (and even rigid rotorheads like the Lynx) the pitch horn ball joint is located on the center of axis of the flapping hinge (or on the center of beam flexure in the case of the Lynx). This allows the blades to flap, cone, and lead-lag without producing uncommanded pitch couplings into the rotor blades during these movements. Again this keeps the rotorhead stable when these movements occur.

In the Robbie rotorhead, there's no such luxury. The pitch horn ball joint is located on the center of axis of the flapping hinge, and this allows the blades to flap without coupling any unwanted pitch changes into the rotor blades. But it's a very different story when the rotorhead goes through teeter angle changes. As you can see, there's no way the Robbie rotorhead can teeter without producing SOME pitch coupling into the rotor blades.

This arrangement is why the pitch horns are located 72 degrees from the rotor blade center line, instead of 90 degrees as on the Bell rotorhead. This is also why the cyclic has to be held to the right a small amount during normal cruise in the R22.

I could get into a discussion as to why this particular design (with its unwanted pitch couplings during teeter angle changes) is the direct cause of the rotorhead separation accidents of the R22, but I'll save that discussion for another time.

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Safe flying to you...

[Grainger]

Yes, I know what the rotorhead looks like. It is possible to attach the lower end of the link at the 90 degree point and keep the ball joint in exactly the same position as it is now. Hence the 'S' bend. Think about it.

Your concerns appear to be based on two factors:

First that the location of the coning hinges somehow forces Frank to design in a phase angle of 72 degrees.

Second that the correct phase angle is precisely 90 degrees.

I'll deal with this second point in a later posting, but for now we can remove the first objection;

As I stated earlier, by using an 'S' curve in the pitch link you could have a 90 degree phase angle on the swashplate (lower end of pitch link) but keep the ball joint at the upper end of the pitch link in the same position, in line with the coning hinge.

If you don't like this solution, there's nothing to stop you putting a mechanical linkage in between the cyclic stick and the swashplate which would allow you to introduce any amount of phase angle you like. That is to say, there is no reason why the swashplate has to tilt forward when you move the cyclic forward. Using this mechanism you could introduce the additional 18 degrees to bring the total to 90 if you wanted.

There is nothing special about the 72 degree angle anyway: it depends to a certain extent on the radius of the swashplate so you could vary it a few degrees either way by changing this radius.

These are just examples. I'm sure there are many other possible solutions. The point is that you could correct for the 18 degrees if you wanted.

So I don't think it is correct to say that the coning hinge design forces the use of a 72 degree phase angle. Neither does it prevent a 90 degree phase angle if so desired.

That's one point disposed of. A lengthy enough discussion for now so I'll be back later with a diatribe on the second point.

[Grainger]

OK, please excuse my limited artistic ability but here goes:

http://www.procyonw.demon.co.uk/images/R22headx.jpg

Now the lower end of the pitch link is attached at the 90 degree point on the swashplate.

I'm not a mechanincal engineer so please don't get hung up on criticising details of the proposed solution. Like I say, the point is that there do exist mechanical solutions that would give you a 90 degree phase angle with the coning hinges .

[This message has been edited by Grainger (edited 29 June 2001).]

[Flight Safety]

Hmmm...Lu, what do you think of this?

It looks like most rotorhead designs have purely vertical pitch links, although some tilt out (from swashplate to pitch horn) and some tilt in. However I don't see any rotorhead designs where the upper (pitch horn) and lower (swashplate) ball joints are out of phase with each other. Is this an issue with centrifugal forces with a spinning rotorhead, and would this cause unwanted pitch changes?

Grainger, it's both the combination of the location of the flapping hinge (and its center of axis) and the LENGTH of the pitch horn (thus effecting the distance of the ball joint attachment point from the blade center line) that establishes the phase angle of 72 degrees. If the pitch horn was shorter, the phase angle would be less than 72 degrees, but if longer, then the phase angle would be more than 72 degrees (and approaching 90 degrees if the pitch horn was say 15 feet long). That's simple trig. FWIW, the Robbie rotor blades do have rather long pitch horns.

I wish I had a good textbook on helicopter design, I'm sure these issues would be covered.

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Safe flying to you...

[This message has been edited by Flight Safety (edited 29 June 2001).]

[Grainger]

... length of the pitch horn....


Exactly, that's what I meant by 'radius of the swashplate' but I think you've put it more clearly.

Thanks for thinking about this seriously. I really really want to understand how this works too...

[Grainger]

OK, I promised you a diatribe on my second point so here goes - should we expect the reaction to take place at exactly 90 degrees from the cyclic input?

We all know that there is a world of difference between text-book physics and practical engineering. We are invited to consider frictionless systems, to ignore air resistance and so on.

A good example of this is the projectile: basic mechanics predicts that a projectile will follow a parabolic path - but these are ideal objects: rigid, pointlike, not subject to aerodynamic forces etc. A real projectile such as a golf ball or a frisbee or an arrow shot from a bow will not follow an exact parabola because it is subject to a collection of forces and movements in addition to the constant acceleration of gravity.

Rotation and precession are affected in the same way. In an ideal system consisting of a completely rigid rotating object subjected to a single torque which is applied precisely perpendicular to the axis of rotation, then the reaction will indeed take place exactly 90 degrees after the point at which the torque is applied provided that there are no other forces or torques acting.

The real system consisting of rotorhead, hinges and rotor is not rigid: it can cone and teeter; the blades can bend and flex. It is not subjected to a pure torque caused by a single force applied at the rim; there is drag, there is lift, there is friction; the aerodynamic forces are distributed non-uniformly along the length of the blade and vary throughout the cycle, rather than being applied at a single point.

The collection of forces acting on the rotor is far more complicated than a pure torque acting perpendicular to the axis of rotation. Just as you wouldn't expect a ball thrown into the air to follow a precise parabola, we shouldn't expect a rotor system to have an exact textbook 90 degree phase shift.

A good example is the boomerang. To throw it correctly, you hold it with the arms vertical - not flat. As you throw it away from you it moves forward, spinning about a horizontal axis, so that the upper blade is moving forward through the air faster than the lower blade. The upper blade therefore generates more lift than the lower one.

For a right-handed thrower the lift is directed to the left and since the lift is greater for the upper limb this generates a torque which, looking from behind as the boomerang flies away from you, acts in an anticlockwise direction. Because the boomerang is rotating, the reaction to this torque occurs 90 degrees after the blade reaches the top of its travel - that is to say the front (far side from the thrower) moves to the left. Thus the boomerang follows a circular path, curving to the left and returning to the thrower.

So far so good, but as those of you who have boomerangs will know, by the time the boomerang returns to you, it is no longer vertical - it is flat, spinning about a vertical axis like a frisbee (or indeed a helicopter rotor !) and you can catch it between your hands using a 'crocodile jaw' motion. As it goes around its orbit, it gradually 'lies down', going from vertical to horizontal.

The point being that the reaction to the torque caused by asymmetry of lift does not take place at exactly 90 degrees - because this is a 'real world' system the non-ideal effects (drag etc.) cause you to have an approximation only to the textbook behaviour.

Now I'm not saying that 72 degrees is or isn't correct for the R22 - all I'm saying is that there are more factors to take into consideration.

[This message has been edited by Grainger (edited 29 June 2001).]

[Lu Zuckerman]

To: Grainger and Flight Safety

The solution Grainger proposed in his modification of the drawing looks as if it would work. Whether it would change the mechanical advantage offered by a vertical pitch link I can’t say. Also, I do not know if it would alter the dynamics of the rotor system or not.

However, no matter what is proposed to alleviate the problems caused by the 18-degree offset nothing will change. Frank Robinson designed this rotor system by incorporating cone hinges and in doing so, he established the physical characteristics of the pitch horn in relation to its’ attachment to the rotor blades. The pitch horn cannot cross the cone hinge. The cone hinges were incorporated to minimize the bending loads on the blades and to allow the blades to flap in relation to the teetering head. This minimized the transmittal of flapping loads into the head. There were restrictions made to the flight envelope to further minimize flapping loads.

Graingers solution to rotate the attachment of the pitch link in relation to its’ attachment to the blade was attempted on the Cheyenne AH-46 Attack Helicopter. On the Cheyenne the Phase Angle (note, in the UK phase angle is the lead of the pitch horn in relation to the blade. In the USA phase angle is the precession angle normally indicated as 90-degrees) would change due to blade loading, maneuvering loads and speed. This problem caused the blades to diverge and hit the fuselage. In one case it killed the pilot and in the other it destroyed a very large wind tunnel in California. Lockheed and Parker Bertea worked for over two years and they finally solved the problem by doing what Grainger suggested in his post (rotate the swash plate in relation to the head). It worked but the control system became so complex and it had so many single point failures that the program was cancelled.

In any case the Robinson head will not be changed and the pilots whether they will admit it or not must adjust the cyclic to the right to compensate for the offset.

Whether the offset contributes to the loss of the rotor or rotor incursion is open for review.

Another point to consider is that with the capability to flap the blades will also lead and lag. The rotor head however has no lead/lag hinges so any tendency to lead and lag will be reacted by the cone hinges and the teeter hinge and eventually grounded through the shaft. The blades on the Robinson are not very robust and as such will suffer spanwise bending which will tend to make them very unstable, which could result in divergence.


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The Cat

[Flight Safety]

One issue I see with Grainger's solution is that it puts additional side (or bending loads) on the pitch link. The conventional vertical pitch link layout (with no phase difference between pitch horn and swashplate attachment points) applies only tension and compression loads to the pitch link without side loading, except that the centrifugal forces of the spinning rotor head applies some side loading to the pitch link, but this loading is spread out over the entire pitch link and is therefore not concentrated.

Grainger's solution, whether involving a straight pitch link to the out of phase atttachment points, or an S shaped pitch link, would create additional side and bending loads on the pitch link. It seems to me that these loads would be concentrated at the pitch link adjustment points, which are the weakest part of the pitch link. An FEA analysis could confirm this. Anyway this would only serve to reduce the mechanical reliablity of the pitch link, the failure of which would be disastrous. You could argue that a more "robust" pitch link might overcome the side and bending loads, but this would only increase the centrifugal loads, thus creating additional stresses.

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Safe flying to you...

[This message has been edited by Flight Safety (edited 29 June 2001).]

[Nick Lappos]

I must point out that there are two issues here, one is the immediate response of the helicopter to control movements (dynamic control response) and the other is the place the cyclic comes to be when the pilot has adjusted for all the aerodynamics of forward flight (static control trim.) Vorticey is noting the fact that the static control trimmed position is moving steadily out to the right as he goes faster and faster. This is due to several factors, not the least is the lateral balance of the helicopter. I suggest that vorticey make careful note of the difference between this trim for times when both seats are filled, and perhaps when he swaps seats while solo (is this allowed?)

We can make large changes in the static trim angle with lateral cg, and also when we trim the high tail rotor to more or less thrust in forward flight (the tail rotor tries to roll the aircraft, and lateral stick is needed to cancel that factor). Also, if we move the horizontal tail from the left to the right side (assuming it is only on one side of the aircraft) we get large roll cyclic trim shifts, as much as 15% of stick travel with everything else left unchanged.

The other posters have focused on the dynamic control response, which is controlled by the relationship between three different control reference frames:

1) the control input axis (forward cyclic stick)

2) the swashplate tilting axis (usually about 90 degrees before the control input axis)

3) the rotor tilt axis (where the tip path plane dips the lowest after a cyclic input)

The pilot might note that when he pushes an inch of forward cyclic, the tip path plane might not go purely forward, but might have some roll as well. This particular effect is due to many contributers, including gyroscopic precession, rotor flapping resonance frequency, rotor blade flapping inertia and flap aerodynamic damping.

The amount of difference between the control input axis and the swashplate tilt axis is called Gamma by designers, and is easy to change with no rotor head modifications. All you have to do is make the pilot's controls mix differently so that the desired tilt is caused. This is done in larger helos by a mixing unit (as in the S-76 and Black Hawk) or by a simple double swash plate (as in the Sea King). Both schemes simply make a forward cyclic tilt the swashplate in a non-forward direction of the designer's chosing. In flight test, we tune the cyclic controls (through adjustment of the variable mixers we have on test aircraft)until the forward stick truly achieves a forward tilt of the rotor plane.

It is surprising to many people to find that the Gamma angle is seldom exactly 90 degrees. If the designer guesses wrong before flight test, and is not too energetic after testing to make it right, you can have non-orthogonal control system that makes the pilot adjust for yet another helicopter control anomoly.



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[Flight Safety]

Nick Lappos, many thanks for your excellent post. Given your background and the company I believe you're employed by, the quality of your post is not too surprising.

I doubt that Nick would say what I'm about to say, but here's a quote from his post:

<font face="Verdana, Arial, Helvetica" size="2">It is surprising to many people to find that the Gamma angle is seldom exactly 90 degrees. If the designer guesses wrong before flight test, and is not too energetic after testing to make it right, you can have non-orthogonal control system that makes the pilot adjust for yet another helicopter control anomoly.</font>

I believe this is the situation with the Robbie.

I also believe that Nick's company goes to the trouble to remove as many control anomolies as possible, before delivery of its products to its customers. But then again, his company builds much more expensive helicopters.

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Safe flying to you...

[Lu Zuckerman]

To: Nick Lappos

Once again, you have baffled your audience with engineering speak and shifted the emphasis on to how you do things at Sikorsky. If the Robinson has a mixing unit it is extremely simple and there is absolutely no compensation in the linkage nor, is there a means of rigging the problem out.

Here is the problem. On the Robinson like the Bell when the pilot pushes forward cyclic the swashplate tips down over the longitudinal axis. If he moves the cyclic to the left or right the swashplate will tip in that direction. The Bell has a 90-degree pitch horn and like all helicopters has a 90-degree phase angle (or damned close to it).

The Robinson on the other hand has a 72-degree pitch horn but it has a 90-degree phase angle. With this combination when the cyclic is moved forward from the rigged neutral position the swash plate tips down over the nose. The blade at this time is advanced 18-degrees forward of the lateral axis and the opposite blade is 18-degrees to the rear of the lateral axis. With a phase angle of 90-degrees the disc will tip to the left and the pilot must compensate by moving right stick.

In advancing this theory I was crucified and everyone told me I was wrong. Several other factors enter into the equation to include transverse flow effect, which causes the helicopter to roll right. To compensate for this the pilot moves the stick to the left. As he passes through this phenomenon he brings the stick to the point where the helicopter is flying straight ahead. It is my contention that when the helicopter is flying straight ahead the cyclic has been moved to the right of the rigged neutral position. When I asked if someone would perform a test to determine if I was correct they drove in a couple more nails really pinning me to the cross.

What vorticey stated was this. In order to fly straight-ahead, he had to move the cyclic stick to the right. If he didn’t and he pushed the cyclic forward the helicopter would fly to the left which is what I had stated above. Vfrpilotpb backed this up as he had the same experience. This can be easily proven if some brave soul would bring his Robbie to a hover and placing the stick in the rigged neutral position and moving the stick straightforward he should fly to the left. There are no dynamics or higher harmonics involved, as he would be moving at walking speed or less.

One other point. the certification requirements state that the helicopter must move in the same sense as cyclic movement. They do allow one or two degrees to allow for coupling but not 18-degrees.


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The Cat

[This message has been edited by Lu Zuckerman (edited 30 June 2001).]

[This message has been edited by Lu Zuckerman (edited 30 June 2001).]

[Flight Safety]

Lu, I don't think Nick is really disagreeing with you. He's just in a position where his language has to be more guarded that's all(flapping dynamics aside).

[Dave Jackson]

As I understand it, there is a direct correlation between an offset hinge dimension and the phase lag angle. The great the offset hinge dimension (distance between center of the mast and flapping hinge) the smaller the phase lag angle (i.e. &lt; 90). This is because the offset increases the speed at which the blade arrives at the desired flap angle.

It appears that the Robinson's flapping/coning hinges cannot be considered as conventional flapping hinges because the teetering hinge negates the effect of the offset.
_____________________

The following modification to the Robinson rotor head is suggested, with tongue in cheek. :)

The two grips be geared together so that they cone in unison. This linked coning will still compensate for the 'lightness' of the blades, while also removing any tendency for these hinges to flap. The pitch horns can then be extended so that the phase angle is 90-degrees and the pitch link is vertical.

The only modification left is to relocate the pitch horns and links at the trailing edge of the blades. Now the rotor will act like a conventional teetering rotor, except that when the blades cone some of their pitch will be pulled out.
________________________

For those that might be interested, the following web page contains Frank Robinson's Nov/2000 posting on this conference. http://www.synchrolite.com/B185.html#Robinson

[Flight Safety]

Yes, but will it fly?

[Dave Jackson]

"but will it fly?"
Now that's asking for too much.

If people have sympathy for Mr. Bell's and Mr. Robinson's difficulties in developing their teetering rotors, they should have the utmost compassion for poor Mr. Kaman. He had to do all of the above, and then, link two of darn things together.

[baranfin]

to Lu Zuckerman: I followed the link and under Frank Robinsons comments it gave a summation of your position. In it you said that one of your fears was when a pilot encountered a low-g if they pulled aft cyclic they could exacerbate the roll.

ok so my first question is what would be the correct cyclic application? would it be to go slightly aft and slightly left cyctic? another thing that i have wondered is what an increase in the collective would do to reload the rotor? what are the problems that i havent foreseen with increasing the collective to reload the rotor.

ok so that was actually 2 questions but i have one more as well.
Why didnt robinson design the rotor system like the bell 2 blade heads?
all of the posts have been really informative and interesting, I appreciate any answers to my questions. thanks you

[Lu Zuckerman]

To: baranfin

The normal thing to do when encountering Zero G is to pull the cyclic to the rear. It was my contention that in order to fly straight ahead the cyclic would have to be displaced to the right of the rigged neutral position. This meant that there was a right roll component in the swashplate. If in encountering Zero G the pilot pulled aft cyclic he would exacerbate the right roll component generated by the tail rotor.

On a previous thread a post was made telling about a training program in the UK conducted by Tim Tucker. Mr. Tucker is somewhat of a consultant to Robinson and he conducts the safety courses for them. During the development of the R22 he was one of the test pilots. In that training program he told his students that when encountering Zero G they should pull the cyclic back and a “Tad” to the left in order to keep from adding to the tail rotor induced right roll. This is not what the POH states. It states that the stick should be brought back gently to the rear or, in the aft direction. I feel if that is done a low time pilot can lose the helicopter. If the stick is moved too far to the left when moving it aft the aircraft will encounter severe flapping loads and could possibly suffer rotor incursion or mast separation.

There is a possibility that if collective is added to load the rotor the increased drag could cause loss of rotor rpm but that is my opinion. Robinson designed the rotor head with cone hinges to reduce the bending loads on the blade from coning and flapping. If he used a Bell design type rotor the blades would have to be so robust as to add a great deal of weight to the airframe. I personally believe that all of the problems of the Robinson breaking up in flight would go away if they used a fully articulated three-blade rotor system similar to that used on the Sweizer 300.


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The Cat

[The Nr Fairy]

Lu :

Well remembered. I posted that bit about Tim Tucker's "aft and a tab left cyclic" some months ago, when this last came up.

To be fair, and I can't remember if this was in the original post, he added the "tad left" bit when he was talking about the test flying he'd undertaken exploring the low-G regime after several related accidents.

However, the POH says "aft cyclic" and unless you're very good, I'd go by that - after all, easier to pull straight back that have to add in a left component as well, and lesser mortals like me like the simple things in life.

[212man]

I haven't flown an R22 for several years, but I do know that you had to apply right cyclic in the cruie. However, whether this was a physical displacement or just a force, I don't recall. I wouldn't have thought there was any disputing this though as Mr Robinson has installed a simple trim system (bungee cord) to alleviate the strain on your wrist. That is hardly a secret.

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Another day in paradise

[Rotor Nut]

This technical stuff is very heavy going so is far from clear for me.

I fly R22 and R44 and can't say that I notice any problem with cyclic positioning. I certainly don't have to compensate 18 degrees to the direction I want to fly in. I push forward - it goes forward - I push left - it goes left etc... Next time I am a passenger in an R22/R44 I will take some digital pictures of the cyclic position in various flight modes and post them here. However I don't think this will prove anything because its where the cyclic feels right rather than looks right that the pilot is concerned about.

A further point - while hovering the R22 I feel there is no cyclic pressure in any direction however in forward flight there is a pressure to the left, which the pilot has to compensate for by pressure to the right. This becomes more intense the faster you go. Surely this is caused by the dissemmetry of lift and is why the cyclic trim is operated in forward flight? Could this be what Vorticey was referring to? Sounds like he is also confusing dissymmetry of lift with flap back too. Flapback requires the cyclic to be pushed thru more forward but not right. In the R44 Raven the hydraulic controls mask any cyclic pressure/vibration anyway but I do remember in training the force needed to hold the cyclic in position in forward flight with the hydraulics off.

(Now is a good time to remind R44 pilots to make sure the hydraulics are ON before you lift).