1 Picture= 1000 words [Archive]

Lu Zuckerman

10th Dec 2000, 22:31

To all of you that have requested a copy of my report, I have made some major changes to one of the three diagrams. Please log onto:

<A HREF="http://205467.homestead.com/diagrams.html" TARGET="_blank">http://205467.homestead.com/diagrams.html</A>

The changes are highlighted in yellow. The changes resulted from my trying to figure out what Frank Robinson had said in response to my postings in the Rotorhead Forum.

From the changes, you can see that the picture originally painted didn't go deep enough in explaining the problems in countering Zero G.

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The Cat

Grisoni

11th Dec 2000, 00:08

Having experimented with Low G, I fail to see how right cyclic will cause immediate loss of control. Adding slight right cyclic should (correct me if I'm wrong)reduce the large left flapping action. I'm sure the FAA had a stickplotter for the Certification.
Second we're talking small control inputs, at no point is it req.to move the stick full travel during a recovery everybody should know not to yank the cyclic during a rather critical phase of flight.Third the flapping angles and distance to the airframe has been measured and published. (RHC Newsletter) during the SFAR ordeal. regards

Lu Zuckerman

11th Dec 2000, 00:36

To: Grisoni

Adding right cyclic in countering Zero G will result in adding to the Right roll component of the tail rotor resulting in loss of control. These are not my words, they are the words of Robinson Helicopters.

The same is true for the warning about not trying to counter the right roll by pulling the cyclic back and to the left as this would cause high flapping angles resulting in blade / fuselage contact.

The purpose of the illustration was to show first of all the relationship of the neutral position of the cyclic stick relative to the allowable travel. The illustration also shows the rigged neutral position of the cyclic stick in relation to the neutral travel position and third, the illustration shows the line the cyclic must travel in order to compensate for the 18-degree offset of the swashplate to allow forward flight.

I was not trying to imply that the cyclic stick must be at the extremes of travel in order to counter Zero G. I was implying that the stick if moved on the lines shown would have the result indicated in the applicable blocks.

Regarding the use of a plotting board , that was a question in my report. Mr. Robinson did not reply to that question. So, I will raise it again. Was a plotting board used during the certification? If it was, did they discover the points made in the Cyclic Control Diagram which is the subject of this post.

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The Cat

[This message has been edited by Lu Zuckerman (edited 10 December 2000).]

Multp

11th Dec 2000, 04:23

Gentlemen. This and associated threads have concerned me for a while. For safety's sake do not experiment unless you are in the experimental flight test business doing a properly constituted trial. By all means discuss things, but PLEASE PLEASE do not go exploring the edges of the flight envelope unecessarily. Fly the aircraft..any aircraft...in accordance with the flight manual. Sorry, but deliberately looking for low g experience in the R22 or similar is a fool's errand! If you have what you think is a genuine concern, then take it up with the appropriate regulatory authority.

Remember: 'There are old pilots and bold pilots. But there are no old bold pilots.'

Lu Zuckerman

11th Dec 2000, 06:34

To: Multp

The material covered in my report has already been turned over to the UK CAA as well as other certification agencies and accident investigation organizations.

Please log on to:

<A HREF="http://205467.homestead.com/caa.html" TARGET="_blank">http://205467.homestead.com/caa.html</A>

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The Cat

Grisoni

11th Dec 2000, 19:50

All these years I've been told that lateral cyclic has Very little aerodynamic effect at lower than 1g. And now Lu says adding right cyclic will increase my roll rate to cause selfdestruction of the AC.
'splain that Lucy.
Also having done a lot of Max aft CG at Vne flying, the cyclic is not at or around 3/4 travel to the right. But I'm starting to think that Frank is really a Commiebastard and is brainwashing us to see it his way?!

Also SN 24 in r22 POH adresses low RPM and susequent fuselage contact.

Merry Xmas Y'all.

Lu Zuckerman

11th Dec 2000, 20:27

To: Grisoni

When in a zero g condition the rotor is unloaded and therefore, difficult if not impossible to control. By pulling back on the cyclic, you load the rotor. Meanwhile the tail rotor is still effective and as a result imparts a right roll component. In the process of loading the rotor, if the cyclic is positioned as indicated in the diagram it can add to that right roll component causing the helicopter to roll right in what might be a non recoverable situation. Not my words, they are in the POHs for the R22 and R44. If you look at the diagram you can see how easy it is to input the additional right roll component.

Regarding SN-24 stating that the rotor stall will cause the blades to tip back similar to that of retreating blade stall. I will agree with that but only if you are talking about a Bell helicopter. The Robinson rotor head incorporates a coning (flapping) hinge and as such it will do the same thing that a fully articulated rotorhead would do in a stall condition. The blades will fold up on the hinge and not flap back as described in SN-24. Now, if Frank Robinson wants to counter that statement he is welcome to do so.

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The Cat

Kyrilian

12th Dec 2000, 02:27

Lu,
You said "When in a zero g condition the rotor is unloaded and therefore, difficult if not impossible to control."

As I understand it, this is not true. The reason zero/low-g is dangerous is that while flying at positive g, the helicopter 'hangs' from the rotor like a pendulum (yeah, it's more complicated, but the point can still be made). When you push over and get into a zero-g condition, the fuselage is no longer hanging--the rotor no longer imparts any significant forces on the fuselage. Now the major forces acting on the fuselage are the tail rotor and any aerodynamic forces due to positive velocity. At this moment removing the main rotor would have no effect (I take that back--there's a lot of torque, but that only plays into how much T.R. force is necessary). The fuselage rolls right because the tail rotor at this moment is above the c.g.

During normal positive-g flight, if something causes the helicopter to roll right the pilot instinctively nudges in a little left cyclic. Since the rotor is still being controlled by the swashplate (and hence the cyclic), doing so will cause the rotor disc to tilt as would be expected. But because the helicopter is not 'hanging' from the rotor, it will not follow. It will continue rolling right. If the angle between the two becomes too great the area around the teetering hinge will come in contact with the rotor shaft (or a rubber bumper if it's attached), and/or the blades will contact the left side of the fuselage.

I don't quite understand how slight right cyclic is bad. It seems that a combination of slight aft cyclic to reload the rotor and make the helicopter 'hang', and slight right cyclic to ensure that as the helicopter fuselage rolls right the disc follows (thus ensuring that the disc plane remains near perpendicular to the rotor shaft), would be the best way to get out of a low/zero-g condition. Even if you end up doing a positive-g diving right turn with very large (>>60°) bank, you're still ok. How is this unrecoverable?

Lu Zuckerman

12th Dec 2000, 05:10

To: Kyrilian

What I meant, was that in the zero g condition the rotor system is unloaded and as such was not completely responsive to cyclic input. It is true, in this situation not only is the helicopter free floating in space the pilot is as well. It is almost as if it were in orbit like a satellite. Until the pilot can get the rotor system under positive control by gently bringing the cyclic stick rearward the rotor is not under control. When the rearward cyclic takes effect, the pilot can then take corrective action by applying some left cyclic to counter the right roll. If in the process the pilot introduces right cyclic and adds to the right roll caused by the propeller effect of the tail rotor the helicopter will imediatly increase the rate of right roll.

What you said in your last paragraph about doing a diving right roll with a very large >> 60 degree bank is a viable alternative. Other pilots described this same maneuver on this thread. However being able to do it depends on two things both being equally important. The first is APTITUDE and the second is ALTITUDE. The ability to perform the maneuver and the altitude necessary to correct for the dive.

The way the POH addresses this matter is to not introduce right roll during zero g recovery and to never introduce left cyclic to counter the right roll when pulling back on the cyclic.

If you looked at the diagram you can see that the pilot must have situational awareness as to where his cyclic is in relation to the rigged neutral position. In accordance with the diagram it can be shown that it is very easy to introduce right or left cyclic during recovery from zero g.

Regarding why right cyclic is bad, please read safety notice SN-11 in the Robinson POH

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The Cat

Grisoni

12th Dec 2000, 07:05

In regards to Rotorstall don't forget that the R22\44 still has a teetherhinge; if one blade creates more lift it WILL force the other one down, so I think I'm safe to assume that it will eventually contact the AC.
SN 11 only says what all the training books say in that prior to lateral cyclic input near positive G should be restored. Now the problem is: if you get to that point, being training or inadvertend encounter of Less than 1 G, the AC tends to roll quite fast.
I've been told it accelerates at @ 120 deg. per second. From experience I can tell you it snaps over and you will find yourself going the other way and 100' lower. But like Mr. K stated by applying right cyclic you will end up closer to 90 to the disc than not. But like the books say: aft cyclic first if you find yourself in a turn go with it don't fight it. Maybe somewhere in your great analytical mind ( and I mean that with respect) you make things more complicated. But if you find yourself looking at the ground through the doorwindow all you (should) remember is the basics.

Lu Zuckerman

12th Dec 2000, 07:45

To: Grisoni

Regarding rotor stall on a Robinson Vs a Bell. What you said is true but it might only apply to a Bell. Regarding the Robinson rotor head it has coning (flapping) hinges which gives it the characteristics of a fully articulated rotor head in that when the blades stall for whatever reason the blades will fold up on the coning hinges if you can't counter the stall condition. It may be true that the entire head pivots on the teeter bearing at some time during the stall but the ultimate action as a result of blade stall is blade fold.

Regarding your comment about the roll rate being 120-degrees per second I am in no position to say that it is wrong so if that is what you heard and you state from your own experience then I must accept it as being correct. If in fact you are correct then how can a pilot react to the roll if he has to pull the cyclic back very gently with no rapid control inputs. If it took him more than 1 second to fully react it would appear to me that he is on his back and out of control. One pilot wrote in this thread that he knew of an R22 that rolled on its' back and the pilot had enough presence of mind to get the dirty side down. The newbie pilot faced with this same situation would be eating a plexiglas sandwich.

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The Cat

Grisoni

12th Dec 2000, 19:21

Mr.Cat
Yes, the blades do fold up, but prior to the "rabbitear" syndrome the disc tends to make fuselage contact usually on the tailcone @ the strobelight. And there is the statistic of most of the inflight breakups of the R22. But rpm is controlled by the pilot and therefore it's considered pilot error. (??)
as far as the high rate of roll during low G ,it has to be pretty extreme to roll that fast, that's why it generally not practiced anymore, rather you practice recovery at the first indication (light in the seat). Now I don't know how rapid other AC roll since most operators don't practice it.
The R22 is widely used as a trainer and you tend to do things that you will not do in other AC, and I believe it is reflected in the accident statistics.
But then there are three kind of lies: Lies, damm lies and statistics.
Happy Holidays.
Talk to you next year.

HeloTeacher

12th Dec 2000, 22:50

QUOTE: "To: Kyrilian
What I meant, was that in the zero g condition the rotor system is unloaded and as such was not completely responsive to cyclic input. It is true, in this situation not only is the helicopter free floating in space the pilot is as well. It is almost as if it were in orbit like a satellite. Until the pilot can get the rotor system under positive control by gently bringing the cyclic stick rearward the rotor is not under control"

Sorry Lu, incorrect again. The rotor is 100% controllable with cyclic, that is the problem. Inexperienced or panic-ed pilots OVER-control the cyclic in response to inadequate seat of the pants feel and visual confirmation of their actions.

If the pilot held the cyclic still during the right roll induced by zero-G the rotor actually would follow the fuselage for a while, as the stick (in space relative to the disc) is moving right with the fuselage. Eventually the excessive bank angle and increasing rate of descent would conspire to make things very lousy, but this is still what would transpire for that brief period. THE PROBLEM IN INAPPROPRIATE PILOT ACTION! I have NEVER gone out with the intent of exploring low to zero G behaviour, but have had too much experience with low-time students (sometimes mid to high time) and demo flight passenger/students who panic and push-over or dump collective. It won't kill you if you react immediately and correctly, and it is very do-able.

You ask incessantly about stick plotting boards, and we keep telling you that the cyclic does NOT drive off to the right in forward flight.

Low RPM results in the high flapping angles because at reduced RPM the flapping angle required to correct for the dis-symmetry of lift in forward flight is increased. The coning angle will be increased as well but that is irrelevant unless you are at the lower speeds to hover.

To Grisoni: The R22 roll rate is high because of the high position of the tail rotor combined with a short longitudinal couple, requiring more thrust per gross weight.

I am amazed that anyone would "practice" this .?.?.?

Lu Zuckerman

13th Dec 2000, 01:14

To: Helo Teacher

I quote from the FAA Rotorcraft Flight Handbook page 11-10

During the low G condition lateral cyclic has little, if any, effect because the rotor thrust has been reduced. Also, in a counter-clockwise rotor system, there is no main rotor thrust component to the left to counteract the tail rotor thrust to the right and since the tail rotor is above the CG, the tail rotor thrust causes the helicopter to roll rapidly to the right. If you attempt to stop the right roll by applying full left cyclic before regaining main rotor thrust, the rotor can exceed its' flapping limits and cause structural failure of the rotor shaft due to mast bumping

Non applicable paragraph omitted.

If you do find yourself in a low G condition which can be recognizes by a feeling of weigntlessness and an uncontrolled roll to the right, you should immediately and smoothly apply aft cyclic. Do not attempt to correct the rolling action with lateral cyclic. By applying aft cyclic, you will load the rotor system, which in turn produces thrust. Once thrust is restored, left cyclic control becomes effective, and you can roll the helicopter to a level attitude.

It seems strange that these exact same words are in Safety Notice SN-11 in the Robinson POH. My question is did Robinson copy this material from the FAA or was it the other way around.

How does the text above differ from what I told Kyrilian?

Now, I would suggest that you look at the diagram to see how easy it is to input the wrong cyclic control. Too far to the right or even just on the centerline of cyclic movement you can add to the right roll. Too far to the left and you have lost your rotor. That is my whole argument relative to Zero G.
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The Cat

[This message has been edited by Lu Zuckerman (edited 12 December 2000).]

[This message has been edited by Lu Zuckerman (edited 12 December 2000).]

Kyrilian

13th Dec 2000, 02:03

Lu,
"During the low G condition lateral cyclic has little, if any, effect [on the helicopter fuselage] because the rotor thrust has been reduced."

To a beginning pilot the main point they're trying to make is that when the rotor on such a helicopter is unloaded, laterial cyclic will have nil effect on motion of the _helicopter_.

The difference between what you, and HeloTeacher and I are saying is between control of the helicopter, vs control of the rotor. The cyclic control is still intact--therefore it will still control the feathering angle of the individual blades and therefore the disc plane. The rotor will not cone, and therefore it will no 'flap back' with forward speed. But this is the only difference between the loaded and unloaded rotor system dynamics on their own. Because the rotor doesn't apply any force in the plane of the disc, and lift that is normally present perpendicular to this plane while in positive-g flight is not there, the rotor does not apply forces to the fuselage of the heli. Since we're talking about a teetering rotor, no moment from the rotor is applied to the rotor shaft. The difference between teetering heads and articulating heads with hinge offset or hinge spring is the reason why low/zero-g flight is so much more dangerous in a teetering rotor helicopter (B206, R22, UH-1, etc). I know that on at least one fully articulating helicopter the restiction is not 1g, but .5g. I imagine on some hingeless heads it's probably 0g.

HeloTeacher

13th Dec 2000, 23:53

"That is my whole argument relative to Zero G.
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The Cat"

What argument, that the lateral component of the [aft] cyclic movement has to be tightly constrained due to the 18 degree offset or the rotor will fall off? It doesn't bear out in practice.

Remember Lu, we are talking about a very brief period, the excessive flapping angles are not brought on by minute movements of the cyclic or fuselage attitude or subtle aerodynamic effects.

The right roll is rapid and disconcerting. The initial action of an uninformed, disoriented or scared pilot is gross overcontrolling. The result is catastrophic.

This is true of any helicopter design where a pilot who is not anticipating enough is faced with a rapidly developing situation that requires prompt and correct action. If the engine fails and he waits too long to salvage and recover RRPM the result is fatal. If he has loss of tail rotor components and flares too agressively, the result is fatal. If he responds to wind gusts or turbulence with excessive corrections the result is fatal. IN ANY TYPE!!

The cure is adequate training, both initial and recurrent, and ongoing awareness of the operating environment. It is NOT a car, don't treat it like one. It is NOT an airplane, don't treat it like one.

Awaiting your reply as always

Lu Zuckerman

14th Dec 2000, 01:49

To: Helo Teacher

I agree with you. I used the term argument and maybe that was a poor choice of word(s).

My point is that I was trying to show that the fact that the control of the Robinson is far from precice and any mishandling of the controls by an experienced or, unexperienced pilot during the countering of Zero G can result in disaster.

The words I used to indicate what was required to counter Zero G were not mine, they were taken from the Robinson POHs. These books tell the pilot what to do in this situation. They also tell the pilot what not to do under these conditions.

Whether you say that my points are not borne out in actual practice may be true but until they change the wording in Safety Notice SN-11 or in the FAA Rotorcraft Handbook I will have to maintain my points of view.

Have you read my new thread on the 18-Degree offset? That will help to drive my point home.

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The Cat