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Certification of Robinson Helicopters (incl post by Frank Robinson)

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Certification of Robinson Helicopters (incl post by Frank Robinson)

Old 16th Dec 2000, 01:53
  #221 (permalink)  
Grey Area
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Imlanphere

I try not to overstress the theory side, although I do cover it. I find that a demonstration is worth far more, but it does help if it is supported by background knowledge. To a basic student at the beginning of the first sortie I simply sit on the ground and demonstrate that as the cyclic is moved so the disk moves in response.

In ground school I work through as follows (assuming a simple freely hinged system):

Revision of the forces on an airfoil, stressing that if lift exceeds mass then the airfoil will climb.

As the airfoil moves upwards there will be two velocity vectors, forward movement and vertical movement, which can be combined through simple vector addition to create a resultant velocity. This resultant “relative airflow” will reduce the angle of attack until the lift created equals mass at which point a stable state is achieved.

Point out that in a helicopter the blade mass and the centripetal reaction will both oppose lift.

At this point, the process becomes known as “flapping to equality” as the system is free to achieve equality of blade lift over mass and centrifugal reaction. One should not go further until the student is comfortable with the concept that constant angle of attack is achieved around the disk in the hover and constant lift moment at all times.

Discuss a tilted disk and compare with a level disk. Draw a graph of vertical displacement. Note that the points of maximum vertical velocity will occur 90 out of phase with the tilt (very easy on a graph). At the 0, 90, 180 and 270 points add angle of attack and examine the total pitch angle. Note that to achieve a disk tilt the max pitch angle must be applied 90 in advance and so on.

Stress that the rotor is a dynamic environment and these changes happen very quickly. (It is possible to demonstrate this with rapid fore and aft control input on some helicopters but if you have hydraulic actuators or jacks they will attenuate the rapid movements.)

Later I discuss the control rigging required to achieve a 90 advance angle.

Much later if the student enquires I will discuss the damping and torsional forces that can be passed to the blades from the head (I instruct on a rigid head aircraft). However, during basic training it is more important that a student accepts that his inputs will be replicated by the disk and that he should simply select and hold attitudes by looking out at the horizon and adjusting the cyclic position as required.

Hope that helps

Grey Area
 
Old 16th Dec 2000, 05:03
  #222 (permalink)  
Lu Zuckerman
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To: Grey Area

I find this very difficult to say, as it may seem like a personal attack on you as an Instructor.

I ran a training program for the U S Army a long time ago. I managed my department, which dealt with flight controls and flight theory for three different types of Sikorsky helicopters. I taught mechanics and maintenance officers. When I started on this job I had just came from Sikorsky and while at Sikorsky I attended seven schools on different helicopters including auto pilot school. After finishing the classroom studies I worked on the shop floor and did everything from riveting to flight test. Oh yes, when I was In the USCG I attended two other Sikorsky schools.


When I started to teach I knew everything there was to know about Sikorsky helicopters. My students were in most cases high school graduates with limited mechanical experience. When they got to my classes they had only completed a two-week course on rotorblade repair. Knowing what I knew of the classes that they were yet to attend, I not only taught flight controls I taught hydraulics, dynamic systems and power train. Some really fantastic training devices that replicated the flight controls and dynamics systems of the three helicopters enabled this.

When I was on the platform I was dead serious and I taught what I had learned which was well beyond the comprehension level of the students. It took several months before I mellowed out and started to use humor as a means of enhancing education.

It is my personal belief that if you are teaching your flight students as described in your post you are talking over their heads. I would strongly suggest that you teach the technical aspects of helicopter flight theory by simplifying that, that is too technical too fully understand by a neophyte student pilot. Look at the posts above. Fully qualified pilots don’t fully understand what you were talking about. I would guess that the only participant on this forum that understands what you said is Kyrilian and he is a grad student in aeronautics at MIT.

I also take exception to your use of the term control rigging necessary to achieve a 90-degree advance angle. There is a control system and there is rigging of the flight control system. The design of the control system is a fixed entity that the mechanic and/or pilot can do nothing about. On the other hand, the mechanic/pilot can rig the control system to achieve the necessary blade angles and control ranges to achieve safe flight. It is a very good thing to teach the student how his control system works and what happens at the top end when the bottom end is moved in any given direction. The results of that movement are manifested in pitch change that results in lift differential and in turn causes the precession of the blade disc. Also, you state that this is done to show the student about how the 90-degree advance angle is achieved. Here you admit to a 90-degree advance angle but above, you stated that there was no precession angle and that disc tilt was achieved by a bunch of mathematical formulae that I didn’t understand no, did many of our fellow postees.

Your bit about flapping to equality can be better described as the coning angle which is the result of weight being lifted and the centrifugal forces developed by the spinning rotor system.

You teach that centripetal reaction and blade mass will both oppose lift. That’s nice to know but nothing can be done about it as these forces were factored into the design and, it clouds the students' mind.

Just one word in closing “SIMPLIFY”

I’ll shut up for now.


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The Cat

[This message has been edited by Lu Zuckerman (edited 16 December 2000).]
 
Old 16th Dec 2000, 16:28
  #223 (permalink)  
Grey Area
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Lu,

I feel we are suffering from a trans-Atlantic terminology problem. In the UK Military, the term “Control Rigging Angle” is used to define the angular offset of the Servo Jacks and “Advance Angle” is used to define the angle between the blade axis and the pitch change rods, which together will equal the “phase lag”.

Coning angle is only one part of “flapping to equality” and was addressed in one of my previous posts (the one with the “mathematical gibberish” as you put it).

My last post was explaining how I teach a basic student, so for instructional purposes we use 90deg; it is a little simplistic. My previous posts have been aimed at a higher level.

Quite a while ago, I spent 5 months at the Central Flying School (Helicopters) on the instructors’ course. Since then, I have successfully instructed UK, US, French, German and Danish aircrew. The syllabus and techniques used have been quite successful for many years and produce some exceedingly capable pilots, who have no trouble understanding the concepts in my last post (I would add that that lesson takes about an hour). I agree that simplicity is good for instruction but one should never fudge or mislead a student. If a student doesn’t understand one must ask simply explain how it affects them and leave technical understanding until later.

I have the utmost respect for your experience and knowledge but I would suggest that we let the subject lie as we are not going to agree.

Grey Area

(Perhaps some UK Military trained chaps might want to comment?)
 
Old 16th Dec 2000, 18:23
  #224 (permalink)  
Lu Zuckerman
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To: Grey Area

An American was criticizing an Englishman about the terminology used to describe the parts of a car. The American says it is properly called a hood and not a bonnet. He went on to say that it is properly called a trunk and not a boot. He then said the rubber parts of the wheel are spelled tire not tyre. He finished up by saying after all we Americans invented the automobile*.

The Englishman replied, “yes, that may be true but we English invented the language".

Your point is well taken. Let’s cross swords or, keyboards on another subject.

* Actually, I think Benz invented it in 1886


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The Cat
 
Old 16th Dec 2000, 19:04
  #225 (permalink)  
212man
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Wink

Talking of the MIT (Lu mentioned it), here's what their helicopter page has to say on the subject of precession:

Gyroscopic precession is a phenomenon occurring in rotating bodies in which an applied force is manifested 90 degrees later in the direction of rotation from where the force was applied. Although precession is not a dominant force in rotary-wing aerodynamics, it must be reckoned with because turning rotor systems exhibit some of the characteristics of a gyro.

Just to put the cat amongs the pidgeons! (no pun intended).



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Another day in paradise
 
Old 17th Dec 2000, 01:02
  #226 (permalink)  
Kyrilian
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212man,
Just fyi, the page to which I think you're referring is hosted by a computer in the Artificial Intelligence Department at MIT--hardly an official view of anyone knowledgeable in the subject. This page was developed by a helicopter pilot (Paul Cantrell) who I think never had any ties with MIT.

I would love to join the debate but at this point I'm rather busy with end-of-term work.

Cheers,
- Kyrilian
 
Old 17th Dec 2000, 03:18
  #227 (permalink)  
Lu Zuckerman
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To: 212 Man

As Kyrillian stated those were the words of Mr. Paul Cantrell. Mr. Cantrell is also the author of a web site that is called Helicopter Frontiers. This web site is actually two web sites both of which are supported by Robinson Sales agents and/or training facilities that use Robinson Helicopters. Here is his complete statement about Gyroscopic Precession.

GYROSCOPIC PRECESSION

Gyroscopic precession is a phenomenon occurring in rotating bodies in which an applied force is manifested 90 degrees later in the direction of rotation from where the force was applied. Although precession is not a dominant force in rotary-wing aerodynamics, it must be reckoned with because turning rotor systems exhibit some of the characteristics of a gyro. This diagram shows how precession affects the rotor disk when force is applied at a given point:

A downward force applied to the disk at point A results in a downward change in disk attitude at point B. And upward force applied at Point C results in an upward change in disk attitude at point D.
Forces applied to a spinning rotor disk by control input or by wind gusts will react as follows:
"table at bottom of page 2-44"
This behavior explains some of the fundamental effects occurring during various helicopter maneuvers. For example, the helicopter behaves differently when rolling into a right turn than when rolling into a left turn. During roll into a left turn, the pilot will have to correct for a nose down tendency in order to maintain altitude. This correction is required because precession causes a nose down tendency and because the tilted disk produces less vertical lift to counteract gravity. Conversely, during a roll into a right turn, precession will cause a nose up tendency while the tilted disk will produce less vertical lift. Pilot input required to maintain altitude is significantly different during a right turn than during a left turn, because gyroscopic precession acts in opposite directions for each.

ME SPEAKING:

I think his statement about gyroscopic precession not being a major factor in helicopter aerodynamics is like saying that Bernoullis' theory is not a dominant force in aerodynamics but it must be reconed with when you design an airfoil. Without gyroscopic precession entering into the design concept of a helicopter then you and everybody on this thread would be in another line of work.

Here is another point. You may have read some of my previous posts where I referenced Mr. Cantrells words on these two sites. The main thrust of these web sites is in support of the Robinson design. This web site is full of a lot of very good pictures of helicopters and diagrams. Just prior to the section on gyroscopic precession Mr. Cantrell was discussing the Robinson rotorhead and compared it to a Bell rotorhead. At the completion of that part of the discussion the reader was directed to the section dealing with Gyroscopic precession. Instead of diagramming a Robinson swashplate, he used the Bell swashplate to illustrate precession (his words above) and in doing that, any one that read his words would believe that he was addressing a Robinson swashplate. If this is the way it is taught at Robinson schools then I can understand why there is so much animosity towards me when I discuss the infamous 18-degree offset.

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The Cat

[This message has been edited by Lu Zuckerman (edited 16 December 2000).]
 
Old 17th Dec 2000, 04:04
  #228 (permalink)  
212man
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I'm afraid I posted that in the spirit of 'devil's advocate' to some extent. Having said that, the author of the site (Paul Cantrell) does give credit for the information coming from the FAA basic rotorcraft handbook. Not having seen that particular manual, I couldn't say whether he has doctored it's contents.

On the subject of precession, does the magnitude of the force being redirected by 90 degrees stay the same? or is there some magnifying effect/absorbtion? Also, to continue with Grey Area's theme with pitch up; if you take an a/c with a very high control power (ie rigid head or large flapping hing offset)eg Bo105 or Lynx, and pitch up rapidly, surely there would be an equally rapid roll reaction too? Conversely, when manouevering in a tactical situation with large rates of roll, would not the pilot have to contend with the a/c 'bucking' up and down in pitch? I don't believe this to be the case.

Totally different topic, Lu have you had any involvement with the EC 155B (Dauphin N4)? If so, any info gratefully read.

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Another day in paradise
 
Old 18th Dec 2000, 01:54
  #229 (permalink)  
Lu Zuckerman
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To: 212 Man

There are two things in play relative to the introduction of gyroscopic precession as a means of changing disc position (tilt direction). 1) The rigidity or the disc in space which is a function of the rotating mass and the speed of rotation. 2) The force necessary to overcome the rigidity and to perturb the rotating mass. On a lightly loaded rotor system like the Robbie it can be done with mechanical input. On larger helicopters hydraulic servos are used. The servos serve two functions one is to supply the perturbing force and the second and from the pilots point of view is most important is the resisting of the feedback forces from the spinning rotor.

In either case the cyclic movement supplies the input or, perturbing force and depending on how that force was applied the resultant of that force will reach maximum response 90-degrees later in the direction of rotation. This input could be a singular input such as left or right cyclic or, fore and aft cyclic or, it can occur in combination when the cyclic is applied in a vector in relation to the X/Y axes of the helicopter. The results are still the same.

As long as the cyclic input is maintained the precession will continue until the rotor reaches the maximum deflection or, when the cyclic hits the stops. However when the pilot reaches the desired disc tilt and stops, then the precession will also stop, as the perturbing force will be removed.

The following is for Grey Area.

In diagramming the A Star dynamic system I came up with the following. If I recall correctly, you stated that when the disc tilted up in forward flight you would expect to get a roll input due to precession but this was not the case. In theory if the helicopter (any helicopter) disc were perturbed due to some external force and the rotor system and the helicopter were moved as a unit then there would be a roll. However on the A Star when the rotor (flaps back for whatever reason) the disc moves in relation to the fuselage and the swashplate is rigid to the fuselage due to the hydraulic lock of the servos. When this relative movement takes place, the movement of the pitch links in relation to the swashplate is the same as if the pilot inputted a cyclic movement in the right forward quadrant, which would nullify any tendency to roll, and would restore the rotor to nearly the position it was originally in, prior to the blow back. A minor adjustment in cyclic would fully restore the disc to the original position. (I THINK)

Regarding the Daufin (HH-65 Dolphin) I'm sorry to say no. I got out of the U S Coast guard about 25 years prior to the HH-65 coming on line.

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The Cat

[This message has been edited by Lu Zuckerman (edited 17 December 2000).]

[This message has been edited by Lu Zuckerman (edited 18 December 2000).]
 
Old 22nd Dec 2000, 20:27
  #230 (permalink)  
SPS
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Well Lu, 212,

I am having a bad week ! I am struggling with my own beleifs and theories this last couple of weeks actually.

It is clear that UK and US use of Gyro P is quite different, polarised if you like. I do not wish to become embrioled in all that because I think that polarisation closes minds to the others point of view, and on balance that is not the best thing (for me only, not instruction or sermon for you guys).

On balance though, nothing wrong with healthy debate either, it is good for your self and keeps the likes of government bodies and aircraft manufacturers honest.

I do think it is likely that US reaches for GP too readily, whereas it must be equally likely that UK considers it too little. It seems reasonable that the truth lies somwhere in the middle?

This proccess I refer to (in my head) has been triggered by the seemingly unsound exam questions I have mentioned (and many I have not)and some of the things that have been simmering for ages under the surface. I crave for one truth and one truth only.
Doubt if I'll get it somehow but I am sure glad to have you guys to bounce some of it off.

Lately apparent anomalies and ambigueties
have been leaping off the pages or whatever and biting me deep. I don't know if it is I that has suddenly changed or just coincidence.

I pick up a Nav. computer with instructions
for simple wind drift calculations on the top of the slider and find that they are technically wrong. There are 1000's of them around the globe.

The instructions?

"1. Place wind direction under true index.
2. Mark wind VELOCITY up from centre."

WHAT??!!

I will not go into where it was produced.

Then I read another sample exam paper and go ahhh....

I teach "Translational lift" to someone with a different attitude lately because I no longer beleive in the stock explanation (the product of a minor study into vortex action).

I don't beleive in the two most popular theories on ground effect because neither fits all of the actual effects when you are flying.

I don't want this to turn into a personal whinge or campaign against the science. I'm just trying to explain things as I perceive them.

The reason I have picked up on your thread
is that I have problems with Mr Cantrell's explanation of disc tilt in turns and I would like to debate that with you.

I don't think that GP has much to do with it.
Trouble is that if I differ with that item it seems I will be differing with the FAA Heli. book. Can it be held in isolation in conditions like this? eg. I would probably appear to be the staunch Brit (although I live in NZ!)telling the US boys they have it all wrong. I'm not, and I won't be. Can I differ with just this one thing without being marked down as an "Anti GP?"

I reckon I will say what I have to say on that particular matter on a new thread, try to stop it spreading and establishing me
as anti GP when I have an open mind on the matter, but a valid opinion to put forward.

BFN
 
Old 22nd Dec 2000, 21:09
  #231 (permalink)  
SPS
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Basic problem -

Gyro properties ;

1. Rigidity
2. Precession

Ok, we all know that, just setting out my stall.

If a rotor disc does not have high RPM (and no one would consider 550 or so "high" when
considering the RPM of instrument rotors) then how can it have rigidity, a product of
a rotating mass and its RPM?

If a heli has a low inertia rotor system, that suggests low mass, also a contributor to rigidity.

So - Low mass and low RPM (both relatively).

If the disc can be tilted by relatively small
control powers and inputs (eg a light heli with no servo assistance) then how can it have true rigidity? Surely if it did have true rigidity then ten pilots on one stick would not shift it from its attitude in space?

It isn't that simple of course, as the pilot does not change disc attitude by using the cyclic as a lever but does so by changing cyclic pitch. But I have to wonder that the amount of effort used to tilt a disc must in some way be commensurate with the amount
(or lack of) rigidity.

Not much rigidity = not much precession?

I'm not saying the disc does not have ANY gyroscopic properties but I sometimes wonder if it only has only SOME in the light of the above?

Is that why some push GP to the back of explanation? I'm not telling you, I'm asking you!

Just some thoughts.

 
Old 22nd Dec 2000, 22:17
  #232 (permalink)  
Lu Zuckerman
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To: SPS

In my email to you as the operator of a web site in New Zealand I asked a question regarding how they teach helicopter theory in your part of the world. Obviously by the questions posed by you in this thread it is obvious that they use the same instruction syllabus in New Zealand as they do in the UK.

On another thread in the Rotorheads forum I got chopped to pieces when I expounded on my theory about gyroscopic precession only to find out that in the UK they discount gyroscopic precession as the moving force in changing the disc tip path plane. Now, if I give an answer to your queries you must understand that I am speaking from my point of view which is based on 51 years of exposure to helicopter flight theory as taught in the United States.

The Robinson helicopter has a low inertia rotor system. What this means is that it has a low amount of stored or potential energy available to carry the rotor through in the event of a power failure. The low inertia has nothing to do with gyroscopic precession. The blade system has very low weight as compared to other helicopters but it has a rotational speed ranging from 495 to 530 RPM, which contributes to one of the gyroscopic characteristics of rigidity in space. Even with this rigidity it does not take a great deal of mechanical force to perturb the disc and thus introducing the other gyroscopic characteristics, which is precession. Look at the Robinson swashplate. The lateral inputs are very close to the centerline of the swashplate and as such do not offer very much mechanical leverage. The fore and aft input is at the front and back of the swashplate and as such have a slight mechanical advantage over lateral input but not by much.

You have to understand that rigidity and precession are characteristics of a Gyro and are separate elements. By applying a mechanical perturbing force the rotor will precess but it doesn’t effect the rigidity in space it simply changes the tip path and the rigidity in space is maintained. Rigidity means that the disc will stay where you place it and precession means that you can command a new position for the disc.

That is the difference in the two methods of teaching rotor theory. The UK method addresses individual blades and not a disc like a gyro. In the states they teach that the disc is like a spinning rotor on a gyroscope and as such has the characteristics of a gyroscope rotor.

Regarding using the cyclic as a lever to change disc attitude in space this is how it is done on autogyros that do not have articulating rotorheads. The gyro rotor also has rigidity is space but the disc can be moved with mechanical force exerted by a pilot with no help from a hydraulic boost system.

On larger rotor systems they use hydraulic boost to effect control. The main reason for the boost is not to input control forces to the rotor but they are there to resist the feedback forces generated during cyclic pitch changes as the blades rotate. Blades as such want to climb or dive depending on the input and they also have large centrifugal twisting moments that want to return the blades to a flat pitch condition.

Here is something you can try. On a larger helicopter that uses a constant pressure variable delivery hydraulic pump as the pressure developer bring the rotor up to speed and the pump will be developing full pressure. Move the cyclic rapidly in a circle and you will see that the pump pressure will drop to a very low level as long as the cyclic movement continues. the pressure reading when you do this test is a reflection of the forces necessary to perform the test. Do the same test when in a hover or when slowly flying forward and the pressure level should be higher than when you performed the test on the ground. The higher pressure reflects the force necessary to not only effect pitch change but to resist the feedback forces. The feedback forces will never be higher than the force necessary to resist those forces. That way the feed back forces will always be stopped by the hydraulic boost system.

Recently Robinson installed a hydraulic boost system on the R44. There was no change in rotor configuration or the force necessary to introduce pitch change; they installed the boost system, as a means of canceling the feed back forces that were reflected in the cyclic stick.

Regarding larger helicopters that you would think would require a boost system to overcome feedback forces you only have to look at the Kaman Sea Sprite or, the K-MAX. They effect the precession of the rotorhead by the use of small servo flaps on the blades. The pilot moves the servo flaps with out hydraulic boost and the servo flaps change the disc position via precession. A very small perturbing force moves a very high mass rotor system. Any feed back forces that are generated by the servo flaps are very small and can be easily countered by the pilot.


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The Cat

[This message has been edited by Lu Zuckerman (edited 22 December 2000).]

[This message has been edited by Lu Zuckerman (edited 22 December 2000).]
 
Old 22nd Dec 2000, 23:29
  #233 (permalink)  
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Lu,
One of the biggest problems I have with the concept of the rotor as a gyro is that effects such as pitch variations when rolling into a turn simply do not seem to happen. I know Ray Prouty has included work on this in his excellent books but I have demonstrated effects of controls too many times to mention on Gazelle, Wessex and Lynx and never have I observed a tendency to pitch nose up or down in any of them. Having taught many student QHIs at CFS there could be no question of cheating or fudging the demos by masking these effects so I am convinced that therefore the gyro concept is fatally flawed.
 
Old 23rd Dec 2000, 01:01
  #234 (permalink)  
212man
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How you can say that a head low inertia, has no bearing on its gyroscopic tendencies is beyond me. Rigidity in space is directly related to the polar moment of inertia and the rotational speed. A low mass two bladed system rotating at 500 rpm will not have a great deal of rigidity (though obviously some, even a Chipmunk or Tiger Moth displays it hence one reason for yawing during T/O and ldg).

The feedback forces are all aerodynamic, and the reason for small control linkages with low mechanical advantage is that the pitch change they produce results in a much larger force being exerted by the blade in total. Use of moveable tabs is just another way to solve the large forces involved in heavier machines in the same way that some older fixed wing a/c (eg Brittania) do.

It is not true to say that with Hydraulics the feedback forces are always shielded from the pilot; ever heard of 'jack stall'? Often the forces are caused by feathering action by 'propellor moment (CTM)'.

On the three 'large' types I have flown, moving the cyclic rapidly in a circle has no effect on the hydraulic pressure, either on the ground or airborne. In fact, if it is possible to demonstrate this effect (accumulators?) the degree and rapidity of movement required would almost certainly result in overstressing the various head components.

I can't believe that the differences in opinion can be blamed on national cultures. How confusing it would be for exchange pilots in general and those attending the 4 test pilot schools in particular, if that was the case. God nows what the French teach! (Grey Area, perhaps you could comment on this?).

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Another day in paradise

[This message has been edited by 212man (edited 22 December 2000).]
 
Old 23rd Dec 2000, 06:00
  #235 (permalink)  
Lu Zuckerman
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To: Crab

I know that Ray Prouty states that the nose will tuck under in one turn and pitch up in the opposite turn. I’m sure it is also stated in the FAA Rotorcraft Handbook but I was unable to locate the specific paragraphs. I’m afraid that until I can find that passage in the FAA handbook we will have to hold this conversation in abeyance.

Regarding trying to demonstrate this phenomenon on the Gazelle, Wessex and the Lynx. I am familiar with the Wessex control system as it is the same as the S-58. I know that the US Navy version, and the later US Army versions, had an Automatic Stabilization System, but normally, when control inputs were being made, the pilot would disengage the ASE and then reconnect it, when the new attitude and direction were reached, so that it would have no effect on canceling the tuck and/or pitch.

The Gazelle I know nothing about. On the Lynx you stated that there was an automatic stabilization system that cancelled the tendency to roll left or right depending upon which way you moved the cyclic. Is there a possibility that this system is restoring the attitude of the helicopter when it tries to tuck or, pitch up?

To: 212man

What I stated that the term low inertia rotor system applied to the minimal stored energy in the rotor system to carry through when you lose power. I also stated that the high rotational speed of a lightweight rotor system created stiffness of the rotor disc or, as you indicated, the polar moment of inertia. I would hazard a guess that with the light weight blade rotating at 500 RPM would reflect a tensile load at the blade attachment of several thousand pounds or, maybe more.

The feed back forces are aerodynamic and I totally agree. You made a statement about why they use small control inputs. You indicated it is that a small change in pitch will result in a much greater force due to the rotational speed of the blade and I agree with that. What we disagree on is the resultant force that changes the disc position. You say it is the individual blades that fly to their positions aerodynamically and, I state that the aerodynamic forces that occur across the disc are not the same, and this resultant force differential causes a perturbing force on the disc and gyroscopic precession causes the tip path plane to tip in the commanded direction.

Regarding the flaps used on the Sea Sprite can you imagine trying to fly a helicopter that is somewhere between a Blackhawk and a Seaking with out hydraulic boost? That has been the patented system used by Kaman since they made their first helicopter. Not having first hand knowledge of the Kaman system I would hazard a guess that the Sea Sprite hydraulic system is used mainly to retract and extend the landing gear plus the powering of several utility system. It is not used as a boost in the flight control system as in other helicopters. The control flaps generate the only aerodynamic feed back that the pilot might feel.

I am not familiar with the term Jack Stall. However it appears to be some sort of an abnormal problem in the control boost system. Tell me more about how it manifests itself and we can go from there. You stated the same things that I did in that most of the forces are due to Centrifugal Twisting Moment. However the present day larger helicopters use blades that are cambered on their upper surface. A blade with this feature is very unstable in that when it pitches down it wants to tuck under. If it is pitched up it wants to keep pitching. These oscillatory forces take place on each blade in its’ rotation and result in oscillating loads being resisted by the servos. This is allowed because the blade has a fixed pitch axis, which anchors the blade in position, and the pitch horn is attached to the swashplate by the pitch link and the swashplate is immobilized by the servos. Now, on the Apache the blades are attached to the rotorhead by flexible metal straps and the blade is attached to the swashplate like the previously discussed system. The blades on the Apache are also cambered on the upper surface and they too are unstable. Since the Apache blades are not rigidly anchored the rear strap reacts to the oscillatory pitching and tucking tendencies and as a result the strap is constantly being hammered against the lower portion of the rotorhead and subsequently has a high rate or replacement.

Again, I can’t address the Gazelle or the Lynx but the Wessex; if Westland did not change the design, used two constant pressure variable delivery pumps to power the two servo systems. I demonstrated the described pressure drop hundreds of times on dynamic simulators and on the actual H-34 during run-up practice. In fact, it is the standard means on Sikorsky Helicopters to rotate the cyclic as a means of detecting if you had a defective damper on the rotorhead. If on the other helicopters, they had an accumulator in the system, you would not get the pressure drop. In doing the demonstration, the cyclic would move faster than the “GYROSCOPIC PRECESSION” would take full effect. The rotor disc would only wobble.

Regarding your statement about the differences in national cultures being the cause of this problem. Just change “national cultures” into “technical approaches” and you have hit the nail on the head. That’s what this whole argument has been about. If a helicopter qualified exchange pilot came over from the UK and was attached to a US Navy Helo squadron or possibly to the US Airforce or US Army and he had to learn a new helicopter and new operational procedures he would not have a single problem. That is, unless he had to learn the aerodynamics of the new helicopter I would venture based on my experience on PPRuNe that he would be pulling his hair out. It is not like US pilots sit around all day and discuss aerodynamic and I would imagine that UK military pilots are the same. It is only in places like these threads that the arguments come forth.


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The Cat
 
Old 23rd Dec 2000, 06:09
  #236 (permalink)  
Lu Zuckerman
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To: 212man and Crab and Baque Flip.

Speaking of exchange pilots please read my post here on Rotorheads titled Do things like this still happen? You might get a kick out of it.

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The Cat

[This message has been edited by Lu Zuckerman (edited 23 December 2000).]
 
Old 23rd Dec 2000, 15:06
  #237 (permalink)  
SPS
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People are often said to "push the boundaries".

Boundaries are not just in front of us, they
are a globe, a sphere all around us.

To 'push' them as we desire, we must be prepared to look sideways, upwards, backwards, maybe in any direction, maybe several directions to succeed.

And remember - Boundaries do not know they are there.

They are only where we placed them.
 
Old 24th Dec 2000, 02:32
  #238 (permalink)  
Lu Zuckerman
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To: All

I believe that somewhere on this thread somone made the comment that the NTSB doesen't get involved in crashes that occur outside of the USA. Here is the response to that statement. It appears that he lost his engine resulting in low rotor RPM with blade fold and subsequent fuselage strike.

Or, maybe it was for another reason.

NTSB Identification: LAX01LA053

Accident occurred DEC-03-00 at CHURCH FENTON
Aircraft: Robinson R-22B, registration: GBNUZ
Injuries: 2 Fatal.
This is preliminary information, subject to change, and may contain errors. Any errors in this report will be corrected when the final report has been completed.

On December 3, 2000, at 1535 UTC, a Robinson R-22B, UK registration G-BNUZ, broke up in-flight over Church Fenton, North Yorkshire, UK, while descending toward the Sheburn Aerodrome for landing. The helicopter was destroyed in the accident sequence and the private pilot and one passenger sustained fatal injuries. The helicopter was operated by Hields Aviation and rented by the pilot for a local area personal flight. Visual meteorological conditions prevailed and no flight plan was filed for the operation. Ground witnesses reported that in the minutes prior to the breakup, the engine was heard to run roughly and the main rotor blades were slow enough to see the individual blades. The accident is under the jurisdiction of, and being investigated by, the Air Accidents Investigation Branch of the United Kingdom's Department of the Environment, Transport and the Regions, Berkshire Copse Road, Aldershot, Hampshire, GU11 2HH, United Kingdom. Telephone number


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The Cat

[This message has been edited by Lu Zuckerman (edited 23 December 2000).]
 
Old 24th Dec 2000, 03:39
  #239 (permalink)  
212man
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And? what's the point?

Of course the NTSB will be involved from time to time in investigations outside the USA. Definately when an N reg a/c is involved, but also when requested by other agencies. The UK AAIB is the same.

On a visit to FSI I was talking to my instructor about input drive shaft failure in the 212, he stated that when in the USMC he had been in a formation of UH1-Ns when an a/c in front had the very same. He witnessed the blades slow down and fold as the thing fell out of the sky. (high inertia head)

About 12 years ago a AS355 crashed in the UK after suffering double engine failure in icing conditions. The AAIB theory was that one engine flamed out and while the pilot had his hand off the collective to do something (frequency selection, engine control movement?) the other engine flamed out too, with not sufficient time to arrest the Nr decay before lowering the 'lever'. (Low inertia head).

It's a fact Lu, when main rotors lose their drive, they slow down. Unlike the profficiency check scenario where the pilot has his hand poised as if on a hair trigger, ready to lower the collective, in real life people's reactions are slower.

I do feel that postings like the above detract from your original intentions and can best be described as red herrings (hope that crosses the pond).

Anyway, glad to hear you are trying to get a trip in an R22, hope you find one soon and make sure the pilot keeps the ball in the middle!

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Another day in paradise
 
Old 24th Dec 2000, 03:53
  #240 (permalink)  
Lu Zuckerman
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To: RW1

The main reason I made the post was to show that the NTSB does get involved when an American designed aircraft is involved in an accident at some overseas location. It is true, they have to be invited but it is usuallly a forgone conclusion that they will be invited. The same is true for the manufacturer. At this time the NTSB is involved in two rotor incursion accidents on Robinson Helicopters one in Scandinavia and the other in Ireland.

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The Cat
 

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