Robinson Safety Courses

[Vortex what...ouch!]

Is the Robinson safety course worth doing and if so what does it teach you?

Thanks

[Brilliant Stuff]

You bet your cotton socks it's worth doing. I have just done the Course, it costs a measely $350, for which you get 3 1/2 day's worth of course including an hour flying, with one of their Pilot's. You will learn a lot about Safety. And the Design of the Robinson helicopter's.

[Vortex what...ouch!]

I have heard there is a lot to learn for low time pilots, I have about 160 in 22, 44 and 206 evenly split and it starts to teach you a bit about how to explore and recognise the limits of flying Robinson helicopters. What are the type of things they teach?

Thanks

[Lu Zuckerman]

To: Vortex What?

I would like to paraphrase your last question. What are the types of things they teach and what are the types of things that they don’t teach?

[Brilliant Stuff]

Vortex,
the course does not differentiate between low and high time Pilot's. They will teach you about Mast Bumping, Low RPM recovery, Blade stall etc.It's all easy enough to learn.

[Rotorbike]

If you want to know what the safety course is all about checkout http://www.robinsonheli.com/training.htm which gives you a full rundown on what is studied during the course.

Your flight time is perfect for the course. A large proportion you will find having less flight time than yourself as many in the US do the course between their private and CFI licences.

Don't hesitate about doing the course it will make you more aware whichever of the three types you mentioned you fly.



Edited so that Lu doesn't correct my grammar!!

[ 24 October 2001: Message edited by: Rotorbike ]

[muffin]

I agree with the other replies. I did the course here in the UK last year - just the classroom part without the flying, and it was excellent. I certainly learnt a lot from it and would definitely recommend it. I had about 100 hours TT at the time.

[Jiff]

I very recently completed the course and if you are a low time pilot I think you would walk away from it a bit wiser.
If you have an engineering background then you may well leave with some unanswered questions and find some of there opinions and statistics questionable.
Also remember that the course is run buy the Robinson Helicopter Company that wants to sell you Robinson Helicopters.
I heard some choice quotes from the lecturer which I had a few problems with,

"The myth of turbine reliability".
I have no problem excepting that a derated recip engine is very reliable but when showed a table of figures that show the R22 engine to be almost five times more reliable than a C250, hmmmmm.
Someone once said there are lies, damb lies then statistics.

"No one has yet to produce a small reliable gas turbine"
I'm sure the French and a few others would have something to say about this.

Being a company course its understandable that they would want to promote there product and a bit of creative license is expectable but directly criticizing other Helicopter companies and there products was a bit much.
Being a PPruner for a while ive read most of the stuff about the R22 and thought that this is a great opportunity to hear it from the horses mouth, so I asked two questions. The first one to Frank Robinson and the second to the course lecturer.

1.
"Frank, there has been a large debate recently on the PPrune web site about the R22 and R44 head design, what's your view point", I asked.
"The person who started that has since been discredited as an engineer", Replied frank.

2.
"If the blades have the ability to flap independently but cant advance or retreat, how does the design of the head work", I asked.
"Its very complicated and you wont be able to understand it", the lecturer replied.

Draw you own conclusions about the answers but I'm damb sure I know which impression I was left with !!!!

Lu,
We were shown a rotorhead which was damaged as a result of continually pulling to much power (cattle mustering in Oz), the area's on the head where the damaged occurred looked unusual and I would like to discuss this directly with you.

Jiff

[Lu Zuckerman]

To: Jiff

Here is my response to your posting. Regarding your last point, I will be most happy to have you contact me directly.

Here are your comments with my responses:

1.
"Frank, there has been a large debate recently on the PPrune web site about the R22 and R44 head design, what's your view point", I asked.
"The person who started that has since been discredited as an engineer", Replied frank.

RESPONSE:
It is nice to know that Frank Robinson thinks of me as a discredited engineer. There are a lot of American and European aircraft companies that seek my services that would disagree with him.

2.
"If the blades have the ability to flap independently but cant advance or retreat, how does the design of the head work", I asked.
"Its very complicated and you wont be able to understand it", the lecturer replied.

RESPONSE:
The responder was an idiot for saying that and only shows that he has limited understanding of the mechanics of the Robinson Rotor head. Because the blades are free to flap the blades will also want to lead and lag but they are restrained by the cone bushings. The energy imparted by the leading and lagging tendency will cause the blade to exhibit spanwise bending. This bending is reacted by the cone bushings and causes them to wear in an elliptical pattern. This cyclical energy transfer is also reacted by the teeter bushings and they wear in the same pattern but to a lesser degree. The total energy is also reacted by the main rotor shaft and it exhibits cyclical torsional bending.


Draw you own conclusions about the answers but I'm damb sure I know which impression I was left with !!!!

RESPONSE:

What were your impressions? I'm sure that the members of this forum woulld like to know as I surely would like to know.

Lu,
We were shown a rotorhead which was damaged as a result of continually pulling to much power (cattle mustering in Oz), the area's on the head where the damaged occurred looked unusual and I would like to discuss this directly with you.

RESPONSE:
Can I assume that the damage you allude to was mainly in the cone and teeter hinges. This will manifest itself to a greater degree on helicopters used in mustering as they are flown with a great deal of maneuvering that manifests itself in higher leading and lagging loads but it can also manifest itself in any Robinson but to a lesser degree. On the Oz Robbies if the bushing wear is excessive the through bolts passing through the bushings will also wear and be significantly weaker. Also the bushing wear will actually allow the blades to lead and lag and since there is no damping effect the blade will come up hard on the bushing (stop) further exacerbating the condition and the ride quality. High power application has minimal effect. It is the high maneuvering loads that cause the problem on the head you describe.

[Jiff]

Lu,
Well the impression I was left with was far from favorable and there was defiantly an arrogance which was along the lines of, We are Robinson Helicopters, we know best, because we say so.

You are correct in your description of the damage to the main rotor head, the teeter bushing had worn to the point of failure in the same plane as head rotation.
Because the aircraft had been used in a high load condition I was expecting to see the wear in a vertical plane, but I except your explanation as to the reasons.
I'm an electrical engineer and mechanical stress isn't really my thing but I do remember the mechanical guys on various projects doing there absolute utmost to avoid cyclical torsional bending on shafts.
This begs the question, if cyclical torsional bending on the mast becomes excessive, can it lead to mast failure without mast bumping ever occurring?
If so how much?
Does flying out of trim increase this?

Jiff

[Lu Zuckerman]

To: Jiff,

In order for the torsional windup to cause mast failure the effects must be accumulative over time. It is my opinion that the mast failures are caused by rotor divergence which is the result of high flapping loads and resulting lead lag energy caused by flying out of trim or by side slipping or for other reasons yet to be explained. That is the reason Robinson included the suggestions (not warnings) in the POHs for the 22 and 44. These high flapping loads can also result from excessive left cyclic when recovering from a zero G situation. That is why Robinson suggested (not warned) against application of left cyclic in the zero G recovery. It is my opinion that the loss of the aircraft can also be the result of pulling the cyclic back during recovery from a zero G incident because of the 18 degree offset. This is still to be proven.

I also feel that the rigging procedure is faulty and it can cause some problems that under certain situations can result in stability problems.

[Lu Zuckerman]

It is my personal opinion that the Robinson safety course is designed to protect Robinson helicopters and not the people that fly Robinson Helicopters. They tell you about mast bumping and how to avoid it. They tell you about low rotor RPM conditions and how to avoid them and they tell you to avoid zero G conditions and how to recover from them yet they do not address the basic design deficiencies of the helicopter that will allow you to get into these hazardous conditions. Zero G is a given in a single rotor design so there is no problem there. However the recovery from this conditions requires that you recover in a way that could get into further trouble assuming there is a relation to the 18-degree offset on the head. If when a Robinson crashes as a result of mast bumping nobody discusses the design deficiencies that led to the failure it is always the pilots fault because he/she disregarded the POH or even worse, what he/she was taught in the safety course.

It has been discussed elsewhere on these threads that one particular instructor in the safety course (Tim Tucker) indicated to his students that in recovering from a zero G situation that in bringing the cyclic back it should be displaced a tad to the left. Being a test pilot for Robinson in the development of the R22 he must have been aware of the problem that if the cyclic were moved straight back it would add to the right roll which could cause loss of the helicopter. Yet, if you look at the instructions in the POH it does not address this tad movement to the left. In fact, it states that if you move the cyclic to the left in recovering from zero G you could increase the flapping loads causing mast bumping and resultant loss of the rotor system. Assuming Tim Tucker was correct and, I do there must be an acceptable amount of left displacement before it becomes excessive and results in mast bumping. It seems to me that there is an inconsistency between the teachings of Tim Tucker and the other course instructors and an inconsistency between what Tim Tucker teaches and what is in the POH. This means that in theory individuals taught by Tim Tucker have a higher probability of surviving a zero G incident than those students taught by other instructors.

Jiff in his posting above indicated that he was not favorably impressed with the course mainly because of two things (my opinion). He has an engineering background and he asked questions that I had posed in my other postings. He also questioned their holier than thou attitude. If you remember in some of my more vitriolic postings I stated that Frank Robinson is not God (nor are his employees) and just because he/they state something it is not necessarily true.

Why don’t you (those that attended the course) revisit it in your thoughts and see if some of your opinions change? Don’t let this thread die.

If Helo Teacher is reading this, please contact me via email.

[vaqueroaero]

Lu Zuckerman,

Having read all the above posts can you answer the following for me:
How does a 'normal' semi rigid head i.e. a 206 compensate for lead lag? What is the difference between this and the Robinson? I asked the mechanics in the hangar but they don't know. From what I can see, although the construction is different, why should the principle of operation be different? Not trying to pick a fight, just a beginner trying to learn.

[Lu Zuckerman]

To: Vaqueroaero

On the Bell rotorheads placing the rotor mass below the teeter hinge minimizes the tendency to lead and lag. This is known as underslinging. When the Bell blade flaps the rotor mass tilts below the teeter hinge which places the rotating axis very close to the driving axis. On a multi blade helicopter that has blade offset from the driving axis when the blade flaps and the disc tilts there is a deviation between the two axes and this results in lead and lag. Some helicopters incorporate a lead lag hinge to permit movement of the blade on this axis. Other systems allow the blade to flex in plane accomplishing the lead lag in this manner or, the rotorhead had a flex beam to allow this movement. On the Bell system the two axes are not totally coincident with each other so there is a minor amount of lead/lag and the drag links on the larger bell models and the blade restraints on the smaller (206) types resist this. This results in a slight amount of in plane bending.

There is another phenomenon at work as well. All helicopters compensate for the tendency for in plane bending by moving the pitch axis slightly ahead of the rotor centerline. This compensates for the fact that it is nearly impossible to get the blade mass CG on the same line as the pitch axis and this causes an in plane bending of the blade. By offsetting the pitch change axis from the rotor centerline the tendency for bending is negated or minimized.

The Robinson helicopter is unique in the fact that the rotor head is not only free to teeter but also it is capable of flapping (on the cone hinges). When the blades teeter they are also free to flap to minimize the bending loads on the blades. The Robinson head is much like the Bell system in the fact that it is underslung for the same reasons as on the Bell. But since the blades are free to flap the disc center of drive deviates from the center of rotation. Here again, this induces leading and lagging. Since the blades are rigidized in the inplane rotation direction the blades will bend inplane at two times the rotor speed. This can cause problems such as flexing on the pitch axis, it can result in the fatigue of the blades and as stated above it can lead to the accelerated wear of the cone hinges and the teeter hinge as well as fatiguing the rotor mast.

[helmet fire]

To Lu:

Welcome back Lu. Where do I start with all of your statements? How about this:

1. The Bell 2 bladed system DOES flap, lead and lag, despite the absence of cone hinges. It is designed to. Same as most tail rotors including Bell and Robinson.

2. ALL teetering head helicopters are subject to mast bumping when handled incorrectly – in utter isolation from the 18 degree offset.

3. The 18 degree offset is part of the ROUGHLY 90 degree phase lag that ALL helicopters have to contend with in designing their control system. It is IRRELEVANT in the context of mast bumping (but I know I am not going to convince you here!)

4. If you tried to roll hard left in a Bell 2 bladed system during a zero G situation you would also mast bump and adopt the glide path of a brick!! And it doesn’t have cone hinges or an 18 degree offset.

5. A good understanding of flapping to equality would help you understand these concepts (I sound like a broken record!)

6. Perhaps Frank was saying that your formal qualifications as a licensed aircraft (helicopter) engineer were discredited when you admitted to not having a licence, rather than discrediting you as an engineer per se. Depends how you read the grammar.

7. Lu Said: >>They tell you about low rotor RPM conditions and how to avoid them and they tell you to avoid zero G conditions and how to recover from them yet they do not address the basic design deficiencies of the helicopter that will allow you to get into these hazardous conditions. <<

Lu, you avoid zero G through piloting technique and avoiding turbulence if you can. Nothing to do with design. Nothing.

8. Lu said: >>Zero G is a given in a single rotor design so there is no problem there.<<

What are you talking about????

9. Here is how I believe the zero G situation works on a 2 bladed Bell. I don’t fly the R22/R44 so I cannot say for sure, but I cannot imagine how the Robinson system would behave any differently, cone hinges or not. As the aircraft approaches zero G, the aircraft can effectively be seen as weightless in relation to the rotor disc, i.e: the aircraft is no longer “hanging” from the rotor disc. This brings the tail rotor drift/roll into play because the fuselage is now free to roll, and so it does – it rolls right (due to the direction of tail rotor thrust) on American direction helicopters. The rotor disc, however, does not know the fuselage has rolled right, and stays (relatively) in the previous plane. Therefore, if we freeze the action at this point, the disc is displaced to the left of the fuselage, i.e.: the state that exists during the initiation of a fast left turn, only because of the zero G situation the fuselage does not “swing” into plane under the displaced disc. Accordingly, if you introduce left cyclic to counter the right fuselage roll (as would be natural) you are actually further displacing the disc to the left rather than countering the roll, the eventual result of which is to induce mast bumping and funerals.

10. Lu, you advocate the introduction of aft cyclic in the zero G situation. I wouldn’t in a Bell, and I suspect the same is true for the Robinson. Let’s revisit the zero G situation above in (9). If the zero G situation was induced by a rapid lowering of the collective or encountering a severe downdraught, it is likely that the fuselage will also pitch nose down due to the sudden lack of rotor downwash on the horizontal stabilizer. Again, as per above, whilst the fuselage pitches nose down, the disc remains in relatively the same plane so if we freeze the action, it resembles the initiation of a fast pitch up where the disc is displaced to the rear. Same as for the roll, an introduction of aft cyclic at this point will displace the disc even further aft rather than counter the pitch down, thus the eventual extreme result of which is to induce mast bumping and funerals. This effect is not as noticeable as the rolling moment.

Due to the above, I would be very careful before advising the use of left or aft cyclic in zero G. One technique I have been taught is to “fly the disc” not the aircraft in this situation. During normal level flight, look up and note the position of the tip path plane in the windscreen, i.e. its “normal” position relative to the airframe. When encountering zero G, position the cyclic to move the disc to the “normal” position in the windscreen, thus maintaining the alignment between fuselage and rotor disc and avoiding mast bump. From here you can very gently reintroduce G and recover. I reiterate – I don’t fly Robbos so I do not know if this technique is valid for them.

Lu might like to know that the Australian accident rates with Robinson’s is reputedly much better than the USA’s, and mast bump accidents are virtually unknown. Frank once put this down to the higher minimum hours requirement of the Australian Instructor course (about 400ish I am told).

In reply to Vortex: Every professional (as opposed to private or recreational) helicopter pilot (as opposed to engineer) I know who has done the Robinson course has said it is excellent, and most agreed it is a “must”. Enjoy it as part of your professional development.

Edited due to some poor spelling and crap arguements (I mean discussions of course! ).

Edited again to include a bit about turbulence.

[ 27 October 2001: Message edited by: helmet fire ]

[ 27 October 2001: Message edited by: helmet fire ]

[tgrendl]

Helmet,

In seven (above) I think the correct sequence of events would be slightly different.

The nose would yaw to the right.

As the barn door side of the fuselage becomes exposed to the wind it would react to that force and roll skids away from the wind. (the pivot would be at the mast)

All this time the disc would stay (with no pilot input) in it's previous alignment with the direction of flight.

Since we still have control of the disc and not the fuselage I think your squaring the disc to the fuselage idea is correct I just don't think the sight picture will be what you expect when you look up.

Thanks for the thought provoking post,

[Lu Zuckerman]

To: Helmet Fire

In response to your comments;

1) It is true that the bell blades flap but unlike blades on other helicopters which flap up and down as individual elements the Bell blades flap in opposition to each other. One moving upward and its’ opposite number moving downward. The Bell blades contrary to your statement do not lead and lag as the underslinging of the rotor head minimizes if not totally eliminates the tendency to lead and lag. Bell and Robinson tail rotors do flap just like the Bell main rotor with this flapping action working in concert with the delta hinge equalizes the lift across the disc. On some larger tail rotors there is leading and lagging and it is allowed by the inplane flexing of the rotor blade or in some cases the tail rotor includes a lag hinge and the blade is restrained by a damper. The Bell and Robinson tail rotor blades do not lead and lag as you have indicated. On the early tail rotor used on the S-58 helicopter the blades literally tore themselves apart because of the tendency to lead and lag. The blade root was redesigned to allow some flexing thus reducing the flexing loads.

2) The 18-degree offset as I indicated is yet to be proven to have an effect in zero G recovery. Regarding mast bumping resulting from recovery from zero g on the Robinson the effect of left aft cyclic in the recovery will introduce very high flapping loads resulting in either mast bumping or rotor incursion or both.

3) See 2) above.

4) I agree, however on the Robinson it is strongly suggested that you do not move the cyclic left in recovering from a zero G situation because of the high flapping loads. See 2) above.

5) Flapping to equality does not enter into this discussion as it is mainly encountered during forward flight and can be countered with cyclic input.

6) Regarding Frank Robinson’s’ comment, I believe he was referring to me being discredited as an engineer and not as a mechanic (engineer). I never stated that I was not a licensed mechanic (engineer) as I obtained my Powerplant license in 1962 and I got my Airframe ticket in 1976 and have carried that combined license (A&P) since 1976.

7) Again I agree with you in your statement about avoidance of the situation. What I was alluding to was that the course tells you about these hazardous situations but they do not tell the students that these situations are the result of the rotorhead design. The rotorhead comprises a coning facility unlike the bell design. This was included to minimize the bending loads when the blades cone. This allows the blades to flap. In a recovery from zero G, the free flapping blades in a sense go wild which can have several effects. 1) They can allow rotor incursion. Or, 2) If the blade flaps down to the point of contacting the static stops (tusk) the energy of the blade can force the rotorhead down and it can make contact with the mast causing mast bumping. It is true that this can be avoided by proper piloting but as I pointed out there is a discrepancy between the teachings of the course instructors and there is a discrepancy between the teachings of one instructor and the POH. Another point is that the Robinson has low inertia blades, which can slow down under maneuvering conditions or mis-application of power. The lightness of the blades dictated the inclusion of cone hinges and all of this effects the way the Robinson helicopter is flown placing a higher burden on the pilot.

8) What I meant was that single rotor designs are susceptible to Zero G where multiple blades with offset are not. I was addressing the recovery from zero G and the resultant effects of application of left rear cyclic in the recovery.

9) Sounds reasonable to me.

10) I can’t fault you for some of the statements you made as you prefaced them with the fact that your experience is on Bell and not Robinson. If you get an opportunity read the POH for either the R22 or, the R44 (section 4) as it specifically states that in recovering from a zero G condition you carefully move the cyclic aft (TO LOAD THE ROTOR) and not to move it to the right which will add to the right roll induced by the tail rotor thrust nor, to move it to the left to counter the tail rotor induced roll which would result in high flapping loads resulting mast bumping.
Regarding the high safety record of Oz Robbies I would refer to the rotorhead used as a training device in another post. I would suggest that this head was removed because of the unbearable ride quality due to excessive lead lag loads at twice the rotational speed of the disc. If the pilots had not complained about the excessive inplane vibration they would eventually have been killed when they lost a blade or if the mast fatigued. A comparable situation is compressor stall on a turbine helicopter. It requires that the entire dynamic system and the driveline be inspected and in some cases all of it being replaced. If they only removed the rotor head and kept the blades and the mast there can be another catastrophe down the line.

Regarding the value of the course I agree but I go back to another post in which I stated that it is not what they do teach you it is what they don’t teach you that can cause problems.

[helmet fire]

Tgrendl: You are right about the yaw, well spotted. I think the roll component is what grabs your attention first, and when you think about it, any right yaw in a right hand angle of bank can be appear to the pilot as dropping the nose downwards similarly to pitching down. I totally agree with your comments on looking up and seeing something unexpected, that although the fuselage rolls right, the disc would be offset to the left. I think that is the exact reason why the technique gets you to look up and fly the disc – because it is not where you expect it to be. Thus my point was that the recovery may not be “natural” in the sense that countering the right roll/pitch down may bring you unstuck.

To Lu:
In response to your comments;

1) Bell blades do lead and lag. I guess we will have to agree to disagree on this one.

2) As you indicated: the 18-degree offset is yet to be proven to have an effect in zero G recovery, thus I would avoid attributing characteristics to it that appear to be shared by other teetering heads without it, i.e. the tendency to mast bump if using incorrect piloting techniques. I cannot agree with your comment that: >> on the Robinson the effect of left aft cyclic in the recovery will introduce very high flapping loads resulting in either mast bumping or rotor incursion or both<< I do not accept that the introduction of cyclic causes flapping, even though I have never flown the Robbie. Flapping is related to induced flow over the blades and will occur during acceleration OR deceleration. Although I would accept that as the blades fly up or down in response to cyclic feathering, there is a change in induced flow, this is irrelevant in the context of mast bumping. See my explanation in the previous post (9) which explains the situation devoid of flapping.

3) See 2) above.

4) Lu, I seem to have put my point across poorly here. My point is that the Bell teetering head is subjected to the exact same dangers of mast bump in zero G even though it does not have coning hinges or an 18 degree offset. I.E. the coning hinges and 18 degree offset are CANNOT be relevant factors in mast bump if it occurs even when they are absent from the design.
5) You state that: >>Flapping to equality does not enter into this discussion as it is mainly encountered during forward flight and can be countered with cyclic input<< When you start talking about flapping loads due to cyclic inputs, etc, in YOUR discussion, I believe you are demonstrating some misconceptions about flapping (see (2) above), hence I again suggest you learn this concept. I have said on previous posts that I believe that the credibility of some of your theories suffers due to the fact that they can sometimes be seen to ignore the basics.

6) Got it.

7) Several issues here, firstly Lu you said: >>free flapping blades in a sense go wild [in zero G] which can have several effects<< This is another disagreement we continue to have. The rotor disc DOES NOT suddenly become wild or unstable during zero G. In fact the problem is the opposite: that it maintains its stability and does not follow the fuselage. If you REALLY subscribe to your gyroscopics theory, wouldn’t that tell you that the disc will maintain stability due to its gyroscopic tendencies?

Also you said: >> there is a discrepancy between the teachings of one instructor and the POH<< Simple problem here Lu, there is no such thing as perfect standardization. I know nothing about the techniques best suited to a Robbo, but I do know as an instructor that if I teach something to a student and it gets him killed, I need to be able to fully and scientifically justify the technique. If the technique is in accordance with the manufacturers requirements, I have justified myself and the problem belongs to the manufacturer. If on the other hand, I have taught a technique in contravention to the manufacturers instructions and I cannot scientifically justify why, then surely there would be a liability (and moral) issue. IMHO. This is not intended to in any way discredit, nor disagree with the instructor you mention as I reiterate I have no knowledge of the ins and outs of the Robbo. Each to his own.

Lu, in regards to your theories of low inertia blades “slowing down under maneuvring”: they dont! This is, however, irrelevant to the discussion, perhaps another thread?

8) I think my misunderstanding is with your grammar. All things flying can be subjected to zero G, but you used the word “susceptible”. Perhaps you meant “susceptible to mast bump in zero G situations?”
9) Thanks.

10) The reason the Oz record was mentioned was that you seem to keep inferring that the Robbie is at fault here, and that inference is continued with your discussion about the training rotor head. I would suggest that ANY rotor head with a severe imbalance would eventually cause mast/head/blade fatigue and possible separation if you had a brain failure that meant you kept flying it. Nothing Robbie specific.

My overall summary was that zero G and its dangers ARE NOT Robbie specific, the course is reputedly excellent. The “what they don’t teach you” appears to be for a very good reason .

[Lu Zuckerman]

To: helmet fire

Many of your objections to my comments are based on your knowledge of Bell flight theory and using that logic; you question or object to my postings about the Robinson.

The following is extracted from the R22/R44 POHs, as the information is the same for both helicopters.

This is a statement appearing on the last page of section 4 of the respective POHs. The page is unnumbered and the material contained on this page was published in accordance with a FAA priority letter AD dated 13 January 1995. For brevity sake I did not reproduce those things that could result from Zero G as they also apply to the Bell system. It addresses Mast Bumping.

…High forward speed, turbulence, and excessive sideslip can accentuate the adverse effects of these control movements. The excessive flapping results in the main rotor hub striking the main rotor mast with subsequent main rotor separation from the helicopter.

Below that statement are 5 items that should be avoided in order to minimize high flapping loads. Statement (4) reads, Avoid sideslip during flight. Maintain in-trim flight at all times.

I now extract material from Safety Notice SN-11 that applies to both helicopters. …. During the low-G condition the lateral cyclic has little if any effect because the main rotor thrust has been reduced. Also 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 will cause the helicopter to rapidly roll to the right. If the pilot attempts 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 or allow a blade to contact the airframe.

It continues. The best way to prevent mast bumping is to avoid abrupt cyclic pushovers during forward flight. Always use gentle sensitive cyclic control inputs, and if you do have a feeling of weightlessness during a maneuver, gently bring the cyclic aft to regain main rotor thrust before
Applying lateral cyclic.

The POH says to bring the cyclic back without adding any lateral cyclic. Tim Tucker who was a test pilot during development of the R22 also teaches many safety courses. He tells the students to add a tad left cyclic when pulling back on the cyclic to load the main rotor. No other instructor has his experience or technical background and in their ignorance or whatever they follow the instructions in the POH. If it can be proven at some later date that the 18-degree offset of the control system is a factor it can be proven that Tim Tucker was right and if you did the control movement in accordance with the POH you could increase the right roll rate. A highly experienced pilot could possibly recover from this situation. However if a newbie enters this condition he will most likely instinctively push the cyclic left and chop off his rotor or the tail boom.

I have one question. How many Bell Helicopters have a restriction against sideslip and out of trim flight?

[ 28 October 2001: Message edited by: Lu Zuckerman ]

[ 29 October 2001: Message edited by: Lu Zuckerman ]

[ 29 October 2001: Message edited by: Lu Zuckerman ]

[Lu Zuckerman]

To: helmet fire

IMHO the reason for the differences in handling between the Bell and the Robinson is in the differences in their respective rotorhead designs. This difference is also the reasons for restrictions placed on the Robinson and not on the Bell.

This fact is not taught in the Robinson safety classes.