Mast bumping, tailboom chops and two bladed rotors (merged)

[SPS]

We all know of the risk of rotor separation resulting from mast bumping if Low or Negative G is allowed to develop whilst flying ANY Helicopter with a teetering or semi-rigid head. But does every Pilot know as much as they possibly could about the problem?

I do NOT wish to become embroiled in any argument as to whether the cause is largely due to ‘Pilot error’, design or anything else. That is not the point here.

I DO wish to draw attention to the danger and hopefully prevent another occurrence. Better awareness must always be sought. With that aim in mind I have decided to take an unusual step -To provide this page in the full knowledge that anyone will be able to print it and share it with their Helicopter flying colleagues.

It is still under my copyright, and will remain so, but anyone who wishes to print it and avoid such problems has my permission to do just that. The only conditions are that it must not be sold, resold or republished by any other and that the source is
credited. The item will be available on this site for the next 10 days, after which I will have to use the space on my ISP for another purpose.

Anyone who doubts my sincerity on this is reminded that (A) I provide a website to improve safety awareness (and host this item for Pprune) at my own cost and (B) that this is not a cynical ploy to sell the book itself. The words ‘Gift Horse’, ‘Mouth’ and ’look’ are operative here!

Understand that I just wish to reduce accident figures by DOING SOMETHING to improve awareness of MAST BUMPING, and as I have this item on hand it seems ridiculous that it is not available to every Pilot in the world.

So, if anyone wants to take me up on this offer than all they have to do is push the button. If anyone thinks this is ‘boring R22’s again’, I ask they to remember that it is relevant to other types as well. In any event, the life of a low time Pilot is worth the same as a Space Shuttle Pilot, i.e. ONE.


Fly safe (fly safer) all.


SPS

The early bird catches the worm, but the second mouse gets the cheese.

http://www.helicopterpilotsguide.com/Low_g.gif


[This message has been edited by SPS (edited 24 February 2001).]

[rotorspeed]

Interested to see that Don'ts include diving. I've always presumed that while the transition into a dive should be gentle to avoid negative G, there is nothing wrong with a steady moderate dive at cruise power, within airspeed limits. Is this hazardous and if so, when and why?

[SPS]

Thanks for the input.

The main reasoning for this advice is to begin with good habits. If a student is allowed to dive in a limited way from early lessons this may be embedded (and very hard to remove) from their resulting flying style. If they meet a situation in the future that COULD be avoided with a dive then they might just reach for that 'tool' and do it much more quickly or agressively depending on the urgency at the time. Disaster awaits.

Of course, a limited 'dive' can be made, we do that every time we transition away or adopt an accelerative attitude if you wish to see it that way, and in types with different head construction/design a much greater dive is a perfectly acceptable maonevure but for students beginning (and maybe never flying another type in some cases) I decided to stick to this, the safest option.

It is probably due to the way I approach this sort of thing when teaching, ie. with negative G. teach what you must not do first (giving the strongest basic pattern or 'habit')and learn later if that can be modified (if necessary).

If no-one ever 'dives' then Negative G is much less of a risk? Just my view....

[Thomas coupling]

SPS You can't get rotor mast bumping in anything other than a teetering head. Gazelles / Squirrells, are semi rigid, they don't suffer from mast bumping.

------------------
Thermal runaway.

[RW-1]

I saw nothing wrong with your graphic SPS.

RS:

Thing about the "gentle" dive in a Robbie, or any other teetering head, is it can be done, just not abruptly, a gentle dive doesn't approach the low "G" condition.

You moved the cyclic forward to begin your descent and you stabilized, so now you are in a cruise descent at whatever airspeed, but at 1 "G".

I'll always take a turn to avoid if possible, a dive is just out of the question (I know I know maybe I'll want to, most who ask me this refer to birds, in which I respond that most birds like to "tuck and drop" anyway, so why dive to meet them? :) )

BTW, thinking about it, I suppose one could make move in forward cyclic, if you are already in a turn and pulling in collective keeping the head under a load, you would be accellerating and diving at that point, but I wouldn't do it from the level attitude.

Real thing is you don't want to have to make any agressive moves on your controls in the first place.

I got a changce in a British Navy Lynx to see how having a solid head differs, after reaching the top of the ridge, they nosed over and we held onto the backside, a bit of 0 "G" for a moment, something you would never do in a teetering head, but fun when you can!

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Marc


[This message has been edited by RW-1 (edited 25 February 2001).]

[SPS]

Thomas,

Point taken.

The fact that a teetering head may also be semi-rigid has nothing to do with the problem and it is not transferred to types
of head that are semi rigid but not teetering.

What I should have said was 'teetering and
UNDERSLUNG'. The diagram shows clearly how the mast can be contacted by an underslung head in some circumstances. This is the real problem with some types. A teetering head that is not underslung (if made)would not be so susceptible to the effects of Negative G as it would not be able to promote or allow mast bumping.

Coupling 1, Sparrow 0.

(Just for today of course :) ! )

[chips_with_everything]

Just an idea or two from an oily rag engineering type...

1) How about a g-meter coupled to a servo which applies aft cyclic (bit like a stick pusher on a jet, but of course it would be a puller)?

or

2) How about a sensor on the mast to catch actual excessive angles, triggering the same servo action? Could be a couple of magnets on the rotating parts and a static hall effect probe or two for instance.

Having said that, SPS has the right idea IMO. Avoidance is the key.

------------------
More volts, Igor

[rotorspeed]

Good use of the forum to get input on issues, thanks all. Glad my normal descent approach technique using 125 kts in a B206 or 135 in a AS350 to claw back a few of those precious seconds lost in the climb doesn't have a mast bumping risk. Certainly having a bit of headroom over fast cruise to Vne/Vma, unlike the 407, does offer a small advantage here; OK it's more psychological but still makes me feel better!

Good point on making it quite clear to any students or low timers that if this technique is used, the initial gentle nose over must be virtually imperceptible.

On teetering types it would be interesting to know what actually is thought to be the catastrophic precipitating manoeuvre - was it intentional or accidental abrupt nose over or was it turbulence. I have certainly had occasional cabin/baggage contents bangs in a 206 from presumably momentary 0 (or worse) G in very turbulent conditions over hills, despite being at reduced power. Presumably mast bumping is aggravated by low G with big cyclic input?

[SPS]

'Presumably mast bumping is aggravated by low G with big cyclic input?'

Fully agree with that Rotorspeed.

Low G involves the disc being unloaded, it therefore has no control over the fuselage which may adopt an attitude placing the mast at far less than 90 degrees to the POR in one sector of the 360 degree head cycle.

Add that to large cyclic inputs (worse if made suddenly, aggressively) which can place the underslung hub even closer to the mast and its goodnight nurse.

Of course, it does not have to be fuselage movement relative to the disc which may cause mast bumping. If the disc flaps violently (turbulence is a major cause)the same can happen. This is more likely to occur at high airspeeds with large pitch angles applied to the blades and is a good reason for reducing speed if flying in turbulent conditions.

Chips, my background too. I will mull it over
and reply later, thanks for the ideas. One thing I can say for certainty rightaway is this - No underslinging = a lot less risk.
I'd err on the side of designing the problem right out in preference to preventative measures that may fail to work. I'm not rubbishing the idea, far from it.

Finally, for completeness, the underslung head (hub) is mainly employed to overcome Hooke's Joint Effect. It does so using less parts, maintenance and initial cost. If the blades had the freedom to lead and lag (or flex) then the head would not have to be underslung.

[The Nr Fairy]

This accident report is worth a look - experienced pilot ( both in hours and on type ) but mast bumping killed him and the observer. The suspected cause was turbulence, not mishandling.

http://www.open.gov.uk/aaib/apr99htm/gshrr.htm

[This message has been edited by The Nr Fairy (edited 26 February 2001).]

[RW-1]

RotorSpeed/SPS:

Rotor's question is a good one. From what I have read it is not really the low "G" condition that kills, it's making the wrong move afterwards. If you give that cyclic (lateral is usually it, one should give aft)
movement prior to loading the rotor back up, boom.



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Marc

[Lu Zuckerman]

To: RotorSpeed/SPS and RW 1

"Rotor's question is a good one. From what I have read it is not really the low "G" condition that kills, it's making the wrong move afterwards. If you give that cyclic (lateral is usually it, one should give aft)
movement prior to loading the rotor back up, boom".

This is probably the very first time I have totally agreed with RW 1 as every thing he said in the above quote it 100%correct

I would like to resurrect the dead horse and beat it a few more times. It is my contention that the 18-degree offset on the Robinson head results in a right cyclic bias in order to fly straight-ahead. When the pilot encounters a zero G condition he is warned to bring the cyclic straight back to reload the rotor disc. When he does this with the right cyclic bias he actually introduces a right roll component. This exacerbates the right roll caused by the tail rotor and he enters into a “violent” right roll. If he counters the right roll by pushing left cyclic he encounters violent flapping loads that result in mast bumping and/or fuselage incursion.




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The Cat

[RW-1]

All I have to say on the ressurection:

I leave it for the engineers to figure out why, and I agree with the contention, however I have not seen the aircraft exhibit it.

Avoidance is the best form. Hopefully one is "Far enough ahead of the aircraft" to not have to make sudden cyclic movements, I expand being ahead to include knowing possible traffic conflicts, etc. This limits you to a reaction to that nasty suprise that no one can forsee.

However I would like Lou's opinion on what I stated earlier. Let's say (without bringing in the robbies offset into the equation)
If I decide to pull a hard cyclic turn, pull in a little collective and then am pushing forward right or left cyclic for the turn, would the rotor stay under a good enough load to prevent getting into the low "G" area? (thinking in terms of a good strategy when one does need to lose alt and turn at the same time. My thought is if I'm shoving 45 degrees right or left but pulling in collective, rotor keeps most of the load though I'll start to drop and gain airspeed.
Is there a balance that can be maintained?


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Marc

[SPS]

I'm more than a little disturbed this morning.

I have had 2 private mails over the last couple of days from private pilots in the UK that fly teetering/underslung a/c and they both say that that they have not been made aware of this peril and want to know more.

No, I'm not berating UK training or any individual organisation or instructor. I make no judgement at all. I was taught in UK and made WELL aware of this. I later taught students in UK and did all I could to emphasise the problem. It may well be that it HAS been explained to them but they 'forgot', and that may happen anywhere.

A previous Chief Pilot (no, I was an AFI) of the organisation I did my training with had been killed a couple of years earlier (RIP Harry Knapp) whilst demonstrating Low G (it was allowed then, or not specifically forbidden) and that left its mark on me. But does someone have to die before all sit up and take notice? I think not, and that is part of the reasoning behind this thread.

Whatever the reasons for these two pilots not having sufficient knowledge, (BTW it does indicate that pilots read prune but choose not to post replies) the effect is the same. It is plain that MORE awareness of mast bumping and its cause(s)is required and I'm sure the need is NOT isolated to one country only. Do not forget that it is also NOT isolated to only one Helicopter type either.

I'm taking things a little further by adding the mast bumping item to my website, make it more available to all who need it.

One of these chaps could not print the item from prune, maybe he will be able to do so from my site. If that fails and anyone wants the info. but doesn't want to buy the book then that's also fine, I'll send a copy out at my own cost, even if it must be snail mail.

At risk of sounding insincere on this, I will mention that there are another 120 pages of similar information in my book and that also should be out there doing what it was intended to do. It most certainly was not about collecting money. But I cannot provide all for nothing (although I wish I could) as economics rear their head(s).
A copy of the book may be ordered through the website.

To all of you out there, let's begin a campaign aimed at making sure EVERY Pilot can give you back chapter and verse on mast bumping, and not focus on why they can't now.

SPS

If a little knowledge is dangerous, greater
knowledge promotes safety.

[SPS]

Nr Fairy,

In providing the aaib link (that I have just read) I think you have greatly assisted the purpose of this thread and I'd like to thank you for doing so. I encourage any member to post similar links.

I strongly recommend that all and any use Nr's link.

Read the report it leads to, it is important.

[SPS]

For anyone interested - The item on Mast Bumping is now also available through my site (address above).

[Outside Loop]

Lu,

At the risk of flogging a dead horse,take another look at the diagram which shows the helicopter in the mast bump situation.

If the disc tilts a little to the right when appling aft cyclic in recovery,it will provide greater clearance between the head and the mast, therefor making mast bump LESS likely.

[The Nr Fairy]

SPS:

I ALWAYS read the AAIB reports the day they come out, and not just the helicopters ones, either. Anything which will help me fill the bag of experience before emptying the bag of luck is most welcome.

On your other post, private emails about people being unaware of mast bumping, I learnt in Oz and was taught about it during training, and also it was stressed during the safety course I went on. It would be nice to know if there's a pattern to where the knowledge isn't being imparted, or whether it's student related.

[SPS]

Nr,

Yes, It was bashed into me in UK and then on UK and US safety courses.

I doubt that the problem can be isolated to areas or even schools. I know that everywhere
I've been ( I've been around a little)
has instructors that give warnings on the subject. I'm not trying to make anyone wrong but I do think it more likely that it has been mentioned and then forgotten by the sutdent in some cases.

BUT! If that is the case, it also means it has NOT BEEN MENTIONED OFTEN ENOUGH!

Interesting point from Outside loop. I've been trying to avoid the 18 deg. issue because I am still 'processing' on it and do not wish to centre on one type (read Nr's link to the 206 occurrence) but this has me thinking....

Hmmmmmm...

[SPS]

Some reflections -

RW 1 - During your 45 degree turn the disc loading has increased way beyond 1G. The limit for the R22 (done well keeping myself off types until now I guess) is 53 degrees angle of bank resulting in the max. permissible disc loading (1.7G). At 45 degrees it is likely to be 1.4 or 1.5 G
and this would give you a greater margin against low or negative G so there is less of a problem than there would be in forward flight.

LZ-

'When he does this with the right cyclic bias he actually introduces a right roll component. This exacerbates the right roll caused by the tail rotor and he enters into a “violent” right roll. If he counters the right roll by pushing left cyclic he encounters violent flapping loads that result in mast bumping and/or fuselage incursion.'

OK. I do see your point but have one problem with this train of thought.

The main reason that low or negative G is a problem for this type of head is that cyclic
(and therefore disc) authority over the fuselage has been lost, This is what enables TR thrust to induce fuselage roll to the right and the possiblity of the hub and mast being in contention exists only because of this.

Why should a right cyclic bias exaserbate right fuselage roll? The cyclic (disc) has lost authority and cannot affect fuselage attitude. Add to that Outside loop's point about the hub/ mast clearance being increased
(not decreased) if any authority were present and it begins to look as if this particular type may have no more of a problem
than any other that is similarly constructed
and designed.

Chips - On balance I think I would go for a 'design out' rather than remedial acton but ecomomics may give it to the latter, as with many things in life.

Keep it coming - It can only be good for everyone in Aviation.

SPS

[steady]

This discussion brings a few points to mind.
Firstly. It would be interesting to have some idea of the terrain and weather factors involved in most -G accidents. As far as I know, the problem first appeared or was recognised in Vietnam where people were attemping to follow terrain for obvious reasons. The fact that it still happens in an aviation enviroment where that kind of flying isn't generally required just points to poor student knowledge of the problem and or students being poorly equipped in the way of alternatives when presented with a situation requiring this type of manouvre.
I personally lead with the collective and adjust airspeed to suit when I need to follow terrain or establish a turn crossing a ridge to stay loaded up.
I'd be interested to hear if there are better ways.

Regards all

[Lu Zuckerman]

To: Outside Loop

“If the disc tilts a little to the right when applying aft cyclic in recovery, it will provide greater clearance between the head and the mast, therefor making mast bump LESS likely”.

It is true that the right cyclic input will increase the angle between the mast and the head but in the application of aft cyclic with a right roll bias you are not introducing any control that would result in mast bumping. If in fact there is a right bias on the control system when the cyclic is moved aft, it will add to the right roll introduced by the tail rotor, and the pilot is specifically warned not to add to the tail rotor induced roll, as he can lose control of the helicopter. On a previous post it was stated that an experienced pilot would be capable of going with the high roll rate and dive the helicopter to the right and regain total control during that dive. However, not all pilots have that experience and capability to respond to the high roll rate.

To: SPS

“Why should a right cyclic bias exacerbate right fuselage roll? The cyclic (disc) has lost authority and cannot affect fuselage attitude. Add to that Outside loop's point about the hub/ mast clearance being increased
(not decreased) if any authority were present and it begins to look as if this particular type may have no more of a problem than any other that is similarly constructed and designed”.

In order to regain control of the rotor system in recovering from a zero G event the pilot is instructed to move the cyclic aft. It is true, the rotor system is at that time uncontrollable, but in moving the cyclic aft, at some time the control will be regained. If at that time the cyclic has a right roll bias it will add to the tail rotor induced right roll and increase the roll rate. If in the process of doing this the pilot gets a bit shaken up and he tries to counter the rapid roll rate with left cyclic he will encounter mast bumping which is caused by high flapping loads/excursions.



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The Cat

[RW-1]

SPS:

I can go with that (for the turn).

I would be moderately upset having people not aware of Low "G" and mast bumping asking too. No excuse for anyone (especially if trained in an underslung teetering head!)
Eeeeeeek !

[Grey Area]

If the disk has been loaded again then the thrust/gravity couple will far outweigh the tail rotor roll component, particularly as in an intentional manoeuvre the pilot will (should) have been off-loading pedal as the torque demand is reduced.

In fact we are talking about two different regimes; intentional flight at low g and inadvertant flight at low g. One could reasonably expect a pilot to offload tail rotor thrust during an intentional manoeuvre, therefore reducing the tail rotor roll, but it is far less likely in an instantaneous situation such as turbulance, thus the control regime will be markedly different.

Perhaps this explains why even a high time pilot can be caught out by turbulance, as his experience would be based on low tail rotor thrust regimes achieved in intentional low g manoeuvres. In an inadvertant case, it would be reasonable to say that a high time pilot could be expected to be equally surprised by the results due to the difference between his mental model based on experience and the reality, he could even apply a control input that works in a controlled environment but would be inappropriate in his unplanned situation.

Furthermore one could also argue that the lower the mass of the aircraft (and kinietic energy in the rotor system) greater the effect of instantaneous gusts and therefore its' suceptability to inadvertant low g. This sadly increases the risk to pilots of the lighter machines, the same ones that tend to have fully articulated heads and suffer from mast bumping problems in the first place.

[Lu Zuckerman]

To: Grey Area

Allow me to address your post as a theoretician and not as a pilot. Regarding the first paragraph of your post you address what the pilot should have done relative to offloading the pitch on the tail rotor to minimize the tendency to roll right during a zero G encounter. My immediate frame of reference is the POH for the R22 and the R44 and this is not even mentioned. The only statement regarding the countering of zero G is to move the stick aft without introducing additional right roll and to not move the stick left in order to not encounter high flapping loads / excursions that would result in mast bumping. I would go out on a limb and state that this is not normally taught in flight school however I’ll leave the final answer up to RW-1 as he is probably the most recent grad from the Robinson flight training program.

You addressed the intentional and accidental entry into zero G. From what has been posted on this and other threads a pilot would have to be an idiot to do it intentionally although others have pointed out that some pilots do it in order to instruct their students.

During the zero G encounters the tail rotor will introduce a right roll. When the control is eventually established the helicopter is rolling to the right. Under normal conditions when control is regained the pilot can counter the right roll by flying out of it or by introducing left cyclic to the point of stopping the right roll.

I believe that the Robinson pilot is instructed to reduce speed and control input during gusty conditions. One final point, I believe you misspoke in saying,” the same ones that have fully articulated heads and suffer from mast bumping problems….”. I believe you meant to say Teetering rotorheads.

Here is another point that is germane to this thread but not to your post. It was alluded to above that underslung rotorheads are susceptible to mast bumping. The original Bell heads used on the Model 47 helicopters were not underslung but were still susceptible to mast bumping. However, the rotorhead was restrained by a Sprague Cable (2) that limited the degree of teetering so that if the pilot put in too much cyclic the cables would come under tension restraining the degree of tilt. When the cables came under tension the pilot was well aware of it, as the helicopter would start to shake. This shaking informed the pilot that he should reduce cyclic input. If the pilot exceeded the tensile strength of the cables they would snap and mast bumping would result.


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The Cat

[Arm out the window]

To add my 2c worth:

I flew UH-1B and H for a good while, mostly in low level terrain-contouring flight.
Having teetering-head underslung rotors they could be mast-bumped if handled carelessly, but our training (RAAF) was thorough in this area, with a number of simple rules that seemed to keep everyone out of trouble. Maybe this is telling people how to suck eggs, but perhaps some may find it useful:

1. Don't bunt.

2. When flying over undulating terrain, use the collective (with appropriate anticipation) to go up and down, and adjust the cyclic to maintain desired speed. Don't use the cyclic as an altitude controller a la fixed wing.

3. When crossing ridges, fly at them obliquely rather than straight on. Use collective to go up at the required rate, and at the crest you can look over the other side and either turn to cross, or turn away.
If crossing, your bank angle combined with lowering the collective will descend you to follow the terrain.

4. As alluded to above, you can be fairly agressive with lowering the collective, but anytime you have less than 1 g, the danger of mast bumping is increased so be careful with the cyclic.

Interesting topic, cheers.
P.S. Seagull 571, if that means anything to anyone!

[Flight Safety]

Excellent topic. I also wanted to add a few thoughts. SPS said:

<font face="Verdana, Arial, Helvetica" size="2">One thing I can say for certainty rightaway is this - No underslinging = a lot less risk. I'd err on the side of designing the problem right out in preference to preventative measures that may fail to work.</font>

I agree that it'd be better to design this problem out, but unfortunately there are thousands of helicopters out there that currently have this issue. That means training and good awareness of "mast bumping" is a must.

The only remedial "fix" that I'm aware of for underslung teetering rotor heads is the "hub restraining springs" used on the Bell 222/230 series. My understanding is that this feature applies spring pressure to the rotor disk if it starts to get too far out of alignment with the mast during a low-G encounter. The spring pressure opposes the rotor disk if it tilts at too severe an angle relative to the mast, and tries to force the rotor disk back towards a 90 degree angle with the mast.

I don't know exactly how effective this remedy is, but I understand that is does work up to a point. It would be interesting to see if similar "spring restraining" devices could be developed for other helos with this type of rotor head.

I think it would be a good thing to have a low range G-meter in all of these helos with a range of say +3/-1G, so the range of 1G to 0G would have a fairly broad sweep, allowing the monitoring of the disk loading in .1G increments or finer. The meter could have a red band extending from say .5G on down to help keep the pilot constantly aware of the state of the disk loading. The meter could also have a red band from the max disk loading value on upwards as well (to help prevent an overload disk failure).

My understanding is that the purpose of the aft cyclc input (with maybe a little up collective) is to get the disk reloaded as quickly as possible, without doing anything like adding lateral cyclic, which could exacerbate the disk/mast angle while the disk is still unloaded.

Other possible causes of mast bumping could be: rapid control reversal, landing on too steep a slope (but at least this would be on the ground), low rotor RPM (causing a low inertia disk more easily upset by wind gusts), loading out-of-CG limits (causing a more severe rotor/mast angle to begin with).

SPS, I think it would be good idea to post of list of all of the helos affected by this problem.

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Safe flying to you...

[This message has been edited by Flight Safety (edited 28 February 2001).]

[Grey Area]

Lou, as a theoretician you would do well to READ MY POST PROPERLY.

My point was that as a pilot reduces the TORQUE demand, by either lowering the collective or bunting, one can reasonably expect that he will reduce the tail rotor pitch by moving the pedals, he does this to keep the aircraft in balance ie to control YAW not roll. He will have had this reaction beaten into him during basic flying training. This will have an effect on the reaction of the airframe if zero g is achieved.

To expand somewhat, from a developmental and instructional point of view it is highly important to consider that when simulating malfunctions in the air the helicopter does not always actually respond as it would in the real instance, often because it is prohibited or plain dangerous to carry out a faithful simulation.

In the case of zero g therefore, the average pilot is most likely to be familiar with a LOW g environment, which to the seat of the pants can feel very similar, however the aircraft can react to pilot input in a different manner, this can often surprise and disorientate the pilot as he has spent his flying career adjusting to and learning reactions to aviation stimuli.

The human brain, when confronted with an unknown stimulus will attempt to find a similar known situation and apply the correct response. In this case the response could be dangerous, and for a high time pilot will be more ingrained and therefore is more likely to be removed from the cognative loop. That is to say even if he recognises a potentially dangerous mast bumping event, his subconcious could well apply an inappropriate learned response. This chain of events could be termed a cognative failure. No amount of ground instruction or reading POH can ingrain these cognative responses into a pilot, they are not engineers. They don't think like engineers and they can't sit back and think the problem through when it all starts going wrong. On top of all that the problem will not be isolated from all other aerodynamic influences and therefore should not be seen in isolation, how he got there has as much bearing on how he is going to get out alive as the textbook response derived from a controlled test flight as part of a certification programme.

PS I intentionally referred to articulated heads because all such heads will lose most if not all authority at zero g, granted I was expanding the topic, but the problem is also dangerous to an allouette/ gazelle/ whatever(*delete as appropriate) pilot.

[SPS]

To all -

There is a wealth of information and experience from some very knowledgable people on this thread.

Why does it have to stop here?

Would it be a good idea to collate all of the information (Avoidance, recovery, solutions for existing a/c and ideas for those yet to be built or designed etc.)
and send it around the globe?

It could be done collectively, on a separate thread. All involved could spend half an hour one evening e mailing the result to every manufacturer,pilot,safety group, authority,magazine, flying club or training organization that they know of and mark it 'please pass around your contacts'.

Then we may have a chance to get this information where it should be - OUT THERE.

I do beleive it would save a lot of loss of life and benefit all in ther industry in the long run. Public perception might improve,insurance rates go down and so on.

Principally, if it saves just ONE life it'll all be worth it, and a small price to pay.

Anyone interested?

[The Silver Fox]

OK, just before I edit my profile, a couple of points from experience and discussions with experience.

A former vietnam pilot I once spoke to counselled me on the subject of avoiding mast bumping with a variety of techniques for maintaining loading on the disk, basically they involved rear cyclic to load the disk, then make corrective movements for attitude. The only alternate came about in tests to verify the existence of the phenomena (I become pale at the very thought of testing this theory) that being that a pushover induces the low-g, a right roll will occur due to TR thrust and position (higher than MR hub in an R22). Consequently right pedal will use TR thrust to counter right roll and allow time for rear cyclic to load disk for recovery. All in all, something this little black duck won't be trying to prove tomorrow....


Experience comes from tracking 2,000 AGL over mountains (I was a student at the time)and experiencing an updraft, a downdraft and another updraft . 2 bouts of climbing @ over 1,000fpm with MAP around 11 inches or less with a brief interlude of similar or higher descent with the pencil from my pocket levitating around eyebrow level. Fortunately the pencil didn't end up in the pedal tunnels & I decided to return home. The lessons from my instructors came through and I froze cyclic in the updrafts & kept the disk loaded during the down. The pucker factor was off the scale.

Incidentally, on the B206 susceptibility, I vaguely recall some years ago of a 'Ranter' bumping during a torque turn with 5 on board. Some very sad families probably just wished the driver had stayed within the flight manual limits.

[SPS]

Looks like this is about to slip into relative obscurity so I write firstly to thank everyone who has contributed to the thread and secondly to give anyone visiting another chance to be safer for reading it.

SPS

If a little knowledge is dangerous, greater knowledge promotes safety.

[engineoff]

I am interested to find out about mast bumping occurences in fully articulated rotor heads. All of the books and documents I have looked at talk about teetering rotor systems when discussing mast bumping/negative g. Is mast bumping exclusive to teetering/2 bladed systems?
I am particularly interested in information relating to the H500 after a recent demonstration of a manouever involving fairly severe negative g pushovers . The pilot claimed that there was no risk of mast bumping in that a/c . Any information much appreciated.

[Lu Zuckerman]

To: Engineoff

The phenomenon of mast bumping is restricted to teetering rotorheads. Articulated multi-blade rotor heads are rigidly mounted on the rotorshaft and in most cases have a splined or flanged coupling that drives the rotor system. The rotorhead is affixed to the mast with some type of mechanical fastener usually a nut and some times a series of bolts. The bolts attach the rotor to a flange on the rotormast and the nut threads onto the splined rotormast. The rotor head being rigidly affixed to the rotor mast can not move in relation to the rotor mast.

Another point to consider is that mast bumping occurs mainly when the helicopter has entered a zero G condition and the rotor is not imparting any thrust forces to the mast. As such, if the control is not properly reestablished by loading the rotor the rotor system it can experience high flapping loads resulting in mast bumping. This can not happen on a fully articulated rotor system, which allows them to enter into maneuvers that result in low G environments.

It relates to what is known as interlock. On a single rotor helicopter the interlock is minimal in that the tilted disc imparts a bending moment to the rotormast and it in turn causes an upsetting moment on the helicopter and it flies in the direction of cyclic movement. On a multi blade system there is significant interlock, which is immediately transmitted to the rotorhead, which in turn aligns the helicopter with the tilted disc. The higher the interlock the faster response to control input. On some rigid rotor helicopters the interlock is so strong that they incorporate a Mast Moment Indicator so that the pilot is warned if he is inputting too much cyclic control.


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The Cat

[Kyrilian]

Strictly speaking, 'mast bumping' only occurs in teetering heads where the two blades are fixed to one another but relatively free to hinge about the teetering hinge. When the limits of this hinge are reached you have mast bumping. This occurs in zero-g conditions because there is no hinge offset, and hence nothing causing the rotor blades and the shaft from remaining perpendicular.

In a fully articulating head (which I believe the 500 has) there is a non-zero hinge offset, so even if at speed there is nothing mechanically keeping the blades from drooping (like droop stops), the centripetal (centrifugal) force will 'pull' the blades outwards. Thus, at zero-g the blades will still try to remain perpendicular to the shaft, and amount relating to the hinge offset distance.

It would seem to me that if an abrupt maneuver is performed, the blades could be pulled out of their normal plane far enough that they may reach their hinge limits, or more likely, will contact the tailboom. So yes, in an extreme case the hinge limits may be reached, but this is more likely due to rapid control movements than a negative-g condition. In fact, I recall reading/hearing of a NOTAR 520 that chopped its tail, I think due to what may have been extreme maneuvering (hot-dogging), but don't quote me on that :)

[SPS]

The problem is restricted to head designs that move in relation to the mast, as they do with a teetering design. It may be further exacerbated by the head (hub) being underslung (for the purpose of cancelling Hooke's joint effect).

If the hub is free to teeter about its connection to the mast the potential for it to strike or 'bump' the mast during extreme flapping of the blades exists.

If the hub does not move in relation to the mast because it is rigidly fixed to it, no teetering of the hub occurs, and no risk of Mast Bumping can exist.

Many other things may happen to various parts of another type of head (hub)as a result of low or negative G but they do not have the potential to damage the mast in the way a teetering underslung system can.

[Lu Zuckerman]

To: Kyrilian (Welcome Back)

It has been many years since I last had a good look at a Hughes rotorhead, but I believe that the strap packs that react the centrifugal forces and allow pitch change are contained, within two profiled clamps which control the flapping of the blades and act as a theoretical flapping hinge so that the flapping is always about the same theoretical axis. Many helicopters have theoretical flapping hinges or points on the rotorhead that flapping takes place about. This could be a rotor that has a flex beam of some sort or a rotorhead that utilizes elastomeric flex bearings.

Regarding droop stops on rotorheads they are normally out of play during rotation of the blades above a set speed. On the Hughes 500 type head if a blade contacts the droop stop it just forces the droop stop out of the way moving it off of its’ central position.

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The Cat

[This message has been edited by Lu Zuckerman (edited 21 March 2001).]

[Dave Jackson]

Lu,

Your informative posting and comments on 'interlock' bring up an interesting consideration.

The following two examples are based on helicopters, with CCW rotation, in which the pilot has just applied forward cyclic.

When considering a conventional helicopter with a teetering or highly articulated rotor hub and very flexible blades; the maximum angle of attack will be on the left-hand side and the maximum blade elevation will be at the back. The forward 'tilt' of the disk will drag the fuselage into a forward leaning position.

When considering a helicopter with *extremely* rigid blades and *extremely* rigid couplings between the blades, the hub and the fuselage/frame; it would appear that the maximum angle of attack must be located at the back - so that it will *pry* the helicopter into a forward leaning position.

Any thoughts.


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Project: UniCopter.com (http://www.synchrolite.com/UniCopter_Index.html)

[This message has been edited by Dave Jackson (edited 21 March 2001).]

[Grey Area]

Dave,

Even with rigid head you will get phase lag. The best way to imagine it is to think of an autogyro in forward flight. All of the blades are fixed in pitch. If you tilt the disk forward then the blades will alter their angle of attack reference the airflow. If you imagine the advancing blade (3 oclock) then with the disk tilted forward the blade will be at a reduced angle of attack, and the converse is true for the retreating blade. To manoeuvre a cyclic controlled disk the pitch angles must be altered so that the angles of attack match those that a tilted fixed pitch disk would produce. (Normally I do this on a whiteboard, so try drawing it, it might help).

GA

[Lu Zuckerman]

To: Dave Jackson

To properly respond to your question I will have to address the phenomenon of Gyroscopic Precession and from past experience this is like a lightning rod for our English brethren. Whether you are addressing a rigid rotor or a flex rotor or even a two-blade teetering rotor the laws of gyroscopic precession apply. The advancing blade has the lowest pitch at the 3:00 position for counter clockwise and at 9:00 the blade has the highest pitch. With a phase angle of 90-degrees the maximum response will be 90-degrees later in the direction of rotation. In the case of a flex rotor the blades will flap up over the tail and flap down over the nose with this flapping being accommodated by hinges or flex beams. On a rigid rotor the situation is the same but the flapping up and down is accommodated via the flexing of the rotor blades.

In your illustration of the rigid rotor helicopter if the blade had the highest angle of attack over the tail the helicopter would fly to the left due to precession. There was only one helicopter that had a rigid rotorhead and very rigid (low flexibility) rotor blades. That was the Cheyenne and depending upon speed, density altitude, gross weight and speed of control input when the pilot pushed forward cyclic the blades would flex down somewhere between the right side of the nose and the left side of the nose and if the cockpit got in the way so be it.

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The Cat

[212man]

By definition you cannot get mast bumping with an articulated head but as stated other things can happen. Perhaps a good example of this is the S61 that, some years ago on the back of a pitching boat, managed to remove the top of the cockpit. The hapless pilot found himself with no throttle quadrant and therefore no means to shut down. The captain of the ship was none to happy and ordered the a/c off the deck. So, the newly cabriolised 61 sat in the water till the fuel ran out. I believe they brought out a mod that allows the fuel to be cut off without the quadrant subsequently.

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Another day in paradise

[Dave Jackson]

To Gray Area and Lu

I agree that a downward force at 3:00 results in a minimum elevation (in respect to the mast plane) at 12:00. This can be attributed to the gyroscopic precession force working on the mass of the blade and/or to the aerodynamic force working on the surface of the blade.

I believe that these forces are probably one and the same. For example, take the preverbal bicycle wheel and replace its rim with little individual unconnected weights at the outer end of each spoke. Then spin this 'wheel'. Have a series of air jets blow down on the weights as they move between 6:00 and 12:00 and up on these weights as they move between 12:00 and 6:00, Also assume that the strongest jets are located at 3:00 and 9:00.

The result of the above example should be representative of both the gyroscopic and the aerodynamic event; I hope :)

________________


In conventional helicopters, the flexible blade plus the rigid, semi-rigid and teetering hub all represent a flexible connection to the mast. In a theoretical helicopter with absolute rigidity between the airfoil and the mast, similar to a propeller with variable pitch, I think that the phasing of the cyclic must be advanced 0 degrees, not the conventional 90 (or less) degrees. http://www.pprune.org/ubb/NonCGI/confused.gif

_______________

Perhaps the Cheyenne, which Lu mentioned, has a phase lag of close to 0 degrees?

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Project: UniCopter.com (http://www.synchrolite.com/UniCopter_Index.html)


[This message has been edited by Dave Jackson (edited 22 March 2001).]

[Lu Zuckerman]

To: Dave Jackson

I posted the following material on Just helicopters last year. It is a part of a much larger posting and it has been reduced to address disc tilt due to gyroscopic precession and any material that applied to retreating blade stall has been removed. It addresses counterclockwise rotation helicopters.

Try to visualize this, or better still, pick up a pencil and paper and draw a picture of a circle. Cut the circle into four equal parts. On the circumference of the circle where one line meets, write the letter N for North. Now, label the other lines East, West and South. North is the direction the helicopter is flying.

I would suggest that you stop thinking of individual blades flying in a circle. Instead, think of the blades as a solid disc just like the rotor on a gyroscope. If you apply a force to a gyro that is on gimbals, the gyro because of precession will react 90 degrees later in the direction of rotation. The same is true on a helicopter. Cyclic input will change the pitch relationship across the disc and will result in an imbalance of forces. If the pilot pushes the stick forward, the greater force is over the West side of the disc. This upward force (due to precession) will cause the disc to raise over South and drop over North. Aerodynamics plays a minimal part in this action. The change in disc position was caused by the change in lift forces but the gyroscopic forces and characteristics of the spinning disc caused the actual movement. These characteristics are rigidity in space and precession.

Blades do not fly to a position, the are moved by gyroscopic forces.

Regarding a helicopter that has blades that are infinitely rigid attached to a rotating mast I really can’t comment. You used a controllable pitch propeller as an example of this type of setup. Propellers are subject to gyroscopic precession but unlike helicopters that introduce the precession with pitch change the propeller wants to precess due to aircraft maneuvering in relation to the fixed rotational plane of the propeller. This tendency for precession is absorbed in the bearings supporting the prop shaft, which results in bending of the blades.

The V-22 has this problem in that the prop rotor is suspended in rubber and when maneuvering causes the disc to precess the flapping sensors in the prop rotor cause the hydraulic servos to adjust the swashplate to counter the precessional movement.


This is purely for American and Canadian consumption, as it does not apply to helicopters in England.






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The Cat

[Dave Jackson]

The thread is moving off topic, but it looks like the original question by Engineoff has been well answered.


For the fun of a discussion and the opportunity to pursue knowledge; :) here goes
_____________________________________
Gyroscopic Precession vs. Aerodynamic Precession

The subject of gyroscopic precession, in respect to rotorcraft disks, appears to be an on going one that never quite gets answered to everyone satisfaction or understanding.

A clear description of basic gyroscopic precession can be found on the .PDF page http://www.gyro-scope.co.uk/how/hagwa4.pdf , or alternatively, the .HTML page http://www.gyro-scope.co.uk/

Gray Area, in an earlier posting, discusses a gyrocopter with teetering rotor, as an example. He talks about tilting the control-plane (hub) forward and that this will cause the no-feathering-plane (angle of attack) to change and subsequently cause the tip-path-plane to tilt forward also. To me, this would appear to be aerodynamic precession.

Assume now that the above gyrocopter had the teetering hinge removed and a gimbal inserted at the base of the hub, adjacent to the 'mast' bearing. Also assume that there are no flapping hinges and the blades cannot flap. In this example, the attempt to tilt the control-plane forward is an example of weight shift control (the CofG of the gyrocopter is being physically pulled forward). This increase of the downward force on the front of the disk will cause the tip-path-plane to tilt to the left. To me, this would appear to be gyroscopic precession.

I think that cyclic control is subjected to aerodynamic precession and gyroscopic precession. Also, I believe that aerodynamic precession is by far the greater of the two, because the *relative* mass of a rotor disk is nowhere near the *relative* mass of a gyroscope.


Lu, do you think that the following two quotations may be in conflict. It appears that the blades are flying to position in the second quotation.

&gt; Blades do not fly to a position, the are moved by gyroscopic forces.

&gt; flapping sensors in the prop rotor cause the hydraulic servos to adjust the swashplate to counter the precessional movement.


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Project: UniCopter.com (http://www.synchrolite.com/UniCopter_Index.html)

[VLift]

This may be a tad late in the discussion. Had a little problem with a password. Never the less, the Hughes 269 and CH-47 both have articulated heads and both will suffer the same problem you asked about but by a different name. In the -47 it's called "droop stop pounding" by at least the U.S. Army. The upper droop stop is mounted on the blade yoke assemble and the lower is on the mast. For all practical and functional purposes what happens to cause the articulated blade droop stops and a rigid or simi-rigid hub and mast to make contact are the same.
And, having sat in the back of a CH-47 during a droop stop pounding event in turbulence in mountainous terrain, I will give you my personal guarantee that the event can occur in rotor blade negative G conditions, regardless what you read above.
Also, an out of trim condition reduces the angular distance between droop stops or hub and mast. We have all been taught to work toward a level fuselage attitude in turbulence, staying in trim as much as possible in a helicopter is just as important in my book.

[Lu Zuckerman]

To: VLift

Droop stop pounding as you called it is common to many articulated rotorheads and it normally results from rapid control inputs that exceed the flapping limits of the blades. On some helicopters the design of the rotorhead will allow the blade to diverge and possibly contact the fuselage if the down limits are exceeded.

The variants of the Bell Model 47 had droop stops in the form of cables that limited the teeter travel of the blades and head. If control limits were exceeded the cable would come under tension and the results were in the form of a very heavy tug on the mast. This would manifest itself as if the head came into contact with the mast but not serious enough to cause damage. When this is going on, the pilot is well aware of the condition and he will modify his cyclic input.


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The Cat

[VLift]

Lu

I wasn't asking how droop stop pounding is manifested in an articulated rotor system. I am all to well familiar with the subject. My 47 reference was to the CH-47. Regardless of the helicopter involved, rapid control inputs are not required. In almost three decades of flight instruction I have watched pilots of both simi-rigid and articulated rotor system equipped aircraft reduce power before the cyclic was close enough to center while landing on a slope and create a droop stop pounding or mast bumping situation.Done it myself as well, I'll admit. You can do the same thing picking the aircraft up from the slope but, I'm convinced you have to try. Conditions that include maneuvering or turbulence and negative G can also cause the problem.


[This message has been edited by VLift (edited 23 March 2001).]

[Lu Zuckerman]

To: VLift

I was aware of your reference to the CH-47. I mentioned the Bell 47 in order to address how they minimized the possibility of mast bumping by incorporating the Sprague cables. They were not only mounted on the rotorhead they were also mounted on the engine which limited engine displacement in the event that the Sprague cables on the rotorhead reached their limit due to excessive cyclic input. Later versions of those same helicopters incorporated what could be called droop stops in that they maintained the rotor in the neutral teeter position until rotor system reached a specific RPM. The stops were spring loaded to the in or locked position and moved outward by centrifugal force as rotor speed built up. Once they were out, the pilot could move the cyclic and tilt the disc.

Some Sikorsky rotorheads incorporated both droop stops and coning stops. Both were centrifugally operated and spring loaded to the return position. In both the Sikorsky and Bell droop or cone stops the rotor system had to be returned to the neutral position before reducing power so that the stops could move back to their static positions. Many times on the Sikorsky the stops did not return due to binding for whatever reason and the stops had to be visually monitored from the ground before the pilots were given the signal to lower the power level dropping the rotor speed. If the droop stops would not return to the static stop the pilot had to be extremely careful in lowering the speed of the rotor due to the possibility of hitting the fuselage.

The droop stops on the Hughes 500 series used a circular droop stop (limiter) that was in some respects self-leveling. With the blades at rest the droop stop maintained all blades at the optimum droop angle. On this type of system if you moved one blade down at the tip the opposite blade would move up at the tip. When up to speed and coning the blades moved away from the droop stop and the droop stop was free to move (providing you could reach in and move it.) If a blade in flight hit the droop stop the stop would shift its’ position until it reached the maximum of its’ movement in this way not restricting blade movement. As the blade lose lift they drop back down on the droop stop and it will shift position until all of the blades are in contact thus restricting the downward deflection of the blade.

When the centrifugal operated droop stops are out of play; excessive control input can still cause fuselage contact.


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The Cat

[The Nr Fairy]

Thankfully this happened on the ground - but see what the suspected cause was.

http://www.open.gov.uk/aaib/mar01htm/gpyob.htm

[Helo]

Basic question, I know, but can someone please explain why a two bladed rotor can end up contacting a tailboom in particular situations.

Can the same happen with a 3 bladed rotor, such as on the H269c? If not, why not?

Thanks.

Helo

[headsethair]

There'll be a more "physics" explanation. But basically, when the rotor is "unloaded" and the craft is in a pushover (coming out of a climb by pushing the cyclic forward before adjusting power - in other words flying in a parabola) there is every chance that the rotor can chop the tail because this situation induces rotors to flop, tail to go up. Hence the large placard on Robinsons saying "pushovers not permitted".

There are some wind conditions (gusts and wave) which can force the craft into a neg g situation whilst cruising - and pilots have to be careful about how they react to such a movement.

In all manoeuvres it is vital to keep "the disc loaded" - this keeps the blades well out of harm's way.

Obviously the length and flex of the main rotor affects the chances of a tail chop. In a craft like the MD/Hughes 500 the multi-blade rotor is too short and stiff to cause a problem.

But most Robinson tailchops are caused by low rotor rpm when a pilot has over-pitched the rotor and it loses speed, dropping beneath the critical limit and the rotor stalls. Once this has occurred the chances are that the floppy rotor will chop through the tail and even the cabin during the next phase of the incident.

And some tail chops have been caused by pilots using the collective to slow the rotor after shutdown of the engine. The added pitch drags the rotor to a halt - but it doing so the rotor goes into a droop and can touch the tail.

If you're not a heli pilot and thinking of becoming one, I may just have put you off!:eek:

[The Nr Fairy]

Helo :

Two bladed / underslung / teetering heads rely on the weight of the aircraft to load the disc and keep it under control. If you get into low / negative G, the disc is unloaded and can effectively do its own thing as it's only connected to the rest of the aircraft by ( effectively ) a simple bolt through the top of the mast.

If you get into negative G, assuming initially wings-level, then the tail rotor tends to roll the body to the right ( I'm remembering this, hoping my memory is right ) but the disc doesn't follow suit as it's off doing its own thing. The unrestrained and uncontrolled disc and the rotor mast come into close proximity. If they get too close, you get contact between the head and the mast at a point below where they're bolted together. This tends to lead to a fairly rapid separation of the two, and if you've got any height then it's not going to be pleasant.

Rigid heads don't suffer from this phenomenon since they're rigidly ( hence the name - geddit ? ) bolted to the mast and can't move except in sympathy with the rest of the aircraft.

Tailboom chops aren't my speciality, but I think headsethair has mostly covered it. But you can chop the boom off with either a rigid or teetering head - most notable recent incident the 520N in Florida flown in a manner such as to cause this.

And you can be really unlucky with turbulence - a Bell 206 doing pipeline or powerline survey was lost to mast bumping and the accident report surmises that the most likely cause was turbulence leading to low G. The report's on the AAIB web site somewhere.

[the coyote]

Helo,

Although an articulated rotor system (H269) has more rotor control force under low/negative G due to its flapping offset from the mast, you can still get droop stop pounding and/or contact the tail boom. Whenever you unload the rotor disc in a teetering or articulated head, the rotor loses its ability to transmit force to the mast (and the fuselage), and thats when the rotor can deviate beyond its normal limits.

headsethair,

I disagree with your comment on the Hughes 500. If you poled a 500 over, left your seat and then pulled the stick back quickly I would hate to see how close those short blades would come to the tailboom, if not contact it.

Only my opinion, but I reckon its like VRS, just dont go there in the first place...

[Lu Zuckerman]

Excessive rearward movement of the cyclic while on the ground with low collective can result in tail cone contact due to blade flexing on some helicopters. This happened on a commercial S-55 operating as Chicago Airways. The pilot sneezed and reached in his back pocket for a handkerchief. In doing so he pulled the cyclic aft and hit the tail cone. Up until that time Sikorsky never considered this as a possible accident waiting to happen. As a result later models of the S-55 had a drooped tail cone. The military variants were H-19 A-C with the straight cone and the H-19 B-D with the drooped tail cone.

:cool:

[helmet fire]

I think we may be connecting several seperate factors here.

Mast bump is nothing to do with Tailboom chop. Mast bump is generally associated with two bladed systems, but is akin to droop stop pounding on applicable multibladed systems (articulated). Severity of the condition is what causes the next problem - damaged mast/droop stop od mast seperation (rare result from droop stop pounding - but common from mast bumping).

Tailboom chop can occur on many helicopters. Two bladed systems are not a determining factor here. It has occurred on fully articulated systems such as the Black Hawk, and even fully rigid systems such as the BK-117. Generally it relates to the pitching moment of the fuslegage, the speed of control movement, and the flexibility and airflow over the blade.

Clear as mud?:)

[Nick Lappos]

Some clarification on why a teetering rotor is more likely to experience a tail boom strike than an articulated one:

An articulated rotor derives control over the aircraft via two fundamentally different methods, both of which sum to give the control power.

The first is the rotor flapping that creates a powerful twisting force at the hub. This flapping harnesses the powerful centripital acceleration of the rotor blade (centrifugal force to Lu and his follower, as he must have a follower, somewhere). This pitch or roll twist (moment) is transmitted down the mast and pitches or rolls the aircraft as directed by the pilot's cyclic. Such flapping control does not rely on the G's or loading of the rotor, and is available at any g, positive, zero or negative.

The second is the conventional tilting of the rotor disk and its thrust, which causes the body to rotate. This is the only kind of control a teetering rotor has, because the blades must flap in pairs, hinged at the hub, so there is no harnessing of the centripital force, it simply passes through the rotor head. When lift is low (low G) the control is weak, when lift is zero, there is no control, and with negative lift (as in a severe bunt or pushover) the cyclic control can work backwards.

I have posted two sketches to illustrate this effect on my web page. See:

http://mywebpages.comcast.net/llappos/teetering.jpg

and

http://mywebpages.comcast.net/llappos/articulated.jpg

The net effect is that a teetering rotor can flap like crazy when unloaded, and the fuselage gets no signal to get out of the way, it just lays there and takes the whack. In an articulated rotor, the flapping makes the fuselage rotate out of the way, even at zero g. Aft cyclic rotates the tail down, away from the blades. Therefore, it is harder to make a fuselage strike in an articulated helicopter. Not impossible, but much harder than in a teetering helo.

For any helo, restraining it against the ground as in a landing can keep the fuselage from rotating away from the blades, so back-stick on landing, as the skids or wheels touch down, is the "best" way to get a tail cone strike.

[What-ho Squiffy!]

I'll wait 'till I see you diagram before I fire a question or two Nick. Excuse my ignorance, but where is your web page?

Ahh, the address has appeared - thanks :)

[Irlandés]

Helo,
if you're still not satisfied after this fairly exhaustive treatment, try reading this previous thread (if you haven't already done so). While not dealing directly with tail strikes it does give a very good insight into the R22 Rotor hub (straight from the Horses's mouth). I found it very helpful.


http://www.pprune.org/forums/showthread.php?s=&threadid=58337

Irlandés

[Helo]

As the title says, this is such a cool forum!

Thanks for the replies and comments. Think I'm almost there in my understanding. Interesting to see that pilots are just like economists (sorry) - eight pilots, eight different opinions :D (or at least variations on a theme)!!

Thanks again.

[headsethair]

"eight pilots, eight different opinions"

Yes, sort of. But united on one front: low g pushovers/bunts, don't go there! Severe turbulence, avoid forward stick. Keep the rotor disc loaded. And never use the collective to stop the rotor at shutdown. (Standby for the disagreements....):cool:

[Lu Zuckerman]

To: Irlandes

Although most of what the writer stated is true relative to the mechanics of the rotorhead he got one thing wrong. If the helicopter is in flight and the rotor speed drops the rotors will not droop. If there is insufficient CENTRIFUGAL force to maintain the blades in the radial coned position the blades will fold up on the cone hinges. If this introduces excessive flapping then maybe the tusk will hit the stops but most likely (even on multi blade flex rotors ) the blades will fold up.

:cool:

[headsethair]

"If you poled a 500 over, left your seat and then pulled the stick back quickly I would hate to see how close those short blades would come to the tailboom, if not contact it."
Presumably leaving your seat means the aircraft seat - rather than parting company with your own buttocks? You see there are scientific and cultural differences between Brits and Aussies.......also, in this hemisphere to be "poled" is something entirely different from your meaning.
:D

[heedm]

Lu, you're correct that a reduction of centrifugal force would cause the blades to rise, but that's only part of the story. Reduction of the centrifugal force also means that flapping angles will increase, therin lies the danger.

Also, reduced Nr reduces lift, so lift reduced and centrifugal force reduced results in ?????

[helmet fire]

Lu,

The bloke in the first Robbo thread is talking about on the ground during shutdown.

Haven't we cured you of centifugal?:rolleyes: :rolleyes:

[Irlandés]

Bloody hell Lu, (excuse my french) all I did was point the guy at another thread for further reading. I expressed no opinion whatsoever. Do you honestly expect me to be responsible for everything everbody says in another thread??? This is the first time I've been corrected for something I didn't even post! If you really think there's a problem with what someone else said in a different thread, well then say it on that thread. I dunno, I should of stayed in bed this morning. :rolleyes:

Irlandés

[Lu Zuckerman]

To: helmet fire

Quote:

“The bloke in the first Robbo thread is talking about on the ground during shutdown”.

Response:

If you mean the first post in this thread the question was non-specific about when the chop would occur.

Quote:

“Haven't we cured you of centrifugal”?

Response:

If centrifugal is good enough for Frank Robinson it is good enough for me.


:D

[Flight Safety]

helmet fire or anyone else...

Can you please explain "droop stop pounding" on the fully articulated rotorhead and most importantly, what usually causes it?

[Nick Lappos]

Flight Safety,
The articulated rotor heads can allow the blades to flap too much when the rotor rpm is low, during engagement or shutdown. We generally use a simple rotating stop mechanism that is held out of the way at 100% Nr, and that falls into place as the rotor winds down. This droop stop prevents the blades from flapping down enough to hit the fuselage. They are quite reliable and can be felt as they fall in during shutdown.

Droop stop pounding at full rpm - If the pilot makes large cyclic inputs while the aircraft is restrained on the ground, he can make the blades hit these droop stops. It is very hard to hit them while fully airborne, since the aircraft rotates rapidly with all that cyclic input. Generally, droop stop pounding occurs while the wheels or skids are on the ground, and the pilot puts in a big cyclic input.

A favorite way to get pounding is to use "aerodynamic braking" with back stick during a running landing. This is poor practice, and not generally recommended. This is a good way to pound droop stops and take a divot out of the tail cone with the blades, and a very poor way to stop, since it is quite ineffective. If your instructor teaches Aerodynamic braking, he/she throws in free tail cone contact lessons.

Droop stop pounding at high rpm also helps stress the blades and rotor head, since while the blade is in contact with the droop stop, it is not articulated any more, and lots of bending is experienced. The droop stops can get dented, and the hinges can crack, too. In especially bad cases, the droop stops can be launched off the aircraft and get tossed onto the ramp!

Droop stop pounding at low rpm - Droop stops also get pounded if the pilot does not find the sweet spot when shuting down and spinning up. The rotor flies downwind, so the pilot must use a little cyclic into the wind to keep the disk level and the stops unpounded. The forces at low rpm are low, so the droop stops can take this milder abuse quite well.

Pilots use the landing light at night to help keep the disk level during enganement and shutdown. I generally use the movable spot light and turn it upward to illuminate the tips.

[Lu Zuckerman]

To: Irlandes

Although most of what the writer stated is true relative to the mechanics of the rotorhead he got one thing wrong. If the helicopter is in flight and the rotor speed drops the rotors will not droop.

I in no way cast any stones or any other kind of aspersions in your direction. I simply stated that the writer was wrong in what he said.

:)

[Flight Safety]

Thanks Nick. :D

[vaqueroaero]

Just a quick query: If you are unlucky enough to experience mast bumping in an R22, and I assume an R44, which part of the head actually hits the mast? My boss was telling me the other day that the tusks (droop stops) snap off and this causes the blades to flap down and voila, hit the mast. However I am not convinced. I always thought that it was the part where the coning bolt goes through the head, and that it comes down and hits those small plastic pads that are attached to the mast. (that's a bit vague, but anyone with a diagram please post it.)

[Lu Zuckerman]

You are correct about the point of contact and your boss is correct as well but only under certain circumstances. If the head and blades are moving as a unit during the flapping excursions the point of contact will be as you described. If the blades are flapping independently from the head the tusk(s) can make contact with the cone stops and the energy of the blade can cause the rotorhead to deviate from the rotational plane and contact will made at the teeter stops against the mast. However, if the energy of the blade is so great as to cause the tusk to fracture one of two things can take place. Either the energy of the blade acting against the inertia of the head can cause the tusk to fracture moving the head to the point that the head contacts the mast or, the blade will continue to flap downwards and make contact with the tail boom or hit the cabin area.


IMHO

:eek:

[Lu Zuckerman]

The question you should also ask is why the flapping excursions occur and under what conditions. What is there about the Robinson rotor system that causes extreme flapping?

The Robinson POH indicates that flapping excursions can occur under three and possibly more conditions. They are, Flying out of trim, Flying in a sideslip or having entered into a zero G condition and under certain conditios of low rotor RPM.

The POH suggests that the pilot avoid these conditions yet the certification documents require that the helicopter demonstrate flying out of trim by 10-degrees and flying in a sideslip by 90-degrees. Since the FAA certification requirements state that these maneuvers must be demonstrated why is the FAA now stating that these maneuvers must be avoided.

One question that I propose is would the Robinson design be certifiable if Robinson told the FAA that the R-22/44 could not be flown out of trim or in a sideslip per the certification requirements?

But then again I have said this in the past and I was crucified for saying it.

:confused:

[Vfrpilotpb]

Mast bumping can occur on any "Teetering Head" system the R22, R44 B206 the Huey and many other well known Helicopters have that sort of head, Lu is quiet right in his explanation of the way to cause mast bumping, it is something that is normally drummed into most Helo pilots to avoid at all costs( the cost being your life and or that of your pax) with the rotor going one way and you and the heli going the other, it is apparantly a problem that is easier for converted fixed wing pilots to get into than pilots trained on Helo from scratch, the main reason being a fixed wing pilots reactions are to push the stick forward at the sign of any problems this action will cause the Teetering head Heli to get into the condition of Negative G, this is a most dangerous position to get into and require's quick but accurate action to recover, it has been demonstrated to me by a very experienced Heli pilot, with the cyclic loosing all feeling of being attached to anything, very dangerous dont go there!:eek:

[Helo]

Can anyone give me a SIMPLE answer in words of one syllable please .....

When in negative g in a Robbo (or similar), why does the craft roll to the right, rather than the left? I understand that in such a situation the disk is not loaded and that the tail rotor thrust then comes to bear on the situation, but when I imagine this situation in my mind, I envisage the tail coming up and continuing to be pushed to the right, with a subsequent roll left and pitch downwards. Trouble is, I know the roll is to the right but I just can't see why.

As I say, a really basic explanation would be great.

Thanks

Helo :confused:

[The Nr Fairy]

Off the top of my head, it's because the T/R is above the vertical C of G. Air pushing to the left ( if viewed from behind ) creates a couple around the C/G which rolls the a/c right.

[Tail Bloater]

Another nail in the coffin for the R22.
I'm surprised that people want to fly it with all the problems that are associated with it's design.
Fit for purpose springs to mind and duty of care by the manufacturer are still relevant with this machine.
However, the boys in Australia herding the cattle seem to be able to fly these in their day to day tasks quite successfully. How do they cope with these problems?

[Lu Zuckerman]

As much as I may criticize the Robinson design I was absolutely amazed at the agility and responsiveness of the R-22 when involved in cattle mustering as shown on a Discovery channel program. It is my opinion that the rotor system does not enter into the regime of high flapping loads at the low speeds encountered in mustering. If they tried the same maneuvers used in mustering at normal flying speeds they might get into trouble.

Then again there is a display flying team in California that flies R-22s in aerobatics flying with the full knowledge of Robinson and they constantly violate the FAA’s recommendations about flying out of trim and sideslipping. On their web site they show an R-22 flying in formation with a Stearman and the R-22 is flying backwards.

One of their high time pilots (20,000 + hours) was killed along with his first flight student when the rotor came off. It was decided that the student over controlled the helicopter when they really did not know who was at the controls. The flight terminated after fifteen minutes of flight.

:eek:

[sneetch]

Hi all,

I have a question regarding Low G and Mast Bumping in R22s. In the R22 POH it states that "Pushing the cyclic forward following a pull-up or rapid climbor even from level flight produces a low-G (weightless) condition"

My question is .... If it's possible to induce Low G by moving cyclic forward in level flight, just how abrupt would the movement have to be? Increasing speed requires a similar action ... doesn't it?

I was taught that to level off from a climb you should apply fwd stick to accelerate to cruise speed and then lower the lever to set cruise power.

Have I misunderstood the POH? Just what would you have to do to induce low G this way.

Paranoid Sneetch

:ugh:

[Jcooper]

It would have to be a very large input. Just imagine a rollercoaster. If it is a gentle downslope from going straight you don't feel much strain. Now if you are moving level and it drops off to a 45 degree slope you will feel quite a strain. Just remember if you ever feel weightless in your seat to pull in aft cyclic until the feeling goes away. If you don't feel weightless you are fine.

[Whirlybird]

I had this demonstrated to me on my instructors course, and cyclic movement does have to be pretty abrupt. It's easy to correct, as Jcooper says, by aft cyclic. The main problem, it seems to me, is that the helicopter begins to roll (left I think, can't remember for certain), and you may instinctively want to correct that. DON'T!!!! Aft cyclic first to reload the rotors, then whatever corrections are needed. So you have to be able to recognise what's happening.

Of course, the best thing is to always make reasonably gentle (ie normal) cyclic movements, and then it won't happen.

[Aesir]

It will roll right, due to the thrust produced by the tailrotor to the right, instinctive correction would be left cyclic.
That would probably be the last mistake you ever make!

Correct recovery is smooth aft cyclic, no abrupt cyclic inputs and when you feel positive G again then smoothly apply left cyclic to level off out of the turn/bank.

The right roll will be quite abrupt, DO NOT APPLY LEFT CYCLIC!

However to initiate the low G in the first place you would have to push the cyclic forward very abruptly, it will not happen when transitioning from normal climb to level flight. So when transitioning first apply forward cyclic to build up speed after level off, then lower collective from climb power to cruise power.

If you encounter heavy turbulence, again no abrupt cyclic inputs will do the trick, let the helicopter ride it out, perhaps put some friction on the cyclic and hold it loose.

[wish2bflying]

This also happens by pushing down the collective suddenly in flight.

To see this in action without killing yourself, get a copy of X-Plane (http://www.x-plane.com/), take a flight in the R22 or Bell206, and drop the collective in flight. You'll find yourself upside down faster than you can say "what the ...", and then good luck getting back to level flight without crashing, especially if you've got it set to break up the aircraft in an over-G situation!

--
Michael.

[Jcooper]

lowering the collective quickly will not cause mast bumping. It will have a very quick low g but nothing dangerous will come of it. If lowering the collective created that in real life it would make autos a new and exciting challenge.

[Flingwing207]

The Low-"G" is caused only very partially by the ballistic trajectory of the airframe after a cyclic pushover. The real unloading of the rotor disk comes from the abrupt and dramatic change in the AOA of the blades.

Instead of the airflow being "pulled" through the rotor by a high AOA (as in a climb or high-speed cruise), it is suddenly trying to "push" through the rotor having a low AOA (due to the large change in disk attitude vs path of travel).

Lowering the collective reduces the AOA, but the disk attitude remains relatively constant. (I realize this is an oversimplified description, but it gets the point across.)

The result of this combined with the ballistic trajectory of the airframe unloads the rotor. Since the underslung rotor is free to pivot about the rotor mast, the airframe is free to roll under the rotor disk, and thanks to the tail-rotor thrust, it will.

[Autorotate]

Is Mast Bump in an R-22 something that, in your opinon, all students should be aware of if they are going to fly that type.

Autorotate.

[wish2bflying]

Autorotate, I believe yes absolutely. I just found this very good description that everyone should read:

http://www.helicoptersonly.com/sayagain_MastBumping.html

--
Michael.

[Aser]

Will help adding a bit of collective ?
Yesterday I was flying in turbulence and when I felt a VERY light low-g instinctively added some collective(mental note:I must remember to use aft ciclyc) but no roll moment developed

[RobboRider]

Autorotate

Not only is it part of the standard training but it is a required for a warning placard to be visible in the cabin " Low G Pushovers Prohibited".
On all the R22s I have flown it was placed on the cross-arm of the T-cyclic so it would be pretty hard for a student to not see it and at least wonder about it if they hadn't yet been told about it.

I had a look in my old notes from when I was training and it was part of one of the lessons.

wish2Bflying:
Unfortunately the helicopter flight models in all those sims leave a lot to be desired. I think if you were learning to fly real helicopters you should probably not use them. Fixed wings is another story, they seem to be reasonable then.
I found they induced a lot of bad habits when I played around with them.
Perhaps others might feel differently.

[wish2bflying]

RobboRider, I agree with you, but the simulators are useful for demonstrating basically what can happen in these manouevres (particularly X-Plane, which if you haven't tried it might surprise you, especially if you have the full size controls).

You're in QLD, so am I (Brisbane). I can arrange a flight for you in our soon-to-be CASA accredited cockpit simulator if you like. It's configured to be a 206 at the moment, but I'd be happy to set it up as a Robinson and get you to help us calibrate it if you like.

--
Michael.

[3top]

Hi all,

don't forget, that although in a low-g the rotor looses control over the fuselage, you still have control over the rotordisc.

You move the cyclic left - the rotor tilts to the left, relative to the fuselage, or forward, back, to the right, according to your input.

The forces needed to flap the rotor so severly as to cause mast bumbing are way up there. I believe the heavier the machine the easier it is to actually exceed these limits by pure aerodynamic forces on the blades/rotor (assuming the pilot keeps the cyclic centered), as the inertia of the fuselage resists to follow quickly to the rotors excursion. A R22 or even a R44 are too light to actually get in trouble, as they will follow the rotor before mast bumbing occures. The resultant ride however is exciting to say the least!
But as mentioned before keep the cyclic inputs small and let the helicopter ride it out.
This all assuming turbulence!!

In a "push-over typ low-g the flapping never gets anywhere near enough to bump the mast on any Robinson, IF the pilot keeps his cool and the cyclic smooth with small inputs.

To make helicopters teeter-rotor free will cost a fortune, which in case will reduce the number of helicopters, reduce the number of those who can afford to fly, etc.

Training is still the best insurance!!

3top



:cool: if

[Lu Zuckerman]

To: 3top

Just about everything you have said in your post above is totally contrary to what is in the POH and what is taught in the Robinson safety course. The flapping excursions on the blades whether flapping about the teeter hinge or the cone hinges can be so extreme as to cause fracture of the spindle tusks resulting in rotor incursion or tail chops.

If you know any psychics that can talk with those that have crossed over due to flapping excursions on a Robinson ask him/her to talk with those individuals asking them what they did to end up on the other side and/or if they would agree with your comments..

:E :E

[bugdevheli]

The words mast bumping seem to be followed by descriptions of how the blades flap, pitch up, diverge, whatever. But if you want a sobering excersize, get an old knackered Robinson attatch a dial gauge to the mast just above to swash plate and get your mate to hang on a blade while you measure the deflection over that inch or so of mast. oooooooohh:D

[Lu Zuckerman]

To: bugdevheli

What you are saying about the deflection of the mast may be true however pulling down on a blade is in violation of the instructions in the POH and could cause damage.

:E :E

[The Nr Fairy]

Lu:

"Old knackered" is idiomatic UK English for time-expired, so not harm done - but you're absolutely right in your advice.

bugdevheli:

Out of interest, are you an engineer, helicopter pilot, both or none ?

[Lu Zuckerman]

To: The Nr Fairy

Out of interest, are you an engineer, helicopter pilot, both or none ?

First things first. There are many Robbies that are old and knackered and have yet to be time expired. How about the mast deflection on those helicopters and how does it effect safety of flight.

To answer your question I am a Reliability, Maintainability, and Systems Safety Consultant and have been doing this work since 1968. My credentials also include an A&P license, California Teaching credentials in Aerospace technology and several other subjects.

In this capacity I have worked on the Cheyenne, AH-64, A-129, EH-101, V-22, A-310, A-340, A-380, Gulfstream GIV X, F-16 and several other commercial aircraft cargo conversions.

If I were in my right mind I would retire but I get bored very easily.


:E :E

[The Nr Fairy]

Lu:

RTFP. My question about credentials was for bugdevheli - I'm well aware of your long and distinguished CV.

[bugdevheli]

Nr Fairey. Qualified pilot? NO. Engineer, yeah. Described by my phsyciatrist as suffering from terminal Sikorskyitis. This condition can only be aleviated by numerous five minute hovers, or on better days a ground run might suffice. A longer flight seems to top me up for about a week. Any offers of help gratefully recieved. As to payment, would a cup of tea and a look round my workshop be acceptable?:D :D