Mast bumping, tailboom chops and two bladed rotors (merged)
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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
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
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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
“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
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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.
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.
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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
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
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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!
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!
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Excellent topic. I also wanted to add a few thoughts. SPS said:
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).]
<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>
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).]
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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.
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.
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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?
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?
Guest
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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.
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.
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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.
SPS
If a little knowledge is dangerous, greater knowledge promotes safety.
Guest
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Mast bumping in a fully articulated head?
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.
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.
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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
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
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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
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
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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.
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.
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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).]
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).]
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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
[This message has been edited by Dave Jackson (edited 21 March 2001).]
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
[This message has been edited by Dave Jackson (edited 21 March 2001).]
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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
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
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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
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
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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
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Another day in paradise