AS 350 Negative "G"
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No mast bumping-BUT!
Given that a fully articulated rotor cannot experience mast bumping under zero/negative G, there is another matter which may "curtail the curious" in the exercise of these manoeuvres.
And that is the consequence of placing the T/R thrust line above the main rotor.
Given that, in order to demonstrate the preamble to catastrophic mast bumping with a teetering rotor it is necessary to have "considerable" power on to cause the heli to roll right, when teaching the correct recovery technique from a low G situation. (I speak of the R22)
The technique of course, being to move the cyclic aft to load up the rotor.
I have always presumed that the right roll so induced was brought about by the T/R thrust being above the M/R. Am I right in so thinking?
If so, it seems as though one could induce inadvertant barrel rolls with a fully articulated rotor, at the very least!
thekite
And that is the consequence of placing the T/R thrust line above the main rotor.
Given that, in order to demonstrate the preamble to catastrophic mast bumping with a teetering rotor it is necessary to have "considerable" power on to cause the heli to roll right, when teaching the correct recovery technique from a low G situation. (I speak of the R22)
The technique of course, being to move the cyclic aft to load up the rotor.
I have always presumed that the right roll so induced was brought about by the T/R thrust being above the M/R. Am I right in so thinking?
If so, it seems as though one could induce inadvertant barrel rolls with a fully articulated rotor, at the very least!
thekite
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I once did a days filming in a 350 that required a low level run at approx 90 kias with an abrupt cyclic climb to about 150 - 200 feet followed by an abrupt level off and entry into auto. We did the shot about 12 or so times. The camera operator and director got very sick but the aircraft handled fine. No dash lights and no control problems that I could tell. But as I said it was just the one job.
'If so, it seems as though one could induce inadvertant barrel rolls with a fully articulated rotor, at the very least!'
No. An articulated head will have sufficient control power to maintain control in roll even under negative G. A teetering head relies on positive G to provide control of the head. Having said that, apart from turbulence and gentle push-overs when low level at fast cruise, negative G is a no-no (IMHO) with 350s, or with any other for that matter.
No. An articulated head will have sufficient control power to maintain control in roll even under negative G. A teetering head relies on positive G to provide control of the head. Having said that, apart from turbulence and gentle push-overs when low level at fast cruise, negative G is a no-no (IMHO) with 350s, or with any other for that matter.
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In a previous life, teaching on the Squirrel, EFATOs to engine off with enough negative G to make the wire cutters in the back smack you in the back of the head as they flew out of the not-so-secure stowage didn't produce any captions. I had one caption once, but the student got thoroughly debriefed afterwards...I think he scared himself enough anyway!
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Had pen momentarily suspended in mid-air with associated caption flicker when encountering severe turb while LL on EMS job (AS 350 B2)......reduce to turb penetration speed......tighten seat belts....calm flight nurse....and dream of days flying Lynx in the Bog. Some are made to do it, some are not...some can - but leave you wondering whether "the total inability to look facts in the face will actually see you through".
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Be very careful quoting FARs as far as load factors are concerned.
They are design requirements, and probably haven't been demonstrated as such - for example, there is a requirement to design for a full pedal displacement at cruise speed, but no-one I know of has ever demonstrated it.
And there is a major concern with negative G - as far as I know the hydraulic tanks are not pressurized. At zero G, you're likely to not have any hydraulics going into the pump...
(happened to an HH-65 at the US Navy Test Pilot School when I was there, and I don't think the AS-350 series has much difference in hydraulic tank design).
So be very, very careful when thinking about doing any negative or even zero G stuff - in any helicopter.
They are design requirements, and probably haven't been demonstrated as such - for example, there is a requirement to design for a full pedal displacement at cruise speed, but no-one I know of has ever demonstrated it.
And there is a major concern with negative G - as far as I know the hydraulic tanks are not pressurized. At zero G, you're likely to not have any hydraulics going into the pump...
(happened to an HH-65 at the US Navy Test Pilot School when I was there, and I don't think the AS-350 series has much difference in hydraulic tank design).
So be very, very careful when thinking about doing any negative or even zero G stuff - in any helicopter.
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At zero G, you're likely to not have any hydraulics going into the pump...
So here's the question. Why doesn't the hydrolics warning light or horn come on at that instance? Because surely that is almost like a hydrolic failure.
negative g...
Short of doing severe manouvers for filming, ie. spectacular effects for the viewing public, tv, display etc.. bringing on warning lights generally means you are a ham fisted git and may need to review your handling skills...
Grow up dude
Grow up dude
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Thanx for all the inputs guys,
Scissorlink, your suggestion is true in theory, but obviously you haven’t done a lot of survey flying because if you had, you would know that to follow your suggestion all the time is not possible. There is a huge difference between the theory and the practicability side of things!
Krobar, the correct term for the situation you are describing is: “servo control reversibility.” The hydraulic system is there to assist the pilot to overcome heavy controls when there is a lot of “load” on the rotor system, therefore you should not get it under low G or even negative G conditions unless the hydraulic reservoir is low on fluid, but that is an entirely different matter.
JimEli, the condition Krobar is describing as in stiffness of controls, is actually feedback from the hydraulic system that you can feel! So is the onset of Vortex, experiencing translational lift, approaching VNE, etc. it is all symptoms that you feel!!! So, your statement: “
Flying by feel? Isn't that how a blind man reads?
Is a bit scary for me, I don’t know if a want to take advice from a helicopter pilot that has NO FEEL for helicopters. By the way, low G in a teetering Rotor head is not such a big issue if the pilot initiates it with collective, and keeps the wings level, but if it is initiated with cyclic you can have problems, but the Squirrels doesn’t have a teetering rotor head, so I don’t know why you brought that in.
Anyway, Thanx again for all the comments, there are some useful ones, but I will however try to get more credible info from Eurocopter, and if I do, I will post it here for people that might be interested.
Again, the idea was never to push limits, but to find out exactly what they are.
Scissorlink, your suggestion is true in theory, but obviously you haven’t done a lot of survey flying because if you had, you would know that to follow your suggestion all the time is not possible. There is a huge difference between the theory and the practicability side of things!
Krobar, the correct term for the situation you are describing is: “servo control reversibility.” The hydraulic system is there to assist the pilot to overcome heavy controls when there is a lot of “load” on the rotor system, therefore you should not get it under low G or even negative G conditions unless the hydraulic reservoir is low on fluid, but that is an entirely different matter.
JimEli, the condition Krobar is describing as in stiffness of controls, is actually feedback from the hydraulic system that you can feel! So is the onset of Vortex, experiencing translational lift, approaching VNE, etc. it is all symptoms that you feel!!! So, your statement: “
Flying by feel? Isn't that how a blind man reads?
Is a bit scary for me, I don’t know if a want to take advice from a helicopter pilot that has NO FEEL for helicopters. By the way, low G in a teetering Rotor head is not such a big issue if the pilot initiates it with collective, and keeps the wings level, but if it is initiated with cyclic you can have problems, but the Squirrels doesn’t have a teetering rotor head, so I don’t know why you brought that in.
Anyway, Thanx again for all the comments, there are some useful ones, but I will however try to get more credible info from Eurocopter, and if I do, I will post it here for people that might be interested.
Again, the idea was never to push limits, but to find out exactly what they are.
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TheKite, the R22 reduced g right roll is caused by the 18 degree delta 3 (Lu Zuckerman championed against). The 18 degrees are there to counter rotor coning (which would otherwise cause right roll in forward flight), and wee-wa (which would cause right roll for forward pitch increase). My brain ached for a while before this made sense to me...
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CGWRA-
Negative G can be obtained in forward flight by pushing over with the cyclic (quite a lot of forward cyclic and more than 60 knots is going to be needed).
Without a forward velocity vector, it's not possible to generate a flight path that would develop a negative G. And at slow airspeeds (below 60 knots typically), you probably couldn't generate enough of a curve in the flight path to take you to the 'things floating in the cockpit' state of 0G.
And for those who claim that an articulated rotor will still be OK at G levels less than 1.0 - you're right, up to a point. At some G below zero, the controls will start to work incorrectly - the G level where this happens will depend on the amount of flapping hinge offset. Something like the Lynx or BO-105 will have it happen at ludicrously large values of negative G, others will have it happen starting at something as relatively low as -0.5G. I hope no-one out there tries this to prove me right!
Negative G can be obtained in forward flight by pushing over with the cyclic (quite a lot of forward cyclic and more than 60 knots is going to be needed).
Without a forward velocity vector, it's not possible to generate a flight path that would develop a negative G. And at slow airspeeds (below 60 knots typically), you probably couldn't generate enough of a curve in the flight path to take you to the 'things floating in the cockpit' state of 0G.
And for those who claim that an articulated rotor will still be OK at G levels less than 1.0 - you're right, up to a point. At some G below zero, the controls will start to work incorrectly - the G level where this happens will depend on the amount of flapping hinge offset. Something like the Lynx or BO-105 will have it happen at ludicrously large values of negative G, others will have it happen starting at something as relatively low as -0.5G. I hope no-one out there tries this to prove me right!
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I still dont see how this will make a negative G. You aren't creating any lift upwards that will force the helicopter down you are just letting the helicopter drop when you push the nose down, even after a climb. Am I wrong? I've had a few "rollercoaster" rides in a helicopters myself although I havent done any myself yet and I know the feeling your talking about like your stomach is trying to escape but its the same feeling you get jumping off a diving board its just freefall.
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CGWRA, dont forget angle of attack is caused by Relative Air Flow. Even though rotors are at positive pitch, tilting the disk forwards presents a downflow though the rotor, hence negative AOA.
theKite, we are talking gyroscopic nutation here - where movement lags 90 degrees azimuth behind force input. Basically the teetering rotor is always trying to flap back against airspeed, with cyclic input overcoming flapback. The coning means front portion of rotor disk causes a force which would produce starboard roll. By vectoring the forward cyclic position 18 degrees to port, this starboard roll can be revectored to foreward pitch. It only works ideally for coning at 1g, and i won't confuse you with inflow roll which is very similar.
The dynamics of "wee-wa" are a little harder to understand. The rotor always wants to pitch or roll at a constant rate, dependant on cyclic position (teetering delays the fuselage response a couple of seconds though). Once the rotor has established a pitch or roll rate, the aerodynamics act in a similar manner to coning. Thus at a forewards pitch rate the rotor also has little starboard roll rate. Again you revector the foreward cyclic to port roll and it all comes true.
Apologies for not taking much rocket out of the science, but it is a complex problem. I recommend getting a gyroscope from these guys to convince yourself about the 90 degree lag between force input and pitch or roll rate.
theKite, we are talking gyroscopic nutation here - where movement lags 90 degrees azimuth behind force input. Basically the teetering rotor is always trying to flap back against airspeed, with cyclic input overcoming flapback. The coning means front portion of rotor disk causes a force which would produce starboard roll. By vectoring the forward cyclic position 18 degrees to port, this starboard roll can be revectored to foreward pitch. It only works ideally for coning at 1g, and i won't confuse you with inflow roll which is very similar.
The dynamics of "wee-wa" are a little harder to understand. The rotor always wants to pitch or roll at a constant rate, dependant on cyclic position (teetering delays the fuselage response a couple of seconds though). Once the rotor has established a pitch or roll rate, the aerodynamics act in a similar manner to coning. Thus at a forewards pitch rate the rotor also has little starboard roll rate. Again you revector the foreward cyclic to port roll and it all comes true.
Apologies for not taking much rocket out of the science, but it is a complex problem. I recommend getting a gyroscope from these guys to convince yourself about the 90 degree lag between force input and pitch or roll rate.
Last edited by Graviman; 19th Oct 2007 at 19:34.
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Neg g - possible or not ?
Re the comments on whether Neg G is possible I think it is but it actually the inertia of the helicopter that causes it. Say for instance you are traveling at 60 knts in the climb at say a 30 degree from the horiz angle. Whilst lowering the lever to zero pitch and nosing the aircraft down the inertia of the aircraft has changed rapidly from the 30 degree up angle to the down path the aircraft is taking (this has to be a relatively severe manoeuvre to create this). This will create the effect of negative g
Any comments ? or am I completely out the ball park ?
Any comments ? or am I completely out the ball park ?
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Wee-wa, Gyroscopic Nutation, ,,!!
I'm clearly going to have to do further studies! Any suggestions re further reading material?
As a training aid for students, I used a bicycle front wheel to illustrate the effects of gyroscopic precession. Held at arm's length it graphically demonstrated both gyroscopic inertia and precession.
But while we are talking about negative G, I have never worked out what went wrong with Hiller 12E that I was flying when I experienced a severe negative G event.
I watched in horror as a geologist's rock pick rose of the lap of the lass along side me, spun slowly in the air, headed towards me then attacked the windsreen instead. By then I was steeply nose down with dust coming up from the floor, and a severe overspeed on both rotor and engine.
I didn't even know about mast bumping in those days (80's), but there was no evidence of damage apart from that to my underwear.
Perhaps the event was intiated by a willi-willi (dust devil), but there had to be much more to it than that. I wondered if the blade sweep had been incorrectly set. Perhaps an LAME with more Hiller experience could suggest an answer.
CASA could throw no light on the matter, nor could any of the Bankstown engineers. I even wrote to Ray Prouty, and he was unable to provide any insights.
So to this day, I am none the wiser.
thekite
As a training aid for students, I used a bicycle front wheel to illustrate the effects of gyroscopic precession. Held at arm's length it graphically demonstrated both gyroscopic inertia and precession.
But while we are talking about negative G, I have never worked out what went wrong with Hiller 12E that I was flying when I experienced a severe negative G event.
I watched in horror as a geologist's rock pick rose of the lap of the lass along side me, spun slowly in the air, headed towards me then attacked the windsreen instead. By then I was steeply nose down with dust coming up from the floor, and a severe overspeed on both rotor and engine.
I didn't even know about mast bumping in those days (80's), but there was no evidence of damage apart from that to my underwear.
Perhaps the event was intiated by a willi-willi (dust devil), but there had to be much more to it than that. I wondered if the blade sweep had been incorrectly set. Perhaps an LAME with more Hiller experience could suggest an answer.
CASA could throw no light on the matter, nor could any of the Bankstown engineers. I even wrote to Ray Prouty, and he was unable to provide any insights.
So to this day, I am none the wiser.
thekite