AS 350 Negative "G"

[Jamaligan]

To all Squirrel polers out there,

How much negative "G" can one get out of a B2? With a little amount there is the MGB P light that comes on, and with a little more the Eng P light illuminates aswell. Both these lights comes on just for a split second, and I pressume its because of the sudden fluctuation of the oil levels.
Is this pushing the heli too far?
Please advise,
thanx

[TRC]

Is this pushing the heli too far?



What do you think?

[northseaspray]

A pilot in a company I used to work for, had an incident where he lost control of the cyclic stick in his B2, (think the report stated that he was busy folding charts..) which ended up going forward, as a result he found him self upside down wings level, with enough negative g's in the manouver to pull the Garmin 120 out of it's socket, and that takes quite a lot i think.. when he landed, there was oil all over the aircraft of course.. Eurocopter had someone to look it over, found nothing wrong, after a while it was back on line.

[Jamaligan]

If I knew, I would not have asked the question!!!

I can also add that there are no pax and it is operational flying contour lines, and that I felt that I am well in the machines flying capabilities.

[Devil 49]

I don't know how much or if I've ever had negative G. Slammed about in some weather that flickered the XMSN caution panel segment- pitch, roll, yaw, vertical- could have been oil slosh instead of negative, although the engine kept running.
I would never try anything aerobatic in one of these. Jack-stall is unpredictable and at the wrong time, in a critical situation, I think it could provide more excitement than I want. Other than that, they're a treat to fly and strong.

[Jamaligan]

I also want to add that there are nothing in the Limitations section of the flight manual. This question is here to get advice, to fly as safe as possible, not to see how far I can push the helicopter.

[skidbiter2]

It's a good question, I was wondering that myself the other day.
There is nowhere that I have seen to say you cant do -G maneuvers, no acrobatics maybe, so?
I don't know about worrying about "jack stall", as the disk is unloaded rather than over loaded?
Hope you get a 'good' answer!

[WylieCoyote]

I've used gentle pushovers when low level to keep the tail away from trees etc, can't say I've had any lights on but I'm carefull not to be to aggresive. Saying that I have seen them done pretty aggressively by others with no probs encountered,heard reports of pressure lights flickering during severe turbulence though.

[northseaspray]

A mechanic once told me that low g maneuvers in the Squirrel would cause small cracks in the star that would shorten its lifetime, other than that no worries..

[ericferret]

Small cracks in the star my arse.

It leads to cracks that right off the star at several thousand pounds a pop.

We had a series of unexplained star failures on a hoist equipped Dauphin.

In an unguarded conversation one of the hoist operators told us about a certain captains
party piece. He would place a pen on the console induce negative g and watch the pencil rise to the amusement of the crew.

The captain had his arse well kicked and the series of star failures stopped surprise surprise.

[SawThe Light]

Just for the record, the limitations in the AS 350 JAA Type Certificate listed +2.67g and -1g. The JAA version has since been superseded by the EASA version it doesn't include that information, nor does the FAA version.

I don't think I want to experiment with this though, as I could picture the engine quitting real quick when the fuel pump inlet became uncovered and sucked air.

[Scissorlink]

If you contour fly using the collective ( I think thats what its there for) and bleeding off airspeed to go up and down steep bits you won't have to worry about negative G

SL

[TRC]

If I knew, I would not have asked the question!!!


Well, keep doing it to see how many more warning lights you can get on......... Those that you've already seen are telling you that all's not well.

By the way - there isn't one to tell you to start when to start reciting the Lord's Prayer...

There's been some good advice offered here - take it. (But not mine).

Good luck.

[JimEli]

>
…I felt that I am well in the machines flying capabilities.
<

Flying by feel? Isn't that how a blind man reads?

The minimum load factor boundary of the maneuver envelope for a helicopter is more restrictive than that of the fixed wing aircraft. The primary reason for this is that control is directly related to the load factor. At zero G, the pilot of the teetering-rotor helicopter has no cyclic control effectiveness, while loss of cyclic control effectiveness occurs at some negative G value for helicopters with flapping-hinge offset. To avoid loss of control the Nz envelope is established at some margin above the value where loss of control would occur. Approximate values for the lower load factor limit for a helicopter is somewhere between -1 and +0.5.

Further minimum load factor limits can arise from aircraft systems (i.e., oil system, fuel system, gps mounts, etc.) which are not designed for negative load factor.

I believe the AS-350B2 TC is based upon the original type certificate, so a little digging uncovers the (US) FAR at the time--

"The rotorcraft must be designed for--
(a) A positive limit maneuvering load factor of 3.5 and a negative limit maneuvering load factor of 1.0; or
(b) Any lesser positive limit maneuvering load factor not less than 2.0, and lesser negative limit maneuvering load factor not less than 0.5, whose probability of being exceeded is shown by analysis and flight test to be extremely remote."

not sure what the corresponding icao reg stated back then...

[idle stop]

To sum up JimEli's response: DON'T TRY IT!
An inadvertant transient excursion into less than 1.0 g, due, for example, to turbulence, may be OK, but most helicopters were not designed for less than 1.0 g.
If you think you have a genuine need (?) to be in this flight regime, then ask the Design Authority /OEM for advice and clarification of the flight envelope. Designers and test pilots do their best to give you the widest possible operating envelope: if what you want ain't there, we probably thought there was a good reason.

[eagle 86]

The only part of most modern helicopters that MAY handle stresses associated with extremes of flight (including those imposed during "aerobatics") is the main rotor system. The rest, ie the fuselage structure, engine/gearbox oil systems, fuel systems, hydraulic systems etc, are not. Leave exploration of the edges of the flight envelope to test pilots!
GAGS
E86

[krobar]

Pushing the envelope as far as G's go has a drawback in the hydraulic system. Extreme manuevers tend to result in 'stiffness' of cyclic control, and the last thing you need while in a extreme manuever, is a lack of control.
I'm sure we've all wanted to 'see what this baby can do', but with experience you learn there are ways of tweaking manuevers to not test the limits.

[manfromuncle]

Can someone tell me what 'jack stall' is?

[Bravo73]

Did you try searching first? :confused:

http://www.pprune.org/forums/showthread.php?t=254165&highlight=jack+stall

[Darren999]

I used to get the transmission light flicker when I was in Alaksa flying round the mountains. The turbulence there could get quite hairy! :uhoh: time to tighten that seatbelt again!! MUMMY!! :sad:

[thekite]

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

[2leftskids]

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.

[oldbeefer]

'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.

[oldpinger]

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!

[If all else fails]

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".:ooh:

[Shawn Coyle]

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.

[Vertical_Reference]

At zero G, you're likely to not have any hydraulics going into the pump...

I know that you get a slight hydrolic failure when pulling zero or negative G's. I have felt it from the certain type of work we do, when pushing over a hill trying to fly the same heigh AGL the whole time. When I wanted to turn around during that push over I had stiff controls like a hydrolic failure. But it was only for that one second until the hydrolics pumped through again.

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.

[griffothefog]

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...:eek:
Grow up dude:=

[Jamaligan]

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.

[Graviman]

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...

[CGWRA]

How is it possible to acheive a negative G in a helicopter?

Unless the blades reverse pitch to force air upwards the lowest G you can acheive is ZeroG or Freefall.

[thekite]

I guess I will to read your message a few more times before it makes sense to me! I'm sure you are right, but the matter puzzles me. :confused:
thekite

[Big Beres]

-If All Else Fails - your not my "Moody" friend from the Manawatu are you? PM me if you are.

[Shawn Coyle]

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!

[CGWRA]

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.

[Graviman]

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 (http://www.gyroscope.com/catalog.asp?catalog=1006) to convince yourself about the 90 degree lag between force input and pitch or roll rate.

[starshiptrooper]

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 ? :8

[CGWRA]

Never thought of it that way but it makes sense. All that forward air hitting the disk straight on would indeed force it down creating a negative G.

[thekite]

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. :uhoh:

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. :confused:

thekite

[ericferret]

Sounds to me like you had a problem with the stick to seat interface unit!!!!!!!!!!!!

I really liked the 12E to work on, much easier than the Bell 47.

[Graviman]

theKite, it may be that a willi-willi could cause a microburst when the thermal that caused it stopped working. Normally it is the ice particles falling, but cold air might have a similar sink effect.

Dave Jackson might point you to the Frank Robinson quote on his website about coning and wee-wa.

That rotor overspeed might be something else the AAED we discussed the other day could eliminate. This would also improve the life of the rotor components...

[RVDT]

Kite,
Simple, let me guess - Stainless blades and collective ballast incorrectly set up. Known problem number 238.

[thekite]

Oh dear, the smugness of a non-flying engineer.....
thekite

[ericferret]

True, true, but I just couldn't resist.

Actually I did think of something similar after my smug post.


We had a 12e that had a tendancy to suddenly drop without warning.

Not funny when crop spraying at 3 feet AGL!!!!!!!!

We tried all the normal fixes, but it continued to happen and the aircraft was grounded.

We eventually found that the cause was badly brinnelled bearings in the main rotor grips.

On most helicopters the result would be an inflight vibration , but with the Hiller set up this didn't occur.

The Hiller has a fully gimballed head like the Bell 47 and therefore the grips only move in response to collective inputs. This lack of movement explains the lack of inflight vibration.

The sudden drop was caused by the roller bearings sitting on the peaks of the brinnel marks and then dropping into the valleys.

Replaced the head and the problem went away.

[thekite]

YES! You just described the situation exactly.
Hard to believe, but the 12E was relatively new in Australia in the 80's.
I could not find a soul who could help me, and my boss sacked me for asking CASA's advice. (I got my revenge, but that is another story)
TK