Sloppy feel in cyclic

[ronnie barker]

Hi i am trying to find out what conditions would cause a lynx or other type of helicopter to have a sloppy feel of the cyclic stick?

your ideas would be much appriciated

[Tourist]

Why don't you just explain why you want to know this and perhaps you will get more response, rather than just repeatedly asking a boring question?

[P6 Driver]

I'll bite - poor maintenance?
;)
(Sorry, engineers)

RD

[ronnie barker]

this is for a assignment i have to carry out. as part of my degree

[Tourist]

The problem is that the question is so wide ranging.
1. The lynx is very different from most helicopters in terms of how the head works, and how movement of the cyclic feels.
2. There are a million failures at all stages of the control runs which will make the cyclic sloppy. (ie loss of all rotor blades will make the cyclic sloppy, but so will a worn bush at various points)
3. Then there are many different trim/autopilot systems ranging from the appalling Gazelle through to decent modern systems, all of which affect the cyclic feel.

[ronnie barker]

i am specifically looking at the lynx. The synerio i was given is that the aircraft was carrying out a max con check and the Nr drooped to 95.5% and that is when the sloppy feel occured and there was also a vibration through the airframe

[minigundiplomat]

Limp Wrists?

[HeliAviator]

Ronnie, having taken a Lynx AH7 to 14000 ft over Crossmaglen at night whilst bored, the cyclic was very sloppy in feel!

[Tiger_mate]

Vortex Ring - 'Settling with power'

The signs of a helicopter encountering settling with power are a vibration in the main rotor system followed by an increasing sink rate and a decrease or loss of cyclic authority, depending on the severity of the condition.[2] The failure of a helicopter pilot to recognize and react to the condition can allow high descent rates and impact with terrain, a frequently fatal accident.

This gets my vote, being overhead XMG at 14k without Oxygen does not.

[FlightTester]

...It's been a long time since I worked anything rotary (20 + years), but if I recall sloppy controls and airframe vibration/buffet/rumble generally means a couple of things:

1. Incipient stall - i.e high blade alpha
2. Incipient GB/drivetrain failure
3. Vortex Ring - i.e. high power on the engine, low forward airspeed and descent through the downwash.

[ronnie barker]

i would probably go for the first idea as it said he was doing 100 knots at 7000 feet. so vortex rings only occur at low speed as far as i am aware.

[Tiger_mate]

I guess thats the punch line before the joke then :ugh:

Being type specific could be a red herring, but more likely to narrow down research to single rotor (as opposed to Chinook for example) and it may be that the Lynx semi-rigid rotor head could feature in your answer.

Probably a duff steer, but mention anyway in case you are expected to have some lateral thinking:

Air Test: - ‘over-pitching’ is often misinterpreted as vortex ring. This is where the pilot rapidly increases collective considerably and the engine cannot produce enough power to overcome the large, swift increase in drag on the rotor system. The result is that the rotor system quickly slows down and loses efficiency causing the helicopter instantly to sink. Again, this is not vortex ring. Not sure if this would necesarily induce vibration and loss of cyclic control, but it seems quite resonable to me, anybody else?

[sycamore]

Ronnie,from what I recall and it`s 38 yrs since I last flew a Lynx,you have 2 hyd. systems,2 generators,and 2 autostabs/autopilots,but await corrections to that. Amax.cont check is usually carried out at a safe height generally at best climbing speed ie appx 60 kts say,with one engine at idle at a time,and power applied until one reaches either a Torque limit/TIT-temp limit/or an Ng limit,before the rotor RPM` droops` significantly. repeat again,then do the other engine. If the RRPM droop significantly,then that engine`s governor needs adjusting,and exercise repeated. I can`t recall the RRPM that the generators drop off-line,possibly 92%, but if the `secondary hydraulic pump is a bit `tired`,or has insufficient fluid,or air in the system,then a collective `demand` may give the autostab/pilot insufficient `power` and make the controls feel `weak/sloppy`,as also there is a reduction in t/rotor performance, requiring a bigger pedal input. There are possibly otherreasons but without a look at the transmission diagrams/hyd /elec systems,this is just a guess..
Tourist; where did you fly a Gaz. with an autopilot ?

[Spurlash2]

Some years ago I was in the back of a Wessex, just about to coast out, when I noticed a ploughed field approaching alarmingly quickly. We made a loverly 3 line groove in the soil. The pilot mentioned a 'floppy cyclic', but not enough time to say 'Mayday', or 'Brace', or something else suitable. Banter aside, he did a good job to cream us on a sloping ground.

Memories fail, (early 80's) but I think it was connected to the Stab, and the yaw, pitch and roll thingies in a cupboard. The Auto Stabilisation Equipment (?) (ASE) pack springs to mind as the source of all things evil to aircrew.

Not much detail, I'm afraid, but that was generally the source of flying control faff's.

[ShyTorque]

Tiger mate, I agree with your (amended ;) ) diagnosis.

Bearing in mind that the scenario, as originally presented, involves a hypothetical Max. Contingency engine check, it's possible that the engine failed to produce the rated power, the Nr drooped and the cyclic control became less effective, rather than "sloppy".

The only other condition I can think of is a flying control friction maladjustment or impending failure.

[Tourist]

sycamore
I felt it was easier to describe the sas as trim/autopilot than try to explain what it was.

[sycamore]

Tourist, I think some civvy Gazelles/usa models were so fitted.

[nodrama]

As stated above, this has nothing to do with vortex ring state.

It has everything to do with the onset of stall on the retreating main rotor blade.

At high forward aircraft speeds (e.g. during max cont check) there is high pitch on the rotors and lift is only being produced by the tip section on the retreating MR blade....due to speed of reverse air flow over that blade.

Reduce the speed of the rotors (i.e. Nr droops) and the reverse flow of air over the retreating blade effectively increases and blade tip starts stalling. At the same time, the other blades are at high pitch with a reduced relative airflow over them. This produces lots of turbulent airflow which causes vibration and the sloppy feel.

Like I said in your max cont thread, this question has set a hyperthetical scenario to get you to think about the condtions and causes.

Ronnie, if you are studying for a foundation degree, maybe you should get a good book on helicopter aerodynamics....where effects such as above are clearly explained.

[RINKER]

Sycamore The civilian Gazelle I did my type rating on definatley had an autopilot and as far as I recall was IFR equipped as the instrument panel extended completley across onto the righthand side, I recall this as it made it hard to see out the front coming into a tight confined area so a bit of left pedal and a quick peek out the rh side was in order.I didn't know how to use it myself but it was demonstrated to me by the IP.
R

[Shawn Coyle]

'Sloppy' is a bit imprecise.
The Lynx has an artificial feel system to provide force with displacement, and if memory serves me right, cannot be turned off. So unless something fails in the artificial feel system, the forces on the controls will always be the same. There should be no sloppiness associated with just the controls - if so, it would be there all the time.

The response of the aircraft to control inputs can be less than precise, however. This is probably what should be looked at. This can be caused by a less than well behaved AFCS or less likely, looseness in the mechanical controls. Looseness in the mechanical controls (worn bearings, etc.) would be evident all the time, where something not right in the AFCS can be temporary.

Also if flying at high altitudes, the Lock number effects on the controls can cause unanticipated reaction from a control input - a fore-aft movement will have some roll associated, for example.

More details of when the 'sloppiness' appeared are needed.

[nodrama]

All this talk about worn controls and AFCS ?

It's a hyperthetical question to do with the effect of Nr droop on MR control and lift....doesn't have to be a Lynx...it's general helicopter aerodynamics. The situation is set whilst doing a max cont check because the aircraft then has high forward speed with relatively high pitch on the blades.

[Sloppy Link]

Because of me!

[wokkamate]

" The synerio i was given"

And you are doing a degree? I don't know why, but this seems a little suspicious to me.....is it because people are too lazy to bother checking their speelling before posting, or are so many people barely literate these days? :rolleyes:

What degree are you doing Ronnie Barker? Sounds like an interesting one!

Happy New year to you all by the way, and good luck in 2010 in all you do.

:ok:

[[email protected]]

It is all but impossible to get a Lynx with CMRB (BERP Blades) into retreating blade stall - it does 160 kts with ease (212kts max speed record) and shrugs off high G manoeuvering with barely a wobble (2.7 G max permitted for aerobatic manoeuvres but it takes 3 with no problems).

The stick feel/trim can be turned off and leaves you with a heavy, pendulous feeling cyclic (like waving a broomstick with a weight on the bottom)but you still have one of the worlds most responsive helicopters on the other end of it.

96% Nr is not enough to cause the hyds to stop producing enough power but it might trip the gennies off leaving you with no stab. 96% Nr is also not low enough to cause any handling problems in terms of RBS - the flyaway manouvre from the ODM recommends drooping the Nr to below that if memory serves. ISTR the critical Nr for Lynx was about 85%, below which it was very difficult to recover it in flight.

Ronnie - I think you need to check who has asked this question and what Lynx knowledge they have - it may be that they have used a generic helicopter aerodynamics question and applied it to the Lynx without really knowing what the aircraft can do.

[nodrama]

it may be that they have used a generic helicopter aerodynamics question and applied it to the Lynx


I believe this is most likely the case.

All of Ronnie's posts have refered to Army aircraft...Apache, Gazelle, Lynx. The course is obviously military orientated and so it makes sense that it focuses on aircraft types that the students can relate to.....shame about the inaccuracy to type performance though when choosing an a/c for a question.

The answer to the subject of this thread is retreating blade stall, nethertheless........the scenario gives high forward speed, high(ish) altitude and Nr droop, which are all contributing factors to this condition.

[MightyGem]

2 Flt AAC(the ones who went to Norway every year) Gazelles had SAS as did the RAF ones at Shawbury.

[nodrama]

2 Flt AAC(the ones who went to Norway every year)


...and one year never came back. :}

SAS was also fitted to all the 3 BAS Gazelles.

[TwinHueyMan]

I'm on board with NoDrama... isn't 5% droop in NR speed enough to make the controls a bit less effective? Even in a mighty lynx?

[FLI]

My guess:
High ROD at normal airspeed (90 - 120 Kts) and heavy aft cyclic will cause this effect. Basically RBS! The cyclic goes 'sloppy' unless one reduces the back pressure AND reduces the collective pitch. A classic situation following a wing over. Not too many Lynx pilots will have experienced it unless they have been Display pilots or......... show-offs. (the same?). Excessive 'G' particulally during a 'pull-up' will have the same effect. NON BERP.

[[email protected]]

Twinhueyman - no.

FLI - all UK Lynx are BERP now and RBS most certainly isn't a factor even at max chat at max AUM. Loops take 2 -2.5G and steep turns at 60 deg are a 2 G manoeuvre and flown regularly during training. Even in a harsh recovery from a descent in a steep turn there is no hint of RBS and 140 kt rapid descents using 30 deg nose down are halted easily, again without any hint of RBS. Even at 10,000 DA with the aircraft full of freefallers ready to jump there is no sloppiness whatsoever in the cyclic.

It would take a massive Nr droop to cause any appreciable change in handling qualities because there is so much control power available - an effective hinge offset of 17% if memory serves.

[Graviman]

Don't think this has come up yet:

How is the pitch link control spider spherical bush and slider mechanism lubricated? If greased i imagine wear will occur, as with any bush. Play might be the first indication of this wear, as i imagine difficult access would make this hard to spot otherwise.

Still not sure of context of original post.

[ShyTorque]

i imagine it is difficult to get to for inspections.

So.....do you think they don't bother......?

[Graviman]

ShyTorque, of course i do not! Reading the initial post it looked to me like the question might be along the lines of "what could it mean if...". So the any play in the cyclic could indicate some wear in the slider mechanism - seems sensible enough. Absolutely i'm not suggesting that there is any lax on the part of the engineers!

[Schinthe]

Could it be the nut holding the controls?;)