French accumulators

[sprocket]

While there is a slight French accent on helicopters here at the moment, I have an open question regarding the provision for maintaining control when hydraulic power is “interrupted”.

The AS350 has an accumulator on each control servo to give control to the main rotor for a limited time; Do later model light French machines with a single hyd. power source, use a similar back-up or do they use a different system to the AS350?

[coalface]

Quite common on Eurocopter a/c. Dauphins have accumulators to avoid "jack stall" as does (I think) the latest Pumas (MK2's). Not sure about the 5 bladed Pumas about to start appearing off the production line.

[TomBola]

Coalface
Which models of the Dauphins have these accumulators to avoid jack stall? Is it the C or N series?

[CRAN]

The key issue with helicopter hydraulic systems with the size of the machine. Small helicopters such as R44, B206, EC120 have control loads that are light engough that should the hydraulic system failure the pilot is physically stong enough to move the controls manually. i.e. Manual Reversion. In bigger machines such as the Squirrel/AStar the loads become too large and the pilot simply wouldn't be able to move the controls. In this case you need a dual fail-safe hydraulic system.

In order to achieve the necessary levels of redundancy in the system accumulators are used to store hydraulic fluid under pressure, to provide a short period of control capability as you point out. As I say this is necessary in big birds as without hydraulic power you can't fly. With little helicopters you can manage without.....though it's not very nice.

Therefore in small helicopters accumulators are seens as needless weight and are not used.

Hope this helps
CRAN
:rolleyes:

[coalface]

Tombola,

Definitely the N, not so sure about the C.

[redandwhite]

Part of the reply from 'CRAN'

Small helicopters such as R44, B206, EC120 have control loads that are light engough that should the hydraulic system failure the pilot is physically stong enough to move the controls manually. i.e. Manual Reversion. In bigger machines such as the Squirrel/AStar the loads become too large and the pilot simply wouldn't be able to move the controls.

'bigger machines?' the squirrel? You've obviously never flown the ubiquitous UH-1, which was quite benign following a failure of the one hydraulic system, and had a MAUW of 9500lb, although the fact that it was a teetering head probably helped. Mind you, if you kept it airborne for too long your right arm used to start vibrating!:D But at least the collective was assisted by the tension/torsion straps.

It wasn't until Bell came to their 'bigger machines' that they gave you the luxury of two 'highbollock' systems!:p

'up the banner' - oops, given the game away to some!

[coalface]

I think there maybe a slight confusion in this thread. I believe the accumulators that were mentioned in the first post by Sprocket refer to the ones which ensure the hydraulic system provides continuing control assistance during extreme flight manouevres which could otherwise cause a hydraulic servo to lock up momentarily (sometimes known as jack stall) due to the inability of the hydraulic fluid pump to provide sufficient flow at times of high demand. A hydraulic jack locking up during an extreme manouevre could cause loss of control until such time as the pump managed to catch up (which may be too late to recover the aircraft to a safe attitude).

These accumulators are not intended to provide hydraulic pressure for extended periods following a pump failure or other system failure. In this event, the aircraft must have an alternative back up (or parallel) system which can allow the pilot to retain control functions. In the case of smaller helicopters with acceptable control forces, the pilot may have to do all the work him/herself following a hydraulic system failure.

I believe that earlier models of the Dauphin without accumulators had a warning light which warned the pilot of incipient jack stall. This would normally occur when the pilot demanded high roll rates at the same time as a rapid pitch change. As I have limited experience on the Dauphin, maybe someone with more knowledge of the type could correct me.

[CRAN]

redandwhite,

I didn't know that about the UH-1 and find it very interesting.

However, the Huey is an old design with relatively low performance compared to contemperary helicopters of that size and weight. One of the reasons for the improvements achieved is advanced rotor aerofoil design and planform design.

Unfortunalety these improvements come at a cost, which is frequently higher pitching moments and control loads than the famous NACA 0012 aerofoil based rotors (at low speed). I'm guessing that this is what they used on Huey, though I haven't checked (It will certainly be something very similar). When mounted about the quater-chord (ish) 0012 produces very small pitching moments and hence finds lots of applications in early helicopter rotor design.

On the assumption that newer designs such as Squirrel use modern proprietory aerofoils (as all Eurocopter helicopters do) then the larger pitching moments created by these aerofoils will require a hydraulic system in machines that are smaller that you might have expected with lower performance earlier rotors. Don't forget the newer rotor have to operate in even more difficult flight conditioopns than the older ones by virtue of the fact that they can fly faster!

CRAN
:rolleyes:

[rotormatic]

The accumulators on the A-Star hydraulic system are installed so that in the event of a failure, the aircraft can be slowed down to an appropriate airspeed, which allows control without hydraulics....

Seems that's the theory anyway...

Heard the 130 has two hydraulic pumps....

[sierra-papa]

sprocket,
I'am not anymore 100% sure about the AS 350, but I know for a fact, that the AS 355 has only 1 accumulator and it is mounted to the tailrotor servo actuator. The AS 355 gets hydr. redundancy from it's tandem servos and 2 hyd pumps. Maybe thats why they have no accumulators at the cyclic servos. I addition they have simple jack stall warning lights, like coalface describes.
sp

[widgeon]

Yep EC130 uses twin star hydraulic system 2 pumps and tandem actuators . Claim you do not need to teach hyd out procedures on this .

[Nick Lappos]

Cran,

The mystery of how a Huey can be controlled even with its nominal 10,000 pounds GW is explained by the pitching moment of the airfoil it has. The trusty old NACA 0012 is kindly, and can be suspended at its quarter chord with almost no moment to feed back to the crew. All the control forces servo out are devoted to creaking the servos and mechanisms around as the pilot muscles the tip path around, little is needed by the blades.

Most modern helicopters have drooped nose assymetrical airfoils that have steady moment outputs even at low thrust, so the servos are needed just to keep the system where it needs to be. Without servos, the systems tend to go to flat pitch.

Even the servo out Huey is a handful if the rotor is taken near stall, where the pitching moment takes a turn, and the airfoil tries to go flat. The airfoil is more like a barn door when deep into stall, so the center of lift is near the mid chord point. Prior to stall, the center of lift is very near the 1/4 chord point, about where we put the feathering hinge.

I used to instruct in the H-13, which is easily flown with servos off, but gets nasty in steep turns, for the above reason.

The jack stall in the 365 family is due to the powerful blade pitching moment near stall overpowering the servos, a factor brought about because the servos were originally sized for the much lighter gross weight of the early 360/365 series, making them more marginal for the increased weight. The earlier model Cobras had jack stall for the same reason.

[sprocket]

coalface: The accumulators which I originally referred to, are the ones used to keep the servos/jacks working for a limited period following hydraulic failure ….. But on reflection, do these accumulators have more than one purpose? On the AS350 (single hydraulics) there is one attached to each servo. As you mentioned ...... do they also assist in preventing jack-stall (transparency)? .... or is that not an issue with the AS350

I had presumed that the principal reason for the light French helos to have accumulators, was because of the extra force required to move the elastomerics in the rotor hub.

CRAN:
Small helicopters such as R44, B206, EC120 have control loads that are light engough that should the hydraulic system failure the pilot is physically stong enough to move the controls manually.

Does that mean the EC-120 does not have elastomerics in its MR head? If so, can the pilot maintain control without hydraulics and accumulators?

[CRAN]

Nick,

Thanks for the affirmation - that exactly what I thought!

CRAN
:)

Sprocket,

The EC120 certainly does have and elastomeric rotor head. It uses LORD spheriflex bearings - as do many modern hubs. The spheriflex bearings don't necessarily increase the control loads substaintially, there purpose is to do the same thing as your flapping, lagging and pitching bearing sets but by eleastic deformation of a single bearing/flexture rather than 3 sets of rolling surface bearings.

They are really neat little things!

Essentially spheriflex bearings are hemispheres made up from laminiates of exquiste hybrid rubber-type materials and high grade metallic materials. The reason why we use them instead of discrete rolling element bearings is principally the reduction in maintenance - no lubrication and massive reduction in hub complexity.

Elastomeric articulated hubs are typically significantly smaller than an articulated hub with 'normal' bearings and also require less maintenance, can be cheaper, lighter, and can be less draggy (by virtue lower area and simpler geometry)

As Nick explained above the main source of control loads in helicopters (with modern aerofoils) is the aerodynamic pitching moments created about the chordwise attachment position, not the nature of the bearings used to accomadate rotor motion.

Though for clarity I guess I should state that because elastomeric bearings require the 'torsional deformation' of a material then of course the material will resist this hence creating a small additional control force that the pilot/hydraulics must over come (in pitch) whenever the bearing is not in its neutral position.

Hope that helps

(I'm sure Nick can add something of value here as the S-92 uses some very nice Elastomeric bearing - and BIG ones too! - Saw them at Farnbourough)

Cheers
CRAN

[soggyboxers]

In simple terms (cos I'm a pretty simple bloke!) the AS 350B has a single hydraulic system and the hydraulic accumulators allow the pilot to set the aircraft up in a suitable configuration to cruise and make a slow running landing in the event of failure of the hydraulic system. The fewer and the smaller the control inputs the pilot makes the longer the accumulator pressure will last.

The Twin Squirrel has a dual hydraulic system for the main rotor controls, but the tail roro is only powered by a single system. When the MAUW was increased by 140kg on the F2 model a hydraulic accumulator was added to the tail rotor control system to assist in maintaining yaw control in the event of a single hydraulic system failure on take off at masses in excess of 2400 kgs. There is a small hydraulic accumulator on top of a lateral servo jack (and when the pressure against this is increased beyond a set value it brings on a 'Limit' warning on the CWP).

The SA 365C2 has a dual hydraulic system, but again only a simplex tail rotor control system. The hydraulics are unusual in that one set of jacks is connected direct to the main rotor control system, whilst the other set is in tandem with it, but separated and under the floor. It too has a hydraulic accumulator system to warn of excessive load on a lateral servo. The SA 365N also has a simplex tail rotor control system and the same 'Limit' warning. The SA 365N1 and N2 models were the first to introduce duplex tail totor hydraulics and both have the lateral servo 'Limit' warning system.

Hope that helps.

[Twisted Rigging]

If memory serves, SA365N and AS365N2 have accumulators only on the aux. hydraulics, purely for lowering the gear in the event of an aux. hyd. failure.

Can someone tell me how an accumulator can prevent jack stall? Surely when a jack has stalled due to excessive loading, it will stay stalled until the load is reduced?

Cheers.

T

[soggyboxers]

Actually TR, the 365N and N2 do, as you say, have a secondary hydraulic system for the undercarriage, but the pump for this is powered by the same shaft as the left hand hydraulic system. They have an electrically powered emergency pump only for lowering the undercarriage in the event of a failure of the secondary system. The only actual hydraulic accumulators are in those aircraft with an assisted braking system, which enables the brakes to be assisted a few times without activating the hydraulic pumps, so that e.g, if the aircraft is parked with the rotors stopped the brakes can still be re-pressurised after moving it.
The so-called 'accumulators' to prevent jack stall are actually incorrectly named. On the top of one of the lateral servos, powered by the right hand hydraulic system, there is a small chamber pressurised to a preset value through a calibrated orifice and holding open a microswitch against a spring. When the aerodynamic loading on that jack approaches a point just short of actual blade stall, the force is sufficient to overcome the hydraulic pressure, close the microswitch and bring on a 'Limit' caution light to warn the pilot of impending jack stall so he can decrease the severity of his manoeuvre and offload the jack.
Hope that helps.

[Twisted Rigging]

Thanks Soggy,

It all comes rushing back to me after all those years.

I have not had a helicopter with brakes for some time.

T

[GLSNightPilot]

The AS350 is actually flyable with a hydraulics failure. It's not easy, but it can be done. Back when I was flying them, we took them to a hover, then landing, hydraulics off in annual recurrent training. Offshore, you may have to land on a small platform, & you can't do a running landing. I know of at least one pilot who died doing it for real, though. As I said, it isn't easy.

[bellsux]

The accumulators for the AS350 are only there to assist the pilot in getting below 60kts and are dumped of all pressure when you have your hand on the situation and gotten over the lights and horn scaring the living out of you. Not that it is a common problem in the Squirrel anyhow no mater how hard you laugh at the Hyd drive belt they are very reliable. You can put the dual hydraulic system in if you have more money than brains and common sense (eg ADF). And as for the why can't you disable both systems gripe... why shut off both when you have a redundancy system? Like have you ever tried to fly a S76 with both systems off? A: No, as not possible to fly with out hydraulics or turn it off. Jack stall can occur in the AS350 but you have to really load up the hydraulics to do so and why on earth you would want to get into that situation in the first place is beyond me as it really takes a bit to do it and is quite scary when it does freeze up on you.

[[email protected]]

Cran, just a small point - the squirrel will fly in manual reversion without hydraulic servo assistance just like the Gazelle did although it is not a very pleasant experience and a running landing is the preferred option. I believe the squirrel's hyd out handling characteristics were the reason the Australian Army didn't select it as a basic trainer.

[Aladdinsane]

Crab,

I think the Oz army utilise ex-RAAF AS350's as ab-initio training helicopters (although I agree not by choice). Graduates go onto Kiowa (B206), Huey or Blackhawk later.

Cheers.

[whatsarunway]

Cran... the 120 has accumulators . just thought you would like to know . does that mean it is still a little heli or does it get a promotion ?