Cruise setting with helicopters with 3axis AP

[ODEN]

Would be interesting to know what different S92 operators (and also other helicopters with 4axis(3cue) AP) use for cruise setting. (Vno)

IAS, power setting or FF.
Fly in 2 cue or 3 cue?

Has any company made calculations for most economical (meaning money not fuel) speed (power) setting? Taking cost of flight hours, maintenance costs per hour/calendar and FF costs into account?

already got a few answers as below:

1360lbs/hr fuel flow, about approx. 140kias with ALT and NAV coupled.
and
2 que, ALT and NAV. Torque either 70% or 75%, depends on weight. Max 145 IAS.

any other cruise settings, recomendations?

[212man]

We use 3-cue with 140 KIAS and 145 when light(er).

On the EC-155, in my previous life, we used 4 axis with the IAS datum set about 5 kts above Vh, to ensure the FLI remained at MCP. This followed the ECF philosophy of using MCP as much as possible and avoided the power hunting in turbulance.

[HeliComparator]

On the 225 we fly 3-cue (4 axis) and set the IAS datum to Vne - 10kts. The autopilot prioritises vertical modes (ie ALT or VS) when above Vy so it holds the height and accepts it can't do 165kts in level flight. In fact it sets torque to around 2% below MCP to give some buffer against exceeding MCP. In strong turbulence, its sometimes necessary to fly 3 axis (2 cue) and manually set the collective below MCP to avoid frequent exceedances of MCP (which apart from anything else triggers a loud gong).

We set the IAS datum to Vne - 10 so that it will accelerate during descent but not get too close to Vne in turbulence. It does sometimes get to 165 at which point it would start to back the power off.

For the 225, surprisingly it gives best range at only slightly below flat out - reducing speed by around 5kts to cruise at typically 145 kts gives best range, any slower and range gets worse even in still wind. If we do this (not often, as you are not often short of fuel in a 225!), as 212 says its best not to cruise in 3 cue to avoid collective hunting.

HC

[ODEN]

Thanks 4 answering.

HC, just soo i understand correct, u mean there is now collective hunting on the 225 even though u r in 3 cue due to the fact that tq is limiting at 2% below MCP before it can reach the set IAS value?

Reg

[HeliComparator]

ODEN, normally we cruise in 3cue/4 axis as the AFCS pegs the collective on MCP-2%. I suppose there must be very slight hunting in turbulence but its not detectable (probably just the series actuator moving slightly, the pilot's collective lever doesn't move).

What I was referring to was when you want to cruise significantly below MCP. If you do that in 3cue/4 axis the AFCS is trying to maintain a particular IAS, so unless its totally smooth there will inevitably be changes in collective to maintain the target speed. This is not a big deal and certainly totally acceptable for instrument approaches, night visual approaches etc, however its probably not best practice for extended cruising. For extended cruising significantly below MCP, ie with IAS datum set to below max cruise speed (eg to save fuel, meet an EAT or in strong turbulence) IMO its best to fly 2 cue / 3 axis, though there is nothing to stop you flying 3cue/4axis if you wish.

We also sometimes use 2cue/3axis when slowing down eg for a rig landing. With the aircraft doing up to 165 on descent it takes a fair bit of slowing to get to 10kts committal speed! If you use the IAS datum to slow down its a fairly vigorous deceleration which is perhaps not optimal for passenger comfort, especially if you stop the deceleration at various points and/or fiddle with the IAS datum. The alternative is to fly 2cue eg with ALT or VS on the cyclic, and slowly lower the collective from cruise power to a low value.

On some helicopters this is a slightly dangerous as you can fall off the back of the drag curve and descend into the sea with the nose up in the air, but 225 has a nice safety feature in that if you are flying 2cue, as the speed drops below about 60 kts the AFCS automatically moves the ALT or VS to the collective and engages IAS on cyclic at the current speed, pulling in some power to stop the speed loss and stabilise the flight path.

HC

[ODEN]

HC txs for the answer.

I understand

Could I ask one more thing...

however its probably not best practice for extended cruising.

I agree on the above...with your knowledge and experience could you elaborate on the reason (thoughts) for this not beeing a good practice?

Reg

[HeliComparator]

Oden - simply because if you are asking the AFCS to maintain both a specific airspeed and a specific height it will inevitably result in continual adjustments to collective pitch (unless its flat calm). That means control runs moving, swash plate etc moving far more than would be required just to maintain attitude, although of course the control systems are designed to be able to do that.

Probably the main reason is to reduce the changes in N1 (Ng or whatever you want to call it) that result from changing power demands. Gas turbine engines like to operate at constant speed - every time the N1 increases or decreases speed, that alters the centrifugal (alright, centripetal for the purists) force on the compressor and turbine blades, effectively stretching the blades more or less. Each change in rotational speed is therefore effectively a tiny fatigue cycle. I am sure engines are able to tolerate this, but if there are 2 ways to do something, both equally effective but one of those ways results in less wear and fatigue, even if only slightly, why not go for that method?

HC

[rotorfan]

every time the N1 increases or decreases speed, that alters the centrifugal (alright, centripetal for the purists) force on the compressor and turbine blades, effectively stretching the blades more or less. Each change in rotational speed is therefore effectively a tiny fatigue cycle.

HC, well put. One of my tasks in my professional life is repair to high-accuracy CNC machining equipment. (Sorry for thread drift here...) There is no question that the reliability and longevity of a machine is directly related to how it's treated or used. If your operation of the machine can either decrease the peaks of the stress or spread it over a longer time period , the aging takes place at a slower rate. Over time, enough small stresses compound into large ones, and any mechanical device WILL fail, sooner or later. When I'm in the pilot's seat, I'll take the "later", thank you.:ok:

rf

[ODEN]

HC,

thanks again for your time and answer.

[ODEN]

I agree, seams like good airmanship and practice not to fly around with a hunting collective during cruise.

so from the answers above...

225 cruising 2% below MCP (aprox 150IAS)

S92 ff 1360, tq 70-75 or IAS 140.

MCP on the S92 is 86%tq.

I would guess that an increase in tq on the 92 would give more economical flight, even tough ff will go up the flight time will be reduced and so the cost of maintenance. Seams like no one has done the calculation on that, but maybe there are other reasons to cruise 11-16% below MCP on the 92???

[ODEN]

225 cruising close to MCP but S92 well below MCP....

...is there something else limiting the S92 to cruise closer to MCP?

Anyone flying SAR S92 maybe can answer what cruise setting you use when going on time critical (endurance no concern) missions and how it perfoms at MCP?

Thanks in advance for any answers.

[[email protected]]

Whilst we have 225 and S92 operators on this thread, what is the maximum speed of each aircraft with a typical SAR role equipment load, 4 crew and 3 hours of fuel? Assume SL and ISA.

Do any of the operators impose a lower speed to reduce fatigue or do they accept that on a rescue that the aircraft will be flown to its limits?

[HeliComparator]

Crab

For the 225 its 175 kts (Vne). Oh, did you mean in level flight? :}
There is a tender in progress so I am not about to give away the answers!

Operators of the 225 normally cruise at MCP so that would be no different for a SAR version.

HC

[[email protected]]

HC, the technical assessment part of the bids is done and dusted so such info is not going to affect the outcome - unless the goalposts have been moved again like suddenly deciding Chiv, Boulmer and Portland would be 12 hour day only operations post 2012 (yes I know Portland is already).

Hardly 'no less capable a service' is it? Unfortunately all the arguments defending it are based on having faster aircraft which if you only include flying time does allow you to make all the medium risk areas in 1 hour - completely ignoring the time to launch - RS 45 at night which leaves you 15 minutes of flying time to make the medium risk area. Even at 150kts you can only go 45 miles which leaves very big gaps in some medium risk areas.

[HeliComparator]

There is more than one SAR tender in this world!

HC

[ODEN]

:)

...anyone care to comment on the 92?

[albatross]

If you want to cruise around at MCP in the 92 - great- it will do it.
We do some long legs and are generally more interested in long range cruise and payload.
Hence the higher altitudes, lower cruise settings on a lot of legs.


Remember it is a draggy schoolbus with a rotor system so yarding in the pole doesn't give a whole bunch more speed for the power setting used and fuel burns generated.

http://i37.photobucket.com/albums/e68/midcanada/92cruise6000plus10.jpg

[212man]

For those who can't be bothered to do the maths (math for those over the pond...) the figures above give about 38 extra nautical miles for a 5000lb fuel load.

At the altitude/OAT shown, the aircraft is engine limited, not torque limited (MCP limit is 86% but only 80% available,) but you are still not really getting the turbine efficiencies you see on smaller types.

[ODEN]

If you want to cruise around at MCP in the 92 - great- it will do it.

ok, so in time critical missions or urgent medevacs there should be no problem getting around 160kts TAS...

Thanks albatross

[albatross]

Hang on a sec there Oden!:ok:

I didn't say that you can cruise at 160 TAS - a lot will depend on Alt - temp and aircraft mass.

What I said is that if you want to zorch around at MCP the aircraft will do it.

I think 160 TAS in level flight will be asking a lot except at low aircraft Masses.


Unfortunaltely I don't have the other performance sheets to hand.

I am sure smarter folks than I will have the correct answer.

Part 2 section III of the ops manual is what you should refer to. Happy digging.

Cheers
Albatross

[212man]

Yes, at high weights and with any engine bleeds, RIPS or air-con (not to mention mission equipment 'sticking out in the breeze') you will be lucky to get much above 150 Kts TAS.

Besides - you should be following the SOPs for your company, surely?

[ODEN]

Ok albatross I rephrase:

According to the chart you supplied (figure 3:16) in those given conditions with an aircraft weight of 22000lbs (yes very light) in theory you should get around 144 IAS and 160 TAS, or I am reading it wrong?

[ODEN]

Im talking about time critical SAR/HEMS missions were you are interested to go as fast as possible, but of course stay within limits but also feel that the aircraft not gona fall apart on you...or that you start feeling worried...

SAC always gona give u happy figures and refer to the FM (FM says MCP 86 tq, but most operators fly well below that, is it only due to ff or?)
I though I might get a comment from someone flying the S92 in the SAR role, and there experience from high cruise settings/speeds.

Always good to know how to get best range but also the best speed.

Alt , press, temp, wind and TQ (and ac weight and penalties from hoist, sx 16 and so on) are all in the equation...

Thanks again for all the comments.

Reg

[albatross]

Remember there are enought charts to choke a horse and that one is ISA +10 @6000.

You had best find a S+R person to give you better info.