Single engine cruise for twins

[Ian Corrigible]

Interesting research topic. Previously discussed in a 2009 'single engine ferry flight (http://www.pprune.org/rotorheads/362784-ferry-flight-single-engine-debate.html)' thread.

Researchers look at single-engine cruise ops on twins
Aviation International News (http://www.ainonline.com/aviation-news/business-aviation/2015-02-14/researchers-look-single-engine-cruise-ops-twins) February 14, 2015

Single-engine cruise flight on twin-engine helicopters, to save fuel, could be possible, according to researchers at the Technische Universität of Munich, Germany. Their work focuses on how to cope with failure of the operating engine when the other has already been shut down. Two elements are key: quick automated restart; and limiting to safe levels the rate at which the engine builds torque during that quick restart.

During most steady flight phases, a twin’s engines are not heavily taxed and one engine could deliver the power required, Professor Manfred Hajek pointed out. Specific fuel consumption falls as the load on the engine rises, said Hajek, so cruising on one engine at a high power setting in suitable areas of the flight envelope would cut down fuel consumption “significantly.” For a given power requirement, he noted, one turboshaft operating at a higher output is approximately 30 percent more fuel-efficient than two turboshafts running at a lower output.

Intentional single-engine operation of a twin would be worth investigating only if the inactive engine could restart and produce power quickly enough to minimize altitude loss after failure of the operating engine, Hajek conceded. As part of the study, his team used an engine test rig and a BO105 flight simulator. One engine (the one that had to restart swiftly) was a Rolls-Royce 250-C20B fitted with a quick-start system designed in previous research work and using a high-pressure air supply. The modified engine can reach flight idle in two seconds, rather than the usual 27 seconds, according to Hajek. The second engine (the one that had to “fail”) was simulated.

The first priority was to limit the rate of torque increase to 65 Nm per second, beyond which the engine surged.

The researchers then compared how pilots and automation handled the restart. A “quick-start controller” with an altitude-hold mode lost 330 feet of altitude; pilots lost about 560 feet. Pilots tended to regain 100-percent rotor rpm more quickly, while the automated system kept collective pitch higher, helping to explain the different altitude losses.

More work needs to be done, Hajek said, especially in assessing the effects on the main gearbox. He emphasized that single-engine cruising would not be suitable for offshore twin operations or for brief EMS flights. He also noted that rolling an engine to idle rather than shutting it down would likely make only a small dent in fuel consumption.

I/C

[krypton_john]

It probably would require too much energy, but if the standby engine could be kept spinning at say 15% then that's a decent time saving.

[hueyracer]

Don´t see the benefit of this…

Every aircraft i have flown so far (with payload in it) wasn´t able to continue cruise flight at given airspeed in OEI.

Airspeed always had to be brought back to about 70 knots…..which means that you might safe fuel with one engine OFF, but you´ll need to fly longer to get to your destination.


Deciding to shut off one engine, but use it later is a decision usually carried out when there is a longer flight between the place the occurrence happens, and the place of intended landing-which also means that you got plenty of time to restart the engine….

[paco]

Wasn't that the point of the Twin Ranger?

Phil

[AAKEE]

Bad idea for a lot of reasons.

For the economic part, technical cost is the bigger part and fuel the minor.
I bet the saving in fuel is overrided by the increased technical cost when flying slower.

[dangermouse]

It's not a problem,

AW101 can achieve its best range speed with one engine deliberately shutdown, the MGB is designed for it ('declutch' the engine before shutdown) and the aircraft tested to allow for recovery time from OEI to TEI in the case of a running engine also failing. Normal SOPs for some operators as it does increase the range a useful amount

I can see issues with going deliberate OEI on a twin though, you would have to have a bl00dy reliable way of restarting the off engine when in autorotation and I guess a minimum AGL to allow the engine to restart and to be able to recover the aircraft to level flight. The mention of a high pressure syetym is fine provided it allows you to have more than 1 shot at it!, probably any weight increase for that system would negate the range benefit

I suspect there would be issues with flying in icing conditions or in moisture with an engine off as well

DM

[vfr440]

Ah (or more correctly aagh), the infamous 206LT. CAA wouldn't buy that when (the only) one applied for UK registration c1995; I don't think FAA were too keen on the idea either. - VFR

[tottigol]

Not to mention how the oil workers unions would react to such an engaging "we would save fuel" argument in light of a possible power off ditching.:rolleyes:
Besides that, I know of at least one medium twin tgan can cruise at about 130 KIAS on one engine.

[MightyGem]

AW101 can achieve its best range speed with one engine deliberately shutdown,
Maybe, but the study was for twins, not triples.

[Jack Carson]

Many years ago I was provided with an opportunity to fly the Wessex 5 off HMS Hermes. To say the least it was a delightful experience. While the ship was in route to Ft Lauderdale, the Wessex was dispatched in advance to Hermes arrival. Due to range requirements the flight was planned and flown single engine. It was explained that this was normal operating doctrine for the Wessex 5. It would be interesting to hear from those with Wessex 5 experience. I am sure that there are many interesting stories of single engine Wessex 5 operations.:8

[hueyracer]

I see the point of saving fuel in the cruise…..but why not work on engines that can go into "power saving mode", aka running at 50% in the cruise and therefor burning less fuel?

[chopjock]

Or perhaps someone could design a system where by the larger main engine
is only used for the cruise and a standby APU is used to power an electric motor to assist in take off and landing. That way an electric engine could start instantly on a battery for a couple of minutes whilst the APU starts up to take over power delivery.

[Fareastdriver]

How are you going to convince ATC that you can fly IFR in controlled airspace. They do not expect twin engined aircraft to change flight levels in the event of a single engined failure. Increased range in any sort of breeze only works if you are flying downwind.

The only time I have known, in helicopters, for a deliberate single engined leg was where the destination and the diversion had socked in and it was a case of just about making it to somewhere where they could land.

Somebody was demonstrating the single engined performance of the Wessex. Easy, look, you can winch with one engine; then the good engine ran down. The bloke on the end of the winch only just got away with it.

'power saving mode'.. Most large twin's engines are running at 50% power in the cruise. The idea on using one engine is because at 50% power they are using 60% of the fuel that they would use at 100% power.

[Dave B]

This used to be carried out by some people on Bristow Wessex ferry flights, I remember talking to Pip smith about, and he was all in favour. My worry as an engineer, was that the inoperative engine would still be windmilling, but not generating enough labyrinth seal pressure to contain the engine oil.

[hueyracer]

but not generating enough labyrinth seal pressure to contain the engine oil.

Interesting point!

[Ian Corrigible]

I know of at least one medium twin that can cruise at about 130 KIAS on one engine.
Totti -- Yes, first thing that came to mind when I read the article. There are several twins in operation that max out (or come close to maxing out) the xmsn with just one of their engines, therefore potentially avoiding the 'part power' SFC issue.

Or perhaps someone could design a system where by the larger main engine is only used for the cruise and a standby APU is used to power an electric motor to assist in take off and landing
A "little & large" twin engine solution is known to have been studied by at least one OEM in recent years. The downside is having to support two different donks on a single platform. The 'oversized APU' idea could offer an alternative approach (similar to the 'APTU' studied for plank applications), though downsides here include more stringent certification requirements, and poor SFC performance when used in 'APU only' mode.

I/C

[timprice]

Might as well just have one engine and save all the complicated extras, like crew etc:cool:

[Fareastdriver]

Might as well just have one engine

Couldn't agree more. 1,500 hrs, mostly over primary jungle with one engine, no problem. 15,000 hrs with two, a pain in the backside.

[Fareastdriver]

but not generating enough labyrinth seal pressure to contain the engine oil.

The Convair B36, six turning and four burning, had an iris in the jet intake. The jets were only used for some take-offs but were there to guarantee target speed and height. In the cruise they were shut down and the iris was closed to stop them windmilling with the associated drag.

[cattletruck]

I recall a twin piston lightie called a Piper Seminole that flew so dandily on just one donk.

If going single on a twin is a growing operational option then manufacturers should legitimise the practice by publishing it as an alternative and acceptably constrained flight mode in their FM.

I'm sure an intelligent FADEC technology could do a lot to help legitimise the practice.

[Gemini Twin]

Now please don't get me wrong and I'm not a saying it's a brilliant idea or that it could be ever economically justifiable. But given the right mix and using proper techniques it does work very well.


In the early 90's we did in fact gain FAA approval for single engine operations on the Tridair's Gemini 206L-1. 9 (Covered in supplement 6 of the Gemini flight manual dated 28 July 1994).


It was the right mix as we were trying to produce the ultimate Cat A helicopter, in that an engine failure on take off had very little effect on the twin engine performance. We had two 450 shp engines and the transmission was rated at 456shp. The systems were developed so that the loss of the engine after decision point, full power was immediately available with out exceeding any engine limit.


Returning from Heli Expo in Miami 1992 we always took off on two and shut down one engine when in cruise. The big impact here was that the C20R at MPC cruise used quite a bit less fuel that the original C28 or C30 at the same power setting.
By alternating the engine shut down on each leg each engine had less time on them than the airframe. Miami to Seattle via LA it made quite a difference.


We always started the "spare" prior to landing and it really was a easily managed process. The technique used to prevent clutch grab at the restart was to beep the engine down prior to shut down so the it was completely off line. We use triple needles on the torque indicator one for each engine and one for what was going into the transmission. So in cruise it would display 85% at the transmission and 42.5% for Eng 1 and Eng 2. As the engine was beeped off line the torque on that engine was drop to 0 while the running engine would rise to 85%. When restarted we would get it running to flight idle delivery zero torque then slowly beep it up to 42.5% while doing this the running engine would drop also to 42.5%.


Bell installed our kits in the Twin Ranger version of the L4, while being an excellent Cat A performer they did not want the single twin option.


It's kind of interesting for us to see the recent announcement mentioned above, more than 20 years after our experiments. Could have saved them some money perhaps. :)

[megan]

In the early days Bristow in Oz used Wessex on a particular offshore contract where it was SOP to shut one down in the cruise. Reportedly I believe to have dropped fuel consumption by 25%, which was the aim as they were operating at their extremes of range. Regulator required flight above a certain altitude so you had room to get the other one started (hopefully) should the operating engine fail. Wonder what the innocent pax would have thought, had they known. Still, different days back then.

[Nigel Osborn]

The twin Wessex fuel consumption was not that much different between 2 engine ops & single engine. I was amongst the original Wessex pilots & we did several tests in both cold & hot weather to get some data. The result was the navy didn't do single engine cruise.

Another important point was that the combining gear box had only a 25 hour life on 1 engine. I would imagine that restriction must have been lifted by the time Bristows got the series 60.:ok:

[John Eacott]

In the early days Bristow in Oz used Wessex on a particular offshore contract where it was SOP to shut one down in the cruise. Reportedly I believe to have dropped fuel consumption by 25%, which was the aim as they were operating at their extremes of range. Regulator required flight above a certain altitude so you had room to get the other one started (hopefully) should the operating engine fail. Wonder what the innocent pax would have thought, had they known. Still, different days back then.

I flew the Wessex 60 for Maine-Bristow and never heard of such a procedure. Single pilot from Broome or Derby to just off East Timor, three refuels out and one back, no way I'd have cruised on one engine. Not deliberately ;)

[Dennis Kenyon]

Not sure its that relevant, but my log book tells me when flying the Meteors at No 209 AFS Westonzoyland circa 1954, I was tasked to carry out a search on Exmoor for a downed aircraft. The brief was to shut down the right hand Derwent. I see the sortie was logged at 1 hour 35 minutes. The standard endurance for the Meteor Mk V11 on both engines was circa 55 minutes. Quite an improvement. Dennis K.

[megan]

Aging brain cells John is my excuse. Checked with one of the people and his replyI personally never flew it single engine with passengers. To do that deliberately would have been against all regulations.

It was done occasionally on long distance ferries crew only, where fuel was critical and I have done that. You would fly twin until the overload external tank was empty then shut one down. 25% sounds about right. The machine flew slowly, maybe 100K single or twin. A very good single engine machine. I flew it back from the Scott reef on one when the number two had a runaway up. We were at MAUW, 13600?? and she just waddled all the way back. 180 NM as I recollect.

Part of the conversion was to go to 3000 (I think) and shut an engine down, set it up for restart then pull the other to idle. It was then a matter of entering auto and getting the shut down engine on line. As I recall (it was over 35 years ago for me too!! ) you could be up and running by 1000 feet. No big deal. It was Mayne Bristow by the way.Nearly time for morning tea, better park the BMW and have coffee and cake.

[John Eacott]

Aging brain cells John is my excuse. Checked with one of the people and his replyNearly time for morning tea, better park the BMW and have coffee and cake.

I'm looking over my shoulder now: as I have the first coffee and cake of the day at the Deloraine Bakery! Battlestar Galactica is safely parked, too :cool:

I did a bit ferrying in the Wessex 60, again the concept of single engine was not on the menu; Broome to Perth springs to mind. But maybe your contact was from an earlier time than mine?

Ask him if he was mistaken for a road train driver; we all had six shades darker tan on the right arm due to North in the morning, South in the afternoon with the door open ;)

[Fareastdriver]

The brief was to shut down the right hand Derwent.

The good old Meatbox. Killed dozens of pilots practising single engined failure with one engine shut down; real engine were failures practically unknown. Endurance, as opposed to range; ie. being stuck above cloud at the end of the QGH line, was markedly improved by slowing down and shutting down an engine. The choice was simple: the generator, radios etc, was on the port engine, and the hydraulic pump on the starboard.

IIRC one had to get 270 knots at the last part the of the QGH to ensure that the engine was windmilling fast enough to relight otherwise there was a lot of pumping.

[Lonewolf_50]

I can't find a reference on current mishap data bases. There are a bunch of SH-2F accidents/write offs from the 80's that have very little detail over at the flight safety web site. All I can recall is that it was a west coast squadron. HSL-31, 33, 35. One of them.
As we got told in the east coast ... there was a reason that singling up to "extend range" was more or less an emergency procedure when using the T-58-8-F engine. The NATOPS manual had some interesting charts on how far one engine could take you. (If you fly over the open seas in single ship ops that kind of info is very handy to have ..)

The data point from the west coast was flight during the early/mid 80's, crew traveling from south to north California.

One engine taken one off line to extend range/extend reserve due to ... memory foggy I cannot recall details well ... perhaps a change in weather and alternates all of a sudden becoming less than desirable.
As it worked out, the one they were running on acted up. They were unable to restart the one that had been windmilling, which seems to have fouled the ignitor plugs ... something like that.
Ended in tears. IIRC, all souls perished, landing in mountainous terrain that went badly.
My takeaway was: use them both, unless it's an emergency.

[AAKEE]

I know there was a discussion some time ago here on pprune about shutting one engine off if in an fuel emergency at sea. Dont remember the helo type though...

If the 'shut-one-down' was for economy it would be a total other angle about it.
All helos i ever flown( military, different sizes), the fuel is a very little part of the total cost. Only have one FM here so I used it for this example(but from memory, about the same proportions for the other Twins, from qiute small to big): I picked a normal weight, about standard day at 3000'.

(Fuel-) Economical cruise 135kt @ 550kg/h
Max cont cruise AEO: 153kt @ 690kg/h
Max cont cruise on one Engine: 107kt @385kg/h

At 5000E/h; The cost for flying 135Nm(equals 1h economical crusie):
Economical Cruise = 5000E
Max Cruise = 4400E + 72E(72L extra) = 4472E
Max single Engine = 6300 - 80E(80L minus) = 6220E

Seems obvius to keep the speed up. As I said, the other twins had about the same proportions.

If development proceeds(which I doubt) , I guess it would be possible to save some Money on the enginge shut down, but not as much as it differs. I also guess the quick start system would add some weight, both slowing single engine cruise and payload...

[Edit] The helo in the example above does about 4.1kg/Nm in economical cruise and 3.6kg/Nm in single engine so in fuel emergency over sea, I see the Point !

[AAKEE]

And as for the BO105 in the OP, it didnt do much more than 60kt on OEI, maybe 70 at best ? (..dont remember but this was on 30min OEI rating ?)

...So speed down to 50%, and a little fuel saved...

Just get a R22 instead and you are that slow, and on one engine :ugh:

[JohnDixson]

Ian C:

Quoting the article that you posted ( in part ): " More work needs to be done, Hajek said, especially in assessing the effects on the main gearbox "

Their research will most likely find, unless the box has been way over-designed structurally, that regular use of much higher than normal power on one input will have an effect on the box TBO. Not something everyone thinks of at first.

[Ian Corrigible]

John,

Point well made. The concept is probably better suited for a dedicated design (i.e. the 'little & large' engine configuration mentioned previously, with input shafts rated accordingly) rather than an adaptation of an existing platform.

I/C

[AAKEE]

The concept is probably better suited for a dedicated design (i.e. the 'little & large' engine configuration mentioned previously, with input shafts rated accordingly) rather than an adaptation of an existing platform.

Yes, I Think your both right Ian and John.

The enlarged MGB and bigger engine weighs more and costs more, and it doesnt add payload. It eats from the payload availabe, as the Quick start mechanism do.
More expensive, less payload.

[Gemini Twin]

No change in TBO on our cobox. Part of the certification program was a gear fatigue test to prove infinite gear life which included 50 hours on one input at 585 shp. We also performed a full 30 min. no oil test at 70 % of mcp.

[Hedge36]

I can't find a reference on current mishap data bases. There are a bunch of SH-2F accidents/write offs from the 80's that have very little detail over at the flight safety web site. All I can recall is that it was a west coast squadron. HSL-31, 33, 35. One of them.
As we got told in the east coast ... there was a reason that singling up to "extend range" was more or less an emergency procedure when using the T-58-8-F engine. The NATOPS manual had some interesting charts on how far one engine could take you. (If you fly over the open seas in single ship ops that kind of info is very handy to have ..)

The data point from the west coast was flight during the early/mid 80's, crew traveling from south to north California.

One engine taken one off line to extend range/extend reserve due to ... memory foggy I cannot recall details well ... perhaps a change in weather and alternates all of a sudden becoming less than desirable.
As it worked out, the one they were running on acted up. They were unable to restart the one that had been windmilling, which seems to have fouled the ignitor plugs ... something like that.
Ended in tears. IIRC, all souls perished, landing in mountainous terrain that went badly.
My takeaway was: use them both, unless it's an emergency.


Digging between the dusty cobwebs formed in the dark recesses of my brain over the past twenty or so years, I recall a conversation with a -35 driver who mentioned some scheduled and unscheduled trials of this sort over at HSL-33.

[ShyTorque]

I once had to carry out a single engined takeoff, transit and landing in a Puma HC1. We practiced minimum power takeoffs quite regularly so I had no real qualms about it, other than that it involved a sea transit. This type of flight had to be approved by the Air Officer Commanding. He willingly gave verbal authorisation in person for it, not surprising seeing as we were on our way to get a serviceable aircraft to fly himself and entourage back to the mainland.

I agree with previous posters about some modern medium twins being very capable on one engine. Having taken off on two, I shut down the one with the stuck oil cooler thermo valve then started it up again for a twin engined landing. It would easily have managed the entire trip on one, far better OEI performance than the old Puma (which didn't have a lot, tbh).

[Lonewolf_50]

Thanks, Hedge.
The number of class A mishaps that H-2 squadrons experienced in the 80's was (in my view looking back) eye watering.
Only by looking back do I understand the significant number of my compadres who left the community (and small deck aviation) for good after one tour.
I, of course, went back for more. (OK, a little bit of crazy isn't bad. :} )

[Hedge36]

I was a Knox class pit snipe during my Canoe Club days, but having come from the oil fields (212s and 412s) my first love was always aviation - I didn't start flying helos until after I removed the uniform. That said, the quirkiness of the H-2 appealed to me for a number of reasons and I always had a stack of request chits on the CHENG's desk prior to getting underway with an air det aboard.

Along the way I formed close friendships with a number of AWs and drivers, and was always amazed by the list of names any one of those guys could rattle off, drunk or sober, lost in H-2s and -3s during the late 80s and early 90s.

Of course, now I'm the head of an organization trying to retrieve a Knox hull from our friends overseas, and a SeaSprite to park on her flight deck. Nobody ever accused me of being very smart, either :O

[Lonewolf_50]

The last Knox I landed on was Ainsworth. 85/86. Med.
Before that, I spent a short while on the W.S. Sims before our squadron assigned me to a different det on a SpruCan
While the QOL on a SpruCan was better, I found the winds around the aft end of a Knox a lot less turbulent, which made for a smoother end game during night landings.
FFG 1's and Garcia class Frigates, and for that matter, FFG 7's ... not so fond of them. :p
Best wishes on getting that Knox frigate back to this side of the pond. :ok:

As to single engine performance, (the topic of this thread) with T-700's and the new gear box, I am pretty sure the single engine stats on the G would have been awesome. Never got to fly it.