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Single engine cruise for twins

Interesting research topic. Previously discussed in a 2009 'single engine ferry flight' thread.

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Researchers look at single-engine cruise ops on twins
Aviation International News 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

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.

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….

Wasn't that the point of the Twin Ranger?

Phil

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.

Well if you have 3 engines ....

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

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

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.

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AW101 can achieve its best range speed with one engine deliberately shutdown,

Maybe, but the study was for twins, not triples.

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

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?

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.

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.

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.

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

Interesting point!

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Originally Posted by tottigol

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.

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Originally Posted by 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

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

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

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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.

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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.

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.