single engine twins

[widgeon]

I recall some time back when the twin engine 206 conversion was launched some discussion that one engine could be placed in ground idle in flight

My Question , do two engines at a low power setting consume more fuel than one engine at a High Power setting ?.

[CRAN]

Very much so! All engines are designed to provide a specific power output efficiently, operating 'off-design' will result in very poor efficiency indeed.

This is true of all types of engine and indeed all kinds of machines. It's easier to illustrate using a reciprocating engine as an example.

If you wanted to generate 15hp, such as a 125cc motorcycle, then the single cylinder lightweight engine would efficiently deliver this power level. The internal losses due to piston inertia, friction and thermal losses minimised by virtue of the system operating at a point where the output power is significant in comparison to the internal power consumed in operating the engine. (I.e: Compressing the air, overcoming frictions, pumping coolant and lubricant etc etc).

If we then use a 20 litre, 10 cylinder truck engine (normally rated at say 450hp) to generate our 15 hp then clearly the power required to operate the engine, (which will be much greater than 15hp!) is extremely significant in comparison to the output power and hence a very poor efficiency and hence fuel consumption is obtained.

This is an extreme case but same is true with turboshaft engines.

By operating two engines in place of one you are making the output of each engine less significant in comparison to the power required to operate that engine. Hence the poorer efficiency.

If you operate both engines at 50% output, they are both clearly 'off-design' if you operate one at full power and the other at idle you can see that you will have to provide fuel to idle the second engine in addition to fuel for the driving engine.

Hope this helps.
CRAN
:)

[Helinut]

The other thing that will happen is that you are carrying around an "unecessary" second engine that will substantially reduce your useful payload, when compared with the single engine version of the type. The single engine version of different airframes is often an overall better bet - except that the mission will sometimes dictate you must have a twin especially if your regulator is obssessed with engine failure.

[ShyTorque]

The inefficiency comes from the power required (= fuel consumed) to run the compressor and accessories and overcome the frictional losses.

If you place an engine in ground idle it will be working at just above self-sustaining with no useful power being produced. The fuel required to keep it running is therefore all wasted.

If fuel is a major consideration on a twin (for example if it is to make the difference between a ditching or making land), purely from a performance point of view it is better to shut down one engine completely and run the other at a high power setting.

As usual, design is a compromise. The designer is torn between over-powering the twin engined aircraft for maximum lift / single engine performance considerations resulting in high fuel consumption or possibly under-powering the aircraft in the event of single engine failure in the quest for better fuel consumption.

Please note that shutting down engines ecxept in a true emergency is not recommended!

[Nigel Osborn]

Interestingly all the posts have talked about fuel efficiency only. Surely what really matters is the range with 1 or 2 engines running? Most twins will fly slower on 1 engine and if you have a strong headwind, you may well not have the range to reach your destination, as the headwind will have more effect on the slower speed. Hence the need to calculate both 1 and 2 engine ranges available.:confused:

[rotormatic]

The reference to the 206L series twin engine conversion is called the Gemini ST.

You can find the specification of the aircraft at:

http://www.tridair.com

This is the only twin engine helicopter that is authorized to conduct flight operations with either one or both engines...

Twin engine service ceiling is 20,000 feet.

Single engine service ceiling is 10,000 feet.

[rotormatic]

When Bell integrated the STC kit into their production system to build new ships at the factory, they called the aircraft a 206LT... Gemini conversion kits are still available for purchase...

They also made a new flight manual for the aircraft......and did not authorized single engine operation for their version of the aircraft.....

If you operate the aircraft as designed, the range is similar to single engine helicopters...but to do that, you can't buy the one from our friends at Bell.....

A 206A did not carry much either...have also heard the 427 is gross weight challenged......

[Nick Lappos]

The efficiency of a turbine engine is driven mostly by the percent of its thermodynamic power that it is using. The hotter it is running the more efficient it is. A good modern turbine should burn about .4 to .45 pounds of fuel per hour for each horsepower while cruising at 75% power. If you put two engines into the helicopter, each as powerful as the one original, they will be operating at about 40% power each, and the fuel consumption could be about .5 or .55 pounds per hour per horsepower. That means that a powerful twin might burn 25% more fuel than a single with the same power. A twin with two small engines could be the same as a single with one big one, of course.

This factor is the reason why helicopters with lots of excess power are relatively big fuel hogs, and have comparatively less range (Wessex, anyone?).

It is also a paradox that the excess power forces the aircraft to carry more fuel, which robs it of performance (that 25% extra fuel to go the same distance means less power can be used for payload or engine out performance).

Putting one engine in idle is almost a sure way to waste fuel. The back engine is in its most inefficient state (some fuel flow and no power) and the increased efficiency of the up engine never makes that up. However, if you have the nerve 9and are willing to see exactly how fast you can get a restart) you could shut one down. This has been done when range was needed and the alternative is a ditch, but it is tempting Murphy.

[Nigel Osborn]

Helidvr
The LT was a bit different to most twins for the reasons you mention, so that can't be used as the norm.
The Wessex V was also a bit unusual in that it is still probably the most powerful twin, in that 1 engine could almost deliver full twin engine power. The limiting factor was that the transmission couldn't take the twin engine power available; from distant memory it was 3200 lbs torque for 2, and 2900 lbs for 1. This meant that you had to be high or hot to have a problem on 1. Some pilots ferried on 1 for economy as the airspeed didn't decrease on 1 until it was pointed out the gearbox had a life of 25 hours on 1 engine. I had enough grunt to pick up 9 paras in Borneo on 1 engine when the other one wouldn't start and it was an emergency. Usually the ram effect after getting airborne would let you start the second engine but not on this occasion.:D

[Nick Lappos]

helidrvr,

You are right, a good twin with one shut down is just like a single from a failure probability, so what's the worry?

Simple, in the case of the single, the pilot had no choice. In the case of a twin with two good engines, when you shut one of them down, you earn the right to explain to every Tom, Dick and Harry why you did it, should Murphy show his hand. Trust me, after you land, you will meet experts who never flew higher than where the top button of an elevator could take them, all quite willing to explain your mistake!

Choices, always choices!

Nick