Altering engine layout

All,

To quote one of the many reports on this subject that have floated about over recent years:

An unsolicited proposal for reengining 94 aircraft in the B-52 fleet was submitted to the Air Force by Boeing North American, Inc. in June 1996. Boeing proposed modernizing the B-52 fleet by replacing the current TF-33 engines with Allison/Rolls Royce RB-211 engines through a long-term leasing agreement, and providing fixed-cost, privatized maintenance based on the number of hours flown each year.

Boeing initially projected reengining cost savings of about $6 billion, but later revised the projected savings to $4.7 billion. An Air Force team formed to study Boeing’s proposal analyzed the lease and purchase alternatives and concluded that both options are cost prohibitive compared to maintaining the existing TF-33 engines.

The General Accounting Office estimated that Boeing’s unsolicited proposal to reengine the B-52 fleet would cost the Air Force approximately $1.3 billion rather than save approximately $4.7 billion as Boeing projected.

It is accepted that the newer engines would be far more fuel efficient than the old TF33s with the added bonuses of getting longer ranges per fillup, or having to haul less fuel overall or even giving a longer on station loiter time and reducing the amount of mid-mission tanking that would be required.

Heres another piece from a discussion thread of just over three years ago that seems to put doubt on the USAFs counter-argument that the cost savings are minimal. As with all things, theres always two sides to an argument !

By any measure, the venerable B-52 bomber is a gas hog. It burns 3,334 gallons per hour through eight jet engines that were designed in the 1950s, when the B-52 first entered service.

The B-52 remains the Air Force’s main big bomber. The Air Force has rejected proposals over the past decade to replace the plane’s eight engines with four new, efficient jets. The Air Force said spending $4 billion on new engines would save only a net $400 million and then only if the plane kept flying for another 40 years.

The Science Board study said the Air Force incorrectly calculated fuel costs. Each gallon of jet fuel was priced as if the airplanes were filled up by tanker trucks at the B-52s’ main home base. However, about 10% of the fuel is delivered by aerial tanker, which costs about 10 times more than regular fueling.

In addition, many of the planes flying with a full load, such as those in use over Afghanistan, take off from remote bases, such as Diego Garcia, an island in the Indian Ocean, where special fuel shipments are required.

The Science Board’s study of replacing the B-52 engines estimated $9 billion in savings, and that was when fuel prices were half their current level.

One benefit would be that regular missions from Diego Garcia to Afghanistan could be done without aerial refueling. Such refueling, which involves a tanker plane extending a fuel line to the B-52 flying below, is often dangerous and exhausting for pilots. It also slows the mission.

I dont claim to know the answer to this question, however, I do know that old stuff costs shedloads more to maintain, and i do find it suprising that the option hasnt been explored further by doing the mod on a couple of a/c and actually trying it!

Lets face it – we’ve all seen what a change the CFM56 has made to the -135 series, i would have thought that that lesson would have been patently obvious.

The re-engined tanker, designated the KC-135R, can offload 50 percent more fuel, is 25 percent cheaper to operate and is 96 percent quieter than the KC-135A.

I wasn’t only referring to the 747 when I asked about altering engine layouts, I am interested to know if altering the positions of engines on any existing design causes any major changes to flying characteristics?

Another example would be moving the engines from any four engined passenger aircraft from the wings to the tail (al a VC-10) cause any major flying changes?

To change the position of an engine:
Major re-design of the wing…….

To change an engine type:
In short, you would need a major re-design of the engine mounts and / or fuselage mounts. So yes, it would affect flying characteristics via weight and aerodynamics.

The proposed re-fit of B52s with 4 high bypass fans will affect its operation – perhaps one reason why it hasn’t happened yet.

Actually the rudder controls on most modern airliners are heavier as they are duplicated because of the dual rudders. They both operate together at low speed and only one at high speed as with the ailerons. OK so the materials usded these days are lighter but not that much.

I was just trying to point out that there are good common sense engineering reasons for the wing mounted engines,the VC10 had a weight penalty because the engines were not acting as an unloading force on the wing structure and therefore had a heavier wing.
Probably no aerodynamic reason not to use grouped engines underwing but I doubt modern safety requirements would allow it.
Moving the engines to the tail on an existing a/c would be a non starter because the complete tail assy would have to be redesigned (T tail) and also would move the centre of gravity rearwards a wee bit.
Having said all that I have to say that the VC10 is a really elegant a/c 😀

cheers baz

The VC10 was designed in the late 50’s-they had different airworthiness criteria in those days.

I have never seen a 747 with a cut down rudder – you must post a picture for us to see :rolleyes: Pilots have the other option of cutting power on the opposite outboard engine-may even be auto these days
The original question was specifically about the 747.

I wasn’t only referring to the 747 when I asked about altering engine layouts, I am interested to know if altering the positions of engines on any existing design causes any major changes to flying characteristics?

Another example would be moving the engines from any four engined passenger aircraft from the wings to the tail (al a VC-10) cause any major flying changes?

The VC10 was designed in the late 50’s-they had different airworthiness criteria in those days.

I have never seen a 747 with a cut down rudder – you must post a picture for us to see :rolleyes: Pilots have the other option of cutting power on the opposite outboard engine-may even be auto these days
The original question was specifically about the 747.

You might have to put an armoured bulkhead between the 2 engines to protect against an uncontained compressor/turbine failure ‘taking out’ the adjacent engine and or fuel system.
I know it is not a flying quality issue but might be an airworthiness consideration
There are 2 good reasons for keeping the original configuration -unloading the wing structure thereby making it lighter and the safety factor of engine separation.

Yes. And VC-10 does have paired engines. Does VC-10 suffer contagious failures?

As Mike said -more assymetric moment during engine failure but that is not the big problem it used to be – modern powered controls and safety systems should cope ok with outbd engine failure.

Powered controls cannot control the plane if the control authority is not there because the rudder has been made smaller to save weight and drag. All that the powered controls achieve is stall the airflow at rudder.

Well pointed out Baz. I neglected to consider the fact that most modern airliners are so automated that an engine failure is almost automatically compensated for by the electronic wizardry. Even without that though the powered controls make it far easier to hold the necessary rudder input long enough to dial in compensating trim.

You might have to put an armoured bulkhead between the 2 engines to protect against an uncontained compressor/turbine failure ‘taking out’ the adjacent engine and or fuel system.
I know it is not a flying quality issue but might be an airworthiness consideration
There are 2 good reasons for keeping the original configuration -unloading the wing structure thereby making it lighter and the safety factor of engine separation.
As Mike said -more assymetric moment during engine failure but that is not the big problem it used to be – modern powered controls and safety systems should cope ok with outbd engine failure.

cheers baz

I should imagine that asymetrical thrust i.e No 1 or 4 engine inop would cause less yaw due to the thrust line not being so far from the a/c centreline. Possibly a small reduction in overall drag due to getting rid of two pylons. Other than that I cannot think of anything else.

you would probably have to strengthen the wing structure a wee bit,the outboard engines ‘unload’ (reduce) some of the bending forces on the wing.

regards baz