Would it be feasible to design a quad with, for example, two Trent xwbs as the inner pair and two much smaller GTFs as the outer pair (or any other combination of engine type) What would the pro's and cons be? how could issues such as engine out and asymmetric thrust problems be resolved?

It’d be difficut to certify its take-off performance—you’d have continue at V1 with one Trent and two GTFs provding thrust. Why would anyone do this?

GF

On the 747, you could do that but just on one engine, IIRC. For example, you could have an R powered engine swapped out with a Q or F engine. I would have to into the MEL to see just what the specific combinations would be. But you couldn't put a Q on a 744 in place of a PW4000 Q, as the engine mounts are completely different.

We just had this discussion a couple of weeks ago.
viewtopic.php?f=5&t=1417305

Sounds like the Lockheed P-2E/G/H Neptune

The point would be to design an aircraft for, say, 500 passengers in 3 comfortable classes, but with much better economics than an A380. The small engines will essentially be used for providing the additional thrust when required, such as at takeoff, but provide much less drag and fuel consumption when the extra thrust is not required. I also like the idea that the smaller engines could retract up into the with when not required, or into a more aerodynamic fairing/pod. Just a thought exercise.

Bigger engines are designed that way because generally they are much more efficient at cruise. The redundant cruise engines will just add huge amounts of drag

Agrajag wrote:

The point would be to design an aircraft for, say, 500 passengers in 3 comfortable classes, but with much better economics than an A380. The small engines will essentially be used for providing the additional thrust when required, such as at takeoff, but provide much less drag and fuel consumption when the extra thrust is not required. I also like the idea that the smaller engines could retract up into the with when not required, or into a more aerodynamic fairing/pod. Just a thought exercise.

LOL! still waiting for the thought part to appear...

Pikachu
Im sure such comments are your way of making yourself feel superior (perhaps making up for shortcomings elsewhere) but its unnecessary. And, as it turns out, after a bit of research, the 'thought' has been deemed good enough to make it into production on several aircraft.

Which ones?

Acording to wiki, The Convair B36, The Martin B4M, The Ryan Fr.

Agrajag wrote:

:roll: Pikachu
Im sure such comments are your way of making yourself feel superior (perhaps making up for shortcomings elsewhere) but its unnecessary. And, as it turns out, after a bit of research, the 'thought' has been deemed good enough to make it into production on several aircraft.

You're using the example of a B36 which first flew in 1946 to further support your "thought" exercise?!

I will continue to patiently wait for more "thought" to appear...

How about your "thought" on retractable engines? LOL!

Gear up, flaps up, engines up!

Captain we have a problem! The engines won't extend. No problem. Pull out the engines won't extend checklist.

Not to mention your title states engine combinations on a "quad". But you go on to reference a ten engine relic to support you cause.

I think I might just abandon the thread as thought might be dangerous here.

Pikachu, you continue to embarrass yourself. Why the frustration? Things not going well.? Retractable engines will happen in commercial flight, youll see.

Agrajag wrote:

Pikachu, you continue to embarrass yourself. Why the frustration? Things not going well.? Retractable engines will happen in commercial flight, youll see.

Well I guess you told me...
Hey can you provide your aerospace engineering background to support your claim?
Have you noticed how many quads are in production these days? And you want to design another one?
Maybe you're right, if the 380 had your concept of retractable engines I bet it would have worked!
LOL!

I have no need to prove anything, the purpose of the thread is quite clear. You seem to be an 'angry' chap who is quite vulnerable.
Anyway, back to my chosen topic. In the future i am certain that 'deployed as needed' engines will be in use.

Agrajag wrote:

I have no need to prove anything, the purpose of the thread is quite clear. You seem to be an 'angry' chap who is quite vulnerable.
Anyway, back to my chosen topic. In the future i am certain that 'deployed as needed' engines will be in use.

Since you have no professional credentials and offer nothing other than referencing an archaic airframe from a bygone era to support you position I will continue to disagree with your unshakable truth.

LOL!

Fine, jog on!
I am thinking, in this thought exercise, about a clean sheet design concept, where optimal cruise engines are supplemented by retractable thrust to be deployed when needed i.e at takeoff. I dont see that as such an outlandish concept for the future despite what that silly sausage Pikachu says above.

Agrajag wrote:

Fine, jog on!
I am thinking, in this thought exercise, about a clean sheet design concept, where optimal cruise engines are supplemented by retractable thrust to be deployed when needed i.e at takeoff. I dont see that as such an outlandish concept for the future despite what that silly sausage Pikachu says above.

So engineers are going to tack on engines to a quad (which I assume is a long haul machine in your clean sheet design?) that are carried around for fourteen hours at a time and ARE NOT OPERATING?

I can see just a hint of outlandish in the concept...

Maybe your thread title should be, "High school students offer amazing insights into future air travel realities".

LOL

ps. please do not refer to me as silly sausage pikachu. I find it offensive.

And yet you are still here... Says a lot.
Making predictions is difficult, especially about the future. Thats why its a thought exercise.

I wonder what happened to you to make you behave like this?

Agrajag wrote:

And yet you are still here... Says a lot.
Making predictions is difficult, especially about the future. Thats why its a thought exercise.

I wonder what happened to you to make you behave like this?

Ok just offer me one small glimpse into the future of air travel with something that can be discussed on technical merit.

What thrust do the non deployable engines produce and what thrust do the "deployable on demand" engines produce?

The advantage is obvious, reduce drag. I am sure that is the very reason the jet engines are retractable on gliders. Not such an amazing leap of imagination to see that concept being applied on larger aircraft IN THE FUTURE! Could even be centre-line thrust, not wing based.

I am interested to know some of the potential technical difficulties. Is that not what this Forum is for? If you dont like my thread, i give you my permission to scroll past.

Agrajag wrote:

The advantage is obvious, reduce drag. I am sure that is the very reason the jet engines are retractable on gliders. Not such an amazing leap of imagination to see that concept being applied on larger aircraft IN THE FUTURE! Could even be centre-line thrust, not wing based.

I am interested to know some of the potential technical difficulties. Is that not what this Forum is for? If you dont like my thread, i give you my permission to scroll past.

What percentage drag reduction would you anticipate on your clean sheet design with "deploy on demand" engines over say a 747-8?

Would this concept technology doom the 777x with it's lonely two engines?

I don't follow your logic that a powered glider will prove the technology works on airliners?

Agrajag wrote:

The advantage is obvious, reduce drag. I am sure that is the very reason the jet engines are retractable on gliders. Not such an amazing leap of imagination to see that concept being applied on larger aircraft IN THE FUTURE! Could even be centre-line thrust, not wing based.

I am interested to know some of the potential technical difficulties. Is that not what this Forum is for? If you dont like my thread, i give you my permission to scroll past.

If it worked and was certifiable , don’t you think it would have been tried already? The current state of the art isn’t because the idiots at the builders have no imagination; it’s because the regulations and physics mean twins using monoplane wings and tubular fuselages are the most efficient designs.

Galaxyflyer, at least i got some sense from you. My questions are not necessarily about what has gone before or even the state of the art now, its a concept for the future.

Galaxyflyer, at least i got some sense from you. My questions are not necessarily about what has gone before or even the state of the art now, its a concept for the future. The notion that it wont ever happen because it hasnt happened already is clearly false.

Agrajag wrote:

Galaxyflyer, at least i got some sense from you. My questions are not necessarily about what has gone before or even the state of the art now, its a concept for the future.

Arkjugag, do you believe i have not pointed out at least one difficulty your future drag reducing concept technology might face?

Wouldn't a pod to house an engine have to be bigger than the engine itself? That's a drag baby! (See what i did there?)

Not much room to store engines inside wings the last time I looked out there.

Agrajag wrote:

Galaxyflyer, at least i got some sense from you. My questions are not necessarily about what has gone before or even the state of the art now, its a concept for the future. The notion that it wont ever happen because it hasnt happened already is clearly false.

Then, something revolutionary in physics and engineering will have to occur. Evolution is a bitch but here we are and plane designs are converging around what we have now. Twins have all sorts of built-in advantages that won’t change without said revolution. Aviation is not revolutionary, it’s slow evolution, at best.


Gf

If we're looking into not-so-near future...
Ramjet is one of (IMHO) most promising things in airspace technology. But apparently those engines cannot run on the ground and at low speed, so staged acceleration is a must.
Maybe there are some frames with multiple engine types to be seen some day...

Agrajag wrote:

I am thinking, in this thought exercise, about a clean sheet design concept, where optimal cruise engines are supplemented by retractable thrust to be deployed when needed i.e at takeoff.

That concept already exists as a possibility for future hybrid aircraft, were electric fans augment a cruise gas turbine during take-off/climb and recharge the batteries during descent.
It *might* became viable in the future.

As for your, 2 big + 2 small/retractable conventional, fuel-burning turbofans, I think you can forget it. It would be a maintenance nightmare, take-off engine out certification nightmare and the fact the "booster"engines would just be ballast for most of the flight would just negate any possible efficiency gains.

PerVG wrote:

Agrajag wrote:

I am thinking, in this thought exercise, about a clean sheet design concept, where optimal cruise engines are supplemented by retractable thrust to be deployed when needed i.e at takeoff.

That concept already exists as a possibility for future hybrid aircraft, were electric fans augment a cruise gas turbine during take-off/climb and recharge the batteries during descent.
It *might* became viable in the future.

As for your, 2 big + 2 small/retractable conventional, fuel-burning turbofans, I think you can forget it. It would be a maintenance nightmare, take-off engine out certification nightmare and the fact the "booster"engines would just be ballast for most of the flight would just negate any possible efficiency gains.

And again.... C-130 JATO comes to mind. Not exactly retractable turbofan, though, but takeoff assistance nontheless... I just saw a mention JATO was considered for 727 hot-and-high.
Advantage here is much smaller weight and smaller drag footprint.

You’d never get JATO certified under today’s Part 25, not to mention get an airline to buy it.

GalaxyFlyer wrote:

You’d never get JATO certified under today’s Part 25, not to mention get an airline to buy it.

If you will, this tells me the concept is not dead crazy, just crazy enough to consider and turn down... at least for now, for airline use. May still worth asking, though.

It’s dead crazy, even the military hasn’t used it for decades.

To achieve what you really want you'd have better luck going down the direction of variable nozzle/inlet/bypass designs that adjust themselves based on the current flight segment. Somewhat similar to the purpose of a variable pitch prop.

GalaxyFlyer wrote:

It’s dead crazy, even the military hasn’t used it for decades.

I believe the 109th Airlift Wing (NY) were still using JATO takeoffs on their C-130s on ski takeoffs on soft snow up until about two years ago. Then they received the 8 bladed prop conversion and don't need them anymore. On another note, the old C-123K sported two turbo jets in addition to two big radials. Of course this is the military. They do things differently with much not applicable to civilian 121 operations.

kalvado wrote:

PerVG wrote:

Agrajag wrote:

I am thinking, in this thought exercise, about a clean sheet design concept, where optimal cruise engines are supplemented by retractable thrust to be deployed when needed i.e at takeoff.

That concept already exists as a possibility for future hybrid aircraft, were electric fans augment a cruise gas turbine during take-off/climb and recharge the batteries during descent.
It *might* became viable in the future.

As for your, 2 big + 2 small/retractable conventional, fuel-burning turbofans, I think you can forget it. It would be a maintenance nightmare, take-off engine out certification nightmare and the fact the "booster"engines would just be ballast for most of the flight would just negate any possible efficiency gains.

And again.... C-130 JATO comes to mind. Not exactly retractable turbofan, though, but takeoff assistance nontheless... I just saw a mention JATO was considered for 727 hot-and-high.
Advantage here is much smaller weight and smaller drag footprint.

‘JATO’ is often used as a term to describe
what is actually RATO or rocket assisted take off


This was installed, certified and used on the B727 aircraft mostly in MEX with the high altitude performance issues