Why do all airliners look alike?

[Say again s l o w l y]

Why is it that most current airliners are just variations on a theme? Mid wing, podded engines, cruise around M0.8.

Would airlines (and pax) be willing to operate radical designs. I'm specifically talking about oblique wing designs (sometimes known as Slew or Scissor wings)
The potential benefits of oblique wings are weight, fuel consumption and most of all speed. They have been designed so that over land they will have a mid sweep(30deg) and cruise at M1.2-1.3 at 36,000ft, at which level the over pressure is not great enough for the shock wave to reach the ground. The over water the sweep is increased and cruise increased to M1.4-1.5.

This is very technically feasible, but is it realistic?

Why aren't commercial aircraft high wing like military transports such as the C-17 and various Russian beasts? Surely it is a more efficient structure having a full span wing as well as not having to have a whacking great spar box in the cabin.

Good website about this (http://adg.stanford.edu/aa241/intro/futureac.html)

[noisy]

It seems to me that modern airliners are optimised for the carriage of passengers and their luggage, sometimes even to the same airport. As such, given the constraints of aerodynamics they will all start to look the same. Also, as you have touched on, it must be acceptable to the passengers who will be the end consumers.

I personally think that airline execs are unimaginative people who wouldn't be interested in anything unusual before someone else has proven it is profitable.

[Say again s l o w l y]

I'm not sure that airliners are optimised for anything really. They seem to be a mish mash of design ideas, that nobody has the bottle to break out of. If they did, then the current ideal would be made to look very old very quickly.
A rounded fuesleage cross section is not really any good for interior space or load lugging. Mind you the only square section a/c I know of is the mighty Shorts 3/30-60 so maybe not the best example!!

[noisy]

Airliners are optimised for making money.

Also, if you remember your primary school maths you will know that a circle encloses the largest area of any shape. :rolleyes:
However, I do agree that this is a far from ideal shape for a fuselage cross section in terms of operation and comfort. I'd say it's an aerodynamic thing.

In modern airliners the below floor area is used for carrying luggage: what struck me when I went aboard a part-stripped 747 was the massive space above the false ceiling.

I wonder if a compromise could be reached such as the oblong fuselage shape of the Avro Lancaster/Lancastrian? Anyway, people seem to be talking about blended wings. (But not doing anything about it.)

[Say again s l o w l y]

What we need is useable area, not just a large total volume. I understand the greatest current problem woud be to design a feasible pressure vessel in any other shape.

The vast majority of cost savings over the previous 40 years has come from the engine design NOT the aerodynamics, why don't the chaps at Boeing and Airbus keep up with the amazing progress of the engine manufacturers. Or even try!

Isn't the A380 an oblong section?

[FlyingForFun]

What struck me when I went aboard a part-stripped 747 was the massive space above the false ceilingI can't help wondering if you're thinking about this the wrong way around?

If I were designing an airliner cabin from scratch (which I'm not, because I'm not in the slightest bit qualified to do so!) then I might be inclined to start off building a shape which was comfortable. It would have to have flat floors so people could walk on them, and a flat-ish ceiling because that's what pax expect.

Then, I'd build around that basic box shape to make it aerodynamic. So if a flat ceiling is too draggy, I'd add a rounded section on top of it. After all, I'm not space-constrained in this direction, am I?

After that, I'd think about construction, and I might realise that it's cheape to build my design with a false ceiling. The false ceiling may make it appear that a box shape has been artificially constructed within the constraints of the fuselage, but actually it's the other way around.

I have no idea how accurate this is, it's just a discussion point. I'm not even sure if it's relevant!

FFF
---------------

[noisy]

Well in the 747 it appears that the air conditioning system lives above the false ceiling, but there's still a lot of empty space up there.
And, yes the A380 does have a kind of egg shaped cross section-I'd forgotten about that.

It's just that in the DH Comet, the below floor/belly area doesn't appear to be used for anything (fuel?), whilst the cabin goes to the full height-so there's no set cross section.
Anyway, where were we? Propulsion?

[Tinstaafl]

Right from when someone says "Build me an aeroplane" and a designer replies "What would you like it to do?" they're the epitome of compromise.

My thoughts:

Circular fuselage is structurally strong to contain pressurisation loads whilst being light and also provides good internal volume. How would you have a strong but light structure in a blended wing to contain the pressurisation loads considering the relatively large surface area:volume ratio? Oval is a compromise that increases wetted area for the volume contained.

What are the options for engine position?

Burried in the wing root makes for structural complexity & weight to route the load path around the engine(s) and also some engine access difficulties. Has the advantage of relatively low asymmetric moments.

Fin mounted also gives similar effects as wing root mounted re load paths. Also more difficult to get to compared to podded underwing. Aft fuselage eg B727 means extra routing for the intakes and some additional access problems. How many will fit? Good for asymmetric considerations though. Also some drag benefits.

Podded rear fuselage mounted: relatively easy to access - but not quite so easy as podded underwing. What if more than two engines are required? The inner mounted is less easy to get to. How to prevent an uncontained failure of one affecting the one next to it? Ditto the asymmetric benefits.

Where else to put them? On top of the fuselage? What about airflow disruption at higher AoA? Underneath? Bit awkward for ground clearance & prone to FOD. Front side of the fuselage? Possible but what compelling advantage is there compared to the pros & minimal cons of podded underwing?

Wing tips? HUGE asymmetric yaw moments. Not desirable at all ==> huge rudder needed ==> what about stability effects?

Podded underwing have the benefits of easy access for maintenance, separates & isolates the engines in case of failure, simplifies some intake considerations by reducing some interference effects and also provides wing bending relief which allows a lighter wing structure. FOD can be a problem as can finding anough clearance under the wing to fit them. Look at the changes on the B737. Low bypass/small fan fitted under the wing OK but when the larger fan engines were fitted they had to be mounted far enough forward of the wing for the fan to be clear. That let the engine be mounted higher to maintain ground clearance. Still had to flatten the bottom of the air intake though. Bit of a problem constraining aerodymic engine/wing interactions though so certainly not perfect.

They could have designed longer undercarriage but imagine the structural ramifications: Where to fit the longer legs after retraction? Major redesign of the fuselage & wing structure would be required. May as well design an all new aircraft. Hmmm.......B757 perhaps?


'Conventional' forward mainplane/aft stabiliser is relatively robust & allows lots of options for wing modification devices for different purposes eg high speed 'clean', low speed/high lift for take off/landing. Canard designs are not so easy to do these things. The canard can also be relatively sensitive to surface deformation/contamination.

Forward sweep offers aerodynamic benefits as well as getting the main spars further aft (giving a larger uninterupted cabin volume) BUT have problems with getting sufficient stiffness to prevent the tip from twisting up or down.

Delta wings have some advantages but look at the trade offs used in the Concorde.

Swing wings add structural & mechanical complexity. Add in the need to keep any pod mounted items aligned correctly.

...and so it goes. Conventional design offers relative simplicity & robustness cheaply.

[Say again s l o w l y]

With the size of some current engines (trent's for example) there are huge issues with ground clearance.

I agree that maintenance is an issue, but it is not a be all and end all problem.

How about engines mounted in the rear of the fueselage, behind the pressure bulkhead.

Aerodynamicists have huge problems with podded engines, specifically in the 737 with the first CFM engines, the flow interaction between the engines and body caused all sorts of issues and took months to solve satisfactorily.

With modern materials and techniques, I don't feel that swing wing a/c are that much of an issue. How much extra weight and complexity goes into a wing for all the high lift devices, engine bits and bobs, fuel tanks etc etc.. That is a seiously compromised design. Airbus are doing O.K in this field, just have a look at the flaps on an A320 compared to a 73. Far less complex, but still pretty effective.

With a simple pivot on the top of the body and engines in the rear and the whole structure comprised of different composites, the a/c could be light years ahead of current macines in terms of weight and performance in both high speed and low speed regimes.

I feel we as pilots should start to think 'outside of the box' in regard to aircraft design. When the potental benefits of new ideas come to light, then airlines will be clamouring for them, just look what happened when jets came along. I feel its about time we took another quantum leap.

[noisy]

Go unto Hendon or Cosford and look at the Tornado. See how the swing wing influences the rest of the design. I think swing wings have too much of a weight penalty for a civil a/c.

[Say again s l o w l y]

I don't mean a swing wing in the traditional sense, they are far to complex, mind you they worked OK in the F14.

I'm trying to find a link to the design that I mean, but until then I'll descibe it.

Long fuselage, at about 2/3 from the nose imagine a larger version of a spitfire wing (elliptical) stuck to the top of the fuselage by a pivot abot which it would rotate. One side would be projected forward and the other back in the same way as if you held one side of a pair of scissors and opened and closed with the freely moving part of the scissors.

NASA Ames have built and flown a small version of this and the handling characteristics were superb as well as stability, it was only tested in the low speed envelope, below 250kts, but is a very promising idea, even Boeing are supposedly looking into the concept!! (not sure about at the moment as the bean counters seem to be in firm charge currently)

[noisy]

Oh! that kind of swing wing. More like a pivot. I'm sure it could be made to work, but you're going to have a huge load bearing hinge somewhere in the cabin. Are the airlines going to pay for the cost though?

[BEagle]

Only since 24 October 2003 have all airliners looked the same....:{ :{ :{

[Say again s l o w l y]

Found a photo of the NASA technology demonstrator.
NASA AD-1 (http://www.aeronautx.net/Historix/content/hist_1003.htm) and of the airliner design by the same engineers, second design down.

BEagle, Know what you mean:{ :{Oblique wing airliners (http://www.geocities.com/asymmetrics/ow.htm)

[chock2chock]

When an Aircraft is designed, I believe that aesthetics might be an impoertant aspect. These huge things must also wear a the airline's livery. Remember when the 757/767 was stlill on the drawing board (7N7 at the time), remember what rhey looked like? I'm sure Boeing that when Boeing proposed a design they would have designed something just as efficient, if not more as what finally rolled off the factory floor. It was when this was proposed to most airlines I guess they liked the numbers, and not so much how they looked. I belive the same thing happened with the 777, I remember seeing a drawing of something like a 767 with some kind of an upper deck:yuk:

I belive what also must be considred is that airplane companies usually try to offer some kind of continuity in design; a family of models which they would also want to produce at minimum penalties in manufacturing processes- Desiging something too radically different would probbly require too much investment in manufacturing. As we all know, airlines benefeit from operating a family of aircraft be it from an engineering standpoint or from a crewing one.

[Say again s l o w l y]

Blimey no wonder the shape of aviation hasn't changed since the '50s. With all the negative feedback and no one willing to even look at concept is it hardly surprising that aviation design is virtually stagnant.

Unfortunately it seems we need another war to kick start some innovation in large aircraft.

Long live the Rutan brothers!!!

[Mac the Knife]

Presumably for the same reason that most of today's small cars look the same.

Convergent evolution towards an optimum solution.

[Say again s l o w l y]

Mac, sorry but I simply don't think that is accurate.
The design of the car has been held back for decades by the oil producing companies, without a manky old ic engine, the designs of modern cars would look totally different. What you suggest is an optimum solution for current technology, I'm banging on about getting new technology out of the research centres and into some proper use.

It's a bit arrogant to believe that we have already reached the pinnacle of design excellence. Look back 10 years and I'll bet you'll have somebody else making the same argument, and things have changed since then haven't they.

I'm not talking about an evolution in design, that's just lazy design and engineering. I'm waiting for a complete revolution in ideas, aviation was always at the forefront of technical revolution, but it has become so tightly screwed by the bottom line that there is seemingly no room for 'blue sky' thinking. (n.b why do you think that term is as it is?)

Pilots are often fairly conservative types, but I am hoping for a bit 'pioneer spirit' amongst people on this board, or have the bean counters already won?:(

What types of aircraft would we like to see flying in the future? Assuming technical issues will be overcome. What sort of time frame do you think is even in the remotest part feasible? What would manufacturers have to do to make airlines sit up and take note?

[PAXboy]

Firstly - I'm not in the biz, as the name says I sit somewhere down the back.

Blimey no wonder the shape of aviation hasn't changed since the '50s. With all the negative feedback and no one willing to even look at concept is it hardly surprising that aviation design is virtually stagnant. There are many reasons for this and the first three are Money , Money and Money. The cost of development means that only consortia can really put the funding together. You then have the problem of consensus. The only point on which companies (i.e. individuals) will agree, is that they want to make money. This is for the simple point that, if they do not, no one will have jobs any more!

Boeing 'bet the farm' on the 747 and they won, but the machine was an enlarged 707. Sure, there were many, many differences but the basic profile of circular fuselage, wing + podded engines, was the same. What they managed was the leap in size to wide body and so forth. Boeing nearly closed because of that. No one is going to make that kind of decision again.

I am sure that there are some new designs waiting to be hatched but they will not emerge from commercial aircraft companies.

Where might they emerge from? The military do not have the budgets for 'blue skies' ideas that they had 20 years ago. They have to concentrate on getting bombs to specific points and (almost) all development is focused on that. Getting the bombs to the place of explosion is to be on unmanned devices wherever possible.

There is no market for a new Concorde, there is no market for anything that will not get it's money back without fail. It is said that a new commercial jet transport, has to sell at least 300 hulls to get back it's development costs (I sit to be corrected on that number).

[Say again s l o w l y]

Unfortunately that will always be the case, especially in the current climate. What I forsee is a time when the passengers and airlines will eventually want something different, speed being key. This isn't about now, but 30 years time.

I don't think that any revolution will be led by the large established manufacturers, they have too much tied into the current way of thinking. GA and specifically biz jets are going to provide the next leap as more speed becomes a quest again.
Look at what the Rutans are doing. They are showing what can be done, one day it may trickle down.

This may be controversial, but I can envisage the Chinese trying to build something totally different, they've just gone into space for no reason I can see other than national pride. This kind of thing might be their bag.

I always thought it was between 400-500 hulls to break even.

[Torquelink]

Aircraft today do look similar for reasons discussed but, of course, under the skin they are very different to their forebears in terms of materials, systems and technology. Even though e.g. the wings look similar, today's wings are hugely more efficient than those of earlier generations due to advances in computational fluid dynamics among other things. But even if there is a revolutiionary technology out there which will transform economics it is unlikely that any manufacturer will take the risk without some kind of government support - either direct or through the military. The B707 was, of course, a development of the military-sponsored KC135 and even the B747 design was part paid for by the US Goverment during the competition which ws eventually won by the C5A. The Blended Wing Body (BWB) seems to offer huge efficiencies but the risks involved (technical and pax acceptance) are enormous. If the military decided to adopt it for their next generation transport, it would ease the path into eventual commercial service. The Rutans are great free thinkers but, on the other hand, it was Burt Rutan who designed the Beech Starship!

Will there ever be another supersonic / transonic transport or will the Sonic Cruiser / 7E7 phenomenon always come into play i.e. airlines will always want the technology to reduce costs rather than increase speed?

[Unwell_Raptor]

If you ask ten computers similar questions you will get ten similar answers.

It is so bad in the car market that designers are having to go to great lengths to differentiate their products - from the downright eccentric (Renault?) to the trend for badges and logos to get bigger and bigger.

[Say again s l o w l y]

The sonic cruiser was nothing but a smoke screen. I don't think Boeing ever looked at it in any real sense. The speed differential was never enough to convince anyone to move from current designs.

CFD is a great tool, but it is in no way perfect yet. Why do manufacturers still spend enormous amounts of time in the wind tunnels and water tunnels? I believe that for the 777 they spent 10x as long in the wind tunnel than they did with the 747.

Seat costs have dropped by a factor of 3 since the inception of the 707, a pretty big advance, but as stated before that is mainly due to the engines and not the airframes themselves. Sure they are a bit better, but the materials used are fundamentally the same and apart from tweaking so are the aerodynamics.

A blended wing body has some pretty serious technical issues to overcome, but I can't see that is totally unfeasible. The same with Slew wing machines, yes the research may have been done in 1982 but the principle is sound. For example the fuel cell was designed in the 50's, but that is seen as the 'latest solution' to overcome piston engines in cars.

[Torquelink]

Say Again

Take your point on CFD/wind tunnel testing but iro Sonic Cruiser I heard that, but for 9/11 and subsequent events, all the internal signs were that Boeing would launch with a good spread of domestic and international launch customers. Post 9/11 all attention went back to stopping bleeding red ink and this has impacted everyone's evaluation of what's important in future aircraft. Maybe if (big if) we had a few years of good traffic, improving yields and no traumatic events the focus would shift again to differentiating the product for high-yield traffic by offering more speed.

[Say again s l o w l y]

I really believe that with a few years of 'normality' we may get back to different desires about a/c than purely cost driven solutions.
The industry has been savaged over the past 2 1/2 years and I don't think it will ever be quite the same again, but I hope that some day we may get some of the spark back.

[PAXboy]

I'd like to think that we will get back to normality but I suspect that it will be a 'new' normality!

In the 1950s and 60s, they thought that there would be much more leisure time and pondered what we would do with the time liberaterd by machines. Huh? They thought that we would have paperless offices. Huh?

There must be many more examples ... so I woul dnot try and imagine what life wil be like in 30 years time. Think to how it was 30 years ago and how we believed it would be now?

Result? More big sausages with wings, proceeding through the air at a steady pace.

[steamchicken]

Both that NASA test plane and the Handley Page SST design are utterly beautiful. Does that make me a pervert?

[Weight and Balance]

Say Again Slowly wrote:

“I really believe that with a few years of 'normality' we may get back to different desires about a/c than purely cost driven solutions.”

I'm not sure how you can separate "desires" and "cost" when you talk about large aircraft development. Sure, your desire can be based on non-profit things like aesthetics or innovation, but you will still have to spend the big bucks to build the thing. Unless you pay for it all out of your left-over cash, you will have to have a method of recouping the development costs. Since all airlines today use basically the same methods of financing, the airliner manufacturers tend to all come up with basically the same solutions. Ones that (we hope) will let the airlines and the manufacturers make enough money to pay off the bank.

There is an interesting new trend, however. We are seeing a very small number of ultra-rich individuals who can spend the big bucks, for totally “non-profit-making”, personal reasons. For example, look at space tourists, the people building replica antique aircraft and cars, and the people lining up to ride the x-plane contestants. Maybe, someday, one of these big spenders will build revolutionary airliners (or electric cars, or whatever your pet project is). And maybe, just maybe, some of these pet projects will turn out to be practical in ways nobody expected.

In the mean time, committees of bankers and accountants will force the committees of engineers to turn out similar products. However, I have to totally disagree with the premise that this means there is no advancement. Instead, it means we all advance at roughly the same rate, and in roughly the same directions. If one day some radical spender actually proves swing wings (or whatever) are better at making money, all new airliners will very quickly adopt the new technology.