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So when I look around on the airport, I don't really see many differently shaped airplanes.
They all look pretty much the same, only subtle differences.
Why is this design so common? Why don't we have more extravagant designs?
Or am I not looking closely enough to appreciate the differences?
It seems like there could be so much potential for a variety of designs, offering more space and comfort for passengers for example... Or is it too risky to try something new?
For most transport aircraft, aerodynamic efficiency is the key parameter as it allows lower fuel consumption. The layouts you see most often are the most efficient known:
For jets low swept wings with engines mounted under and ahead of the wings and conventional tail is most efficient known layout. Tail-mounted engines used to be common on smaller jets, but as that puts lot of bulk far aft, they are worse with respect to Whitcomb area rule, worse for weight and balance, and wing-mounted engines also help damp flutter. So new regiojets are switching to engines under wing too.
Propeller aircraft fly slower, so they have straight wings. Low mounted wings are slightly more efficient, but then the plane has to have long gear to maintain sufficient ground clearance for the propellers, so high wings are more common. T-tail is then simply used to put the elevator above the most turbulent wake of wing and engines.
The only deviations from these three basic layouts are on special-purpose plane that have other more important concern. The most notable is military cargo planes are generally high-wing so they can sit low on the ground for easy loading and unloading using built-in ramp. Their marked anhedral is to avoid overstabilizing the aircraft in roll as both high wing and wing sweep increase roll stability. Earlier regiojet designs used tail-mounted engines for the same reason; to sit lower on the ground so they can be easily loaded via built-in airstairs and these are not too heavy and bulky.
The aircraft industry did the bulk of its research in the 1920s and 1940s. There were literally dozens of variations of airframe and wings. After some experimenting and theorizing (at NACA, Farnborough and elsewhere), for each design goal/market niche there emerged one dominant design that fit the payload to fuel and other constraints and minimized cost.
Pretty much in passenger airplanes was decided by the DC-3 design, and in the jet era, by the Comet.
EDIT: Jan Hudec points out two significant improvements that the Comet did not have:
As an aside, there's another widespread addition to designs - winglets.
References:
If you look around a reasonably active hub airport, you will actually see many different design choices for passenger aircraft, all being produced today:
If you look at planes optimized for flying similar distances with similar numbers of passengers (compare, for example, Airbus A320 with Boeing 737) they will have the same choices in most of these categories. That's because the choices do make a difference, and for that particular use a particular combination turns out to be economically advantageous.
However, once you look at planes with different roles, things change. A 70-seater regional such as the ATR 72 does not look like a scaled-down A320 at all.
(Except very coarsely at the level of "long narrow fuselage, one pair of main wings near its center, a group of stabilizers all the way aft, tricycle gear").
Every time a new aircraft is introduced, the marketing department of the respective manufacturer claims that now a new era of air travel has begun. Remember the claims made at the time when Boeing launched the 747: We would enjoy a big-screen cinema and a bar in the sky, and much the same (plus the option of a flying spa) happened with the A-380.
In the end, customers vote with their wallets, and airlines need to earn money, so all those extravagant options wither and everyone comes back to what works best. Aircraft design has matured, and all those claims of blended wing-body configurations will only keep marketing and the press occupied.
Add to this the constraints of existing infrastructure and excessive regulation (thanks to which we enjoy unparalleled safety in air travel, btw.), and the potential advantage of new configurations will quickly vanish. Just try to find a way to quickly evacuate one of these blended wing-body configurations with 20 seats in a row, and any claimed aerodynamic advantage (which it doesn't have) will be moot.
It's called "convergent evolution". If you have one task to perform, and that task has physical requirements, all designs will eventually look and work alike. There are many side-trips along the way and those side-trips tend to stay around servicing their corner of the species.
Examples:
Large aircraft are low wing. Exceptions are heavy cargo and short / rough field props like the Dash-8 (big props don't fit on low wings) ( a 747 is not a heavy cargo aircraft. each piece is small)
Everyone puts their engines on the wings. The ones that don't sit very low to the ground and don't have enough clearance. Next best place: the tail. Good luck hanging a Trent 900 on either side of the tail. And the elevator has to move out of the way. The number of engines you have is purely a function of needed vs. available thrust. Notice how the trijet configuration went extinct as more powerful (and reliable) engines evolved.
all pressurized aircraft have circular cross sections and rounded windows, easier to inflate. Non-pressurized planes still use lots of flat panels.
fish-type creatures use a single large fin for propulsion. True fish wiggle it side-to-side, aquatic mammals that returned to the sea wiggle it up and down, like the legs they used to have.
Things like winglets, canards and other protuberances are bolt-on fixes for aerodynamic issues discovered after the initial design was completed. They likely won't be there the next time a blank-page design is started.
The design is popular because it happens to be the best so far for subsonic large-capacity air travel. Other designs would either be less safe or have more drag, plus, there is not reason to fix what is not broken.
Boeing has flirted with different airliner designs in the past few years, but it seems that for the time being the most extravagant designs we will have will be the Boeing 787 and Airbus A380.
It would be a risk to try something new. The airframe builder's goal is to make money and they can do that by improving efficiency vs the previous generation and occupying slightly different niches in the market compared to the competition.
What are the alternatives to the tube and low mounted swept wing?
High wing. This often necessitates a T-tail which has inferior safety. With the low wing the wing and landing gear can be mounted to strong points of the airframe.
Mid wing. Fighters use this because it's more efficient but it would reduce cargo volume on an airliner.
Blended Wing and Body (BWB). This design has many passengers away from windows and presents evacuation problems. It's also easier to pressurize a sphere or cylinder than the BWB shape.
Delta wing. http://seattletimes.com/html/businesstechnology/2002973147_boeingconcepts05.html
The Boeing Honeydew concept uses a delta wing. Evidently the low speed handling/safety issue is considered too big a problem.
I have seen others answer the question of why this shape (aerodynamic efficiency) but not why it is aerodynamically efficient.
The key in producing an aircraft with high Aerodynamic efficiency(lowest drag) is lowest possible wetted area and frontal area while simultaneously generating maximum lift. Frontal area is just what you think, the lowest possible cross section that you try to push through the air. Whetted area however is the the area that gets "wet" ie the total surface area of the aircraft. This means that the best possible shape is a long cigar: the thinner the better. Any area that you add to the "tube" is wasted energy.
Simultaneously the wing shape that generates maximum lift with lowest drag is a long thin shape. This is because wings cause lift primarily by creating negative air pressure by making air flow faster over the top than the bottom. This effect is destroyed by having a wide, fat wing because the wing can't create the same tight pocket of air differential between top and bottom that a thin wing can. This is what kills Delta wings and lifting bodies.
http://www.discoverhover.org/infoinstructors/guide8.htm
http://en.wikipedia.org/wiki/Lift_(force)
Once the speed of the aircraft goes to a larger percentage of the speed of sound however other factors come into play. The wing must be swept and the wing must be stronger to handle the forces in play so it must be thicker.
The last portion is the tail in back. This is just mimicking the arrow: putting the fletching in the back of the arrow creates natural stability by making sure the front of the aircraft stays pointed forward.
There are many advantages to other designs, Canards have great stall behavior, lifting bodies have superb strength and therefore safety and tandem wings with struts have great strength and great lift characteristics. But the airline business is a high volume low margin business so any tiny bit of drag gets voted down when decisions get made.
As others have noted, the current general design of passenger aircraft is the result of decades of engineering and research. When all of the requirements are analyzed, this basic layout is the one that provides the best balance of advantages and disadvantages. The infrastructure has also been built around this configuration, which introduces another limitation that a new design would have to work around. While airports were willing to make changes in order to accommodate the A380, a completely different design would probably involve a more drastic change which would be harder to sell to airports.
Over the many years in which this design has evolved, many good lessons have been learned. Today's safety record in aviation is a result of these lessons being put into practice to improve the design. For a radical new design, much of this would have to be discarded or at least deeply reviewed. Research and testing would have to be done in many areas to ensure that the important aspects of the design are correctly understood.
Another key element is certification. In order for the FAA, EASA, etc., to grant a type certificate on an aircraft, there is a long certification process. For a completely different design, the certification would also be completely different. Not only would the designers have to be confident in the new configuration, but they would also have to convince these authorities. This would involve introducing a lot of new information. Recent new aircraft have already been taking longer and longer to certify. Introducing a completely new configuration could take even longer.
From the 30s to the 60s, the standard for transoceanic travel was a 4-engine aircraft. If you'd asked certain airlines if they'd buy a 3-engine Lockheed or Douglas airliner, they'd say "NO" because their customers would refuse to fly with less than four engines!
But gradually 3 engines became standard for oceanic travel.
And in the early 80s, ETOPS came along because engines had become extremely reliable.
So the design process is evolutionary; most changes happen pretty slowly, and designers often copy the successes of other designers.
Image: Aircraft Recognition
