Originally Posted by
Hussar 54
A bit off thread but.....
In these days of super sophisticated computers and software producing most of the structural design and theoretical testing of performance at this design stage, I've always wondered why all aircraft don't end up being identical....
For much the same reason as, if you were to ask ten different people for directions on how to get from A to B, you may get up to ten different answers. Because the answer isn't just a function of the question, but of how the person answering understands the question.
Let's say a design team is given the objective: design the best short-haul airliner you can. What is "best"? Most fuel efficient? fastest? Most pax? Fastest to turn around? Most reliable? Safest? Cheapest? How a given team balances all those competing criteria biases design choices. And there is no right answer to the question of balance - because there will be a customer who puts each individual item first.
Do you want an airliner that can do LCY? If I'm Southwest Airlines, no, of course not. If I'm BA, maybe it's my #1 priority?
The reason there are many different designs, is that there are many different design objectives. (Indeed even with identical design objectives, the exact same end result is unlikely. X-32 and X-35 were designed for the same contract, after all... )
To answer the specifics.
Why would the wing for the 787 and A350 not be exactly the same other than for size/scale differences ?
Ah, but WHICH scale difference? You can't just scale everything, even if it was the optimum. Materials come in finite sizes, as do fasteners and the like. Do you scale to keep constant wing loading? Or scale lengths (to keep cg loadability the same)? A thinner wing, in absolute terms, may be more susceptible to ice catch. Boundary layer issues arise.
Why would the location and length of the pylons ( relative to the size of the wing ) not be exactly the same ?
Do you have a "rubber engine"? You likely don't have that degree of flexibility.
Why wouldn't the airframe's ribs and spars and the profile/shape of the v/s be exactly the same, again allowing for size/scale ?
Again, not everything CAn scale, and how shall we scale? Cabin deltaP is the same - some structure may need to be identical, not scaled. The doors have to be the same (or similar) size.
And why were some aircraft conceived, designed and built with underwing mounted engines and others with fuselage mounted engines and T-Tails ?
And others designed with high wing construction rather than conventional low wing construction ? Presumably one method or combination of the these alternatives produces the better aerodynamics compared to the other(s) ?
No. Each configuration has good and bad points. The role of the aircraft may determine which is the best COMPROMISE. There is no absolute right answer.