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.
Last edited by Tinstaafl; 28th October 2003 at 21:08.