a reply to: Zaphod58
I was just figuring that they were looking at the 737 class as it's the design family that's closest to the hard limit of what they can do to upgrade
it due to the engine clearance issues.
The 777 still has many years left in it, and the 787 may well have many decades left if they choose to go the 737 route with it.
Though ironically, now that the 737MAX has replaced Y1 and the 777MAX has replaced the Y3, in a couple years time, the oldest/most outdated aircraft
in Boeing's stable will be the 787!
So after making a fool of myself on the truss wing issue, I have a legitimate question. Why the high wing?
One of the reasons why the low wing has stayed so popular for jetliners is that it's a really elegant solution. For one, landing gear can be
accommodated in the wing, which allows for a wider, more stable main gear arrangement while also eliminating the need for dedicated fairings (and the
drag that they produce) in which to house the main gear.
In terms of weight, the main spar now does all the work, carrying the weight of the aircraft on the ground AND in the air, while also handling
thrust/braking loads. Compare that to a high wing design where you now need a second spar to adequately spread the main gear and handle their
rolling/braking forces when on the ground, while the wing has to handle all those same forces in the air.
Furthermore, the low wing design does wonders for cabin environment, effectively shielding the passenger area from the engines minimizing cabin noise.
Compare that to a high-wing design, where there will be a night and day difference in cabin noise for passengers who are ahead vs behind of the
engines, unless airlines are willing to pay a major weight/cabin width penalty due to acoustic insulation.
Finally, I'd imagine that there could be a maintenance/safety penalty to pay for the high-wing truss-braced design. Accidents happen, and jetliners
are constantly bumping into stuff on the tarmac. With a low-wing design, the non-structural aerodynamic elements like leading edges, engine intakes,
and control surfaces serve almost like crumple zones to keep all of the critical stuff (spars, engines, fuel tanks) protected when someone makes a
whoops and drives a tug/baggage train/food truck/other airplane into a parked or moving airliner. Now, with this high-wing truss-braced design, the
lowest hanging element is now critical structural element that is essentially unprotected to minimize drag. On a presumably composite design where
repairing it, much less replacing it, will be a relative nightmare compared to doing the same on a Cessna, Skyvan, or Twin Otter. Right now, a
baggage van driving into the wing means a lot of incident reports and a day or two worth of sheet metal repairs. On a high-mounted truss-braced wing,
it could mean taking an aircraft out of service for a month or more while the truss spar is replaced.
With that all being the case, might it be more likely that we will instead see an upside-down truss-braced wing with significant dihedral, where the
engines hang below the wing to shield them from passengers, the main gear are mounted to the wing spar, and the truss spars now reach upwards to mount
to the top of the fuselage tube? I'd imagine that such a design would also save you the extra weight of that T-tail, since a conventional tail would
now be extended into essentially clean air.
Airliners would now all look like giant Cessna 188s, but it seems like it would solve a bunch of potential design problems.