Angle of attack (aircraft)

No, not bombing just the pitch angle of the plane relative to the flight path. I just flew across the country in an Embraer ERJ145. Not a bad plane except for the seats being somewhat narrower than my ass but I noticed that it like many other passenger jets cruises with the fuselage pitched up slightly - nose higher than the aft end - while on the ground it is slightly pitched down though not as much. I know there is a reason but I want to know why they didn’t just increase the incidence of the wing so that the fuselage is level at cruise speed. What factor is more critical to flight than keeping my diet coke from sliding backwards on my tray table?

The fuselage produces some lift itself when pitched up. If it were level, the wings would have to produce more lift, and therefore more induced drag. Fuel efficiency is absolutely critical for an airliner (fuel alone is typically a full third of the company’s entire operating costs), and that flight attitude might be the best tradeoff to maximize it. You do like those low fares more than your Diet Coke, don’t you?

The fuselage is pitch-down when on the ground so that the wing will have a negative angle of attack immediately after landing, pushing the plane onto the ground and reducing landing roll. For planes that often go into smaller fields, that’s more important than passenger comfort during boarding. It also helps taxiing stability.

Several possible reasons:

In the ERJ145’s case, it might be related to the fact that it started life as a modification of the wing-engine turboprop ERM120, and commonality of parts is a possible design goal.

In the general case, most medium passenger aircraft (and some large ones) have a cruising-level positive AoA to maximize the safety of the landing attitude–that is, when the pilots and landing gear are level to the ground, the aircraft is in slow descent by default. This allows for safer, higher-engine-power approaches to the airfield, rather than the “traditional” high AoA, low-lift, low-power approach that lowers pilot visibility and puts the aircraft much closer to the low end of the flight envelope.

Granted I don’t know for sure, but it seems likely based on the design classes I’ve had. (was aero engineer for two years before I decided I liked computers better).

Elv1slives probably hits it closer to the mark for airliners, my experience with the subject is more or less concentrated in low-power and unpowered one/two-place gliders.