Cruise AOA varies significantly
I agree completely with the reply made by HazelNuts39. Let me offer a little more explanation.
A key point that HazelNuts39 points to is the AOA reference being the fuselage. AOA is the difference between pitch attitude and flight path angle. In steady cruise flight, the passenger cabin deck angle will match the AOA. I'm sure most SLF have noticed that during cruise the front of the passenger cabin is slightly uphill when compared with the back. For long range flights, very observant SLF will note that the deck angle is greater at the beginning of cruise when the airplane is heavy (lots of fuel) vs. at the end of cruise when the airplane is lighter (not much fuel left). This will be slightly offset by increases in altitude (and thus decreases in equivalent airspeed at constant Mach number) that are typical as the cruise progresses.
A very important parameter in the design of an airplane is the relative rotation of the wing as mounted on the fuselage. With the fuselage at zero AOA, the wing could be set such that it sees positive AOA and thus generates significant lift even though the fuselage (and thus the airplane) AOA is zero. Wing rotation angle with respect to the fuselage has a significant impact on cruise drag. The best arrangement for minimum drag usually will have the cruise fuselage AOA in the neighborhood of +2 deg.
Earlier entries in this thread have suggested that zero lift occurs at an AOA that is well below 0. For most modern commercial transports the flaps up cruise zero lift AOA is very close to 0 deg. With the wing's high lift devices deployed for lower speed flight, the wing geometry usually changes such that the trailing edge is lowered, effectively rotating the wing relative to the fuselage. As a result, at landing flaps the zero lift AOA will typically be several degrees below 0. Approach and landing touchdown pitch attitude are important considerations that play into configuration definition and landing reference speed selection.
The bottom line is that cruise AOA will vary with all of the parameters that HazelNuts39 referenced above. The following changes will tend to increase cruise AOA:
- increased weight
- reduced speed
- increased altitude at a given Mach number
- more forward cg