# Is it possible to fly horizontally with zero angle of attack?

If an aircraft has a zero angle of attack and the wing doesn't, is it possible to fly horizontally? Because as the book says, there is no lift in this condition,

Is this zero angle of attack of the airframe or the aerofoil?

Can you give a simple explanation?

• depending on the airfoil profile (non-symmetric cambered ones) you can have lift also with 0 AoA
– Federico
Jan 31 '17 at 9:17
• What book? I'm pretty sure that's wrong. Also the wing angle AoA is what's important for lift, not the aircraft AoA. Jan 31 '17 at 10:59

Unless you tell us which book you read this in (and what exactly it said), it is not possible to answer your question (the answer depends on whether the zero angle of attack is that of the airframe or the airfoil (wing)). We can try for a general answer though.

As far as angle of attack is concerned, there are two kinds- geometric and absolute. First, angle of attack is the angle between some reference line and the freestream (i.e. the velocity vector). The selection of reference line determines the 'kind' of angle of attack.

Geometric angle of attack, $\alpha$ is the angle between the chord and freestream; this is the one usually used. Absolute angle of attack, $\alpha_a$ is the angle between the freestream and the zero lift line (the line, which when parallel to the freestream, produces no lift in the wing).

The geometric and absolute angles of attack can be related through $\alpha_a = \alpha - \alpha_{L=0}$, where $\alpha_{L=0}$ is the angle between the chord and zero lift line.

Relation between geometric and absolute angles of attack, image from gtae6343.wikia.com

For a cambered airfoil, the zero lift line is negative i.e. the airfoil will produce lift even when the geometric angle of attack is zero (in this condition, the absolute angle of attack is positive). For a symmetric airfoil, of course, the zero lift line coincides with the chord and as such, the airfoil produces no lift when the geometric (or absolute) angle of attack is zero.

As far as the lift is concerned, what matters is the absolute angle of attack, while we usually use the geometric one. Simply put, what matters is the absolute angle of attack of the wing. If this value is positive, lift will be produced and level flight is possible, whatever the aircraft attitude.

• You might add something about angle of incidence in case the book is referring to airframe AoA Jan 31 '17 at 18:09
• And talking about airframe AoA is rather common, because with wash-out, the wing AoA is not all that well defined. Jan 31 '17 at 18:15

Yes, with fuselage angle of attack zero. It makes a lot of sense to design the aircraft such that at cruise speed, the fuselage is aligned with the free stream so that there is minimal drag. The wing Mean Aerodynamic Chord then usually has an incidence to support enough lift to support aircraft weight, although if the wing has plenty camber, it is also possible to fly with zero MAC angle of attack.