# Is it correct to say “the up-elevator position decreases the camber of the elevator”?

Aft movement of the control column deflects the trailing edge of the elevator surface up. This is usually referred to as the up-elevator position. The up-elevator position decreases the camber of the elevator (my emphasis) and creates a downward aerodynamic force, which is greater than the normal tail-down force that exists in straight-andlevel flight. The overall effect causes the tail of the aircraft to move down and the nose to pitch up. PHAK 6-5

This is an excerpt from PHAK on the elevator. Do you agree with the statement "the up-elevator position decreases the camber of the elevator"? I've always thought the camber of control surfaces indicates their curvature, which is designed by aircraft designers or controlled by pilots using high-lift devices, and it is the angle of attack that that changes when pilots give control inputs. So isn't it more correct to say "The up-elevator position decreases the angle of attack of the elevator"?

• @mins The author of the handbook confused the term "elevator" (= moving part) with "empennage" (= the whole shebang). The correct sentence is of course: The up-elevator position decreases the camber of the empennage. – Peter Kämpf Oct 3 '17 at 22:33

You're absolutely correct, camber equals curvature. The camber of the horizontail tail is changed by deflecting the elevator, not the camber of the elevator itself. And yes the local angle of attack changes when the elevator deflects, resulting in an aerodynamic moment.

Both of the approaches are correct.

The camber is a basic geometric concept, useful for understanding flow around airfoils. The airfoil can be the wing, and the deflected trailing edge can be the aileron, flap, elevator, or rudder, etc.

The angle of attack description, that the aoa decreases with up elevator, is also correct. In fact, many basic calculations and preliminary aircraft sizing includes empirical correlations of Deflection to AoA ratio.

Deflection of a trailing edge surface, effectively (though not physically) alters the flow such that the AOA of the undeflected airfoil is increased or decreased, somewhat proportionally.

DATCOM and ESDU are some resources that include empirical approaches to these approaches.

However, you're talking about the stabiliser/elevator, which is an INVERTED airfoil. So elevator-up INCREASES the camber of the airfoil, which generates more (inverted) lift, and forces the tail downwards.