Stall Warning System
Introduction
Although the Grummans typically give a good tactile warning when a stall is imminent, the stall horn adds an additional aural warning. For those that have not noticed, the tactile warning is the shaking of the yoke due to wing wake impingement on the horizontal tail near and while in the stalled condition. In this article, the applicable regulations are presented, basic theory, why preflight checks of the stall horn sometimes fail, adjustments, and general troubleshooting.
Applicable Regulations
14 CFR 23.207 states that
- (a) There must be a clear and distinctive stall warning
- (b) The stall warning may be furnished either through the inherent aerodynamic qualities of the airplane or by a device that will give clearly distinguishable indications under expected conditions of flight
- (c) ... the stall warning must begin at a speed exceeding the stalling speed by a margin of not less than 5 knots and must continue until the stall occurs.
Because of the tactile feedback due to the aerodynamic qualities of the stalls in Grummans, an audible stall warning may not be technically required. However, the Grummans were certificated with the stall horn.
Basic Theory
The images to the right show the streamlines around an airfoil at progressively higher angles-of-attack. However, they do not show the separated streamlines typical of a stall. Notice that there is a streamline that terminates near the nose of the airfoil. The point of termination is called the stagnation point because this is where the flow reaches zero speed. More to the point, any streamlines above the stagnation streamline flow above the airfoil and any below the stagnation streamline flow below the airfoil.
Stall warning devices take advantage of this phenomenon. On the Grummans, a small tab sticks out of the leading edge. For normal angles-of-attack, the tab is positioned so that it is located below the stagnation streamline so that the normal flow forces the tab down and keeps the stall horn switch in the open position. As the angle-of-attack increases to stall, the stagnation point moves below the tab, and the streamlines act to lift the tab and close the switch thus activating the stall horn. Cessnas use a pressure activated switch rather than a tab but basically measure the location of the stagnation point in a similar way.
Preflight Checks
To check the condition of the stall horn on Grummans, the master switch must be on. For some models of Grummans, the stall horn is designed to have a several second delay ostensibly to avoid spurious activation of the horn near but above stall. Some have preflighted their stall horn and determined that it does not work without knowing about the several second delay.
Troublshooting
To be completed ...
