Plane & Pilot
Tuesday, July 9, 2013

The Reality Of Deep Stalls

Stalls are simply stalls, except when they’re not

Contrary to popular belief, there are a significant number of general aviation models that fly beneath tails in T-formation. At one time back in the late '70s and early '80s, it was considered stylish to design the vertical tail with horizontal stabilizer mounted high across the top.

The obvious downside was higher bending loads on the vertical stab that dictated structural beef-up, often adding weight. Several manufacturers tried T-tails and liked the modern, updated look of the airplane, even if there was little aerodynamic benefit. That's not to suggest there aren't some very real advantages to T-tails, but just like winglets, stall strips and leading edge slots, they don't work on all airplanes.

Among the Pipers, T-tails included the Tomahawk, Arrow IV, Lance II and Seminole, plus the turboprop Cheyenne III and 400LS. (Today, the Seminole is the only Piper that remains T-configured.) Beech had the Skipper, Duchess and most of the King Air series; Diamond has the C1 Eclipse, DA40 Star and DA42 Twin Star; Piaggio has the P-180 Avanti; Pilatus has the PC-12 and, depending upon how you count them, you could add at least another half-dozen models to the list.

Canard-equipped airplanes that don't sport T-tails also may be susceptible to deep stalls by the very nature of their design. Models that fly behind canards typically enjoy a stall-immune existence—most of the time.

Designers configure the canard to meet the relative wind at a slightly higher angle of attack than the following wing. This means, by definition, the canard will always stall first, the nose will drop, and the wing will never be able to reach its critical AOA. Beware of the word "never." If the pilot does induce a high pitch attitude that stalls the wing, it may be extremely difficult to recover as the canard is even more deeply stalled.

Stall Aerodynamics
In order to understand deep stalls, we need to review normal stalls. As angle of attack increases, the wing assumes more and more lift, but it also begins to pick up additional drag. As AOA increases further, both lift and drag increase, but the drag curve begins to catch up with the lift curve as more airflow separates from the top surface of the wing and flow becomes progressively more turbulent. When drag finally exceeds lift and the airfoil reaches its critical angle of attack, the wing finally stalls and pitches forward. At least, that how it's supposed to work.

Most aircraft wings stall at or below 20 degrees angle of attack. Deep stalls can occur when the airfoil is forced into an attitude greater than its critical AOA. Depending upon wing location, this can place the T-tail in the airflow shadow of the wing, effectively blanking the tail and significantly reducing elevator effectiveness. With improper management, especially in certain CG situations, this can result in a stabilized deep stall. (It's also possible to experience a deep stall in a low-tail airplane, but that's an extremely rare event.)


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