The stalling characteristics of the sweptwing jet airplane can vary considerably from those of the normal straight wing airplane. The greatest difference that will be noticeable to the pilot is the lift developed vs. angle of attack. An increase in angle of attack of the straight wing produces a substantial and constantly increasing lift vector up to its maximum coefficient of lift, and soon thereafter flow separation (stall) occurs with a rapid deterioration of lift.
By contrast, the sweptwing produces a much more gradual buildup of lift with a less well-defined maximum coefficient. This less-defined peak also means that a swept wing may not have as dramatic loss of lift at angles of attack beyond its maximum lift coefficient. However, these high-lift conditions are accompanied by high drag, which results in a high rate of descent. [Figure 15-13]
The differences in the stall characteristics between a conventional straight wing/low tailplane (non T-tail) airplane and a sweptwing T-tail airplane center around two main areas.
- The basic pitching tendency of the airplane at the stall.
- Tail effectiveness in stall recovery.
On a conventional straight wing/low tailplane airplane, the weight of the airplane acts downwards forward of the lift acting upwards, producing a need for a balancing force acting downwards from the tailplane. As speed is reduced by gentle up elevator deflection, the static stability of the airplane causes a nose-down tendency. This is countered by further up elevator to keep the nose coming up and the speed decreasing. As the pitch attitude increases, the low set tail is immersed in the wing wake, which is slightly turbulent, low energy air. The accompanying aerodynamic buffeting serves as a warning of impending stall. The reduced effectiveness of the tail prevents the pilot from forcing the airplane into a deeper stall. [Figure 15-14] The conventional straight wing airplane conforms to the familiar nose-down pitching tendency at the stall and gives the entire airplane a fairly pronounced nosedown pitch. At the moment of stall, the wing wake passes more or less straight rearward and passes above the tail. The tail is now immersed in high energy air where it experiences a sharp increase in positive AOA causing upward lift. This lift then assists the nose-down pitch and decrease in wing AOA essential to stall recovery.
In a sweptwing jet with a T-tail and rear fuselage mounted engines, the two qualities that are different from its straight wing low tailplane counterpart are the pitching tendency of the airplane as the stall develops and the loss of tail effectiveness at the stall. The handling qualities down to the stall are much the same as the straight wing airplane except that the high, T-tail remains clear of the wing wake and provides little or no warning in the form of a pre-stall buffet. Also, the tail is fully effective during the speed reduction towards the stall, and remains effective even after the wing has begun to stall. This enables the pilot to drive the wing into a deeper stall at a much greater AOA.
At the stall, two distinct things happen. After the stall, the sweptwing T-tail airplane tends to pitch up rather than down, and the T-tail can become immersed in the wing wake, which is low energy turbulent air. This greatly reduces tail effectiveness and the airplane’s ability to counter the noseup pitch. Also, if the AOA increases further, the disturbed, relatively slow air behind the wing may sweep across the tail at such a large angle that the tail itself stalls. If this occurs, the pilot loses all pitch control and will be unable to lower the nose. The pitch up just after the stall is worsened by large reduction in lift and a large increase in drag, which causes a rapidly increasing descent path, thus compounding the rate of increase of the wing’s AOA. [Figure 15-15]
A slight pitch up tendency after the stall is a characteristic of a swept or tapered wings. With these types of wings, there is a tendency for the wing to develop a spanwise airflow towards the wingtip when the wing is at high angles of attack. This leads to a tendency for separation of airflow, and the subsequent stall, to occur at the wingtips first. [Figure 15-16]
In an unmodified swept wing, the tips first stall, results in a shift of the center of lift of the wing in a forward direction relative to the center of gravity of the airplane, causing a tendency for the nose to pitch up. A disadvantage of a tip first stall is that it can involve the ailerons and erode roll control. To satisfy certification criteria, airplane manufacturers may have to tailor the airfoil characteristics of a wing as it proceeds from the root to the tip so that a pilot can still maintain wings level flight with normal use of the controls. Still, more aileron will be required near stall to correct roll excursion than in normal flight, as the effectiveness of the ailerons will be reduced and feel mushy. This change in feel can be an important recognition cue that the airplane may be stalled.
As previously stated, when flying at a speed near VMD, an increase in AOA causes drag to increase faster than lift and the airplane begins to sink. It is essential to understand that this increasing sinking tendency, at a constant pitch attitude, results in a rapid increase in AOA as the flightpath becomes deflected downwards. [Figure 15-17] Furthermore, once the stall has developed and a large amount of lift has been lost, the airplane will begin to sink rapidly and this will be accompanied by a corresponding rapid increase in AOA. This is the beginning of what is termed a deep stall.
As an airplane enters a deep stall, increasing drag reduces forward speed to well below normal stall speed. The sink rate may increase to many thousands of feet per minute. It must be emphasized that this situation can occur without an excessively nose-high pitch attitude. On some airplanes, it can occur at an apparently normal pitch attitude, and it is this quality that can mislead the pilot because it appears similar to the beginning of a normal stall recovery. It can also occur at a negative pitch attitude, that is, with the nose pointing towards the ground. In such situations, it seems counterintuitive to apply the correct recovery action, which is to push forward on the pitch control to reduce the AOA, as this action will also cause the nose to point even further towards the ground. But, that is the right thing to do.
Deep stalls may be unrecoverable. Fortunately, they are easily avoided as long as published limitations are observed. On those airplanes susceptible to deep stalls (not all swept or tapered wing airplanes are), sophisticated stall warning systems such as stick shakers are standard equipment. A stick pusher, as its name implies, acts to automatically reduce the airplane’s AOA before the airplane reaches a dangerous stall condition, or it may aid in recovering the airplane from a stall if an airplane’s natural aerodynamic characteristics do so weakly.
Pilots undergoing training in jet airplanes are taught to recover at the first sign of an impending stall instead of going beyond those initial cues and into a full stall. Normally, this is indicated by aural stall warning devices or activation of the airplane’s stick shaker. Stick shakers normally activate around 107 percent of the actual stall speed. In response to a stall warning, the proper action is for the pilot to apply a nose-down input until the stall warning stops (pitch trim may be necessary). Then, the wings are rolled level, followed by adjusting thrust to return to normal flight. The elapsed time will be small between these actions, particularly at low altitude where a significant available thrust exists. It is important to understand that reducing AOA eliminates the stall, but applying thrust will allow the descent to be stopped once the wing is flying again.
At high altitudes the stall recovery technique is the same. A pilot will need to reduce the AOA by lowering the nose until the stall warning stops. However, after the AOA has been reduced to where the wing is again developing efficient lift, the airplane will still likely need to accelerate to a desired airspeed. At high altitudes where the available thrust is significantly less than at lower altitudes, the only way to achieve that acceleration is to pitch the nose downwards and use gravity. As such, several thousand feet or more of altitude loss may be needed to recover completely. The above discussion covers most airplanes; however, the stall recovery procedures for a particular make and model airplane may differ slightly, as recommended by the manufacturer, and are contained in the FAA-approved Airplane Flight Manual for that airplane.