As we saw last time, some airplanes exhibit degraded pitch stability and a tendency to pitch up as the angle of attack increases up to and beyond the point where the stall begins. This is never a desirable characteristic, and if it is severe enough it can be extremely dangerous since the airplane will pitch up spontaneously and may enter an unrecoverable deep stall.
There are two primary aerodynamic causes for this kind of stability change at high angle of attack.
Wing/Body Lift Shift
Stall effects can sometimes increase lift forward of the CG and decrease it on surfaces farther aft. This moves the total lift forward, producing a nose-up change in pitching moment. How serious this is depends on the magnitude of the change in wing/body pitching moment and whether the tail can still stabilize the airplane sufficiently and produce enough nose-down moment to recover.
Tail Effects
The tail surfaces of an airplane operate in the flow field behind the wing. The way the wake of the wing affects the tail can change both the stabilizing influence of the horizontal tail and the control power of the elevator. Interaction with the wake of the wing can affect both pitch stability and the ability of the elevators to drive the nose down to recover from high angle of attack.
If early flight testing reveals a pitch-up tendency or a lightening of pitch forces with increasing angle of attack, the first step in addressing the problem is to determine which of these aerodynamic effects is causing the problem. Depending on the configuration of the airplane, it can be either one or both in conjunction.
For straight-winged general aviation configurations it is more likely that nonlinear behavior near the stall will be primarily due to the interaction of the tail with the wing wake, so we will turn our attention there first. Both stability change and pitch-down recovery elevator power are concerns.
The significant parameters that determine how the tail interacts with the wing wake are the geometry of the tail and the relative positions of the tail and the wing. The vertical position of the horizontal tail relative to the wing is most important.
Configurations where the tail is significantly above the wing, particularly T-tails and V-tails, are more likely to have stability and control issues at high angles of attack. These high-tail or mid-high-tail configurations sometimes suffer from significant nonlinearity in pitch characteristics.
At low angle of attack, the wing downwash is relatively low and the tail is above the majority of the downwash. As angle of attack increases two things happen. The downwash angle in the wake behind the wing increases, and the horizontal tail moves down relative to the oncoming flow, into the downwash behind the wing.
The increased downwash produces a negative angle of attack increment on the horizontal tail. This negative angle of attack change on the tail produces a nose-up pitching moment increment.
The result of this is that the airplane loses pitch stability as angle of attack increases, resulting in lightening of pitch stick force and in some cases an uncommanded pitch-up.
As the wing starts to stall, the horizontal tail may end up in the turbulent wake behind the stalled wing. The separated wake has less energy, and therefore the effective airspeed at the tail is decreased. The loss of effective airspeed diminishes both the ability of the tail to stabilize the airplane and the control power available from the elevators to push the nose down and recover.
By contrast, when a low-mounted tail moves down relative to the wing with increasing angle of attack it will move farther away from the downwash of the wing and will be below the separated wake when the wing stalls. Accordingly, a low tail is more likely to retain both its control power and its stabilizing influence.
Depending on the constraints on the design and the severity of the problem, there are several approaches that have been implemented successfully to alleviate the problem of tail-induced pitch-up or stability degradation.
Enlarging the Tail
If the nonlinearity in pitch is relatively mild and the tail does not become fully immersed in a turbulent wing wake, one solution may be to increase the size of the horizontal tail. Although this will not necessarily make the characteristics of the airplane more linear, it will increase the baseline stability so that the loss of stability at high angle of attack is less of a concern. The larger tail will also have more elevator control power, improving its ability to push the nose down and break a stall.
This approach has been successful on multiple airplanes, including several single-engine production airplanes that sprouted T-tails in the late 1970s and early 1980s. At the time, the idea was that having the tail up above the wing downwash would improve the tail’s ability to stabilize and therefore allow the horizontal tail to be smaller, thus reducing drag. In practice, the airplanes had nonlinear pitch forces, and the smaller horizontal tales were deficient in pitch control power. It was necessary to enlarge the tail to make the airplanes acceptable, and several types started out with low-mounted tails, were produced with T-tails for a few years, and reverted to low tails later in production.
Another example is the Douglas DC-9 airliner, which was initially designed with a tail optimized for minimum drag. Before the prototype DC-9 flew, a BAC 111 was lost in a deep stall accident. Douglas designers reexamined their data and realized that the DC-9 could be vulnerable to a similar accident under extreme conditions. They redesigned the tail, making the horizontal tail and elevators larger before the airplane flew.
Aft Body Strakes
Enlarging the horizontal tail on a high-tail configuration requires a significant redesign of the tail and the structure it attaches to. Any production tooling for the original tail must also be scrapped and replaced with tooling for the new tail. This can be quite expensive, and the structural changes can add significant weight.
Another approach is to add additional aerodynamic surface to produce stabilizing forces at high angles of attack. The auxiliary surfaces can clean up the pitching moment characteristics of the airplane at higher angles of attack sufficiently to give the airplane satisfactory flying qualities while retaining its original tail. Low-aspect-ratio horizontal or canted strakes mounted to the aft fuselage are often used to improve longitudinal stability at high angle of attack.
Lift on the strakes produces a stabilizing nose-down pitching moment increment as angle of attack increases, and this increment persists and increases as angle of attack increases up to and beyond the angle of attack at which the wing stalls. They are particularly effective at higher angles of attack because instead of stalling, they generate strong vortices that produce lift at angles of attack well above the stall angle of attack of the wing.
Aft-body strakes are common on business jets and some other T-tailed and V-tailed airplanes. Among these are the Lear 60, Cessna Citation Mustang, Piaggio Avanti, and Cirrus Vision Jet.
The Vision Jet actually takes the concept a step beyond being a passive stabilizing device. It has control surfaces on the trailing edge of the strakes that work along with the V-tail ruddervators to improve the controllability of the airplane.
Tail Configuration Change
If enlarging the tail or adding auxiliary surfaces like strakes does not work or is not viable because of other considerations, more comprehensive changes to the configuration of the tail might be necessary.
Since the primary cause of the pitch stability issue is often interaction between a high-mounted tail and the wake of the wing, tail configuration changes that move the horizontal tail down relative to the wing can be the solution.
Several of the single-engine production planes that experimented with T-tails reverted to a more conventional low-mounted horizontal tail in later models. While the Piper PA-38 Tomahawk was only produced in its T-tail configuration, in 1984 another company flew a modified Tomahawk with the horizontal tail moved down to the “conventional” low-tail position. Although that company’s ambitions to restart Tomahawk production with the low-tail configuration came to naught, a pilot report published in the December 1984 issue of AOPA Pilot reported significantly better pitch handling for the modified airplane.
Another example of tail modification to move the tail down below the wing wake is the anhedral on the horizontal tails of the F-4 Phantom fighter. Pictures of the original mockup of the Phantom show a flat horizontal tail and very little wing dihedral. The final configuration as flown had considerable anhedral in the tail and dihedral in the outer wing panels outboard of the wing fold. It seems likely that both of these configuration changes were to address stability and control issues that appeared in the wind tunnel before the configuration was frozen.
Next time we will turn our attention to some wing/body aerodynamics that can cause pitch-up and deep stalls and how to fix them.
