Last month we started our look at testing the airplane in slow flight and exploring stall characteristics. We discussed lateral/directional behavior last time, so we now turn our attention to the pitch axis and we look at pitch characteristics.
The pilot trims the airplane to fly at a higher angle of attack in order to reduce speed in flight. The slower the commanded airspeed, the higher the angle of attack. At some critical angle of attack the airflow over the wing begins to separate and the airplane enters an incipient stall. We consider the airplane to be at a “high angle of attack” when it is approaching or beyond the angle of attack at which the flow starts to separate on the wing.
As the stall develops, the airplane should exhibit these behaviors in pitch:
- Aerodynamic buffet as the airplane approaches stall angle of attack to provide the pilot with warning of the impending stall.
- A stable nose-down pitch break at the stall so the airplane naturally pitches down and reduces angle of attack when it is stalled.
- Linear stick force or increasing back pressure needed to drive the airplane to a higher angle of attack.
- Monotonic variation of elevator deflection to trim with airspeed. The stick position to trim should move steadily aft as the commanded airspeed decreases.
Approach stall gradually during testing. One knot/second deceleration is typical for stall demonstrations and initial testing.
Gradual deceleration is important for safety, because it gives the pilot time to perceive how the airplane is behaving and recover if the airplane exhibits any undesirable behavior. The test program should move on to more aggressive tests including accelerated stalls and departure stalls only after the airplane shows acceptable characteristics in level-flight 1G stalls.
During initial stall tests, two major factors are of concern.
High AoA Pitch Stability
As the angle of attack approaches stall, the airflow over the wing begins to separate. This flow separation changes the aerodynamic characteristics of the wing, and this change increases as the stall progresses. The primary effect of flow separation is loss of the wing’s ability to generate more lift with increasing AoA, but this change in wing forces and the effects of the turbulent wake behind the stalled portion of the wing can also change pitch stability and trim.
The airplane should at least retain its pre-stall level of pitch stability as the stall develops. It should naturally pitch down at the stall, and the variation of pitching moment with angle of attack should remain stable (more nose-down as angle of attack increases).
Pitch-Down Recovery
The pilot should have the ability to command a nose-down pitch rate at all angles of attack. The elevators should have enough control power to drive the nose down below the critical angle of attack to break a stall and restore level flight with attached flow.
On an airplane with good bare-airframe stall characteristics that breaks nose-down at the stall and remains stable post-stall, pitch-down recovery is not likely to be a problem. Usually all the pilot must do to recover is relax back pressure on the stick. If the airplane has a post-stall degradation of pitch stability or a pitch-up, elevator power becomes a more serious concern.
It is a major concern for airplanes that are intended to fly at high angles of attack and airplanes like modern fighters that are unstable in pitch and rely on full-authority digital flight control systems. The flight-control computer can augment the characteristics of the airplane by very rapidly commanding the control surfaces, but the aerodynamic controls must have the control power to produce the effect the computer is commanding. At high AoA a modern fighter is likely to be unstable and have a nose-up pitching moment with the controls neutral. To drive the nose back down the aerodynamic pitch-control surfaces must be able to generate enough nose-down moment to make the total pitching moment negative and command a nose-down pitch rate.
During the approach to a stall, the pilot should be alert for any indication that the airplane might have a tendency to pitch up spontaneously. There are several warning signs to watch for.
Lightening Pitch Force
The airplane should retain a stable stick-force gradient through the stall. The pilot should have to pull progressively harder to raise the nose to increase angle of attack and/or reduce airspeed.
It is a warning of a potentially dangerous characteristic if the pull force required to trim the airplane does not increase or gets lighter as the nose rises. If this happens during an approach to a stall while testing, recover immediately and do not proceed with the stall or try to decrease airspeed below where the reduction in stick force first appears.
It’s vital to figure out why the stick force is getting lighter and only proceed to lower airspeed when and if the stick-force variation is well enough understood for it to be safe to do so.
The unstable stick-force gradient will need to be fixed in any case, but if it is primarily a control-surface issue on an otherwise stable airplane it may be safe to explore closer to the stall.
Stick Position to Trim
To see if it is a stability problem, we need to consider the variation of elevator position to trim. The elevator deflection required to trim a stable conventional aft-tail airplane should be progressively more trailing-edge-up as the commanded airspeed decreases and angle of attack increases. The stick trim position should move progressively aft with decreasing trimmed airspeed.
If the elevator deflection to trim the airplane does not move trailing-edge-up to increase commanded angle of attack it is an indication that the pitch stability of the airplane is decreasing with angle of attack. Even worse is a situation where the trim position of the stick is moving forward as airspeed decreases. This is an indication that the airplane is unstable, and there is a serious risk it will pitch up by itself in spite of the pilot’s elevator input.
Degradation of pitch stability, or the onset of actual instability with increasing angle of attack, is very dangerous and may be a warning of an incipient pitch-up that can lead to an unrecoverable deep stall condition.
Uncommanded Pitch-Up
If the airplane becomes unstable as it approaches the critical angle of attack and the pitching moment becomes nose-up, it will continue to pitch up and worsen the stall unless the pilot is able to force the nose down with the elevator.
This is extremely dangerous since the airplane will pitch up spontaneously and may enter an unrecoverable deep stall. At the first indication of pitch-up the pilot should immediately apply nose-down elevator to recover to level flight and discontinue stall testing until the behavior of the airplane is understood and fixes are implemented. Avoid the temptation to further explore the initial pitch-up or repeat the test without landing and analyzing the situation thoroughly first.
Pitch-up can appear in two ways. The first is a clearly discernible pitch-up where the nose visibly rises and the pilot can perceive a nose-up pitch rotation. The uncommanded nose-up rotation is a clear indication of trouble and should cue the pilot to immediately push the stick forward and recover from the incipient stall.
The second mode where aerodynamic pitch-up manifests is more subtle and can be very dangerous because there is no clear acceleration or rate cue that the airplane is pitching up. As the airplane approaches the stall, and the angle of attack increases, drag increases rapidly as the flow over the wing starts to separate. The airplane will develop an ever-increasing sink rate that further increases angle of attack.
If the airplane has a stable pitch break at the stall, it will naturally nose down, decrease angle of attack, and recover.
If the airplane has an aerodynamic pitch-up the increasing sink will increase angle of attack and the airplane may pitch up relative to the incident airflow just enough so that the pitch attitude relative to the earth will not change much but angle of attack will continue to rise.
This is harder to detect than an abrupt pitch-up, but it is very dangerous because the airplane is transitioning into a steep descent at high angle of attack in an approximately nose-level attitude without a strong pitch-rate cue to the pilot. The airplane can progressively lock into a steeply descending deep stall as sink rate increases and airspeed decreases. This particular characteristic caused accidents with some early swept-wing T-tailed airliners. Both the BAC 111 twinjet and the Hawker Siddeley Trident trijet had fatal deep-stall accidents caused by this.
Next time, we will continue with a look at the aerodynamic causes of instability and pitch-up at high angle of attack and some aerodynamic fixes that might be implemented to cure the problem.
