Initial flight testing explores the characteristics of the airplane as it is initially configured. At this point, the designers have done their best to create a machine that is safe to fly, meets all of the mission-performance requirements and satisfies the requirements imposed by certification standards.
Flight testing will often uncover things that need to be fixed or improved to finalize the configuration of the airplane. The most common changes required are to those needed to correct aerodynamic issues that affect the safety and flying qualities of the airplane.
Stall Characteristics
Good stall characteristics are essential to flight safety. Regardless of the maximum performance of the airplane, it must fly slowly at angles of attack near the stall during takeoff and in the traffic pattern and landing approach. Most stall/spin accidents happen either on the base-to-final turn approaching to land or on takeoff when the pilot tries to force the airplane to climb more steeply than it can and allows the airspeed to decay. What makes stall behavior even more of a concern is that inadvertent stalls very often occur at low altitude where the pilot has little time to recognize the stall or altitude to recover.
During the first flight of a new airplane it is important to do an initial exploration of slow flight. After climbing to a safe altitude, the pilot should slow the airplane enough to establish a safe approach speed. This should be the lowest airspeed on the entire first flight to ensure a safe margin from stall. An unexpected stall at low altitude can be deadly.
There has been at least one serious accident on the first flight due to a low-altitude stall. The airplane was intended to be a record-breaker, so it had a high wing loading and an airfoil optimized for minimum drag rather than benign stall characteristics. After a successful takeoff, the engine temperatures began to rise and the pilot elected to land rather than allowing the engine to overheat. He did not have the time or altitude to explore slow flight before attempting an emergency landing. Unfortunately, the airplane stalled at a higher-than-expected speed in the final stage of the landing flare. The airplane dropped a wing and hit the runway inverted with fatal results.
This sad story emphasizes that even if there is a chance to explore slow flight on the first hop it is advisable to plan on flying final approach to the first landing at a slightly higher speed than what is expected to be the “normal” approach speed. Even though it means a longer landing roll, it is the safe thing to do to avoid an unexpected stall at low altitude.
Continuing Development
After first flight, the test program will include full stalls in a variety of conditions: power off, power on, in straight-ahead flight and turning, as well as accelerated stalls.
Ideally the airplane should exhibit these behaviors as the stall develops:
- Aerodynamic buffet in the incipient stall 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 its angle of attack when it is stalled.
- Lateral and directional stability and stable roll damping through the stall and recovery.
- Acceptable stall speed.
If all of these requirements are satisfied no modifications are needed. Sometimes, though, the stall tests reveal characteristics that are not acceptable or need improvement.
Wing Drop or Roll-Off
A common problem is a tendency to roll abruptly as the airplane stalls. Most airplanes will roll if they are yawed at the stall but a large roll-off or a sudden roll-off before the pitch break is dangerous and makes the airplane unacceptably prone to spinning out of an incipient stall.
The propensity to depart in roll at the stall is caused by either abrupt stalling of a large portion of the wing or a stall that starts outboard on the wing. In both cases, a very slightly asymmetric flight condition like a gentle roll or sideslip leads to one wing letting go first, and since the stall is outboard the airplane will roll sharply toward the stalled wing. In many cases, the outboard stall will also cause the ailerons to lose effectiveness or even experience control reversal.
Common Solutions
Short of a complete redesign of the wing there are several common fixes that can tame the stall and eliminate the abrupt roll departure. They all have the same objective: Ensure that the stall starts inboard on the wing and develops symmetrically.
Stall strips are the most common of these. They are small devices, usually with a triangular cross-section that locally sharpens the leading edge. The local sharpening of the leading edge trips the flow at a lower angle of attack than the bare airfoil would exhibit, causing it to stall sooner. By adjusting the spanwise position of the stall strip and its vertical position on the leading edge, we can force the stall to start behind the stall strip at the correct angle of attack.
Stall strips work by locally decreasing the stall angle of attack of the inboard portion of the wing. They are simple and easy to implement and adjust. Their one disadvantage is that they reduce the maximum lift coefficient of the wing and slightly increase stall speed.
There are other approaches that focus on increasing the stall angle of attack of the outer part of the wing so that it maintains attached flow to a higher AoA than the root. These are not as simple as stall strips but do increase the maximum lift coefficient of the wing and slightly reduce the stall speed.
Fixed leading-edge slots delay stall by helping to guide the flow around the leading edge of the wing at high angles of attack. Partial-span fixed leading-edge slots are used on some airplanes to keep the outer panels flying and maintain aileron effectiveness at the stall. They are sometimes referred to as “letterbox slots” and can be found on, among others, the Globe Swift and Stinson 108.
While full-span vortex generator (VG) installations have become popular to increase maximum lift and lower stall speed, it’s also possible to add VGs to the outer portion of the wing only. These keep the outer panel flying to a higher angle of attack than the untreated inboard part of the wing, forcing the stall to start inboard.
An outboard leading-edge droop cuff that increases the camber of the outer panel of the wing has a similar effect. NASA developed this option to stabilize the stall and make the airplane spin-resistant. One critical feature for spin resistance is that the inboard end of the cuff must be cut off parallel to the centerline of the airplane. Fairing the inboard end of the cuff is detrimental to spin resistance.
Nose Slice
Sometimes the airplane will yaw abruptly or lose directional stability as the AoA approaches stall. Asymmetric stalling of the wings is one possible cause. If one wing stalls first the lift of that wing will decrease and its drag will increase relative to the other wing. The airplane will roll toward the stalled wing but the drag increase will also cause it to yaw in the same direction as the roll. The fixes used to tame the roll-off will also reduce the yaw asymmetry.
Even if the wing stalls symmetrically and the roll behavior is stable with positive roll damping, there are other phenomena that can lead to large asymmetric yawing moments at high angles of attack. Sudden yawing at a critical angle of attack is called “nose slice.” It usually appears on configurations with long noses or long, slender fuselages. Fighters designed to fly supersonic and also maneuver at high angles of attack are particularly prone to nose slice, but they are not the only airplanes that sometimes exhibit this problem.
The yawing moment that causes the nose slice is the result of asymmetric flow separation on the nose. At high angle of attack, there is a significant cross-flow on the nose, and at some point, this causes the flow to separate. Once the flow separates on one side of the nose, the side force on the side with attached flow is larger than that on the side with the flow separation. This causes a net side force on the nose toward the side with attached flow that pulls the nose that way. The yawing moment develops quite suddenly and can lead to a departure in yaw.
Fighter designers are very aware of this and put a lot of effort into shaping the noses of their airplanes to ensure that the flow over the nose stays symmetrical.
Sometimes nose slice happens on a subsonic airplane with a simple cylindrical fuselage. One example of this was the MD-80 airliner. During stall testing, the airplane exhibited a large uncommanded yaw approaching stall AoA. The fix was to place a pair of horizontal strakes on the nose below the cockpit. These strakes force the flow to separate symmetrically from the nose and eliminate the nose slice.
So far, we have discussed ways to fix lateral/directional issues at the stall. In our next issue, we will switch our attention to how the airplane behaves in pitch at high angles of attack.
