Thin Margins In Wintry Takeoffs
istockphoto
For many years the application of the “clean wing policy” has allowed aircraft to take off when conditions are conducive to icing. While anti-icing fluids negate the serious effects of wing icing, recent research has determined that the fluids themselves extract a significant penalty from the wing’s aerodynamic capabilities, especially from high-performance wings commonly on jets that cruise at transonic speeds. The effect is manifested most during the takeoff rotation.
In addition, swept-wing aircraft experience crucial aerodynamic penalties due to ground effect. And if crosswinds are present, further aerodynamic compromise can lessen the margin of safety even more.
High-Performance Wings
Most commercial aircraft that cruise at transonic flight speeds are fitted with supercritical airfoils whose stall behavior is unlike that of the thick, generic wings used in many ground training programs. The boundary layer along the leading edge of a supercritical wing begins thin and laminar at low angles of attack (AOA). Then, above a certain AOA the laminar boundary layer partially separates, forming a “short bubble,” behind which the turbulent boundary layer reattaches.
The bubble has an overall negligible effect on the pressure distribution around the wing and remains stable without creating any discernable handling or performance effects while the airfoil remains in the normal (linear) portion of its lift curve. However, as AOA increases further, an adverse pressure gradient builds, and within the thin boundary layer, at high AOA a shockwave can form even though the aircraft’s speed may be relatively low. A critical point is reached at which the short bubble “bursts” and the airflow detaches suddenly and completely from the leading edge to the trailing edge. A serious consequence is the lack of aerodynamic stall warning and an abrupt loss of lift.
Clinton E. Tanner, Bombardier’s senior technical advisor in flight sciences, presented “The Effect of Wing Leading Edge Contamination on the Stall Characteristics of Aircraft” at the SAE Aircraft & Engine Icing International Conference in September 2007, further discussing the aerodynamic characteristics of high-performance wing sections.
The accompanying figure is a typical lift-curve slope of a “hard” wing, or one without movable leading edge devices, often found on regional and business jets. Notice the abrupt loss of lift at the stall AOA, a characteristic of thin wings exhibiting a leading-edge stall behavior. Wings of this design tend to stall abruptly without warning such as airframe buffet. Accordingly, such wings require a stall protection system such as a stick pusher triggered at a below-stall AOA to meet certification requirements.
Incidentally, an aircraft’s field performance would be based on the lift obtained at the stick pusher firing angle, not on the maximum lift attained at the point of natural stall. V2+10 is the normal takeoff speed of the aircraft in an all-engine condition. The AOA for normal operations is well below the critical AOA for natural aerodynamic stall. Auto-ignition protection is provided to the engine because at high AOA, unstable airflow ingestion into the engine could cause compressor stalls. It is calibrated to trigger at a lower AOA than the stick-shaker warning.
Ground Effect
When an aircraft is close to the ground, negative changes occur to its aerodynamics, especially on swept-wing jets. This is particularly true during the landing flare and takeoff rotation when the aircraft is at a precarious energy state with very little margin for error. As the aircraft rotates, the tips on a swept wing are momentarily closer to the runway, changing the airflow significantly, and further increasing the negative impact of ground effect.
The tragic loss of a Gulfstream G650 during certification flight-testing at Roswell, New Mexico, in April 2011 highlighted some of these characteristics. The aircraft was conducting a planned one-engine-inoperative (OEI) takeoff when a stall on the outboard section of the right wing produced a rolling moment that the experienced flight test crew was unable to control. The right wingtip contacted the runway and the aircraft departed the right side of the runway. It then struck a concrete structure and an airport weather station, resulting in extensive structural damage and a post-crash fire that completely consumed the fuselage and cabin interior. The NTSB’s investigation found that the airplane stalled while lifting off the ground and noted some of the common misconceptions and misunderstandings about ground effect. (See the “Ground Effect and Airflow” sidebar for further description of the changes to the airflow in ground effect.)
The NTSB’s John O’Callaghan, a national resource specialist in aircraft performance, noted that all aircraft stall at approximately 2-4 deg. lower AOA with the wheels on the ground. Flight test reports noted “post stall roll-off is abrupt and will saturate lateral control power.” The catastrophic roll-off of the wing in the Roswell accident was due in part to the absence of warning before the stall in ground effect.
Before the accident, Gulfstream estimated that the in-ground-effect stall AOA would be 13.1 deg. and set the AOA threshold for the activation of the stick-shaker warning at 12.3 deg. The company’s post-accident computational fluid dynamics (CFD) analysis indicated that the maximum lift coefficient of the G650 in ground effect was actually lower than the maximum lift coefficient in free air and found that the decrement from the free-air stall AOA to the in-ground-effect stall AOA was about 3 deg. Flight test engineers had incorrectly assumed that the maximum lift would be the same both in and out of ground effect.
Because the maximum lift and stall AOA in ground effect were overestimated, the airplane’s AOA threshold for stick-shaker activation and the pitch limit indicator were set too high. Moreover, the flight crew received no tactile or visual warning before the actual stall occurred. The airplane stalled at an AOA that was below the in-ground-effect stall AOA predicted by Gulfstream and the AOA threshold for the activation of the stick-shaker stall warning.
The NTSB determined, through conversations with Gulfstream, other manufacturers and the FAA, that the potential for the maximum lift coefficient in ground effect to be reduced might not be recognized industry-wide. Given the results of Gulfstream’s CFD analysis and the findings of the NTSB’s accident investigation, it was determined that the maximum lift coefficient for at least some airplanes could be reduced in ground effect. Further, assumptions to the contrary could result in an overestimation of the stall AOA in ground effect and could increase the risk of a stall in ground effect with little or no warning.
During application, gravity causes the anti-icing fluid to naturally flow around the leading edge and to the wing’s lower surface. As the airplane accelerates in the initial stages of takeoff, the shearing forces near the leading edge are relatively low due to the low AOA and low initial speeds. There is some shearing of the fluid as the airplane’s speed rises, resulting in the primary wave of fluid flowing downstream. Upon rotation, the shear forces near the leading edge increase significantly, forming a secondary wave of fluid that flows around the leading edge.
Wind-tunnel testing has found that the formation of the separation bubble further promotes accumulation of fluid in a critical location near the wing leading edge. This would increase the height of the secondary wave and contribute to the observed reduction in maximum lift and stalling angle for fluid/contamination cases. According to “Aerodynamic Characterization of a Thin, High-Performance Airfoil for Use in Ground Fluids Testing,” a study by NASA Glenn Research Center and the National Research Council of Canada, this secondary wave of anti-icing fluid can have a significant impact on aerodynamic performance because it is located close to the wing leading edge at higher angles of attack.
The study, which employed aerodynamic performance measurements, flow visualization and boundary-layer surveys to better understand the adverse aerodynamic characteristics of anti-icing fluids on thin, high-performance wings, discovered a decrease in a wing’s maximum lift coefficients ranging from 1.91 to 1.95 compared to the clean wing value of 2.2. Correspondingly, the stall angle was reduced to 15.3 deg. compared to the clean value of 20 deg. The study concluded that secondary wave effects could have a significant impact on the maximum lift coefficient and stall angle for anti-icing fluid tests on the thin, high-performance wing.
Incidentally, during preflight inspections you should examine the condition of the aerodynamic seals on your wing, particularly on “hard” wing jets. Deteriorated aerodynamic seals, particularly those near the leading edge of the wing, also cause a significant loss in the maximum lift of a wing as well as decrease the stall AOA.
Crosswinds can likewise create a stall at a lower AOA. During crosswind takeoffs and landings in a swept-wing jet, the upwind wing experiences airflow that is more direct (i.e., perpendicular) to the wing’s leading edge, and this generally improves the wing’s performance. Conversely, the downwind wing experiences the airflow at a greater angle (essentially increasing the “sweep” of the wing), which decreases its lift, increases drag, promotes the span-wise flow of air and thereby reduces its stall AOA.
For example, a crosswind from the right effectively increases the sweep of the left wing and reduces the sweep of the right wing. Bombardier’s Tanner cites flight test results showing that sideslip reduces the stall AOA of the left wing by up to 3.5 deg. when it experiences a sideslip of 20 deg. Large rudder applications during a highly dynamic stall event will also generate high sideslip angles. Either of these conditions may result in asymmetric stall of the downwind wing.
The in-depth engineering studies already cited focused solely on ground effect, crosswinds or anti-icing fluids. Tanner is concerned about the combined effects of all three on lowering the overall margin of safety during takeoffs.
Given the very real possibility that the three have an additive effect on the reduction in stall AOA, the margins over an actual aerodynamic stall during a takeoff decrease and a stall could result without aerodynamic warning. The AOA has to be reduced several degrees below the AOA at which the stall first occurred to fully re-attach the airflow. This is called aerodynamic hysteresis. Altitude may have to be sacrificed to recover the aircraft from the stall, even when the aircraft is flying close to the ground.
The airline industry has operated with relatively few incidents due to ground icing in recent decades because of the adoption of the widely accepted guidelines on ground deicing and anti-icing procedures. Airlines also benefit from operating at larger airports that have vehicles with elevated platforms and numerous personnel who are specially trained in the application of deicing and anti-icing fluids. Air carriers are required to address their ground deicing procedures during training, and the wings on larger transport aircraft are less affected by smaller-sized contaminants. This support infrastructure and focused training isn’t as readily available in the business aviation environment.
A takeoff in conditions conducive to ground icing requires a methodical process to assure that contamination on critical surfaces has been eliminated. If an aircraft has been sitting on the ground in conditions conducive to wing contamination, a preflight tactile check of the airfoil is necessary. Proper application of deicing and anti-icing fluids as well as observance of the hold-over times are vital in such conditions.
And since the ground crew applying the fluids may not be familiar with which areas to avoid on your aircraft, it is incumbent on your part to familiarize them. Following the aircraft flight manual’s procedures for the proper configuration during deicing and taxi is important, but these often call for taxiing in a non-takeoff configuration for the flaps, which is contrary to standard. The selection of proper takeoff speeds and pitch limits is also a necessity. As this recent research has shown, the safety margins during takeoffs in these conditions are thin.
Ingen kommentarer:
Legg inn en kommentar
Merk: Bare medlemmer av denne bloggen kan legge inn en kommentar.