Active High-lift System Research Articles

Experimental validation of a compact mixed-flow compressor for an active high-lift system

Compact, electrically-driven compressors are a core component of a novel active high-lift system for future commercial aircraft. A newly-developed aeromechanical optimization process was used to design the compressor stage. The optimization resulted in an unusual mixed-flow compressor design with very low aspect ratio blades and a high rotational speed of up to 60,000 rpm. Due to the unusual design, experimental validation of the performance predictions by means of CFD is necessary. This paper presents the first experimental results obtained using a preliminary prototype at part-speed, i.e. rotational speeds from 20,000 to 30,000 rpm. The experimentally-determined pressure ratios deviate up to 1.5 %, the polytropic efficiencies up to 4 percentage points from the CFD predictions. Besides the deficiencies of available turbulence models, the underestimation of overall losses is presumably due to the omission of the volute in the CFD model. An experimental validation of the CFD predictions at full-speed is under way.

Controllability of an aircraft with active high-lift system using a segmentwise controllable flap system

Active high-lift aircraft needs exceptional aerodynamic control performance to operate at low airspeed. In this study, a new concept for the improvement of controllability is investigated, incorporating the special capabilities of a blown Coanda flap system. The blowing system along the flaps is divided into 12 independently controllable segments. This provides the opportunity to influence the lift distribution along the wingspan by individual blowing performance settings for each segment without changing the flap deflection. This offers the chance to control the airplane in the final approach phase with fully deflected flaps using only the blowing system and to dedicate specific control tasks to particular segments. A model for the blowing system influence on the local wing aerodynamics is implemented in an existing nonlinear full aircraft flight mechanics model. The system capabilities in terms of roll control and the climb performance are investigated by criteria’ evaluation and dynamic simulation assessment. The ability to fly turn/altitude change maneuvers by utilizing the active high-lift system is proven and a corresponding control concept is presented. It also includes the compensation of different blowing failure cases, which leads to acceptable but still improvable aircraft reactions.

Design Considerations for the Electrical Power Supply of Future Civil Aircraft with Active High-Lift Systems

Active high-lift systems of future civil aircraft allow noise reduction and the use of shorter runways. Powering high-lift systems electrically have a strong impact on the design requirements for the electrical power supply of the aircraft. The active high-lift system of the reference aircraft design considered in this paper consists of a flexible leading-edge device together with a combination of boundary-layer suction and Coanda-jet blowing. Electrically driven compressors distributed along the aircraft wings provide the required mass flow of pressurized air. Their additional loads significantly increase the electric power demand during take-off and landing, which is commonly provided by electric generators attached to the aircraft engines. The focus of the present study is a feasibility assessment of alternative electric power supply concepts to unburden or eliminate the generator coupled to the aircraft engine. For this purpose, two different concepts using either fuel cells or batteries are outlined and evaluated in terms of weight, efficiency, and technology availability. The most promising, but least developed alternative to the engine-powered electric generator is the usage of fuel cells. The advantages are high power density and short refueling time, compared to the battery storage concept.

Flight mechanics model for spanwise lift and rolling moment distributions of a segmented active high-lift wing

In this study, the aerodynamics of wings using an active high-lift system are investigated. The target is the flight mechanical description of the spanwise forces and resulting moments and the influence of the active high-lift system to their distribution. The high-lift system is a blown flap system divided into six segments per wing. Each segment is assumed to be individually controlled, so the system shall be used for aircraft control and system failure management. This work presents a flight mechanical sub-model for the simulation of flight dynamics, which has been derived from high-fidelity CFD results. An assessment of single-segment blowing system failures will be presented including recommendations for compensation of either lift or rolling moment loss. For this investigation, the compensation is required to act at the same wing side on which the failure appears. Thus, the potential for an increase of system reliability shall be proven. The results show that less performance investment in terms of pressurized air is necessary to compensate the rolling moment of a failing segment instead of its lift. However, large blowing performance increases for the remaining wing segments that occur for some of the failure cases.

Bürgernahes flugzeug: testing technology for the high power propeller of a wind tunnel model

The research project “Burgernahes Flugzeug (BNF)”, funded by the federal state of Lower Saxony, is set to study future commercial aircraft. Main task is the development of technologies for quiet aircraft with short take-off and landing capability. A wind tunnel model is built for testing the active high-lift system on a semispan wing, equipped with a scaled high power propeller which is driven by an electric motor. The electric motor was specially developed to meet the required high power density. In 2012 and 2013 several measurement campaigns were carried out in the Low Speed Wind Tunnel Braunschweig (NWB) of the German-Dutch Wind Tunnels (DNW). An operation procedure was set for the measurement campaigns, which allowed testing on varying operation points.

Aerodynamic Effects of Propeller Slipstream on a Wing with Circulation Control

The German Bürgernahes Flugzeug (Citizen-Oriented Airplane) research program advances technologies for high-lift systems using circulation control Coandă-type flaps. The interaction of these high-lift systems with a propeller engine is the subject of the present work. A large and complex wind-tunnel model equipped with an active high-lift system and a powered propeller was designed, manufactured and instrumented. The wind-tunnel data show the potentials of the active high-lift system in three-dimensional wing applications. The results characterize and quantify the interactions between a propeller and a high-lift wing by using systematic variations of blowing momentum and propeller thrust. The results represent a unique database for future efforts to improve the efficiency of active lift and for validation of numerical simulation methods.

Synergies between suction and blowing for active high-lift flaps

The present 2-D CFD study investigates aerodynamic means for improving the power efficiency of an active high-lift system for commercial aircraft. The high-lift configuration consists of a simple-hinged active Coanda flap, a suction slot, and a flexible droop nose device. The power required to implement circulation control is provided by electrically driven compact compressors positioned along the wing behind the wingbox. The compact compressors receive air from the suction slot, which also represents an opportunity to increase the aerodynamic performance of the airfoil. The present work investigates the aerodynamic sensitivities of shape and location of the suction slot in relation to the maximum lift performance of the airfoil. The main purpose of the study is the reduction of the compressor power required to achieve a target lift coefficient. The compressor power requirements can be reduced in two ways: obtaining a high total pressure at the end of the suction duct (compressor inlet) and reducing the momentum needed by the Coanda jet to avoid flow separation from the flap. These two objectives define the guideline of the suction slot design. As a result, a jet momentum reduction of 16 % was achieved for a target lift coefficient of 5 with respect to the same configuration without suction. Furthermore, the study yielded physical insight into the aerodynamic interaction between the two active flow control devices.

Effect of an active high-lift system failure during landing approaches

Simulation results of the longitudinal motion of a civil twin-engine aircraft with an active high-lift system are presented. The investigated system uses the lift-increasing effect of blowing over Coandă flaps along the wing. The core elements of the nonlinear model describing the dynamic behavior of an aircraft with this specific type of active high-lift system are explained. The main focus lies on the analysis of the aircraft’s reaction to an instant symmetric total failure of such a system. Initial investigations analyze the outcome of such failures, if throttle is set to maximum and elevator is controlled, without using stabilizer or flaps. Subsequently, an investigation has been performed estimating the effect of flap reconfigurations and stabilizer adjustments. The paper also considers a temporary failure of the system due to an engine failure combined with a too low power setting of the remaining engine which has to be increased first before the restart of the blowing system. The investigated situations vary from inconvenient to unrecoverable. Recommendations for backup systems and procedure changes are made to prevent such situations. A procedural approach is analyzed and respective simulations prove the efficiency of this solution.

Analysis of trimmable conditions for a civil aircraft with active high-lift system

This paper outlines recent flight dynamic analysis results of a civil aircraft with active high-lift system using blown Coandă flaps. The main focus lies in the trim analysis of the aircraft. Therefore, the basic structure and core elements of the nonlinear model, describing the dynamic behavior of an aircraft with this specific type of active high-lift system are presented. The center of gravity range allowing controllability and static stability of the aircraft is determined, and the resulting characteristics of the aerodynamic model and their impact on the trim results of the aircraft are analyzed. The results show specific flight dynamic difficulties related to the active high-lift system, namely in the flying characteristics necessary for safe take-off and approach procedures. The flight physics is explained and discussed. The necessity of the application of a wing leading edge device is outlined by preliminary studies and further remedial means are proposed.

Assessment of leading-edge devices for stall delay on an airfoil with active circulation control

The use of active, internally blown high-lift flaps causes the reduction of the stall angle of attack, because of the strong suction peak generated at the leading-edge. This problem is usually addressed by employing movable leading-edge devices, which improve the pressure distribution, increase the stall angle of attack, and also enhance the maximum lift coefficient. Classical leading-edge devices are the hinged droop nose or the more effective slat with a gap. The flow distortions generated by the gap become an important source of noise during approach and landing phases. Based on these considerations, the present work aims at evaluating the potentials of gap-less droop nose devices designed for improving the aerodynamics of airfoils with active high lift. Both conventional leading-edge flaps and flexible droop noses are investigated. Flexible droop nose configurations are obtained by smoothly morphing the baseline leading-edge shape. Increasing the stall angle of attack and reducing the power required by the active high-lift system are the main objectives. The sensitivities of the investigated geometries are described, as well as the physical phenomena that rule the aerodynamic performance. The most promising droop nose configurations are compared with a conventional slat device as well as with the clean leading-edge. The response of the different configurations to different blowing rates and angles of attack are compared and the stalling mechanisms are analyzed.