Flight Testing of Anti-Icing and Anti-Contamination Systems for HLFC Surfaces

Kurzfassung

The feasibility of achieving laminar flow has been demonstrated numerous times both in the laboratory and by flight tests but for it to be considered as a viable method for production aircraft, robust and mature systems and structural concepts need to be developed and demonstrated. To achieve laminar flow on a large aircraft with swept wings such as a commercial or military transport, through skin suction will be required in the leading edge region (Hybrid Laminar Flow Control). Ice protection will be required to ensure a clean wing after climb to cruise flight level and to guarantee flight safety under icing conditions. At low flight levels between ground and 5000ft within the thermal convection zone methods to control roughness due to impacted of insect debris and dirt will be necessary. These systems will have to be contained mainly within the leading edge and must be compatible with appropriate leading edge high lift devices. The primary object of this flight experiment was to gain experience with integrating and operating typical systems necessary for a laminar flow airfoil, to evaluate their performance and to observe their interactions. The entire leading edge, outboard of the engine, of the right wing of a Do228 test vehicle was replaced. The test leading edge included regions to evaluate two combinations of systems together with control zones. One region combined suction with transpiration thermal anti-icing and a Kruger high lift device configured as a shield against insect contamination. The second test region combined suction with foamed liquid for ice protection and insect contamination control. Neither transpiration thermal anti-icing nor liquid foam has been flight tested for this application in Europe. The foam-protected region was divided into two sections. During icing tests the entire span of this region could be operated to ensure that the airflow over the aileron was not disrupted but for insect contamination tests the outer section could be turned off to act as a reference region. In order to determine the effectiveness of the suction and contamination control systems the test required that the boundary layer would be naturally turbulent and only laminarised by suction if the systems operated efficiently and correctly. That part of the span in the propeller slipstream did not form part of the test and was provided with a simple liquid ice protection system for safety and opertional reasons. In order to allow the detection of laminar flow the test aircraft was equipped with an infrared system which measures the small surface temperature difference between laminar and turbulent flow. The pressure distribution is measured by means of a row of pressure taps between the outer two test panels. The flight tests have been conducted in four test campaigns starting in December 2001 and being completed in September 2002. In January and February 2002 the certification flights could be finished with a preliminary certification for further test systems investigation. In a second step the effectiveness of the suction system has been tested. These tests showed that depending on flap deflection and suction velocity transition locations between 25% and 50% of chord length could be achieved. Without suction no laminar flow could be observed. In three test campaigns between March and September 2002 the effectiveness of the anti-icing and anti-contamination systems have been investigated. The anti-icing tests have been performed in the wake of the icing aircraft Do228 of the Fairchild-Dornier company. This aircraft is equipped with a water spray nozzle system and can load 1 cubic metre of water enabling a spraying time between 30 minutes and one hour. Three hours of anti-icing tests with different liquid water contents (LWC) from 0,3 up to 1.7 g/cubic metre clearly showed that the bleed air anti-icing system on the suction surface had not only a faster response due to the better heat transfer by blowing through the porous surface but also a faster de-icing effect due to the overpressure being effective against the ice sheet. The effectiveness of the fluid/foam de-icing system depends strongly on the capability of the foaming device. After modification of the foaming device the fluid/foam system shows a reliable de-icing capability. It's effectiveness was close to that of the standard de-icing system of AS&T. The anti-contamination tests in June and August 2002 have been as successful as expected. On the fluid/foam protected test panel only 10% of the insects sticking on the unprotected surface could be counted on the surface with fluid protection. The shield protection of the Kruger has an effectiveness of 100%. No insects could be counted on the protected surface for all test flights. Summarizing it could be stated that all three systems tested be these flight tests worked as effective as expected or even better. They allow to protect an HLFC wing reliably against icing and contamination.