AEROPROPULSION SYSTEMS TEST FACILITY ICING
Published February 19, 2009
The Air Force's Arnold Engineering Development Center offers icing testing, as well as other essential test and evaluation services, required for Federal Aviation Administration and Joint Aviation Authority certification of full-scale aircraft turbine engine propulsion systems, subsystems and components.
Under existing FAA and JAA certification regulations, an airplane must demonstrate the ability to operate safely in a wide range of icing conditions. Engine icing occurs as aircraft fly through clouds of supercooled liquid water droplets. The water droplets impact the forward facing surfaces of the aircraft and the engine and, when the heat of fusion of the liquid water is removed from the surface, the small water droplets freeze. Ice adhering to airfoil surfaces will distort the airflow and lead to altered performance of the airfoil. In turbine engines and inlets, ice collection can eventually, sometimes in a matter of a few minutes, alter or block enough airflow to cause engine operability problems.
AEDC's Propulsion Development Test Cell C-2 is 28 feet in diameter and 85 feet long, large enough to test several full scale elements of an aircraft propulsion system or, in some cases, the complete system. Test Cell C-2 now offers aircraft manufacturers icing test conditions with conditioned airflow rates up to 1,600 lbm/sec. The ASTF icing system was designed to provide the complete range of atmospheric icing conditions occurring in nature, with the system capable of providing simulated-altitude icing testing of large turbine engines, inlets, windshields, wings and other test articles.
Airflows to 1,600 lbm/sec can be obtained for icing testing in C-2 depending on temperature and pressure. Higher air mass flow rates through the test cell are available without the spray system operating. For example, an engine can be accelerated to higher power to shed ice from the fan after exposure to icing conditions at low power, as might occur in an aircraft ìgo-aroundî following descent in bad weather.
AEDC Test Cell C-2 The key element of the ASTF icing system is the spray apparatus, which has 17 spraybars with 199 possible nozzle positions. AEDC engineers can vary the number and position of the spray nozzles within the array to provide a uniform icing spray cloud at the test section.
AEDC also has the expertise to control the principal factors governing the mechanics and thermodynamics of icing: water droplet size, liquid water content, cloud temperature and pressure, and cloud exposure time. Spray nozzles are available that produce mass median droplet diameters of 15 to 35 microns. The system can simulate flight in conditions representing liquid water contents from 0.2 to greater than 3 grams of supercooled liquid water per cubic meter of air.
During operation of the ASTF icing system, water droplets of the proper diameter are sprayed into the test cell airflow. The droplets are produced with water-atomizing spray nozzles calibrated for droplet mass median diameter production as a function of the ratio of atomizing air-towater mass flow rate. The amount of liquid water added to the airstream is set to provide the required test cell liquid water content.
The control system for the spray system interfaces with the existing test cell services control system. Predetermined cloud conditions are programmed in the control system. The system then monitors the airflow rate and the temperature and pressure of the airstream, and adjusts the water flow and the atomizing air mass flow rates to deliver the proper liquid water content and droplet size to the test article.
The ASTF icing spray system uses an AEDC-developed cloud simulation computer program specifically designed to emphasize transient capabilities. The control system is capable of delivering a steady-state cloud to within 5% of target liquid water content and within 3 microns of target mass median water droplet diameter in less than 10 seconds. Cloud terminations can be accomplished within 5 seconds of termination command.
The icing system can also provide an accurate transition from one icing cloud condition to another very rapidly, within 10 seconds of the command to change levels. This state-of-the-art feature provides unprecedented transient capabilities for AEDC customers.
The icing system in C-2 offers commercial and military aircraft customers safe, accurate, and closely controlled environmental icing testing. Icing testing in ASTF offers seasonal independence, repeatable results collected under closely controlled simulated icing conditions, and extensive data acquisition and analysis.
Performance of such tests at AEDC rather than in flight also eliminates threats to flight test crew and aircraft safety.