Service history has shown that in-flight snow conditions have caused power interruptions on some engines with air intakes that incorporate plenum chambers, reverse flow, or particle separating design features. For instance, in the early 1980s, U.S. Coast Guard HH-65A Dolphin experienced a few unexpected cases of engine surge during demonstration tests and the problem took two years to correct.
To comply with certification requirements, manufacturers need to substantiate that each engine and its air inlet system can operate throughout the flight power range of the engine (including idling) in snow, both falling and blowing, without adverse effect on engine operation (power or thrust loss, surge, stall or flameout), within the established limitations (CS25/29, §1093(b)). The available regulatory, research and guidance documents define approximations of snow conditions to be tested when testing is required. However, there are no validated engineering tools (test facility and numerical tools) available to support design of power plant systems by assessing the risk of snow accretion or accumulation. Demonstration is thus performed at the end of the program development during certification flights. Any issue found at this stage of the development can lead to significant delay and cost to redesign the air inlet or integrate protection systems and can even impact the entry into service of new product. Therefore, to secure future program development and certification, there is a need to develop snow test capability to de-risk power plant system design before in-flight demonstration.
A few test facilities, such as RTA or CSTB, are already able to generate artificial snow using atomising nozzles and by controlling the ambient temperature, wind speed, and water and air supply. However, the generated artificial snow does not match natural snow properties (e.g. size, shape and density) and as such this capability cannot be used as a sole means of compliance or development tool during the design phase. To show compliance to CS 23/25/27/29, flight tests in natural snowstorms, beside their intrinsic risk, are difficult to schedule due to the rarity of events, fewer than 4% of all snowstorms conform to the requirements reported in the AMC, and cannot be used during the preliminary design phase.
Investigating a technology able to generate naturally equivalent snow is, as such, one of the main objectives of the ICE GENESIS project.
Deliverable D3.5 Definition of the target requirements for test facilities operating envelopes for snow provides the technical requirements for falling and blowing snow conditions to be reproduced in ground wind tunnel facilities. These requirements intend to cover all the different parts of the rotorcraft and aircraft affected by snow with a focus on power plant system.
The current document describes calibration methodology for snow test facilities. It has been derived from the SAE ARP 5905. In particular, relevant parameters and acceptance criteria for snow cloud calibration have been updated. The basic procedure remains unchanged including particle size distribution, particle characteristics, water content and cloud uniformity measurements but the instrumentation and techniques to be used for the calibration of snow conditions change. The characteristics of this new instrumentation are further detailed in this document. Finally, a continuity check test is introduced as main acceptance criteria by testing a model (simple configuration e.g. NACA airfoil) to demonstrate the ability of the test facilities to reproduce snow accretion phenomena, to allow an inter-comparison of the test facilities or/and to assess the impact of any change in the test facility configuration.
This document could be the baseline for a future update of the SAE ARP5905 “Calibration and Acceptance of Icing Wind Tunnels”.
