NASA is expanding the use of Unmanned Aerial Vehicles (UAVs) fabricated from composite materials as aerial platforms to carry scientific payloads for science and environmental missions. The UAV brings an unprecedented capability for extended flight duration (over 24 hours on station) to provide uninterrupted monitoring of emergency situations with real-time information for emergency response commanders.
The UAVs at Dryden have ice detection sensors, but do not have anti-icing or de-icing capability. Buildup of ice on external surfaces of aircraft can disturb the normal aerodynamic flow conditions to the point that vehicle stability margins are reduced or, in some cases, the aircraft may stall or become uncontrollable. These potential hazards are compounded since UAV pilots and operators in ground cockpits lack traditional situational awareness cues associated with manually piloting an aircraft. In severe icing situations, when the vehicle suffers substantial performance degradation or becomes uncontrollable, there is a high probability of aircraft loss. Without protection from the effects of icing, the effectiveness of UAV operations is limited and risks the loss of critical data, which is needed to assess threats to public safety.
What is NASA Doing?
The current practice is to use weather forecasts for scheduling missions to avoid entering regions of icing conditions. The FAA, when authorizing UAV flights in the National Airspace System (NAS), specifically prohibits UAVs from flying in areas of known or forecasted icing conditions. Frequent updates to forecasts are used to provide warning of icing in UAV operating areas so that the aircraft can exit the region. This is a straightforward remedy; however, mission limitations, schedule dependence, and unplanned interruptions in mission staging are unfortunate consequences. It may not be possible to timely acquire critical science data or to observe ground images to strategically assess threats to public safety when icing conditions develop.
What are the Challenges?
The conventional approach would be to develop an automated ice protection system for composite UAV aircraft. Current systems for manned aircraft require the pilot to activate the ice protection function and necessitate substantial aircraft modification to be effective (i.e. installation of pneumatic boots or electromechanical ice expulsion devices, or electrical/bleed air heaters with ducting, or weeping wing installations). In the absence of a pilot, a reliable ice detector would be required to initiate the ice removal. Installation of additional equipment would add to the overall weight and reduce payload. Further, depending on the chosen ice removal method, installing the expulsion mechanism and modifying the composite structure may be difficult when the structural design is optimized for minimum weight and aerodynamic performance.
The fact that the UAV is constructed with composite materials, including its aerodynamic surfaces (wings, horizontal stabilizer, and rudder), mandates that proposed ice protection systems are compatible with the fabrication technique for the composite material and ideally would be a part of the initial UAV design. Because the composite structures achieve their outstanding structural efficiency from embedded fibers, ice protection systems must not disturb the internal layup of embedded fibers. Retrofitting a system without proper precautions risks interfering with the composite structural design and may reduce strength and affect performance and airworthiness.
An alternative approach would be a remote sensor system capable of detecting icing conditions at a sufficient distance for the aircraft to evade the hazard. Multi-frequency polarized radar and other multi-sensor remote sensing technologies have been tested for identifying icing conditions from a distance, but have not reached the stage of being a mature, commercially available technology. This approach would reduce (but not eliminate) the impact to mission scheduling and staging from potential or real icing encounters.
A completely passive technique that would prevent ice from adhering to the aircraft would be a highly desired approach, but no known technique to implement this approach is known to exist.
What Applications Benefit from Meeting this Need?
In addition to resolving or eliminating the in-flight icing issue with NASA UAV aircraft, private and commercial aircraft would benefit from a simplified, low-cost ice protection system. A short list of other beneficiaries would include elements of the public infrastructure that are incapacitated by coatings of ice, including highways and power lines.