N13A-T015 TITLE: Airborne Sensing for Ship Airwake Surveys

TECHNOLOGY AREAS: Air Platform, Ground/Sea Vehicles, Sensors

OBJECTIVE: Develop and demonstrate a technique for in-situ experimental measurement of full-scale ship superstructure airwakes using small air vehicles.

DESCRIPTION: Much of the challenge associated with recovering sea-based aircraft to Naval vessels stems from the complex airwake generated by the ship superstructure and convecting downstream into the aircraft's approach path. While considerable progress has been made in the computatational evaluation of ship airwakes [1, 2], advances in predictive methods are hampered by the difficulties involved in obtaining experimental data for validation. Subscale wind tunnel testing provides some data but introduces difficulties related to modeling detail, scaling and blockage effects, and instrumentation. Full-scale testing with fixed anemometers mounted to the ship deck requires installation of a number of masts, instrumentation cabling, and signal processing and recording hardware. Practical difficulties arise in scheduling of the ship, installing the equipment, and the fact the all aviation operations are precluded while the anemometers are installed. Above all, this approach does not provide any data for the airwake aft of the ship's stern, or above the height of the highest anemometers. Various remote sensing approaches that would address the off-deck measurement limitation are under consideration but at present time it is unclear if these will lead to a viable solution.

A recent methodology validation effort employing a remotely-operated helicopter behind a full-scale vessel demonstrated low-resolution mapping of the ship airwake by a small unmanned aircraft traversing the airwake [3]. This topic builds on that concept, envisioning high-resolution airwake measurements throughout a 3D field obtained using small air vehicles. The vehicles may be fixed-wing, rotary wing, tethered, or free flying. Tethered vehicles (kites, gyro-gliders [4] etc.) may be advantageous in terms of cost, lack of onboard propulsion system, and ease of setup, operational footprint, and launch & recovery. In addition, the constraint of a towline of known length may be simplify determination of the position behind the ship. The vehicle will serve as a platform for air data and/or other sensors and either record the data onboard or transmit them to an operator on the ship. The entire system should be easy to transport, operable by a crew of minimum size and quickly deployable and stowable while underway. When stowed, the system should not interfere with flight operations. Postprocessing should allow timely reconstruction of the airwake. The unsteady nature of the airwake will require development of advanced mathematical techniques to provide useful time-history reconstructions and/or statistical representations of the airwake field from the discrete asynchronous samples obtained with the vehicle. Multiple-vehicle concepts may provide significant improvements in the ability to reconstruct the unsteady 3D airwake.

Airwake velocities may be measured using air data sensors carried by the air vehicle, by inference based on air vehicle flight mechanics, a combination of these, or other innovative means. The vehicle should be capable of sampling velocities anywhere within a volume bounded by lateral angular displacements from the ship's stern of at least +/-30 deg, vertical angular displacements of zero to at least 20 deg, and downrange from as close to the ship's stern as possible, to as far astern as possible. Air velocities must be resolved into a ship coordinate system; hence if velocities are measured while the air vehicle is in motion, means to measure the vehicle's motion relative to the ship must be provided. Measurements should be possible from range starting at 10 kt or less out to at least 50 kt. Velocities should be resolved to within at least +/- 5% in speed and +/- 5 deg in direction. The measurement bandwidth, spatial resolution, and volume over which the air velocity is averaged should be stated. The ability to map the field autonomously without operator intervention would be desirable. The method will ideally be applicable to any air-capable ship.

Proposals should provide a credible review of the existing technologies and clearly outline the reasons underlying the choice of solution. While this topic is not focused on vehicle launch and recovery, proposals must clearly identify a viable means for launch and recovery of the aerial vehicle.

PHASE I: Define and develop a concept for an economical air vehicle with associated sensor and data processing techniques for measuring 3D unsteady ship airwakes. Demonstrate technical feasibility of the vehicle concept using a shore-based ground vehicle. Demonstrate viability of critical sensor and data processing capabilities in laboratory testing and/or simulation.

PHASE II: Produce prototype hardware based on Phase I work. Complete qualifications and certifications required for operation aboard a Naval vessel. Demonstrate on a Naval vessel for which CFD predictions and, if possible, previous experimental data are available. Compare results with analytical and previous experimental data to demonstrate efficacy of technique. Document validation study, data processing methods and algorithms, and all hardware elements.

PHASE III: Refine vehicle and supporting equipment based on experience gathered in Phase II and produce a limited number for use by NAVAIR and NAVSEA in ship airwake mapping.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: This technology will enhance our understanding of large structure, bluff-body aerodynamics such as encountered on commercial buildings, in particular those with integrated helicopter landing facilities.

Low-cost air vehicles, in particular tethered aircraft, could provide commercial vessels with an as-needed over-the-horizon surveillence capability (weather; other shipping). Such aircraft could also provide commercially successful devices for scientific/educational recreation.

REFERENCES:
1. Polsky, S., Imber, R., Czerwiec, R., and Ghee, T., "A Computational and Experimental Determination of the Air Flow Around the Landing Deck of a US Navy Destroyer (DDG): Part II," Proceedings of the AIAA Applied Aerodynamics Conference, Miami, FL, Jun. 2007. AIAA 2007-4484

2. Shipman, J., Arunajatesan, S., Cavallo, P.A., Sinha, N., and Polsky, S. A., "Dynamic CFD Simulation of Aircraft Recovery to an Aircraft Carrier," Proceedings of the 26th AIAA Applied Aerodynamics Conference, Honolulu, Hawaii, August, 2008. AIAA 2008-6227

3. Metzger, J. D., Snyder, M. R., Burks, J. S. and Kang, H. S., "Measurement of Ship Air Wake Impact on a Remotely Piloted Aerial Vehicle," Proceedings of the 68th Annual AHS Forum, Ft. Worth, Texas, May 1-3, 2012

4. Young, A. M., "Captive Helicopter-Kite Means," US Patent 2,429,502, October 21, 1947