N15A-T009 TITLE: Towed Magnetic Anomaly Detection (MAD) Aerodynamic Modeling for Rotary Wing Platforms

TECHNOLOGY AREAS: Air Platform, Sensors, Electronics

OBJECTIVE: Develop and validate aerodynamic models and dynamic simulations of Towed MAD systems from rotary wing aircraft.

DESCRIPTION: Magnetic Anomaly Detection (MAD) systems are used on U. S. Navy airborne Anti-Submarine Warfare (ASW) platforms to detect and track submerged submarines. Normally fixed wing platforms such as the P-3C and its predecessors use an inboard MAD system in a non-magnetic tail boom or stinger to help isolate the aircraft magnetic noise from the sensor. Rotary wing platforms, such as the SH-60B and predecessors, tow the magnetometer approximately180 feet from the aircraft in a non-magnetic tow body, to isolate the sensor from the aircraft magnetic noise. Currently, NAVAIR is proposing the integration of towed MAD systems on MQ-8C Fire Scout and MH-60R/S. The possibilities for a MAD sensor include a sensor similar in size and weight to the ASQ-81 used on the SH-60B and much smaller magnetometers that only weigh a few pounds. There are expressed concerns because these tow bodies are so light and will have minimal drag that they will be adversely affected by the rotor wash when near the aircraft during launch and retrieval causing unstable flight which could damage the tow body and/or aircraft and may trail dangerously close to the tail rotor.

There has been much research on towed decoys from fast moving fixed wing platforms such as the F-18 (see references), but very little on towed bodies from rotary wing platforms which are much slower and the rotor wash can affect the tow body when it is near the helicopter. Additionally, the MAD sensor requires much more stable flight compared to the towed decoys in order to reduce the motion generated magnetic noise and thus maximize the detections range. Vertical motion and pendulum motion of the tow body (magnetometer) in the Earth�s field creates magnetic noise as well as attitude (Roll, Pitch & Yaw) changes and vibration. The ASQ-81 is not as sensitive as today�s state-of-the-art technology, and motion noise was not a major issue. The state-of-the-art magnetometers are more than 100 times more sensitive than the ASQ-81 so this motion generated noise needs to be reduced as much as possible. The stability goal of the towed body, when the towing aircraft is straight and level at constant airspeed, is to maintain attitude (Roll, Pitch and Yaw) within 0.5 degrees and altitude changes to less than 1 foot. Second order effects such as strum on the cable and helicopter vibrations telegraphing down the cable, can cause magnetic noise on the sensor which can also reduce the Signal-to-Noise Ratio; thus, detection range should also be investigated and modeled to determine the vibration and acceleration effects on the tow body.

The model should include input variables such as:
� Tow Bodies: ASQ-81 size (7 inch (") diameter, 60" long, 24" diameter drag skirt, Center of Gravity Towed, 30 pound sensor (lb) weight) and next generation magnetometers (2" diameter, 18" long, 4 lb weight estimated)
� Tow Body Design: Combinations of Center of Gravity towed, nose towed, drag skirt, cruciform tail, and aerodynamic wings and other passive designs that may improve stability.
� Tow Cable: Nominal 0.125 to 0.25 inch jacketed cable, smooth jacket and modified jacket to reduce strum. Magnetometers require power and return (10 � 28 watts @ 28 VDC) and Ethernet (copper or fiber-optic)
� Tow Cable Length: 180 to 300 foot from tow aircraft
� Airspeed: MQ-8C (Bell 407) and MH-60 operational envelope (Approximately 60 � 120 Knots (Kts))
� Aircraft Tow Point: Under-slung or outboard pylon

The model should simulate and determine:
� Roll, Pitch and Yaw stability (PSD from 0.05 to 10 Hertz (Hz))
� Vibration (XVZ Accelerations, PSD from 0.05 to 100 Hz)
� Damping or pendulum motion (When the helicopter comes out of a turn, determine how long it takes to tow body to return to straight and level flight).
� 3 Dimensional position of tow body relative to the aircraft at the airspeed range listed.
The goal of the MAD towed body design is when the helicopter is in straight and level flight to maintain attitude (Roll, Pitch & Yaw) to within 0.5 degrees, altitude change less than 1 foot, and to damp out pendulum motion as quickly as possible, and to reduce any cable strum or vibration interaction with the tow body.

PHASE I: Determine feasibility of proposed methodology to model and simulate the towline shape, towed body 3 dimensional position relative to the tow aircraft, and tow body stability during deploy, recovery and fully deployed for various size tow bodies, tow lengths, cable diameters, cable outer jackets and tow speeds. The algorithms should also determine the drag (weight) at the aircraft and any airspeed related instability in order to size the reeling machine accordingly.

PHASE II: Develop prototype algorithms and demonstrate simulations on various configurations of tow bodies, cables, cable lengths, tow speeds, and aircraft attachment points. Through simulation, determine the most stable design(s) within Size Weight and Power (SWaP) and tow requirements. Construct non-magnetic tow body or bodies and perform wind tunnel and flight tests to validate the design and refine algorithms.

PHASE III: Partner with Towed MAD system developers to provide design and stability data of the Towed body assist, with wind tunnel and/or prototype flight tests. Validate and refine algorithms based on wind tunnel and flight tests. Work with helicopter and Towed MAD System primes and NAVAIR to provide data for flight safety qualification and testing.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: The geophysical community uses towed magnetometers for geophysical research and mapping. These algorithms could be used to refine the design of the towed system to improve performance and update the towed magnetometer systems to the much smaller units currently being developed.

REFERENCES:
1. Barnes Jr., B., & Pothier, J. (1971, June). Wind Tunnel Measurement of Airborne Towed Cable Drag Coefficients. School of Engineering, Air Force Institute of Technology (AU), Wright-Patterson AFB OH. Retrieved from: http://www.dtic.mil/dtic/tr/fulltext/u2/754889.pdf

2. Richardson, T., (2005, June). Parametric Study of the Towline Shape of an Aircraft Tow Decoy. School of Engineering, Air Force Institute of Technology (AU), Wright-Patterson AFB OH. Retrieved from http://thehuwaldtfamily.org/jtrl/research/Aerodynamics/Parametric%20Study%20of%20Tow%20Line%20Shape%20of%20an%20Aircraft%20Decoy,%20Thesis,%20Richardson.pdf

3. Leijonhufvud, M., Lindberg, A. (2008). Dynamic Simulation of a Towed Decoy. International Congress of the Aeronautical Sciences. 26th Ed.

KEYWORDS: Tow cable; Anti-Submarine Warfare (ASW); Magnetic Anomaly Detection (MAD); Aerodynamic Modeling; Towed Sensors; Fire Scout MQ-8C; MH-60R

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