EM Experiments for Computational Model Validation for AH-1S Cobra Helicopter

Scott D. Nelson
Lawrence Livermore
National Laboratory

8th National High Power Microwave Symposium, April 8-10, 1997
The Johns Hopkins University Applied Physics Lab, Laurel, Maryland

Traditional finite difference time domain electromagnetic models for airframes have utilized cubical or rectangular-solid cell structure. For typical airframe structures, this is geometrically limited and requires an excessive number of cells to model the curved surfaces and small components found in typical aircraft. Helicopters serve as difficult problems in RF coupling analyses because of dimensional changes in the joint structure due to vibrations, the open access structure of helicopter main bodies, and the high density of easily exposed wiring.

The effort presented here describes a set of outdoor experiments performed on the AH-1S Cobra helicopter in order to validate and compare to the computational electromagnetic models of the bulk structure of the airframe in the frequency bands up to 4 GHz. Also included in this were coupling measurements to wires and cables inside of the airframe and various cavity coupling measurements. Experimental work during the Summer of 1997 will expose these aircraft to HPM pulses in this frequency range as part of other activities. Additionally, the low power modeling comparison will be completed in this time frame.


This test is being sponsored by the Live Fire Test and Evaluation (LFT&E) Office which is part of the Office of Test and Evaluation (OT&E) under the Office of the Secretary of Defense. The test consist ed of a test suite performed in FY96 encompassing EM susceptibility tests. The HPM Test Facility, Junction Ranch, NAWC, operated by the Radar Cross Section Outdoor Branch, Electronic Combat Range, at the Naval Air Warfare Center Weapons Division, China Lake, California, was selected as the location to conduct this test in cooperation with the Helicopter Dynamic and Static Rotor Blade Ballistic Vulnerability Test, Task I, FY96. The AH-1S Cobra Helicopter was selected as the test object for coupling analyses because it was available in quantity, was suited for external and internal coupling measurements, provided access to subsystems, and was available for this type of testing. The purpose of these tests is to confirm computational model pre dictions for the system susceptibility and to then ex trapolate that knowledge to computational models of other systems about to enter production; specifically the RAH-66 Comanche helicopter.

The general electromagnetic environment of the aircraft includes both externally-generated high intensity radiated fields and internally-generated radiated or conducted emissions. Part of the tasks of this test suite was to evaluate not only the external threats, but to also gauge the severity of internally generated threats for comparison with the model's predictions. The ability to withstand internally-generated emissions is often referred to as electromagnetic compatibility.

Figure 1. the test site during low power testing


The test series had three principal motivations:

  1. to provide specific susceptibility information on the AH-1S,

  2. to evaluate and compare the computational electromagnetic model to the test data, and

  3. to extrapolate these test results and computational data to the RAH-66 for use in that phase of the second part of this effort.
As such, this test series had different motivations than those needed for an assessment for the AH-1S. It should be re-empha sized that the intent of this testing was not to per form an assessment on the AH-1S Cobra helicopter.

The purpose of LLNL's low power experimental tests was to validate its computational models for its OSD activities in support of the Live Fire test pro gram.

``The data from the live fire tests are not in tended primarily as a means of improving the computer modeling capability of the design com munity although that is certainly a spin-off. Rather, the primary purpose of these tests is to gather first- order insights into the total ability of a given system to withstand and/or inflict combat damage, includ ing all of the synergisms. Live fire tests are neces sary, and so are computer models. One cannot substitute for the other.'' -- James F. O'Bryon, As sistant Deputy Under Secretary of Defense, Aug. 1996.

Several previous classified and unclassified1 experiments illustrated the need for combined experiments and analyses. Specifically these results, when extrapolated to the RAH-66, will aid in the development of that system. Based on previous experiments2, the wide frequency bands used in this experiment have been found to be susceptibility points for a broad class of electronics systems and represents a peak of the coupling band obtained by combining the coupling into air frames and the sus ceptibility of components/subsystems. Thus, the main coupling points and bands which were isolated as part of this experiment, were those which (a) provide sufficient coupling to the air frame body, (b) are able to penetrate into the interior of the air frame, (c) are able to couple to the electronics of the aircraft, and (d) are susceptibly points. The consequences of (d), will be applied to the electronics proposed in RAH-66 as part of the later analyses. It is generally easier to separate the stress (fluence coupled to assemblies) and strength (failure limit of assemblies) portions of the EM testing since strength testing can be done on the subsystem or component level in a bench top environment and does not require the full aircraft after the localized stresses have been established.

Testing on the RAH-66 has previously been limited to environmental concerns (lightning and static charge), lower frequency measurements3 on the air frame, and higher frequency measurements on the flight control system (Dahlgren, VA). The prime focus of LLNL's experiments was centered on the higher frequency bands, those that have subsystem and component level susceptibilities which have been demonstrated previously4, and those where air frame coupling is expected to be high. An initial question that was examined as part of low power material testing5 indicated that the composite skin of the RAH-66 would not couple high frequencies much differently than non-composite materials. On the other hand, one issue that was not directly addressed by the experimental test series was the required minimum windscreen metallization needed to provide effective shielding for the interior. This process will be better evaluated as part of the computational analysis of the RAH-66.

Note that seam and crack penetration effects due to vibration (in-flight dynamics) of the air frame were not performed due to time and logistical constraints. This issue is important to resolve since it greatly impacts the computational comparison effort. It was felt at the time that the process of accessing the internals of the airframe was similar to the kinds of day-to-day variances that vibrational effects could have provided. This issue will be addressed in more detail during the operational testing scheduled for Summer of 1997.


The experiments were conducted on an electrically complete AH-1S aircraft (except for the battery). It is critical for this series of electromagnetic tests that the air frame be in its ``pre-modified'' configuration; e.g. the standard operating condition before modification for the Rotor Blade Ballistic Vulnerability Test series. This was especially important for the avionics sections and the additional shielding that the BVT added to the air frame. This did not pose a problem since the aircraft used in the electro magnetic testing were succinct from those used in the ballistic testing.

Figure 2. AH-1S with weapons pods

The helicopter was tested in three configurations which allowed for from below illumination (45o declination), from above illumination (45o elevation), and nose-on illumination (0o elevation). In particular, the from above orientation was expected to be most susceptible to windscreen coupling and access lines via rotor ports while from below coupling was expected to be induced through instru mentation and weapons bays. The coupling and susceptibility analysis consists of several instrumentation geometries as described below. These geometries are designed to

  1. provide preferential coupling into the main body of the air frame,

  2. provide optimal resonant coupling onto feed lines, and

  3. deposit energy on-target to susceptible sub systems. The combination of these three is easily recognized as some of the goals of a typical RF as sessment analysis for weapons and communications systems.

In the course of testing the aircraft, it was necessary to expose the helicopter from a vari ety of orientations. These orientations can be achieved by rotating the helicopter around the cen ter point. The goals of changing the orientation are three fold:

  1. to align major feed lines with the polarization of the incident wave field;

  2. to orient the major cavity coupling paths to the direction of the incident wave field; and

  3. to reduce standing waves formed between the air frame with the surrounding platform, the ground, and the RF source trailer.

Figure 3. inside of tower looking at optical fibers

The 9000 lb. weight of the helicopter posed struc tural requirements on the support tower which were addressed through the use of high denisty urethane foam. This tower formed the base of the support structure and was topped by a spherical vaulted cav ity and a wooden platform to distribute the weight of the helicopter. The tower had the following prop erties suited for electromagnetic testing:

  1. low scattering cross section (foam construction)

  2. very low metal parts count

  3. minimal wavefront blockage between the source and the object under test

  4. the ability to rotate the object-under-test on the platform

  5. the ability to withstand the vibrational modes of operation of the test object


The major cavities and compartments of the air frame were instrumented using a variety of RF probes, including non-obtrusive monopoles and field probes, to measure volumetric resonances in the internal cavities. Strategic feed lines were in strumented using high frequency current probes.

Figure 4. test points in the low power test

Signals from the various instrumentation packages were conveyed from the aircraft to the recording computers through a series of fiber optic transceivers to avoid the severe losses and intrusiveness into the test volume that would have occurred by using coaxial cables.

Incident RF Power

The incident RF power was provided from the RF source (see fig 5) in the LLNL portable source trailer. The transmitted energy was launched from the broadband antenna attached to the top of the extendable mast shown previously in figure 1.


Figure 5. 200W TWT sources generate output RF

The RF sources in the portable source trailer were swept from 100 MHz to 4 GHz in order to get the broad spectral characteristics of the helicopter in the instrumentation configurations mentioned above. The source was connected to the transmit antenna on the top of the extensible mast through the con necting coaxial cable. The output power density from the source was a maximum of 1 W/m2 at an operational distance of 10 meters.

Transmit antenna

The output power was radiated through broad band transmit antennas that had a spectral response covering the operating frequencies. The gain, and hence the beamwidth, of the antenna varied as a function of frequency but was broad enough to cover the test region at a distance of 10 meters. The antenna is positioned at a slight angle to prevent resonances between the air frame and the antenna itself. The mutual coupling between the antennas was taken into account during the measurement us ing both the low-frequency antenna pair and the high-frequency antenna pair. The initial operating polarization was vertical but was alternated between vertical and horizontal during the tests. The interaction of the antenna polarization with the ground and the tower determined which polarization not to use. The interaction of the polarization with the helicopter assemblies determined which polarization to use.

Figure 6. low-freq. broadband antennas

Incidence angles

There was a need for three incident angles as part of this test suite which included:

  1. a look down angle from above with the heli copter on the tower to aid in windscreen coupling mechanisms for the dominant coupling angle.

  2. a look up angle from below with the helicopter on tower to utilize weapons pod and structural member coupling and included that from feed lines and interconnects. This mechanism was expected to be of secondary nature due to the existing shielding of the air frame external structural members but, unlike the windscreen effects, is prone to buildup of standing waves corresponding to the physical lengths of the connecting lines.

  3. a nose-on illumination with the helicopter on the tower to examine main body and front compartment coupling mechanisms.

Test Aircraft

This test used the AH-1S helicopter in an instru mented configuration. The aircraft was in a fly-able condition and was intact, operational, and contained cabling and wiring assemblies in their original positions. In general, test specimens were selected from the body of samples such that:

To be explicit, it was specifically desired that the aircraft used as part of this electromagnetic testing contain its avionics packages and not have additional armor (added to many of the samples for the Helicopter Dynamic and Static Rotor Blade Ballistic Vulnerability Test, Task I). Coordination between the two tests occurred such that the goals of both tests could be achieved without disruption to either.
Table 1
EMC from main cavity (cockpit) to main body assumes that cavities are resonant or highly over moded. determine if internal coupling is appreciable
coupling from transmit antenna to main cavity specific window details are not important if main window coupling dominates measure the level of window coupling and its affect on the cockpit cavity modes
coupling from transmit antenna to feed lines dominant modes of feed lines can be set up through the shielding measure the feed line coupling for (later) subsystem analysis
air frame body resonances subject air frame has to be accurately modeled geometrically in order for good agreement to occur calculate the body resonances and compare with impulse ``ringing'' signature
weapons compartment coupling paths compartment can be accessed for probe placement and is mechanically stable (consistent) measure the secondary path into the cockpit for subsystem coupling analysis


For the purposes of the electromagnetic testing, a real crew was not necessary nor used. The computational portion of this effort will determine whether a crew simulation is required due to significant alterations of the modal patterns. Previous efforts by LLNL did include a simulated crew (see fig 7)

Figure 7. slice of 3D model showing cellular profile

The requirements and goals for the the data acqui sition series is shown in Table 1.

Field Mapping

A limited field mapping that covered the target area was performed to determine the uniformity of the illumination in the presence of the boom, tower, surface effects, etc. This involved a 2D scanning of the planar region normal to the direction of propagation at a point above the foam tower. The results represented a flatness to within +1.5dB. The specific cause of the excursions was not isolated during testing.

Figure 8. field amplitude across target area


Since personnel access to the helicopter needed to occur throughout the test suite, adequate precautions were made to insure that both physical and electromagnetic safety guidelines were followed.

For physical safety, the requirements of Naval Air Warfare Center, China Lake, Naval Weapons Center (Instruction 5100.6C, Safety and Industrial Hygiene Manual), and the Lawrence Livermore National Laboratory, (LLNL Health and Safety Manual, UCRL-MI-118839) were observed. Since the tower was specifically designed not to interfere with the electromagnetic wavefront, then the amount of physical barriers that were possible was limited, thus access to the tower platform was lim ited to essential personnel. These personnel were attached to the bucket truck via a harness at all times while on the tower.

Although the radiated power levels were low, for electromagnetic safety reasons precautions were taken to ensure unintentional exposure and incidental exposure would be in accordance with the limits in LLNL's Health and Safety Guidelines[8,9].


Preliminary results from some of the test points show the kinds of shielding results obtained from the main body of the Cobra. Not surprisingly, the equipment in the cockpit sees no shielding since the canopy structure and materials provide no shielding. While the tail section contains the radios, RF equipment, and is structurally rigid to support the forces involved and has higher effective shielding. On the other hand, the mid section appears to be getting gain from the concentration of energy from outside the craft to the inside via the metal structures.

Figure 9. (a)-(d) shows some of the coupling data from four of the test points in the helicopter. Plots (a) and (b) are points in the cockpit; plot (c) is a point in the mid-section; plot (d) is a point in the tail section.

The complete analysis of this data is part of the FY97 activities which started in early April 1997 when this phase of the project began.


Preliminary results for the RAH-66 Comanche are shown in the figures below. This illustrates the cubical cell structure mentioned above and shows the EM waves impending on the front of the helicopter. In this case, a CW wavefront is being used.

Figure 10. EM wave propagation incident from the left

Figure 11. solid model of helicopter

The CAD model of the Cobra helicopter shows the main features of the helicopter. The computational EM modeling portion of this effort, and the continuation of the RAH-66 Comanche efforts, are part of the FY97 activities.

Figure 12. thick slice of computational model of helicopter


The author would like to thank Gen. Samuel Cockerham (Ret.) for his insights into operational requirements for testing motivations, Mark Henderson, David Schriner, and the JR support crew for their efforts in connection with the low power testing, and the LLNL EM Lab staff for their diligence in completing a successful field experiment.

Work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract W-7405-ENG- 48.


  1. Balanced Technology Initiative Comparison of Ultra Wide band (UWB) and High Power Microwave (HPM) Susceptibility of Rochelle Salt, Lawrence Livermore National Laboratory, Jan. 1992, UCRL-ID-111676.

  2. A. Poggio, R. Zacharias, S. Pennock, C. Avalle, R. Sharpe, K. Kunz, C. Meissner, ``NASA FBL / PBW Program NASA Boeing 757 Low Power On-the-Ground-Tests,'' Lawrence Livermore National Laboratory, UCRL-ID-118775, Oct. 1994.

  3. B. Smith, ``Low frequency testing of the Electromagnetic [physical] model of the Comanche Airframe [frequencies > 100 kHz]'', Army Research Lab, 1996.

  4. W. G. Magnuson, R. J. King, W. C. Ng, ``An Overview of Circuit Analysis and Experimental Validation of HPM Upset and Burnout,'' 4th National Conference on High Power Mi crowave Technology for Defense Applications, Naval Post Graduate School, Montery, CA, May 9-12, 1988. Lawrence Livermore National Laboratory UCRL-97845.

  5. W. Ng, R. Zacharias, ``Composite material shielding effectiveness for RAH-66,'' Lawrence Livermore National Laboratory, part of the briefing package presented to James O'Bryon, Live Fire Test Office, Office of the Secretary of Defense, Jan. 17, 1996, and Comanche TIWIG, Jan 24, 1996.

  6. W. Ng, ``HPM Testing of Electronic Components,'' Lawrence Livermore National Laboratory, UCID-21687, May 1989.

  7. K. Griffin, ``EMP Susceptibility of Various Cables,'' Lawrence Livermore National Laboratory, LEN 22380, Oct. 1989.

  8.  ``Nonionizing Radiation and Fields,'' from the LLNL Health and Safety Manual, section 26.12.

  9.  ``Electrical Safety,'' from the LLNL Health and Safety Manual, Chapter 23.

For further information contact:

Scott D. Nelson, nelson18@llnl.gov

Last Modified: Apr 4, 1997

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