acfd challenge ii revisited after 15 ... - bombs-r-us.com

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8th ANKARA INTERNATIONAL AEROSPACE CONFERENCE AIAC-2015-022 10-12 September 2015 - METU, Ankara TURKEY

ACFD Challenge II Revisited after 15 Years

M.G. Tutty and G. Akroyd, RAAF, Canberra and Adelaide, Australia

A. Cenko, Bombs-R-Us.com, Huntingdon Valley, PA, US

ABSTRACT

The Applied Computational Fluid Dynamics (ACFD) program was a US Department of

Defense sponsored tri-service effort to insert CFD into the aircraft stores separation/certification

process. ACFD sponsored two American Institute of Aeronautics and Astronautics conferences

to determine how well CFD could match wind tunnel/flight test results: ACFD Challenge I in 1992

and ACFD Challenge II in 1999. Since that time, ACFD Challenge II has become the default

benchmark standard for comparisons for CFD predictions to flight test results. Several of the

organisers and participants are still active in the profession of arms and aircraft stores separation

/ certification and describe how this challenge has affected that process over the last fifteen

years.

Introduction

Since the time that computer based modelling and simulation (M&S) and then

Computational Fluid Dynamics (CFD) was first found potentially capable of representing the

geometric complexity of an aircraft with external stores, there has been the desire to reduce or

even replace the need for the more traditional wind tunnel testing. The three detriments for full

utilization of CFD in this fashion have continued to be computational speed, computer resources

and accuracy of the solution. For example, PANAIR Application to Weapons Carriage and

Separation or AWCAS configuration in the early 1980’s[1], just one solution using a linear code

with less than 1000 panels(maximum), required full utilization of the CDC 6600 supercomputer of

that time for more than twenty-four hours. Clearly, the wind tunnel was in no danger at the time.

As a metric of where we are now, the same solution will now run in less than a minute on a

standard Dept. of Defense issued PC.

Over the past quarter of a century, the US Air Force, Army and Navy, Royal Australian Air

Force (RAAF), and Royal Canadian Air Force, amongst others primarily in Europe, have made

concerted efforts to accelerate the validation and verification necessary to enable the insertion of

the latest CFD methods into the aircraft stores certification process. There have been numerous

organized international conferences for this purpose using the American Institute of Aeronautics

and Astronautics (AIAA), International Test & Evaluation (T&E) Association (ITEA), the US Joint

Ordnance Commander Group (JOCG), the Five Eyes (Australia, Canada, New Zealand, UK and

US) Air Standardization Coordinating Committee (ASCC) for Air Armament, The Technical

Cooperation Program (TTCP) and the NATO Science and Technology Organisation (STO).

The first of these conferences was for a ‘typical’ Wing / Pylon / Finned-Store, which

occurred in Hilton Head, SC in the summer of 1992. One of the important results from this initial

conference was the discovery that full potential methods [2, 3] gave answers equivalent to those

provided by an Euler code [3] for the wing lower surface in the presence of the store.

The second conference was sponsored by the US Office of the Secretary of Defense

(OSD) funded Central T&E Investment Program (CTEIP) Applied Computational Fluid Dynamics

(ACFD) program. This was for the F-16 / Generic Finned Store; the conference took place in

New Orleans in the summer of 1996 and became known as ACFD Challenge I. For this meeting

lower order [4] solutions again exhibited good agreement with Euler and Navier Stokes codes. For

both these challenges, the participants were provided with the wind tunnel test data at the

beginning of the efforts.

AIAC-2015-022 Tutty etal

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The ACFD Challenge II was also sponsored by the OSD CTEIP ACFD program at the

39th AIAA conference at Reno, NV in January 1999 where the aircraft stores separation

configuration was the F/A-18C / Joint Direct Attack Munition (JDAM)i. Australian involvement in

the Challenge was jointly funded by the RAAF and the U.S. Office of Naval Research

International Field Office Asia. Large sets of wind tunnel and flight test data existed for the

F/A18C JDAM configuration, Figure 1, and all the participants showed excellent correlation with

both the wind tunnel and flight test results. A detailed summary of the results for ACFD

Challenge II is available [5, 6]. This configuration has become the standard for store separation

code validation, with many new participants during the past two decades. However, the original

national and international participants did not have the advantage of knowing what the answers

were prior to conducting the calculation.

Figure 1. F/A-18C JDAM aircraft stores separation configuration for ACFD Challenge II

Aim

The aim of this paper is to revisit the Applied CFD Challenge II and discuss how it helped

establish CFD as an accepted tool in the aircraft stores compatibility modelling and simulation,

experimentation, test & evaluation (T&E) and certification process.

Background

Aircraft stores separation forms a key part of establishing the compatibility of an aircraft

stores configurationii to be operationally suitable and effective to perform testing, training and

conduct operations. Traditionally the Five Eyes and many NATO nations use MIL-STD-1763[7]

and MIL-HDBK-1763[8] / MIL-HDBK-244A[9], NATO STANAG 7068[10] and Science and

Technology (STO) AGARDOgraph 300 Vol 29 [11] as the basis for conducting modelling and

simulation (M&S), laboratory qualification wind tunnel tests prior to ground and flight

experimentation, and Test and Evaluation (T&E) to establish the certification basis for the aircraft

stores configurations needed.

The assessment of aircraft stores compatibilityiii includes an engineering review (called a

Judgement of Significance in Australia) by qualified ASC Design Engineers has occurred in the

following disciplines for each aircraft stores combination required to determine if a ‘significant

change’ (as defined in MIL-HDBK-1763) is made to an aircraft stores configuration in the areas of

physical, information, cognitive and social domains of war and their operational suitability and

effectiveness[12]:

I. Function;

II. Form and Fit;

III. Structural & Environmental;

IV. Aeroelasticity;

V. Captive Carriage, Handling/Flying Qualities & Performance;

VI. Employment & Jettison;


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VII. Information Suitability: External Interfaces, Mission Planning, Ballistics and

OFP Validation & Verification, Safe Escape & Danger Areas (Safety Templates)[12];

VIII. Cognitive Suitability: Procedures, Tactics, Techniques and Procedures and

Human Factors;

IX. Emergent Properties for Critical Operational/Technical Issues / Measures of

Suitability/Effectiveness: Experimentation and T&E.

Engineering Review

The engineering review is most important for establishing such a degree of

interoperability i.e. compatibility and assessing interchangeability should commonality of doctrine,

equipment or processes not be agreed. Use of the ‘significant change’ criteria in MIL-HDBK-1763

now gives the design engineers and operational users some tolerances that enable minor

changes to be progressed without the huge systemic and organizational overheads of traditional

‘point design’ engineering done without interchangeability and prior thinking in mind. Use of such

methodologies clearly shows the maturity of any organization’s processes and leadership.

Figure 2. An Aircraft Stores Configuration Operating Limitations for

Carriage and Employment (Jettison not shown here).

Depending on the maturity of the stores and/or aircraft, there are four separate compatibility situations involved when authorization of a store on an aircraft is required. The four situations, in order of increasing risk, are:

• Adding ‘old’ in-service stores to the authorized stores list of ‘old’ aircraft.

• Adding ‘old’ stores to the authorized stores list of a ‘new’ aircraft.

• Adding ‘new’ stores to the authorized stores list of an ‘old’ aircraft, or adding new aircraft

stores configurations and/or expanding the flight operating envelope.iv v.

• Adding ‘new’ or modified stores to the authorized stores list of ‘new’ or modified aircraft.

The assessment of aircraft stores compatibility will determine the operating limitations that

will then be used by the aircrew in their Flight Manuals, as shown at Figure 2. The aircraft stores

configurations and expected operating limitations are always included in a good Concept of

Operations (Conops) as they may not need to be the maximum that the aircraft and stores can

achieve (i.e., Parent pylon versus multiple ejector rack configurations typically will have different

limits). For more mature aircraft and/or stores, and consequently those with less risk, the process

is specifically tailored against the OCD / Conops such that only those phases required to be

conducted to introduce the store into service need to be undertaken. For example, if all the

aircraft stores configurations have been successfully demonstrated or certified by known

ALITITUDE ( ft)

30,000

0.4 0.6 0.8 1.0 1.1 1.2 MACH NUMBER

700 KCAS

600 KCAS

500 KCAS

400 KCAS KCAS 300

CARRIAGE

EMPLOYMENT

0.9 0.7 0.5 0 0

20,000

10,000


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Experimentation, T&E and (airworthiness) certification agencies to operating limits that satisfy the

User’ s Operational Requirement, an aircraft stores combination could be introduced directly into

service with minimal risk. While this strategy has been extremely successful in minimising the

work with specific aircraft stores configuration in an acquisition process that is platform-centric, it

is often thought less successful when viewed in the context of designing interchangeable stores

on fewer platform types. This warrants investment in trade-off studies to determine future

Armament Integration Mission Environment needs and often a capability realisation plan/strategy

with specific interoperability (covering the systems / System of Systems / Families of SoS [12],

their operating limits and the levels i.e., compatible versus interchangeable or common)

requirements in mind that meets both national and NATO/Five Eyes needs.

Certification approval. The agency requesting aircraft stores certification should be as

specific as possible as to their requirements in each of the areas above to assist the certification

agencies in establishing the criteria to be used in the clearancevi and certification effort. Through

the initial certification request and, if necessary, subsequent follow-ups, the certification agencies

will determine the appropriate criteria to be applied to the specific store certification program.

These criteria will include (but not be limited to) essential and desired aircraft stores

configurations (including any mixed load configurations) and the essential and desired operating

limitations such as: carriage speeds and accelerations, dive (or climb) release angle, release

modes, speeds, intervals and accelerations, selective and emergency jettison speeds,

accelerations, flight path angles, and required levels of accuracy, etc., as required for the aircraft

stores combination to be operationally effective.

Formal approval for certification of an aircraft-store/suspension equipment configuration is accomplished through publication of operational data in appropriate technical manuals. These are:

a. Navy: NATOPS Flight Manuals and Aircraft Tactical Manuals.

b. Army: Technical Manuals (Operators, Maintenance and Parts).

c. Air Force: Aircraft Technical Orders (-1, -2, -5, -16, -25, -30, , -33, -34, and -35).

d. Joint-Service Technical Data publications

Predicting safe and acceptable aircraft stores separation trajectories. As noted in MIL-HDBK-1763 etal [7-12] predicting accurate store separation trajectories on today’s high speed aircraft under the varying conditions of altitude, Mach number, flight path angle, load factor, and other factors related to delivery techniques (particularly where multiple carriage of stores is involved), is an extremely difficult task, requiring a skilled and experienced analyst. Several techniques are available for store separation analysis, and these are documented throughout the scientific literature. There are well proven wind tunnel and Computational Fluid Dynamic M&S experiences that have supported advanced weapon development and integration. Most Five Eyes and NATO nations use a variety of unique CFD codes to augment wind tunnel testing. These techniques have been extensively validated for external store separation. During the past decade, various AIAA Challenges have seen great progress and the US, under the auspices of the DoD High Performance Computing (HPC) Modernization Program Office have combined each of the Services’ initiatives to establish an Institute for HPC Applications to Air Armament (IHAAA) which has included key NATO and Five Eyes nations. Some are purely analytical in nature, utilizing theoretical aerodynamics and complex mathematical manipulation and analyst interpretation. Others utilize wind tunnel testing of small scale models of the store and aircraft, while still others involve a combination of theoretical and wind tunnel data, utilizing a high speed digital computer for data reduction. Wind tunnel test data for store separation may be obtained from one, or a combination of, the following:

a. Captive trajectory. This test uses a strain gauge balance within the separating store to

continually measure the forces and moments acting on the store. An on-line computer

simulation determines successive positions of the store through its trajectory.

b. Grid data. An instrumented store or pressure probe is used to measure the forces and

moments acting on the store in the flowfield through which the store must separate.


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Trajectories are calculated off-line using this information as inputs to a trajectory program.

c. Dynamic drop. The dynamic drop tests use dynamically scaled models that are physically

separated in the wind tunnel. Data can either be photographical or telemetry. (This

method is generally limited to simulated level flight releases only.)

d. Carriage loads. In this test forces and moments are measured on the store, with the store

or weapon attached to the aircraft in its correct carriage position. These data are used as

inputs to trajectory computation programs.

No single technique will suffice for all cases. Rather, the analyst must examine the

particular case to be analyzed and select the technique that, in his opinion, offers the most

advantages for his particular situation. Most purely theoretical techniques available today suffer

severe degradation when applied to transonic store separation, or where multiple stores carriage

is involved.

The recent advances in CFD and semi-empirical techniques provide excellent tools for

engineering estimations or for use in conjunction with experimental data. Captive trajectories and

dynamic drops are expensive in that they are only for a specific flight condition. [6] Grid data are

superior because many flight and store characteristics can be changed while the grid data are

used as input for 6 degrees-of-freedom equations or other analytical tools. Grid data are also

required when considering the effects of changing the store’s automatic control system

gains/logic. Wind tunnel testing is cheaper than flight testing when the cost of aircraft flight time,

weapons assets, telemetry packages, and photogrammetric analyses are considered. For these

reasons, most analysts today employ hybrid methods which reduce costs while retaining wide

applicability.

Several AIAA stores separation workshops have seen M&S predictions challenged by blind

(hidden) actual flight test results. CFD has matured so that, given one

has enough time (and funds for advanced computer time for the number

of cases required), CFD can now clearly predict trajectories – however,

time and cost effective M&S are still a trade-off against accuracy and

fidelity (imagery at right courtesy of USAF SEEK EAGLE Office).

Aircraft stores separation analyses. As a first step in store separation analysis, all

available flight test and predicted data pertaining to the separation characteristics of the store in

question, either from the aircraft being examined or others with similar installations, should be

accumulated and screened for completeness of flight envelope coverage and for trends. If

existing data covers the store’s separation characteristics from the proper aircraft throughout the

desired flight envelope, delivery conditions (speed, dive angle, load factors, altitude), delivery

configuration and mode (single, pair, ripple, etc), little or no additional testing may be required to

allow certification. If this is not the case, however, additional data must be obtained in accordance

with the method of store separation prediction chosen. The mainstay analysis tool of the

weapons clearance community is the Captive Trajectory System (CTS). [12, 13] This tool was / is

usually the right level of fidelity for external clearance problems, because it has matched a very

rapid prediction capability (due to the assumption of quasi-steady flow), with an external store

flowfield which was quasi-steady. External flow over an aircraft in steady level flight (conditions

when you typically release weapons) is designed to be non-separated and steady. If the aircraft

is well-designed, then that is precisely what the external flow will be – attached, non-separated,

and relatively steady. For external weapon carriage and release, the flowfield that the store is

immersed in is therefore predominantly steady, and CTS works well for the prediction of store

trajectory in the majority of those cases. The exception (where CTS may not work well) for

external store integration is cases where aeroelastic effects predominate, and the carriage

structure movement itself is partly responsible for driving the flow unsteadiness.

Weapons bay cavity flows, on the other hand, are naturally and rather dramatically

unsteady, due to a robust self-reinforced acoustic resonance phenomenon, coupled to and driven

by an equally robust free shear layer instability. Within the vicinity of the open weapons bay

cavity, the quasi-steady assumption (the assumption that the store sees a single value of forces

and moments at each position and orientation which is constant and independent of time) does


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not hold. All weapons bays with dimensions typical in modern aircraft installations exhibit this

self-sustained acoustic resonance, to varying degrees. This would lead one to conclude that it

may not be very conservative to use the CTS process to clear weapons released from within

weapons bays. The exception to the strong unsteady weapons bay behavior are cases where

effective flow control devices (such as properly designed leading edge spoilers) have been

employed to suppress self-sustained oscillations, and the resultant high acoustic levels and

unsteady loads have been suppressed. Unsteady pressure levels in unsuppressed weapons

bays can be high enough (160 to 180 dB) to damage aircraft bulkheads, or to “instantly” tear off

weapon components

It is useful at this point to clearly define what is meant by unsteady weapon trajectory

effects. It is easiest to describe the most dramatic case – which was termed a “bifurcation” [13].

The unsteady shear layer in the weapons bay is the dominant source of flow unsteadiness.

When a weapons bay experiences strong acoustic resonance, the flow tends to take on a

twodimensional character, with the formation of coherent 2D “rollers” which span the cavity. In

this situation, an unsteady component of normal force is created along the bay which changes

sign from instant to instant – from “into the bay” to “out of the bay”, and vice versa. It is possible

in this circumstance, depending on the time of release of the weapon, for the store to tend to fly

toward or away from the bay, depending on the time of release. This pitch “bifurcation” behavior

is the most dramatic example of unsteady weapon trajectory effects.

Separation operating limitations. Store employment covers separating the store from

the aircraft in its normal operational mode. It should cover separations at all speeds up to the

allowable speed in level and maneuvering flight, both in the single release mode, and in multiple

release (ripple) mode down to the minimum release interval. Particular attention should be given

to releases of unpowered stores in large dive angles (60° or greater) at the attendant low g

(cosine of the dive angle). Such separations can be, and often are, extremely dangerous,

particularly for unstable or low density stores. In determining the separation envelope, the review

should also consider that some parts of the flight envelope will not require analyses due to a more

restrictive dive recovery or safe escape limitation. It should also be kept in mind that proper store

employment denotes not only safe separation from the aircraft, but also that the separation be

relatively unperturbed so as to assure rate capture and not to adversely affect delivery accuracy.

Analyzing the launch transient phase of store separation is extremely difficult. It generally

involves guided stores, such as electro-optical guided bombs, which contain autopilot and

guidance systems that are active during store separation to avoid target breaklock or radical store

movements caused by release perturbations. If every component functions properly, separation

will be completely safe and unperturbed. However, control failure or spurious guidance signals

causing abnormal control deflections at release can cause high-energy collisions with the aircraft.

Because of these possibilities, a reliability analysis of the store guidance and control system will

be performed, and the results of possible failures identified and examined for probability of

occurrence and effect on store separation. Although no specific pass-fail criteria can be used in

all cases, probabilities of failure of a single component causing an impact on the aircraft should

be kept in the realm of 10-6. If this cannot be done, the effects of control failure modes on

separation trajectory should be analyzed or store redesign should be effected prior to flight

testing.

Internal weapons carriage is being used to improve the aircraft aerodynamic performance and

low observable characteristics. The separation of stores from a weapons bay may be

significantly impacted by the unsteady flow in the bay. These temporal effects may not be

captured during wind tunnel testing which use a quasi-steady approach to run the CTS to

determine the store trajectories. One of the first IHAAA tasks undertaken by the store

separations team was the Store Separation from Cavity project. The goal of this project was to

determine CFD application best practices for the separation of stores from weapons bays. This is

a current real-world problem that can benefit from the optimal application of HPC technology.


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Jettison criteria. Jettison of a store (or a store/suspension equipment combination)

involves the releasing of items from the aircraft during emergencies (emergency jettison) or as

normal operation after expenditure of cargo or submunitions (selective jettison). Examples of

these would be weapons, fuel tanks, gun pods, dispensers and multiple bomb racks complete

with some or all of its weapons. The primary concern of any jettison is to separate the item, or

items, from the aircraft safely, without collision, because there is no requirement for accurate

delivery. This phase of store separation is by far the most dangerous to the releasing aircraft

since many items jettisoned are aerodynamically unstable, usually of low density, and their

separation behavior is generally erratic and unrepeatable. If at all possible, the jettison envelope

of a store should be close to the full authorized carriage flight envelope. Jettisons are, however,

commonly limited to level flight (plus and minus a reasonable g tolerance, and sideslip). Jettison

envelopes that are limited to a single speed, or those that specify a very narrow speed, altitude or

dynamic pressure band, should be avoided, if at all possible.

ACFD Challenge II Discussion

Large sets of wind tunnel and flight test data existed for the F/A-18C JDAM configuration

as a result of USN store certification effort. During the flight test phase, both photogrammetrics

and telemetry were used to track the position of the store during releases. Out of these tests, two

release conditions were selected for this CFD Challenge. The basis for these two cases included

the following considerations: 1) matching aircraft and store geometry in both wind tunnel and

flight tests, 2) correlation between wind tunnel data and flight test data, 3) possession of both high

transonic and low supersonic cases with interesting JDAM miss distance time histories, 4) ability

to publicly release the wind tunnel and flight test data to an international audience.

The test cases for the F/A-18 GBU-38 JDAM configuration at Figure 1 were M = 0.962 at

6,382 ft. and M = 1.05 at 10,832 ft. Both cases were for the aircraft in a 45-degree dive. For

these two test cases, the configuration geometry for the wind tunnel and flight test were with the

JDAM mounted on the outboard pylon, with the 330-gallon fuel tank on the inboard pylon. The

SUU-63 BRU-32A/A ejector rack provided a nominal peak force of 7,000lbs for both forward and

aft ejectors. The implementation of ejector force modeling was left at the discretion of the

participants. Both CTS grid data, and store aerodynamic force and moment data, measured on

the wing pylon, were available for these aircraft configuration. These data were input into a

sixdegree-of-freedom trajectory code before the flight tests were performed. Parametric

variations on flight conditions and store aerodynamic forces were performed to ensure that the

flight test could be safely accomplished. After the flight tests were completed, the trajectory

simulations were again performed, with the actual flight conditions used to try to match the flight

test results. Case 1 was flight test #13 conducted on July 10, 1996. The store was released in

a 43 degree dive at 6,382 ft. at M = 0.962. Case 2 flight test #14 was conducted on August 29,

1996. The store was released in a 44 degree dive at 10,832 ft. at M = 1.055. [6] Each participant

was requested to include in their paper.[6]

1) a description of the CFD and trajectory integration methods used to produce the estimates

of the trajectory;

2) a description of the methods and resources required to produce the computational grid;

3) estimates of carriage loads, the position and attitude of the store throughout the computed

trajectories and an estimate of the miss distance versus time; and

4) metrics of the CFD process used, including convergence rate, man-hours and time

required for grid generation, computer resources used and an estimate of the expertise of

personnel required to replicate the results.

One of the key, unique feature of ACFD II was that it was intentionally run as a blind test

comparison, i.e. the participants were provided with the aircraft and store geometry, and the flight

test release conditions, and were requested to provide their solution before the actual flight test

results were released. All the papers presented exhibited good to excellent agreement with the

flight test results. [6]


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Eight papers by Cenko [14], Hall [15], Tomaro [16], Woodson [17], McGroy [18], Fairlie[19],

Welterlen [20], and Benmeddour [21] were submitted for the ACFD Challenge II. The meeting was

held at the AIAA Annual meeting in Reno Nevada on January 12th, 1999. Due to the interest in

the Challenge, the timing of the session and the venue were changed to enable seating for

around 200 people; despite this, the room was filled to capacity with over 50 people having to

stand in the back for four hours. The first paper by Cenko described the wind tunnel and flight

test results, while the other seven described the application of seven different CFD codes to the

problem. Two of the papers[15, 18] were not ready in time to be included in the meeting

proceedings, but all eight papers were either presented at the meeting, or the results were

provided at a later date. [6]

The quality of the invited papers and presentations reinforced the approach used by the

AFCD Challenge sponsors. However, taking these presentations as representative of state of the

art for applying current CFD-based tools for stores carriage and separations indicated that wind

tunnels would still be relied on for the provision of the major part of the aerodynamic data on

which stores certification are to be safely based. Indeed it was acknowledged that the CFD

solutions were in the majority of cases within the error range of the wind tunnel and flight test

data. Accuracy would not therefore seem to be issue, but rather the time required to produce a

solution needed to be decreased significantly. The conclusion was that CFD-based tools should

become far more prevalent in use during Requirements Definition and Systems Engineering

trade-off studies for the aircraft and stores thereby reducing the likelihood of expensive aircraft

and/or store redesign after hardware has been made. [6]

One other general result was the consensus that improvements in the ejector modelling

and ejector foot/store interaction during the ejection needed to be accomplished. [6] One of the

principal drawbacks of CFD Challenge II was that all the CFD results, using both Euler and

Navier Stokes, as well as a simulation that ignored the JDAM canards gave similar results. Did

that mean that Navier Stokes formulation does not have to be used, or were the test cases

selected fortuitous for the inviscid formulation? Indeed, Welterlen showed that the inviscid

calculation was superior to the viscous one. Since diagnostic data were not available, it was

impossible to say whether the SPLITFLOW viscous formulation was at fault, or that the inviscid

results had a fortuitous canceling error. It was the consensus of the participants that another

CFD Challenge, one that would have diagnostic data (store and wing pressures) was merited.[6]

Another unique feature of this challenge was that representatives of national agencies in

Australia, Canada and the US formed a judging panel that reported the results [6, 12]

Accelerated development of store-trajectory-prediction techniques using flight

measurements

The previous F/A-18/JDAM CFD Challenge example showed that while gross forces /

moments and trajectory traces are useful for establishing global agreement in store-trajectory

prediction, they do not provide the insight into the detailed flow physics required when analysing

the differences between CFD codes or experimental results.


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Figure 3. F/A-18C/MK-83 Test Configuration and CFD vs PSP CP comparisons

In the collaborative program, Accelerated Development of Store Trajectory Prediction

Techniques Using Flight Measurements (KTa 2-18) [22, 23], pressure-sensitive paint (PSP) was

used at the Canadian National Research Council’s (NRC) High-Speed Wind-Tunnel; and

flighttest-trajectory data for the release of a single MK 83 1000lb class low-drag store from a

vertical ejector on the wing of an F/A-18 were available. The results were compared with CFD

predictions using a range of flow solvers.

Pressure-coefficient (Cp) data derived from PSP measurements enabled better insight into shock

locations and highlighted the issues involved in the use of inviscid codes for release predictions

with viscous effects. Good agreement was achieved both with the pressure comparisons, as

shown in Figure 4, and with the flight-test trajectories. A major benefit of the collaborative activity

was the access to a richer ground-based experimental dataset with flight validation data. The use

of PSP, and the availability of extensive comparative CFD data highlighted limitations of the

experimental technique, such as surface contamination and deterioration [22], as well as minimum

flow solver requirements.

Figure 4. B-1 GBU-38 JDAM aircraft stores separation configuration

B-1 weapons bay releases of GBU-38

The last such Challenge was for the release of the GBU-38 version of JDAM from the

B-1B bomb bay, Figure 4. One advantage of doing a blind comparison was that all the

participants, at first, came up with the wrong result and gained a deeper understanding of the

discipline before making predictions for use in flight clearance and test purposes. [13, 24]

Warfighter operational needs.

The first direct operational impact of ACFD Challenge II was the quick response to support

an immediate US Navy warfighter need. The operational requirement was for the flight test

clearance of the sequential release of two CVER mounted GBU-12 Laser Guided Bombs from an

F/A-18C aircraft with an adjacent 330 gallon tank to support Operation Iraqi Freedom, Figure 5. Because of time constraints, a wind tunnel test could not be conducted. Without supporting wind

tunnel data or analysis, the typical flight test approach is referred to as a buildup approach (also

known by some neophytes as the hit or miss method). With this approach, store drops are first

started at relatively benign condition and then additional flight tests are performed at increasing

Mach numbers and dynamic pressures while gradually approaching the desired flight release

condition or until it is determined that it is unsafe to continue. This is a costly and time consuming

approach.


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Figure 5. F/A-18 / Canted VER GBU-12 aircraft stores separation configuration and Miss Distance As a result of the ACFD Challenge II, the USAF Seek Eagle Office already had the Beggar [25]

grids for the GBU-12 and for the F/A-18C aircraft. They led a cooperative effort with NAVAIR


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to demonstrate that using computer resources and tools could impact a time critical flight test

program. Within a short period of time, the team was able to perform time-accurate CFD

trajectory simulations and to supply results that compared well with flight test drop at a lower

Mach number. They also simulated the store trajectories at the desired maximum release

condition - a higher Mach number. Although the miss distance during the flight test at the more

benign condition was too close to authorize a flight clearance to proceed to the next condition,

Figure 5, the CFD time-accurate trajectory showed that the store would clear at the desired

condition and authorization to continue flight testing was granted. The computations mitigated the

risk and allowed the flight test program to achieve its goals. The CFD predictions were in

excellent agreement with the flight test results. Based on this, the flight test program proceeded

to the transonic end point, M = 0.97, 45 degree dive. Further details about this project are

available [26]. Figure 5 also shows the computed GBU-31 trajectory from the F-18C/D aircraft.

One additional outcome from the CFD Challenges was an improvement [27] in wind tunnel

testing techniques for store separation.

The success of the CFD Challenges led to joint participation in several further Key Technical

areas (KTa’s) under the auspices of TTCP Panel WPN-2, Launch and Flight Dynamics, as

described in the following sections.

AEDC wind tunnel

Figure 6. F-111 Weapon bay with miniature munitions (PLOCAAS) and ASRAAM in-flight.

© AOSG-RAAF Analysis of the acoustic suppression, active separation control and

release of miniature munitions from RAAF F-111 aircraft

With the advent of the F-35 Lightning II JSF, P-8 Poseidon, and concepts for future Remote

Piloted Aircraft / UCAVs, all designed with internal weapons carriage, forward-looking US and

Five Eyes research programs focused on the understanding of the complex aerodynamics and

aeroacoustics of weapons bays. The RAAF was still operating the F-111, and the Australian – US

collaborators saw opportunities to use a flight-test F-111 to investigate the phenomenology of

cavity flows with the Small Smart Bomb (SSB) in 2001 [28, 29, 30, 31] and, in 2005, Powered Low Cost

Autonomous Attack System (PLOCAAS) shapes from a Boeing preumatic ejector rack using

active separations control [32, 33], shown in Figure 6.

In the collaborative program, analysis of the Release of the SSB from the F-111 Aircraft

(KTa 2-22) [32 - 34], neither the wind-tunnel, nor CFD results matched the flight-test results. Not

unexpectedly, the wind tunnel results did not reflect the carriage to initial release trajectory

because the aft store trajectories started some two feet (at full scale) from the carriage position, as

shown in Figure 8 at right.

Because a trajectory is largely determined by initial conditions, if these are wrong, the

prediction will be in error. The forward store was tested at the end-of-stroke position; and, although

Aft - sting arrangement F - 111 – SSB in the US


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those trajectories seemed to compare better, sting interference effects in the cavity might have

corrupted the subsonic and transonic results. Although this collaborative program did not resolve

the issue of CFD applicability to internal weapon bays, it helped determine the windtunnel-testing

methodology for the F-35 JSF and P-8A Poseidon programs.

Further, the work indicated that the lack of a priori information on sting effects could be overcome with CFD techniques; in this way, stings could be designed for minimal, or at least known, impact.

For these reasons, a new collaborative program, Weapon and Cavity Aerodynamics and

Aeroacoustics (KTa 2-26) [34, 35] was initiated in 2008. The work in this case was based on the

UCAV 1303 geometry [36]. This configuration has been widely studied, and significant experimental

testing has occurred [36, 37] for a generic store in a rectangular weapon bay, along with

complementary CFD.

RAAF F/A-18 JASSM clearance and certification

Prior to the ACFD II challenge there had been some scepticism as to the ability of CFD to

contribute meaningfully to the store separation clearance process at meaningful airspeeds. In

Australia CFD had been successfully used in the late 1990’s to clear the current Mk65 mine from

AP-3C aircraft for mine trials as shown at Figure 7. This was more to validate the USN flight

clearance against the current aircraft and weapon types rather than ab initio effort. The fact that

ACFD II was a blind test and still gave good results established confidence in the CFD methods

within the Australian stores clearance community. This led to greater acceptance and use of CFD

for RAAF stores integration projects including the F-111C integration of GBU-24 and AGM142

missile, also shown at Figure 7. Another example of which is the integration of the AGM-158

JASSM on the RAAF F/A-18A/B.[38]

Figure 7. DSTO - RAAF AP-3C / Mk 65 mine and F-111C AGM-142 missile carriage and

separation using CFD

The small size of the Australian transonic wind tunnel at DSTO Melbourne drives the use of

half aircraft models for stores integration testing. Data from this arrangement has been

consistently proven to be of high quality but obviously suffers from an inability to measure the

effects of sideslip and effects on whole aircraft configurations. Furthermore, the maximum Mach

that can be achieved in the tunnel is limited to just over Mach 1.1 for practical aircraft stores

configurations. CFD has been used to augment the wind tunnel data by filling in the “gaps” in wind

tunnel data.


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Figure 8. F/A-18 A/D AGM-158 JASSM CFD.

The AGM-158 presented a significant integration challenge. This is a large missile of

noncircular cross section with highly non linear aerodynamic characteristics and is asymmetric due

to the upper fin folded to one side for carriage. It is also initially very unstable and requires the

upper fin to deploy and wings to be partially deployed very early to achieve a level of stability

during separation. On the RAAF F/A-18 the primary carriage configuration is on the outboard wing

pylons with External Fuel Tanks on inboard wing stations. The proximity of the EFTs causes

significant aerodynamic interference effects and represents a collision risk for deploying fin and

wings.

Figure 9. RAAF flight test F/A-18 Hornet with AGM-158 JASSM. © AOSG- RAAF

Grid loads were generated for the missile in several stages of wing and fin deployment using

the half aircraft model in the DSTO Transonic wind tunnel. By testing the store model configured

with the upper fin folded to left and also to the right side the effect of the store asymmetry could be

accommodated in the tunnel data. However the effect of sideslip for the potentially sensitive and

risky separation next to the EFT could not be determined in the wind tunnel with a half aircraft

model. To overcome this deficiency, CFD was used to provide the incremental effects of sideslip

on the carriage and grid loads at Figure 9.

The hybrid use of wind tunnel data supplemented by CFD proved to be very effective and

culminated in a series of successful flight tests such as is shown at Figure 10.

Figure 10. F/A-18 Litening Pod with MK84 stores separation F/A-18 separation effects with

targeting pods

As the military and political requirement for precision strike has increased, the requirements

for precision targeting pods, such as the AN/AAQ-28 Litening Pod and AN/ASQ-228 Advanced

Targeting Forward-Looking Infrared (ATFLIR) Pod, have had significant impacts on


P a g e | 14

ASC programs. The pods modify aircraft external geometries and, in many cases, decrease the

store-to-aircraft distances in critical areas. In 2005, Northrop Grumman marketed the Litening Pod

to the Australian and Canadian governments for use on their F/A-18A/B/C/D aircraft.

Northrop Grumman contracted NAVAIR, via a commercial-services agreement, to support

flight certification of the Litening Pod and the associated pylon-mounting system on station 4,

illustrated in Figure 10. The goal was to clear the GBU-12, GBU-38, MK-84, Dual AIM-120, and

330-US-gallon FPU-8 fuel tank adjacent to a Litening Pod to the present TACMAN limits (with an

adjacent ATFLIR).

The following discussion illustrates a number of examples of weapon/pod mixes that

demonstrate the evolution of the tools and techniques applied to the ASC problem.

CFD-based clearance of stores on F/A-18

During the first phase of the project, the lessons learned from the F-18/JDAM CFD

Challenge, as well as from the TTCP “Accelerated Development of Store Trajectory Prediction

Techniques Using Flight Measurements (KTa 2-18)” allowed CFD to be used to clear the MK 65

mine from the RAAF AP-3C for a flight test demonstration and GBU-12, the GBU-38, MK-82, MK-

83, and MK-84 from the F/A-18 parent pylon without the need for wind-tunnel testing[37, 40, 41]. Figure

11 shows a comparison between the predicted pitch, yaw and roll attitudes and the flighttest results

for the MK-84 trajectory. This validated the approach used and determined the next steps to

augment the CFD with targeted wind tunnel data for more complex configurations and conditions,

such as missile launcher assembly jettison.

Importantly, results from the RAAF – USAF F-111 miniature munitions program indicate

that CFD can be used to account for sting-interference effects in the cavity [40], as well as to predict

the weapon-bay aerodynamics and aeroacoustics.

CFD Applications to Aircraft/Weapon Integration

A novel use of CFD in aircraft/weapon integration was achieved early in the JSF Program.

The effects of different fuel tank designs were examined. Considerable improvement in the

separation characteristics of adjacent stores were achieved, Figure 11 [51].

Figure 11 JSF Fuel Tank Re-Design to Improve Store Trajectories

Future multi-disciplinary armament systems compatibility approach for capability

preparedness of joint task forces

The previous sections have used a number of examples to illustrate the RAAF/NAVAIR

collaborative programs that have helped both partners build techniques and tools and issue


P a g e | 15

clearances. However, future weapons clearances in a more complex, network-centric-warfare

space will add complexity to the currently stove-piped process; hence, a framework will be

required. This will address in particular the network-enabled operations between systems that at

the time of the release MIL-HDBK-1763 [8] was not required. The NATO Air Launched Weapons

Integration study in 2004 [42] recommended that a NATO STANAG be developed over the next 10–

20 years to improve the reusability of aircraft-stores-certification criteria and to streamline the

approaches used. The use of a NATO ‘CODe of practice for Experimentation’ (CODEx) for the

testing of joint fires[45 - 47] operational capabilities in a new Joint fires Armament Integrated Mission

Environment’

(JAIME) with ‘network-centric complex, adaptive mission capabilities’ employing both kinetic

(weapons), non-kinetic (electromagnetic) directed energy and cyber effects could assist in this,

based on the successes with the use of MIL-HDBK-1763 [8, 12] for what are considered simple and

complicated ASC flight clearance and certifications in today’s language.


P a g e | 16

Figure 13. Capability Preparedness Levels and OPCF P6 Framework [12] vii

Research using grounded theory and case studies investigated use of MIL-HDBK-1763, the

TTCP GUIDE to Experimentation (GUIDEx) [48] and as a result the JAIME CODEx has been

proposed [49, 50] as a disclosure draft for further development by NATO STO. [12, 49] The research

was conducted in collaboration with over 300 Five Eyes and NATO STO members and other

subject-matters experts. As part of that effort, McKee and Tutty [49] reported on the current

methods used nationally and internationally for capability preparedness/management,

systemsengineering, T&E and project-management practices. They identified the key elements

that will increase the confidence in future military capabilities being operationally suitable and

effective that are evidence-based and scientifically defensible.

Figure 1 2 . Systems, systems of systems (SoS), and family of SoS (FoS)


P a g e | 17

Figure 14. Operational Capability and Preparedness ET&E Framework

Figure 15. Operational Capability and Preparedness P6 Framework

A conceptual framework for network-enabled, force-level armament systems compatibility

has been proposed [12, 47 - 50] to achieve balanced capability management that integrates the

experimentation, systems engineering, test and evaluation, and system-safety communities, as

shown in Figures 12 to 17 throughout the life of the capability and that ET&E and certification is

synchronized to ensure operational commanders have confidence in the capability, at least at the

JTF level.

To effectively deal with the increasing complexity and interdependence of current and future

network enabled military systems, experimentation and testing and evaluation (ET&E) must

evolve and mature so as to detect undesirable and/or unexpected results, e.g., interdependencies


P a g e | 18

of safe-separation certification with seemingly unrelated upgrades to mission-systems software.

Surprises in this already complex environment will increase as the complexity of the systems of

systems (SoS) and family of systems of systems (FoS) increases with national and international

interoperability expectations of operational commanders and users.

Figure 16. US Distributed M&S LVC operational view via InterTEC, Tutty[12, 44]

To implement this strategy, a change in focus by both the systems engineering and the

experimentation and T&E organisations will be needed, so that they are able to also conduct

scientifically rigorous testing, training, and experimentation that build confidence and remove risks

in capabilities for conducting secure, network-enabled real-time kinetic and non-kinetic effects.

Figure 17. JAIME Weapon Danger Area for Yin safety and Mission Success Regions of

Significant Influence (RoSI) conventions for Yang at the Mission Level

The ability to independently test systems, SoS, and FoS using a scientifically defensible

approach using the LVC environment is critical. As predicted by and Cenko etal [51], in the aircraft-

stores-separations arena, scientists and engineers will see a new higher-level systems

engineering level approach for wind-tunnel and flight tests with increased use of CFD. Steinle et

al [43] for example also propose numerous improvements in wind-tunnel testing and CFD modelling

with the Live Virtual Constructive (LVC) ‘simulation’ worlds via use of the joint-T&E methods

discussed in Tutty [12], while also performing other, more mundane roles.


P a g e | 19

Conclusion

Over the past three decades, collaboration between the Five Eyes in the area of aircraft stores separation has considerably improved the capabilities of each nation. These joint efforts have established the credibility of new tools, eliminated duplication, and provided significant cost savings.

These collaborative efforts were the result of predominantly Five Eyes and NATO, ASCC

and TTCP international agreements and specialist conferences (AIAA, ICAS, ITEA), as well as

agreements between individuals to do interesting work that would complement their respective

agencies’ priorities. Future joint task forces using families of systems of systems will require even

more collaborative and cooperative systems for aircraft-stores configurations to be part of a greater

framework that has armament systems compatibility across the systems of systems and are

operationally suitable, effective and prepared.

Movie 1. Joint fires LVC animated view, [12] viii

CFD has become an increasingly accepted tool in the aircraft stores separation and

certification process. The paper discussed how ACFD Challenge II helped advance this process,

and described how more recent efforts help explain why Euler solutions for this configuration agree

reasonably well with the flight test results. [27]

MIL-HDBK-1763 has been critical to this revolution in air armament affairs until now, to which

ACFD has been a common initiative. To address the network enabling of joint fires operational

capabilities, the Five Eyes and NATO need to urgently develop and implement use of a

replacement based on the research underpinning the proposed JAIME CODEx to ensure that

armament system compatibility is established and maintained for increasing the confidence of

commanders and operational users in what levels of interoperability and capability preparedness

are demonstrated and are scientifically based. The tools developed in use of CFD in the aircraft

stores separation and certification area are long overdue for use in other domains such as

nonkinetic electromagnetic compatibility, directed energy and cyber operations to achieve this.

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51. Cenko, A., Piranian, A., and Talbot, M., (1997), Navy integrated T&E approach to store

separation. IAC, Chester, UK.

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International Aircraft Stores Compatibility Symposium, April 13, 2007. Endnotes:

i Unknown at the time was that this would be the last external ACFD Challenge, well at least, so far. ii Aircraft Stores Configuration. An aircraft stores configuration refers to an aerospace platform,

incorporating a stores management system(s), combined with specific suspension equipment and aircraft

store(s) loaded on the aircraft in a specific pattern. An aircraft stores configuration also includes any

downloads from that specific pattern resulting from the release of the store(s) in an authorised

employment or jettison sequence(s) All definitions are from MIL-HDBK-1763, unless noted otherwise. iii Aircraft Stores Compatibility. The ability of each element of specified aircraft stores configuration(s) to

coexist without unacceptable effects on the physical, aerodynamic, structural, electrical, electromagnetic,

optical or functional characteristics of each other under specified ground and flight conditions. iv

It can also be argued that depending on the novelty / technology readiness level (TRL) of the ‘new’ aircraft

or ‘new’ store i.e. the degree of analogy basis - that the second or third situation may actually need to be

reversed. Store performance/integrity and unique (but undiscovered) aircraft characteristics/environment can increase/decrease the risks between these two scenarios. This may be

the case for any complex adaptive system and aircraft using active separation control techniques. v Analogy. A form of reasoning in which similarities are inferred from a similarity of two or more things in

certain particulars. Analogy plays a significant role in problem solving, decision making, perception,

memory skills, creativity, explanation, emotion, and communication. [12] vi

Aircraft Stores Clearance. Primarily a systems engineering activity used in most Five Eyes and NATO

countries to formally document in a Flight Clearance, or similar document, the extent of aircraft stores

compatibility within specified ground and flight operating envelopes determined by the Technical

Airworthiness Authority [typically at the Engagement and System of Systems (SoS) level]. Aircraft Stores Compatibility Flight Clearance. A document issued by the Technical Airworthiness

Authority that explicitly defines the extent of aircraft stores compatibility to safely prepare, load, carry,

employ and/or jettison specific aircraft stores configurations within specified ground and flight operating

envelopes. This document is a mandatory basis required by most NATO nations for release to service of

the aircraft stores configurations. [12] vii The following definitions are proposed by the author for future SoS & FoS use in Joint Fires operations:

• Ops Category A – mission and safety critical operations. • Ops Category B – mission critical – safety affected operations. • Ops Category C – mission affected/advisory – ‘non-safety critical’ operations. Such a taxonomy closely aligns with the systems, SoS and FoS views and the three V&V implications

levels as proposed at Tutty (2015 Table 6.1). This is vital to delineate those SoS and FoS that are

OPS CAT A and safety critical, complex and adaptive in nature versus OPS CAT C engineered systems. viii

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