Technical Description

Evaluation of Store Separation Characteristics

from Aircraft Using Aerodynamic Technology

The latest military aircraft is equipped with stores in

the internal weapon bays to improve stealth and speed
performance.
In this context, we developed a store separation
characteristics evaluation system to analyze the flow-
fields of the subsonic to supersonic ranges and
simulate them at the wind tunnel test to identify the
flow around the internal weapons bay and how the
store is separated from the aircraft in flight for the first
time in Japan. This system has been integrated into the
Tri-sonic Wind Tunnel in the Chitose Test Center of the
Acquisition, Technology & Logistics Agency.

before flight. For this purpose, the CFD (computational

Introduction
fluid dynamics) analysis is used in addition to a wind tunnel
One purpose of military aircraft is to carry and drop test such as the CTS (captive trajectory system) test.
stores such as missiles and bombs in flight. If stores are The MPA (maritime patrol aircraft) P-1, shown in Fig. 1,
attached to the bottom of the fuselage or underwing has an internal weapons bay that keeps stores in the
hardpoints, however, they reflect radar signals and fuselage. When using an internal weapons bay, the P-1
generate aerodynamic drag resulting in decrease stealth flies at a relatively low speed, while fighters fly at transonic
and speed performance. To avoid this problem, many of the or supersonic speed. As Fig. 2 shows, an air-flow layer
latest military aircraft feature an internal weapons bay that called the shear layer, forms between the slow air flow in
houses stores to improve performance. the internal weapons bay and the fast air flow outside the
aircraft. In addition, in the velocity range from transonic to
supersonic speeds, the flow field becomes complicated as
1 Background
When the weapons bay door is opened, it exposes the
concavity of the internal weapons bay and generates a
cavity flow, which is a complicated air-flow pattern, around
the bay. A cavity flow varies greatly in flow velocity and
pressure, and ever-changing aerodynamic forces are
exerted on stores that pass through this flow. Moreover,
cavity-flow conditions for subsonic flight speeds often
differ from those for supersonic speeds. Therefore, as the
aerodynamic forces on stores vary by flight speeds, it is
important to predict trajectories on release from the
internal weapons bay.

2 Store separation from aircraft

When a store is separated from an aircraft, there is a
risk that it may rise due to aerodynamic force to hit the
aircraft. To prevent such accidents and secure safe flights, Fig. 1 Example of internal weapons bay (P-1)
it is essential to verify the store separation characteristics

53
 transonic and supersonic speed ranges, as shown in Fig. 3,
one needs to understand the aerodynamic phenomena and
Shear layer conduct store separation tests using a wind tunnel.

(1) Understanding the aerodynamic phenomena

To separate stores from the internal weapons bay
safely, it is important to understand the aerodynamic
phenomena around the bay1).
Expansion wave Shock wave At transonic and supersonic speeds, as shown in Fig. 2,
Air flow
when the supersonic flow along the aircraft is distorted by
Fig. 2 Example of air flow around internal weapons bay the internal weapons bay, expansion waves and shock
waves occur. Also, a shear layer forms between the flows
inside and outside the bay, around which occurs an
it involves shock waves and expansion waves, and no unsteady flow that varies greatly with time. When a store
analysis of cavity flows with stores has been carried out in separates from the internal weapons bay and enters this
Japan until today. unsteady flow, the store may change its behavior and hit
While low speed wind tunnels are available in Japan the aircraft. Therefore, to separate stores safely when
and have to date been used for store separation tests, flying at a transonic or supersonic speed, it is necessary to
overseas facilities had to be used for transonic and understand the aerodynamic phenomena around the
supersonic velocities. internal weapons bay that affect store behavior. However,
no-one has addressed this issue up to the present in
Japan.
3 Technological challenges related to
evaluation of store separation (2) Wind tunnel test of store separation
characteristics Although CTS are available in Japan for tests in a range
To evaluate store separation characteristics in the from low to high subsonic speeds, few of them have

Understanding aerodynamic phenomena Wind tunnel test on store separation

Store separation characteristics

evaluation system (CTS)

Tri-sonic wind tunnel

Standard cavity
model Cross-verification

CFD analysis of cavity flows

Fig. 3 Concept of store separation characteristics evaluation

Kawasaki Technical Review No.179 54

October 2018
 Technical Description

conducted simulations on separating stores from the

internal weapons bay.
To secure high-quality test results when evaluating Expansion wave Expansion wave
Shock wave
store separation characteristics in the transonic and
supersonic speed ranges, the most appropriate facility is
the Tri-sonic Wind Tunnel 2) at ATLA’s (Acquisition,
Technology & Logistics Agency) Chitose Test Center. This
facility is capable of generating air flows equivalent to
(a) Schematic diagram of flow field
those of real flights (a wide Mach range and high Reynolds
number). This tri-sonic wind tunnel, which covers a wide
1
range from low to supersonic speeds, needed the

Pressure on the cavity bottom, Cp

development of a CTS that can be used for simulating 0. 8
CFD analysis
separation of stores. 0. 6
Wind tunnel test
0. 4

4 Technological development related 0. 2

to evaluating store separation
characteristics 0

−0. 2
(1) Understanding aerodynamic phenomena
( i ) Development of a cavity-flow analysis tool −0. 4
Position
To acquire basic data on cavity flows in the wind tunnel (b) Distribution of the pressure on the cavity bottom, Cp
test, we fabricated a model simulating an internal weapons
Fig. 4 Example of cavity flow (transitional type, Mach 2.0)
bay and created a CFD analysis tool, and then carried out
cross-verification between the analysis and the wind tunnel
test data.
We designed and fabricated a standard wind-tunnel
model for studies of cavity flows. This model is designed to upstream of the cavity an expansion wave occurs when
form a uniform flow over a plate that simulates the the air flow turns around the leading edge of the cavity and
fuselage surface around an aircraft’s internal weapons bay. then changes direction to enter the cavity. The air is
In this model, a cavity with a range of depths and lengths compressed inside the cavity to generate a shock wave,
is placed to simulate various internal weapons bays, and which changes the air-flow direction, and the flow leaves
measured pressure3). the cavity. At this point, an expansion wave occurs at the
In creating the CFD analysis tool, we applied the grid- trailing edge of the cavity, and the air flow changes
generation technique nurtured through the development of direction to downstream. The average pressure distribution
aircraft such as the P-1 to analyze complicated shapes of on the cavity bottom surface shows that the expansion
internal weapons bays and stores. This CFD analysis tool wave accelerates the flow velocity at the leading edge of
consists of grid data that work on FLUENT, commercial the cavity to make the pressure on the cavity bottom, Cp,
CFD analysis software, and a file for setting calculation negative. After that, the flow is compressed by the shock
conditions. This tool’s accuracy has been improved through wave to increase Cp downstream.
several wind-tunnel tests4). We compared the three flow-field types for the CFD
(ii) Cavity flows, which are sensitive to cavity dimensions analyses results and the pressure distribution obtained
and the Mach number, are difficult to predict or simulate from the wind tunnel tests, and then confirmed the CFD
aerodynamically. analysis could predict cavity flow fields.
Cavity flow fields are categorized into three types: the (iii) Evaluating the effect of leading-edge devices
open type, the closed type, and the transitional type, which We conducted an investigation on control capability of
is classed between the former two. They are known to the cavity flow by the leading-edge devices shown in Fig. 5.
represent different pressure distributions on the cavity’s Without a leading-edge device, as Fig. 6 (a) shows, the
bottom surface. shear layer enters the cavity to form a transitional-type flow
Figure 4 shows a schematic diagram of a transitional- field. On the other hand, Fig. 6 (b) reveals that a leading-
type flow field and an example of its average pressure edge device deflects the shear layer. In this case, the flow
distribution on the cavity bottom surface obtained by the field is the open type, which definitely differs from that in
CFD analysis and the wind-tunnel test. the former case. This difference in shear-layer positions
The diagram shows that as an air flow runs from also appears in the total pressure distribution, and both the

55
 Fig. 5 Shape of leading edge device

Mach number
High

Low
Air flow
Shear layer

Cavity
(a) Without a leading-edge device

Air flow Shear layer

Cavity
Leading-edge device
(b) With a leading-edge device

Fig. 6 Effect of leading edge device on cavity flow (Mach contour, free stream Mach 0.85)

wind tunnel test and CFD analysis data show it as well. evaluating store-separation characteristics. We applied this
Thus, by verifying the CFD analysis results with the know-how and technique to development the CTS for Tri-
wind tunnel test data, we confirmed its capability to sonic Wind Tunnels shown in Fig. 7.
appropriately analyze the flow fields around the internal The CTS for Tri-sonic Wind Tunnel is a CTS with three-
weapons bay with stores in it. axis robotic arm that supports the store model and varies
the model’s attitude. This system features a slim shape
(2) Wind tunnel test on store separation due to an offset rotary-joint mechanism that has one of its
In 2003, our company developed the CTS for High- three axes inclined, reducing its aerodynamic influence in
Speed Wind Tunnels for the first time in Japan using our the wind tunnel. Before introducing this system, we
own robotic technology, and introduced it into the conducted research and performance verification tests6).
transonic wind tunnel at the Gifu Works5). We used this Using this CTS verified that the store-separation tests
system to evaluate the store-separation characteristics at can be conducted in the speed range of Mach 0.3 to 2.5,
high speeds in the development of the P-1, and nurtured assumed for future fighter aircraft. Also, to establish the
the know-how of system design and the technique for technique that simulates the separation of stores from an

Kawasaki Technical Review No.179 56

October 2018
 Technical Description

Fig. 7 CTS for Tri-sonic Wind Tunnel

Store model
Z
Air-flow direction D

Parent aircraft model

Without a leading-edge device
With a leading-edge device

24
10
16
8
8
θ〔deg〕

6
Z/D

0
4
-8
2
-16
0
-24
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.0 0.1 0.2 0.3 0.4 0.5 0.6
Time〔s〕 Time〔s〕
(a) Position in the vertical direction〔Z/D〕 (b) Pitch angle〔θ〕

Fig. 8 Store separation trajectories with or without leading edge device

internal weapons bay, we conducted store-separation leading-edge device did not affect the vertical positions
tests. These tests used the standard cavity model, and the (Z/D) of the store, whereas it so greatly affected the pitch
results showed that it is possible to simulate the angle (θ) that the sign was reversed.
separation of a store from its initial position inside the bay
for the range covering transonic to supersonic speeds.
Figure 8 shows the separation simulation results with
Conclusion
and without a leading-edge device. The difference between By creating a CFD analysis tool that evaluates the air
shear layer positions due to the presence/absence of the flow around an internal weapons bay, we clarified the

57
aerodynamic phenomena related to cavity flows. This
achievement can be applied to R&D for aircraft such as Seiichi Sonoda
Technology Development Department,
future fighters. In addition, by developing the CTS for Tri- Engineering Division,
sonic Wind Tunnels, we became the first in Japan to Aerospace Systems Company
demonstrate that the system can simulate store separation
for flight speeds up to the supersonic range. We plan to
use this system to simulate store separation in the Doctor of Engineering
development of future aircraft. Akio Ochi
This development was conducted as a part of the Technology Development Department,
Engineering Division,
ATLA's project “Research on the Aerodynamics in and Aerospace Systems Company
around Weapons Bays.” We would like to express gratitude
to the Air Systems Research Center for the kind
cooperation. Wataru Suzuki
MPA&C-X Project Engineering Department,
Defense and Aerospace Project Division,
Aerospace Systems Company
Reference
1) Udagawa, Kikumoto, Sonoda, Ochi, Hashioka,
Shirogane, and Kawamura: “Multi-Approach Research Toshiyuki Kimura
on Aircraft Store Separation - Research Program Technology Development Department,
Engineering Division,
Overview,” Proceedings of the 54th Aircraft Symposium
Aerospace Systems Company
(2016)
2) Sugita: “The Development of MOD Trisonic Wind
Tunnel,” Proceedings of the 45th Aircraft Symposium
(2007)
Takahiro Hashioka
Technology Development Department,
3) Kikumoto, Udagawa, Takao, Sonoda, Suzuki, Kitagawa, Engineering Division,
Kimura, and Ueda: “Multi-Approach Research on Aerospace Systems Company
Aircraft Store Separation - Cavity-Flow Wind-Tunnel Test
with Internal Stores,” Proceedings of the 54th Aircraft
Symposium (2016) Hideyuki Shirogane
4) Ochi, Nagata, Udagawa, Kikumoto, Osawa: “Multi- Aerodynamic Machinery Department,
Energy System Division,
Approach Research on Store Separation - Comparison Energy System & Plant Engineering Company
between cavity-flow CFD analysis and wind-tunnel
test,” Proceedings of the 54th Aircraft Symposium
(2016)
Masataka Koyama
5) Hamada, Hayama, Ochi, Tsujiuchi, and Suzuki: Manufacturing Automation Improvement
“Aerodynamic and Aeroacoustic Fundamental Department,
Technologies Applied to Aircraft Development,” Manufacturing Improvement Center,
Corporate Technology Division
Kawasaki Heavy Industries Technical Review, No. 171,
pp. 43-50 (2011)
6) Hashioka, Shirogane, Koyama, Kubota, Tekenaka, Doctor of Engineering
Udagawa, Kikumoto, and Kaneko: “Multi-Approach Tetsuya Kubota
Electromechanical System Department,
Research on Store Separation - Development of CTS System Technology Development Center,
(Captive Trajectory System) for Trisonic Wind Tunnel,” Corporate Technology Division
Proceedings of the 54th Aircraft Symposium (2016)

Kawasaki Technical Review No.179 58

October 2018