A device is introduced for reducing drag and store separation difficulties caused by shallow cavities on aircraft in supersonic flight consisting of a group of hollow pipes the same length as the cavity. The pipes are attached to the cavity floor so as to allow air to flow through the pipes. This device allows air to flow through the pipes opposite to the direction of flow outside the pipes. This results in reduced drag and improved store separation characteristics

Stallings, Robert L., Jr.

Wilcox, Floyd J., Jr.

English

NASA Technical Reports Server

United States Patent [191 [11] Patent Number: 4,863,118 
Stallings Jr. et al. [45] Date of Patent: Sep. 5, 1989 
[54] PASSIVE VENTING TECHNIQUE FOR 
[75] Inventors: Robert L. Stallings Jr., Yorktown; 
SHALLOW CAVITIES 
Floyd J. Wilcox, Jr., Hampton, both 
of Va. 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[73] Assignee: The United States of America as 
[21] Appl. No.: 252,081 
[22] Filed: Sep. 30, 1988 
[51] Int. (3.4 ......................... B64C 23/00; B64D 1/02 
[52] U.S. C1. .................................. 244/130; 244A37.4 
[58] Field of Search ............... 244A37.1, 137.4, 118.1, 
244/207-209, 198, 130; 84/1.51, 1.54 
~561 References Cited 
U.S. PATENT DOCUMENTS 
1,817,089 8/1931 McFadyen .......................... 244/198 
2,451,475 10/1948 Diehl ................................... 244/130 
3,010,680 11/1961 Kaplan ................................ 244/207 
4,552,360 6/1985 Barnwell et al. . 
4,671,474 6/1987 Haslund .............................. 244/209 
4,697,864 10/1987 Hardy et al. ..................... 244A37.4 
Primary Examiner-Galen L. Barefoot 
Attorney, Agent, or Firm-George F. Helfrich; John R. 
Manning; Charles E. B. Glenn 
t571 ABSTRACT 
A device for reducing drag and store separation diffi- 
culties caused by shallow cavities on aircraft in super- 
sonic flight consisting of a group of hollow pipes the 
same length as the cavity. The pipes are attached to the 
cavity floor so as to allow air to flow through the pipes. 
This device allows air to flow through the pipes oppo- 
site to the direction of flow outside the pipes. This re- 
sults in reduced drag and improved store separation 
characteristics. 
3 Claims, 1 Drawing Sheet 
4 
https://ntrs.nasa.gov/search.jsp?R=19900010763 2020-03-24T03:15:13+00:00Z
US. Patent Sep. 5,1989 4,863,118 
4.863,118 
m 
PASSIVE VENTING TECHNIQUE FOR SHALLOW 
CAVITIES 
ORIGIN OF THE INVENTION 
The invention described herein was made by employ- 
ees of the U.S. Government and may be manufactured 
and used by or for the Government for governmental 
purposes without the payment of any royalties thereon 
or therefor. 
CROSS REFERENCE TO RELATED 
APPLICATION 
This application is related to our copending applica- 
tion Ser. No. 250,468, filed Sept. 28, 1986 entitled “Pas- 
sive Venting Technique for Shallow Cavities”. 
TECHNICAL FIELD OF THE INVENTION 
The present invention relates in general to shallow 
cavities on aircraft (for example, bomb and missile bays) 
and in particular to a device for reduction of drag and 
store separation difficulties caused by shallow cavities 
through the use of pipes set in the floor of the shallow 
cavity. 
BACKGROUND OF THE INVENTION 
During supersonic flight, cavities on aircraft affect 
the airflow across the surface of the aircraft depending 
on the length to height ratio of the cavity. If the length 
to height ratio is greater than about 12, the cavity is a 
shallow cavity and the airflow around the cavity is 
“closed cavity flow”. If it is less than about 12, it is a 
deep cavity and the airflow is “open cavity flow”. Al- 
though test results show that a length to height ratio of 
12 is the approximate boundary between closed cavity 
and open cavity flow, this invention is also beneficial for 
length to height ratios between 9 and 12. 
One difference between the two types of cavity flow- 
fields is the pressure regions formed at the ends of the 
cavity in supersonic flight. A low pressure region is 
formed at the front of the cavity and a high pressure 
region is formed at the rear. In shallow cavities, these 
pressure areas are separated by a large enough distance 
that the high pressure region cannot vent to the low 
pressure region. This results in a large difference in the 
pressure between the two regions and leads to increased 
drag and to difficulty in separating stores (bombs or 
missiles) from the cavity. 
These problems are not characteristiciof deep cavities 
which demonstrate “open cavity flowy. In deep cavi- 
ties, the airflow is essentially undisturbed over the cav- 
ity. This is because the high and low pressure areas are 
not separated by a large distance and the pressure is 
essentially the same along the floor of the cavity. This 
means that the cavity causes very little drag and that 
there is no difficulty separating stores from the cavity. 
Because it is not feasible for all cavities on aircraft to 
be deep cavities due to space and size limitations, the 
following are objects of the present invention. 
An object of the present invention is to provide a 
device to cause shallow cavities on aircraft to behave 
like deep cavities during supersonic flight. 
Accordingly, an object of the present invention is to 
provide a device which will cause the air in the high 
pressure region at the rear of a shallow cavity to be 
vented to the low pressure region in the front of the 
cavity. 
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SUMMARY OF THE INVENTION 
According to the present invention, the foregoing 
and additional objects are obtained by providing hollow 
pipes which are set in the floor of a cavity on an aircraft. 
The length of the pipes is slightly less than the length of 
the cavity and the combined diameters of the pipes 
equal between 50 and 100% of the width of the cavity. 
The diameter of the pipes must be large enough to allow 
air to flow freely through them. The pipes should have 
as small a diameter possible so that the storage space in 
the cavity is maximized. A large diameter does not 
affect the usefulness of the invention. The pipes are 
shaped to allow air to enter and exit the ends. This 
allows air to flow from the high pressure region at the 
rear of the cavity to the low pressure region at the front 
of the cavity which occur during supersonic flight. Any 
means may be used to attach the pipes to the floor of the 
cavity so long as air is not blocked from entering and 
exiting the ends of the pipes. A shallow cavity is one 
with a length to height ratio greater than 12 although 
some beneficial results occur in cavities with a length to 
height ratio between 9 and 12. 
BRIEF DESCRIPTION OF THE DRAWINGS 
In the drawings: 
FIG. 1 is a side view of a pipe mounted in a cavity. 
DETAILED DESCRIPTION OF THE 
INVENTION 
Referring to FIG. 1, hollow pipes (6) are provided. 
The length of the pipes (6) is 0 to about 20% less than 
that of the cavity (2) and the diameters of the pipes (6) 
combine to equal from between about 50 to 100% of the 
width of the cavity (2). The ends (12 and 16) of the pipes 
(6) are shaped in such a manner that air is allowed to 
enter (la) and exit (12) the pipes (6). The pipes (6) must 
be made from a material strong enough to withstand the 
difference between the air pressure inside the pipes (6) 
and that outside the pipes. A means to attach the pipes 
(6) to the floor (14) of the cavity (6) is provided. The 
pipes (6) must be of a large enough diameter to allow air 
to flow freely. However, because the area outside the 
pipes (6) is used for storage purposes, the pipes (6) must 
be designed to be of as small a diameter possible. Large 
diameter pipes do not negatively affect the usefulness of 
this invention, they only affect the amount of storage 
space left in the cavity. 
The pipes allow air to flow in the direction (8) oppo- 
site to the air flow (4) on the surface of the aircraft (10). 
This is important in shallow cavities because during 
supersonic flight, a high pressure area is formed at the 
rear of a shallow cavity and a low pressure area is 
formed at the front. In shallow cavities, these two areas 
are too distant to allow the air in the high pressure area 
to flow to the low pressure area. This causes “closed 
cavity flow” which results in increased drag and store 
separation difficulties. Shallow cavities which have 
pipes set in the floor exhibit characteristics of “open 
cavity flow” such as reduced drag and the elimination 
of store separation difficulties. 
EXAMPLE 
A small scale of this invention has been tested in a 
wind tunnel. A cavity having a length to height ratio of 
12 was used for test purposes. The cavity height was 
2.441 inches and the length was 29.362 inches. Tests 
were conducted at Mach numbers ranging from 1.6 to 
3 
4,863,118 
4 
2.65. Similar results were obtained at the different Mach 
numbers. Semicircular copper pipes having a wall 
thickness of 0.035 inches, a radius of 0.25 inches and a 
length of 28.362 inches were attached to the floor of the 5 
cavity. The pipes were mounted in the cavity with both 
ends set a small distance away from the sides of the 
cavity. The end of each pipe at the rear of the cavity 
was cut diagonally from a point 0.5 inches from the end lo 
to allow air to enter. At the front of the cavity, air was 
allowed to exit through the end of each pipe as well as 
through a 0.035 inch high by 2.5 inch long slot in the 
The results of this testing show that the pipe vents 
result in a modest improvement in the separation char- 
acteristics of stores dropped from the center of the 
cavity and a ma,or improvement in the separation char- 
acteristics of stores dropped from the sides of the cav- 
ity. 
1. A device for reducing drag and store separation 
difficulties caused by shallow cavities on aircraft in 
supersonic flight which comprises: 
a. a cavity, which is defined as one having a 
length to height ratio greater than about 9, 
b. a group of hollow pipes whose length is between 
about 80 and 100% of the length of the cavity, 
whose combined diameters equal between about 50 
and 100% of the width of the cavity and whose 
ends are shaped to allow air to enter and exit, 
where the pipes are constructed from a material 
strong enough to withstand a pressure difference 
between inside and outside the pipes, and 
c. a means to mount the pipes in the floor of the cav- 
and in the opposite direction to the airflow outside 
of the pipes. 
2. A device according to claim 1 where a shallow 
cavity is defined as one having a length to height ratio 
where the pipes are 
side of each pipe adjacent to the floor Of the cavity* 15 ity such that the airflow the pipes is parallel 
20 greater than about 12. 
3. A device to claim 
What is claimed is: 
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mounted in a planar co&guration. * * * * *  
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Passive venting technique for shallow cavities

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