) 

NASA CASE NO. I AR 15746-1 ' 

i 

PRINT FIG. 2- 

NOTICE 

The invention disclosed in this document resulted from research in aeronautical and 

space activities performed under programs of the National Aeronautics and Space 

Administration. The invention is owned by NASA and is, therefore, available for 

licensing in accordance with the NASA Patent Licensing Regulation (14 Code of 

Federal Regulations 1 245.2). 

To encourage commercial utilization of NASA-owned inventions, it is NASA policy to 

grant licenses to commercial concerns. Although NASA encourages nonexclusive 

licensing to promote competition and achieve the widest possible utilization, NASA 

will provide the necessary incentive to the licensee to achieve early practical 

application of the invention. 

Address inquiries and all applications for license for this invention to NASA Patent 

Counsel, Langley Research Center, Code 21 2, Hampton, Virginia 23681 -0001. 

Serial No.: 

LaRC 

(NASA-Case-LAR-15246-1) BASE N95-14183 

PASSIVE POROSITY FOR DRAG REDUCTION 

Patent Application (NASA. Langley 

Research Center) 13 p Unc l as 
LAR 15246-1 AWARDS ABSTRACT 

BASE PASSIVE POROSITY FOR DRAG REDUCTION 

The present invention relates generally to controlling the pressure load on ground vehicles, 

and more particularly to the passive control of drag on ground vehicles using porosity. 

A device for controlling drag on a ground vehicle. The device consists of a porous skin 

mounted on the trailing surface of the ground vehicle. The porous skin may be separated 

from the vehicle surface by a distance of at least two times the Boundary layer formed on 

the surface. Alternately, the trailing surface of the ground vehicle may be porous. The 

device minimizes the strength of the separation in the base and wake regions of the ground 

vehicle, thus reducing drag. 

Novel aspects of the present invention include reducing the drag on ground vehicles without 

adding and significant weight to the vehicle. This reduces fuel consumption and the effect 

large ground vehicles have on other vehicles. 

Inventor: Steven X. S. Bauer Richard M. Wood 

Address: 3 14 Brook Lane 700 Berkley Court 

Yorktown, VA 23692 Virginia Beach, VA 23452 

ss: ss: 

Employer: NASA NASA 

Initial Evaluator: James L. Dillon 

Serial No.: 08/327,06 1 

Filed: 1011 9/94 
LAR 15246-1 -1 - PATENT APPLICATION 

BASE PASSIVE POROSITY FOR DRAG REDUCTION 

Origin of the lnvention 

The invention described herein was made by employees of the United 

States Government and may be used by and for the Government for 

governmental purposes without the payment of any royalties thereon or 

therefor. 

10 Backqround of the lnvention 

- I. Technical Field of the Invention 

The present invention relates generally to controlling drag on ground 

15 vehicles, and more particularly to the passive control of drag on ground 

vehicles using porosity. 

- 2. Discussion of the Related Art 

Previous devices for controlling drag on ground vehicles have involved 

active devices which attempted to inhibit separation on the base to create a 

predetermined best situation. These devices consist of flaps, turning vanes, 

vortex generators, and a wide range of suction and blowing concepts which 

add considerable weight to the overall vehicle weight and increase fuel 

25 consumption. 

It is accordingly an object of the present invention to control drag on 

ground vehicles. 

It is another object of the present invention to achieve the foregoing 

objects with a passive device. 
LAR 15246-1 -2- PATENT APPLICATION 

It is a further object of the present invention to achieve the foregoing 

objects with minimal spatial and mass requirements. 

It is yet another object of the present invention to accomplish the 

foregoing objects in a simple manner. 

5 Additional objects and advantages of the present invention are 

apparent from the drawings and specification which follow. 

Summary of the Invention 

10 According to the present invention, the foregoing and additional 

objects are obtained by providing a drag reduction device for use on a ground 

vehicle. This device consists of a porous outer skin having a thickness 

between about 111 6" (0.16 cm) and 1" (2.54 cm), preferably between 1116" 

(0.16 cm) and 114" (0.635 cm), which is fixed to the trailing surface of the 

15 ground vehicle (i.e. the outer surface of the doors). The porous outer skin 

may be separated from the trailing surface of the ground vehicle by a 

distance equal to at least two times the thickness of the boundary layer 

formed on the surface of the ground vehicle (i.e. 0.25 inches (0.635 cm) or 

more). The surface porosity of the porous outer skin is between about 10 

20 and 20%. The porous outer skin may be fabricated from a porous material or 

by creating a number of perforations in a solid material. If the perforation 

method is used, the maximum dimension of the perforations is about 0.025 

inches (-0635 cm) on the sides and top of the ground vehicle and 0.25 inches 

(.635 cm) on the trailing surface of the ground vehicle. Alternatively, the 

25 trailing surface of the ground vehicle may be porous. Again the surface 

porosity should be between 10 and 20%. The porosity may be caused by 

perforating the ground vehicle's existing trailing surface and, depending on 

the thickness of the existing surface, the maximum dimension of the 

perforations is about 0.25 inches (.635 cm) for a trailing surface with a 

30 thickness of 112 inch ( 1.27 cm) or more. 
LAR 15246-1 -3- 

Brief Description of the Drawings 

PATENT APPLICATION 

Fig. 1 is a side view showing a porous skin mounted on the trailing 

surface of a ground vehicle; 

5 Fig. 2a is a cross sectional view across line 11-11 of figure 1 for a ground 

vehicle having a porous skin separated from the trailing surface; 

Fig. 2b is a cross sectional view across line 11-11 of figure 1 for a ground 

vehicle having a porous trailing surface; 

Fig. 3 is a perspective view of the trailing end of the ground vehicle; 

I0 Fig. 4 is a side view showing the wake formed on a ground vehicle 

without a porous skin mounted on the trailing surface of a ground vehicle; 

Fig. 5 is a graph showing drag characteristics of a ground vehicle 

having a porous skin and drag characteristics of a ground vehicle without a 

porous skin; and 

15 Fig. 6 is a graph showing drag characteristics of a ground vehicle 

having a porous skin and drag characteristics of a ground vehicle without a 

porous skin. 

Detailed Description of the Invention 

Figure 1 shows a ground vehicle 12. A trailing surface 16 of the 

ground vehicle 12 is covered with a porous skin 14. The thickness of the 

porous outer skin 14 is between about 111 6" ( 0.16 cm) and 1 " ( 2.54 cm), 

preferably between 1116" ( 0.16 cm) and 114" (0.635 cm) and should be I 

25 chosen based on structural requirements of the ground vehicle. The porosity 

of the porous skin 14 is optimally between 10 and 20%. The porous skin 14 

may be made from a porous material, such as sintered metal, or from a solid 

material, such as steel or fiberglass, which has been perforated, however the 

maximum dimension of the perforations should be no more than about .025 

30 inches (.0635 cm) on the sides 24 and top 22 of the ground vehicle and 0.25 
LAR 15246-1 -4- PATENT APPLICATION 

inches (.635 cm) on the trailing surface 26 of the ground vehicle (as shown in 

fig. 3) in order to minimize the disturbance of the external flow field. 

Figure 2a shows the porous skin 14 mounted on the trailing surface 16 

of the ground vehicle, forming a plenum 20 between the porous skin 14 and 

5 the trailing surface 16. The porous skin 14 must be mounted over the plenum 

20 such that there is minimal blockage in all directions. The depth of plenum 

18 should be no less than about twice the thickness of the boundary layer 

formed on the surface of the ground vehicle in motion, generally about 0.25 

inches (0.635 cm). 

I0 An alternate embodiment of the present invention is to adapt the 

existing trailing edge 16 of the ground vehicle 12 by, for example, perforating 

the rear doors of the ground vehicle as shown in figure 2b. The surface 

porosity should be between about 10 and 20% and the size of the 

perforations depends on the thickness of the doors or trailing surface. For 

15 doors having a thickness of 112 inch (1.27 cm) or more, the maximum 

dimension of the perforations is about 0.25 inches (.635 cm). 

Figure 4 shows a ground vehicle without a porous skin attached. The 

wake 18 is larger and stronger than the wake 18 shown on figure 1. 

This invention allows the flow over the ground vehicle 12 to turn 

20 around the corners at the base of the vehicle reducing the strength of the 

separation in the wake 18 of the vehicle. This allows the wake 18 to be 

shorter and reattachment of the flow in the wake 18 occurs sooner, reducing 

drag. This phenomenon occurs at all speeds with reductions in drag of 20% 

or more shown in wind tunnel tests. 

25 Figure 5 shows a comparison of a tractor-trailer with a short cab and a 

porous skin mounted directly on the trailing surface of the cab and 10% 

forward of the trailing surface and a tractor-trailer with a short cab with no 

porous skins. 

Figure 6 shows a comparison of a tractor-trailer with an extended cab 

30 and a porous skin mounted directly on the trailing surface of the cab and 5% 
LAR 15246-1 -5- PATENT APPLICATION 

forward of the trailing surface and a tractor-trailer with an extended cab with 

no porous skins. 

Many improvements, modifications, and additions will be apparent 

to the skilled artisan without departing from the spirit and scope of the 

5 present invention as described herein and defined in the following claims. 

What is claimed is: 
LAR 15246-1 PATENT APPLICATION 

PASSIVE CONTROL OF PRESSURE LOADS USING POROSITY 

Abstract of the Disclosure 

5 A device for controlling drag on a ground vehicle. The device consists 

of a porous skin mounted on the trailing surface of the ground vehicle. The 

porous skin may be separated from the vehicle surface by a distance of at 

least two times the boundary layer formed on the surface. Alternately, the 

trailing surface of the ground vehicle may be porous. The device minimizes 

10 the strength of the separation in the base and wake regions of the ground 

vehicle, thus reducing drag. 


Fig. 3 

Fig*