Boundary layer control system

A boundary layer control apparatus including a duct having a streamlined shape which is mounted in spaced apart relation upwind from the wing and extending generally parallel to the leading edge of the wing. The duct has a thickness substantially less than the thickness of the wing to which it is attached. The duct of the present invention includes an opening on the downwind side of the duct for injecting compressed air toward a stagnation line associated with the aircraft wing. The duct according to the present invention can be pivoted during flight so that the air injected from the duct is always directed at the stagnation line, which moves relative to the aircraft wing as aircraft speed increases.

BACKGROUND OF THE INVENTION 
This invention relates to apparatus for controlling the boundary layer of 
air moving adjacent to a wing as an aircraft moves through the air. 
Apparatus for controlling the boundary layer on an aircraft wing are known. 
See, for example, U.S. Pat. No. 3,917,193, to Runnels, which describes an 
apparatus wherein a leading edge flap is used for directing a stream of 
air across a wing surface. This apparatus is used during low speed 
operation, when the leading edge flap is extended. Other patents showing 
leading edge flaps for controlling a boundary layer include U.S. Pat. No. 
4,099,691 to Swanson, et al., and U.S. Pat. No. 4,285,482 to Lewis. An 
early patent, U.S. Pat. No. 1,887,148 to Ganahl, described an airplane 
propulsion apparatus which included apparatus for injecting a stream of 
air across the top of a wing surface. 
The present invention overcomes a number of problems associated with 
conventional boundary layer control systems. Namely, the known systems 
only operate effectively at low aircraft speeds. No apparatus is known 
which effectively controls boundary layer growth at all aircraft speeds. 
SUMMARY OF INVENTION 
The present invention is directed to a boundary layer control apparatus 
wherein a duct having a streamlined shape is mounted in spaced apart 
relation upwind from the wing and extending generally parallel to the 
leading edge of the wing. The duct has a thickness substantially less than 
the thickness of the wing to which it is attached. The duct of the present 
invention includes an opening on the downwind side of the duct for 
injecting compressed air toward a stagnation line associated with the 
aircraft wing. The duct according to the present invention can be pivoted 
during flight so that the air injected from the duct is always directed at 
the stagnation line, which moves relative to the aircraft wing as aircraft 
speed increases. The duct according to the present invention can be 
pivoted either manually or with a servomechanism. The servomechanism 
utilizes two input control signals created by a pair of wind-flow sensors, 
one sensor mounted on top of the leading edge of the duct, and the other 
sensor mounted on the bottom of the leading edge of the duct. By pivoting 
the duct and measuring the wind-flow across these two sensors, the 
position of the stagnation line can be determined and the duct pivoted 
accordingly.

DESCRIPTION OF A PREFERRED EMBODIMENT 
A preferred embodiment of the boundary layer control system 10 is shown in 
FIG. 1 as installed on an aircraft wing. 
To understand the present invention, reference is made to FIG. 2, which 
shows a schematic cross-section of a wing 12. The wing 12 has a straight 
line chord A connecting the two points spaced furthest apart as measured 
between the leading edge and the trailing edge of the wing. The thickness 
B of the wing is the maximum height of the wing measured perpendicular to 
the chord line. As wing 12 flies through ambient air, relative wind 20 
strikes the wing. An angle-of-attack of is formed, which is the angle 
between the changeable relative wind vector 20 and the fixed chord line A 
of the wing. During flight, at high air speeds will be low; at low air 
speeds will be high. A specific streamline 24 in relative wind 20, strikes 
wing 12 at a leading edge stagnation line 26 extending along the wing 
(shown as point 26 in FIG. 2). The stagnation line 26 occurs where 
streamline 24 divides so that a portion of the streamline goes over the 
top of the wing and a portion goes beneath the wing. 
The velocity of relative wind 20 also drops to nearly zero in a very small 
volume extending along the leading edge stagnation line 26. The change in 
momentum of ambient air molecules changing from aircraft velocity to 
nearly zero is felt as drag coming from the wing. As the angle-of-attack 
.varies. changes during flight, the leading edge stagnation line 26 
changes position along the leading edge of wing 12 so as to move closer or 
away from the top of the wing. 
The present invention is directed to apparatus for injecting high speed air 
at the stagnation line 26 to reduce drag by enabling ambient air to flow 
around the stagnation line without reducing its velocity to zero. Further 
the present invention is directed to a leading edge duct for injecting air 
toward the stagnation line which duct has a thickness substantially less 
than the wing to which it is attached. 
As best seen in FIG. 3, boundary layer control apparatus 10 includes a 
streamlined duct 14, mounted to a wing 12 in spaced apart relation upwind 
from the leading edge of the wing and extends generally parallel to the 
leading edge of the wing. 
Duct 14 is preferably streamlined in shape and has a thickness 
substantially less than the thickness of the wing to which it is attached. 
A source of compressed air (not shown) is connected in fluid communication 
with air supply 32, which in turn is connected in fluid communication with 
duct 14. Duct 14 further includes an injector opening 34, which in a 
preferred embodiment is a slit. Other types of openings, such as nozzles, 
may be used with beneficial effect. The injector opening 34 is located on 
the downwind side of duct 14 and is positioned to inject compressed air 
from duct 14 to the leading edge of stagnation line 26. The pressure of 
the compressed air directed to duct 14 is adjusted with means (not shown) 
so that the air exiting injector opening 34 has a velocity greater than 
the relative wind. In a preferred embodiment, this exit velocity is 
adjusted to be 20-30% greater than the relative wind velocity. When 
opening 34 is a slit, compressed air exits slit 34 as a high velocity 
sheet of compressed air aimed at the stagnation line 26. 
As best seen in FIG. 4, duct 14 is held in position by support arm 16. 
There is a similar support arm 16 located at the opposite end of duct 14. 
Support arms 16 are fixedly attached to each end of conduit 14, and are 
pivotally attached to wing 12 with axles 38. Axles 38 are held in position 
by a fixed flange 40. Opening 41 in wing 12, through which a support arm 
16 extends, is sized to permit support arm 16 to pivot duct 14 between an 
aircraft's smallest angle-of-attack .varies. 22 and its largest 
angle-of-attack. Optionally, a sliding cover, a rubber boot, or any other 
device well known in the art, can be used to cover opening 41 while 
allowing support arm 16 to pivot between the two extremes of the 
angle-of-attack. 
Compressed air supply tube 32, extending in the interior of wing 12, has a 
flexible hose 42, which is connected to compressed air support tube 32 to 
provide fluid communication between the supply tube 32 and duct 14. Hose 
42 is flexible to allow movement of support arm 16. 
It is desired to move duct 14 as the angle-of-attack changes during flight, 
so that air injected through opening 34 is oriented to insert air at the 
stagnation line. Any suitable mechanical, hydraulic, or electromechanical 
apparatus may be used, as is well known to those skilled in the art, to 
pivot duct 14. 
One such manual system is shown in FIGS. 4 and 5. A spring 46 is attached 
between support arm 16 and a fixed spring flange 48 attached to a spar 30 
of wing 12. The support arm 16 acts as a bell crank. A spring return 46 is 
attached to support arm 16 on the side of axle 38 opposite duct 14 and 
acts to bias duct 14 to a position where air is inserted at the stagnation 
line corresponding to a minimum angle-of-attack. A cable 50 is attached to 
support arm 16 as shown in FIG. 4. Cable 50 is led through any appropriate 
number of pulleys 52 or guides (not shown) so as to emerge through the 
cockpit floor as shown in FIG. 5. The free end of cable 50 is then 
attached to a conventional lock and release handle 56. By pulling lock and 
release handle up and locking it, duct 14 is pivoted to a greater 
angle-of-attack 22 (lowered relative to the top of wing 12) and held 
there. When it is desired to raise duct 14 relative to wing 12, button 58 
is pushed on lock and release handle 56 so that spring 46 pivots support 
arm 16 and conduit 14 back to a position closer to the top of the wing. 
A second embodiment of a system for pivoting duct 14 is shown in FIGS. 6-8. 
In this embodiment, a servomechanism is used to control the positioning of 
duct 14. The mechanical structure shown in FIG. 6 is similar to that shown 
in FIG. 4, but without the spring and cable to move duct 14. As duct 14 
encounters relative wind, a dividing line 60 develops along the leading 
edge of duct 14 that is similar to leading edge stagnation line 26. 
Two wind-flow sensors 62 are provided with one positioned on either side of 
dividing line 60 as best seen in FIG. 6. In a preferred embodiment, the 
wind-flow sensors 62 are conventional resistance-temperature sensors. 
Resistance-temperature sensors have the property that as the temperature 
of the sensor changes the resistance also changes by a corresponding 
amount. The sensor 62 that has the most airflow over it becomes cooler 
than the other sensor of the pair, and this is reflected in the electrical 
resistance of the sensor. Each sensor 62 provides an electrical output 
signal dependent on the resistance of the sensor. A three-wire cable 63 
connects the pair of output signals of wind-flow sensors 62 with an 
electromechanical circuit 64. This circuit 64 is schematically shown in 
FIG. 8. 
The electro-mechanical circuit 64 is used to pivot duct 14 automatically 
until the wind-flow across one sensor 62 is equal to the wind-flow across 
the other sensor 62. When the signals of the two sensors 62 are equal, the 
injector opening 34 is properly oriented so that air existing opening 34 
is aimed at the stagnation line 26. 
With reference to FIG. 8, schematically showing the elements of 
electro-mechanical circuit 64, the cable 63 is led to a direct current 
amplifier 68. Amplifier 68 is powered by power supply 70. The output of 
amplifier 68 is led through mode selector switch 78, and from there to 
pulse generator and amplifier 80. If the pilot selects a manual mode by 
using selector switch actuator 74, located in the cockpit, selector switch 
78 is switched to receive manual inputs rather than the outputs of 
amplifier 68. Manual switch 76, also located in the cockpit, which has 
either plus or minus direct current voltages available at the terminals, 
can be toggled by the pilot to provide plus or minus input voltages to 
pulse generator and amplifier 80. 
Whenever pulse generator and amplifier 80 have a voltage input, it sends an 
appropriate pulse to stepping motor and speed reducer 82 to actuate the 
motor in the appropriate direction. Stepping motor and speed reducer 82 
then turns worm gear shaft 86 which in turn turns worm wheel 88. Worm 
wheel 88 is attached to axle 66; and axle 66 is fixedly attached to 
support arm 16. As worm wheel 88 turns, so does support arm 16. 
A digital pick-up device 90 is attached to axle 66 to sense the shaft 
position of axle 66 and provide an electrical signal to readout indicator 
72 in the cockpit to provide instantaneous information to the pilot as to 
the angular location of duct 14. 
In operation, boundary layer control system 10 is used to direct 
high-velocity compressed air at the leading edge stagnation line 26 that 
extends along wing 12. As flight attitudes change during flight, 
compressed air exiting slit 34 may be redirected at the relocated leading 
edge stagnation line 26. Support arms 16 are pivoted using any of the 
means well known in the art to pivot and hold conduit 14 in a changed 
position. This position is then held until another change in position of 
leading edge stagnation line 26 occurs. 
While the fundamental novel features of the invention have been shown and 
described, it should be understood that various substitutions, 
modifications and variations may be made by those skilled in the art 
without departing from the spirit or scope of the invention. Accordingly, 
all such modifications or variations are included in the scope of the 
invention as defined by the following claims.