Apparatus and method for increasing the angle of attack operating range of an aircraft

An apparatus and method for increasing the angle of attack operating range of an aircraft is disclosed. The invention comprises a pair of substantially vertical tip fins. Each vertical tip fin has a main portion and an aft portion. The main portion is securely connected to the tip of the airplane wing and has a swept back leading edge. The aft portion comprises a substantially vertical aileron hingedly connected to the main portion of the tip fin for controlling the roll of the aircraft at high angles of attack. A hinge line is formed at the hinged connection which is swept back. The sweeping back of the leading edge and the hinge line is such that at high angles of attack the oncoming airflow becomes nearly perpendicular to the leading edge and the hinge line, thereby increasing the perforamnce of the vertical tip fin when deflected.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to increasing the performance of aircraft 
and, more particularly, to an apparatus and method for increasing the 
angle of attack operating range of an aircraft. 
2. Description of the Related Art 
When aircraft wings stall due to flow separation, roll control devices such 
as conventional ailerons or spoilers become ineffective. By increasing the 
angle of attack operating range over which aircraft aerodynamic roll 
control surfaces are effective, maneuverability is enhanced, the aircraft 
can fly slower, and it may utilize shorter runways. 
U.S. Pat. No. 3,152,775 issued to J. A. Boyd, entitled "SUPERSONIC 
AIRCRAFT", discloses movable control surfaces carried by the wing tips for 
providing roll control for the aircraft. The movable control surfaces are 
shown in a generally vertical orientation relative to the wing. Thus, 
although deriving benefits for supersonic aircraft, the Boyd invention 
would not serve to allow an increase in the angle of attack. 
U.S. Pat. No. 4,247,062, issued to H. Brueckner, entitled "HIGH EFFICIENCY 
VERTICAL TAIL ASSEMBLY COMBINED WITH A VARIABLE WING GEOMETRY", discloses 
a vertical tail unit combined with a variable wing geometry for large 
angles of attack. The vertical tail unit is hinged for adjustment about an 
axis substantially parallel to the longitudinal axis of the fuselage. The 
outer wing portions are connected, at an angle, with a vertical tail unit 
which is directed vertically downward in its normal position. The 
Brueckner, et al. reference does not provide for directional (roll) 
control. 
U.S. Pat. No. 3,025,027, issued to P. F. Ferreira, entitled "VERTICAL 
AIRFOIL" discloses an aircraft using three vertical airfoils for enhancing 
lift. The vertical airfoils produce a lifting force the sum of which is 
located substantially over the center of gravity of the aircraft. 
U.S. Pat. No. 3,826,448, issued to S. M. Burk, Jr., entitled "DEPLOYABLE 
FLEXIBLE VENTRAL FINS FOR USE AS AN EMERGENCY SPIN-RECOVERY DEVICE IN 
AIRCRAFT", discloses a flexible fin device for mounting to an aircraft to 
affect spin recovery. The device may be selectively deployed to provide a 
triangular platform of flexible material to provide spin recovery, and 
retracted for compact storage during non-use. A single flexible fin may be 
deflected in a specific direction depending on direction of the spin 
rotation, or two flexible fins forming an inverted "V" configuration may 
be used according to the invention. The device is mounted on the underbody 
of the aircraft. 
U.S. Pat. No. 3,845,918 issued to R. P. White, Jr., entitled "VORTEX 
DISSIPATOR" discloses the use of a fixed plate secured to the tip of an 
airfoil and aligned with the free stream direction. The plate extends 
forward from the trailing edge of the tip and outward from the section 
side of the lifting surface and is configured so that the resultant of the 
velocity of the free stream flow and the rotational velocity of the vortex 
produces a stall angle of attack relative to the plate and dissipates much 
of the strength of the vortex. 
U.S. Pat. No. 3,834,654, issued to L. R. Miranda, entitled "BOXPLANE WING 
AND AIRCRAFT" discloses an aircraft wing system wherein a first pair of 
rearwardly swept wings is interconnected at the wing tips to a second pair 
of forwardly swept wings, the interconnection being accomplished by a pair 
of vertically swept fins. The two pair of wings are horizontally and 
vertically staggered relative to one another. Each pair of wings is also 
structurally attached to another aircraft component. 
OBJECTS AND SUMMARY OF THE INVENTION 
It is therefore a principal object of the invention to increase the angle 
of attack operating range over which aircraft aerodynamic roll control 
surfaces can be effective. 
The present invention comprises a pair of substantially vertical tip fins. 
Each vertical tip fin has a main portion and an aft portion. The main 
portion is securely connected to the tip of the airplane wing and has a 
swept back leading edge. The aft portion comprises a substantially 
vertical aileron hingedly connected to the main portion of the tip fin for 
controlling the roll of the aircraft at high angles of attack. A hinge 
line is formed at the hinged connection which is swept back. The sweeping 
back of the leading edge and the hinge line is such that at high angles of 
attack the oncoming airflow becomes nearly perpendicular to the leading 
edge and the hinge line, thereby increasing the performance of the 
vertical tip fin when deflected. 
Other objects, advantages and novel features of the present invention will 
become apparent from the following detailed description of the invention 
when considered in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings and the characters of reference marked 
thereon, FIG. 1 illustrates an aircraft with a wing 12 and a conventional 
aileron 14. A tip fin, designated generally as 16 is connected to the 
outboard portion of the wing 12. The tip fin has a main portion 18 and an 
aft portion 20. The main portion 18 is securely connected to the wing 12 
preferably by an adjustable hinge 22. However, it is within the purview of 
this invention that the wing 12 be rigidly attached or integral to main 
portion 18. 
The aft portion 20 comprises a substantially vertical aileron connected by 
a hinge 24 to the main portion 18 of the tip fin 16 for controlling the 
roll of the aircraft at high angles of attack. The aileron 20 is 
preferably adjustably pivotable in a range between -30.degree. and 
+30.degree. of the plane defining the general orientation of the main 
portion. The vertical aileron preferably defines an area which is in a 
range between 20% and 60% of the total area of the tip fin. Referring now 
to FIG. 2, it can be seen that the leading edge 26 is swept back at an 
angle .theta. from an orthogonal of the wing root. The angle at which the 
leading edge 26 is swept back is in the approximate range of 25.degree. to 
50.degree. (from the orthogonal) depending on the design requirements of 
the particular aircraft such as maximum and minimum speeds, Mach numbers, 
and angle of attack range. Furthermore, the hinge line 24 of the vertical 
aileron 20 is swept back at a similar but slightly lower angle (i.e. 
15.degree.-45.degree.). As a result of the sweeping back of the leading 
edge 26 and the hinge line 24, the oncoming airflow becomes nearly 
perpendicular to these items at the angle of attack where the wing stalls, 
thereby increasing the performance of the vertical tip fin 16 when 
deflected. 
Referring now to FIG. 3, a front view of the aircraft wing 12 is shown with 
the tip fin 16 of the present invention. As can be noted by reference to 
this figure, the tip fin 16 of the present invention is disposed at an 
angle of .beta. at high angles of attack. The angle .beta. is defined 
relative to the wing 12 and is approximately 90.degree.. In this respect, 
the tip fin 16 is described as being "substantially vertical". As used 
herein the term "substantially vertical" is defined as an angle .beta. in 
an approximate range between 90.degree. and 120.degree.. 
Conventional actuation means may be utilized to orient the tip fin 16 to 
its correct orientation relative to the wing 12 and similarly, such 
conventional actuators (not shown) may be used to adjust the vertical 
aileron 20 relative to the main portion 18 of the tip fin 16. 
When the ailerons 14, 20, are deflected, the pressures produced by the 
vertical aileron 20 on the underside of the wing produce a rolling moment 
for control. When the wing is at stall or at greater angles of attack, the 
vertical aileron 20, due to its orientation to the oncoming airflow, 
cannot stall. This deflected vertical aileron 20 remains effective, while 
the wing 12 upper surface produces highly separated airflow which makes 
the normal airleron 14 ineffective. 
FIG. 4 illustrates the operation of the present roll control innovation at 
high angles of attack when the wing flow is separated. A front view of the 
subject airplane wing 12 is shown while at a relatively high angle of 
attack. At such a high angle, without the apparatus of the present 
invention, the wing 12 would be at stall. Generally, this occurs at angles 
of attack of approximately 15.degree.-30.degree., depending on the wing 
shape. 
As can be readily seen, at such high angles of attack, a rolling moment may 
be created in the direction of arrow 30. In such instances, aileron 20 is 
deflected with its trailing edge directed out. The normal aileron 14 is 
directed up and away from the trailing edge of vertical aileron 20. 
Therefore, at a high angle of attack, where deflecting the normal aileron 
14 has little effect on the wing pressures, a relative area of low 
pressure is formed inside tip fin 16 and a relatively high pressure zone 
is established on the outside of tip fin 16, as shown in this figure. This 
low pressure also acts on the wing lower surface. Thus, the negative lift 
results, as illustrated by arrow 32. This negative lift acting on the wing 
tip produces a rolling moment 30 about the center of the aircraft. On the 
other hand, on the opposite side of the wing 12, the normal aileron 34 is 
deflected downward. Furthermore, the trailing edge of the associated 
vertical aileron 36 is directed toward the center. A region of increased 
pressure is formed on the inside of tip fin 38 and a region of relatively 
low pressure is formed on the outside of the tip fin. As a result, a lift 
is created as shown by the arrow designated 40. This produces a rolling 
moment contribution that adds to the rolling moment contribution from the 
other side of the wing to produce a net rolling moment 30. 
The principles of the present invention have been tested at NASA/Langley on 
a 0.2 scale model of a Rockwell International Corporation fighter equipped 
with such a vertical tip fin having a vertical aileron. The test was 
conducted in the Langley 30'.times.60' wind tunnel. FIG. 5 presents the 
data which was derived from these tests which shows the effectiveness of 
the invention at high angles of attack. The vertical surface comprisres 
3.5% of the wing area. The graph shows the rolling movement coefficient, 
C.sub.1 as a function of the angle of attack, .alpha.. The upper graph 
shows the aircraft equipped with only conventional normal ailerons, the 
ailerons, each being deflected at 20.degree. in opposite directions 
(Right, +20.degree.; left, -20.degree.). The lower curve illustrates use 
of both conventional ailerons and the vertical ailerons of the present 
invention. All the ailerons are deflected at .+-.20.degree. to produce the 
maximum rolling moment coefficient. Thus, the effect of the vertical 
ailerons can be seen as the difference in the rolling moment coefficients 
at a particular angle of attack. The comparison of these curves 
illustrates that the utilization of such vertical ailerons is most 
effective between angles of attack of 35.degree. and 50.degree., even 
though substantial benefits derive at any angle of attack above stall. 
Obviously, many modifications and variations of the present invention are 
possible in light of the above teachings. Accordingly, it should be 
understood that, within the scope of the appended claims, the invention 
may be practiced otherwise than as specifically illustrated and described.