High drag airfoil apparatus

Disclosed are airfoil structures which provide two distinct mean centers of lift and high induced drag. Also disclosed are aircraft structures employing such high-lift high-drag airfoils for both the lifting wing and the horizontal stabilizer with the center of gravity disposed between the mean centers of lift of the fore and aft airfoils.

This invention relates to aircraft structures and methods of maintaining 
aircraft stability in relatively low relative wind conditions. More 
particularly, it relates to aircraft employing at least two longitudinally 
displaced airfoil lifting surfaces, each incorporating structure to 
produce relatively high induced drag, with the mean center of lift of one 
lifting surface disposed forward of the center of gravity of the aircraft 
and the mean center of lift of another lifting surface disposed aft of the 
center of gravity. 
Since ancient times kites of various design have been flown for amusement 
and other purposes. Kites generally comprise a relatively lightweight body 
presenting at least one relatively flat surface upwind at a relatively 
high angle of attack, an anchor harness such as string, and some 
drag-inducing dampening means such as a tail. Kites do not usually employ 
an airfoil in the conventional sense, but instead employ a relatively flat 
plate with a high angle of attack presented to relative wind to generate 
lift. 
Since the invention of the glider and powered airplane using airfoil wings 
to generate lift, attempts have been made to fly unpowered airfoils in 
tethered arrangements such as conventionally used for kites. An unpowered 
craft using airfoils for lift which can be flown in a tethered arrangement 
has particular appeal as a novelty or toy if the unpowered craft can be 
made to simulate or duplicate the physical appearance of a conventional 
airplane in flight. However, because of the aerodynamically clean design 
of airplanes, a scale model airplane cannot readily be flown as a captive 
glider using merely a tether without artificial drag-inducing means such 
as a tail or the like or other means to provide lateral stability at the 
relatively low airspeed or relative wind conditions in which tethered 
unpowered craft must perform. Obviously, the attachment of a drag-inducing 
tail seriously detracts from the desired visual illusion. However, without 
artificial drag-inducing means or other stabilizing means scale models of 
airplanes suffer severe lateral instability and/or overfly the fixed 
remote end of the tether line to enter a steep uncontrolled dive, thus 
they cannot ordinarily be successfully flown in tethered arrangements. 
In accordance with the present invention, apparatus is provided which in 
many respects resembles conventional airplane configuration but which 
deviates from conventional airplane design by incorporating an airfoil 
producing high lift and high induced drag in the horizontal stabilizer and 
also using high lift and high induced drag airfoils for the lifting wings. 
By locating the center of gravity of the craft between the center of lift 
of the wings and the center of lift of the airfoil horizontal stabilizer, 
unusual flight characteristics are obtained which cause the aircraft to be 
extremely stable in pitch and yaw at extremely low airspeeds. Because of 
the unique flight characteristics of aircraft employing the principles of 
the invention, simulated aircraft which have the general configuration and 
appearance of conventional aircraft may be flown in tethered arrangements 
on a single tether line in relatively low wind conditions. Furthermore, 
tethered aircraft employing the invention respond to changes in relative 
wind to automatically reduce angle of attack with increased airspeed. 
Therefore, the craft may be controlled by manipulation of the single 
tether line to climb, cruise in level flight, descend and even perform 
aerobatic maneuvers with such realism that the maneuvers emulate the same 
maneuvers performed by conventional aircraft. Additionally, by appropriate 
positioning of the thrust (tether) line, the aircraft may be readily 
rigged to maintain level flight almost directly overhead of the remote 
fixed end of its tether line and thus never overfly the operator. Other 
features and advantages of the invention will become more readily 
understood when taken in connection with the appended claims and attached 
drawing in which:

The principles of generating lift with an airfoil moving through a relative 
wind component (with either the aircraft or the wind component moving or 
fixed with respect to earth) are, of course, identical. Accordingly, the 
principles of flight for an aircraft wherein thrust is provided by an 
onboard motive force which moves the aircraft through the air and a 
tethered aircraft wherein the craft is tethered to a fixed point or 
another towing craft are generally equally applicable. Of course, in a 
tethered aircraft flight control surfaces may be much more simplified. 
As the principles of flight become better understood, means for generating 
lift (generally referred to as airfoils) of various configurations have 
been designed and utilized to take advantage of various phenomena. In the 
most basic of aircraft designs, the aircraft comprises a longitudinally 
extending fuselage with substantially laterally extending wings to provide 
the necessary lift. Furthermore, in the most basic airfoil design, the 
linear distance measured along the top surface of the airfoil is 
substantially greater than the chord of the wing and the undersurface of 
the airfoil is approximately the same as the chord. Accordingly, air 
travelling over the wings travels faster over the upper surface than the 
lower surface, thus providing lift. The center of lift (the average or 
mean point of vertical lift forces across the top surface of the wing) 
will, of course, move fore and aft along the wing with changes in angle of 
attack but, in level flight, is substantially at the same point (fore to 
aft) as the center of gravity of the aircraft. The trailing edge of the 
airfoil is usually tapered to a point to reduce drag. Roll stability of 
the craft is provided by the dihedral of the wings and pitch control is 
provided by a horizontal stabilizer generally positioned aft of the wings. 
Yaw control is provided by a vertical stabilizer, usually also aft of the 
wing, and may incorporate a moveable rudder to variably control the 
direction of flight of the aircraft. 
In accordance with the invention, aircraft employing the same general 
components may be designed which, when modified in accordance with the 
principles of the invention, exhibit flight characteristics heretofore 
unattainable. For example, as illustrated in FIG. 1, a tethered aircraft 
employing the invention is shown having a fuselage 10 of conventional 
configuration. The craft employs an airfoil wing 11, a horizontal 
stabilizer 12 and a vertical stabilizer 13 with the vertical and 
horizontal stabilizers disposed well aft of the wings as in conventional 
aircraft configurations. The aircraft of FIG. 1, however, departs 
substantially from conventional design in the airfoil configuration of the 
wing 11, the configuration of horizontal stabilizer 12, and the placement 
of center of gravity. Since center of gravity can be controlled by 
appropriate distribution of mass, center of gravity does not affect 
outward appearance of the craft. Therefore, the only visible deviations 
from conventional craft exhibited by craft employing the principles of the 
invention are the configurations of the airfoils used for the wings and 
the horizontal stabilizer. These deviations, as will be explained in 
detail hereinafter, are immediately apparent upon close inspection of the 
craft but are not readily visibly apparent to the naked eye when the 
aircraft is in flight. 
In order to produce an aircraft capable of flight in extremely low relative 
wind conditions, the airfoil used for the wing must be designed to produce 
relatively high lift. In accordance with the invention, the wing airfoil 
produces not only high lift, but produces a first lift component on the 
top surface and a second lift component resulting from the configuration 
of the lower surface and also produces a relatively high induced drag. One 
preferred embodiment of the airfoil design of the invention is illustrated 
in FIG. 2. It will be observed that in the airfoil design of FIG. 2, the 
upper surface 14 of the wing is convexly curved and thus longer than the 
chord of the wing. The lower surface 15 of the wing beginning at the 
leading edge and traversing a major portion thereof is substantially flat 
and parallel with the chord of the wing so that the upper surface 14, in 
cooperation with the lower surface 15 of the forward section of the wing, 
defines a substantially conventional airfoil for providing lift. For 
purposes of discussion, the lift generated by the airfoil of FIG. 2 as 
described hereinabove will be referred to as the first lift component. The 
first lift component is, of course, distributed over the curved surface 
and has a mean or average point which moves fore to aft with decrease in 
angle of attack. 
The airfoil of FIG. 2 deviates from conventional airfoil configuration by 
the inclusion of a downwardly diverging section 15a on the lower surface 
15 adjacent the trailing edge of the wing. The downwardly diverging 
section 15a radically departs from the line parallel with the chord and is 
not paralleled by the corresponding upper surface. Accordingly, as the 
lower surface defined by the downwardly diverging section 15a departs from 
the curvature of the upper surface 14, the trailing edge of the wing 
becomes relatively thick and terminates in a thick blunt trailing edge 16, 
the vertical surface of which is substantially normal to the chord line of 
the wing. The downwardly diverging section 15a near the trailing edge 
results in a thickened trailing edge for the airfoil and, since the 
curvature of the downwardly diverging section 15a also departs radically 
from the curvature of the upper surface 14, the thickened trailing edge of 
the wing defines an airfoil providing a lift component separate and 
distinct from that defined by the upper surface 14. Since the downwardly 
diverging section 15a departs radically from the chord line of the wing, 
the lift component provided thereby (hereinafter referred to as the second 
lift component) is distinct from the normal lift component provided by the 
upper surface 14. Furthermore, the fore to aft shift of the average or 
mean center of lift provided by the downwardly diverging section 15a with 
change in angle of attack moves only slightly with changing angle of 
attack. Thus as the angle of attack of the airfoil with respect to 
relative wind decreases, the mean center of lift provided by the first and 
second lifting components is displaced substantially aft of the mean 
center of lift of the first lift component. Furthermore, the relatively 
thick trailing edge of the wing having a blunt surface normal to the chord 
line of the wing provides a very high induced drag factor. Therefore, the 
airfoil as shown in FIG. 2 produces extremely high lift and also extremely 
high induced drag at relatively low airspeeds. 
It will be readily appreciated that an airfoil designed to produce first 
and second major centers of lift and high induced drag as described 
hereinabove need not be configured precisely as illustrated in FIG. 2. For 
example, a modified version thereof is illustrated in FIG. 2a wherein the 
upper surface 14 is curved with respect to the chord as illustrated in 
FIG. 2 but wherein the downwardly diverging section 15a, instead of being 
a curved surface of constant or changing radius, is defined by a 
relatively straight line diverging from the plane of the forward portion 
of the lower surface 15. 
Similarly, in FIG. 3 a modified airfoil in accordance with the invention is 
illustrated wherein the upper surface 14 and the lower surface 15 are 
substantially parallel with each other and the chord line of the wing. The 
structure of the wing of FIG. 3 forward of downwardly diverging section 
15a thus is essentially a flat plate and will produce high lift at high 
angles of attack. However, as the angle of attack approaches zero, the 
lift provided by the wing forward of the downwardly diverging section 15a 
also approaches zero. The downwardly diverging section 15a, however, still 
continues to provide lift and the thickened trailing edge still produces 
high induced drag. Accordingly, the airfoil of FIG. 3 still produces a 
high lift high drag airfoil at high angles of attack with a rearwardly 
shifted center of lift and high drag in level flight. Similarly, as 
illustrated in the lifting surface of FIG. 3a the downwardly diverging 
section 15a need not be a curved surface but may be a relative flat 
surface which deviates from the plane of lower surface 15. Nevertheless, 
the structure performs aerodynamically in much the same manner as the 
structure shown in FIG. 3. It will thus be observed that by employing any 
of the airfoil designs illustrated in FIGS. 2, 2a, 3 or 3a in the aircraft 
of FIG. 1, a wing having first and second distinct centers of lift and 
high induced drag is realized. Use of curved upper surfaces as illustrated 
in FIGS. 2 and 2a contributes somewhat to the realism of visual effect of 
the aircraft since conventional airfoils include curved upper surfaces. 
The curved upper surface of the airfoil also obviously contributes to the 
lift component. Use of a curved upper surface as illustrated in FIGS. 2 
and 2a also provides an airfoil wing of thicker cross-section, thus 
simplifying structural stressing of the wing by permitting use of a 
thicker wing. 
In order to produce an aircraft which automatically responds to variations 
in relative wind by varying the angle of attack of the airfoil wing, the 
aircraft of the invention also employs a lifting airfoil for the 
horizontal stabilizer. In order to produce an unpowered aircraft for 
tethered flight in extremely low relative wind conditions, an airfoil 
employing a similar configuration as described above is used for the 
horizontal stabilizer 12. 
As illustrated in FIG. 1, the horizontal stabilizer 12 employs the airfoil 
of FIG. 3. It would be readily recognized that since the center of gravity 
of the craft is aft of the main center of lift of the wings as described 
hereinafter, the airfoil used for the horizontal stabilizer must provide a 
certain amount of lift in level flight (zero angle of attack) and also 
produce high induced drag. 
In order to produce a tethered craft in which the angle of attack of the 
aircraft is automatically reduced with increasing relative wind (as would 
be required to prevent the aircraft from overflying a fixed tether point), 
the center of gravity of the aircraft in level flight must be aft of the 
mean center of lift of the wing and forward of the mean center of lift of 
the horizontal stabilizer. This configuration radically departs from 
conventional aircraft design wherein the center of gravity is ordinarily 
directly below the average center of lift of the wing in level flight. 
As noted above, the center of gravity can be determined by appropriate 
distribution of mass of the aircraft. In models for flying on a tether, 
weights or the like may be appropriately positioned within the fuselage to 
appropriately position the center of gravity as desired. In the preferred 
embodiment, the center of gravity is approximately at or slightly aft of 
the trailing edge of the wing. Thus, by positioning the horizontal 
stabilizer toward the rear of the aircraft, the moment arm of the fuselage 
between the center of gravity and the horizontal stabilizer permits the 
use of relatively low collective lift on the horizontal stabilizer and 
therefore the horizontal stabilizer can be of scale dimensions 
substantially corresponding to the horizontal stabilizer in conventional 
aircraft. 
Where the aircraft is to be flown in a tethered arrangement, the tether 
line is attached at the forward end of the fuselage. Obviously, whether 
the aircraft is powered, towed or tethered, forward thrust is determined 
by relative wind. When the aircraft is flown in a tethered arrangement, 
the length of the tether line, the angle of the tether line and the weight 
of the tether line will all affect performance characteristics of the 
craft under certain conditions. However, for relatively small lightweight 
craft used as kites, extremely lightweight line such as ten pound test or 
the like is suitable. Thus when the line is relatively short, the aircraft 
is sufficiently near the operator that the operator can directly exercise 
some control over angle of attack. However, as the length of the line 
increases, relative thrust is provided by playing out or reeling in the 
tether line and by natural changes in relative wind. Thus the thrust line 
is determined by the azimuthal angle between the point of attachment of 
the tow or tether line and the zero angle of attack center line of the 
craft. By varying this angle, the response of the craft to changes in 
relative wind will vary as desired. It should be noted that with a 
substantial length of tether line between the fixed remote end (operator) 
and the aircraft, the tether line will assume a substantially fixed angle 
with respect to the thrust line and the aircraft may thus be flown to a 
point almost directly overhead of the remote end (either fixed or 
controlled by an operator). 
It will be noted that as the aircraft attitude changes from climb to level 
flight, the centers of lift of the wing and the horizontal stabilizer both 
move aft. Since the center of gravity is fixed, the center of gravity thus 
moves forward with respect to the center of lift. Furthermore, as the 
angle of attack is reduced, lift is reduced but high drag conditions are 
maintained. Thus as the aircraft approaches level flight, the high drag 
component prevents the aircraft from increasing its forward velocity. 
Instead, the lifting components of the wing and horizontal stabilizer 
counterbalanced about the center of gravity tend to force the craft into 
stabilized level flight. The high induced drag of the airfoils also 
induces lateral stability. Therefore, the craft automatically becomes 
stabilized in level flight. Furthermore, if the craft is forced into a 
nose-down attitude (as might occur by attempting to overfly the remote end 
of the tether line), the horizontal stabilizer is forced into a negative 
lift condition and, since the center of gravity is aft of the wings, the 
tail immediately drops so that the craft resumes level flight conditions. 
By automatically dumping lift from the horizontal stabilizer, the craft is 
prevented from diving and overflying the tether point. Furthermore, since 
the lift component of the horizontal stabilizer approaches zero (or merely 
balances the rearwardly displaced center of gravity), the aircraft resists 
entering a dive attitude. Instead, the aircraft maintains level flight 
with a slight tendency to climb. If excess thrust is provided (as by a 
gust of increased relative wind) little or no attitude change will result. 
If relative wind is decreased, however, the rearwardly displaced center of 
gravity causes the craft to settle tail first, thus increasing angle of 
attack. Thus attitude of the aircraft can be controlled by controlling 
relative wind. This may be accomplished, for example, by varying the 
length of the tether line. Accordingly, the aircraft may be caused to 
climb, cruise and even make stall landings by appropriately controlling 
the single tether line. 
It will be observed that the aircraft configuration described hereinabove 
is uniquely suitable for use in tethered arrangements since the relative 
lifting airfoil configurations can be fixed and the aircraft remain 
extremely stable in low relative wind. In the configurations discussed, a 
tethered craft can be flown and maneuvered in relative wind as low as 
three to eight knots. However, since the only flight dynamics penalty for 
the fixed configuration is high induced drag at level flight, the craft 
can also be towed for use as a target or the like. The aircraft may also 
even be flown in powered configurations for low speed drones, if desired, 
for target practice or for low speed stable observation or photographic 
missions. 
It is essential that the airfoil of the invention provide at least two 
distinct lift components, one of which moves fore to aft with changes in 
angle of attack while the other remains relatively fixed (fore to aft with 
changes in angle of attack) and must produce a relatively high induced 
drag. Within these limitations, however, the airfoil may take various 
forms. For example, FIG. 4 illustrates a more conventional appearing 
airfoil with a convexly curved upper surface 14 and a relatively flat 
lower surface 15 joined to form a leading edge. However, at the trailing 
edge both the upper and lower surfaces deviate sharply downwardly 
depending trailing edge 20. Downwardly depending trailing edge 20 
functions in much the same manner as described hereinabove since the 
airfoil still includes the same sharply downwardly diverging section 15a 
on the lower surface which produces the second distinct component of lift. 
However, the curved upper surface of section 20 will produce less induced 
drag under most conditions. 
Where it is particularly desired to produce an aircraft in accordance with 
the invention but which visually appears to employ a conventional airfoil, 
the trailing edge portion of the airfoil may be made substantially 
transparent by using clear plastic material or the like. This may be 
accomplished by forming any of the modified airfoils discussed herein 
using clear materials and painting or otherwise coloring only that portion 
of the airfoil which provides conventional shape. Alternatively, a 
conventional airfoil may be used if modified as shown in FIG. 5. In this 
embodiment a substantially L-shaped (in cross-section) and inverted member 
of substantially transparent material is attached to either the upper or 
lower surface at or near the trailing edge of the airfoil to form a 
sharply downwardly depending flange 21. The flange member 21 thus 
positioned provides both the required substantially fixed (fore to aft) 
component of lift as well as high induced drag. However, since it is 
substantially transparent it is virtually invisible to the naked eye at a 
distance from the aircraft. Thus an aircraft employing this embodiment 
will, when viewed from a distance, appear to employ only conventional 
airfoils. 
In view of the foregoing, various applications of the principles of the 
invention will readily be apparent to those skilled in the art. It will be 
understood, therefore, that although the invention has been described with 
particular reference to specific embodiments thereof, the forms of the 
invention shown and described in detail are to be taken as preferred 
embodiments thereof. Various changes and modifications may be resorted to 
without departing from the spirit and scope of the invention as defined by 
the appended claims.