Patent Application
20100303634
The present invention relates to improvements to foils and related fluid dynamic structures for influencing or controlling aerodynamic or hydrodynamic flows to reduce drag, such as drag and turbulence induced by wingtip vortices of an aircraft, cooling and ventilating drags, or such as is found in the wake of a watercraft. The invention also relates to slot-like structures for fluid dynamic foil bodies.
Slots have been used for years in many aircraft wing designs in order to prevent the airflow from separating prematurely from the wing at high angles of attack, and thereby reduce the stall speed and improve handling at low speeds. Air from below the wing accelerates through the slot and exits rearward and substantially parallel to the upper wing surface, energizing the boundary layer and delaying separation. A similar approach provides a pressurized discharge from wing surface openings, either from compressed air tanks, pumps, or from the jet engines, to accomplish a similar goal. Because many of the slot designs also tend to contribute to drag at cruising speeds, retractable slats may be used instead to provide a leading edge opening that can be closed when not needed. Slots may also be used to reduce wave drag under transonic conditions to distribute pressure and increase the critical Mach number, thereby improving performance at such speeds. Variations of this basic theme of using slots, slats or other openings to ensure more attached flow across the wing surfaces and delayed boundary separation are described in U.S. Pat. Nos. 2,041,786 (Stalker); 2,507,611 (Pappas et al.); 2,571,304 (Stalker); 2,587,359 (Milans); 3,208,693 (Riedler et al.); 3,363,859 (Watts); 3,897,029 (Calderon); 4,641,799 (Quast et al.); 4,664,345 (Lurz); 5,255,881 (Rao); 5,788,190 (Siers); 6,293,497 (Kelley-Wickemeyer et al.); 6,328,265 (Dizdarevic); 6,905,092 (Somers); 7,048,235 (McLean et al.); and U.S. Patent Application Publication No. 2007/0034746 (Shmilovich et al).
Another problem in aircraft design is the production of trailing vortices and wake turbulence during flight. Throughout the history of aeronautics, aircraft designers have had to deal with the energy consuming effects of wingtip vortices, which form at the tip of a wing where higher pressure air from beneath the wing flows in a generally span-wise direction around the wingtip to the lower pressure region above the wing. Wingtip vortices have been considered by many experts to be an essentially unavoidable consequence of a wing producing lift. Due to the nature of these vortices, they are associated with lift-induced drag and are a major component of wake turbulence. Smaller vortices are induced at other points on an aircraft wherever there is an abrupt change in planform or contour, such as at the outboard tips of wing flaps, ailerons, horizontal stabilizers, elevators and other flight control surfaces. The drag and wake turbulence from the vortices have a negative impact on fuel efficiency and flight performance of the aircraft, and also pose a safety hazard to any aircraft that follow too closely or otherwise cross the wake.
Various solutions have been developed in an effort to reduce the production of trailing vortices, especially wingtip vortices. Since wingtip vortices only affect that portion of a wing closest to its tip, one partial solution is to use a higher aspect ratio wing (longer wingspan and/or reduced chord), but this also tends to reduce aircraft maneuverability and adds structural weight. Another approach is to modify the lift distribution along the span to generate more lift at the wing root and less toward the wing tip, either by using an elliptical or tapered wing planform, increasing the mean camber or asymmetry of the airfoil section near the wingtip, or using a washout (span-wise twist of the wing) to reduce the incidence near the wingtips. However, these solutions do not make full use of the wingspan to efficiently produce lift.
A number of wingtip devices have been designed to allow nearly the entire wingspan to produce lift, while simultaneously altering the airflow near the wingtips in order to partially interfere with the production of vortices or to change the pattern of vorticity so as to reduce the associated drag. The intended result is an improved lift-to-drag ratio, with a corresponding improvement in fuel efficiency. Drooped (Hoerner) wingtips focus the vortex away from the upper wing surface. Winglets, a near-vertical upward or downward extension of the wingtips, cause the vortex to strike the outboard surface of the winglet so as to generate an inward and slightly forward force, and thereby converts some of the vortex energy into an apparent thrust. Wingtip fences are winglet variations that may have surfaces extending both above and below the wingtip to reduce the span-wise component of airflow that leads to wingtip vortices, but create vortices of their own at the fence tips and interference drag, albeit displaced from the main wing. Blended winglets smoothly curve up, increasing their cant gradually to reduce interference drag that would otherwise occur at the wing/winglet junction. Raked wingtips have a higher degree of sweep than the rest of the wing. Various wingtip devices are described, for example, in U.S. Pat. Nos. 5,039,032 (Rudolf); 5,634,613 (McCarthy); 6,722,615 (Heller et al.); and 6,892,988 (Hugues).
In U.S. Pat. No. 5,823,480, La Roche provides a wing grid having at least two parallel staggered “winglets” extending outward from the free end (tip) of the wing. The winglets, which in this case are essentially parallel to the main wing rather than upward extending, subdivide the air circulation at the tip so that the span-wise lift distribution is more regular, decreasing induced resistance. In one embodiment, this wing grid can be retracted into the end of the wing.
In U.S. Pat. No. 4,478,380, Frakes discloses a wingtip vortex suppressor that utilizes a scoop having an inlet at a lower leading surface and an outlet at an upper trailing surface to reduce the pressure differential at the wing's trailing edge. The scoop is inboard of a turbulence fence at the wingtip. In another arrangement, U.S. Pat. No. 5,806,807 to Haney, has both a deflector extending from the top surface of the wing inboard of the tip and an air passage extending from an inlet on a high pressure side of an airfoil through the airfoil to an outlet on a low pressure side of the airfoil, with the outlet positioned between the deflector and the wing tip. The deflector and air passage work in combination to attenuate the wingtip vortex. In both patents it appears that the fence or deflector is an essential component for vortex reduction.
In U.S. Pat. No. 5,791,875, Ngo describes a system providing a source of positive fluid pressure interior to the wing and a fluid router (curved slots in lower surface of the wingtip) that directs the fluid inboard against the outward airflow in order to reduce the wingtip vortex. Similarly, in U.S. Pat. No. 7,134,631, Loth provides a tip circulation control that blows air out from the very end of the wing in order to cancel opposing vorticity. In U.S. Pat. No. 7,100,875, Shmilovich et al discharge a jet air stream from a set of nozzles at the wingtips and moves them cyclically back and forth in order to dissipate and scatter wingtip vortices. Although from a safety standpoint any reduction in trailing vortices is an improvement, from an energy efficiency standpoint the amount of energy required to sustain such counteracting airflows tends to defeat any fuel savings that might be achieved by a decrease in drag.
Trailing vortices and wake turbulence can also cause drag in other fluid dynamic structures, such as in rotor blades in helicopters and wind-power turbines, sails (which are also airfoils), as well as underwater surfaces such as hydrofoils, hulls, centerboards, keels, rudders and screws of various watercraft including submersibles. U.S. Pat. No. 5,374,013 to Bassett et al. describes one approach to reducing drag in a truck by providing a pressure shell around the rear of the truck body with better boundary flow around the truck. In any of these fluid dynamic structures, it is desired to realize fuel savings and safety improvements by reducing trailing vortices and wake turbulence, whether from wings and other flight control surfaces on aircraft, or from any of the other foil-like or bluff bodies.
A fluid dynamic section in accord with the present invention comprises a foil body for use in a fluid dynamic environment and having one or more fixed size escapelets in the form of static passages integrated into the foil body and leading from a first higher pressure surface of the body to a second lower pressure surface of the body. Each escapelet is configured in size, shape, location, orientation and number so as to redirect any span-wise component of energy in a kind of shortcut from the high-pressure side to the low-pressure side of the structure, beneficially releasing energy and preempting the normal formation of vortices. Whatever span-wise component of fluid flow that might leak around an end of the foil body gets caught up in the high velocity current exiting longitudinally from the escapelet outlets. Unlike conventional slots, each escapelet is sized and positioned to minimize its impact on the longitudinal component flow, thereby improving the lift-to-drag ratio and resulting in dramatic fuel savings. Despite creating passages from the higher-pressure side to the lower-pressure side of fluid dynamic structures, the elimination of unproductive components of fluid flow from the use of these escapelets results in an overall increase in the effective aspect ratio by as much as 50% or more. Thus, the escapelets act to harness, redirect and release fluid dynamic energy in a manner that prevents chaotic energy dissipation and minimizes energy loss. As an additional benefit, escapelets located near wingtips effectively increase control authority to ailerons, thereby improving such authority even at high angles of attack up to and including spin conditions, while escapelets located inboard on the wings near the fuselage and/or on the horizontal tail sections may further improve spin recovery whenever needed.
Escapelets have fixed sizes that are typically such that their widths range from 1% to 15% of the foil body's local chord. Escapelets are generally installed between the structural ribs of the foil body, so lengths of the individual escapelets do not extend a full span. The cumulative span of a set of escapelets along a common planform of the foil body is usually less than a half-span of the foil body. In one embodiment, the locations of the escapelets may be confined to an outer 15% of the foil body half-span. Escapelet outlets from the lower-pressure side of the foil body usually have a height normal to the surface that may range anywhere from 10% to 110% of the length of the corresponding escapelet inlet on the higher-pressure side of the foil body, with the higher values generally applying to craft of higher design speeds. Escapelet outlets are located downstream or aft of the escapelet inlets, with a center-of-inlet to center-of-outlet mean line of an escapelet oriented chord-wise ranging anywhere from 10° to 80° relative to the chord line, with the lower value generally applying to craft of higher design speeds. The span-wise orientation of the escapelets generally follows the planform of the foil body. For example, where the escapelet openings include one or more leading escapelets and/or one or more trailing escapelets, the leading escapelet openings may be substantially parallel to a leading edge of the foil body, while the trailing escapelet openings may be substantially parallel to a trailing edge of the foil body. The escapelets preferably have a smooth shape conducive to streamlined efficient fluid flow, and may include a preformed liner insert to eliminate any discontinuities or sharp intersections and ensure such streamlined smoothness.
The fluid dynamic structures encompassed by this improvement may include not only aircraft wings, but also aircraft control surfaces, such as ailerons, flaps, aircraft empennages, canards, including stabilizers, elevators, fins, rudders, together with rotor blades, propeller blades, turbofans, ducted fans, and wind tunnel drive fans, as found on both helicopters and wind-power turbines, as well as foil bodies on various watercraft, including sails, wing sails and various underwater structures like keels, and also cables, guy-wires, poles and towers, as previously mentioned. The presence of the escapelet openings in the various foil bodies significantly reduce trailing vortices and wake turbulence, with a consequent reduction in drag and corresponding improvements in energy efficiency of more than 12%, a fourfold or better improvement compared to most prior approaches.
In order to better understand the improvements, we note first that slots, slats and slotted flaps have the purpose of increasing the maximum lift coefficient, thereby reducing the stall speed, while the escapelets provided within the various fluid dynamic sections in accord with the present invention have instead the purpose of preemptively redirecting fluid flow energy before it can cause fluid dynamic havoc manifesting as vortices, turbulence, wake and drag with consequent wasteful energy dissipation. The escapelets harness, redirect and release the energy beneficially so as to minimize such energy losses. In addition to the benefits already discussed, escapelets may also be employed to reduce turbulence due to flow separation, thus making the associated flows more efficient.
“slot” (noun)—a long and narrow opening as between a wing and a Fowler flap; specif. a long and narrow spanwise passage in a wing usually near the leading edge for improving flow conditions at high angles of attack.
“slat” (noun)—any of certain long narrow vanes or auxiliary airfoils, e.g. as used in an automatic slot.
“automatic slot” (noun)—a slot in the leading edge of a wing created by the movement of a slat that is retained in the leading edge contour of a wing at most angles of attack, but automatically lifting away to create a slot as the stalling angle is a approached.
“fixed slot” (noun)—a slot that remains open at all times; distinguished from an adjustable or an automatic slot.
A slotted flap is an adjustable mechanism on the trailing edge of a fluid dynamic foil that moves into position when deployed. In contrast, an escapelet is a static passage built into a fluid dynamic foil.
The structural design differences lead to different modes of operation. In all cases, while in function, slots, slats, and slotted flaps will change the geometry, mean camber line(s), and dimensions of the original foil section, amounting to an adjustable geometry by extending the chord, reshaping the mean camber line(s), from the leading and trailing edges respectively. Since a slat is permanently or adjustably positioned ahead of the leading edge of a fluid dynamic foil, its implementation will always result in a change to the fluid dynamics of the foil, increasing drag while extending the chord and altering the original section profile and camber of the foil. In contrast, the integral passages that form the escapelets through the foil body act to improve fluid dynamic characteristics without physical changes in chord or camber of the foil, acting to reduce induced and interference drag by preempting or significantly reducing the formation of vortices. Additionally, the changes to fluid flow arising from the use of escapelets in some locations improve control authority of ailerons and/or other control surfaces and high-lift devices and enhance spin recovery. The end result is a device that accomplishes aerodynamic results never before achieved in the history of aviation.
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In both cases, the escapelets generally follow the planform of the wing. In the case of the unswept wing 11 in
The positions of the escapelets 17, 19, 37, and 39 on their wings 11 and 31 do not necessarily line up with the ailerons 23 and 43 or any other flight control structure of the wing. In the first case, the outer ends of escapelets 17 and 19 do happen to align with the outer edge of the aileron 23, but this is not required. The aileron 23 is seen to extend inboard beyond the views and beyond the inner ends of escapelets 17 and 19, but again, this is not necessarily required. In the swept wing example, the ends of the escapelets 37 and 39 do not align with either edge of the aileron 43, but rather the escapelets 37 and 39 are positioned to overlap the outer edge of the aileron 43. These escapelets 37 and 39 could just as easily align with say the outer edge of the aileron, as in the unswept wing example. Generally, it may be preferred for optimum performance that one or more escapelets be associated in some manner, either by being' aligned or overlapping with, the outer edge of the % aileron 23 or 43, but it is not an absolute requirement. The mere presence of the escapelets and their action upon the airflow across the foil or foil body helps to transfer better control to the ailerons and other similar foil structures.
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Fluid dynamic structures having foil bodies with escapelets need not be confined solely to aerodynamic structures, but can also apply to hydrodynamic structures. As seen in
A variety of keel types can employ such escapelets. As seen in
Tests were devised to demonstrate the effectiveness of the escapelets. Model test aircraft were built, and test flights of the models were performed over a realistic flight sequence from takeoff to level flight at specified cruising speeds to landing, both with and without the equipping of the aircraft with escapelets. Introduction of the escapelets did not require any trim changes.
In order to assure the veracity of data collected in the test flights, a flight profile was developed that would be followed for each test. Comparative flights with and without the escapelets were conducted immediately after each other, minimizing the possibility for atmospheric change. All times were recorded with a stopwatch, and all fuel measurements were taken with a digital scale. The test flight profile was as follows:
1) Start, radio check, taxi, hold (2 minutes)
2) Takeoff
3) Fly one lap around field at full power
4) Reduce to cruise power
5) Time lap speed and fly several laps in a racetrack pattern
6) Land, taxi, stop
7) Accurately measure weight of fuel required to refill tank
Fuel burn was measured to the nearest gram. Some of the SERS flights with equipped escapelets ran a few seconds longer than standard flights with the unmodified aircraft. Therefore, fuel burn data on the equipped aircraft was adjusted to match the equivalent flight time of the unmodified vehicle, when necessary. Then, fuel consumption comparisons were made.
The recorded flight speed for Test Vehicle One was an average of 50 mph, and the coefficient of lift (CL) was calculated to be 0.25.
Wind: 0-5 mph gusting
Temperature: 68° F.
Barometric Pressure: 29.92 in Hg
(Conditions were recorded but may have changed during testing, and are therefore considered approximations.)
Standard Flight (without escapelets):
Duration: 900 seconds
Fuel Burn: 107 grams
SERS Flight (equipped with escapelets):
Duration: 900 seconds
Fuel Burn: 93 grams
12.54% overall
16.99% cruise
The recorded flight speed for Test Vehicle Two was an average of 60 mph, and the coefficient of lift (CL) was calculated to be 0.5.
Wind: 0-2 mph
Temperature: 27° F.
Barometric Pressure: 29.92 in Hg
(Conditions were recorded but may have changed during testing, and are therefore considered approximations.)
Standard Flight (without escapelets):
Duration: 606 seconds
Fuel Burn: 178 grams
SERS Flight (equipped with escapelets):
Duration: 606 seconds—adjusted from 616 seconds
Fuel Burn: 138.71 grams—adjusted from 141 grams
22.08% overall
29.92% cruise
The recorded flight speed for Test Vehicle Three was an average of 78 mph, and the coefficient of lift (CL) was calculated to be 0.3.
Wind: 0-2 mph
Temperature: 82.6° F.
Barometric Pressure: 29.82 in Hg
(Conditions were recorded but may have changed during testing, and are therefore considered approximations.)
Standard Flight (without escapelets):
Duration: 509 seconds
Fuel Burn: 228 grams
SERS Flight (equipped with escapelets):
Duration: 509 seconds—adjusted from 515 seconds
Fuel Burn: 192.7 grams—adjusted from 195 grams
15.48% overall
20.98% cruise
These fuel savings results correspond to a drag reduction of from 25% to 50%.
