Aircraft Design

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(12) (io) Patent No.:

United States Patent US 9,382,000 B1

(45) Patent: Jul. 5,2016

Bowers et al. Date of

(56) References Cited (54) AIRCRAFT DESIGN U.S. PATENT DOCUMENTS (71) Applicant: The United States of America as Represented by the Administrator of 6,722,615 B2 * 4/2004 Heller et al. ................ 244/45 R NASA,Washington, DC(US) 6,970,773 B2 11/2005 Phillips 7,883,060 B2 * 2/2011 Phillips ......................... 244/203 (72) Inventors: Albion H. Bowers, Lancaster, CA (US); 8,123,160 B2 * 2/2012 Shepshelovich et al. ... 244/45 R Edward Uden, Bamstedt(DE) OTHER PUBLICATIONS (73) Assignee: The United States of America as Ludwig Prandtl, NACA Report No. 116, 1920.

Represented by the Administrator of Nax Munk, NACA Report No. 120, 1920.

the National Aeronautics and Space Ludwig Prandtl, On the Minimum Induced Drag of Wings, Administration, Washington, DC(US) Zeitschrift fur Flugtecknik and Motorluftschiffahrt, 28 XII 1932.

Robert T. Jones, The Spanwise Distribution of Lift for Minimum (*) Notice: Subject to any disclaimer, the term ofthis Drag of Wings Having a Lift and a Given Bending Moment, NACA patent is extended or adjusted under 35 Technical Note 2249.

U.S.C. 154(b) by 42 days.

* cited by examiner (21) Appl. No.: 14/297,186 Primary Examiner Tien Dinh (22) Filed: Jun. 5,2014 Assistant Examiner Richard R Green (74) Attorney, Agent, or Firm Mark Homer (51) Int. Cl.

B64C 3114 (2006.01) (57) ABSTRACT B64C 23106 (2006.01) (52) U.S. Cl. The present invention is an aircraft wing design that creates a CPC ................. B64C 23/065(2013.01); B64C 3/14 bell shaped span load, which results in a negative induced (2013.01) drag(induced thrust)on the outer portion ofthe wing; such a (58) Field of Classification Search design obviates the need for rudder control of an aircraft.

CPC ................ B64C 3/52; B64C 3/14; B64C 3/10 See application file for complete search history. 13 Claims,6 Drawing Sheets

Patent Jul. Sheet 1 of6 U.S. 5,2016 US 9,382,000 B1 <1w✓ LL

Patent Jul. Sheet 2 of6 U.S. 5,2016 US 9,382,000 B1

Wing Tip

spanload

FIGURE 3

Centerline Load (lb/ft)

Wing Tip d w

FIGURE 4

Centerline Oownwash (degrees)

Wing Tip

Cdi

FIGURE 5

Centerline Coefficient of Induced Drag

Wing Tip

twist

FIGURE

Centerline Twist (degrees)

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AIRCRAFT DESIGN as the "sharp tipped wing") and that the new span load has 22% more span and 11%less induced drag than the elliptical STATEMENT OF GOVERNMENT INTEREST span load, but the same lift and the same integrated wing bending moment (the same structure). Although the paper The invention described herein may be manufactured and 5 does not disclose this, it implies that the induced drag begins used by or for the Government of the United States of at the wing centerline, decreases moving outboard and America for governmental purposes without payment of any becomes negative induced drag at the wing tips (negative royalties thereon or therefor. induced drag is induced THRUST). Therefore, the span load contemplated is a bell shape, rather than elliptical.

BACKGROUND OF THE INVENTION 10 In 1934,two teenage brothers, Walter and Reimar Horten, begin building a series of gliders that use Prandtl's proposed 1. Field of the Invention span load. Reimar Horten coins the term "bell shaped span The present invention relates generally to aircraft design, load" for this shape. Over the next 20 years, they attempt to specifically to aircraft wing design, and more specifically to develop the idea. The Hortens never fully explain how to aircraft wing design which would significantly increase air- 15 create the wings associated with the proposed bell shaped craft efficiency and obviate the necessity for aircraft adverse span load. Their work is documented in the book"Nurflugel" yaw controls.

by Reimar Horten,Peter Selinger,and Jan Scott(H Weishaupt 2. Description ofthe Related Art Verlag, 1993).

When Wilbur and Orville Wright test flew gliders in 1900 Robert T Jones of the NACA Ames Aeronautical Labora- and 1901,they discovered a problem with the control oftheir 20 tory publishes a paper, NACA Technical Note 2249 "The gliders. When they attempted to put in roll control, the wing Spanwise Distribution of Lift for Minimum Induced Drag of they would increase the lift on would move backwards. In Wings Having a given Lift and a Given Bending Moment."

other words,the aircraft would roll one way,but it would yaw This problem solution is nearly identical to the one Prandtl the opposite way, causing the gliders to crash. This is called had solved 18 years earlier, but Jones was unaware of adverse yaw, yawing the opposite direction to the roll com- 25 Prandtl's solution. Jones' solution also produced a bell mand to turn. In 1902, the Wrights solved this problem by shaped span load, a similar distribution ofdownwash/upwash adding a rudder. The Wrights were awarded a patent for this (with induced thrust at the wingtips), and a similar distribu- design in 1906.

tion of induced drag as Prandtl's 1932 solution. Jones solu- Current aircraft design includes two methodologies to con- tion uses 26% more span, has 17% less induced drag, the trol the adverse yaw identified by these aircraft pioneers. The 30 same lift and the same wing root bending moment as the first is the tail/rudder developed by the Wright brothers and elliptical span load it is derived from. Jones also proposed to the second is create devices at the wingtips that allow the use planform to produce this new span load.

aircraftto manipulate drag atthe wing tips (split elevons at the Although some ofthis early research described a potential tip like the B-2 Spirit aircraft, for example).

for reducing induced drag on an aircraft wing by creating a The wing designs of current aircraft that employ these 35 bell shaped span load, little serious design and development types of yaw control are based,in part, on a paper published work resulted from these theoretical findings, partly due to by Ludwig Prandtl in 1920(NACA Report No. 116) which the impracticality of using planform to produce such a span describes the theory called the Lifting Line, which becomes a load.

mathematical tool by which the calculation of a wings' per- Finally, one recent technique has been developed to use formance was first set forth. Other theories exist, but are too 40 twist distribution along the wing in order to minimize induced cumbersome to use, or too simplistic to be of value. Prandtl's drag by varying the washout(U.S.Pat. No.6,970,773). How- Lifting Line is the first tool that provides meaningful results ever, this technique employs a linear twist that still results in for wings.In this paper,Prandtl also introduces the concept of an elliptical span load and, therefore, does not provide yaw the elliptical span load as being the minimum induced drag control without a standard rudder.

for a given lift and a given wingspan. 45 Therefore, it is desired to provide a wing design that can Shortly thereafter Max Munk,Prandtl's student, published create a bell shaped span load, thereby reducing drag on the a paper, in NACA Report No. 120, that also describes a wing, without relying solely on planform techniques and, in stagger biplane solution (often referred to as the stagger addition, create yaw control without the need of a plane biplane report). This report describes that the elliptical span rudder or tail.

load results in a constant downwash behind the wing,and that 50 the induced drag along the span ofthe wing is approximately SUMMARY OF THE INVENTION elliptical as well.

In 1932, Prandtl published a paper on the minimum The invention proposed herein comprises an aircraft wing induced drag of wings, "Uber Tragflugel Kleinsten Induz- design that creates a bell shaped span load, which results in a ierten Widerstandes" [this translates as: On the Minimum 55 negative induced drag(induced thrust)on the outer portion of Induced Drag of Wings] (Zeitschrift fur Flugtecknik and the wing.

Motorluftschiffahrt,28 XII 1932; Munchen,Deustchland).In Accordingly, it is an objective ofthis invention to provide this paper,Prandtl attempts to determine a span load that uses an aircraft wing design that significantly reduces induced the same amount of structure and produces the same lift, but drag on the wing, while maintaining lift and thrust.

has less induced drag than the elliptical span load. Prandtl 60 It is another objective to provide an aircraft wing design uses the structure as the constraint, along with the lift by that provides adverse yaw control, obviating the need for any enforcing the same integrated wing bending moment of the separate adverse yaw control(such as a rudder).

elliptical on a new span load.Prandtl shows that this new span This invention meets these and other objectives related to load produces a downwash at the centerline, but the down- more efficient aircraft wing design by providing an aircraft wash decreases moving outboard and becomes an upwash at 65 that produces adverse yaw control without a rudder. This is the wing tip. Prandtl proposes that the wing planform be used accomplished by a wing design having a span load that to create this new span load(Prandtl refers to this wing design changes from downwash to upwash at a location from about

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60% to about 80% of the way from the aircraft centerline to the aircraft. For example,ifthe aircraft flies in a sideslip, the the aircraft wingtips. Such a design comprises a total wing trailing wing presents a larger area of twist to the oncoming twist percentage of about 10 to about 30 times the aircraft's air. This generates greater induced thrust compared to the design lift coefficient. This total wing twist is nonlinear such leading wing, which has a smaller area exposed and generates that from about 10% to about 35% of the total wing twist 5 a correspondingly smaller induced thrust, so the aircraft auto- occurs from the aircraft centerline to about halfway to the matically corrects itself from the sideslip as though the air- wing tip and the remainder of the total wing twist occurs craft had a vertical tail. Because a dynamic dutch-roll(trading along the remainder of the wing.

yaw and pitch in aperiodic cyclical motion)would also result in stronger thrust on the trailing tip and weaker thrust on the BRIEF DESCRIPTION OF THE DRAWINGS leading tip, the dutch-roll motion would quickly damp out.

This results from the structural twist and would not occur in In the drawings, which are not necessarily to scale, like or the case ofa planform only bell shaped span load, as proposed corresponding parts are denoted by like or corresponding by both Prandtl and Jones as discussed above.

reference numerals.

15 Referring to FIGS.1 and 2, an aircraft body 100 comprises FIG.1 depicts an aircraft incorporating the present inven- a centerline 102. Wings 104 extend from the aircraft body tion.

100. A total wing twist 106 is applied to the wings 104.

FIG.2 depicts an aircraft wing from FIG. 1.

The total wing twist 106 is very large by comparison with FIG. 3 depicts a graphical representation of a span load current aircraft designs. In general, the total wing twist com- resulting from the present invention.

prises values well in excess of10 degrees, while conventional FIG. 4 depicts a graphical representation of downwash wings as used now seldom have more than 2 degrees oftwist.

resulting from the present invention.

The wing twist 106 is also unusually nonlinear, with most of FIG.5 depicts the induced drag coefficient across the wing of the present invention. the twist occurring in the outboard part ofthe wing, near the FIG. 6 depicts a graphical representation of the preferred wing tip 108; common practice is to use a linear wing twist.

total wing twist of the present invention.

25 For the presentinvention,it is only in the aboutthe outer 20 to 40 percent of the wings span 104 that the induced thrust DESCRIPTION OF PREFERRED exists. It is necessary for the control surfaces for roll to be EMBODIMENTS placed in this outer portion of span, near the wing tips 108.

This is the region of negative induced drag (induced thrust) The present invention is a novel aircraft design that pro- 30 and the cross-over the flow to negative downwash(upwash).

vides yaw control without a rudder by creating a bell-shaped This is the inflection (about 60%-80% semispan) where the span load through a non-linear wing twist.

wing 104 vortex would roll-up.

Almost all aircraft have vertical tails, or as a minimum they In a preferred embodiment of the invention, use of a have some method of direct yaw control; so why don't birds 35 straight taper aft-swept wing 104, with small tips 108(a large have vertical tails or exhibit some method of direct yaw taper ratio),results in tips 108 that are very small. The amount control? The present invention provides a wing design which ofstructural twist 106 is dictated by the design lift coefficient is a direct analog to the wing of a bird, and, observationally, ofthe aircraft. The aspect ratio, the twist 106, the taper ratio, exhibits the same direct characteristics. The resulting flight the wing 104 area,and the wing 104 mean aerodynamic chord behaves as birds do,and eliminates the need for a vertical tail.

40 all combine to create the characteristic flying qualities for the Rudders have been a fact oflife in aircraft design ever since invention. The size ofthe aircraft/payload and the strength of because adverse yaw has been an accepted fact ofaeronautic the wings 104 dictate the resulting center of gravity location, life. There have been flying wings since 1912 (aircraft with- out fuselages), and designers since then have occasionally and the wing 104 sweep then dictates the longitudinal stabil- sought to eliminate vertical surfaces altogether,but with little ity. Preferably, the twist 106 will result in trim at the design success; no matter what they do, designers have had to resort 45 point that coincides with the maximum lift-to-drag ratio (or to one contrivance after another to deal with adverse yaw. with the desired design point).

As a result, almost all aircraft have vertical tails, or at a The total wing twist106in degrees is about 10-30 times the minimum,have some method of direct yaw control; yet birds design lift coefficient, with the total wing twist 106 being have no vertical tails and exhibit no method of direct yaw preferably 20 times the design lift coefficient. As an example, control. The present invention provides a wing that is analo- 50 if an aircraft has a design lift coefficient of0.6, the total wing gous to the wing of a bird in that it has no vertical tail and twist would be about 12 degrees. The total wing twist 106 is exhibits no direct yaw control and,yet,is entirely controllable the sum of the aerodynamic twist (the offset of the zero lift in flight. angle ofattack due to selection ofthe airfoil) and the geomet- In describing the invention herein, it should be understood ric twist(the twist ofthe wing as measured by the first point that that certain aircraft are designed with two separate wings 55 ofthe leading edge to the last point of the trailing edge).

extending from a central aircraft body and others are designed Also, as noted above, the total wing twist 106 is strongly with a "single"wing with the aircraftbody attached below the nonlinear. The twist 106 between the centerline 102 to about centerline thereof. The present invention can be incorporated halfway to the wing tip 108 is only about 10 to about 35 into any winged aircraft design and when discussing wings percent of the total twist 106, and the remaining 65 to 90 herein, the term "wings" both configurations (wherein the 60 percent ofthe twist106 is inthelasthalfofthewingl04tothe "single" wing configuration would merely be used as two tip 108. In a preferred embodiment ofthe invention, the total separate wings originating from the "single" wing center- wing twist106between the centerline 102 to about halfway to line). the wing tip 108 is 30 percent ofthe total wing twist 106. This The present invention provides a novel aircraft design total wing twist 106 results in wings 104 having a span load employing wings that include a structural twist across each 65 that changes from downwash to upwash at a location from wing. The twist is strongly nonlinear. The wing twist about 60 to about 80 percent of the way from the centerline increases the directional stability and directional damping of 102 to a tip of the wings 108.

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The wings 104 can be designed with aspect ratios from as downwash, the local induced drag, and the local angle of low as about 2 to any higher aspect ratio that can be built. attack (the induced angle of attack).

From a practical perspective, the useful range ofaspectratios is from about 2 to about 60.

(6) Any non-zero design lift coefficient can be used for the 5 1 (Y)= -a 2n[a(Y) i(Y)1 present invention. The design lift coefficient should be in a range of from about 0.05 to about 2.0 and more preferably from about 0.2 to about 1.0 where a is the angle of attack and the a_i is the local angle While the wing 104 taper ratio can be any value for the of attack (or the local induced angle of attack).

invention to operate, the preferable range is from about 1.0 to 10 The symmetrical bell shaped lift distribution is defined by: 10.0(that is the tip chord ofthe wing 104 is equal to the root chord down to the tip chord of the wing 104 being 10 times smaller than the root chord).

( Y"2~ ( 7 ) In addition,in a preferable embodimentofthe invention the ~(s~2-Y~2) Y-s(Y)= TMR C 1-sA22 reduced induced drag can be matched by a corresponding 15 reduction in profile drag/skin friction. This is accomplished by reducing the wing 104 area by the same amount as the FIGS.3through6 show graphical depictions ofsome ofthe induced drag reduction. Present aircraft designs emphasize aspects of the present invention where the left side of the that the drag coefficients, profile (cdp) and induced (cdi), depictions coincide with the centerline ofthe aircraft and the need to be matched in order to achieve maximum lift to drag 20 right side of the depictions coincide with the wing tips.

ratios, but this solution can be forced by changing the corre- FIG.3 shows the span load of an aircraft incorporating the sponding wing 104 area to force matching of the drag coef- present invention. FIG. 4 shows the downwash related to an ficients. In a preferred embodiment, the wing 104 area is aircraft incorporating the present invention. The span is reduced from a standard wing 104 area by about 5 to about 15 depicted using 21 stations and one can see that the downwash percent. 25 becomes negative(upwash)at around station 14(at approxi- Also,it is preferable that a small amount ofdihedral is used mately 70% ofthe wing span). FIG.5 shows the induced drag for the wing 104 designs to achieve lateral-directional stabil- coefficient (Cdi) across the wing. This graph depicts that ity. Low lateral-directional stability can be used if control where the upwash exists (from FIG. 4), there is a negative augmentation is added. However,ifthe preference is to gain induced drag (negative induced drag- induced thrust) which the maximum aeronautical performance, minimum structural 30 is located near the wing tip. Finally, FIG. 6 depicts the wing weight, and maximum stability through static natural means, twist necessary to obtain the results depicted in FIGS. 3-5).

such artificial control means should only be used when nec- What is described herein are specific examples of many essary. possible variations on the same invention and are notintended Below is a mathematical description to clarify certain in a limiting sense. The claimed invention can be practiced aspects of the invention described above: 35 using other variations not specifically described above.

Given: What is claimed is: b-wing span (1) 1. An aircraft that provides adverse yaw control without a The coordinate system defines y-0 at the centerline and rudder, comprising: y=s at the right wing tip (it is the usual custom for aircraft 40 wings having a span load that changes from downwash to coordinate systems to define y as positive out the right wing upwash at a location from about60%to about80% ofthe tip) way from an aircraft centerline to a tip of the wings by providing a total wing twistpercentage ofabout 10times s=br2 (2) the aircraft's design lift coefficient to about 30 times the the local wing chord is 1(y)and AR is the aspectratio defined 45 aircraft's design lift coefficient wherein from about 10% as: to about 35% of the total wing twist occurs from the centerline to about halfway to the wing tip and the AR=b'2/S (3) remainder of the total wing twist occurs from about where S is the wing area(notto be confused with s, orlower halfway to the wing tip to the wing tip.

case "s").

50 2. The aircraft of claim 1, wherein the wings comprise an CL is the total wing lift coefficient, and a(y) is the local aspect ratio offrom about 2 to about 60.

angle of attack 3. The aircraft ofclaim 1, wherein the design lift coefficient Therefore: comprises from about 0.05 to about 2.0.

4.The aircraft ofclaim 3, wherein the design lift coefficient a(y) - 2CLI(jtARs`2)1[(s"2-2y"2)+(8 , 1(s"2-y"2))1(3ii1 55 comprises from about 0.2 to about 1.0.

(Y))1 ( 4 ) 5. The aircraft of claim 1, wherein the wings comprises a The above is given by the Prandtl Lifting Line Theory, and taper ratio offrom about 1.0 to about 10.0.

is a good approximation for wing sweeps not exceeding 20 6. The aircraft of claim 1, wherein the wings surface com- degrees (a simple approximation can be used to improve the prises an area that is reduced from a standard wing area for a correlation for sweep, which we have linearized).

60 specific aircraft, employing a concept of approximating an elliptical span load, by about 5% to about 15%.

7. The aircraft of claim 1, wherein the location where 1 +sdcI l dy' (5) a i(Y) (Y) wings' span load changes from downwash to upwash is about = dY 8n dy' —f Y -Y' s 70% from the centerline ofthe wing to the tip of the wing.

65 8. The aircraft of claim 1, wherein the total wing twist This is the induced angle of attack from the lifting line percentage comprises about 20 times the aircraft's design lift theory of Prandtl. It allows calculation of the local upwash/ coefficient.

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9.A method ofproviding adverse yaw control to an aircraft without a rudder, comprising the step of: providing wings having a span load that changes from downwash to upwash at a location from about 60% to 5 about 80% ofthe way from an aircraft centerline to a tip ofthe wings by providing a total wing twist percentage of the aircraft's wings that is about 10 to 30 times wherein from about 10% to about 35% ofthe total wing twist occurs from the centerline to about halfway to the io wing tip and the remainder ofthe total wing twist occurs from about halfway to the wing tip to the wing tip.

10. The method ofclaim 9, wherein the wings comprise an aspect ratio offrom about 2 to about 60.

11. The method of claim 9, wherein the design lift coeffi- 15 cient comprises from about 0.2 to about 1.0.

12. The method ofclaim 9, wherein the wings comprises a taper ratio offrom about 1.0 to about 10.0.

13. The method of claim 9, wherein the wings surface comprises an area that is reduced from a standard wing area 20 for a specific aircraft, employing a concept ofapproximating an elliptical span load, by from about 5% to about 15%.