Aircraft Conceptual Design Lecture 3: PDF

Aircraft Conceptual Design Lecture 3: Airfoil and Wing Geometry Airfoil section Airfoil is the heart of the airplane. It affects cruise, takeoff, landing, maneuvering etc. 2 Lift, drag and pitching moment By definition, Lift force is always perpendicular to flight direction. Drag force is alw...

Aircraft Conceptual Design Lecture 3: Airfoil and Wing Geometry Airfoil section Airfoil is the heart of the airplane. It affects cruise, takeoff, landing, maneuvering etc. 2 Lift, drag and pitching moment By definition, Lift force is always perpendicular to flight direction. Drag force is always parallel to flight direction Nondimensional Section Lift (2D airfoil) = qcC  1 V 2 cC l l 2 1 Nondimensional Section Drag (2D airfoil) = qcCd  V 2 cCd 2 Nondimensional Section Moment (2D airfoil) = 1 qc Cm  V 2 c 2Cm 2 2 For a 3D wing, the lift, drag and moment coefficients are CL, CD, and CM Pitching moment is usually negative (nose down) 3 Lift, drag and pitching moment  Angle of attack C l Lift curve slope = 2 for theoretical thin airfoil 4 Laminar bucket: There is a range of Cl for which the flow remains laminar over most of the airfoil Airfoil families NACA four digit airfoils (0012, 2412 etc.) indicate the percent camber (first digit), the location of the max. camber (second digit) and the max. thickness in percent (two digits). They are used mostly in tail sections. There are 5 and 6-digit NACA airfoils as well. In the past, designers would select an airfoil from catalogs, i.e. the number of airfoils available were limited . In today’s design environment, it is possible to design and build wings with custom-designed airfoils. However, existing airfoils are assumed at the conceptual design stage. Airfoils are designed towards achieving greater Lift, without flow separation at the trailing edge. Having laminar flow throughout the airfoil is also desired. 5 Design lift coefficient The value of Cl at which the airfoil has the best L/D. For most aircraft, it is around 0.5. In practice, the airfoil is initially selected based on experience or a similar successful design. 6 Design lift coefficient Selecting the airfoil at conceptual design: 1. For your aircraft, assume wing lift coefficient equals airfoil lift coefficient: CL=Cl For level flight, lift equals weight; W  L  qSC L  qSCl 2. 1W  Cl  q S Look up the catalogs of Cl – Cd curves for airfoils that have the Cl found above as their design Cl (highest L/D ) 7 Stall There are three types of stall: Fat airfoils (t/c > 14% with round leading edge) stall from trailing edge Thinner airfoils (t/c about 6-14%) stall from the leading edge. Flow separates at the leading edge at small angle of attacks, but reattaches itself immediately; until high angle of attacks, at which lift drops abruptly In very thin airfoils, mechanism is similar, but the stall occurs gradually 8 Preventing and delaying stall For wing sections, have different angle of attacks (AOA) throughout the wing to avoid instant stall. Twist the wing such that tip sections have lower AOA. Likewise, incorporate different airfoils on the wing sections, having tip airfoil that stall at higher AOA Stall is a bigger issue for unswept, high aspect ratio wings. Low AR or swept wings have 3D effects affecting stall characteristics. 9 Airfoil thickness ratio (t/c) Affects drag, lift, stall and weight. Drag increases with increasing thickness due to increased separation Structural weight increases with decreasing thickness . Statistically, half t/c will increase wing weight 2 (1.41) times as much. The wing is about 15% of empty weight, so empty weight will increase 15% x 41% = 6.15% 10 Airfoil thickness ratio (t/c) Lift initially increases with thickness for high aspect ratio, moderate sweep angle wings (See figure). • This behavior depends on the nose shape. For low aspect ratio, highly swept wings, a sharper leading edge provides greater lift due to formation of vortices behind the leading edge, delaying wing stall 11 Airfoil thickness ratio (t/c) Usually, Thickness is selected based on the trends in the figure. Also, thickness may be varied from root to tip with as much as 20-60% thickness difference between the two. 12 Wing Geometry The basic wing to start with is a trapezoidal wing that extends through the fuselage The root airfoil is the one at the centerline of aircraft The shape of the wing depends on aspect ratio, taper ratio and sweep. Reference wing area depends on takeoff GW 13 Aspect ratio High aspect ratio wings generally have greater wing span. Therefore the amount of the wing affected by tip vortices is lower. Hence High AR wings are more efficient For constant Swet/Sref, max. subsonic L/D increases with AR, but wing weight increases as well. Due to lower lift at the tips, a low AR wing stalls at a higher AOA (see figure) This fact resulted in low AR tail designs to have control even when wings stall Aspect ratio Use the table for initial AR calculations 15 Wing sweep Against adverse effects of transonic and supersonic flow. Shock formation on a swept wing is determined by the velocity of the air perpendicular to the leading edge. Shock waves form only when that component of the velocity becomes supersonic. Sweep forward and backward has the same effect on shock formation. However, forward swept wings have structural divergence issues. 16 Variable sweep Swept wing is desirable for high speed, but an unswept wing is desirable for cruise and take off / landing. Variable sweep provides both. Disadvantages include design difficulty due to shifting aerodynamic center of the wing and CG of aircraft, complex pivot mechanism and its added weight 17 Taper ratio tip chord  centerline root chord Affects distribution of lift along the wing span. Minimum drag due to lift occurs when the lift is distributed in an elliptical fashion. For an untwisted unswept wing, wing planform should be like an ellipse, as shown below. A rectangular wing (λ=1) would have 7% more drag for the same lift. It is difficult to produce, so may be approximated as tapered rectangular wing with λ=0.45, resulting in only 1% more drag. The usual value of λ=0.40 provides more weight savings for unswept wings. Swept wings have λ=0.2-0.3 , because the sweep diverts the air outboard, causing more lift at the tip. Taper must be increased to get elliptical lift distribution. 18 Twist Twist is another way to achieve elliptic lift distribution. Also prevents tip stall. • Twist angles are usually 0-5 degrees. 3 degrees of twist is typical. Geometric twist is the change of airfoil angle of incidence. Measured w.r.t. root section. • If the tip section is at a negative angle (nose-down), the wing has washout. • A wing with washout stalls starting at the root, which improves control during stall. Aerodynamic twist is the angle between zero-lift angle of tip airfoil and that of root airfoil. • If airfoil is the same along the span, then aerodynamic twist is same as geometric twist. Generally , • Total aerodynamic twist = geometric twist + (root airfoil zero-lift angle – tip airfoil zero-lift angle) Since lift of an airfoil depends on Cl, hence, AOA, optimizing lift distribution by changing twist only works at one Cl, and has negative effects at others. Therefore, avoid twisting too much. 19 Wing incidence Pitch angle of the wing w.r.t. the fuselage. If the wing is unwisted, angle of incidence is as shown below. For twisted wings, it is usually defined w.r.t. the chordline at the intersection of wing and fuselage. It is chosen to minimize drag during operation, mostly cruise. It is usually 2 degrees nose-up for general aviation aircraft, 1 deg. For transport and 0 deg for military. 20 Dihedral Angle of the wing w.r.t. the horizontal, viewed from front. Positive direction is wing tips upwards. Positive dihedral provides roll stability during turns: • If a disturbance causes an aircraft to roll away from its normal position, the aircraft will sideslip in the direction of the down-going wing. • This creates an airflow component along the length of the wing from tip to root. • The dihedral angle can be seen as presenting a positive AOA to this lateral flow, hence generating some additional lift. • It is this lift which restores the aircraft to its normal attitude. Dihedral angle is initially chosen based on historical data, shown below. 21 Dihedral Wing sweep also produces rolling moment due to side slip, due to change in air velocity, therefore relative sweep of the left and right wings. Every 10 deg of aft sweep creates an additional effective dihedral of 1 deg. Forward sweep causes negative dihedral (a.k.a. anhedral). Anhedral causes roll instability - used in fighter aircraft Wing placement on fuselage is also important. During sideslip, the air is pushed over and under the fuselage. If the wing is high-mounted, the air going over pushes up the wing, providing increased dihedral. Too much dihedral causes Dutch roll, a repeated side-to-side motion involving yaw and roll : http://www.youtube.com/watch?v= oLe8ajpGNTs&feature=related 22 Wing vertical location It is decided based on operating environment and practicality. Cargo aircraft have high wing, in order to have a lower fuselage for ease of loading heavy items. • A high wing also enables propellers with big radii and large wing flaps. In addition, wing tips are less likely to hit the ground in a rolled attitude, and on rough airfields, engines and propellers would be away from rocks and debris. Landing gear weight is lower as well. Disadvantages of high wings include • increased fuselage weight due to wing support structures, • additional weight and drag because of landing gear housing, • more drag if fairings are used at wing-fuselage junction, and • flat fuselage base (heavier than circular) 23 Wing vertical location The mid-wing has the lowest drag for circular fuselage and no fairings. • It provides space under the wing for bombs and missiles in military aircraft. • A high wing would obstruct the pilot’s view, making him more vulnerable. • Also, a high or low wing would have dihedral/anhedral effects in combat maneuvers. A setback of the midwing configuration is the problem of carrying structural wing loads through the fuselage. • It is impractical to have structural members going through the cabin of a cargo/passenger aircraft. • It is also difficult to incorporate in a fighter, in which most of the fuselage houses jet engines and inlet ducts. 24 Wing vertical location The low-wing provides space and structural support for the landing gear within the wing box. • However, landing gear gets heavier because it has to provide more ground clearance to the wings. • Large passenger aircraft have wing structures below the passenger compartment, leaving uninterrupted space for commercial use. Low wing aircraft has to have dihedral to avid touching of the wing tips during bad landing. Precautions must be taken to avoid Dutch roll. • Propellers must be placed higher on the wing, but that increases interference between the wing and the propeller, resulting in more fuel consumption. 25 Wing tips Wing tips mainly affect the tip vortices. A rounded tip permits air flow from the bottom to the top easily, which is not desired since it causes more induced drag. A tip with sharp edges performs better. A common design is Hoerner wingtip. Sweep of the tip also matters. The tip vortex is located near the trailing edge, so an aft swept tip would also work, with lower drag. However, that increases wing torsional loads. 26 Wing tips A cut off, forward swept tip is used in supersonic aircraft. • The wing is cut off at the supersonic Mach cone angle to avoid wave drag. Another way of preventing air flow towards upper surface is simply placing a plate at the tip. • The plate increases wetted area, hence drag, although it provides no additional lift. • It would be more beneficial if the plate area is added to elongate the wing. An advanced form of end-plate is a winglet. • It uses the energy in the tip vortex to increase the L/D. • However, it only works optimally at one velocity. • Also, it adds weight behind the elastic axis of the wing, giving way to flutter. 27 Tail functions Tails are necessary for trim, stability and control. Trim - An aft horizontal tail exerts negative lift to compensate for wing pitching moment. Vertical tails are for yaw control, and not used for trim during normal flight. On the other hand, propeller aircraft during maneuvers need to be trimmed in yaw. Also, multi-engine aircraft will need substantial yaw correction in case of inoperative engine. Tails stabilize the aircraft in case of an upset in pitch or yaw. Tail size is based on adequate control power needed to maneuver the plane safely 28 Tail arrangement Conventional design is the lowest-weight, most common design. T-tail is heavier because of structural components to carry the horizontal tail on top, but it allows for smaller horizontal and vertical tails. Cruciform tail is lighter than T-tail, but it won’t allow for a smaller vertical tail. H-tail is used to position vertical tails in undisturbed air at high AOA. It allows for smaller horizontal tail. H-tails and triple tails also provide lower height to allow fitting aircraft in confined spaces 29 Tail arrangement The V-tail • • • is combination of horizontal and vertical tails, with less area The components of V-tail forces can be calculated separately. It has reduced interference drag, but complex control mechanism. In addition, it has adverse roll-yaw coupling: when yaw motion is generated, a roll motion in the opposite sense occurs. Inverted V-tail has beneficial roll-yaw coupling. But it may cause difficulties in providing ground clearance. Y-tail is a V-tail with reduced tail dihedral, and added rudder. Has less complexity. A twin tail may be used to keep rudders away from aircraft centerline, because it may be ineffective if blanketed by the wing or fuselage at high AOA. It helps reducing height as well. Boom mounted tails allow utilization of pusher propellers or heavy jet engines located near CG. They are heavier than conventional fuselages Ring tail is a round airfoil section, acting as both horizontal and vertical tail. It has been shown to be inadequate. 30 V-Tail examples Global Hawk Eclipse 400 31 Cirrus Vision SF50 Beechcraft Bonanza Tail positioning Location of aft tail is crucial to recover an aircraft from stall. Figure below shows boundaries for tail locations. Low tails are best for stall recovery. Hence, T-tail aircraft must be designed to recover from stall even when the tail is blanketed by the wing wake. 32 Tail geometry Tail surface areas are determined by wing surface areas, which in turn depend on TOGW. Tail aspect ratio and taper ratio can be assumed as given below. In order for the tail to stall later than the wings, horizontal tail sweep angle should be selected 5 deg more than wing sweep angle. • Also provide a straight hinge line for the elevator for lowspeed aircraft to avoid vibrations Vertical tail sweep can be less than 20 deg for low –speed, and about 35-55 deg for high-speed aircraft. Tail thickness ratio is determined using historical guidelines for that of wings. 33