Outline: Wing Design Factors

Today, I continued working on my outline.

1. Wing Designs
1. Major decisions: Wing area/wing loading, Span, aspect ratio, Planform shape, Airfoils, Flaps and other high lift devices, Twist
2. Wing Area

                                                               i.      Restraints affecting wing area: Cruise Drag, Stall Speed, Take off and landing distance, Maneuver (Instantaneous or Sustained), Fuel Volume, Hangar size

                                                             ii.      Cruise Drag

1.      Low altitude cruise favors high wing loading and low wetted area

2.      Higher altitude cruise favors lower wing loading and greater span.

                                                            iii.      Takeoff and Landing

1.      Increasing wing loading increases takeoff and landing roll

2.      Roll is proportional to the square of the takeoff or landing speed

                                                           iv.      Maneuvering

1.      Favors low wing loading, particularly for instantaneous turn rate.

                                                             v.      Stall Speed

1.       Most light airplanes wings are sized by stall speed requirements

2.      Survivability

                                                           vi.      Normally: the smallest wing area allowed by the restraints

1.      But this is not always true; sometimes the wing area must be increased to obtain a reasonable lift coefficient at the selected cruise conditions, which are critical parts of wing design process

3. Wing Span considerations

                                                               i.      Selecting the wing span is one of the most basic decisions to made in the design of a wing

                                                             ii.      The span is sometimes constrained by contest rules, hangar size, or ground facilities

1.      But when it is not, use the largest span consistent with structural dynamic constraints (flutter). This would reduce the induced drag directly

2.      However, as the span is increased, the wing structural weight also increases and at some point, the weight increase offsets the induced drag savings.

3.      This point is rarely reached, though, for several reasons.

a.       The optimum is quite flat and one must stretch the span a great deal to reach the actual optimum.

b.      Concerns about wing bending as it affects stability and flutter mount as span is increased.

c.       The cost of the wing itself increases as the structural weight increases. This must be included so that we do not spend 10% more on the wing in order to save .001% in fuel consumption.

d.      The volume of the wing in which fuel can be stored is reduced.

e.       It is more difficult to locate the main landing gear at the root of the wing.

                                                            iii.      Climb

1.      Induced drag important at climb airspeeds

2.      Greater span good for rate of climb

                                                           iv.      Cruise

1.      High altitude: induced drag significant, greater span preferred

2.      Low Altitude: parasite drag dominates, span less important

3.      Selecting cruise conditions is an integral part of the wing design process. It should not be dictated a priori because the wing design parameters will be strongly affected by the selection, and an appropriate selection cannot be made without knowing some of these parameters. But the wing designer does not have complete freedom to choose these, either. Cruise altitude affects the fuselage structural design and the engine performance as well as the aircraft aerodynamics. The best C_L (lift coefficient) for the wing is not the best for the aircraft as a whole. An example of this is seen by considering a fixed C_L, fixed Mach design. If we fly higher, the wing area must be increased by the wing drag is nearly constant. The fuselage drag decreases, though; so we can minimize drag by flying very high with very large wings. This is not feasible because of considerations such as engine performance.

                                                             v.      Weight

1.      Increasing span and aspect ratio makes the wing heavier.

2.      Optimum is a compromise between wing weight and induced drag

                                                           vi.      Ground Handling

1.      Taxiways and runway lights

2.      Hangar size

                                                          vii.      Flaps and Slats: "During takeoff and landing the airplane's velocity is relatively low. To keep the lift high (to avoid objects on the ground!), airplane designers try to increase the wing area and change the airfoil shape by putting some moving parts on the wings' leading and trailing edges. The part on the leading edge is called a slat, while the part on the trailing edge is called a flap. The flaps and slats move along metal tracks built into the wings. Moving the flaps aft (toward the tail) and the slats forward increases the wing area. Pivoting the leading edge of the slat and the trailing edge of the flap downward increases the effective camber of the airfoil, which increases the lift. In addition, the large aft-projected area of the flap increases the drag of the aircraft. This helps the airplane slow down for landing."

                                                        viii.      Wing deflection

1.  Large wing structures as they are used in modern passenger and transport airplanes can not be considered as rigid anymore. Due to their size and their limited stiffness they have a considerable deformation when air load is applied. Therefore, during the design phase special emphasis has to be placed on the aeroelastic behavior of such structures. Besides the prediction of the deflection and the influence of this deflection on the aerodynamic behavior and efficiency there is a great necessity to be able to control the deformation of the structure. This can be realized by a special distribution and orientation of the structural stiffness, the so-called aeroelastic tailoring [1], [3]. With the introduction of fiber reinforced polymers as a light and stiff material in aeronautical applications the opportunities of aeroelastic tailoring have increased enormously. Now it is possible to align the direction of the structural stiffness, using a certain fiber orientation, without the disadvantage of increasing weight due to additional structural parts [7]. (http://www.dlr.de/fa/Portaldata/17/Resources/dokumente/publikationen/2003/01_anhalt.pdf)

2. affects stability and flutter mount as span is increased