DRAG REDUCTION METHODS
Four main types of drag are encountered in aerodynamics-Namely
- skin-friction drag,
- form drag,
- wave drag,
- induced drag.
The methods in use for reduction for the reduction of each type of drag are discussed in turn
REDUCTION OF SKIN- FRICTION DRAG
In broad terms, skin-friction drag can be reduced in one of the two ways. Either laminar flow is maintained by postponing transition, this is so called laminar- flow technology, or ways to found to reduce the surface shear stress generated by the turbulent boundary layer.The laminar-flow can be maintained passively by prolonging the favourable constant-pressure region over the wing surface. Active control of transition requires the use of suction, either distributed or through discrete spanwise slots. Often the suction is used in conjunction with the favourable pressure distributions. The basic principle of maintaining laminar flow by means of suction has been known for at least thirty-five to forty years. However, the problems with the practical implementation on aircraft, either real or perceptual, have prevented the widespread of the technology. It seems increasingly likely, however, that the considerable gains in efficiency which would result from the use of laminar -flow technology, will ensure that it will be much more widely exploited on commercial aircraft in the near future.Other methods for maintaining laminar-flow have been developed, but as yet, have not been seriously considered seriously for practical application in aviation.
A moderately effective method for reducing turbulent flow friction which has been developed in the recent years involves minute modification of the surfaces so that they are covered with riblets.Riblets can take many forms but essentially consist of streamwise ridges and valleys, as shown. These triangularly shaped riblets are available in the form of the film.
This corresponds to the actual spacing of 0.025 to 0.075 mm for flight conditions. This 3M riblet film has been tested on an in-service Airbus a 300-600 and in other aircrafts. The skin friction drag reduction observed as of the order of 5 to 8 percent and the skin-friction drag 30 to 40% of the total aircraft drag.Thus the overall drag reduction is modest but, nevertheless, represents a very considerable economic benefit with little in the way of a penalty.It is very likely, therefore, that riblets will be widely used on commercial aircraft In the future.The basic concept behind riblets had many origins, but it is probably the work at NASA Langley in the USA, which led to the present developments .he concept, was also discovered independently in Germany through a study of hydrodynamics of riblet –like formations on shark scales. A plausible explanation for the effect of riblets is that they interfere with the rear wall structure of the turbulent boundary layer in the region where the turbulence is mainly generated.The flow field in the turbulent boundary layer is highly complex, but the form of near wall structures has been elucidated in the seminal study of Kline et al at Stanford university.It appeared from this work that ‘hairpin’ vortices form near wall as shown these vortices then continue to grow until a point is reached when the head is violently ejected away from the wall and, simultaneously, the contra-rotating streamwise oriented branches of the vortices come together, inducing a powerful downwash of high-momentum fluid between the vortices towards the wall. This sequence of events constitutes what is termed a ‘near =wall burst’. The riblets act as barriers which prevent the free spanwise movement of the hairpin vortices.It is thought that owing to this the vortices cannot approach each other closely thereby weakening the near wall bursting process. It should be noted, however, that this explanation is not universally accepted
2. REDUCTION OF FORM DRAG
Form drag is kept to the minimum by preventing boundary-layer separation. Streamlining is vitally important for reducing form drag. It is worth noting that at high Reynolds’s numbers a circular cylinder has roughly the same overall drag as a streamlined aerofoil with a chord length equal to 100 cylinder radii. Form drag is overwhelmingly the main contribution to the overall drag for bluff bodies like the cylinder, whereas the predominant contribution in the case of the streamlined body is skin-friction drag, form drag being less than ten percent of the overall drag.For bluff bodies even minimal streamlining can be very effective.
3.REDUCTION OF INDUCED DRAG
Induced drag falls as the aspect ratio of the wing is increased.It was also shown that for a given aspect ratio elliptic-shaped wings have the lowest induced drag. Over the past fifteen years, the winglet has been developed as a device for reducing induced drag without increasing aspect ratio.Typical example is illustrated in the figure .winglets of this type have now been fitted too much type of commercial aircraft.The physical principle behind the winglet is illustrated in the figure. On all sub-sonic wings, there is a tendency for a secondary flow to develop from the high-pressure region below the wing around the wing tip to the relatively low-pressure region on the upper surface. This is the part of the process of forming the trailing vortices if a winglet of the appropriate design and orientation is fitted to the wing-tip, the secondary flow causes the winglet to be at an effective angle of incidence, giving rise to lift and drag components Lw and Dw relative to the winglet, as shown in the figure. Both Lw and Dw have components in the direction of the drag of the aircraft as a whole.Lw provides a component counter to the aircraft drag, while Dw provides a component which augments the aircraft drag.for a well-designed winglet, the contribution of Lw predominates, resulting in a net eduction in overall drag, or thrust, equal to delta T.
4.REDUCTION OF WAVE DRAG
To some extent in the discussion of a supercritical aerofoil, it was found that keeping the pressure uniform over the upper wing surface minimized the shock strength, thereby reducing the wave drag. A somewhat similar principle holds for the wing-body combination of transonic aircraft. This was encapsulated in the area rule formulated by Whitcomb. It was known that as a wing-body combination passed through the speed of sound of sound, the conventional straight fuselage, shown in the figure., experienced a sharp rise in wave drag. Whitcomb showed that this rise in drag could be considered reduced if the fuselage was Waisted as shown in the figure, in such a way as to keep the total cross-sectional area of the wing-body combination constant. Waisted fuselages of this type are now a common feature of aircraft designed for transonic operation.