Effect of Sweep - Lecture notes 3 PDF

Preview

Summary

This article presents a study of the effects of wing sweep on the aerodynamic performance of a blended wing body (BWB) aircraft that is based on an aerodynamically optimized design with a fixed planform and pitching moment constraint. Sixteen BWB geometries with varying wing sweep angles ranging fro...

Description

SWEPT WING A swept wing is a wing that angles either backward or, occasionally forward, from its root rather than in a straight sideways direction. DIFFERENT TYPES OF SWEEP 1.

Sweep back:

A wing platform favoured for transonic and supersonic flying to reduce the wave drag. Swept back wings have the following advantages;    2.

Extra lateral stability. Longitudinally stable on its own. Delay the compressibility effect when flying at transonic speed.

Sweep forward:   

Allowing more space in generally cramped corporate jet cabins Relatively expensive. Highly manoeuvrable at transonic speeds

3. Variable sweep: Also known as swing wing. That may be swept back and then returned to its original position during flight. Variable swept wings have the following advantages;      

Aero elastically enhanced manoeuvrability Smaller basic lift distribution Increased trailing edge sweep for given structural sweep - lower CD Easy gear placement Good for turboprop placement Subsonic stability and supersonic abilities

REASON FOR HAVING SWEPT WINGS: As the air passes over the wing accelerates to near sonic speeds, shock waves form and compressibility effects become apparent; the drag increases, buffeting is felt and changes in lift and Centre of Pressure occur. The speed at which these compressibility effects first become apparent is the Critical Mach number (MCRT). By sweeping a wing significantly the velocity vector normal to the leading edge is made less than the chord wise resultant. As the wing is only responsive to the velocity vector to the normal leading edge, for a given Mach number the effective chord wise velocity is reduced. This means the airspeed can be increased before the effective chord wise component becomes sonic and thus the critical Mach number is raised. From Raymer, Wing sweep is used primarily to reduce the adverse effects of transonic and supersonic flow. Theoretically, shock formation on a swept wing is determined not by the actual velocity of the air passing over the wing, but rather by the air velocity in a direction

perpendicular to the leading edge of the wing. At supersonic speeds the loss of lift associated with supersonic flow can be reduced by sweeping the wing leading edge aft of the Mach cone angle The exact wing sweep required to provide the desired Critical Mach Number depends upon the selected aerofoil(s), thickness ratio, taper ratio, and other factors. There are other reasons for sweeping a wing. For example, the fuselage layout may not otherwise allow locating the wing carry-through structure at the correct place for balancing the aircraft.

Effects of sweep The wing sweep affects the slope of the lift curve (CLa), the maximum lift coefficient CLmax, the induced drag coefficient (CDi), the drag divergence Mach number (MD), the wing weight and the tip stalling.

i.

Effect of sweep on slope of lift curve

Lift curve slope is reduced by the cosine of the quarter chord sweep angle. This means more angle of attack for takeoff, which requires a longer take-off run and a longer landing gear to avoid a tail strike on rotation.

ii.

Effect of sweep on maximum lift coefficient (Clmax)

It is shown in the equation that the CLmax of a swept wing decreases in proportion to sweep angle, CLmax kept constant

iii.

Effect of sweep on induced drag

The induced drag coefficient of typical jet airplane (subsonic speed) is given by:

Since Cl decreases as sweep angle increases so as CDI .

iv.

Effect of sweep on divergence Mach number (MD)

For a swept wing the change in drag divergence Mach number due to sweep angle ˄, is given approximately by the following equation

Where, (Md) and (Mdn) are the drag divergence Mach numbers of the unswept and the swept wing respectively; ˄ is quarter-chord sweep in degrees.

The critical Mach number, in connection with the aerofoil, is defined as the free-stream Mach number at which the maximum Mach number on the aerofoil is unit. This quantity can be obtained theoretically by calculating the pressure distribution on the airfoil, but cannot be determined experimentally. However, when the critical Mach number is exceeded, the drag coefficient starts to increase. Making use of this behaviour, the term ‘Drag divergence Mach number (MD)’ is defined as the Mach number at which the drag coefficient shows an increase of 0.002 over the subsonic drag value, (drag starts to increase rapidly).

v.

Effect of sweep on wing weight

Swept wings are complicated in structure compared to plain wings, the more the complexity; the more is the weight of the structure. These increase in weight come about as way to balance and reinforce the structure.

CONCLUSION From the above discussion it is noted that the sweep has the beneficial effect of increasing MCRT. However, it has the adverse effects which are:   

increasing CDI increasing weight Decreasing CLmax....