Sweepback – is when wings are not at 90 degrees at the wing leading edge from wing root to the wing tip and is inclined backward. It increases lateral stability as well as longitudinal stability.
It causes Critical Mach No. to increase, since the relative airflow on hitting the leading edge of the wing gets divided into two components. One component flows over the chord line, while the other component causes airflow to flow chord wise because of the Sweepback shape and flows along the Chord.
The increase in the Critical Mach No. can be done with a simple formula as follows-
Mach No. = Critical Mach No. ÷ Cos Z
Where Z is Angle of Sweep back
Example- If Critical Mach No. of an Aerofoil is 0.80 Sweepback Angle is 45°
0.8 ÷ COS 45 = 1.13
The speed of 1.13 can be reached before the A/c experiences adverse flight conditions due to on setting of shock waves.
Since the sweep back wing A/c provides less lift due to less span area, it also requires high speed operation for approach and landing because of reduced lift.
But since it gives less lift, it is also less sensitive to the changes in pitch moment that are caused by gust, or turbulence etc, therefore it can also be a benefit because it gives smoother ride in adverse weather conditions.
It increases Weather Cocking stability.
Weather cocking stability – is when in a sideslip the A/c yaws, the wing into the wind has less swept wing exposed and causes high Drag bringing the A/c back to the level flight. The wing that is away from the wind has more sweep exposed and causes less drag, this combined effect causes opposite yaw causing A/c to return to the level flight.
It also increases Lateral and Directional stability.
Sweepback increases Lateral stability same as Dihedral, When a Sweepback wing is in a sideslip the wing into the wind causes an increase in the Lift since the less swept wing is exposed and the wing away from the wind causes less lift since the high sweep is exposed, this causes the A/c to return to the level flight.
Sweepback wing does not show a clean stall break.
Sweepback when combined with taper wings causes wing tip to stall before wing root, giving little or no pre stall indications.
Sweep forward – is when wings are not at 90 degrees at the wing leading edge from wing root to wing tip and is inclined forward.