NEW GENERATION SWEPT FORWARD WING
The biggest disadvantage of a conventional sweep on the wing is its tendency to tip, stall happens due to span-wise flow. Engineers have come up with a lot of solution to avoid the vortex generation. Wing washouts are all designed to avoid the tip from stalling first. If the tip is stalled, there will occur a separation of flow above the surface of airfoil. This may cause the effectiveness of ailerons. On the swept back wing, the value of induced drag is relatively high due to the span-wise flow towards the wing tip. This result may cause in lift performance at the specified thrust value.
Forward-swept wings have span-wise flow towards the fuselage (root). So the root naturally stalls first; this keeps the separated air well away from the ailerons. The shock wave will be formed at the root of the wing so; there is no conflict on stability or controllability in the ailerons. It is exceptionally good for the high speed supersonic aircraft. Due to the span-wise flow of air towards the root, there is no formation of tip vortices. This will decrease the induced drag. It also decreases the parasite drag (i.e.; pressure drag & skin friction drag) as the wing surface area is decreased.
For a swept back wing, the location of the effective lift force causes the wing to twist near the tip of the wing, so as to decrease the angle of attack of the outer portion of the wing. This tends to unload the wing when lift is increased at a stable situation. In contrast, for a swept forward wing, the location of the effective lift force causes the wing to twist near the tips so as to increase the angle of attack of the outer portion of the wing. This causes lift to increase. If wing twists is further increased; this is an unstable situation that tends to twist the swept forward wing right off the airplane. The lift slope is smaller for the swept back wing than the swept forward wing.
Effect of wing sweep on the lift slope at supersonic speed is shown below
- The wing spar carrying through can be placed after of the cabin, so cabin height can be increased. This is important for business jet sized aircraft.
- Boundary layer at the tips is not affected by the inner wing; controllability can be maintained up to stall.
- Aero-elastic effect will increase control commands. This results very responsive air frame.
- Stall will occur inboard first, which will result in pitch up. If the tail cannot compensate, the stall is unrecoverable.
- The aero elastic effects will encourage flutter. If flutter speed is found to be made stiffer it will result increase in weight.
Article: Dinesh Rawal (4th year)
Department of Mechanical Engineering,
IOE, Pulchowk campus, lalitpur