Some conventional aircraft, such as fixed wing planes, have leading edge flaps or slats, known as Handley-Page slats, to improve the airflow around and across the fixed wings during take-offs and landings.  These flaps/slats add weight to the aircraft and increase the complexity of the wings.  Furthermore, the slats and must be carefully stowed in the wings for cruise flight to avoid drag while being capable of extension into the airflow during take-off and landing.  Some aircraft, such as the V-22 Osprey, have large propellers and avoid ground clearance issues by tilting the heavy engines and propellers.  No improvement to the airfoil shape across the wing, however, is obtained by this tilting design.