Correct Answer : Subsonic flow
Correct Answer : Velocity potential equation
Correct Answer : Thin airfoil, small angle of attack
Explanation : The Prandtl – Glauert compressibility equation is more or less accurate for small perturbations which occur for thin airfoils kept at small angle of attack. One added condition is that this equation is inaccurate for transonic and hypersonic flows.
Correct Answer : Mach number at which sonic flow is achieved on the surface
Explanation : When freestream air flows over an airfoil at suppose Mach number = 0.3, there’s a point on the top surface of the airfoil where the pressure is minimum and the Mach number is maximum such as 0.43. When this freestream velocity is increased, there’s a certain Mach number for which sonic flow is achieved at the top surface. This is known as the critical Mach number.
Correct Answer : Supersonic region on top and bottom region of the airfoil
Explanation : When the Mach number is increased beyond the point of drag divergence, the drag decreases by a factor of 10 due to the formation of supersonic region in both top and bottom surfaces of the airfoil. This supersonic region is followed by a shock wave which leads to flow separation causing higher drag.
Correct Answer : Increase drag divergence Mach number
Explanation : The purpose of a supercritical airfoil to delay the drag divergence Mach number so that there is a delay in wave drag formation which begins at the transonic speed. It aims to increase the Mach number increment between the critical Mach number and drag divergence Mach number.
Correct Answer : Flatter top surface
Explanation : The supercritical airfoils have a flatter top surface compared to the NACA 64 series airfoil. This leads to lesser local Mach values at the top even when it is at supersonic speed. In turn, there is a weaker terminating shock leading to less drag.
Correct Answer : Counter highly negative camber at the front end
Explanation : Due to the presence of flatter top surface in supercritical airfoil, the 60% of the front end of airfoil is highly cambered resulting in less lift. To counter the reduced lift, positive camber is provided at the bottom aft section of the airfoil by giving it a cusp-like shape. This increases the lift.
Correct Answer : Better takeoff and landing performance
Explanation : Supercritical airfoils work efficiently even in the transonic regime. It’s unique geometry of large leading edge, flatter top surface and cusp shape at the bottom aft section all result in high lift. This leads to very good take-off and landing performance as during this flight regime, higher lift is required. For this reason, cargo flights make use of supercritical wings.
Correct Answer : Positive
Explaination : When there is freestream flow towards a biconvex supersonic thin airfoil, using linearlized theory, the sign conventions states positive value of Cp for the forward surface. The surface is inclined into the free stream velocity.
Correct Answer : 0.0541
Explanation :
Correct Answer : Shock waves
Explanation : Wave drag is a result of pressure distribution which is exerted over the airfoil. This is a consequence of formation of shock waves and expansion wave pattern due to the presence of supersonic flow.
Correct Answer : Decreases
Explanation : For linearized supersonic flow, the coefficient of pressure is inversely proportional to Thus, when the Mach number increases, the coefficient of pressure decreases. This in contrast to the trend followed by the subsonic flow where an increasing Mach number results in decreasing coefficient of pressure.
