Generally, a fully subsonic fan would be more efficient. In the fan you can indeed match the local angle of incidence to the airspeed so the fan will work well over its design range. This is done by the pressure field in and around the intake which will spill excess air overboard at high speed or suck extra air in from the sides at low speed. It will have no p-factor, but can run at only one speed for a given flight speed.Ĭontrast this with a turbofan: The intake makes sure that the flow speed and direction at the face of the fan is the same no matter what the flight speed is. Also, the angle of attack must be compensated for by sviveling this propeller axis into the direction of flight. But to be able to fulfill this condition you need to match your propeller speed to the flight speed and twist distribution. Since the blade is uncambered, this means that the change in local flow direction at the leading edge can be minimized to the amount which is required to create the desired thrust. But there is more to it: encouraged me with his comment to dive a little deeper.Ī supersonic propeller will work well when the direction of flow at every station along the propeller blade is about equal to the local airfoil chord. The XF-84H's noise made people literally sick. Thus, a fan engine requires less torque per blade to reach supersonic tip speeds on the fan blades.Īlso, the shroud of a turbofan engine helps a lot to make the noise from supersonic tips manageable. On the other hand, the big diameter of a prop requires proportionally more torque to keep the prop rotating against the drag from the supersonic tips. There is nothing inherent in propellers which prevents their tips from moving faster than the speed of sound. Note that the propeller on the XF-84H Thunderscreech did move at supersonic speed. The very thin, uncambered airfoil of a supersonic propeller and the added wave drag lower the maximum efficiency, but hold efficiency up into supersonic air speeds. The plot for fan blades would look not much different. Propeller efficiency over speed (picture source). This makes the high thrust levels of modern turbofans possible. But in turbofans it is a price worth paying, because the faster tip velocity means higher dynamic pressure, and the pressure difference between both sides of the fan blade grows with the square of their velocity. It is a bad thing to have supersonic fan blade tips, just like supersonic propeller tips are best avoided. Propellers can turn at supersonic speeds, but since flow conditions are less controlled, the penalty for doing so is much higher than the penalty for a fan.Efficiency for propellers and fan blades is highest at subsonic flow conditions.Turbofans can tolerate supersonic speeds because the intake creates constant flow conditions irrespective of flight speed.Turbofans need supersonic speed at the fan blade tips to create their high thrust.If M < 0.2–0.3 and the flow is quasi-steady and isothermal, compressibility effects will be small and simplified incompressible flow equations can be used. As the Mach number is defined as the ratio of two speeds, it is a dimensionless number. The boundary can be the boundary of an object immersed in the medium, or of a channel such as a nozzle, diffuser or wind tunnel channelling the medium. The boundary can be travelling in the medium, or it can be stationary while the medium flows along it, or they can both be moving, with different velocities: what matters is their relative velocity with respect to each other. The Mach number is primarily used to determine the approximation with which a flow can be treated as an incompressible flow. The local speed of sound, and hence the Mach number, depends on the temperature of the surrounding gas. Pilots of high-altitude aerospace vehicles use flight Mach number to express a vehicle's true airspeed, but the flow field around a vehicle varies in three dimensions, with corresponding variations in local Mach number. At Mach 0.65, u is 65% of the speed of sound (subsonic), and, at Mach 1.35, u is 35% faster than the speed of sound (supersonic). Where: M is the local Mach number, u is the local flow velocity with respect to the boundaries (either internal, such as an object immersed in the flow, or external, like a channel), and c is the speed of sound in the medium, which in air varies with the square root of the thermodynamic temperature.īy definition, at Mach 1, the local flow velocity u is equal to the speed of sound. Ratio of speed of an object moving through fluid and local speed of soundĪn F/A-18 Hornet creating a vapor cone at transonic speed just before reaching the speed of sound
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