| MadSci Network: Engineering |
Michael, The design of a helicopter rotor blade has a number of issues to look at. Some of these factors are planned performance of the helicopter, engine power, and noise/vibration considerations. The first consideration is usually the rotor tip speed. Since the rotor is spinning, the speed at the tip will be very fast. For hover flight, the tip speed should be somewhere around .5-.7 Mach. The reason for this is that as the helicopter flies forward, the advancing tip speed will be faster than the retreating tip speed. This causes three primary effects that can limit the performance of the helicopter. First is advancing blade compressibility. The advancing blade tip speed will be the rotational speed PLUS the forward speed of the helicopter. If the helicopter flies fast enough, the tip speed of the advancing blade will approach the speed of sound. This comes with a signicant increase in drag that is tough for the engine to overcome. Second is retreating blade stall. The retreating blade tip speed will be the rotational speed MINUS the forward speed of the helicopter. The faster the helicopter flies the slower the airspeed past the retreating blade. If this airspeed becomes slow enough, the retreating blade will loose lift causing the helicopter to roll. Third is asymetric lift. The faster the helicopter flies, the greater the difference in the speeds of the advancing and retreating blades will be. The difference in lift from one side to the other will cause the helicopter to roll toward the retreating side, preventing proper directional control. For these reasons, helicopters are limited to flight speeds of below 230 mph, with most only achieving about 100-160 mph. After calculating tip speeds, the next concern is the blade area. The area required increases as design speed increases, because the retreating blade will operate at decreasing relative speed. Calculations using the tip speed and aircraft weight are used to determine the required blade area. Once blade area is determined, rotor radius can be calculated based on the engine performance. First, power loading is calculated as engine horsepower divided by aircraft weight. Then charts are used based on the power loading to determine allowable disc loading, which is the aircraft weight divided by the area of the disk that rotor path creates. Once this area is determined, the radius can be found. The final thing to look at is noise and vibration. Noise will increase with larger advancing tip Mach numbers. Also, vibration will increase with speed because of effects of asymetric lift on the rotor, similar to disturbing a gyroscope and watching it ocsilate as it tries to find a comfortable balance. Aside from being irritating, the vibrations will cause structural fatigue in the helicopter parts that could lead to failure. For practical applications, you could consider blade designs used on several production helicopters. A few good web sites to look at for more information on helicopter aerodynamics and other helicopter info are: http://www.a i.mit.edu/projects/cbcl/heli/helo_aero.html (Helicopter Aerodynamics - very good) http://www.treasure- troves.com/physics/Helicopter.html (another helicopter aerodynamics site - very technical) http://www.pra.org/ (Popular Rotorcraft Association - homebuilt helicopter society) http://www.ro binsfyi.com/aviation/rotor/helicopter.htm (large list of helicopter related links) htt p://www.geocities.com/CapeCanaveral/Hangar/1425/section1.html (very good helicopter information) Hope this is helpful, Todd
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