Quote:
Originally Posted by PA32R
At 28.8 r.p.m. you can have a blade diameter of almost 110 meters and have a tip speed of less then mach 0.5. That's a pretty big fan. I suspect stresses might be a problem on a thin airfoil 54 meters long. The A380 wingspan is
not quite 80 meters, so the wing itself might be about 37 meters. I'm not sure about the stress calculations for a wind turbine blade, but I'm guessing that that's the limiting factor. From what I can see googling around, it looks like 100 meter turbines with 49 meter blades are the upper end at this time. Tip speed would be well below mach 0.5 at 28.8 r.p.m.
Here is a picture of a huge turbine with rotor diameter of 126 meters and blade length of 61.5 meters. At 28.8 r.p.m, the tip speed would be about 190 meters/second or a little under mach 0.6. Transonic speed is considered to begin at mach 0.8. So, there is some room to grow. |
But compressibility effects in fluid dynamics begin to become significant at or below M0.3 IIRC from my own courses in turbomachinery design and fluid dynamics.
Your aircraft wingspan analogy ignores that in an airplane the lifting force is roughly perpendicular to the direction of motion whereas in a turbine the lift force is tangential to the direction of motion. This has significant influence on the available cross-sections to resist the bending moment. A turbine blade can have a much larger supporting base in the direction of loading to improve its section modulus without negatively impacting its drag ratio. This is simply not as practical in aircraft.