They are all approximations. Fortunately blades are rather forgiving things. If you entered every possible variable, and obtained an exact answer, well, you can't, because no perfect model exists, but if you could, you would probably only obtain a slight improvement. Adding 10% more to the blades length would probably give more power.
Joke -
What's the difference between a good blade and a perfect blade? Answer = 5%
- the good blade might have to be about 5% longer to get the same power.
Don't worry, I won't quit my day job. Well not yet anyway.
That is because the 5% longer blade sweeps 10 and a quarter percent more area which makes up for quite a bit of difference in efficiency in energy conversion.
(I'm sure we could mess up a blade good enough to make the required difference in length bigger)
Rich