Why Use Backward Blades in High-Speed Compressor Impellers?

[eaaaazizo]

I neeed help in terbomachinary !

I need answer to a specific quistion . Why we use backward blade in impeller design for high speed compressors ??

 

[ank_gl]

make a velocity triangle at the blade outlet, you ll understand why?

And make one for forward curved vane too.