Yes it is important to check the deflection at tip because  this deflection reduces the clearance between the blade and tower ...so improper deflection will lead to collision with the tower. 

For clearance between tower and blade or what you called " a maximum ratio for tip deflection", read IEC-61400 standards especially part 1. 

In general, stiff blades are desirable, particularly to reduce vibrations and mechanical loads on other components of the turbine. It's more a question of weight/costs why these are not used (at least for larger turbines). The effect on the power output is difficult to quantify, but if you had stiff blades, there is no need to incline the rotor plane (to prevent the rotor tips from colliding with the tower) and this might give some advantage aerodynamically.

Bendable blade can be exploited to increase efficiency of the wind turbine. However, you need to specify working conditions in great detail to exploit changes in aerodynamics arising from bending of the blades. Anyway when wind is blowing too fast rotor will stop in home position to prevent damages. 

The force exerted by wind on turbine blade depends on blade configuration, which in turn affects the aerodynamics of the blade. Higher deflection of blade will lead to bending stresses and cause severe vibrations (as rightly mentioned by Dr. Jochen, D)  and it reduces clearance between the shaft and blade (as rightly pointed by Dr. Ahmed H.). However, with bendable blades (to some extent) advantage can be taken from aerodynamics as air gets slides over the blade leading to increase area of contact and hence more force can be exerted on blades by the wind (as suggested by Dr David B)

Thanks for your sharing your kind expertise,

so for safety measures and to increase the clearance between the blade tip and the tower, the rotor should be either coned or tilled for some degrees,

will these solutions effect the rotor performance ? especially the tilted rotor.

Dear Dr. Haseeb Shah,

There are number of books available on design of wind tunnel . List is endless. I am giving the particulars of one book for your immediate reference

Design and Evaluation of a Low-Speed Wind-Tunnel with Expanding Corners

by

Bj¨orn Lindgren & Arne V. Johansson

Department of Mechanics

October 2002

Technical Reports from

Royal Institute of Technology

Department of Mechanics

SE-100 44 Stockholm, Sweden

Dear Dr.Maddali  VS Murali Krishna, 

Thanks for the reference, Can you kind share the  details of  the references- Dr.David B, Dr. Jochen, D, Dr. Ahmed H - mentioned in your first answer.  

Thanks

Tilting the rotor maybe a solution to ensure that no collisions will happened due to deflection of the blade. 

However, I think this is increasing the costs of design and manufacturing (not sure). 

Another point, you should make CFD to determine the aerodynamics loads on the blade when rotor is tilted and un tilted then calculate the deflection of the blade due to these loads. 

If you find the deflection is so dangerous then you should think of modifying the tip by inserting some webs beside the main spar of your blade. 

Another option, Try to design different types of the spar I mean:

1- use box girder.

2-one shear web.

3-two shear webs

4-two shear webs with additionally web at the trailing edge.

Try and check the deflections. 

Dear proffesor

In fact you must do a conceptual design.

in a similar case for a reactor I did an investigation and answer shows me that we cant export a comprehensive conclusion.

you may need to use CFD and then compare.

One of the areas of research in Wind turbine blade design is aeroelastic tailoring wherein the stiffness of the blade is designed such that after a certain aerodynamic loading is exceeded, the blades deform, resulting in attenuating loads.

Flexible blades are also suppose to flex and reduce the bending moments at root in high wind speeds. Ofcourse, the blade tower clearance is kept sufficient to accommodate this flexing 

Agree with Prof. Maddali V S Murali Krishna