What is the recommended angle for twisting a blade?

[satchurated]

I'm looking through some books a guideline or a rule of thumb for twisting a blade. I've read that is related to angle of attack and speed, but i cannot find something clear. For what I've understood, there is not a rule but every manufacturer decide which angle choose ( internal know how). Do you have some reference or rule of thumb?

 

[Baluncore]

Would I be correct in guessing that you are asking about a propeller or airscrew?

Every propeller is a compromise.

Is it a fixed or variable pitch propeller?

 

[satchurated]

I'm referring to a fixed pitch propeller. I'm asking for knowledge not for a specifical need

 

[Baluncore]

First consider a blade section with symmetrical profile with zero lift. It is possible to compute the angular twist for any radius given the airspeed, propeller RPM and radius of the point from the axis. That angle is the arctangent of the velocity ratio.

Now consider the propeller section is under centrifugal tension due to rotation and must support the mass of the blade further from the axis. That requires the section of the propeller blade should reduce along the blade. The cantilevered section will also be required to resist the lift and drag forces along the blade.

Next, at each radius, the vector sum of rotation and airspeed gives an airspeed for the optimum airfoil section of the blade. You must change the airfoil profile along the blade from thick at the root to where it is very thin near the tip.

Knowing the cross section and profile at each radius, you can then change the twist slightly to set the airfoil angle of attack needed to give the maximum lift to drag ratio of the profile.

 

[satchurated]

Ok thank you by now...i will try and then let you know