Blade twist of helicopter blade 1

[seepratik]

Hello everybody,

I am trying to model a helicopter blade with a twist angle if -12 degree. To understand the features of the twisted blade, I was searching on the internet and I have found that (in faa.gov)the blade twist angles are made in such that at the root the angle is greater and as one approaches to the tip, it becomes smaller. My question is to model a negative angle of twist (given in database as "linear blade twist: -12 degree"), does it mean that the blade angle is zero at the root and it gradually becomes larger and larger with leading edge down(nose downwards) and finally, at the tip, ended with a negative 12 degree? If it is, then its a violation of the convention as written in the document in faa.gov. Can anybody explain that I am right or wrong? What actually is a blade twist angle (positive or negative)? I appreciate your help.

Thanks

Pratik

 

[Sparweb]

Basically, the tip is "flat" to the plane of rotation, and the root is "pitched". It helps to think about the flow of air through the rotor (downward) and the blade meeting that flow at a reasonable angle of attack, while the tangential speed of each section of the blade is very different as you go from root to tip.

STF

 

[sechstemann]

Hello seepratik,

I haven't done simulation about helicopter blade before but have some experience in wind turbine blade design. In wind turbine design there exists also pitch along the blade length. The purpose is to get reasonable angle of attack for each section in the span direction. Suppose we know incoming flow (fixed speed value and direction) and the angular velocity of the blade is given. Then the tangential velocity of the blade increases linearly with increasing radius. Considering the blade as fixed, then the relative velocity of the flow to the blade is the vector sum of the flow velocity and the negative tangential velocity (because now the blade is viewed as fixed). The angle between this velocity and the negative tangential velocity decreases with increasing radius. For simplification consider a fixed angle of attack as the optimal one for each section along the span. Then the outer one would be more flat as the inner one is more twisted. Normally by defining the pitch angle of the root and the twist of each section could the local pitch (pitch angle of the root+the relative twist) of each section be obtained.

Yijun

 

[rb1957]

careful ... I think wind turbine blades are designed quite differently to helicopter blades.

Wind turbines maximise torque and minimise thrust, yes?

helicopters obviously maximise thrust.

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