Pitch axis location of aerodynamic profiles of wind turbine blades

[Jess1980]

TL;DR Summary

How is the pitch axis location of an aerodynamic profile decided?

Summary:: How is the pitch axis location of an aerodynamic profile decided?

Hi,

I drew a wind turbine blade in a CAD software. I used information on the pitch axis location I had on the various aerodynamic profiles I used to draw the blade. I understand that it allowed me to align the various profiles on a single axis. What I don't understand is how this pitch axis location is generally decided, that is, is this number proper to a specific aerodynamic profile, or can it vary depending on the exact shape of the blade, even if the aerodynamic profile used is the same? If anyone has information on this, that would be great. I wonder in particular if this location is in any way related to either the aerodynamic centre or the centre of pressure of the airfoil, or if there is no particular link.
Thanks for anyone attempting to give me an answer, I haven't been able to find a clear answer to this by googling it.

 

[Klystron]

I worked at NASA Ames Research Center wind tunnels as a software engineer for several years beginning in 1984. One of the most interesting projects involved programming the integrated systems tests (IST) of the refurbished full-scale wind tunnels described in this PDF file.

Several of the fan blade parameters you mention such as pitch and camber vary depending on experiment conditions. Even the original wooden blades from 1940-1950 wind tunnel designs could vary geometry similar to 'feathering' an aircraft propeller and varying helicopter rotors depending on flight regime.

Can you provide more information on the type, scale and application of the wind tunnel you are designing?

Online searches located numerous papers describing miniature wind tunnels powered by computer cooling fans intended to test tiny models up to giant wind tunnels that can test entire aircraft and/or locomotives. Other designs include tunnels for testing automotive aerodynamics, sports equipment, even clothing.

Also consider the related field of air conditioning designs for large buildings. I have toured HVAC (heating, venting, air conditioning) facilities at new hotels that strongly resemble wind tunnel installations including baffles and other geometry meant to smooth airflow and regulate plenum pressures.

 

[Baluncore]

The pitch axis of a blade can have several interpretations.
One interpretation is the point on the airfoil sections that is the centre of lift should lie on the pitch axis. There should then be no torque applied along the blade, which should help reduce angle-of-attack flutter and reduce feathering forces. But the exact position of that point on the profile will depend on angle-of-attack and airspeed, etc.

Note that for an asymmetric profile the twist is reversed between a propeller and a turbine.

 

[Jess1980]

The pitch axis of a blade can have several interpretations.
One interpretation is the point on the airfoil sections that is the centre of lift should lie on the pitch axis. There should then be no torque applied along the blade, which should help reduce angle-of-attack flutter and reduce feathering forces. But the exact position of that point on the profile will depend on angle-of-attack and airspeed, etc.

Note that for an asymmetric profile the twist is reversed between a propeller and a turbine.

Hi, thanks for answering. It seems coherent indeed that one would place the pitch axis in a way that no or very little moment is created on the blade by aligning it with the centre of lift (I suppose this is the same as the centre of pressure?). I suppose therefore that when designing a wind turbine blade, one would chose a pitch axis location adapated to the most frequent use of it.

 

[Jess1980]

I worked at NASA Ames Research Center wind tunnels as a software engineer for several years beginning in 1984. One of the most interesting projects involved programming the integrated systems tests (IST) of the refurbished full-scale wind tunnels described in this PDF file.

Several of the fan blade parameters you mention such as pitch and camber vary depending on experiment conditions. Even the original wooden blades from 1940-1950 wind tunnel designs could vary geometry similar to 'feathering' an aircraft propeller and varying helicopter rotors depending on flight regime.

Can you provide more information on the type, scale and application of the wind tunnel you are designing?

Online searches located numerous papers describing miniature wind tunnels powered by computer cooling fans intended to test tiny models up to giant wind tunnels that can test entire aircraft and/or locomotives. Other designs include tunnels for testing automotive aerodynamics, sports equipment, even clothing.

Also consider the related field of air conditioning designs for large buildings. I have toured HVAC (heating, venting, air conditioning) facilities at new hotels that strongly resemble wind tunnel installations including baffles and other geometry meant to smooth airflow and regulate plenum pressures.

Thanks for answering. I did use a wind blowing system on the model wind turbine I was testing. It was an open jet blower type, so not a closed circuit. The fans however were provided by a company, I didn't have to design them (Sorry, it was not clear from your reply whether you thought that I needed to design a wind tunnel or whether you were giving me the information you had on fan blades, which I could apply by analogy to a wind turbine blade).

 

[Baluncore]

I suppose therefore that when designing a wind turbine blade, one would chose a pitch axis location adapated to the most frequent use of it.

That seems logical in the first analysis, but the priority must be avoiding positive feedback flutter at all points in the operational envelope. At the same time, minimising the worst case feather control force would reduce the peak power needed by a control system.

Many factors becomes blurred as development decisions reduce the freedom to change. Reduce the weight of the blade, verify stability and survival, examine failures, then strengthen or protect those parts. Repeat the process.

 

[cjl]

To elaborate on the answers above, at a very broad level, it's a balance between minimizing aerodynamic torques, minimizing gravitationally driven torques, and maintaining the blade's overall shape to give the behavior desired. Blades are not necessarily straight - they frequently have a small amount of sweep or curvature, and their deformed shape under load is quite different than the static shape. They also have large amounts of prebend and they deflect a large amount in the flapwise direction, so there's quite a variance in cg location and aerodynamic moments depending on the operational regime. As Baluncore said, a lot of thought is definitely put into optimizing for the mean operational point, but you also have to look at the entire envelope and make sure you aren't putting the turbine into a bad condition where it is unable to pitch if it is hit by an unusual gust, for example.

In addition, the pitch balance is usually done such that an operating wind turbine has a slight bias towards feather (and the pitch system has to fight this to keep the turbine pitched in). This is for obvious safety reasons, since this way aerodynamic and gravity loading helps the turbine to shut down if needed.

It's also worth noting that even if you ignore the non-straightness of the blade, and the deformation, the camber, thickness, and airfoil properties vary pretty dramatically from the root to the tip. Is this just a fun exercise in CAD modeling, or is there something specific you're trying to achieve here?