Blade twist

thread6-175834
In Ray W. Prouty', VOL 1 Aeordynamics book Pg 248, he talks about blade twist. He says that there is a nonlinear "ideal twist" which in theory will even up induced velocity distribution. This type of twist to a blade is to difficult to produce. So it is common to have a linear twist. You can get the most efficiency from a negative 20 degree twist. But the first 10 to 12 degrees is the greatest gain in efficiency. Blade twist can give you a five percent increase in figure of merit. This is an increase of about 20 percent payload capability. On the UH-60, we have a negative 18 degree twist. You can also gain another 2 to 4 percent inrease in figure of merit by adding anhedral tips.

 

[SAITAETGrad]

With composites, non-linear twist can be achieved a bit easier than twisting an aluminum extrusion (which would require clamping and re-clamping along the length to literally apply a series of twists on the section). But, since a linear twist can closely approximate the capability of an ideal rotor for say the outer two-thirds of the blade (where it really counts), one would question the value.

See Leishman Principles of... 1st Ed. Figure 3.8.

 

[Sparweb]

You might find it easy enough to do the "back-of-the-envelope" calculations yourself if you consider that you're looking for a relatively constant inflow velocity across the span of the blade, an ideal angle of attack of the airfoil for certain conditions, and a constant RPM. This does zero you in on a vector diagram that gets you in the ballpark. Make a new vector diagram for each station along the blade, say, divided in 10 pieces or so, and you get a progression of twist angles to ensure the angle of attack is constant.
Some heli manufacturers do pretty crude things to adjust the twist and/or airfoil profile of the blades from root to tip. Perfect is sometimes not as good as "good enough".

- from another SAIT AET grad. (are there too many or not enough of us in the world?)

Steven Fahey, CET