negative blade Angle on the tip

[luckym]

Hello.

Is it ok if blade angle is negative?

for example:

n=600 min-1 (generator speed)

D=4 m

alfa=5

windspeed=10ms-1

...->  TSR=12.5

       blade_set_ang(tip) = atan(D/(3*r_tip*TSR)) - alfa  = - 1.96 deg

Should I make the blade with this angle?

Thank You for any opinion.

regards

michal

[Flux]

I doubt if you will have much luck with a tsr of 12.5 even if you can live with the noise.

Flux

[gibsonfvse]

Is alfa the angle of attack of the blade root, relative to a plane containing the drive shaft?  Or perhaps relative to the plane containing the rotor?  Or....?

[baggo]

Hi Luckym,

If you lower the TSR to a more realistic figure of 7 this will give a tip angle (Beta) of 0.4 and an rpm of 340 in a 10m/s wind (assuming 3 blades). This would give you a quieter, slower running genny that will be less liable to self destruct in a decent wind and less chance of annoying the neighbours!

John

[jacquesm]

a tsr is not just a number that you plug in without a fair understanding of what it does. On the upper end the TSR figure of a blade is limited by the drag of the blade. No matter how 'smooth' you make your blade the air will always be dragging it down in speed. As mentioned above a tsr of 7 (for a three blader, a little higher for a two blader) is attainable and will give you reasonable start up torque. Also, the rpm is not really a factor here, it's a result of the setup, not an input parameter. Also, faster blades are noisier than slower blades.

If you need lots of RPMS you probably want a smaller machine.

[luckym]

thank You All for explanation.

I'll decrease TSR -> (n)

[hvirtane]

I think that it is a good idea

to study TSR, pitch angles,

airfoils, etc of other people's

machines, which were actually built.

As you probably know it is impossible

to get arbitrary high TSR speeds,

whatever airfoils you are going to use.

There are practical limits you can achieve.

Good engineering on this field is based on experience.

- Hannu