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rotor blade axis
By methanolcat, Section Wind Posted on Thu Jul 26th, 2007 at 02:07:13 AM MST
would there be any advantage to tilting the rotor axis upwards?

Is the rotor blade axis in this picture just tilted to give clearance for the blades and tower or is there something more?
For the most part does the wind blow more horizontal or does it blow more of a downward angle, does anyone know, has anyone tested this and would it make any kind of a significant difference?
Matt

rotor blade axis | 14 comments (14 topical, 0 editorial)

Re: rotor blade axis (3.00 / 0) (#1)
by Nando (nando37-at-tx-dot-rr-dot-com Correct theanti-spam) on Wed Jul 25th, 2007 at 08:18:48 PM MST
(User Info)

Yes common trick

Nando

Re: rotor blade axis (3.00 / 0) (#2)
by disaray1 on Thu Jul 26th, 2007 at 05:42:13 AM MST
(User Info)

Blade clearance.

David

Re: rotor blade axis (3.00 / 0) (#3)
by MaryAlana on Thu Jul 26th, 2007 at 10:06:08 AM MST
(User Info)

Many of the commercial designs have a tilt back plus they use a pivot that lets the generator head move up and down in response to different forces on the blades. Not sure how they dampen it but for bigger machines it could help with stresses on the machine.

Re: rotor blade axis (3.00 / 0) (#4)
by SparWeb (sparweb at ANTISPAM_hotmail_com) on Thu Jul 26th, 2007 at 01:29:22 PM MST
(User Info)

Mine is tilted up 5 degrees.
The cosine of 5 degrees is 0.996
The tilt costs me 1.5% of the available power.

Steven Fahey

Re: rotor blade axis (3.00 / 0) (#5)
by methanolcat (methanolcat (at)yahoo(dot)com) on Thu Jul 26th, 2007 at 03:36:52 PM MST
(User Info)

in what way does it cost you power? and how do you know this?

Matt

[ Parent ]

Re: rotor blade axis (3.00 / 0) (#6)
by johnlm on Thu Jul 26th, 2007 at 04:58:20 PM MST
(User Info)

Its the same principle as the power loss that happens during furling. As the blades become non- perpendicular to the air flow, the swept area is reduced and it follows the cosine of the angle difference between perpendicular and the tilt.
This lowering in apparent swept area relative to the flow of the air reduces the power.

As for you initial questions, I believe the average airflow direction is parallel to the ground. I say average as there may be some localized directional changes caused by trees, ground irregularities or other obsticles. But in general the airflow is not downward toward the ground. The tilt back on horizontal mills is usually for clearance reasons.

[ Parent ]

Re: rotor blade axis (3.00 / 0) (#8)
by Ungrounded Lightning Rod on Thu Jul 26th, 2007 at 07:05:00 PM MST
(User Info)

As the blades become non- perpendicular to the air flow, the swept area is reduced and it follows the cosine of the angle difference between perpendicular and the tilt.
This lowering in apparent swept area relative to the flow of the air reduces the power.

Actually it's a tad different:

As you become non-perpendicular the component of the airflow along the axis is lower. You've slowed the wind seen by the turbine by that cosine factor.

The available power is the CUBE of the airspeed, so the reduction in potential power is 1 - cos(angle)^3.

However it's not that simple for two reasons:
- A windcharger does not load the turbine optimally, so the output power is not a cube function of the wind. All that matters is current, which is proportional to torque (and to (gen voltage - (battery voltage + diode drop))/series resistance). So it's pretty close to a first order function of (wind - cutin wind speed).
- The cross-axis component of wind interacts with the blades in a complex way.

Still, a 5 degree tilt probably doesn't make enough difference to measure with ordinary test equipment.

[ Parent ]

Re: rotor blade axis (3.00 / 0) (#7)
by Ungrounded Lightning Rod on Thu Jul 26th, 2007 at 06:54:23 PM MST
(User Info)

The cosine of 5 degrees is 0.996
The tilt costs me 1.5% of the available power.

1-.996 = .004 * 100 = 0.4

Costs you 0.4%

[ Parent ]

Re: rotor blade axis (3.00 / 0) (#9)
by Nando (nando37-at-tx-dot-rr-dot-com Correct theanti-spam) on Thu Jul 26th, 2007 at 09:40:47 PM MST
(User Info)

You have one formula then you apply another to obtain the results

You said:
The available power is the CUBE of the airspeed, so the reduction in potential power is 1 - cos(angle)^3.

> 1-.996 = .004 * 100 = 0.4 %

Is this the correct calculation THEN ?

In one 1- cos(angle)^3 ( to the cube of the speed)

so 1 - 0.996^3 = 1-0.98805 = 0.01195206 % wise = 1.19952 %

Nando

[ Parent ]

Re: rotor blade axis (3.00 / 0) (#10)
by SparWeb (sparweb at ANTISPAM_hotmail_com) on Fri Jul 27th, 2007 at 12:58:47 PM MST
(User Info)

Still wrong!

Not only is the velocity through the disk reduced by 0.996 to the power of 3, but the swept area is also reduced by 0.996.

cos(0.996) ^ 4 = 0.985

1-0.985 = 0.015 or 15%
Steven Fahey
[ Parent ]

Re: rotor blade axis (3.00 / 0) (#11)
by scottsAI (user name at eml dot cc) on Fri Jul 27th, 2007 at 06:53:59 PM MST
(User Info)

SparWeb,

You are joking about the wind speed decreasing by the cos of apparent angle?
Scott.

[ Parent ]

Re: rotor blade axis (3.00 / 0) (#12)
by johnlm on Mon Jul 30th, 2007 at 08:32:15 PM MST
(User Info)

I agree. Please explain how the velocity decreases through the prop.
also when I was in school 0.015 was 1.5% not 15%
johnlm

[ Parent ]

Re: rotor blade axis (3.00 / 0) (#13)
by SparWeb (sparweb at ANTISPAM_hotmail_com) on Tue Jul 31st, 2007 at 12:07:44 AM MST
(User Info)

Sorry that was the pet monkey pushing buttons on the scientific calculator. I'm also sorry for the glib second posting where I still didn't catch my mistake.

The plane of the disk is inclined to the airflow by 5 degrees. Its swept area is reduced by 1-cos(5). That's 0.4%. Any other effects, like changes to the apparent angle of attack are also negligible, so ignore them unless the angle is much greater.

Boy I wish I could delete a posting today!
Steven Fahey
[ Parent ]

Re: rotor blade axis (3.00 / 0) (#14)
by scottsAI (user name at eml dot cc) on Tue Jul 31st, 2007 at 07:45:23 AM MST
(User Info)

SparWeb,

Your posts are excellent.
You had me scratching my head!

Looking forward to your future excellent post.
Thanks for letting us know:-)
Have fun,
Scott.

[ Parent ]

rotor blade axis | 14 comments (14 topical, 0 editorial)

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