Windmill Riddle...

[darrellwittke]

After spending several hours perusing windmill websites (something I should have done years ago!) I consistently found windmill efficiencies at extracting power from the wind to 30-40%. This is the same percentage which Chuck B. and others stated that our gyro rotors perform at. This all makes sense because Chuck B. stated (I believe) that our gyros are really windmills.

The question is this; How can the gyro rotors attain the same efficiency as a horizontal axis windmill when it does not see the same volume of air. The rotor disc area is tilted up 10-11 degrees from horizontal and the relative wind where as the windmill is 90 degrees to the relative wind. It seems to me the windmill just has more air molecules striking it, so why does the gyro rotor have the same efficiency?

(I started searching windmill blades to help my understanding of rotor blades, however I also started wondering why there aren't gyro type windmills that use the lifting power of the rotor blades to power a generator or something. If the rotor extraction efficiency is the same, why wouldn't the work accomplished be the same (minus the little bit of loss in turning pull energy into rotational energy?)

Any comments or answers to the above are greatly appreciated. Thanks, darrellwittke

[Brent_Brown]

If the windmill was rolling at 45 mph it could lay the blades down to, but it has to get the power from 5 to 15 MPH most of the time.

[darrellwittke]

Are you saying the 30-40 percent efficiency is for a certain speed range (ie 5-15 mph for windmills, 45-50 mph for gyro's) and they differ for each blade set?
What I mean is efficiency rating taken across the whole envelope, or only a best speed (and efficiency) rating? Thanks.

[Victor Duarte]

Darrel, i may be wrong, but i also compared the windmill numbers to a gyro rotor.
they simply dont work the same way.. a windmill doesnt work in autorotation, it is spinned by the relative wind, the gyro rotor is spinned by the relative wind + the initial spin (thats why it must be pre rotated).
btw, the goal of the wind mill is to have more spin than lift (less drag on the pylon), the goal of the gyro rotor is to have a lot of axial drag (lift).

if you look at a wind mill from above, the blade pitch is negative, in a gyro the pitch is positive.

may be doug, al or others could answer better.

[StanFoster]

What a coincidence.....I was flying today and saw this windmill set up on a farm. I posted this in the warp drive thread.

[Al_Hammer]

In most all cases, the efficiency of the wind turbine depends on the
actual wind speed. For the three blade design the efficiency curve
looks like this:
http://zebu.uoregon.edu/disted/ph162/cp.gif

The maximum efficiency of 44% is reached in a 9 m/s wind (18 mph) and
falls sharply at higher wind speeds. For a reasonable range of winds,
the average effiency is around 20%,
(from http://zebu.uoregon.edu/disted/ph162/l11.html)

The gyro rotor also has an airspeed at which it is most efficient, which is usually expressed as some percentage of tip speed.

They are both designed for different speed ranges.
I don't think there would be any advantage in using a gyro rotor as a windmill
Even if you could sustain autorotation at those low windspeeds. Of course the wind would no longer be coming flat to the rotor, it would be coming staright at it like a gyro in a vertical descent.
Also, efficiency goes down as the ratio of tip speed to airspeed goes up.
The typical windmill might have a tipspeed ratio of 6, where in a gyro it is 5 times that. I'm referring to the airspeed going up through the rotor. (The 40% efficiency figure given for rotors may refer to the efficiency with which it converts the engine power into lifting power, not the efficiency with which it extracts power from the air. You'd have to ask Chuck to explain how he got those figures.)

Much of the the lift in the gyro rotor is not helping to turn the blades. It is directed along the axis. The windmill has almost all the lift going to turn the blades so it is better suited to powering a generator.

Victor, The blades in a windmill appear to have a negative pitch. but the Angle of attack when they are spinning is in the same range as a gyro, about 2-10 degrees positive. The rotational speed is much lower, so the blades need to be angled more towards the oncoming wind to keep the AOA low.

[Victor Duarte]

thanks Al, i didn't think in that.. i didnt go further on windmills.. i tried to find a formula for the lift, and found one to calculate the lift on a windmill, it looked so simple, i think it was not applicable to gyros..

... amazing the number of amateur buildings and mono-blade windmills..
thanks

[darrellwittke]

I didn't think about the efficiency being for certain speeds, but it is so obvious now that you have pointed it out. Thanks again.

On another note, my gyro windmill idea wasn't to use the rotor blades on a windmill (ie. 90 degree relative wind as in a vertical descent) but rather have a normal rotor head (with centrifugal teeter stops to prevent flapping on start up and shallow pitch untwisted blades for easy start up) lifting on a cable pulling straight up. At the end of the lift, a mechanical linkage would decrease the rotor disk angle/pitch, allowing the rotor/pulling mechanism to descend and take another bite on the cable for another pull.

Not a very good description of my daydream, but it doesn't matter. I think you answered my question as to why I haven't seen a design like this. The rotor disk would still be suited for a gyro flight envelope (even a light disk would be 30-35mph?) and bending the power stroke and converting to rotation has to take away from efficiency. About the only advantage would be the ease of working on it I suppose. Ah well, at least it's brain exercise to think it through. Thanks again for time and effort gentlemen, best regards.

Also, for those who were interested in the positive/negative twist argument, perusing windmill blade (there are lots of them) websites may shed some extra light.

[Alan Coats]

After reading the great discussions here about how autogyros work, I would suggest that, because the small diameter of the disk on the old farm windmills, the outer tips of the blades don't get enough velocity to become begin generating lift. They stay in the "driving" mode, so none of the energy goes towards generating lift. There isn't a "driven" section on the blade. All the energy goes into torque, not into lift.

On the modern electricity generating windmills, even though the tips do reach great velocities, the tips are twisted so they are still not generating lift, just torque.

In other words, the apparent wind direction is always coming from "below" the blade, not meeting it head-on like a wing, as it would need to to generate lift.

If that makes any sense, maybe someone else can put it into better words for me.

Alan

[birdy]

Clear as mud Alan.{joke}

I'v never thought of use'n one of my 24' comet mill fans for a replacement rotor.

[Alan Coats]

Well, I guess now it is crystal clear why I only post questions.

Alan

[devon532]

Gee, my first post and I'm probably going to make a complete idiot out of myself, but here goes......... Your mention of windmills brings to mind the ones you see in Amsterdam and such (the Don Quixote ones!), which rotate slowly but produce a lot of power. Gyro rotors, on the other hand, turn a lot faster (on my first flight last week we were turning around 330rpms in an RAF2000).

Therefore, what I think is that the lack of angle to the wind is made up by rotor speed. I'll bet if you do a calculation like [rotorspeed x angle to relative wind] for a gyro and a windmill, the answer might be relatively the same, or at least in the ballpark.

Again I remind you I have absolutely no scientific basis to this, it's just what occured to me when I read your post. Let me know what you think.

Vince

[darrellwittke]

The problem (and the answer to my question, "why does the modern windmill blade have 40% efficiency at 90 degrees to relative wind while gyro rotor has 40% efficiency while lying flat and tilted up 9-11 degrees to relative wind) lies in the definition of efficiency. We (or I) do not know if the efficiency of the windmill rated by turning power is the same as the gyro rotor rated by lifting power. It may be comparing apples to oranges.

Also, I believe modern windmill rotors will have a driven region when the windspeed (and RPM) move out of their optimized design range (best efficiency) and that is why their efficiency starts to go down.

On the twist note, here is what I found on www.Windstuffnow.com as typical numbers for a three blade modern windmill with 5 ft rotors. Blade degree is 19 degrees at root decreasing out to 1 degree at tip (keep in mind this may not be what angle of attack is at the varying rotor tip speeds) and with a chord of 9 inches decreasing to 2 inches at tip. Lastly, what I found curious was thickness varies from 1.4 inches to 1/3 inch on the airfoil.

Several questions come to mind, particularly positive twist on the blade. I was told to think of the positive twist on the blade as not really being more twist, rather the root being more flat (ie 0 degrees on dragon wings). I still haven't reconciled this view (with gyro rotors functioning the same as windmill blades, which I know is true) with the above figures. Maybe in time or from insight provided here.

Devon, I think you have a good point (and welcome to posting) about the RPM's. Although if you look at the above windmill site listed, you will find the rpm's roughly the same. The big difference is in rotor size (10 ft vs. 22-26ft typical) which would translate to more swept area and greater tip speed (therefore the same number of air molecules.)

Birdy, clear as mud to me to obviously, out of pure curiosity what speed does your comet old style windmill shut down (or furl?)

[birdy]

It'll depend on wot type of mill your refering to too.
I'v got a wind turbine here charge'n a bank of batteries with a two blade 8' rotor.It needs a min of 6' per sec to start up,but feathers at 60kmph,and the rpm its do'n at feather'n speed I don't know,I cant see the blades coz of their speed,but I can definatly hear it.[sounds like an over speed';n R22 when its feathered.]

I'v also got a few water pump'n mills,up to 24' with 24 indervidual sails,designed for max torque with little wind.The bigger ones feather at bout 60 rpm,but start up,producing alot of torque, at less than 15rpm on only a fart of wind.

Both types are pitched negative,with the pitch varying according to torque required from an average wind.

IOW;Wind mills and [gyro] rotor blades = appels 'n' oranges.

[Victor Duarte]

HUGE ! wow ! i hope they don't have a 3./rev shake !!
is it an illusion or have they orientable tips ? perhaps a mean to start in lows winds ?
thanks, good pics

[birdy]

Yeh,theres used to be one of them big suckers at Coober Peady SA,was about 700 yards off the road and if you stopped to have a leak you could hear the blades whistl'n from the road,not the sort of thing you'd want to stagger into when walk'n home after a hard night on the piss.

[Victor Duarte]

I ant imagine a gyro passing through (or just not passing !)
have they strobes ?
Birdy , you're prospecting opals ?

[birdy]

No Victor,the main north south highway passes through CP.
Dig'n for opals dosn't include fly'n gyros,so I'm not interested.