autorotation

[limlom]

Hello!
I am a 500h pilot just doing my fi(h) rating and I have a aerodynamic question for you all! my instructor tells me that a windmill or a windpower generator autorotates - does it? or does it just "windmill"? / Jon :confused:

[Matthew Parsons]

If you get down to the basics, both cases use relative airflow through the blades to generate forces that rotate the blades. In the case of a helicopter, those forces also create a lift component. If a windmill generated an equivalent lift force then it wouldn't be operating at its peak efficiency.

Windmills tend to have a negative pitch, something which is rare for helicopters. So there is a substantial difference in the force diagrams.

It really comes down to how your textbook defines autorotation, and windmilling for that matter. I think most make such a special case for autorotation that the two are not the same, but I wouldn't call your instructor on it.

[NickLappos]

Autorotation and windmilling are the same thing, in fact, designers call autorotation "windmill brake state" in that a brake is needed to draw off the energy the rotor is producing as it autorotates, otherwise the rotor will overspeed.

Matthew is right, the rotor on a windmill has a different design point, but that is because that rotor is designed to autorotate only, so it has no need for a low drag powered cruise mode. My guess is that most autogyros and windmills have similar designs.

BTW, the designs of the several Dutch windmills that I have studied are simply awesome. In the 1600's they had variable blade area and radius, variable angle of attack and slotted leading and trailing edge high-lift devices. A windmill engineer back then probably had all the sophistication of a rotor designer today, but without any Microsoft programs to help him!

One can draw lift diagrams to show the torque produced by the blade section that drives the rotor, but frankly I have never got much out of lift diagrams for rotors, and generally use the pinwheel/windmill analogy to satisfy myself as to how the rotor is eating a stream of air that is forced into it, and in the process it produces both lift and torque. I am also an anti-bernoulli guy, since I never explain that lift is produced by curves. It is far easier to see how lift is produced by a wing that "planes" on the air and makes a bunch of air go down so the momentum of that air makes the wing go up. A waterski and a wing have much in common, and are much more intuitive to budding aviators.

[slowrotor]

I drove over to watch a large wind turbine in Minnesota and just sat there and watched and listened. It was very quiet, woosh....wooosh.... wooosh as each blade passed. I was entralled. They should build more.

But it took some time to understand the blade pitch angles. I think it had blade twist unlike a helicopter, but rather had less angle near the hub than at the tip. This is the apparent negative pitch Matthew referred to above.

But the blade sections are flying at a positive angle relative to the wind.
I see the windmill as a sailplane optimised for rotation. The windmill tower provides the same force as gravity in the case of a free glider.

Be careful, they are tall obstructions for low flyers. The windmill will win.

[Arm out the window]

That blade twist is washout, and lots of helicopters have it.

Oops, should have said wash-in, which helicopters don't have - didn't read slowrotor's post well enough.

[outofwhack]

[I meant attack !]

Do any engineering types amongst us know how many degrees of pitch the blades of a helicopter typcially have when the collective is bottomed.

Just a rough answer is fine. I guess its still 2 or 3 degrees positive but would like a little confirmation.

I am returning to flying model helicopters as well as full size and would like to master the art of autorotations with models [to save me a few quid if engine stops].

Many in the model fraternity seem convinced you have to have actual negative pitch but every vector diagram I have studied for fullsize shows some amount of positive pitch in the simplified diagrams [that neglect washout etc].
For interest and from my own observations it appears model helicopters use a range of 5 to 9 degrees for hover and max power.

OOW

[Gaseous]

Hi A,

Enstrom uses pitch angles of 6.5 to 14 degrees positive, measured at the grip, with the cyclic central. Pitch only goes negative at extremes of cyclic excursion, and then, obviously only on one side of the disk.

Cheers. Phil

[Graviman]

The rigid hubs of these 2.3MW turbines (http://www.bonus.dk/) were the size of a Transit van!
Notice the door at the base of the one in the foreground...
http://publish.hometown.aol.co.uk/martingarrish/images/windmill.jpg
Blade camber is wrong way to allow for autorotation from rotor top. They take time to respond to wind change so get noisy until "pilot" adjusts collective. The advantage being here that a generator forces constant RRPM by varying power adsorption.

Do any engineering types amongst us know how many degrees of pitch the blades of a helicopter typcially have when the collective is bottomed.

Depends on aerofoil lift and drag vector angle to chord. Combined lift and drag vectors have to point straight up to keep RRPM constant. Raise collective vector points backwards, RRPM decreases. Lower collective vector points forwards, RRPM increases. You need to add a pitch increase since finite rotor size causes local downwash over each blade (induced drag from tip vortices).

The practical upshot is that you are probably correct. Long time no study aero. :}

Mart

[topendtorque]

“Long time no study aero.” Yep me too.

limlom

I’d always thought that where the real action is out on the end of the blade that AOA there at collective bottom should be zero, up or down.

To see one which is set at negative settings when it is running at flight RPM on the ground is to see an unhappy helicopter trying to beat itself to death while it works in vain to screw downwards thru the earth. This situation also leads to the crash report which starts with; - the A/C had high RPM upon impact-.

The A/C which is set right (at zero) will ALWAYS demonstrate the lowest power setting at flat pitch, remember - basic engine trend check number one.

Bernoulli can be a confusing gentleman, after studying for some years how he is employed to confuse inbound lightning strikes on electric fences so as mr lightning doesn’t blow the electric gizmo to smithereens, I confess, I agree, I am as confused as Bernoulli.

However when a careless pilot bottoms the collective in a real auto on a machine with the auto RPM set too high, I.E. with negative pitch at collective bottom, then mr coriolis will come to haunt one especially after first pitch pull and give an RPM tending upwards - off the clock - which in turn contributes to,
1) Spindle bearings brunelling,
2) Useless waste of large amounts of energy winding up said RPM, which in turn contributes to, high ROD as that energy has been stolen from the one and only energy source, mr gravity.
3) Crash report which starts with, yep you guessed it – the A/C had high RPM upon impact.

To see a machine with a too high AOA at the collective bottom will lead to no crash report from the pilot at all. Possibly only one from any casual spectator who may be around which will say something like, “why it just stopped right there and plum fell outa the sky.”

Anyone who has done an R22 saftey course will testify that finger trouble can easily cause that same vertical descent even with correct auto RPM being set.

My advice would be – have a look at your FAA basic helicopter handbook- its all there.

Its all just a tad different from the old steel blade windmill or sailing boats where the wind has to divert around an object thus creating a vacuum from behind which then draws the object forward.
Cheers tet

[ShouldItDoThat]

[I meant attack !]

Do any engineering types amongst us know how many degrees of pitch the blades of a helicopter typcially have when the collective is bottomed.

Just a rough answer is fine. I guess its still 2 or 3 degrees positive but would like a little confirmation.

I am returning to flying model helicopters as well as full size and would like to master the art of autorotations with models [to save me a few quid if engine stops].

Many in the model fraternity seem convinced you have to have actual negative pitch but every vector diagram I have studied for fullsize shows some amount of positive pitch in the simplified diagrams [that neglect washout etc].
For interest and from my own observations it appears model helicopters use a range of 5 to 9 degrees for hover and max power.

OOW

Not engineer, but fairly sure you're right about very low positive. Model heli's do however often have -ve pitch for acrobatics - have you not seen them hovering upside down?

cheers
ian

[NickLappos]

The typical collective pitch is usually described for designers at 75% of span, where one presumes the lift comes from, and is usually about +2 degrees at bottom collective to +18 to 20 degrees at max, depending on the max speed and the max power of the aircraft (since speed washes out collective pitch, and more power needs more pitch to absorb it).

In rigging, a good flat spot is found on the blade grip, and the angle is corrected to this spot for maintenance manual rigging.

In Navy helos, the minimum pitch is sometimes down to -2 degrees, so that the helo actually sticks itself down to the deck.

[Dave_Jackson]

http://www.windpower.org/res/turbtubx.gif

[Graviman]

Yes, the autorotation is as if the rotor was at the helicopter base! They are switched off below ~5kph wind, so no useful power generation.

Proven energy (http://www.provenenergy.com/) offer teetering rotors, which behave more like a heli autorotating. They actually use flapback to align on the wind, so rotor always downwind of mast.

This does beg the question: Any difference between autos in teetering and rigid?

The typical collective pitch is ... about +2 degrees at bottom collective to +18 to 20 degrees at max ...

So 1g hover about +7 to 8 degrees @75% span (assuming max 3g margin).


Mart

[slowrotor]

Vortex ring state is not possible with unpowered autorotation. There must be another reason they shut down at low wind.

[3top]

Probably just not enough airflow to keep the thing turning.
An airplane needs a minimum airspeed, so does a helo-rotor, a gyro-rotor, a windmill-rotor....
I assume every plant is designed to be most efficient over the typical year, so one would adjust the design for the average wind.

Some will cut out earlier, some later.
Best yet, when things go hurricaning, they stop them too :)

3top