Prop Moment Questions?

[Baling Wire]

is key to longivity, success, and happiness with a wind project.  Failure for hard work IS depressing!  Obtaining proper furling appears to be a complicated process, the techniques look simple; but in reality, often hard to impliment correctly.  It seems as if there is a successful design; then IT must be followed 100%?  Less than 100% Function is not an option.  I would like to see the implimentation of Furling [allowing variables]get to be a "science" rather than a drawn-out "art".  IT may not happen?

Here are a few questions to consider.

1. Does the gyroscopic force of the prop come into play?
   
2. Does direction of rotation of the prop in relation to the direction of the rotor offset factor-in?
   
3. Being in the Northern or Southern Hemisphere matter in relation to 1. or 2.
   
4. Could someone explain or define "prop-seeking force"?
   
5. How does one go about measuring or calculating prop-seeking force?
   
   

Anxiously awaiting the knowledge of the board.

BW

[Flux]

These are my thoughts, others may think differently.

1. Gyroscopic force affects whether the blades move towards or away from the tower during yaw. It adds to yaw pivot friction and causes vibration on less than 3 blade props. It probably has little real effect on furling.
   
2. Rotation decides whether the prop moves towards or away from the tower as it yaws into furl. No real other effect.
   
3. I can't and there are not many of us even prepared to accept that the phenomenon exists.
   
4. With a machine with fixed tail and no offset you can get some idea by pulling the tail with a rope to see how much force is needed to get it out of the wind. Compare this with the tail force with prop tied stationary.
   
   

You can get a fairly good idea by comparing the thrust with the effective furling force as deduced by the tail moment against its stop.

This force vanishes at some critical angle ( in the order of 45deg). At this point it suddenly pulls out of the wind and there is a dramatic reduction in power out.

This largely accounts for the falling output in high winds as most machines furl, much more so than the peaking of the tail moment at about 45deg with the common scheme.

The larger the offset the more the power tries to stay constant after furling .( Take it too far and you need a monster tail to keep it into the wind below furling. Make the offset too small and it doesn't furl).

If you want to resolve this to a precise mathematical figure, abandon furling and design a pitch controlled hub, replace the tail with a servo motor and it will be analysable and you remove all the nasty gyroscopic forces, . You will have a nicely defined maximum power, no problem of over speed with loss of load and lots of other good things, but you have a mechanical challenge that will defeat the basic home constructor.

Flux

[Flux]

Sorry I missed out your question 3.

It makes no difference which hemisphere you are in. It seemed irrelevant but leaving it out mucked up the following numbers.  3 & 4 become 5 & 6.

Sorry

Flux

[Baling Wire]

Take a look at the Demo on this site.  http://science.howstuffworks.com/gyroscope1.htm

If Most? props turn CW from the front and are attached to a hub/string{from behind}; then the Precession appears to me to be CCW from zenith?  If that is the case, then would not that effect the prop moment?  The precessional rate depending on the prop rpm?

BW

[RP]

Precession only applies if the rotor is pivoted so that gravity can turn it downward.  If the rotor is fixed to the yaw axis then there will be no (gravity) induced precession.

If there were then all box fans would slowly rotated on the floor until they unplugged themselves.  :-)

Precession is an issue when the yaw axis turns suddenly though like during a wind gust.  the quick rotation tends to make the horizontal turbine want to rotate to a vertical axis.  Although the bearings in the hub prevent this, the blades will flex in the attempt.  This flexing can cause the tips to actually collide with the tower.  For this reason it's worth some thought when planning blade direction and which side to put the furling offset on.

[Baling Wire]

Thanks for straightening me out on precession.  I jumped to a conclusion too soon. hi ! hi!

BW

[ULR]

Gyroscopic precession appears because a force on a rotating mass accellearates it as it goes through a half turn, so it's moving most as it reaches the end of that half turn.  So the motion from the force is "carried a quarter-turn by the rotation".

Thus you want your mill to furl so the downward-moving side of the blade goes upwind.  This causes the gyroscopic forces to push it away from the tower a quarter turn later.

Of course it works the other way when UNfurling.  But furling can be abrupt and occurs when the mill is rotating at essentially the max speed, so gyroscopic effects are strong.  UNfurling happens more slowly and the mill has probably slowed down from being furled.  So you design to avoid problems when its furling and let the unfurling take care of itself.

 

[TomW]

ULR;

Sorry about that getting blocked from posting thing. I just doubled the amount of posts before that triggers.

It was tight to prevent massive amounts of spam we were getting at one time. The block is just temporary but somewhat long. It is also cumulative in that if someone keeps trying to post when blocked it extends the time.

Anyway, sorry for the inconvenience. Hopefully you can post away as much as needed now.

Cheers.

TomW