Wind flag disasters

Greg and Mike,
I have formed some more coherent thoughts on wind flag stability. Center of pressure behind center of gravity promotes static stability of untethered flying objects. No argument here. Wind flags, however, are tethered to a pivot. In this case, static stability requires only that the center of pressure be behind the pivot (that the wind drag on the tail be greater than the wind drag on the nose). It doesn't matter where the center of gravity is. All wind flags have this, otherwise they don't work.

Dynamic response is what we are more interested in when we try to reduce fishtailing during a change in wind direction. Dynamic response for a wind flag is described by a second order differential equation like that for the classic spring-mass-damper system, only for rotational motion: http://faculty.ifmo.ru/butikov/Applets/Manl_1.pdf, and is characterized by two parameters - the undamped natural frequency and the quality factor, which is a ratio of the natural frequency and the damping coefficient. For wind flags we want a high natural frequency, so that the flag responds quickly, and a quality factor of 0.5, which corresponds to a critically damped system. A critically damped system neither overshoots nor undershoots the new wind direction. There could be some significant gains to be had in flag performance by analyzing flags this way, even considering that our current flags work pretty darn well.

Cheers,
Keith
 
Atta' boy!

mks said:
Greg and Mike,
I have formed some more coherent thoughts on wind flag stability. Center of pressure behind center of gravity promotes static stability of untethered flying objects. No argument here. Wind flags, however, are tethered to a pivot. In this case, static stability requires only that the center of pressure be behind the pivot (that the wind drag on the tail be greater than the wind drag on the nose). It doesn't matter where the center of gravity is. All wind flags have this, otherwise they don't work.

Cheers,
Keith
Click to expand...


Good for you Keith! :) Well done. I'm not surprised that you caught that. I saw it but was just too lazy to comment.

Gene Beggs
 
Keith, I'm going to hijack your thread a minute:

STOP, OR I'LL PEE ON YOUR SHOES!

There. I'm trying to determine if anyone has ever build a benchrest rifle using the recoil isolation device Harold Vaughn called "flexures."

If anyone has, I figure you'd be one of the usual suspects. Even a simple yes or no would help.

OK, back to wind flags.
 
Kudos, Keith! I think you have hit the nail squarely.....in the center:eek:. I'll do some math and change the size of my vane and/or pinwheel very slightly and test it. I'm still extremely happy with the way that they work. They must be extremely close because I only move the pivot about 1/8" from balanced. Shouldn't take much to bring it into complete harmony without the need for more than the absolute minimum amount of weight. But again, I don't think that you can do away with having some side pressure(friction) within the pivot due to drag varying with wind speed. It will only be a very slight, at best, improvement in my flags due to them having so little weight and friction already. I firmly believe that weight and friction are the enemy to a good flag. That said, I want to make the very best product I possibly can and will do whatever that takes. I haven't cut any corners so far and won't start now.
Rick, I apologize for making a technically incorrect comment about your flag design. I hope you can see to forgive me for it.
Ain't this fun? --Mike
 
Last edited:
Charles,
Check your PM's. (Sorry, I haven't checked mine in months).

Keith
 
I've made a few flags and learned that if you're gonna sell them you have to make them somewhat....well....foolproof. There's no tellin' where the end user will decide to place the pivot. I put enough vane to make them work regardless of where the pivot was placed and however much "bind" resulted. Certainly, the available length of pivot adjustment was a consideration as well.

Hall of Famer Ed Watson would literally "wear your a$$ out" with these:

http://www.benchrest.com/flags/

He did it over and over, rain or shine, gale or calm....
 
My point is twofold:

1. There are two forces. One in front of the pivot and one in back. The force is the wind pressure multiplied by the length of the lever arm formed by the shaft. To put it in layman's terms, it's like a see saw. If you want tubby and slim to see saw together, tubby moves toward the pivot, or vice versa. The bottom line is that you want the greater force in the rear. Well, I guess to be more accurate, the greater force WILL BE in the rear.

2. If the flag is windshield wiping, it is approaching instability and and you need to increase the force behind the pivot point. You can do that by increasing the sail area in the rear. That's kind of tough to do at a match. You can also do it by lengthening the lever arm. That's easy to do at a match. The downside is that the flag is no longer balanced. Fortunately, it doesn't take much and, if the flags are light...and they *should* be...the detrimental effects of being out of balance won't be enough to matter.
 
SGJennings said:
My point is twofold:

1. There are two forces. One in front of the pivot and one in back. The force is the wind pressure multiplied by the length of the lever arm formed by the shaft. To put it in layman's terms, it's like a see saw. If you want tubby and slim to see saw together, tubby moves toward the pivot, or vice versa. The bottom line is that you want the greater force in the rear. Well, I guess to be more accurate, the greater force WILL BE in the rear.

2. If the flag is windshield wiping, it is approaching instability and and you need to increase the force behind the pivot point. You can do that by increasing the sail area in the rear. That's kind of tough to do at a match. You can also do it by lengthening the lever arm. That's easy to do at a match. The downside is that the flag is no longer balanced. Fortunately, it doesn't take much and, if the flags are light...and they *should* be...the detrimental effects of being out of balance won't be enough to matter.
Click to expand...

Bingo! But the pivot needs to offer as low a friction coefficient as possible...because there WILL BE side contact, even on a flag that is pefectly balanced in a true calm(static) condition,it won't be once the wind offers enough force to move it. Drag will make it tail heavy.
 
After trying to muddle through all the physics here I have a question or two, that admittedly may have already been answered but I missed it.
1. CP must be behind CG as I understand it. If we use flag brand X and set it out,,,,, does the CP change from moment to moment based on the velocity of the wind?
2. My vains are 12 x 18. and the over all length of the assembly is 32". If perfectly balanced what does weight have to do with performance?
I ask this based on having observed some extreeeemly heavy things being perfectly balanced being able to be moved by just a slight touch.
 
Vern,

No matter how perfectly balanced, you can't cheat friction and you can't cheat Newton's First Law of Motion. A heavier flag makes for more friction and a heavier flag makes for more inertia.

Well balanced is better. Lighter is better. More visible is better. Simpler is better. More robust is better. More adaptable is better.
 
What does the wind do to the balance I.E. when the flag is at rest it is perfectly balanced but yet as the wind blows it puts more "pressure" on the vein portion of the flag if for no other reason the tail(s) cause drag and or friction that pulls on that part of the flag. Wouldnt it be reasonable to think that the moment the wind blows the actual balance of the flag has changed and the harder it blows the more it changes?
 
Vern said:
What does the wind do to the balance I.E. when the flag is at rest it is perfectly balanced but yet as the wind blows it puts more "pressure" on the vein portion of the flag if for no other reason the tail(s) cause drag and or friction that pulls on that part of the flag. Wouldnt it be reasonable to think that the moment the wind blows the actual balance of the flag has changed and the harder it blows the more it changes?
Click to expand...
Yes, that is what happens.
 
Mike from that statement I would then have to gather that flags can never be properly balanced in variable winds since the velocity of the wind changes the CG because of the CP.
Would I then also be correct in the presumption that a flag in order to be balanced when in actual use should be heavy toward the nose/front?
 
Vern said:
Mike from that statement I would then have to gather that flags can never be properly balanced in variable winds since the velocity of the wind changes the CG because of the CP.
Would I then also be correct in the presumption that a flag in order to be balanced when in actual use should be heavy toward the nose/front?
Click to expand...
Yes, but how much. Maybe a ball or wing on the shaft, under spring tension, that could vary the cg with the varying drag would work. But honestly, my flags move inside...how much better do you need? Another thing I've considered is a pivot pin and a magnet with a hole for the pin in it, both either positively or negatively charged to oppose one another. Hmmm.
 
Vern, if you need more magnets, I've got plenty. I went to a thrust bearing instead. It works better in the light breeze at Boerne.
 
I see two potential issues with the magnets:

1. Weight. Nuff said.

2. Zero friction also means zero dampening. Like a car with no shock absorbers.
 
I believe Vern is using them to support the verticle load only, not centering support. His tops are on the heavy side. The problem with magnets is with light flag tops. They work great with heavy tops. There is a flux cutting action that reduces sensitivity which makes them impractical without a lot of vane area. For light tops a slippery plastic pivot on a polished 1/8" 300 series stainless is what I have used for 20+ years. I have not been able to come up with a better solution for light tops. It will move when most commercial flags lag badly in light winds. Thrust bearings have to be protected from dust and moisture ingress and still have more friction than plastic/steel combo. So they are not without problems as well. Which application is best depends on the overall design. When the top weight goes up, a support bearing of some type is better, but inertia starts revealing it's ugly head. I personally like the lightest top I can make and still be reliable with reasonable durability.
 
SGJennings said:
My point is twofold:

1. There are two forces. One in front of the pivot and one in back. The force is the wind pressure multiplied by the length of the lever arm formed by the shaft. To put it in layman's terms, it's like a see saw. If you want tubby and slim to see saw together, tubby moves toward the pivot, or vice versa. The bottom line is that you want the greater force in the rear. Well, I guess to be more accurate, the greater force WILL BE in the rear.
Click to expand...

Yes.

SGJennings said:
2. If the flag is windshield wiping, it is approaching instability and and you need to increase the force behind the pivot point. You can do that by increasing the sail area in the rear. That's kind of tough to do at a match. You can also do it by lengthening the lever arm. That's easy to do at a match. The downside is that the flag is no longer balanced. Fortunately, it doesn't take much and, if the flags are light...and they *should* be...the detrimental effects of being out of balance won't be enough to matter.
Click to expand...

No. Fishtailing is not a balance nor a static stability problem. It is a different problem, a dynamic problem involving the sail area, rotational inertia and damping. Yes, you can reduce fishtailing by increasing sail area or its distance from the pivot, but you can also reduce it by decreasing inertia and increasing damping.
 
You must log in or register to reply here.
Back
Top