Stability, fin size and weathercocking

[BrianY]

I have recently been fooling around with the RockSim design software and have come up with sevreal designs that appear to be decent. My question is about stability, weathercocking and fin size.

There is an inverse realtionship between the amount of stabilty (as determind by the size and placement of the fins) and the degree of weather cocking in the simulations. If I reduce the stability of the rocket to minimize weathercocking, the fins end up being very small relative to fins on the kits I've built and the pictures I see of other rockets. The stability numbers are around 2 and the simulations run fine, so the models do not appear to be dangerously unstable, but they are also not "overstable" . 'Overstable" versions of the same models have significant amounts of weathercocking - 40 - 70 feet or more - but the fin sizes seem to be more in line with what I've seen on other rockets.

I have built one of my designs but because of persistent bad weather have not had a chance to fly it to see how it behaves.

So, I guess my question is this: when designing a rocket, which is more desirable: more stability and greater weathercocking or less weathercocking and lower - but still sufficient - stability ?

[CPMcGraw]

I have recently been fooling around with the RockSim design software and have come up with sevreal designs that appear to be decent. My question is about stability, weathercocking and fin size.

There is an inverse realtionship between the amount of stabilty (as determind by the size and placement of the fins) and the degree of weather cocking in the simulations. If I reduce the stability of the rocket to minimize weathercocking, the fins end up being very small relative to fins on the kits I've built and the pictures I see of other rockets. The stability numbers are around 2 and the simulations run fine, so the models do not appear to be dangerously unstable, but they are also not "overstable" . 'Overstable" versions of the same models have significant amounts of weathercocking - 40 - 70 feet or more - but the fin sizes seem to be more in line with what I've seen on other rockets.

I have built one of my designs but because of persistent bad weather have not had a chance to fly it to see how it behaves.

So, I guess my question is this: when designing a rocket, which is more desirable: more stability and greater weathercocking or less weathercocking and lower - but still sufficient - stability ?

It's always a compromise, Brian. It's a ratio of Performance:Stability. You give up a measure of performance to get added stability in a given power range; you give up a measure of stability to achieve performance for a given power range. That last part is an important factor. What may be "on the numbers" for one motor may be way off the chart for another motor.

The "magic number" is to have a stability margin of 1.00 or better. This is also called "1 Caliber Stability", and represents a value measured in body tube diameters between the Center Of Gravity and the Center Of Pressure at lift-off. Note that the CP (Pressure Center) stays in one place throughout the flight of a (single stage) rocket, while the CG (Gravity Center) shifts forward during the flight from ignition through deployment. Why? The fuel has weight (mass), and it's being burned off. In all cases, the CG must start out in front of the CP, as measured fron the tip of the nose cone, and needs to have at least 1.00 caliber (largest body tube diameter) separation between the two points. Keep this as a "written-in-stone" rule, and your rockets should generally fly well.

Fin size is only one way to improve stability, however. Fin shape can also make a big difference. I found this out with a commercial rocket kit, the Estes Hi Flyer. This model is very unstable (it has less than 1.00 caliber margin) with a C6 motor. A number of people have written this up on the internet already about how the model can go horizontal immediately after leaving the rod, or begin to twitch badly during the powered ascent. I played around with the fins using RockSim and found that a fin with no greater area, but with a double-delta 'step' in the leading edge, brought the stability margin to just above 1.00 with the C6. The additional drag from the 'step' shifted the CP rearward just enough to compensate.

I look forward to seeing your first RockSim-to-real-world rocket. Post it on this forum when you can and let us look it over.

Do you have a copy of The Handbook Of Model Rocketry (Seventh Edition), by G. Harry Stine and Bill Stine? It's a good book to work with, and should be in the library of every model rocket builder and designer. Be sure to get yourself a copy if you don't already have one.

[garmtn]

Would someone define "weathercocking"?

[CPMcGraw]

Would someone define "weathercocking"?

Same idea as a weathervane in a breeze. The nose of a 'fixed-fin' rocket will point into the wind as is flies. The larger the fin area, the more visually obvious it becomes. There is a point when the fins can be too large, and the rocket tracks nearly horizontally into the prevailing wind, almost immediately upon leaving the rod. Too much fin area can be as bad as not enough; the model becomes so hyper-stable it becomes a wind-controlled cruise missile.

[garmtn]

Interesting point. Posted my "unique fin design" on this site a few weeks ago. "Main" is like a rudder on boat. It's 7"x 3 3/4 x 3/16". The other two "flair down on either side, along side of 3, 24"BT-20 " body tubes. The fourth connects the two, horizontally. just wondering if a little "weathercocking" is happening, causing it to roll 180 degrees? Thanks, Glenn ;)

[mojo1986]

Glenn.................you should be able to determine that. If it goes straight up on a perfectly windless day, then it is weathercocking on breezy days. But if it still does its 'Space Shuttle Roll' on a windless day, then it is probably due to the asymmetry of the model as a result of its unique fin arrangement (possibly an effect of the CG being offset along its long axis).

Joe

[CPMcGraw]

Glenn.................you should be able to determine that. If it goes straight up on a perfectly windless day, then it is weathercocking on breezy days. But if it still does its 'Space Shuttle Roll' on a windless day, then it is probably due to the asymmetry of the model as a result of its unique fin arrangement (possibly an effect of the CG being offset along its long axis).

Joe

Or something even more simple, like one or more tubes being slightly canted relative to the centerline. One way to determine if this is what happened is to watch the direction of rotation. If it reverses from time to time, it's likely weathervaning; if the rotation remains constant, to the same direction each time, then it may be a tube vectoring the slipstream and acting like a thruster.

[JoeLaunchman]

Another Estes kit with inherent instability was the Sprint. I launched one in 1985 and saw something I hadn't seen before or since. Upon reaching the top of the rod it immediately nosedived into the ground, hitting a point within 3 ft. of the pad. Needless to say, it never flew again. Borderline stability seems to be an issue with models about 12" or shorter (standard engine). Anyone else experience that with the Sprint? I'm currently cloning one but I'll be sure to add noseweight this time.

[Shreadvector]

Another Estes kit with inherent instability was the Sprint. I launched one in 1985 and saw something I hadn't seen before or since. Upon reaching the top of the rod it immediately nosedived into the ground, hitting a point within 3 ft. of the pad. Needless to say, it never flew again. Borderline stability seems to be an issue with models about 12" or shorter (standard engine). Anyone else experience that with the Sprint? I'm currently cloning one but I'll be sure to add noseweight this time.

Famous problem. I never saw the instructions, so I can't tell if they warned about it.

http://www.dars.org/jimz/estes/k-49.pdf

OK, now I've seen them. It did include nose weight, but perhaps not enough for C motors.....

[tbzep]

I never had a stability problem with my Sprint, even with C6-7's (yep, it has the lead weight on the nosecone). I have had trouble retrieving it on occasion, though. It flies high enough that it has time to drift into trees or onto school buildings even with a streamer. :mad:

I'll be sure to add noseweight this time.

Just noticed this when I reread the post. I wouldn't consider the model to be inherently unstable. The instructions say to use the weight and that makes it stable. Leaving off the weight of this design is no different than leaving off a fin. :eek:

[JoeLaunchman]

I did use the noseweight with my Sprint, tried a C6-5, and still experienced the instant death dive. Sure, I could've made some minor construction errors, i.e. engine block too far back, fins too far forward, but not by much. I guess my point is, there was not much margin for error with the Sprint, or with any of the 12" long 1 oz. "high fliers."