Diameter size OR Fin style?

General question for building a "Wind Cock Proof" rocket. (straight and true, every time)

I want to build a rocket that wind cocks "none at all" or very, very, very little.

Assumptions are:
Rod length and Motor are sized for stable flight upon exit of launch rod/rail.
Rocket stability margin at "least" 1. (most likely overstable, 2-3 margin)
Fins are aligned parallel to center line of sustainer. (no spinning from canted fins)

Everything else is up for "best" building techniques for "Wind Cock Proof" rocket

Is the diameter size of the rocket (seems smaller rockets are less prone to wind)
OR
Fin style & Fin size more important?

If it's fin style and/or size, please share your knowledge.

Signed,
I want to fly in "any" wind (less than 15 mph)

The body diameter and the fin shape and size are already included when calculating the stability margin. If the rocket sims as stable, then those variables have already been included. I think the main consideration for preventing weathercocking is speed off the rod/rail, not stability. In fact, it is the rocket's stability that causes it to aim into the wind. What you want is a fast motor that kicks hard off the pad and a decent length of rod or rail. You can have a very stable rocket that will aim straight into the wind if it is launched too slowly. For example, a rocket that will weathercock badly on an E9 will fly straighter on an E12 and even straighter on an E20.

There is no such thing as "Weathercock Proof" for a rocket. However, you can make a rocket more "Weathercock Resistant".

The further your rocket gets from 1 caliber, the more the fins will be able to "respond". Keep it as close to 1 as possible where it remains stable.

Generally denser rockets will be less responsive to wind than rockets that are less dense. I'm not talking how heavy a rocket is, but its mass per volume.

Use a rail rather than a rod. Rods whip, that's just what they do. Rail movement is practically nil.

Use a long rail if you can.

Use a template to make sure that your fins are perpendicular to the axial centerline of the airframe (more important) and perpendicular to the surface of the airframe (less important).

My :2:

YMMV

Greg

Tube fin rockets don't weathercock. Found out the hard way when I launched my Groove Tube on a J90 on a breezy day assuming it would weathercock. When it didn't, landed in the trees. Otherwise, as ThirstyBarbarian stated, the faster off the rail, the less weathercocking.

The fin size will have more influence than the shape, because a change in the area of the fin is able to greatly effect the location of the CP.

The fin shape can effect the CP, but it's effects are usually less pronounced than changing the area. For example, if I change a fin size by upscaling it 10%, that will be a greater effect than changing the shape, say by changing the angle of the leading edge from 40° to 50° and holding the area of the fin the same.

Greg

+1 for what Greg has posted.

All Free flight rockets weathercock due to disturbed flight paths caused by Side wind hitting the vehicle causing it to undulate around the CG in a Pendulum motion back and forth until it returns to its starting point... a little over stable the vehicle will stop its swinging before it reaches the original direction starting point (weathercock) in the direction of the wind. A bit on the neutral side of stable.. the model veers off coarse in an undetermined direction... Not really weather-cocking but a far worse effect. One Caliber Stability has proven over the decades to be the proper CP-CG relation giving the least diversion for the original flight path.

You mention fins perfectly aligned with no spin. Well my friend...If your looking for Straight and True Flight paths.
Spinning the model greatly dampens out the undulations caused by side wind disturbances. Canting All fins at the same .5 to 1° angle in the same direction will straighten out your windy day flying. The down side is spinning also creates drag that lowers the achieved altitude possible with a given motor impulse but the flights will always be far straighter then non-spinning vehicles.
Ever wonder why Full Size unguided Sounding Rockets are almost always high spin rate launched vehicles? Now you know.

Higher liftoff speed leaving the rod or rail is a big help and/or longer launch rods... but you'll never get a truly weathercock proof flying model. It is the nature of the beast.

I built a 6.5 times upscale of the hi flyer , the fins are big . I canted all the fins 1° to the right . Flew straight

GregGleason said:

The further your rocket gets from 1 caliber, the more the fins will be able to "respond". Keep it as close to 1 as possible where it remains stable.

Generally denser rockets will be less responsive to wind than rockets that are less dense. I'm not talking how heavy a rocket is, but its mass per volume.


Greg

Click to expand...

"respond" - meaning the more stable, the more it will have a tendency to weather cock?
Mass per volume - so a 18mm rocket that has a mass of 100g will cock less than the same rocket that has a mass of 50g?

thanks for the replies

6x6 said:

"respond" - meaning the more stable, the more it will have a tendency to weather cock?
Mass per volume - so a 18mm rocket that has a mass of 100g will cock less than the same rocket that has a mass of 50g?

thanks for the replies

Click to expand...

"Yes". The further the CP is from the CG the more it will tend to weathercock. The CG is like a pivot point, and the further it is from the CP the more quickly it will pivot, like a weather vane.

With regards to the mass per volume, a penny and small plastic cup may weigh the same but will have very different responses to a 10 mph wind blowing on them. Not that a less dense rocket will weathercock more, just that it will be more influenced by wind in general. Also, in general, heavier rockets are less influenced by wind than lighter ones.

Greg

Fly warp-9 and v-max motors

so my assumption (which seems to be wrong) that small rockets tend to weathercock less is more likely to the fact that they leave the rod with a much higher velocity. (seems like VMax is the way to go )

I'm still puzzled as to why the fins don't really make a difference. Not so much the shape (style), but the dimensions.
Doesn't the root length, span, thickness have a contributing factor? Or do those "only" factor into where the Cp is on the rocket?

6x6 said:

so my assumption (which seems to be wrong)

Or do those "only" factor into where the Cp is on the rocket?

Click to expand...

Precisely! Model rocket stability is the direct relationship between the models CP-CG with I-caliber being the preferred "Norm". Everything that happens Pitch, Yaw & Roll in free flight happens around the models CG.
Varying the shape or dimensions effects the CP location of the model thereby increasing or decreasing the "Static Margin"(caliber) between CP & CG.

Micromeister said:

Precisely! Model rocket stability is the direct relationship between the models CP-CG with I-caliber being the preferred "Norm". Everything that happens Pitch, Yaw & Roll in free flight happens around the models CG.
Varying the shape or dimensions effects the CP location of the model thereby increasing or decreasing the "Static Margin"(caliber) between CP & CG.

Click to expand...

Technically, varying the shape and size of the fins also affects the CG location too. It probably affects the CP more, but it does affect them both. The key is, as you said, it changes the static margin.

WRT the fins, it's all about the surface area in relationship to the rest of the rocket. The greater the fin area, the more potential for the effect.

What may be a helpful exercise is do the Barrowman equations on a simple rocket. I can speak about the relationships all day long, but when you actually do the math you can see where changes can have large or small effects on the rockets CP. A good reference is found with this link:

https://my.execpc.com/~culp/rockets/Barrowman.html

You can even use the "cardboard cutout" method of determining the CP is a helpful exercise, or using OpenRocket for that matter.

Fin shape is important and each fin set will have its own set of unique set of flight characteristics.

The Big Picture Problem: What we are trying to do is manage that first half second or so of flight where airspeed is the lowest.

That is why the 5:1 thrust ratio ROT is for winds at <= 5 mph. The ratio goes to 6:1 and up as the wind speed increases to mitigate the effects of wind when the rocket velocity is low.

HTH.

Greg

6x6 said:

so my assumption (which seems to be wrong) that small rockets tend to weathercock less is more likely to the fact that they leave the rod with a much higher velocity. (seems like VMax is the way to go )

I'm still puzzled as to why the fins don't really make a difference. Not so much the shape (style), but the dimensions.
Doesn't the root length, span, thickness have a contributing factor? Or do those "only" factor into where the Cp is on the rocket?

Click to expand...

I bet there are some small factors involving shape, style, dimensions, etc. of fins that go beyond affecting the CP and are a contributing factor. But those factors are probably heavily outweighed by the CP CG margin.

GregGleason said:

That is why the 5:1 thrust ratio ROT is for winds at <= 5 mph. The ratio goes to 6:1 and up as the wind speed increases to mitigate the effects of wind when the rocket velocity is low.

Greg

Click to expand...

Is there a table for optimal thrust ratio vs wind speed

6x6 said:

Is there a table for optimal thrust ratio vs wind speed

Click to expand...

I've found these charts from the old info-central site to be helpful: https://www.info-central.org/?article=121

Maybe some of the long time TRFer's can speak to how this data was assembled.

6x6 said:

Is there a table for optimal thrust ratio vs wind speed

Click to expand...

See also ...

https://www.nar.org/pdf/launchsafe.pdf

https://www.rocketryforum.com/showthread.php?1799-Rule-of-Thumb

Greg

thanks everyone for the replys
thanks greg for the chart!

I have the perfect solution.

Don't fly when there is Wind.

Just wondering, why do you want the rocket to not bank into the wind? If the rocket goes straight up on a windy day, it means it will drift much farther downrange. When there is wind, I find it useful to let the rocket travel into the wind, and it's standard procedure to angle pads into the wind on windy days at LUNAR. If you launch into the wind, the rocket will drift back towards you after deployment.

SCrocketfan said:

Just wondering, why do you want the rocket to not bank into the wind? If the rocket goes straight up on a windy day, it means it will drift much farther downrange. When there is wind, I find it useful to let the rocket travel into the wind, and it's standard procedure to angle pads into the wind on windy days at LUNAR. If you launch into the wind, the rocket will drift back towards you after deployment.

Click to expand...

If you want to reach a specific altitude every time (same rocket same motor), the only way I can see it happening is to make sure it flies in a straight line, "every" time (regardless of wind speed).
I was asked the question by a TARC team and I didn't have a firm answer to give them, but said I'll bet I can find out. (hence the assumptions)

I've let the team read the comments and come to their own conclusions on how to solve the problem. (which is more complex than I first thought :surprised: )

SCrocketfan said:

Just wondering, why do you want the rocket to not bank into the wind? If the rocket goes straight up on a windy day, it means it will drift much farther downrange. When there is wind, I find it useful to let the rocket travel into the wind, and it's standard procedure to angle pads into the wind on windy days at LUNAR. If you launch into the wind, the rocket will drift back towards you after deployment.

Click to expand...

I used to angle, but for the most part now go straight up (if I do angle, it's just a "tad"). I find that more often than not, if there is any weathercock at launch it will tend to come back near the pad.

Greg

I understand, I'm actually considering entering TARC sometime if I can get a few other people to participate. I's definitely an interesting challenge making sure you hit the same altitude every time. Won't wind still affect the rocket though even if it goes straight up every time?

GregGleason said:

I used to angle, but for the most part now go straight up (if I do angle, it's just a "tad"). I find that more often than not, if there is any weathercock at launch it will tend to come back near the pad.

Greg

Click to expand...

I do the same --- aim the rod straight up in almost all conditions and let weathercocking compensate for drift. At most I'll use a very slight angle if I'm not sure what will happen, and I'd rather have it land "over there."

SCrocketfan said:

I understand, I'm actually considering entering TARC sometime if I can get a few other people to participate. I's definitely an interesting challenge making sure you hit the same altitude every time. Won't wind still affect the rocket though even if it goes straight up every time?

Click to expand...

A rocket goes 100+ mph, a sideways wind force of 5-15 mph will give a small added amount of friction, but mostly negligible where altitude is concerned. (assuming it goes straight up)
However if the rocket weathercocks "into" the wind, the resulting angle of attack WILL result in an altitude drop. Hence the need to build a wind "proof" rocket.

6x6, is your goal in this to read a specific altitude every time with the same engine regardless of windspeed (up to, say 20 mph)?

Cosermann said:

6x6, is your goal in this to "read" a specific altitude every time with the same engine regardless of windspeed (up to, say 20 mph)?

Click to expand...

I'm sure a good programmer could tell a altimeter to "read" a specific altitude every time. :wink:
I'm more interested in the same rocket reaching the same altitude with the same motor, every time - regardless of wind speed, yes that's the goal.

6x6 said:

I'm sure a good programmer could tell a altimeter to "read" a specific altitude every time. :wink:
I'm more interested in the same rocket reaching the same altitude with the same motor, every time - regardless of wind speed, yes that's the goal.

Click to expand...

Motors have a statistical variation, so pick a motor with a lower "sigma" value.

Greg

I have had two squat rockets that in my experience have ignored wind and not weathercocked. Both follow the formula "Fin area, not nose weight". One had fins that overhung the back significantly, and the other simply has massive, enormous fins.

When you do this, the CG ends up very far back on the rocket, and the body tube ends up approximately balancing the fin area when exposed to large angles of attack. They are never squirrelly, and they never weathercock from zero up to 15mph winds.