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Why did it weathercock?

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dhkaiser

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Flew my Scorpion yesterday and it weather cocked noticeably. We measured the wind at a max of 8mph, could have gusted but not much more. OpenRocket says with the G80 motor and the BRB900 stability was 2.13 cal. I photographed the launch and there was no indication of the 1010 rail moving nor the tripod.

Any ideas what could have happened?

Edit: Velocity off the rod according to OR = 44.4ft/sec


_49A2156_DxO.jpg
 
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Steve Shannon

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Nothing happened, it just weathercocked. The higher your "stability margin", the greater the effects of the wind.
 

woferry

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I think some consider 44 ft/s to be marginal for leaving the rail. ThrustCurve for example requires a minimum 50 ft/s before it will consider a motor as being suitable for a rocket. I don't think OR gives any warning at all, at least I've sim'ed motors that it suggests are barely 30 ft/s without seeing any warnings. Though OR defaults to a fairly short rail of 1m (TC is also like 3 feet I believe), not sure if you adjusted the length or not. AFAIK it's not the full length of the rail that matters but rather the length above your highest rail button/guide/etc., or at least above your second lowest button if you have more than 2, as once there's only one button on the rail the rocket can start to pivot if the wind is blowing the right way.
 

timbucktoo

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Yea, I think your velocity off rail was low. I've flown rockets with stability of 4 or greater in10-15 mph winds and they have minimal weather cocking and most of what I fly has a stability of 2 or greater and they fly straight & true with same wind. My personal goal is 50 fps or greater off the rail.
Light rockets are probably more prone to weather cocking too.
 

dhkaiser

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My launch rail is 6' of 1010 rail. I did adjust the length in OR to 68"...

_49A1953_DxO.jpg
 

Buckeye

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I have no data to back this up, but to my eye, the simulators seem to underpredict the horizontal distances due to wind and weathercocking.
 

RocketFeller

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I launched a rocket (shortened Estes Argent) on both an F15 and an F50 last sunday in 8-10mph winds. The F50 flight was straight up while the F15 weathercocked considerably, which leads me to believe that speed off the rail (rod in my case) has a lot to do with it.
 

Steve Shannon

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I launched a rocket (shortened Estes Argent) on both an F15 and an F50 last sunday in 8-10mph winds. The F50 flight was straight up while the F15 weathercocked considerably, which leads me to believe that speed off the rail (rod in my case) has a lot to do with it.
Absolutely right, the slower the vertical velocity, the greater the effect of wind. Also, over the years we have had people try to lessen drift after deployment by tilting their rockets toward the wind. That's a BAD mistake that can result in a nearly horizontal flight.
 

rharshberger

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Absolutely right, the slower the vertical velocity, the greater the effect of wind. Also, over the years we have had people try to lessen drift after deployment by tilting their rockets toward the wind. That's a BAD mistake that can result in a nearly horizontal flight.
How ever you can increase the chances of a vertical flight on slower rockets by tilting the rocket WITH the wind, it seems counter-intuitive, but with slow, rockets the weathercocking can help to push the rocket toward vertical as it gains velocity.
 

Flyfalcons

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The slower a rocket is off the pad, the greater the angle of attack between the fins and the wind. This means more weathervaning.
 

Steve Shannon

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How ever you can increase the chances of a vertical flight on slower rockets by tilting the rocket WITH the wind, it seems counter-intuitive, but with slow, rockets the weathercocking can help to push the rocket toward vertical as it gains velocity.
Exactly right.
 

rocketguy101

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How ever you can increase the chances of a vertical flight on slower rockets by tilting the rocket WITH the wind, it seems counter-intuitive, but with slow, rockets the weathercocking can help to push the rocket toward vertical as it gains velocity.
you can actually see the results of this in OR by plotting "Flight Side Profile" and varying your launch rod angle and change wind velocity. It is counter-intuitive (until you think about it)...here are some plots of a Leviathan with 8 mph wind and launch angles of 0, -10, and +10 degrees. [edit] Wind is coming from the right side of the graph.[/edit]

I thought it would be interesting if the OP re-simmed his rocket with actual launch parameters, and plotted this side view to see what it looks like.

2016-06-18 11_15_53-Edit simulation.jpg


Sim3.jpg


Leviathan_F20-7_WV 8mph_LA 0.jpg


Leviathan_F20-7_WV 8mph_LA -10.jpg


Leviathan_F20-7_WV 8mph_LA +10.jpg
 

rharshberger

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OpenRocket has many neat simulation options that people can take advantage of. My entire L3 sim process was done in OR and RASAeroII. The weathercocking is pretty accurately simmed, as I found when I went back and plugged in corrected wind data for my cert flight, not only did it reflect the weathercocking but also the loss of altitude related to weathercocking.
 

tomsteve

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im no rocket genious and don't use computer simulations(yet), and though of something.
is it possible that weight of the rocket can play into it? is it possible to have 2 <script id="gpt-impl-0.8658271274816731" src="http://partner.googleadservices.com/gpt/pubads_impl_89.js"></script>rockets with same speed off the rail yet the weight of the rockets make for 2 different tragectories?
like a lighter rocket( or heavier?) will be effectd greater by wind even though they are both traveling the same speed off the rail?


heck I cant e en spell genious write!LOLOLO

im also wondering in the flight for the OP if the ignitor leads held on the motor for a bit causing the rocket to be pulled into an unverticle trajectory. or if the motor had a slight chuff just off the rail- slowing it down for a split second.
 
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RocketFeller

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I have found weather cocking to be helpful as often as not. On the F50 flight I had to walk about 3/4 mile to recover my rocket, on the F15 flight that weather cocked the rocket landed a couple hundred feet from the pad. With smaller LPR rockets we often get the same results our Rocket Club launches.
 

ksaves2

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How ever you can increase the chances of a vertical flight on slower rockets by tilting the rocket WITH the wind, it seems counter-intuitive, but with slow, rockets the weathercocking can help to push the rocket toward vertical as it gains velocity.
Like Steve and Rich mention, pointing the rocket a few degrees downwind is the proper way to do it. How many degrees? One gets their rocket into a simulator and they sit there and run sim after sim after sim getting the hang of what it takes to get the velocity at the apogee as low as one can get it at various windspeeds. Why do all this work? Well, if flying cardboard rockets can determine an optimal delay for motor deployment if not using electronics and secondly it helps avoid zippering because of
a low velocity deployment. I disciplined myself to do this for various rockets I use a magnetic anomaly detection unit for apogee deployment. The angle varies for a north-south or east-west deployments and by necessity I want to try to get the rocket to
stop, flop over and deploy. Yeah, that's the ideal but highly unlikely. Trying to achieve it and get as close to it as possible really helps. I've had 14 flights on MAD apogee deploy cardboard rockets and there isn't a single notch, rip, tear, or even hint of
a zipper in the ends of the cardboard sustainers. If the wind is such that one is angling upwind to get the rocket to land onsite, one shouldn't be flying the bird that high. Besides too much angle and weathercocking in an overstable rocket will lead to
a high speed deployment and zipper. How do I know? Been there, done that many years ago and learned my lesson. Well................ Only time I will bend that rule is if the angle is towards the flightline. Big no, no. In that case I point it one degree upwind (into the wind) and use a shorter delay or I fly a fiberglass rocket and put the cardboard ones away. Kurt
 

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