Weathercocking basics?

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billdz

billdz

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On a windy day, many folks aim their rockets into the wind, on the theory that, once the chute opens, the wind will blow it back towards the pad and reduce the length of the recovery walk. I've read in this forum that this is not necessarily a good idea, because aiming into the wind promotes weathercocking. Some have even suggested aiming the rocket with the wind, for a straighter flight.

Yesterday's launch seemed to confirm this. It was the windiest day on I which I have flown. Most people aimed 10 degrees or so into the wind, and several flights weathercocked in the direction of the launch (opposite the direction of the wind). A few aimed with the wind, which seemed to produce straighter flights (but much longer walks).

This got me wondering how the weathercocking process works, as it seems counter-intuitive. If a rocket has any tendency to bend over, one would think that a flight in the direction of the wind would increase the bend, while against the wind the breeze would tend to keep the rocket straighter. But it appears to be just the opposite. What's the science behind this?
 
The rocket fins (with a proper stable CP/CG) will always try to fly straight into the air that it is flying in.

If there is no wind, the only air movement is due to the rocket's flight and it will go straight up.

If there is a wind, the rocket has the air moving over it combined with the wind so it will try to fly into the wind.

Let's assume there is a constant wind (even though it may change at different altitudes or with variable gusts)

If you aim into the wind, the rocket will weathercock into the wind causing it to fly at an angle. You will lose altitude but when the chute pops the wind will bring it back.

If you aim with the wind, the rocket will still weathercock into the wind, but since you aimed it with the wind it will cause it more to fly straight up. You will get a higher altitude, but a much longer walk to get it back

See next post for a crude pix

wind.jpg
 
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To put it another way, aiming upwind does not, as you wrote "promote weathercocking." Weathercocking will happen no matter how the rocket is aimed; it is, as Les described, the phenominon of turning into the wind that is an inevitable consequence of stabe flight. So you have the choice whether to aim upwind, which adds to the weathercocking, or aim downwind, which subtracts from it.
 
This really speaks to understanding basic aspects of rocket design. The CG is where the mass balances, so this is effectively the point about which the rocket will pivot. The CP is where the aerodynamic loads balance around the vehicle. The rocket is designed such that the aerodynamic load center is safely aft of the CG so that load pulls the airframe straight in relation to the vehicle's intended path relative to the medium it is moving through.

If the CG & CP where in the same spot, the cross wind would (theoretically) not influence the direction of travel, because the aero load is balanced about the pivot point. But the rocket would also not travel in any sort of predictable direction, and would likely tumble across the sky. By extension, when rockets are over-stable (the CG & CP have long separations, above 2.5:1 or so), the tendency to weathercock becomes more pronounced.

Another way to think of it is, in its simplest form, the rocket is a weather vane (there's a reason the condition is called "Weathercocking". The weather vane's mounting stick is at the effective CG. The large vanes on one end move the CP away from the CG, and when the wind blows over it, it pulls the pointer into the wind. The same exact phenomenon is what happens to a rocket in a cross wind.
 
Bear in mind this "pointing" rockets on the pad does not work in all cases.
The design has much to do with the reaction.
Compact designs vs standard longer.
How hard you hit "it" with a high or low thrust motor, makes the largest difference. [for me]

If flying in wind over 10-12, I use thrust ratio of at least 10-1.
15-20 winds , I use 20-1 or more. This keeps them flying pretty darn straight.:wink:
 
As far as why some rockets were veering in the opposite direction one would expect (away from the wind rather than into it), could be some other phenomena was at work here. Many times surface winds do not coincide with wind directions at altitude. The design of the rocket could be a factor. Were they marginally stable? Some rocket designs, particularly those with large mid fins like the Sparrow missile, or winged rockets also tend to weathercock more.
On windy days, I would continue to incline my rods to cut recovery drift. Also would consider reefing my shroud lines (tying or taping lines to shorten them and prevent chute from fully opening), or adding dual deploy or a JLCR if your rocket is big enough. Of course, if it's too windy, best not to fly at all.
 
I always liked this drawing in the Estes Technical Report No. 1 to illustrate the rocket as weathervane metaphor:

EstesTR1.png

https://www.ninfinger.org/rockets/EstesTR1.pdf


Taking advantage of weathercocking tendencies in order to shorten the recovery walk is certainly a time-honored technique. What you want to watch out for is the odd flight going full-on cruise missile mode and leaving the field. I like CJ's method of increasing the thrust-to-weight ratio as the wind conditions change.
 
blackjack2564 said:
Bear in mind this "pointing" rockets on the pad does not work in all cases.
The design has much to do with the reaction.
Compact designs vs standard longer.
How hard you hit "it" with a high or low thrust motor, makes the largest difference. [for me]

If flying in wind over 10-12, I use thrust ratio of at least 10-1.
15-20 winds , I use 20-1 or more. This keeps them flying pretty darn straight.:wink:
Click to expand...

Jim, I agree 100%. Spank the rocket and they tend to go straighter.


Sent from my iPhone using Rocketry Forum
 
Here's another way to look at it.

When the wind blows at your rocket, which part of it "catches" the wind? The widest part, i.e., the fins. Wind blows against the fins, pushing the fins away from the wind -- which tips the nose cone into the wind. Rocket flies into the wind. Simple!

weathercock.png
 
samb said:
What you want to watch out for is the odd flight going full-on cruise missile mode and leaving the field.
Click to expand...

If your rocket is too stable and/or aimed too much into the wind, you could end up with a high-speed recovery deployment along your ballistic arc that will tear the shroud lines out of your parachute (especially kits with plastic parachutes). Your rocket may also make it to the ground with the delay still burning and that, or the ejection charge, could start a fire in the field you are using or, worse, the field next to the one you are using where the owner had no expectation of peril.

Use a good thrust-to-weight ratio, fly straight up or as a slight angle and take the long walk.
 
If the surface area of fins is small & airframe large, it can just blow sideways, while pointing vertical.
 
KennB said:
1. If your rocket is too stable and/or aimed too much into the wind, you could end up with a high-speed recovery deployment along your ballistic arc that will tear the shroud lines out of your parachute (especially kits with plastic parachutes).

2. Your rocket may also make it to the ground with the delay still burning and that, or the ejection charge, could start a fire in the field you are using or, worse, the field next to the one you are using where the owner had no expectation of peril.

3. Use a good thrust-to-weight ratio, fly straight up or as a slight angle and take the long walk.
Click to expand...

Not sure I follow point 1. Why would weather cocking affect timing or velocity at deployment?

2. Makes sense. I have seen rockets get up off the pad and turn 90 degrees off vertical so straight into wind, parallel to the ground, then after thrust burnout arc down during delay, deploying near or at ground level.

3. Makes sense as well.
 
BABAR said:
Not sure I follow point 1. Why would weather cocking affect timing or velocity at deployment?

2. Makes sense. I have seen rockets get up off the pad and turn 90 degrees off vertical so straight into wind, parallel to the ground, then after thrust burnout arc down during delay, deploying near or at ground level.

3. Makes sense as well.
Click to expand...

The first two points, as you've parsed my post, refer to the statement I quoted from samb. Ideally, you've matched your motor and delay to the rocket such that the ejection charge will go off right at or near apogee with the rocket at a very low velocity. When your rocket flies off at a large angle from vertical, gravity hasn't decelerated it very much. Weathercocking doesn't impact the the timing of the engine/delay burn, it just puts your rocket in a bad situation as far as speed goes.
 
BABAR said:
Not sure I follow point 1. Why would weather cocking affect timing or velocity at deployment?

2. Makes sense. I have seen rockets get up off the pad and turn 90 degrees off vertical so straight into wind, parallel to the ground, then after thrust burnout arc down during delay, deploying near or at ground level.

3. Makes sense as well.
Click to expand...

About point number 1, when the rocket is going perfectly straight up, gravity slows it to a dead stop at apogee. If the rocket is going sideways, then its speed through the air is only partially going up and partially going to the side — it’s not traveling up as fast at burnout, so it will reach apogee sooner and start falling and picking up speed sooner. Also, when it is traveling sideways, it doesn’t come to a dead stop at apogee — it stops going up, but it continues traveling sideways. Add together the speed traveling sideways and the speed of falling after an earlier-than-expected apogee, and it can be traveling really fast at deployment.
 
Good answer Thirsty, thanks. :) And thank you Kenn for listing some of the negative outcomes that can occur when a flight goes "full-on cruise missile mode".
 
KennB said:
Your rocket may also make it to the ground with the delay still burning and that, or the ejection charge, could start a fire in the field you are using or, worse, the field next to the one you are using where the owner had no expectation of peril.
Click to expand...
A bit off topic (I was unstable rather than overstable) but I had one come down in a tiz and hit the sod nose in with the delay very nearly done burning. A second or less later the ejection charge popped everything except the nose a few inches back up. And I know because it came down right next to us. I've never has so strong a laugh-and-cry-at-the-same-time experience before or since. No one was hurt, and there was no fire.
 
jqavins said:
A bit off topic (I was unstable rather than overstable) but I had one come down in a tiz and hit the sod nose in with the delay very nearly done burning. A second or less later the ejection charge popped everything except the nose a few inches back up. And I know because it came down right next to us. I've never has so strong a laugh-and-cry-at-the-same-time experience before or since. No one was hurt, and there was no fire.
Click to expand...

We get a few of those every year. Also amusing is when the rocket is underthrusted or there is gook on the launch rod, and the rocket stays on the pad burning propellant until the delay burns through. Then pop goes the nose cone!
 
https://www.houzz.com/product/33537...MIge-RpeuR2QIViuNkCh3lkgIREAYYASABEgL9WPD_BwE

Understand the name and where it comes from. many weathervanes have a rooster on them.. Then understand how a weathervane works. A rocket will turn just like a weathervane.


billdz said:
On a windy day, many folks aim their rockets into the wind, on the theory that, once the chute opens, the wind will blow it back towards the pad and reduce the length of the recovery walk. I've read in this forum that this is not necessarily a good idea, because aiming into the wind promotes weathercocking. Some have even suggested aiming the rocket with the wind, for a straighter flight.

Yesterday's launch seemed to confirm this. It was the windiest day on I which I have flown. Most people aimed 10 degrees or so into the wind, and several flights weathercocked in the direction of the launch (opposite the direction of the wind). A few aimed with the wind, which seemed to produce straighter flights (but much longer walks).

This got me wondering how the weathercocking process works, as it seems counter-intuitive. If a rocket has any tendency to bend over, one would think that a flight in the direction of the wind would increase the bend, while against the wind the breeze would tend to keep the rocket straighter. But it appears to be just the opposite. What's the science behind this?
Click to expand...
 
There are some oversimplifications in this thread, IMHO.

Sequence of events:

1) Rocket stationary on pad, in a breeze. Fins are small wings, and they are stalled.
2) Motor lights; rocket travels up rail.
3) Only bottom lug is attached to rail. Rocket is free to pivot about this lug to some extent, up to the point where it either binds or gets ripped off or otherwise damaged. Wind blows on rocket but fins do NOTHING to stabilize at this point. Rocket tilts in the direction of the prevailing wind. Also note that the rail guide causes drag. The drag is off center of the thrust, causing the rocket to tilt a bit towards the rail. The final pointing direction is determined by wind, off-center thrust to drag, and rail whipping. But note the rocket leaving the rail has a combination of linear velocity, angular velocity about the center of mass.
4) Rocket clears the bottom lug. If the velocity is sufficient compared to the wind speed, such that the fins are no longer stalled, then the fins provide a strong restoring force to align the rocket with the freestream. If not, then the fins provide a weak restoring force... It is quite possible that the CP has shifted forwards sufficiently to put it in front of the CG even. That is, the rocket could continue to tend to rotate in the prevailing wind direction even after it leaves the rail, if it is not going fast enough.
5) Rocket is fast enough to be stable, and commences oscillating about the freestream. If the rocket is sufficiently stable, the oscillations dampen out quickly. If the oscillations are bad enough and the rocket has an odd number of fins, then the rocket could go into precession. If the precession is bad enough, it may be stable in that configuration.

Gerald
 
G_T said:
There are some oversimplifications in this thread, IMHO.

Sequence of events:

1) Rocket stationary on pad, in a breeze. Fins are small wings, and they are stalled.
2) Motor lights; rocket travels up rail.
3) Only bottom lug is attached to rail. Rocket is free to pivot about this lug to some extent, up to the point where it either binds or gets ripped off or otherwise damaged. Wind blows on rocket but fins do NOTHING to stabilize at this point. Rocket tilts in the direction of the prevailing wind. Also note that the rail guide causes drag. The drag is off center of the thrust, causing the rocket to tilt a bit towards the rail. The final pointing direction is determined by wind, off-center thrust to drag, and rail whipping. But note the rocket leaving the rail has a combination of linear velocity, angular velocity about the center of mass.
4) Rocket clears the bottom lug. If the velocity is sufficient compared to the wind speed, such that the fins are no longer stalled, then the fins provide a strong restoring force to align the rocket with the freestream. If not, then the fins provide a weak restoring force... It is quite possible that the CP has shifted forwards sufficiently to put it in front of the CG even. That is, the rocket could continue to tend to rotate in the prevailing wind direction even after it leaves the rail, if it is not going fast enough.
5) Rocket is fast enough to be stable, and commences oscillating about the freestream. If the rocket is sufficiently stable, the oscillations dampen out quickly. If the oscillations are bad enough and the rocket has an odd number of fins, then the rocket could go into precession. If the precession is bad enough, it may be stable in that configuration.

Gerald
Click to expand...
That doesn't quite fit what actually happens- 99.9% of the time rockets go INTO the wind.

Sent from my LGL44VL using Tapatalk
 
That's part of #5, oscillating about the freestream. The rocket rotates windward, but due to momentum overshoots, oscillates back, and hopefully the oscillations are dampening down (generally they do). But with odd number fins if the oscillations are large enough that a single fin stalls, then it will go into precession and might not come out of it. That's the cork-screw flight you sometimes see. Also the greater the angle between the rocket and the freestream, the more forward the CP. Not enough velocity and it is unstable and can rotate rather badly. The angle is determined by the arctangent of the ratio of the instantaneous rocket speed and the windspeed perpendicular to the rocket. You can run some RasAero sims to see the CP shift with rocket's angle of attack.

Gerald
 
G_T said:
That's part of #5, oscillating about the freestream. The rocket rotates windward, but due to momentum overshoots, oscillates back, and hopefully the oscillations are dampening down (generally they do). But with odd number fins if the oscillations are large enough that a single fin stalls, then it will go into precession and might not come out of it. That's the cork-screw flight you sometimes see. Also the greater the angle between the rocket and the freestream, the more forward the CP. Not enough velocity and it is unstable and can rotate rather badly. The angle is determined by the arctangent of the ratio of the instantaneous rocket speed and the windspeed perpendicular to the rocket. You can run some RasAero sims to see the CP shift with rocket's angle of attack.

Gerald
Click to expand...
Sure, CG shifts with angle of attack, but the effect is also seen with rockets that are stable at 90 degrees AOA and rockets with only one launch lug.

My version of the story is this:

Rocket leaves rod vertically but slowly. This gives it a bit of angle of attack, so it points upwind- into its airspeed axis, as it should. But now it's accelerating upwind, so the combination of wind and gravity reducing the vertical acceleration adds to the angle upwind, leading to cruise missile mode.

Sent from my LGL44VL using Tapatalk
 

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