Launch Rod 'Pointing'

[Milo]

It was mentioned in another thread about 'pointing' a launch rod into the wind. I'm just curious if this is helpful when flying on a windy day in a 'short' field.

What's the going opinion on this?

As you may know a rocket is going to turn into the wind when under power from the motor. I assume this is because the fins are catching the wind and moving the bottom of the rocket out forcing it into the wind (I could be wrong here). Because of this natural weathercocking, does pointing the rod into the wind help that much or is it better to launch as near vertical as possible?

(Do rockets with front fins like an AMRAAM weathercock?)

 

[KermieD]

I think you described the weathercocking mechanism accurately, and from what I know (which isn't much), a rocket is going to weathercock in direct proportion to it's stability. I think as well that the thrust/weight ratio is going to affect that. My personal experience is that a fast moving rocket doesn't weathercock as bad as a slow moving one. Also, I think the front fins such as on the AMRAAM should help offset the effect.

That being said, I think the pointing into the wind is a balancing act. You obviously don't want to point the rocket so far into the wind that weathercocking is going to send the rocket horizontal (happened to me on my Initiator's maiden flight). On the other hand, how much do you like to walk? A rocket weathercocking isn't going to completely compensate for the wind, and if you point it straight up, you're still going to have a hike downwind on a windy day.

I personally point my kid's smaller Estes stuff into the wind to save the walk, but my MPR (and soon to be HPR) stuff gets pointed more straight up. I figger there's only so much damage a 4 oz model can do even if it is going in ballistic, but a 1 lb rocket going down in a clean aerodynamic configuration has the potential to get ugly.

I'm sure some of the others have more knowledge of this than I do and will probably have better thoughts, but I got here first...neener neener boo boo!

 

[rstaff3]

Here's what I seem to remember....

The standard Barrowman CP/CG comutations we use assume a zero angle of attack (going straight up) and with no wind. A sideways wind vector will change the angle of attack, which has the effect of moving the CP forward. This change in stability can cause the rocket to crash, go 'cruise-missile', or turn slightly into the wind (the norm). Acceleration is certainly a factor, as the fins work better at higher speeds.

So, the answer on the launch rod angle depends on the characteristics of the rocket. In the general case, this will probably be fine for both lpr and hpr rockets (ie those that have enough static stabilty margin and speed), and will save your walking.

If you do this, remember to consider engine delay. A delay that is perfect for a vertical launch is too long for a large weather cock.

 

[KermieD]

...someone would have it down better. Thanks for the info Dick. I hadn't even considered wind moving the CP and affecting it from a stability point of view.

On a little side note, this is such a neat group in that everyone here seems to have a field of expertise and has something unique to contribute. I've gotten some great finishing tips from Carl, Dick always seems to have the physics of flight down, Sandman has been a great help with construction techniques, Eugene with his video, and, of course Phil making the sacrifices to the rocket gods so that the rest of us don't have to.

I'm really glad I found this forum. Thanks for letting me be a part!

 

[rstaff3]

Thanks for the compliment, KermieD. Glad to help out. I wish I remembered/knew more in this area. The details of my explanation are tied up in factors such as moments of inertia, damping, vectors, etc. Without a detailed description in front of me, all I can muster is some general hand waving, and my personal experience. I often move the launch rod angle into the wind for my more conventional mid-powered rockets. For odd designs like my Fireball, I'm less apt to.


This is indeed a great forum!