Weathercocking

by Warren Massey, LUNAR #007

Did you know that you may be doing exactly the wrong thing when you aim your rocket into the wind? On a windy day many of us incline the launch rods into the wind so that the rocket will not drift as far downwind from the launch site. This will work fine as long as the rocket gets up to a fairly high speed while still on the launch rod. If, on the other hand, the rocket is moving fairly slowly as it comes off the launch rod then a phenomenon called weathercocking occurs. Like its namesake, the weathercock found on barn roofs, the rocket will attempt to point into the prevailing wind, which means it tries to turn horizontal and fly parallel to the ground. If you started out with the launch rod pointed into the wind and the rocket weathercocks that way even more, you can create a dangerous situation rather rapidly. Actually the fast rockets weathercock too. It's just that the force of the weather-induced wind (as opposed to the rocket-motor-induced wind) is a much smaller percentage of the total induced wind forces on the fins, resulting in a gentle arc into the direction of the wind up to apogee. For a slow rocket, the force of the weather-induced wind on the sides of the fins is a much greater percentage of the total wind force on the fins and can cause an abrupt change in direction as soon as the rocket is clear of the launch rod.

If you have a rocket that is slow to leave the launch rod, then you should either refrain from flying on windy days or you should make sure to point the launch rod vertically or even point it down wind slightly; the weathercock effect will turn it vertical or slightly into the wind as it leaves the launch rod. As a club, LUNAR is going to be more strict about monitoring this in the future. If you say this will make your rocket drift too far down wind then perhaps you should not fly the rocket in the wind in the first place.

Information date: September 14, 1997 lk