Vent hole size

I am building a 4" Dia IRIS to use as my level 2 Rocket. The only thing that I am somewhat undecided on is the diameter of the vent hole. I was thinking that one 1/4" hole should be fine for my flights. Anyone have any other thoughts on this subject?

Don't put the vent hole directly below the nosecone. Make sure its a couple calibers below the cone. You don't want the air presure changes around the cone to fool the altimeters.

For altimeter vent holes, use odd numbers - like one or three. I prefer three myself. The formula for vent holes is H=D*L*.006. Where H = diameter of the hole, D = diameter of the body tube, and L = the length of the altimeter compartment. The formula gives you the diameter for a single hole. If you are going to use 3 holes, then divide the area of the single hole by three and make each hole that size. It is OK to have the holes a bit oversize. Using multiple holes will minimize the pressure variations due to wind currents perpendicular to the rocket's direction of travel.

HTH
- Ken

I agree with Ken, I use three vent holes on all my rockets with Altimeters.

If the vent holes are to equalize pressures to prevent the nose from blowing early, make sure they are as far up each seperating compartment as possible without being blocked by a shoulder or coupler.

Thanks for the info guys. That helps me out.