We recommend a circumferential vent hole pattern to counteract any side-to-side unbalanced pressure waves (e.g. a breeze). You don't want a breeze to ram air into your vent, or "coke bottle" the pressure out. Adding additional holes prevents those imbalances.
In a payload section, the more holes the merrier. No sense not venting the bejeezus—if you believe in bejeezus—from a payload section.
If you are a fuselage flyer, without vent holes you'll probably get a fairly good apogee reading, unless the delay is really late. Most fuselages aren't that airtight, but some really are, and those are the cases we see the most false-triggering of both altimeters and Chute Releases as the nose cone or top section is forced on or off at the pad. If you've got a rocket that feels springy when you put the nosecone on, it's possible to false trigger Chute Release on the ground. Not super easy, but possible. Add vents in that case.
Without venting, you're also much more prone to weird pressure artifacts. Effects like "pistoning" where during high boost the inertia of the internal air in the altimeter section presses downward, which can lead to weird bumps in your altimeter data. These manifest as an odd jump upward (top of the bay) or downward (bottom of the bay) in the altitude graph during the boost phase. I have seen graphs where the altitude goes negative at launch, as well as ones with a weird upward hump in that same location. And when you don't see those you always you can observe "pressure lag" where the altitude graph only sluggishly syncs with the acceleration graph.
The best reference for venting I think is by Vern Knowles
here.
And I have heard that there can be induced base drag from the rear of the rocket that tries to suck the air out of the fuselage during high speed, though I have not observed that one in modern rockets myself. That's something Tommy Billings of Adept seems to focus on in his reference guide (link
here) for venting, despite the fact that he warns against "too big" vent holes (I don't think that's ever an issue unless they are ginormous). I feel he over-hypes the aforementioned base drag (most rockets are well sealed at the rear), and his disdain for altimeters in the fuselage (hey, the vast majority of rockets don't have a payload section; let's not let the perfect be the enemy of the good).
If you're still reading, one little physics thing that should be appreciated (in the same spirit that we appreciate center of pressure, center of mass, and stability) is that vent holes should be sized based on how much air is inside the section, and how fast it needs to escape. If you have a very tightly packed compartment with little air, you don't need as much venting. Bigger sections with lots of air need larger vents. Rockets that "jump up" to high altitude need larger vents than slow and low rockets. It's all a question of how much + how fast.
Not that anybody on this forum cares, but these effects are very pronounced in water rockets, since they accelerate much more violently than most of the rockets the folks on this forum fly.
UPDATE: in the first version of this I co-mingled Vern and Tommy's guides. Fixed.
