Altimeter Vent Hole Location

Hi,
Just started laying out my fiberglass 4" DX3 for drill holes (launch lugs, vent holes)

Where is the proper location for the vent holes in the payload bay?

I know to stay away from other objects to facilitate clean airflow, but there seems to be several schools of thought on the placement on the airframe.

I usually place them back about 5-6" aft of the nose cone/payload airframe seam.

I have heard of putting them just aft of the nose cone, about 1-2" aft of the nose cone/payload airframe seam.

I have also heard of putting in the middle of the payload space. For instance, if the payload volume is say 14" in length, then put the vent holes at 7" of this volume length.

What is the correct method?

Any help would be GREATLY appreciated!

Brian

Read up in your manual for your altimeter. As I recall, Perfectflite recommended staying away from transitions and nosecones by one body tube diameter as a rule of thumb. 4" rocket = sample holes need to be 4" away from the base of the nose cone longitudinally. BUT, I know folks have had great success with alt bays in their cones so there's that.

I just put the sample holes right smack dab in the middle of the ebay for simplicity. I don't think there really is a right or wrong answer here just so long as the holes are big enough to allow for free airflow to the alts with consideration to the size of the ebay. Also, I would recommend putting in 3 holes evenly spaced around the rocket diameter just to make sure a cross breeze cannot go straight through. And that's one of those things I am not sure is entirely but I just do it cause its a simple precaution to execute.

For a neater appearence I like to aliiiign my vent holes with my fins, 3 fin gets 3 holes, 4 fin gets 4 holes. Sized accordingly. 1" switch band, holes are centered on the 1/2". I like to make sure thy arrrre drilled so as either the main nor the drogue will block the vent holes.

Many people have screw switch access holes around the avbay switch band that can also double as pressure vents. All my dual deployment avbays have this arrangement and I have never had an issue. It is often recommended that an odd number of holes is used.

It almost sounds like he’s not asking about vent holes for the altimeter pressure sensor. I think he’s asking about vent holes to equalize pressure in his payload bay/parachute compartment.

If that’s so, placement is not critical.

If you mean the vent holes for your altimeter, those need to go in the av-bay, usually a caliber or more away from obstructions or protuberances like rail guides. More importantly, keep the hole edges clean, not ragged.

For venting airframes, it doesn't matter.

Frankly, the need for these airframe vent holes is overstated. Most rockets have enough leakage and not enough pressure differential to separate themselves. Hell, Jolly Logic Chute Release doesn't even mention needing vent holes for its altimeter!

Anywhere works fine, try not to cover it up with the laundry if possible. And like Buckeye said, there's probably enough leakage in your airframe anyway... although if you're flying with a "whomp" motor (VMax, ST, etc.) you definitely need a vent hole.

Brian-Tampa said:

Hi,
Just started laying out my fiberglass 4" DX3 for drill holes (launch lugs, vent holes)

Where is the proper location for the vent holes in the payload bay?

I know to stay away from other objects to facilitate clean airflow, but there seems to be several schools of thought on the placement on the airframe.

I usually place them back about 5-6" aft of the nose cone/payload airframe seam.

I have heard of putting them just aft of the nose cone, about 1-2" aft of the nose cone/payload airframe seam.

I have also heard of putting in the middle of the payload space. For instance, if the payload volume is say 14" in length, then put the vent holes at 7" of this volume length.

What is the correct method?

Any help would be GREATLY appreciated!

Brian

Click to expand...

I just received my Altimeter Two, and was wondering the same thing. I know how big (and how many) the holes should be for the area they need to vent, but none of the instructions mention where to place the holes, other than to state the holes should be at least 2" below where the nose cone attaches, but I think that recommendation is just so that the hole will clear the shoulder of the nose cone - not because it's optimal for equalizing the pressure.

I have decided that I'm going to do what looks best to me. For my LOC Lil Diter I think it looks best in the middle of the payload section. For my LOC Wiesel and my LOC EZI-65, I thought they looked better 1/2" above the bottom of the body tube (not the coupler) for the pay load section. With my Wiesel I also had to clear the decals I have, so near the bottom of the section was the best choice. All but the EZI-were retro-fit. I have a LOC Starburst yet to drill vent holes for and I haven't decided where I want to place them. I think in most cases the middle of the payload section looks too prominent, so I'll probably go 1/2" up from the bottom of the payload section on the Starburst.

Buckeye said:

For venting airframes, it doesn't matter.

Frankly, the need for these airframe vent holes is overstated. Most rockets have enough leakage and not enough pressure differential to separate themselves. Hell, Jolly Logic Chute Release doesn't even mention needing vent holes for its altimeter!

Click to expand...

IIRC JLCR has accelerometer data internally

joe7 said:

IIRC JLCR has accelerometer data internally

Click to expand...

JLCR uses a pressure sensor.

Those two possibilities are not mutually exclusive.

The JLCR only cares about the pressure when it's turned on, when it gets above that at apogee, and then when it gets to the release altitude. All of those things happen outside the airframe.

QFactor said:

JLCR uses a pressure sensor.

Click to expand...

Which it doesn't need until it's spit out of the airframe at apogee. It's turned on outside the air frame, shoved into the air frame, then spit out of the airframe where it then measures pressure until it gets to the desired altitude for release. It doesn't need a vent hole in order to detect apogee.

Vent holes...
I normally hide them either in my launch lugs or in line with them. They just have to bleed enough air to not come apart, which is not a tall order. Best on cardboard to drill, wick in CA, and re-drill and sand.

For an electronics bay, I typically have at least one 3/8 hole for accessing switch screws, and on bigger ones (6-10" rockets), the hole(s) might be a little larger, and likely at least two holes.

loopy said:

Which it doesn't need until it's spit out of the airframe at apogee. It's turned on outside the air frame, shoved into the air frame, then spit out of the airframe where it then measures pressure until it gets to the desired altitude for release. It doesn't need a vent hole in order to detect apogee.

Click to expand...

Correct.

The only other advice I'd pass along in addition to the good advice above already given is that it's hard to OVER vent, and venting guidelines (such as vernk's very thorough page here) should be taken to be MINIMUM sizes needed to avoid overly slow pressure equalization.

Also, remember that the size of ports is related to how much air needs to escape (how "full" the section is) and how rapidly it needs to do so (related to thrustiness of motor and diameter of the rocket). So if are in a Nike drag race, you'll need big holes to let that air get out quickly enough to keep the pressure equalized continually on the way up.

Vern Knowles placed a great article on altimeter port size and number years ago on his website. It’s still a valuable resource to many flyers.

https://www.vernk.com/AltimeterPortSizing.htm