I found it odd that the RRC2+ user manual suggests much larger holes than other's, so does the rrc3 quick start guide, here is a cut and paste.
Vent Holes
Vent holes in avionics bay allow the altimeter to sample air pressure. Here are typical some 3-hole example sizes:
A. 38 mm by 6 in. 3 x 1/16” holes
B. 54 mm by 7 in. 3 x 1/8" holes
C. 3 in. by 9 in. 3 x 5/32” holes
D. 4 in. by 12 in. 3 x 3/16” holes
E. 6 in. by 18 in. 3 x 1/4"holes
Make sure when drilling holes they are clean and clear from fuzz and debris for smooth, clear airflow. Locate your vent holes
equally spaced around the avionics bay in a single circumference. If you want more information about proper venting sizes,
refer to the RRC3 User Manual section on Static Pressure Ports
The list sizes are not linear in respect to the av-bay volume, which is OK, but I would like to know why and how those sizes were arrived at.
Well......................... I wrote the quick-start guide & the "other Jim" Amos wrote the manual.
How & Why...
I did almost 2 years of testing comprised of 38 fights of the RRC3 & 22 of the RRC2+.
Add into that over 200 more flights of all those exact size av-bays, which are the sizes that Wildman uses in his kits, & I use in scratch built rockets, many Rocketry Warehouse sizes were the same or close ay the time. Plus the fact other bays may be a bit longer or shorter. This would be a basic guide that will work for those not wanting to take the time and figure it out for themselves.
The vents holes were sized for each different bay after 5 yrs of testing. All this was done with the designer of MW altimeters [Jim Amos] on board. Things that were considered were:
Thickness of airframes.
Possibility of wires sleds, tape,rods, etc. partially blocking one or more holes constricting airflow. Internal volumes.
The partial blocking airflow if using "twist & tape" which runs the switch wire through one or two vents. [depending on use of 1 or 2 altimeters] The fact that on small bays with tiny holes many will make one hole larger so a "switch' wire can pass through. It's hard to try and figure in, every crazy thing flier's might come up with, and still make this guide cover most of it. It's never as simple as one size fits all...lol
The speed of flights depending on size, weight, and propellent used, can make evacuation of internal air fast or slow.
Most common size of bits found in "everyday" packages sold & used. Yes you can get any size if ordered or go to specialty tool shops, but most of us just have the common packages found at HD or Lowes etc.
Analyzing the graphs produced by RRC3's and"other" brands of altimeters that were flown side by side for comparison.
After all this Jim agreed the sizes chosen were close enough to his "formula" sizes that the chart would be the perfect compromise.
The fact of the matter is to get perfect port size holes, you must design them around a specific rocket going to a specific altitude range & know the humidity, temp, etc. with a specific volume in the bay, etc
Bottom line we tried to come up with something simple...reliable and works in a variety of situations & it does.
I'm sure there may be some inconsistencies in my explanations, but I am NOT a designer nor claim to be. I just tried to explain things in laymen's terms. For more exacting theory on how it was done , just call Missleworks and ask, but this is the gist of it.:grin: