Need confirmation of the number of holes needed for Avionics Bay

[conneryc]

So I'm working on my L3 project (L3 7.5" Little John) and I was doing the calculations for the number of 1/4" holes I need in the switch band of the Av-Bay and just wanted some other "eyes" to make sure my calculations are correct.

The equation I am using is from this site: https://vernk.com/AltimeterPortSizing.htm

Dn = Dt*sqrt((Aref/Vref)*(L/N))

Where:

Dn = Diameter of holes required
Dt = Inside Diameter of tube
L = Length of tube
N = Number of holes.

Here are my values:

Aref = 0.04909"
Vref = 100 cubic inches
Dt = 7"
L = 13"
N = 5

The equation simplifies down to:

Dn = 0.02216*7*sqrt(13/5)

Where Dn = 0.2501 or 1/4" - So making the number of 1/4" holes necessary to be 5. Correct?

Thanks

 

[kalsow]

Your calcs look OK. With an AV bay that large, I would fill half the volume with some discs of 1-2" thick pink foam. You could easily cut the number or size of the holes by a factor of 2 or more. It's your choice.

 

[JackC]

I would only go with three holes. That seems to be the number that works best. However, you should check the altimeter’s instruction manual and use the number and sizes recommended by the manufacturer.

 

[tim cubbedge]

Here’s a great article by Vern Knowles. I’ve used 3 holes on most of my av bays and on rare occasion used only 1.
https://www.vernk.com/AltimeterPortSizing.htm

 

[David Schwantz]

This is just me, but if I have 4 fins, it gets 4 holes. If I have 3 fins, it gets 3 holes. I like to line everything up with the fins, along with all my shear pin holes and my alignment holes and tabs. I think it makes everything neater and it make finding them damn tiny holes easier to find

 

[conneryc]

Here’s a great article by Vern Knowles. I’ve used 3 holes on most of my av bays and on rare occasion used only 1.
https://www.vernk.com/AltimeterPortSizing.htm

That is literally the same web site that I referenced in my original post. Based on the calculation from THAT website I came up with a total of five 1/4" holes based on an AV bay that is 13" long with a diameter of 7".

 

[tim cubbedge]

That is literally the same web site that I referenced in my original post.

Right. That’s what I get for not reading it thoroughly.

 

[Handeman]

This is just me, but I believe those calculations for determining the vent hole size are to get some arbitrary equalization speed based on the flow area through the holes and pressure differential between inside and outside. Too small of holes and the equalization will be slow and sensing apogee will probably be past actual apogee and the main deploy will be slightly lower than the set point. Too large of holes will sense apogee more accurately, along with main deploy but possibly cause other issues. The formulas are a compromise of small holes vs. fast enough equalization.

I know folks talk about possible issues with too large of holes, but I've never had any issues with large holes, or know of anyone that has, at least with any newer altimeters that don't require a mach delay jumper to be set.

I tend to use one more hole than the formulas say so I get faster equalization, or use the recommend number of holes, but about 25% to 50% larger diameters. That has been working for me for 16 years now.

My recommendation is that if the formula gives you a hole size between two drill sizes, always go with the larger drill.

YMMV

 

[pathtouch]

Aside from a bunch of math here’s a real world example from my 7.5” LOC Doorknob.

I ended up going with (4) 7/32" diameter holes, and then just used the same bit to make vents for the booster and payload bay as well.

The only caveat was on a windy day the four holes were actually whistling and simulated a 107 foot high flight before it even left the launch pad. This armed the electronic and set off my charges on the pad.

Lessons learned, keep minimum launch detect altitude settings to 200 feet and use an odd number of holes.

Other flights on this rocket up to an L1090 were all fine and altimeters had no issues.

For my 6” diameter x 16” L3 AV bay I’m going 3x5/16” diameter.

 

[pathtouch]

BTW, most people building LOC AV bays split the stiffy coupler and put a bulkhead inside so the AV bay is only 7” long. That’s how mine in the above example is configured. Also gives you more room for recovery gear.

 

[rfjustin]

Aside from a bunch of math here’s a real world example from my 7.5” LOC Doorknob.

I ended up going with (4) 7/32" diameter holes, and then just used the same bit to make vents for the booster and payload bay as well.

The only caveat was on a windy day the four holes were actually whistling and simulated a 107 foot high flight before it even left the launch pad. This armed the electronic and set off my charges on the pad.

Lessons learned, keep minimum launch detect altitude settings to 200 feet and use an odd number of holes.

Other flights on this rocket up to an L1090 were all fine and altimeters had no issues.

For my 6” diameter x 16” L3 AV bay I’m going 3x5/16” diameter.

Just sharing this as a separate data point, but I have been running a single vent hole for many years. For a big 16" long bay, a single 5/16" or 3/8" hole should be way more than enough. You are venting ground level, "thick" air out on the way up into thinner air.

 

[Art Upton]

Just sharing this as a separate data point, but I have been running a single vent hole for many years. For a big 16" long bay, a single 5/16" or 3/8" hole should be way more than enough. You are venting ground level, "thick" air out on the way up into thinner air.

Same here, since I bought my First MAWD altimeter in 2003.

 

[Art Upton]

Same here, since I bought my First MAWD altimeter in 2003.

And that was based on this in the user manual for it:

 

[John Beans]

Yep, recommended hole sizes are always "minimums" to reduce the pressure lag to an acceptable level. "Bigger is better," at least until they interfere with something else (like motor ejection if the holes are in the fuselage). A "puff test" by blowing on the motor mount can assess that pretty well.

One not-often-mentioned technique: reduce the air that needs to escape from the compartment by filling unneeded space with an incompressible filler, like a 3D printed spacer cylinder when practical. Less air that needs to escape, much smaller holes. Can double as holders for batteries.

 

[JackC]

There are many differences in the number of holes. For PML rockets, there are no holes. I suppose the air that gets in through the slide switch is enough air for the altimeter. This was true for the PML Co-pilot altimeter and the Perfectflite Stratologger that I now use for my PML altimerter bay rockets. For my other rockets, I find that 3 holes seem to work best and I have used Vern's process to calculate the size of the holes.