SAMPLING PORT QUESTION

I'm building a 7.5" avionics bay for my LOC Doorknob dual deploy mod - largest one I've built so far. I'm reading the PerfectFlite manual and it says to use four .5" holes. On my 5.5" avionics bays, I use four 9/32 holes.

Guess I'm having a hard time getting my head around four .5" holes - that is the size I drill to install rotary switches.

Before I start drilling this thing, does anyone have any input?? Seriously?? Four .5" holes??? A full 1/2" drill bit??? That just seems HUGE!!!! I realize its a huge avbay, but the holes are just so much bigger than the ones on my 5.5" bays.

I don't have my computer with me so I don't know the reference and / or source of the spreadsheet I use, but for sure the hole area goes down proportionally as overall volume goes up, so you are right to be suspect.

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

Does this help?

BLKKROW said:

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

Does this help?

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Well..... Yes and no.

The table on that link only calculated the port size for a 7.5" bay up to 9" long. Mine is 12" long. For the 9" long, it recommended four 1/4" ports. Which is half of what the PerfectFlite recommends. Hard to believe that an additional 3" length in av-bay requires doubling of the hold size??? Now I'm more confused. :-(

neond7 said:

Well..... Yes and no.

The table on that link only calculated the port size for a 7.5" bay up to 9" long. Mine is 12" long. For the 9" long, it recommended four 1/4" ports. Which is half of what the PerfectFlite recommends. Hard to believe that an additional 3" length in av-bay requires doubling of the hold size??? Now I'm more confused. :-(

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If you look at the second table. This is using 4 holes.

It states Body tube up to 7.512 can use 1/4" holes in a length up to 9.03 inches.

BLKKROW said:

If you look at the second table. This is using 4 holes.

It states Body tube up to 7.512 can use 1/4" holes in a length up to 9.03 inches.

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My avionics bay is 12" long. That is why I was confused, as a 3" extra av-bay tube length requires doubling the port size (according to the PerfectFlite manual). I just wanted to get this nailed down before I drilled. I can always make the ports bigger..... much harder (and messier) to make them smaller. ;-)

neond7 said:

My avionics bay is 12" long. That is why I was confused, as a 3" extra av-bay tube length requires doubling the port size (according to the PerfectFlite manual). I just wanted to get this nailed down before I drilled. I can always make the ports bigger..... much harder (and messier) to make them smaller. ;-)

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Oh I am sorry, I was reading your thread while doing physics homework and missed the details.

Your AV bay has about 530 cubic inches of volume. On the link I posted it states "A good rule of thumb for the static pressure port is to use a 1/4" diameter hole for every 100 cubic inches in the altimeter bay compartment that is being vented."

With this logic you could use 5 holes of 1/4", that would be theoretically close enough.

Rocket Calculator for iPhone says four 0.29" ports. I'd say that is fairly close although you'd probably be OK going smaller.

dixontj93060 said:

Rocket Calculator for iPhone says four 0.29" ports. I'd say that is fairly close although you'd probably be OK going smaller.

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Excellent, thank you. I'm considering four 1/4" holes - everything I've heard always say its better to be a bit on the small size than too big.

On the other hand, using the formula for four ports, where each hole = diameter x diameter x length x .0008 does come out to .54.

Unless I'm really stupid, rounding down .54" to .5" equals a 1/2" drill bit, correct??

neond7 said:

Excellent, thank you. I'm considering four 1/4" holes - everything I've heard always say its better to be a bit on the small size than too big.

On the other hand, using the formula for four ports, where each hole = diameter x diameter x length x .0008 does come out to .54.

Unless I'm really stupid, rounding down .54" to .5" equals a 1/2" drill bit, correct??

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Again, you have to quit going back to the Perfectflite formula. It is too simplistic and doesn't apply properly to larger airframes.

dixontj93060 said:

Again, you have to quit going back to the Perfectflite formula. It is too simplistic and doesn't apply properly to larger airframes.

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Ah.... ok, now I understand, the formula doesn't scale up well. So a 1/4 inch hole every 100 cubic inches is correct, according to the Vernk.com site. So a 1/4 hole is just a tad too small since I'm using a 12" long bay.... but once again, better too small than too large.

Thank you everyone! I'll consider this a done deal.

neond7 said:

Ah.... ok, now I understand, the formula doesn't scale up well. So a 1/4 inch hole every 100 cubic inches is correct, according to the Vernk.com site. So a 1/4 hole is just a tad too small since I'm using a 12" long bay.... but once again, better too small than too large.

Thank you everyone! I'll consider this a done deal.

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Also I did my volume measurement with an ID of 7.5 inch. The coupler will have a different ID so less volume, 1/4" vent hole would make sense to me. I would just make sure the altimeter(s) are as close to the vent holes as possible.