Altimeter holes

Discussion in 'Rocketry Electronics and Software' started by JCRL, Sep 26, 2019.

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  1. Sep 26, 2019 #1

    JCRL

    JCRL

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    Is there a minimum diameter for holes in the payload bay for altimeters? I have a Perfectflite Firefly and drilled 1/16 holes into the payload section. Should I bore it out to 1/8 or larger, or will 1/16 be adequate to collect reliable data from the Firefly?
    IMG_20190825_171141.jpg
     
  2. Sep 26, 2019 #2

    mbeels

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    How many holes do you have? 1/16" will be adequate for a very small volume like that.
     
  3. Sep 26, 2019 #3

    JCRL

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    Two holes; one on each "side".
     
  4. Sep 26, 2019 #4

    timbucktoo

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  5. Sep 26, 2019 #5

    JCRL

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    oh man, thanks Timbucktoo! I don't know why it didn't occur me to check for instructions (doh!)
     
  6. Sep 27, 2019 #6

    BEC

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    In a compartment that size, four pinholes would have been enough. So you should be fine. Basically you want the holes to be big enough to have the pressure inside the compartment and outside to equalize in a short time (less than a second) at apogee.

    FYI - some instructions for altimeters specifically say NOT to use two holes. I'm not entirely sure what is behind that recommendation.

    Happy data collecting!
     
  7. Sep 27, 2019 #7

    cerving

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    Eggtimer Rocketry has always recommended three holes, based on the idea that opposing holes increases the possibility of turbulence/crossflow inside the AV bay, which may lead to transient pressure errors. There's a pretty good (albeit technical) paper that Gary Stroick (from Off We Go Rocketry) did on this awhile ago, you can find it at http://www.offwegorocketry.com/userfiles/file/Documents/Static Port Holes.pdf
     
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  8. Sep 27, 2019 #8

    BEC

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    Thanks, Cris. I knew it was something like that but somehow couldn't articulate that yesterday. And thanks for the link to the paper. I didn't delve into the math but followed the discussion and the conclusions. Interesting stuff.

    It's too bad that he doesn't delve into why the number of ports but recommends three or four and says "Conventional wisdom is that multiple evenly spaced static port holes will reduce the amount of turbulence experienced inside of an AV Bay." in the paper.

    But that will certainly do in this case.

    It is Adept's instructions that, in boldface, said "never use two" static ports. They, too, recommended three or four evenly spaced ports. Their sizing recommendation is the "rule of thumb" cited in the paper you linked to that the author went on to disagree with via his analysis.

    Interestingly, Adrel's instructions say to use two or three holes.....
     
    Last edited: Sep 27, 2019
  9. Sep 28, 2019 #9

    gtg738w

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    If you’re just looking for apogee, almost any hole size is fine. At the top of the flight you are usually so slow that any vent will give you an accurate reading. If you’re looking for velocity data from the pressure profile it gets more interesting. Holes that are too small will introduce a time delay and you will under predict the max speed. Holes that are too big will introduce turbulence and odd spikes in the data. Properly sized vents aligned with the altimeter in the middle of the bay will give the best readings. 1/16 is a good place to start for something that size.

    Russ
     
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  10. Sep 28, 2019 #10

    Steve Shannon

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    I’d be careful about aligning the sensor on the altimeter with one of the ports. You want the pressure of the entire chamber to be about the same but more importantly you do not ever want airflow through the port to directly strike the baro sensor. When that does happen you see things like apogee charges going off in wind gusts while waiting on the pad or false apogee detection during flight. You want the baro sensor out of the airflow.
     
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  11. Sep 28, 2019 #11

    gtg738w

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    True - I meant “aligned” as at the same axial station & centered in the bay, not pointed at the sensor. Good catch.
     
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  12. Sep 28, 2019 #12

    BEC

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    I did an R&D for NARAM-61 that looked at port sizing indirectly, as I was trying to show that some venting is necessary for good apogee readings and that if you don't you're cheating yourself of some "credit" for altitude attained. This all descended from, I believe, an awkward translation from Polish in the Adrel instructions that uses the term "hermetic" with respect to altimeter bays in competition-type model rockets. This in turn led some to say that there is no need to actually put vents in at all and two prior years' worth of R&D reports using bench testing to prove that and then show that there was nothing special about Adrel units in this respect. I just did actual flight testing version of the same experiment flying Adrels, FireFlys and MicroPeaks in multiple compartments on the same flight of the same model.

    Of course some path for the pressure to equalize at apogee is needed or the pressure in the compartment never catches up to the outside air and you cheat yourself out of some meters on your altitude score. (See my graph in Dan Wolf's altimeter article in the Jan/Feb 2019 Sport Rocketry). But I found it was almost impossible, using traditional materials, to not get a reading at all (no launch detection), and that having enough pressure leakage to get good altitude readings required only the slightest venting, really—which corroborates Russ' first two sentences in post #9. Bigger vents (in this case 3 3/32 inch vent holes in a 2 1/2 inch section of BT-50 as the altimeter bay) made no real difference.

    But when higher-than-model rocket speeds and bigger, heavier rockets that are depending on the electronics not only to report the apogee and perhaps give enough data to show altitude vs. time but to deploy the recovery system reliably, things need to be a little more carefully done. This leads to the report Cris linked to and Steve's comments just above.

    This, of course, is way beyond what the OP was looking for to see if he was going to get reasonable readings from his FireFly :)
     
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