maybe the cutouts of the payloadbay are to big, the highspeed air flowing along the cutouts could cause a low pressure in the payloadbay.
You can see similar peaks in your graph when the rocket is tumbling down, sometimes the wind will blow directly into the cutouts causing a high pressure in the payloadbay and somtimes wind will flow over the cutouts causing a lower pressure in the payloadbay.
I agree with arjan that it's probably your pressure sampling "holes". If I understand correctly, your only holes to the outside are at the bottom of the fairing between the payload bottle and the pressure bottle? That looks like a spot that would be subject to a lot of turbulence to me. I would cover up the holes in the neck of the payload bottle and drill some small holes in the side. It might not be the cause of your entire problem, but I think it's the most likely cause and it's easy enough to try out.
From the paint, I take it that your altimeter is not exposed to direct sunlight? I found out through trial and error that the pressure sensors are really sensitive to direct sunlight, but it shouldn't cause anything like the shape of your graph.
Thanks, I'll try drilling a hole in the side . I was wondering if that configuration might create a slight vacuum on launch and that appears to be the case- providing a false early high that corrected as it slowed down?
You need to have the location of the air holes for the altimeter in an area of the rocket body with straight sides, and one of more rocket diameter away from any body transition or seam. Use one or more evenly spaced air holes at the same place around the circumference. Always make sure these are the only air holes that can reach the altimeter.
U.S. Water Rockets1 wrote:You need to have the location of the air holes for the altimeter in an area of the rocket body with straight sides, and one of more rocket diameter away from any body transition or seam. Use one or more evenly spaced air holes at the same place around the circumference. Always make sure these are the only air holes that can reach the altimeter.
Thanks that helps. So sounds like if I drill the holes in the middle of the 20oz bottle and seal the bottom fairing with a silicone glue I should be good- maybe.
Actually scratch that. after doing some measurements, I'm going to build another upper section with the longer 24oz bottle for the payload. That will definitely give me the "1 diameter away" distance from the transitions.
Nick B wrote:A peak altimeter would eliminate all them nasty spikes.
You have a very valid point. The reasons for using a logging altimeter are 1. I am showing my kids how we can use math to calculate top speed and 2. I needed to prove that the parachute-less design is coming down at a speed within safety rules.
Nick B wrote:A peak altimeter would eliminate all them nasty spikes.
And that's why they aren't allowed in the class A/B competitions.
These spikes can easily give a false peak altitude.
To clarify, you're saying the Peak Altimeters aren't allowed for the competitions (except the unreinforced 2liter as I remember they were specifically allowed)?
Nick B wrote:A peak altimeter would eliminate all them nasty spikes.
And that's why they aren't allowed in the class A/B competitions.
These spikes can easily give a false peak altitude.
To clarify, you're saying the Peak Altimeters aren't allowed for the competitions (except the unreinforced 2liter as I remember they were specifically allowed)?
Bugwubber
Yes that's what I meant to say.
From the class A rules:
Rocket must carry onboard a mass produced recording ("Logging") altimeter to document the entire flight on a time vs. altitude graph to nonvolatile logging memory or telemetry to relay the altitude information to a remote logging receiver. Ground based and/or timing based altitude measurements are unacceptable.
So-called "Peak Altitude" altimeters are unacceptable as they are subject to false peak readings caused by:
· Recovery system deployment.
· Landing impact.
· Improper installation.
· Unstable flight characteristics.
Ok well perhaps a discussion for another thread if you don't mind. I really didn't want to get in to debating rules and requirements here. Just asking for technical assistance on designing my rocket so the altimeter works correctly.
bugwubber wrote:Ok well perhaps a discussion for another thread if you don't mind. I really didn't want to get in to debating rules and requirements here. Just asking for technical assistance on designing my rocket so the altimeter works correctly.
Thanks,
Bugwubber
So back on topic.
bugwubber, is your new payload ready for launch?
