To all you Pitot/Static tube users - beware. Three out of three of the 3DR pitot/static probes I purchased recently have had air leaks where the internal tube joins to the machined aluminium tip. In one instance this resulted in a 20% under-reading of airspeed that was not consistent from day to day. When I went back to my retailer for a replacement he discovered that the probes he had in in stock had a similar problem so I elected to fix it myself.
You need to do the following check (not rigorous, but the best that can be done without making a test fixture)
1) disconnect the probe from its silicone tubing
2) Place your finger over the centre pipe coming from the back (if you have truble sealing with your finger, wearing a latex glove can help)
3) Blow into the front of the probe, sealing around the end with your mouth. Make sure when you do this that you are not also blowing air into the four small static holes further down the end of the tube.
4) If you can generate any airflow then the probe is faulty and needs to be fixed.
My fix.
1) Hold the tip of the probe in a gas flame (you could use a hot soldering iron) for a few seconds and remove the aluminium tip with a pair of small pliars when the glue softens.
2) After its all cooled down, apply a small amount of thick adhesive to the probe tip where it interfaces to the outer and inner tubes and push it together. I use CA adhesive because I was in a hurry , but there are other choices that may be better if you have time, but you need to make sure that whatever glue you use can be cleared from the centre tube if it leaks in and blocks it. With CA this is easy - you insert a heated length of piano wire which will vaporise out any excess adhesive.
3) After the glue has dried test for air flow and leaks. You should be able to blow through the probe tip when the rear tube is uncovered, but not when it is. It is VERY important that the probe is not blocked. A blocked probe causes the airspeed reading to rise with altitude and could cause your plane to stall and crash during ascent.
It takes 5 minutes to test and fix this way which is a lot easier than going through the whole RMA process, however I am sure 3DR will organise a swap if you explain the problem.
Regards,
Paul
Replies
Before i took my pitot apart my ARSPD_RATIO was 1,7
But now it is around 8!
Is it normal?
Where did you set yours?
Gábor
My pitot had no glue at all where the inner tube joins to the front part.
So thx! :)
Thanks for the heads up, mine was faulty too.
I took a different route, detached the tip as described, pushed a 1 mm drill bit partly down the center tube, globbed the end with epoxy, pushed the tip back on, held the probe vertical with the tip down, twisted the bit out once set :-)
This pitot tube problem have been going on for quite some time now.
For detailed photos, see this rather old post. Pitot Tube
cheers
Mine was also faulty , thanks.
I tested my tube, and surprise surprise, it was leaking too. But it was not the top end but bottom where the two pipes are glued to the main tube. The glue was not good, it was kind of cracked and leaked. Easy to fix, just add more glue over.
But seemingly there is a consistent quality issue with these tubes.
Great write-up; and complete with a fix! I found that all 3 of my probes leaked somewhat. This one thing that may have contributed to measuring only about 1/2 of the dynamic pressure on my North Texas Near Space 2 flight. My fix was a little different since I was also interested in getting rid of the joint between the silicon tube and pitot probe due to the extreme temperatures. I like your fix for the stock probes and I'll use it on my UAV's. I now have 3 probes that I can remove from the "retired" bin.
If other users want to make their own probes it's not that hard. Just tube, a really small drill bit, and some red RTV for the static tube end. The dual probes sold with APM1 are no longer available.
Thanks for the write-up Paul!