Is it safe to extend the airspeed sensor with a straw?


I experienced airspeed/ground speed variance as above link.

I guess the reason of problem is tilt prop air flow is affecting airspeed sensor

So I attached straw to airspeed sensor to extend it, and attach toothpick to strengthen its structure.

Will there no problem with this?

I will try this configuration very carefully soon.

From a physics standpoint of view I can see two problems arising:

First, the front nozzle of the pitot tube has a very specific diameter that it has to be calibrated for. With the larger opening of the straw extension you effectively widened the front nozzle resulting in a higher static pressure (aka ram pressure) than normal.

Second, the pitot tube’s front end is rounded, to ensure smooth (read: as laminar as possible) flow over the side holes. With the sharp leading edge of the straw and the skewer at the side you will probably now see a lot of turbulence at the side holes which will mess with the dynamic pressure reading.

As far as I understand it, the props will throw the most air outwards (instead of backwards), when the difference in airspeed and theoretical prop airspeed are the greatest (aka when the plane is far slower than the prop’s spin rate would suggest). Therefore, if the props are really the problem, you should be able to see a drop in airspeed when you hold the plane steady and spool up the motors.

A better fix than the straw, if possible, would be to undo the glue and pull out the tube a bit more, or, if there isn’t much tube left inside, to use a longer tube.

Also, have you thought about wind? ~10m/s windspeed is not unusual and would explain the difference between air- and groundspeed. I just read that the airspeed is always lower, which isn’t entirely impossible with 100% tailwind, but unlikely indeed.

And last, please don’t double post. Since this is related to your last week’s problem, you should have extended that post.

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I tried not to double the post,

but the title of this post should be different.

original post’s title is very difficult to imagine this problem.

left tube inside is very little, so maybe I have to purchase longer one.

but before purchasing them I will test this setup

and for your reference, pitot tube diameter is 0.8cm and I bought 0.8cm diameter straw

but the hole diameter is different(straw is wider) like you said.

I changed the install order of straw and pitot tube.

I installed hardened straw first and installed airspeed on the tip of straw.

It solved problem and the airspeed sensor works well, but I encountered another problem.

I will post another post for that.

the prop wash on side hole was the real problem,

so I couldn’t test the straw is not good for airspeed sensor,

but anyway the result with straw was same with original setup. it didn’t mess up largely. maybe I have to test in other time.

now airspeed sensor is on the tip of straw, so now my setup is safe.

Sounds good! :+1:t2:

Would you mind sharing a picture? I’d love to see that solution :wink:

Now airspeed sensor give correct reading. It is satisfying.

but should be careful with its strength of structure.

Good job :grinning:

If you are concerned with the structural integrity, you can always add more supports around the sensor.

Being mounted not so rigidly also has an upside: In an unscheduled high-velocity landing (read: crash) the skewer or the glue should give way first, protecting the tube. In other words, the tube will probably break off but not break itself.

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