I'm working on my next UAV. Since I'm not planning FPV flight with this one I'm using a 6 ft Tellemaster with power in the nose. I was wondering if anyone has any numbers or ideas about how much friction loss can be tolerated in the pitot tube without affecting the airspeed calculation. How would I adjust the code to compensate. I'm estimating 16" of tubing from the sensor to the pitot.
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Friction is practically irrelevant regarding measured pitot pressure. For all practical purposes the air does not flow in the tube, it only breathes a little as speed and altitude change. Long pipes, narrow inlets, and excess volume in the system will cause sluggish response, but no change in steady state reading.
Worry more about getting the pipe into undisturbed air, avoid leaks, and recognize it is only really accurate at exactly one angle of attack, thus one airspeed / weight combination. If you want the "downtown" version, decide if you want to mount the tube straight ahead at cruise speed or straight ahead at, for example, landing speed, (which zone of operation is it most important for you to have accurate readings) and then use a calibration table to correct outside this range. Or, if you are a practitioner of TLAR engineering, don't sweat the small stuff, just stick it on and go flying!
For our 8-foot Telemaster, we've been putting the pitot out on the wing, with a similar amount of tubing -- as long as you can get the end of the pitot far enough out in front of the leading edge, it works fine. We use a piece of brass tubing as a connector so the wing can be dismounted.
I haven't heard of this being an issue, but we do tend to use small diameter tubes in our stuff. As far as I know, the pressure will equalize in the tube to that at the mouth of the tube, and length only adds insignificant lag in time for that to happen. Funny, I'm working on my pitot now!
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Worry more about getting the pipe into undisturbed air, avoid leaks, and recognize it is only really accurate at exactly one angle of attack, thus one airspeed / weight combination. If you want the "downtown" version, decide if you want to mount the tube straight ahead at cruise speed or straight ahead at, for example, landing speed, (which zone of operation is it most important for you to have accurate readings) and then use a calibration table to correct outside this range. Or, if you are a practitioner of TLAR engineering, don't sweat the small stuff, just stick it on and go flying!