Pitot Tube

[Scott Hersha]

Here's a question for the aero engineers in the group - or anyone else that knows.

Why do we have a pitot mast that is up to 6 inches long? Does it really need to be that far away from the bottom of the wing to get undesturbed air? I think it's not to clear boundary layer air because that is pretty thin. The reason I asked is I would like to use a shorter one on my RV14. I found one that is only 2.3 inches long, heated, and very light weight with a static port that is also heated. Sometimes I think we do things because that's the way we've always done it. If I can get a fairly accurate reading at 2.3 inches, why do I need one that looks like it came off a B-52?

Experts and others please respond.
Thanks

[p51dplt]

I too am interested in the answer to this. I don't want to hang a long pitot mast off the bottom of my wing either.

[BobTurner]

I don't know the exact answer, but certainly the wing affects the airflow for some distance out. Remember, the lift equations we use are for incompressible flow, so if they are a good approximation it must be that there is a large volume of air involved. Exactly how far out you need to go probably depends on the wing cross section profile you are using.

There have been some posts previously complaining about pitot tube static ports as not working very well. I would discount the "heated static" as a big advantage; a simple alternate vent to the cabin works very well as a backup.

[penguin]

Its an experimental airplane - you can do what you like! The tube needs to be far enough away that the flow field around the wing has a minimal effect.

The tube that Van's provides works well. It may be that a shorter tube will work equally as well - several production airplanes have shorter pitot tubes (eg Cherokee, or find a picture of a Robin DR400, the pitot is a couple of feet outboard of the left gear leg). Do you feel lucky?

Static ports are completely another story. Plenty of posts here from builders who have inaccurate (+/- 10 kts) airspeed systems because of an inaccurate static system for various reasons. I would build in the Van's static ports (as the drawings show) so that they are available if this P-S tube doesn't work out.

Pete

[Scott Hersha]

Good point Pete - don't gamble on 2 systems related to each other. They sell this tube as a pitot only, or pitot/static (Aero Instruments, Cleveland, OH). It's $100. cheaper without the static port. I think this tube is used primarily on UAVs. I'm going to try to call someone there and get some info on installation.
Thanks