Pitot Mount Angle

[TCONROY]

I just received my Gretz Chome pitot tube mount and am contemplating the installation. At what angle should the vertical part of the mount be relative to the wing? I'm building an RV-7A (could be a -7..) and am not even sure what the angle of incidence is supposed to be (haven't gotten that far in the plans yet). Should I figure out the angle of incidence and then draw an extended chord line from the wing tip and adjust from that point or is there an easier way (such as a specific angle relative to the bottom wing skin??)

Any tips/advice welcome!!

[roee]

One of the serious aero guys please correct me if I'm wrong... I think that ideally you want the pitot tube pointed directly into the airflow, but that it's not very sensitive to some angular variation. In fact, the angle of the pitot tube relative to the airstream will vary under different flight conditions, i.e. with angle of attack. So probably anywhere around the same incidence angle as the wing should be fine, which also means the mount will be approximately perpendicular to the tangent of the wing surface near the spar.

In any case, nice thing about the Gretz mount is that the angle is adjustable and re-adjustable by adding or removing shims/washers between the mounting flanges. So you could fit the mount to the wing any time (add the doubler, cut the holes, etc.), but not have to worry about the "final" angle until later (and you could always re-adjust it if need be).

[splevy]

written in the 30's about the effect of incidence angle on pitot accuracy it turns out to be pretty insensitive. If I remember correctly, ten degrees results in only a couple of percent error. This is at similar speeds to our planes. Above Mach 1 it is probably a lot different....

Stan

N67SL (reserved)
RV-7A (fuse)

N6085L
Grumman AA5

KCCB

[dmaib]

I remember Kevin Horton talking about this issue a few months ago, and as I recall, it is not very sensitive to minor angle changes. I just mounted mine perpendicular to the wing bottom skin in the main spar area and it is very accurate.

[RV8R999]

during Aero Masters thesis worked this problem in a wind tunnel with all sorts of various shaped pitot tubes.

BLUF: between +-15 deg of AOA the error is inside that of your instruments resolution (particularly since Ps, static error, will vary more for changes of AOA). Outside +- 15 deg the error rises almost parabolically with angle.

I haven't flown my RV8 yet so I can't tell you what the normal range of AOA is but someone should be able to tell us, but I doubt it gets much if at all higher than 15 or so. Take care to understand how that AOA is measured before you use the data to set your PITOT. Many are units of AOA and not degrees of AOA, meaning it is arbitrary and relative to the angle of the probe and not to the cord line of the wing.

One of the best PITOT tubes I tested had a swiveling tip that would adjust to AOA and Beta (sideslip angle). Its big, ugly, and required maintenance. No thanks.

Im in post op surgery so if what I wrote makes no sense blame the doctor!