Yes, positive, in relation to ambient. Since the fuselage has a shape, = bernoulli's (sp?) principle still applies. Since there are curves on = the fuselage, the air takes longer to travel from the nose of the = aircraft to the tail around the curved surface rather than a straight = line, thus, that air will be at a lower pressure, no different than the = top surface of the wing. There is positive pressure inside the fuselage = since air going past the fuselage is at negative pressure. In my airplane the canopy skirts bow out slightly in flight, due to the = positive pressure inside the cockpit, even with the vents closed (I = experimented with plugging any holes in the cockpit where air could seep = through, to keep the canopy from bowing out, nothing worked; air always = manages to get in). The cockpit of an aircraft is not a good place for = static pressure due to the reason above, having positive pressure = inside. Most IFR-certified single-engine aircraft such as the Cessna = 172 etc. have an alernatic-static selector valve, that switches the = static source to the cockpit if the one on the aft fuselage becomes = iced-up or blocked otherwise. Not the best solution, but it is better = than nothing. Or, you could break the glass of one of your pitot-static = instruments, such as the vertical-speed indicator, and by breaking the = glass you in effect have a static source inside the cockpit. You can compensate for static errors by experimenting with different = locations along the aft fuselage, or by raising or lowering the static = port from the surface, or by putting an air dam in front of the static = port. Bob > -----Original Message----- > From: Pic Dude [mailto:picdude@AVN-TECH.COM] > Sent: Wednesday, June 19, 2002 12:43 PM > To: PICLIST@MITVMA.MIT.EDU > Subject: Re: [PIC]: Autopilot landing system for model aircraft >=20 >=20 > "positive"??? You mean positive relative to ambient, > rather than positive absolute, right? >=20 > If so, then why? I would expect that it should just be > ambient, unless the gaps are in such a location that it > has a ram-air effect. In either case, should it be > easy to calibrate/compensate for this in code? >=20 >=20 >=20 >=20 > -----Original Message----- > From: pic microcontroller discussion list > [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Bob Japundza > Sent: Wednesday, June 19, 2002 12:30 PM > To: PICLIST@MITVMA.MIT.EDU > Subject: Re: [PIC]: Autopilot landing system for model aircraft >=20 >=20 > Because pressure is usually positive inside the fuselage. >=20 > The best place for a static port is about 1/3 the way between=20 > the trailing > edge of the wing and the horizontal stabilizer. >=20 > Bob >=20 >=20 > > -----Original Message----- > > From: Pic Dude [mailto:picdude@AVN-TECH.COM] > > Sent: Wednesday, June 19, 2002 12:23 PM > > To: PICLIST@MITVMA.MIT.EDU > > Subject: Re: [PIC]: Autopilot landing system for model aircraft > > > > > > Missed a bunch of this thread, but just looking at this... > > Why can't you just use the pressure *inside* the aircraft > > for the static pressure (assuming it's not airtight)? If > > so, then it would be really easy to do using any of the > > low-cost gauge or diff pressure sensors out there. > > > > Cheers, > > -Neil. > > > > > > > > -----Original Message----- > > From: pic microcontroller discussion list > > [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Jeremy Furtek > > Sent: Wednesday, June 19, 2002 12:14 PM > > To: PICLIST@MITVMA.MIT.EDU > > Subject: Re: [PIC]: Autopilot landing system for model aircraft > > > > > > ----- Original Message ----- > > From: "Walter Banks" > > Sent: Wednesday, June 19, 2002 4:41 AM > > Subject: Re: [PIC]: Autopilot landing system for model aircraft > > > > > > > "Andrei B." wrote: > > > > > > > > Such a small scale air speed sensor is a challenge > > itself (I think). > > > > > > > This may be one of the easier parts to build. Airspeed > > measurement is > > > a function of ratio between ram air pressure and static pressure. > > > There are lots of pressure sensors these days. > > > > > > w.. > > > > I think that the subtle and challenging part of doing this > > (accurately) is > > measuring static pressure. The presence of the aircraft > > influences the flow > > field. For full size aircraft I think that they expend some > > effort to find a > > position on the aircraft that will give an accurate reading=20 > for static > > pressure under a representative set of operating conditions. > > > > A wind tunnel test ($$$) of an aircraft may include placing > > pressure taps > > along the model along the centerline. If you plot the > > pressure coefficient > > (comparing measured pressure to the static pressure measured > > well upstream > > or far away from the aircraft) along the axis of the aircraft, good > > locations for the static pressure tap would be places where > > this value is > > zero. > > > > For a flying model, to determine the static pressure, I think > > that they may > > sometimes drag an instrumented object far behind the > > aircraft, and compare > > that to the taps on the aircraft. > > > > For an R/C, you could do something similar. Another > > alternative would be to > > determine the best location computationally, a la CFD. > > > > Just my $.02. > > > > Jeremy Furtek > > mailto:jfurtek@agames.com > > > > -- > > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > > > > -- > > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body > > > > > > >=20 > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body >=20 > -- > http://www.piclist.com#nomail Going offline? Don't AutoReply us! > email listserv@mitvma.mit.edu with SET PICList DIGEST in the body >=20 >=20 >=20 -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body