Hmmm.... I know that if I have a port on the side of a car, it will usually have lower pressure on that side than ambient. I'm thinking of a way to create a system with ram-air and suction-air (patent pending :-). Pretty much a ram-air tube as on a typical pitot-static system, and a similar tube in reverse (facing the back of the aircraft). With similar sized tubes, and one butted right behind the other, static should be the midpoint of the 2 readings, or more importantly, airspeed would be half of the new value. Something like that. > "Or, you could break the glass ...". Exactly why I thought of using the pressure inside. Cheers, -Neil. -----Original Message----- From: pic microcontroller discussion list [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Bob Japundza Sent: Wednesday, June 19, 2002 1:06 PM To: PICLIST@MITVMA.MIT.EDU Subject: Re: [PIC]: Autopilot landing system for model aircraft 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 > > > "positive"??? You mean positive relative to ambient, > rather than positive absolute, right? > > 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? > > > > > -----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 > > > Because pressure is usually positive inside the fuselage. > > The best place for a static port is about 1/3 the way between > the trailing > edge of the wing and the horizontal stabilizer. > > Bob > > > > -----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 > 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 > > > > > > > > -- > 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 > > > -- 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