Aw, darn it, Sam, I was going to mention interior cabin pressure as an alternate static source and then you went and beat me to it!
If you do use a static port you do need to check it before flight to make sure nothing is clogging it. I have had pitot-static failure while in flight - annoying more than anything else. Didn’t need to smash anything. The weather was severe clear so I just eyeballed it around the airport and set it back down.
Although GPS is a wonderful high-tech toy, it, too, has a certain error rate. It might be interesting to compare a pressure altimeter to a GPS unit.
It might also be interesting to compare my financial situation depending upon whether I’ve used a GPS or air pressure based altimeter, after my amateurish attempts at a parachute release mechanism have failed. :eek:
I’ve done some of this. Now that SA is turned off, GPS altitudes are routinely accurate to something like 20 meters (obviously depends on signal quality and satellite position). Aircraft altimeters vary, but 5-7 meters is common. However, they are subject to errors (e.g. temperature fluctuations) that GPS isn’t. At something like 12-15,000’, they are probably about the same as GPS. Above 20,000’, they can be worse. A poorly mainted instrument may be badly wrong. A fairly new, well maintained, recently calibrated IFR unit can be surprisingly good. YMMV.
This model rocket altimeter seems to measure static pressure and double-integrates acceleration, both presumably with piezoelectric crystals, to give an altitude reading after recovery. That has to be a lighter and smaller, not to mention cheaper, package than a GPS system would be. Smaller rockets that don’t carry these (and the smaller ones were all we had back in the day) need triangulation methods from ground observers to get altitude.
Princhester, looks like you’re unclear on the difference between static and total pressure. Think of the law of conservation of energy applied to air (which is all Bernoulli’s principle is, btw). Energy can take the form of static pressure or velocity, and those components add up to total pressure (yes, this is a simple version).
A pitot tube points directly into the airstream so that incoming air is stopped - its velocity pressure is converted to static pressure, which is then equal to its total pressure. The static port, which faces sideways to the airflow, does not collect and stop air, so none of the pressure it measures represents velocity pressure - all it measures is the static pressure of the airstream. The difference in the pressures in the pitot and static ports (which are both static by then) directly indicates airspeed. The static port itself gives altitude.
The static port’s hole is small to minimize error from the local turbulence that a large hole would create. The pitot tube is much larger to minimize error from its own boundary layer, and to reduce susceptibility to icing (stopping the local airflow also stops its water content).
A venturi effect is the difference in static pressure caused by accelerating the airflow through a constriction (converting static pressure to velocity). It doesn’t exist in either the pitot or static lines because there’s essentially no airflow in either.
It has been ages since I have flown last, but I seem to recall having to set the airport base altitude in the altimeter prior to takeoff. Is this a false memory? FWIW, I was flying Cessna 152s.
It’s an accurate memory, regardless of the type of plane. If you set the altimeter to read the correct field elevation before takeoff, it will show you a reasonably correct version of your altitude above sea level for some time afterwards.
Weather changes will mess things up. If you’ve been in the air for any real time, or if you’ve flown into different weather, it makes sense to get an altimeter setting from a local ground station. They will report a pressure setting that represents their current pressure (typically expressed in inches of mercury) corrected to sea level. You set your altimeter until this value appears in the “Kollsman window” and your altimeter is now correct for the local conditions.
As noted above, with a rocket altimeter you’d deal with this by comparing readings taken in flight with those on the ground before or after launch. Thus, if it said the ground was at 800’ and apogee was 4900’, that’s a 4100’ flight. The fact that the actual ground elevation is 932’ doesn’t matter.
You remember correctly. Changing weather conditions mean that the pressure that means sea level one day, could mean 200 feet above sea level the next. It would suck to crash simply because a high pressure system moved in.
Someone mentioned calibration. There is a machine that can be hooked up to the static and pitot ports that can simulate various air pressures and speeds. Calibration is simply a matter of injecting a known value and adjusting the output to match. The airport tower provides a reference pressure.
The real problem comes when pressure decreases. Maintaining a constant pressure altimeter reading means keeping a fixed amount of air above you. When pressure drops, so will your height above sea level.
I was thinking not of a venturi effect in the static line, but rather of a static pressure drop caused by a venturi effect of air flowing past the port, and that pressure drop affecting the pressure inside the port.
Of course as you and others have now explained, that is nonsense because there is no restriction to create any venturi effect in the air moving past the port.
