As I understand it, altimeters use air pressure to calculate air pressure. No problem so far.
But how do they do so for an aircraft or (more importantly for my present purposes) in a model rocket, when air is rushing past, what with the various effects air movement has on air pressure?
How are they able to get an accurate reading?
Uhhh…your understanding might be off.
Please clarify your question. Are you asking how the altimeters in rockets work? Are you asking how to determine the altitude of a model rocket?
Aagh. “Altimeters use air pressure to calculate height”.
Typing too fast, too little time.
This might help.http://ukdscenery.mybravenet.com/custom.html
If you want to predict the altitude that will be achieved bya model rocket, try this.
If you want to measure the altitude of a model rocket in flight, here’s a link.
Other resources are available as well. I googled “model rocket altitude.”
As for big, non-model rockets, their altitude is determined largely by ground-based radar.
By careful placement of the instrument so as to minimize errors caused by air movement past the sensor(s).
Although I know little of model rockets and airplanes are more my thing, the problems posed by movement through air are similar in kind if not degree. When installing or replacing an altimeter in an aircraft some testing and calibration is required…
Weather changes can also cause changes in altimeter readings, sometimes quite significant changes. For this reason, altimeters installed in aircraft are provided with a means of adjusting to compensate for weather changes while in flight.
For a model rocket, this may require some adjustment prior to launch, based on the current barometric pressure.
Look up “Pitot Static Tube”
or, look up “pitot tube” and “static tube” - they are quite different, and single devices are not used very often. The pitot is mounted directly in the airstream, the static in “undisturbed” air (or as close as can be found on an aircraft).
Then there are radar altimeters, GPS position indicators, glideslope arrays - bunches of ways to tell aircraft how high above sea level and/or ground they are.
OK, so I’ve looked up pitot tube and static tube etc. All the sites I’ve looked at say that the static pressure port is placed in an area of undisturbed airflow over the aircraft hull, placed so that airflow is over the port rather than into it.
Why don’t you then get a venturi effect, reducing pressure in the static port, varying with airspeed across the port?
As has been noted, for an accurate reading of pressure altitude you need a good static source - a place where, despite air moving past, pressure is equal to ambient. In general (and oversimplified), at the nose pressure will be higher than ambient, at the tail it will lower, and somewhere in the middle will be about right. It’s not necessarily easy to find a good static location. Airflow tests might be needed. The alignment of the rocket to the airflow may be important.
It’s worth noting that, presuming the flight path is just about vertical, at the top the airspeed will be low and so errors due to a poorly located static source will be too. Thus, readings taken then (probably when you care most) are likely to be accurate. If the rocket will descend by parachute, the airflow-induced errors should be low all the way down. So if all you care about is measuring the max height, you may be able to ignore static source issues and just carry the altimeter as payload.
You will presumably be determining height by comparing the inflight pressure readings with those taken on the ground before and after flight. This will pretty well eliminate errors due to weather changes.
Calibration. For instrument use, the pitot-static system must be calibrated and certified every x years. I don’t know exactly how they are tweaked, but they are.
So Extraneous you calibrate dynamically, to account for varying airspeed over the port?
Xema, I’m not actually at the stage where I’m building rockets that are worth using an altimeter on anyway, I"ve just been reading up on websites where altimeters are used by hobbyist rocketeers.
The designs I’ve seen do not suggest any special placement of the altimeter, or any special ports or whatnot, they just put it in as payload.
I’d thought of your point that the altimeter would have no difficulty getting an accurate reading at apogee, but there are websites where people post charts (from data downloaded from recording altimeters) showing height/time etc in nice curves with no suggestion the data would all be inaccurate but for the part of the flight when the rocket was not moving fast.
And all this got me to thinking about air pressure and air movement and venturi effects and hence this thread.
So I’m left wondering if the data shown on such websites is (but for the max. height) hooey, or whether altimeters are less affected by the factors I mention than I would think.
No, the IFR pitot-static system calibration and certification has nothing to do with it.
Consider this quote from http://www.aa.washington.edu/faculty/eberhardt/Lift_AAPT.pdf
Interesting paper, Berkut. I’ll read the whole thing later.
I understand what it is saying about there being no lowering of pressure in fast moving air as such, but there is no explanation of why there is no venturi effect inside the static port.
No doubt if I understood that, I’d have an answer to my question
There is very little air movement inside the static port.
Actually now that I think about it, what I am missing is the comment about the exception being when air is confined in a pipe. I couldn’t get over the feeling that the venturi effect would cause the fast moving air outside the port to suck on the port, reducing pressure inside it. But now that I think about it, I suspect that the venturi effect invariably involves a sucking effect caused by fast moving air in a pipe. Where the fast moving air is not confined and not bending, you get no venturi effect.
It’s all beginning to become clear.
This suggests to me that they are using either GPS- or accelerometer-based instruments, not aircraft-style pressure-based altimeters. I’m no longer heavily into model rockets, and when I was there was no GPS, so I’m not as authoritative as I’d like on this subject. I think that makers of larger model rockets used to use pressure altimeters with pitot tubes, but they were relatively bulky and expensive, so I’d wager most use these electronic systems now.
A few weeks ago I was watching a series of shows on Discovery or TLC about a big model rocket convention in Kansas, and they had tiny GPS systems that transmitted coordinates in real time to a computer. Pretty cool, and probably the kind of thing you were seeing on those Web sites.
The trick to maintaining stable pressure over the static port is that the static port is very small - almost a pinhole.
The air directly against the fuselage of the airplane actually isn’t moving - it’s stuck to the skin from friction. As you move away from the skin, the air moves faster and faster, until at some point you reach the free stream velocity of the air. This region of slowed airflow is known as a boundary layer.
A static port is flat and small, and sits inside the boundary layer in relatively undisturbed air. This of course assumes that the air is travelling 90 degrees to the port. If you sideslip an aircraft so that the relative wind is ramming into the port, you will see your altimeter drop due to the increased pressure. So sometimes there are dual static ports, one on either side of the fuselage. They’re connected together with a manifold line so that slips are compensated. Sometimes static ports are also located on the same head as the pitot tube. But one thing is constant - the static port is very small, and it’s located perpendular to the airflow.
Sometimes as static port gets plugged or iced over. When that happens you can use the cabin air for an emergency static source, either by opening an alternative static vent put into place for that very purpose, or by smashing the faceplate of a pitot-static instrument (the interior of the instrument is connected to the static system.
Now we’re getting down to it, thanks Sam. So are you saying there would be a venturi effect, if the port wasn’t sitting inside the boundary layer?
No, commasense, the sites I was looking at were using pressure based altimeters. I know this because (amongst other things) they talked of the “spike” on their readings caused by the sudden change in pressure as the chute was popped out of the tube.
No, there wouldn’t be a venturi effect. The reason the port is very small is because at those sizes air is very viscous and just flows right over the port.
There’s not always a suction due to venturi effect anyway. I owned a Grumman AA1 that had a retractable canopy. It could be opened in flight at speeds up to 135 kts, and I flew with it open several times. As long as the canopy was only opened to the stop line, there was no ‘suction’ in the airplane. Just a lot of noise.