speed of flying machines

[srimannarayanakarthik]

how can we measure the speed and altitude of flying abjects i mean areoplanes and gets.for vechiles the speed is measured by the RPM of the whell but for them how can we measure? atlitute as far as i know can be calculated by the preassure changes but what about the velocity

 

[dknguyen]

How convenient, I was just researching into this stuff the other day for a project:

**broken link removed**

 

[Papabravo]

Pitot tubes are great except in the winter when you bring a snow leaden airframe into a semi-warm hanger to get the ice and snow off. When you return it to the cold outside, the residual moisture freezes in the tube causing a very interesting set of airspeed indicator behaviors after takeoff!

Not to panic in CAVU conditions, but get caught between layers and the results were very nearly fatal.

 

[Madhouse]

Depending on what you plan to fix it to, you could use a GPS module and microcontroller as a source. There are plenty of modules that are compact and lightweight, and output altitude to within a few metres as well as change in geographic location over time (=speed). I currently have an old Navman GPS module feeding serial output to a PIC 16F876 for short duration logging of movement (data stored to external EPROM), but for a model a/c there is no reason why it couldn't be transmitted back to base instead....

 

[srimannarayanakarthik]

if you are using a pitot tube,which mainly depends on pressure then if some place if the air density ratio changes then how can it becoeme the perfect method for predicting the above mentioned parameters.

 

[dknguyen]

There is always variability in all sensor measurements and no sensor measurement is ever perfect. Gyroscopes for example drify and change sensitivity with temperature. Planes also fly over different levels of terrain, so it's most useful for measuring how high you are above sea level or some other poin. The height you are at is usually much greater than the changes in terrain so it doesn't really matter all the little changes. If you want to measure how high you are above the terrain that is directly beneath you (so you can "hug" the ground), you are probably flying very low and you need to use radar, but only cruise missiles do that. Not normal airplanes. Plus people have eyes- near the ground you can tell how high you are if someone is ever flying close enough to the ground.

It's what we have figured out, so we use it.

 

[jpanhalt]

There seems to be two issues being discussed here. One is altitude and the other is airspeed. All airplanes eventually land and only a small percentage have radar altimeters. Premature contact with the ground is a real possibility ("When flying from a high to a low, lookout below"). To help avoid that, which is usually caused by pilot stupidity, but can be contributed to by changes in air density, pilots receive local altimeter settings. In instrument conditions, there are other aids to maintain a safe altitude besides radar; however, the altimeter is still critical until within 200 feet of the ground or so.

As for airspeed, remember that the pitot tube also has a reference port, called the static port. Air density does make a difference and can be corrected for by altimeter setting and altitude. However, since all airplanes at the same altitude and location would have the same correction for density, pilots and controllers generally rely on the indicated airspeed and forget about the corrected airspeed.

 

[Madhouse]

Maybe it would be good to establish what sort of "airframe" we are discussing!

A model, unless a large one a Pitot tube solution is fairly impractical due to cost / manufacture / interface, whereas GPS/uC is viable and cheap.

A self build aircraft, if designed by yourself you need to think about instrumentation very carefully according to conditions you may *possibly* encounter - two solutions (Pitot/static tube and GPS I personally believe are essential, the latter as a back up).

A kit aircraft - stick by the letter to the manufacturer's instructions until built, then consult with them before modifying their design!

A commercially available aircraft - don't mess! If nothing else because of insurance problems etc etc. If it works, don't fix it. Just carry something in your pocket as a "back-up back up device"! I do - its made by Garmin, and the only time I used it was to check against the instruments on my plane (to see how good the Garmin was).

Maybe you could expand a little on your project srimannarayanakarthik...

 

[kcflyer]

Pitot tubes are great except in the winter when you bring a snow leaden airframe into a semi-warm hanger to get the ice and snow off. When you return it to the cold outside, the residual moisture freezes in the tube causing a very interesting set of airspeed indicator behaviors after takeoff!

Not to panic in CAVU conditions, but get caught between layers and the results were very nearly fatal.

Papabravo is obviously a pilot, but for the non-pilots reading this, that's why IFR-equipped planes have pitot heat.

You would definitely need either a barometric altimeter and pitot tube, or a GPS-based approach. As others have stated, the alitimeter needs to be frequently corrected for local barometric pressure. Human pilots get that information by radio. An automated system would need some similar way to stay updated.

Here are formulas for converting indicated airspeed to true airspeed. They are pretty hairy:

https://www.mathpages.com/home/kmath282/kmath282.htm

A GPS approach will tell you the ground speed, not the airspeed. Some more advanced GPS equipment can compare the ground track to the heading and calculate the effect of wind and thereby derive an airspeed. I'm not sure of the accuracy of that approach.

 

[Madhouse]

Not to panic in CAVU conditions, but get caught between layers and the results were very nearly fatal.

I assume this was from experience! Ouch.... How low was the layer break? Saw a learner drop a Cessna into the field whilst practising go-arounds once, walked away with his life but got a stern lesson in use of flaps and observation of "hidden" weather conditions afterwards. Club got a new a/c so all were happy at the end!

 

[Oznog]

A pitot tube reading also has to be compensated for changes in air pressure due to altitude, barometric reading, temp, and humidity to give an exact airspeed. Lower density makes the reading lower.

However, often you don't want to compensate it. An aircraft which has a 50 kts stall speed at sea level may have a stall speed of at 60 kts at a higher altitude. But stall speed and most other flight characteristics are affected by density it in the same way as the pitot instrument. The uncompensated pitot, originally calibrated for sea level, would read 50 kts at the stall speed for any altitude. However, an uncompensated gauge is not a literal airspeed and would cause trouble if you used it as a speed for say navigation.

 

[Papabravo]

I assume this was from experience! Ouch.... How low was the layer break? Saw a learner drop a Cessna into the field whilst practising go-arounds once, walked away with his life but got a stern lesson in use of flaps and observation of "hidden" weather conditions afterwards. Club got a new a/c so all were happy at the end!

Aircraft was a Bellanca Decathelon. I was instrument rated but the aircraft was not instument equipped. I had needle-ball and airspeed. Location was the south shore of Lake Erie near Port Clinton. It was twilight. There was an overcast at 10,00 feet and a fogbank with a maximum of 300 feet rolled in undeneath. The airspeed needle was jumping all over the place, but with full power, some nose up trim, and the wings level I knew I was climbing. A call to Cleveland Center produced a suggestion to tune to a local VOR and fly direct. In laymans terms he was telling me to "fly inland - ya durn fool" In 20 minutes or so it was ceilings unlimited and 50 mile visibility.