How do airplanes and helicopters measure their speed?

[jamesbond007]

Hey guys. I was watching Gone in Sixty Seconds, gotta love Jolie , anyways. It was during the part where the chopper gets called in just before Cage hits the "Go Baby, Go!" button and induces Nitrous Oxide into his car. From there, the helicopter guys were saying how fast HE was going by using radar...that got me to think. How do helicopters and planes measure their speed? I know how cars and boats do it, but airplanes and helicopters? They have, or at least should have, a constant blade rotation rate, where cars use rotations and the diameter of the wheel, and how many times the wheel will turn, etc. Do they have some hole or something that measures the speeds of the air going through? I mean, really. How do they do it?

Thanks.

 

[dbcrossfire]

gps

 

[geno]

On top of that, I wanna know how they measure their altitude too? I was thinking that they use air pressure, but that varies at times at the same altitude. Then I was thinking radar, but how would that work when you're way up there? That's another question that's stumped me.

 

[wedi42]

linky
this talks about a air pressure gauge
but i bet GPS is more reliable these days
but planes would have both

 

[wedi42]

<< On top of that, I wanna know how they measure their altitude too? I was thinking that they use air pressure, but that varies at times at the same altitude. Then I was thinking radar, but how would that work when you're way up there? That's another question that's stumped me. >>


they use air presure
you calibrate it before you take off based on the airport elevation

 

[notfred]

assuming we're talknig about a basic airplane here, and not one with high tech GPS and radar on it:

Airspeed: measured by how fast the wind is going past the plane, groundspeed is interpolated based on the weather report, and what the wind's supposed to be like in the area on that day.

Altitude: using air pressure, calibrated for barometric pressure (based on the weather report again) tells altutude MSL (Mean Sea Level)

 

[lsd]

They have pitot and static ports which allow ram air and static air to be measured by an Air Data Computer (for primary instruments). The Barometric pressure at the airport is used to determine the current elevation.

 

[jamesbond007]

So they nevet get an *exact* figure? It sounds like pilots would only get estimates.

 

[jamesbond007]

johngute, thanks for that AWESOME link! I've added that site to my bookmarks. VERY informational and I'm sure I'll be reading things from there quite often.

Thank you VERY much!

w00t!

 

[KDOG]

assuming we're talknig about a basic airplane here, and not one with high tech GPS and radar on it:

Uh, you can get GPS for $150 or less. Even at Walmart...

 

[sharkeeper]

<< Uh, you can get GPS for $150 or less. Even at Walmart... >>



Yes and I'm surprised you don't see them on these modded cases with neon tubes, radiation signs, dog ears, etc. A trimble GPS to monitor fan speed of 33 fans, Alnor velometer to measure 3500 cfm of air to cool Athlon running at 4V!

Pitot tubes are the standard to measure aircraft speed. Measuring ground vehicular velocity is another thing altogether. I suppose a multimode LIDAR could do it, but polizi choppers certainly aren't equipped with such esoteric instruments!

Cheers!

 

[Dunbar]

<< They have pitot and static ports which allow ram air and static air to be measured by an Air Data Computer (for primary instruments). The Barometric pressure at the airport is used to determine the current elevation. >>


Actually only the most expensive aircracft use air data computers, the less expensive A/C (like a Bell helicopter) uses a mechanical gauge which compares ram pressure (pitot) to static pressure. This isn't very accurate (indicated) and must be correct for altitude and temperature (true airspeed.) Altitude just using very sensitive diaphrams to measure (static) air pressure and it's referenced to MSL (mean sea level.) So if you land at Denver your altimeter will show 5000ft.

 

[AndrewR]

GPS is notoriously inaccurate when giving elevations.