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GPS As Pitot Tube Backup?  
User currently offlineJawed From United States of America, joined Sep 2006, 482 posts, RR: 0
Posted (5 years 6 months 3 weeks 3 days 1 hour ago) and read 13484 times:

If the pitot tubes are plugged up with ice or whatever, why don't modern airliners use GPS as a backup option to measure their aircraft's current speed? Wouldn't a GPS-estimated speed be a lot better than no information at all?

119 replies: All unread, showing first 25:
 
User currently offlineA10WARTHOG From United States of America, joined Jul 2004, 325 posts, RR: 0
Reply 1, posted (5 years 6 months 3 weeks 3 days 1 hour ago) and read 13498 times:

Well the chances of all 3 pitot tubes being inop, I think is very slim. You are more likely to lose your GPS data.

User currently offlineJawed From United States of America, joined Sep 2006, 482 posts, RR: 0
Reply 2, posted (5 years 6 months 3 weeks 3 days 1 hour ago) and read 13488 times:

It sounds like a lot of recent problems have been caused by inaccurate pitot tube readings. I would imagine that some kind of sanity-check against GPS would be useful, at least in these emergency situations when the pilot doesn't know what data to trust.

User currently offlineJoecanuck From Canada, joined Dec 2005, 5478 posts, RR: 31
Reply 3, posted (5 years 6 months 3 weeks 3 days ago) and read 13483 times:

GPS only shows ground speed, though combined with other information, (such as power settings and heading), it could be used to estimate airspeed.


What the...?
User currently offlineAirframeAS From United States of America, joined Feb 2004, 14150 posts, RR: 24
Reply 4, posted (5 years 6 months 3 weeks 3 days ago) and read 13469 times:

While the GPS technology is a really neat and cool fad, it is not totally 100% reliable.

Case in point: I have a GPS for my car and the speed that my GPS tells me is not the same as what my car's speedometer says. GPS is not 100% accurate.



A Safe Flight Begins With Quality Maintenance On The Ground.
User currently offlineDescendVia From , joined Dec 1969, posts, RR:
Reply 5, posted (5 years 6 months 3 weeks 3 days ago) and read 13456 times:

Raw GPS is not very useful for altitude and stuff. Plus it would take a major modification to the ADC (Air Data Computer) plus some time of augmentation system to be able to tie GPS in.

User currently offlineFLY2HMO From , joined Dec 1969, posts, RR:
Reply 6, posted (5 years 6 months 3 weeks 2 days 23 hours ago) and read 13441 times:

Quoting Jawed (Thread starter):
ouldn't a GPS-estimated speed be a lot better than no information at all?

Yes. IIRC the Garmin 430 GPS has a function where you can calculate your TAS knowing your pressure altitude and temp. IAS need not be known.

Quoting AirframeAS (Reply 4):
GPS is not 100% accurate.

Far from the truth. GPS is extremely accurate, more so than any speedometer out there. My handheld Garmin has a speed accuracy of better than .0001 of a mi, position accuracy is 3ft or better. And it's just a cheap WAAS enabled handheld. Aviation GPSs are much much more accurate and reliable.

Also, your speedo is wrong. No car (other than calibrated police cruiser speedos) has an accurate speedo and its on purpose by design. Typically speedos will show a speed up to 3mph than what you're actually doing. Also changing the dia. of your tires will also affect readings.

[Edited 2009-06-07 16:31:31]

User currently offlineLowrider From United States of America, joined Jun 2004, 3220 posts, RR: 10
Reply 7, posted (5 years 6 months 3 weeks 2 days 23 hours ago) and read 13434 times:

This has been done well before the advent of GPS. In the event that both airspeed indicators become inop for any reason DME or INS groundspeed along with pitch and power information can be used to approximate airspeed.


Proud OOTSK member
User currently offlineN353SK From United States of America, joined Jun 2006, 833 posts, RR: 0
Reply 8, posted (5 years 6 months 3 weeks 2 days 23 hours ago) and read 13424 times:



Quoting FLY2HMO (Reply 6):

Yes. IIRC the Garmin 430 GPS has a function where you can calculate your TAS knowing your pressure altitude and temp. IAS need not be known.

Take a look at your E6-B. In order to calculate true airspeed you need to know your calibrated airspeed. Calibrated airspeed is based off of indicated airspeed, which can only be ascertained from the pitot-static system.


User currently offlineDw747400 From United States of America, joined Aug 2001, 1265 posts, RR: 1
Reply 9, posted (5 years 6 months 3 weeks 2 days 22 hours ago) and read 13410 times:

In theory, Sat WX with winds aloft could be linked to the GPS in order to compute a rough approximation of TAS/IAS. Of course, this would depend on the reliability of the wind forecasts. I'm not sure how useful this would be when winds are rapidly shifting, though newer GPS has impressive refresh rates.

As others have mentioned GPS reliability isn't perfect, but there is no reason not to use it as a backup when all other options fail.



CFI--Certfied Freakin Idiot
User currently offlineStarlionblue From Greenland, joined Feb 2004, 17186 posts, RR: 66
Reply 10, posted (5 years 6 months 3 weeks 2 days 21 hours ago) and read 13382 times:

Would there be an issue with altitude? That is, does GPS assume you are moving at ground level and base speed measurements on this?

Quoting AirframeAS (Reply 4):
Case in point: I have a GPS for my car and the speed that my GPS tells me is not the same as what my car's speedometer says. GPS is not 100% accurate.

As mentioned, it's not the GPS. Your speedometer is inaccurate. The method of measurement (based on the wheels) introduces all sorts of inaccuracies.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineAirframeAS From United States of America, joined Feb 2004, 14150 posts, RR: 24
Reply 11, posted (5 years 6 months 3 weeks 2 days 21 hours ago) and read 13378 times:



Quoting FLY2HMO (Reply 6):
GPS is extremely accurate, more so than any speedometer out there.

My readings comeout totally different from each other.

Quoting FLY2HMO (Reply 6):
Also, your speedo is wrong.

I disagree. My car is pretty brand new, the speedometer was calibrated by the manufacturer.

Quoting Starlionblue (Reply 10):
As mentioned, it's not the GPS. Your speedometer is inaccurate. The method of measurement (based on the wheels) introduces all sorts of inaccuracies.

See above.



A Safe Flight Begins With Quality Maintenance On The Ground.
User currently offlineFLY2HMO From , joined Dec 1969, posts, RR:
Reply 12, posted (5 years 6 months 3 weeks 2 days 20 hours ago) and read 13365 times:



Quoting AirframeAS (Reply 11):
My readings comeout totally different from each other.

And your GPS is correct, not the speedo.

Quoting AirframeAS (Reply 11):
I disagree. My car is pretty brand new, the speedometer was calibrated by the manufacturer.

There is ALWAYS some error in it, it's impossible to avoid, and manufacturers lean towards a faster reading if there is an error. So your speedo will always read slower than what you're going.

http://en.wikipedia.org/wiki/Speedometer


User currently offlineAirframeAS From United States of America, joined Feb 2004, 14150 posts, RR: 24
Reply 13, posted (5 years 6 months 3 weeks 2 days 20 hours ago) and read 13359 times:



Quoting FLY2HMO (Reply 12):
There is ALWAYS some error in it...

Same with the GPS. If the speed on cars was not correct, then we would be seeing so many folks being pulled over for speeding maybe?  sarcastic 



A Safe Flight Begins With Quality Maintenance On The Ground.
User currently offlineMandala499 From Indonesia, joined Aug 2001, 6972 posts, RR: 76
Reply 14, posted (5 years 6 months 3 weeks 2 days 20 hours ago) and read 13360 times:



Quoting FLY2HMO (Reply 6):
Yes. IIRC the Garmin 430 GPS has a function where you can calculate your TAS knowing your pressure altitude and temp. IAS need not be known.

And wind...

Quoting FLY2HMO (Reply 6):
Far from the truth. GPS is extremely accurate, more so than any speedometer out there. My handheld Garmin has a speed accuracy of better than .0001 of a mi, position accuracy is 3ft or better. And it's just a cheap WAAS enabled handheld. Aviation GPSs are much much more accurate and reliable.

In fairness, GPS is only good when used in conjunction with other stuff and backup when it comes to aviation.
If you've descended based on your GPS expecting a coastline only to find that once U're below the clouds you found NOTHING but water, U'd start wishing you got something else other than your GPS. This happened to a friend and luckily, he only had to wait for 15 mins until the local VOR switched on on schedule... and guess what, the GPS threw him 40NM off course. The airline later sent back the GPS and antennae back to the shop and they found nothing wrong. Another friend felt something was not right on his 777 one day and decided to look at the POS page in his FMC and saw that 1 GPS detected his position about 30NM away from his position. That gave him cold sweat for a couple of hours.

As a backup or used in conjunction with other stuff, GPS is wonderful... but, it is not all the hype people believe it to be... but it is very useful... no doubt.



When losing situational awareness, pray Cumulus Granitus isn't nearby !
User currently offlineAirframeAS From United States of America, joined Feb 2004, 14150 posts, RR: 24
Reply 15, posted (5 years 6 months 3 weeks 2 days 20 hours ago) and read 13356 times:



Quoting Mandala499 (Reply 14):
As a backup or used in conjunction with other stuff, GPS is wonderful... but, it is not all the hype people believe it to be... but it is very useful... no doubt.

 checkmark  You said it better than I could. Thanks!  Smile



A Safe Flight Begins With Quality Maintenance On The Ground.
User currently offlineNomadd22 From United States of America, joined Feb 2008, 1903 posts, RR: 0
Reply 16, posted (5 years 6 months 3 weeks 2 days 20 hours ago) and read 13356 times:

With flying through storms and in and out of the jet stream you air speed can be more than 140 knots off your ground speed.
Saying that, GPS can be a lot more useful than most people realize. A three antenna system with a tiny little gyro for temp loss of signal backup and attitude information is a whole different animal than a K-Mart handheld.
My Iphone map keeps showing me in Vietnam. Dumb thing confuses west latitude with east.



Andy Goetsch
User currently offlineTdscanuck From Canada, joined Jan 2006, 12709 posts, RR: 80
Reply 17, posted (5 years 6 months 3 weeks 2 days 19 hours ago) and read 13341 times:



Quoting Jawed (Thread starter):
why don't modern airliners use GPS as a backup option to measure their aircraft's current speed?

I'm pretty sure some do.

Quoting Starlionblue (Reply 10):
Would there be an issue with altitude?

Nope. As long as you've got 5+ satellites, you can get coordinates in 3D. You can technically do that with 4 for pure geometry, but then you don't get accurate time.

Quoting Starlionblue (Reply 10):
That is, does GPS assume you are moving at ground level and base speed measurements on this?

Aeronautical GPS's shouldn't do this. Some handhelds might.

Tom.


User currently offlineStarlionblue From Greenland, joined Feb 2004, 17186 posts, RR: 66
Reply 18, posted (5 years 6 months 3 weeks 2 days 18 hours ago) and read 13312 times:



Quoting AirframeAS (Reply 11):
Quoting FLY2HMO (Reply 6):
Also, your speedo is wrong.

I disagree. My car is pretty brand new, the speedometer was calibrated by the manufacturer.

Quoting Starlionblue (Reply 10):
As mentioned, it's not the GPS. Your speedometer is inaccurate. The method of measurement (based on the wheels) introduces all sorts of inaccuracies.

See above.

Just to cite one example, your tires change size with changes in temperature. So after driving for a while, your tires have heated up and the circumference of the tires has changes. This results in a small error.



"There are no stupid questions, but there are a lot of inquisitive idiots."
User currently offlineL-188 From United States of America, joined Jul 1999, 29840 posts, RR: 58
Reply 19, posted (5 years 6 months 3 weeks 2 days 16 hours ago) and read 13287 times:

I have a Boeing anti-ash video and they talk about the possiblity of loosing all three pitots in the advent of a volcanic ash encounter.

They do mention using groundspeed readings from ATC to compute airspeed. No reason why those readings have to come from ATC radar vs. GPS.



OBAMA-WORST PRESIDENT EVER....Even SKOORB would be better.
User currently offlinePhilSquares From , joined Dec 1969, posts, RR:
Reply 20, posted (5 years 6 months 3 weeks 2 days 16 hours ago) and read 13277 times:



Quoting L-188 (Reply 19):
I have a Boeing anti-ash video and they talk about the possiblity of loosing all three pitots in the advent of a volcanic ash encounter.

They do mention using groundspeed readings from ATC to compute airspeed. No reason why those readings have to come from ATC radar vs. GPS.

I am sure the 330, like every other transport aircraft, has in it's QRH an abnormal procedure that will cover flight with unreliable airspeed indications. The procedure would have each pilot check their airspeed and ADC to try to determine which airspeed indicator is the most reliable. Absent that, there is a listing of "typical" pitch and thrust settings that the crew can use. With those you can then get the GS from the FMC which has a GPS input.


User currently offlineStar_world From Ireland, joined Jun 2001, 1234 posts, RR: 0
Reply 21, posted (5 years 6 months 3 weeks 2 days 11 hours ago) and read 13208 times:



Quoting AirframeAS (Reply 11):

I disagree. My car is pretty brand new, the speedometer was calibrated by the manufacturer.

As other have said already, this is completely incorrect - the speedometer is wrong, and the GPS is correct. If you want to add some doubt to the conclusion you have come to, try driving with 2 GPS devices in the car, from 2 different manufacturers. I think you will eventually agree  Smile


User currently offlineNomadd22 From United States of America, joined Feb 2008, 1903 posts, RR: 0
Reply 22, posted (5 years 6 months 3 weeks 2 days 11 hours ago) and read 13194 times:



Quoting Nomadd22 (Reply 16):
Dumb thing confuses west latitude with east.

Dumb poster confuses latitude with longitude.

The biggest single difference between good GPS and cheap GPS is probably receivers that scan channels compared to receivers that monitor channels simultaneously. Good ones with real multi channel capability can compare data streams real time and discard one that doesn't add up with the others. Errors on cheap ones can affect all channels and not get caught. The technology is extremely reliable. Some of the hardware isn't.



Andy Goetsch
User currently offlineTrex8 From United States of America, joined Nov 2002, 4884 posts, RR: 14
Reply 23, posted (5 years 6 months 3 weeks 2 days 8 hours ago) and read 13143 times:
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Quoting AirframeAS (Reply 11):
Quoting FLY2HMO (Reply 6):
Also, your speedo is wrong.

I disagree. My car is pretty brand new, the speedometer was calibrated by the manufacturer.

car manufacturers are allowed a certain inaccuracy in their speedos, no more than a few mph either way for US and Japanese manufacturers and the Europeans never allow the indicated speed to be under the true speed (so your porsche is never going100mph when the speedo says it is, by EU law the true speed will have to be at best just under!)

there's a reason the Europeans are developing Galileo GPS, do you really trust the Pentagon in a wartime to keep Navstar GPS transmissions accurate for everyone all the time? let alone the paranoia the Chinese and Russians have for keeping or developing their own GPS constellations.


User currently offlineWPIAeroGuy From United States of America, joined Aug 2007, 240 posts, RR: 0
Reply 24, posted (5 years 6 months 3 weeks 2 days 7 hours ago) and read 13111 times:



Quoting Tdscanuck (Reply 17):
Nope. As long as you've got 5+ satellites, you can get coordinates in 3D. You can technically do that with 4 for pure geometry, but then you don't get accurate time.

Aren't only four satellites needed, with the fourth being the for the time calibration?



-WPIAeroGuy
25 Post contains images Phoenix9 : I took these pictures last year on an AC flight. The ground speed readings on the IFE unit and my GPS unit were fairly consistent (just have to conver
26 Post contains links N353sk : Not for aviation. At least five are needed, and six is ideal. Take a look at this if you'd like: http://en.wikipedia.org/wiki/RAIM
27 DescendVia : 4 or 3 and a Baro altimeter for basic RAIM. That in turn allows LNAV MDA approaches and terminal capability if the database is kept current. 5 or 4 w
28 Zeke : Airbus have a backup speed tape on new aircraft these days that replaces the airspeed information with an angle of attack fast/slow indicator, the alt
29 Flighty : Yup, that seems to be exactly what the thread starter had in mind... it would require some revision of the information systems on the plane. But the
30 L-188 : And that is why GPS isn't great, There are spots north of the Brooks Range where you can't get 4 birds. GPS is good, but it isn't infallible.
31 WPIAeroGuy : I'm not familiar with RAIM (I've never heard of it until now) but from the article the 5th or 6th satellite seems to only be needed as a backup, and
32 Nomadd22 : I can't really make any sense out of that. GPS sats are at a 55 degree inclination. There should be more available at the poles than at the equator.
33 TSS : One of the car magazines (Car And Driver, I believe) used to post a "speedometer error" table in their road test results. Most cars showed a percenta
34 PPVRA : Why not also use the RAT as an emergency air speed sensor? Shouldn't be too complicated, no?
35 Mandala499 : Better still, have deployable pitot static tubes for backup !
36 Osiris30 : Apart from drag, that's brilliant.
37 DaBuzzard : Hmmm, now that just might be doable. Would be working off air flow, so as long as you know the load and rpm, you could work a system out. Would it be
38 Tdscanuck : An orbit at 55 degree inclination means the satellite loops between 55 N and 55 S latitude. If you're below those latitudes, the satellites will go d
39 N353SK : All I can tell you is that you need at least 5 to have RAIM and that you need to have RAIM to use your GPS under IFR.
40 DiamondFlyer : Or WAAS, as was previously stated. Ultimately it comes down to if your GPS box is installed to the TSO 129 standard versus the TSO 146 standard. But,
41 DescendVia : 5 or 4 with a baro altimeter setting for RAIM and 6 or 5 with a baro altimeter setting to have RAIM delete corrupt signals.
42 Tietkej : Hate to be naggy, but your GPS speed is more likely to be correct than your speedometer in the car (provided you are going straight on flat land). Ma
43 Faro : How about pressure-sensitive sensor pads on the wing leading edges? I would imagine that these can be made pretty sensitive these days. After all, for
44 Nomadd22 : Satellites directly overhead are acutally the least usefull, since they have the smallest movement compared to your position. The satellites are arou
45 Zappbrannigan : Yep, I've checked my car speedo with good handheld units, and it overreads by around 3 km/h at just about every point on the dial I tested, up to aro
46 GLEN : To get your indicated airspeed you need the difference between dynamic air pressure (from pitot) and static air pressure (from static source). With b
47 Nomadd22 : GPS doesn't have tires that change diameter throughout their life, manufacturers who think 5% off is accuracy enough, different size tires than the fa
48 Faro : Quite the item, yes, I would prefer that too but then that would also be susceptible to extreme icing/contamination conditions if we -very very tenta
49 Mandala499 : Well, with all this talk about GPS as Pitot Tube Backup... Why not the BUSS/BackUpSpeedtapeSystem that's available on the Airbus? It will allow you t
50 Faro : Very interesting, does it simulate indicated airspeed? Or does it -somehow- render indicated airspeed unnecessary to maintain controlled flight? Faro
51 Post contains links David L : Someone kindly posted the following in the AF447 thread: http://aviationtroubleshooting.blogs...liable-speed-by-joelle-barthe.html See Section 5.
52 Faro : Thanx for the link. I would not be surprised if some manner of BUSS becomes mandatory for all EFIS types some time in the future; very smart idea tha
53 Nomadd22 : I've acutually cobbled together something like that. The signal through one sensor measuring the difference in pressure run through another sensor me
54 DaBuzzard : In stable flight (minimal control inputs) the hydraulic load would be relatively stable, and could be measured and accounted for as could the electri
55 Nomadd22 : Now I'm trying to wrap my brain around a simple acoustical system. Transmit a ping at one place and recieve it at another. Not being photons, you woul
56 Lowrider : Not to be overly simplistic, but couldn't this also be solved with a seperate, independent pitot static system in a location far enough from the prima
57 TristarSteve : If we are worried about pitot tubes icing up, why not put a back up probe somewhere warmer. It would not be as accurate, but why not in the engine? So
58 Tdscanuck : They're called pressure belts. They are wonderful instruments, but incredibly intolerant of moisture and temperature...not exactly great equipment fo
59 Nomadd22 : You mean all these years I thought the speed of sound varied with air density I was wrong?
60 Zappbrannigan : The speed of sound varies with temperature. Check any ATPL textbook, it's pretty much the first thing covered in transonic aerodynamics.
61 Zeke : You are 100% correct, the speed of sounds varies with the inverse of density.
62 Zappbrannigan : It does change as density changes, but is it not said to vary overall with temperature, for the purposes of determining M1.0 for given atmospheric co
63 Nomadd22 : I realize there's a misapprehension that sound speed is always related to density, when it's actually related to medium compressibility, and with gase
64 Lowrider : I am aware. I have flown a few. Usually they are mounted near the nose and in relatively close proximity to each other. However if the concern is tha
65 Tdscanuck : Yes. No. If you have an unconstrained ideal gas undergoing isentropic changes, density varies with inverse of the temperature, which will cause a cor
66 GLEN : Well, that is wrong and in contradiction to the rest of your (correct) comment. You wrote yourself, that temperature is changing the speed of sound.
67 Post contains links Zeke : The general assumption most people use is that air is an ideal gas, and dry, i.e. no relative humidity. Air is normally not dry, and density does cha
68 Post contains links Nomadd22 : My trouble might be using tables, like the one at Glenn http://www.grc.nasa.gov/WWW/K-12/Win...unnel/Activities/knots_vs_mph.html If it was just altit
69 Tdscanuck : Yep, it's completely wrong. I'm honestly not sure what I was thinking when I wrote that sentence. The one above (different density, same temperature)
70 L-188 : Yes but what ends up happening is that the Brooks Range (figure 68N) gets in the way. GPS signals don't propagate well through rocks. Get high enough
71 Post contains images JetMech : Dynamic air pressure is the one that you are trying to use to calculate airspeed. The total tube measures the total pressure of the ambient atmospher
72 GLEN : You're right. That's what I meant, but my memory was a bit careless with the correct definitions.
73 Post contains links JetMech : I had a look at a compressibility factor chart for air. It appears the behaviour of air adheres reasonably close to that of an ideal gas for temperat
74 Tdscanuck : That's about right. It holds up to pretty high pressure (higher than you'd typically see in commercial aeronautica) and is good for temperatures up u
75 Zeke : That is the backdoor way of saying a change in air density.
76 Tdscanuck : How do you figure? For any molecular weight I can give you any density. Molecular weight is constant for a gas mixture, density is not. Tom.
77 Zeke : Suggest you actually read and understand the second link I provided in reply 67, clearly lets you know how the air density changes in air, and also t
78 Nomadd22 : More water means you're dealing with a different gas. You can't change the composition of the air and then use charts to prove that density is what ca
79 Tdscanuck : I did. I'm not sure why you assume I didn't. Actually, it explicitly says that, for a given composition, it depends on temperature only. If you chang
80 JetMech : Out of curiosity, is relative humidity or suspended water content measured or taken into account for any data or performance calculations? Regards, J
81 Nomadd22 : Actually, I was wondering the same thing. How suspended or falling liquid water affects sound speed. I know it increases attenuation.
82 Zeke : The speed of sounds in any form of matter, gas/liquid/solid is equal to the square root of the ratio of the elastic modulus of the medium (that is, t
83 TUNisia : Why not make a backup Pitots in the same form as the RAT (ram air turbine)? In the event that the Pitots are faulting have the backup Pitots engage fr
84 Tdscanuck : Like Zeke said...not for performance calculations. Suspended or falling (i.e. liquid) doesn't change the speed of sound, since that's running in the
85 WPIAeroGuy : I'm gonna back up Tom on this one and say that speed of sound depends only on temperature, ratio of specific heats (gamma) and the gas constant.
86 Zeke : That is an approximation, as it assumes an ideal gas, you have substituted Boyles law in for density.
87 Tdscanuck : Yes. But it's an *extremely* good approximation. There's no such thing as a true ideal gas, but air is so close for the pressures and temperatures of
88 Zeke : Most engineering calculations do involve the use approximations of some form or another, it is up to the engineer to know where the limits of these a
89 Tdscanuck : Agreed. However, I think this is somewhat tangential to the initial issue of what determines speed of sound. Humidity absolutely does change speed of
90 WPIAeroGuy : I think an easy way to picture it is have a sealed box with a constant mass of gas. By changing the temperature, your density will stay the same but t
91 Post contains links Zeke : Every time I have seen the speed of sound derived from first principles, it has always been done in the same way, by using mass continuity equation t
92 Tdscanuck : It absolutely is, provided you include the changes in R and gamma as you go through the engine. Although that's really a continuous function, you're
93 Nomadd22 : That's kind of why I was wondering about the speed through air with a lot of suspended liquid water (heavy rain or fog). I know the speed doesn't cha
94 Tdscanuck : Even in really heavy rain, I'm not sure that the liquid/gas fraction gets that high. As noted above: I think this is probably the dominant effect. Al
95 Zeke : I beg to differ with the "no difference" comment, it does not make a significant difference, but it does make a difference. I have clearly shown in r
96 Tdscanuck : Not one that you can measure outside a very finely tuned lab. Absolutely no difference for the purposes of aviation. You showed how it varies *with g
97 Post contains links Zeke : Incorrect, you can measure it, and as far as I know the US Army and Navy now use an optical air data system on some of their helicopters because of p
98 Tdscanuck : A brownout is air full of sand and dust...naturally, this messes with the measuring equipment. That doesn't mean the speed of sound in the air actual
99 Nomadd22 : Vapour wise, at 40C you can get around 5% water in air. I've been in 8 inch per hour rain and it seemed like I was underwater. But, to keep it on my l
100 Tdscanuck : Just for kicks, I went and pulled the NASA chemical tables...not derived from theory, but the honest to god experimental data. For dry air at 20 degC
101 Post contains links and images Zeke : Huh ? It is not longer just air anymore, how much sand is in the international standard atmosphere ? Changes with the inverse of density, speed of so
102 WPIAeroGuy : From Anderson's "Fundemental's of Aerodynamics-Fourth Edition" Part 3 Chap. 8.Eq 8.25 a=sqrt(gamma*R*T) and quote "...it clearly states that the spee
103 Post contains links Tdscanuck : But the sand doesn't change how fast the pressure wave is propagating in the air. It does attenuate it though. Yes. And the square root of pressure/d
104 Tdscanuck : I just thought of a much simpler way to say this... If you know what gas you have, and you know the density, that's not enough information to know the
105 Nomadd22 : It would be a lot easier talking about precipitation if some long word loving folks hadn't started misusing it until everybody took it to mean both t
106 Zeke : My quote was in response to "real world speed of sound in air", I was talking about reality. Air in the atmosphere is not ideal/calorically perfect,
107 Tdscanuck : The air is ideal and calorically perfect (to such a close degree as you can't tell the difference). The liquids and solids, obviously, do not obey th
108 WPIAeroGuy : I don't believe anyone is saying air is ideal, we're just saying that the ideal gas approximation is good enough for the vast majority of flight cond
109 Tdscanuck : This definitely would work but, once you start introducing materials that aren't gasses, you lose the physical basis for R and gamma and they become
110 Zeke : That is a common APPROXIMATION. Many papers have been written on this subject, it has many real life industrial application in particular if you want
111 Tdscanuck : Yes, it's an approximation. But, as has been stated several times: You seem to be significantly underrepresenting how accurate the approximation is.
112 WPIAeroGuy : Very true, I forgot sand wasn't a gas for a moment. Everything in engineering is an approximation. Pendulum theory is based sin(theta)=theta for smal
113 Zeke : Not at all, in fact I have been the only one to show the difference above, I have shown it is about 1 kt difference at sea level, constant temperatur
114 WPIAeroGuy : Because its not necessary. The gas constant varies very little because N2, O2, and CO2 all have a gamma of about 1.4, so any slight variation isn't g
115 Tdscanuck : Yes. Nobody (that I can tell) is suggesting the speed of sound stays constant if you alter the composition. Yes. You should expect that. You've chang
116 Zeke : I was keeping everything else constant. (i.e. pressure/temperature), I didn't see the need to explicitly state that fact. Just like if I was explaini
117 Tdscanuck : It's not physically possible to keep pressure and temperature constant and vary density unless you also change composition. We both agree that if you
118 Tdscanuck : I forgot at least one other...that the density is high enough or, equivalently, that your control volume is large enough that the continuity assumpti
119 WPIAeroGuy : Plus if you look at it from a real world perspective, sensors are only so accurate. There's no need be able to calculate out to the 6th decimal place
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