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INDICATED AND TRUE AIRSPEED vs GROUNDSPEED


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Have a question as to the reasons for the following.

 

I was flying today at 6000ft heading 350 indicated airspeed 110 and GPS was showing G.S of 80 knots. Turned aircraft onto heading 170 indicated airspeed 110 and GPS was showing G.S of 140 knots giving a wind component of 30 knots. Question is my aircaft has an indicated airspeed of 120 at 1000ft so my true airspeed should be 120 at 6000ft with an indicated airspeed of 110. If you try to work out the wind speed from TAS the answer is incorrect. 120 TAS heading 350 GPS groundspeed 80 knots wind component 40 knots. Heading 170 TAS 120 groundspeed should be 160 but GPS shows 140 knots ?

 

 

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One thing you do know. The GPS gives you groundspeed. The rest you will have to work through. It

 

s unlikely your airspeed is accurate. You have TAS (derived) CAS and IAS. may have unknown errors . Nev

 

 

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In your explanation you used the term heading, I would like to check that by heading you mean magnetic compass heading, or do you mean GPS derived ground track? one is wind adjusted and one isn't.

 

But as DJP says doing the exercise on a nil wind day is the go.... or doing it where you have absolute known wind vs altitude.....I seem to recollect that there are a number of wind traces that the glider guys use like this one for Adelaide http://slash.dotat.org/cgi-bin/atmos?loc=94672&latest=1 as you can see the traces show wind changes around a lot more than you would think given the few ARFOR altitude wind steps covered. I note that the website has locations other than Adelaide and if you look at Brisbane for today you can see that the wind direction linearly changed 315 degrees in 4500ft, albeit a bit higher than we can go but it just shows that ARFOR spot heights can sometimes hide significantly what is really happening....

 

Andy

 

Andy

 

 

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Have you had CAO 100.5 done to check instruments? Cheap instruments can have lots of variance at different alititudes. I know a guy with a similar name to me that hangs out at YLED that can do that stuff for you.

 

Jim (Shameless).

 

 

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One thing you do know. The GPS gives you groundspeed. The rest you will have to work through. Its unlikely your airspeed is accurate. You have TAS (derived) CAS and IAS. may have unknown errors . Nev

True. You should probably start way back with your tachometer - by comparison to optical tacho if you are planning to make any comparisons to manufacturers performance numbers. You need to check that IAS is accurate. You need to check OAT accuracy as well. When checking RV's we usually fly a equilateral triangular 'course' and then average the numbers. Others do a 4 leg course.

 

Then maybe you need to accept that a headwind will always be stronger than a tailwind! (old aviator tale).096_tongue_in_cheek.gif.d94cd15a1277d7bcd941bb5f4b93139c.gif

 

happy days,

 

 

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"Question is my aircaft has an indicated airspeed of 120 at 1000ft so my true airspeed should be 120 at 6000ft with an indicated airspeed of 110."

 

Are you using your prayer wheel to get the actual TAS from your IAS i.e. Correction for alt and OAT?

 

 

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"Question is my aircaft has an indicated airspeed of 120 at 1000ft so my true airspeed should be 120 at 6000ft with an indicated airspeed of 110."Are you using your prayer wheel to get the actual TAS from your IAS i.e. Correction for alt and OAT?

I am using the Dynon D100 that gives the TAS correction for IAS. The temperature probe is on the belly skin under and behind the passenger seat.

 

 

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Have you had CAO 100.5 done to check instruments? Cheap instruments can have lots of variance at different alititudes. I know a guy with a similar name to me that hangs out at YLED that can do that stuff for you.Jim (Shameless).

No the instruments need to have there 2 year calibration done.

 

 

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What are you actually trying to achieve? Firstly, the indicated airspeed is subject to "position error" (otherwise known as "airspeed system error") - if your aircraft is a certificated type, this error will normally be given in the Flight Manual. Secondly, indicated airspeed is subject to any error in the ASI instrument itself - and the permissible maximum for the instrument to be "servicable" is plus or minus 4 knots. Thirdly, indicated airspeed assumes standard sea-level air density.

 

If you correct for those three things (assuming there is nothing wrong with your airspeed system, like water in the plumbing, leakage, a distorted or misaligned pitot, etc) , you will get True Airspeed.

 

True airspeed differs from groundspeed by the vector sum of the wind velocity. Not by the simple sum, unless you happen to be flying directly upwind or downwind.

 

The net result of all this is that the groundspeeds one calculates using the forecast wind and the normal traditional forms of mechanical computer, is at best a first-order approximation only.

 

 

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I

 

What are you actually trying to achieve? Firstly, the indicated airspeed is subject to "position error" (otherwise known as "airspeed system error") - if your aircraft is a certificated type, this error will normally be given in the Flight Manual. Secondly, indicated airspeed is subject to any error in the ASI instrument itself - and the permissible maximum for the instrument to be "servicable" is plus or minus 4 knots. Thirdly, indicated airspeed assumes standard sea-level air density.If you correct for those three things (assuming there is nothing wrong with your airspeed system, like water in the plumbing, leakage, a distorted or misaligned pitot, etc) , you will get True Airspeed.

 

True airspeed differs from groundspeed by the vector sum of the wind velocity. Not by the simple sum, unless you happen to be flying directly upwind or downwind.

 

The net result of all this is that the groundspeeds one calculates using the forecast wind and the normal traditional forms of mechanical computer, is at best a first-order approximation only.

I was flying directly upwind and downwind using a burning stubble paddock as a wind reference and the GPS to indicate the minimum and maximum ground speeds in the applicable direction. Maybe I should forget about TAS and just work out wind speed from difference between IAS and groundspeed for low performance aircraft ?

 

 

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Have you had CAO 100.5 done to check instruments? Cheap instruments can have lots of variance at different alititudes. I know a guy with a similar name to me that hangs out at YLED that can do that stuff for you.Jim (Shameless).

Can I get his number please ?

 

 

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II was flying directly upwind and downwind using a burning stubble paddock as a wind reference and the GPS to indicate the minimum and maximum ground speeds in the applicable direction. Maybe I should forget about TAS and just work out wind speed from difference between IAS and groundspeed for low performance aircraft ?

I gather you were trying to discover the wind speed and direction at your height?

A flight data computer (which needs GPS or inertial input, as well as flux-gate compass data, and pitot and static inputs) will give you a windspeed. Problem is, they usually cost almost as much as a recreational aircraft. (Maybe there's an affordable one by now, but it will still need a flux-gate compass input.) Some gliding computers get the wind by averaging the GPS ground speed over a period of thermalling, assuming the rate of turn is fairly constant.

 

Flying VFR cross-country in the days before GPS, one could eliminate the need to guess the wind, by using a time-scale, which has an elastic scale that you can stretch so it reads the distance across the map in minutes, rather than in hours. How much you stretch the scale is indicated in groundspeed, i.e. the groundspeed you are actually achieving - which is what you want to know, for fuel usage calculations.

 

 

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The area 500 Garmin coupled to the EFIS gives you a winspeed and direction. Seems pretty accurate. Not on the older set ups like mine. I presume calculated from TAS, GND SPEED, HGD and TMG.

 

The question arises what use is it really. You already have a track, a heading for TMG, HDG to destination, and ground speed. Anything else is just a gimmick

 

 

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