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Air Speed

PostPosted: Mon Mar 17, 2014 10:20 pm
by Dan Jaeger
I am having an issue with the air speeds and the GPS ground speed matching up and making sense in my Velocity. I purchased the airplane in November 2013 and I started noticing that I always had a head wind. I did the True Air Speed calibration on the EFIS and it changed the TAS value and It now makes sense with the ground speed but it doesn't make sense compared to the Indicated air speed. I am trying to rule out anything that I could have wrong in the EFIS before I go Looking at the IAS system. I have had the pito static system checked and the AP said my air speed was within 2 kts of his test box but when I fly at altitude my IAS is a few kts higher than TAS on the GRT Sport SX. Does anybody have any trouble shooting ideas?

Re: Air Speed

PostPosted: Tue Mar 18, 2014 2:21 am
by jclark
Try this simple test.

1. Go flying and notice IAS, TAS, "Headwind" or "Tailwind" component. "OAT" and "Ground Speed".
(Be sure to turn this on and and show all components.)

2. IAS + "Tailwind" = "Ground Speed"????
3. IAS - "Headwind" = "Ground Speed"????

4. If these do NOT add up. "calibrate" heading **AND** verify "Outside Air Temperature (OAT) is correct.

James

Re: Air Speed

PostPosted: Tue Mar 18, 2014 4:06 am
by Bobturner
No, IAS plus or minus wind does not equal ground speed, it is true airspeed plus minus wind equals ground speed.
Try this: slowly circle and watch the gps ground speed, highest is straight tail wind, slowest is headwind. Fly a straight line into the wind, let ground speed stabilize, write it down. Do a 180 and repeat. Average the two ground speeds (direct headwind, direct tailwind): this is TAS, true airspeed. Note pressure altitude (set altimeter to 29.92), note temperature, and use an old fashioned E6B to calculate IAS from the measured TAS, compare to airspeed indicator.
Now what is working, what is not?

Re: Air Speed

PostPosted: Tue Mar 18, 2014 3:05 pm
by Dan Jaeger
I have done almost exactly the flight you described. I used the EFIS to match ground track and heading which should put me either straight into or with the wind. at a heading of 295 I had TAS 150 and Ground Speed of 136. Then turn 180 degrees and to 115 and I had TAS 148 and ground speed of 173. Average GS gives 154.5 and average TAS is 150. I then turned 90 degrees so I was flying with a direct cross wind and showed TAS of 152 and GS of 154. One more 180 degree turn to fly with cross wind going the opposite direction showed TAS of 150 and GS of 153. Averaging all 4 directions I get TAS of 150 and GS of 154. I am calling the 4 knot difference within the margin of error and I say that the TAS calculation on the EFIS is good. Do you agree?

The confusing part for me is the IAS readings of 152, 155,157 and 154. This flight was at 10,500 feet MSL. Outside Air temp according to the EFIS was 1 Degree Celsius, and the pressure altitude reading off of the altimeter with it set to 29.92 was 10,400. If I am reading the E6-B correctly the IAS should have been about 127. The Pito Static system has been recently checked and the airplane air speeds matched the test box air speeds within 2 kts. So If the mechanic did the testing correctly then the airplane has a major difference between IAS and Calibrated air speed. But 25 kts seems like a lot. I remember from pilot training that CAS usually varies just slightly from IAS and its basically zero at cruise speeds. Does the GRT Sport use the IAS for the TAS calculations? Is IAS that is out of whack causing calculation errors in the EFIS? Seems to me that the EFIS is fine and the problem is in the IAS system. I think my next step is to discuss it again with the A&P that did the pito static test. Can you see anything I am missing that would help me when I talk to the mechanic?

Thanks,

Dan

Re: Air Speed

PostPosted: Tue Mar 18, 2014 6:12 pm
by Bobturner
Okay, something's not right here, for sure.
I'm 90% certain the GRT uses IAS to get TAS. How else would it know?
Are you sure you aren't displaying TAS in knots and IAS in MPH?

BTW, if you hold a heading 90 deg to a cross wind, the ground speed should be higher than the TAS, just as you show. You cannot average the 90 deg heading to the wind ground speed data to get TAS, that's not how it works.

Re: Air Speed

PostPosted: Wed Mar 19, 2014 1:08 am
by Dan Jaeger
I never thought about the system being set up in Kts and MPH I will check that out. Airplane has a back up IAS way over on the right side of the panel and it reads the same as the EFIS but maybe it is MPH?? Thanks for the idea and I will let you know what I find.

Re: Air Speed

PostPosted: Thu Mar 20, 2014 2:53 pm
by jclark
Bobturner wrote:No, IAS plus or minus wind does not equal ground speed, it is true airspeed plus minus wind equals ground speed.
Try this: slowly circle and watch the gps ground speed, highest is straight tail wind, slowest is headwind. Fly a straight line into the wind, let ground speed stabilize, write it down. Do a 180 and repeat. Average the two ground speeds (direct headwind, direct tailwind): this is TAS, true airspeed. Note pressure altitude (set altimeter to 29.92), note temperature, and use an old fashioned E6B to calculate IAS from the measured TAS, compare to airspeed indicator.
Now what is working, what is not?


You are **CORRECT**!!!

I was WRONG. Brain and fingers not properly engaged and connected.

What I meant to enter was TAS not IAS.
Also, was trying to say if all is correct and these are same units and if OAT and magnetometer are correct, they **should** add up or at least be close.

As mentioned, 2 runs 180 degrees apart or 3 at 120 or 4 at 90 should be tried.

Re: Air Speed

PostPosted: Thu Mar 20, 2014 6:56 pm
by Bobturner
Off topic, but to calibrate the TAS, only in the case of flying with a direct headwind, then direct tailwind, does the average ground speed equal the TAS. However, if you do three runs 120 degrees apart, you can use the data to solve for the wInd speed and direction, and then calculate the TAS. Flying more legs than three the problem is over-determined but again, you can solve for the wind speed and direction in a 'best fit' sense, and then calculate TAS.

Still waiting to hear if IAS is in knots or MPH.