Airspeed Calibration

[istrumit]

Hello all -

I know this subject has been covered many times over in this forum and I have read all of the posts.

But, here you go anyway.

I had a chance to fly a a three-hour trip yesterday that was an out and back (direct return flight), with a 1000 foot altitude difference (8500 out and 7500 back).

My ground speed in level flight was 164 knots outbound 180 knots on the return.

Same power/mixture settings...etc

My question is, does this mean that my TAS was 172 kts ? (assuming that the winds aloft were not changing much).

I ask because, since I have owned the plane, my headwinds have always been 6-7 knots less than forecast and my tailwinds have always been 6-7 kts more than forecast, making me think the TAS is not accurate.

Thanks,

Scott

[Auburntsts]

Maybe. 172 was your avg ground speed for the flight for sure. Normally we'd have to know the altimeter setting, temp, and IAS, and the wind direction and speed to figure the TAS and ground speed relationship.

Remember that ground speed is equal to TAS minus the headwind/plus the tailwind component. TAS and ground speed are only equal in no wind conditions, so TAS will be greater than ground speed with a headwind and less with a tailwind.

[istrumit]

Originally Posted by Auburntsts

No. 172 was your avg ground speed for the flight. We'd have to know the altimeter setting, temp, and IAS, and the wind direction and speed to figure the TAS and ground speed relationship.

OK...assuming that baro , wind speed/direction, and IAS where all the same (reasonable, since it was a direct return, two hours later, along the same route), then the only variable would be OAT (which did change by 4-5 degrees).

But, "if" OAT was the same, then it would seem to approximate a simple GPS based calibration run.

[BobTurner]

1. If you made the same power on both runs, you'll be faster at 8500' than 7500' - less drag.
2. If you had a direct tailwind/headwind, then the average ground speed is your TAS (IF you had been at the same altitude). If there was a crosswind component then your TAS will be more than that average since you will be flying in a crab.
3. If you suspect you always have a tailwind or low headwind, look for a static leak. Or maybe a poorly designed static port, giving you a lower than correct pressure.
4. There are lots of descriptions on how to do a three-way calibration run(s). Start there.

[tomkk]

Not sure it's quite that straight forward. Seems like there could be a wind difference over the time period and altitude difference.

There's another thread here that talks about GPS TAS verification (http://www.vansairforce.com/communit...AS+spreadsheet). Spreadsheet link in Post #2.

[istrumit]

[quote=tomkk;1087119]Not sure it's quite that straight forward. Seems like there could be a wind difference over the time period and altitude difference.

Yea...I agree. There would be differences. Wind could shift. OAT def changed. Altitude was 1000 feet different.

Overall,not a perfect test. But, I have been suspecting TAS was 5-7 knots low for a while since the winds aloft calculation on the G900 is always more favorable than what the pre-flight data says it should be.

This is just one more datapoint that suggests I have an incorrectly calibrated TAS.

[Auburntsts]

Originally Posted by istrumit

OK...assuming that baro , wind speed/direction, and IAS where all the same (reasonable, since it was a direct return, two hours later, along the same route), then the only variable would be OAT (which did change by 4-5 degrees).

But, "if" OAT was the same, then it would seem to approximate a simple GPS based calibration run.

As the others have posted, it's not that simple. TAS changes with temp and altitude. That's why airspeed calibration via GPS is done at a constant altitude to eliminate the temp and altitude changes, constant power settings, and on a 3 or 4 course pattern to factor out the wind effects.

[istrumit]

Originally Posted by BobTurner

1. If you made the same power on both runs, you'll be faster at 8500' than 7500' - less drag.
2. If you had a direct tailwind/headwind, then the average ground speed is your TAS (IF you had been at the same altitude). If there was a crosswind component then your TAS will be more than that average since you will be flying in a crab.
3. If you suspect you always have a tailwind or low headwind, look for a static leak. Or maybe a poorly designed static port, giving you a lower than correct pressure.
4. There are lots of descriptions on how to do a three-way calibration run(s). Start there.

Could be a static leak....the autopilot hunts +/- 150 feet also, over about a 1 minute cycle, which seems like static related...could all tie together.

[Auburntsts]

[quote=istrumit;1087121]

Originally Posted by tomkk

Not sure it's quite that straight forward. Seems like there could be a wind difference over the time period and altitude difference.

Yea...I agree. There would be differences. Wind could shift. OAT def changed. Altitude was 1000 feet different.

Overall,not a perfect test. But, I have been suspecting TAS was 5-7 knots low for a while since the winds aloft calculation on the G900 is always more favorable than what the pre-flight data says it should be.

This is just one more datapoint that suggests I have an incorrectly calibrated TAS.

So run a real airspeed calibration test to find out. If you suspect the G900's calculated TAS is low, then most likely your IAS is reading low which is most likely due to a pitot-static leak or possible there's an issue with the pitot itself.

Have you had a static system check done by an avionics shop?

[istrumit]

[quote=Auburntsts;1087127]

Originally Posted by istrumit

So run a real airspeed calibration test to find out. If you suspect the G900's calculated TAS is low, then most likely your IAS is reading low which is most likely due to a pitot-static leak or possible there's an issue with the pitot itself.

Have you had a static system check done by an avionics shop?

I have not had a static check completed. Also, I think the static ports are not in the Vans recommended location.

All good info.