istrumit

istrumit

Well Known Member
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
 
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.
 
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Auburntsts said:
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.
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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.
 
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 said:
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.
Click to expand...
 
istrumit said:
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.
Click to expand...

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.
 
BobTurner said:
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.
Click to expand...

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.
 
istrumit said:
tomkk said:
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.
Click to expand...

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?
Click to expand...
 
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Auburntsts said:
istrumit said:
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?
Click to expand...

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

All good info.
Click to expand...
 
istrumit said:
Auburntsts said:
I have not had a static check completed. Also, I think the static ports are not in the Vans recommended location.

All good info.
Click to expand...

A static check isn't required for VFR flight, only IFR (every 24 months), but it might be helpful if no other smoking guns materialize. Also, besides location, the actual physical shape for the ports are important. If the ports are flush to the skin, most likely you'll have static port induced errors.
Click to expand...
 
It's not hard to check for leaks (finding them can be!). Get some vinyl tubing of just the right diameter for your static port or pitot tube. Hold it in a U shape and add water and fill the U up 3 or 4 inches. Force one end over the pitot or static port, use tape to make it air tight. Before hand tape over any other static ports if you have more than one, tape over the pitot drain hole. Now raise the open end of the tube a few inches. The water should not flow towards the pitot or static port (the water should be higher in the end you raised). If it the water quickly comes back to level on both sides of the U, you have a leak.

BTW, if there is no leak on the pitot side, you can check your airspeed calibration this way. There are published charts showing how many inches difference in the water level equals what indicated airspeed reading. If that checks out and you still have a problem in flight, it must be in the static side.
 
The average ground speed from two runs on reciprocal headings only equals the TAS if there is zero cross wind. If there is crosswind, the TAS will be higher than average ground speed.

If reciprocal tracks are used, the TAS will be higher than the average ground speed, if there is any crosswind.
 
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