Airspeed and Altitude Calibration Procedure and Spreadsheet

[Dynon101]

I am FINALLY getting around to doing the airspeed and altimeter calibration in my airplane.

Does anyone have a procedure and an associated spreadsheet that I can use to do this test?

I have seen procedures where you (I) fly one leg on a given TRACK (and some use a HEADING) lets say use 000 (Due North at level altitude and do not make changes in power and let the IAS stabilize (at about the 30 second point should do it) and then I record the GPS Speed and the TRACK (or HEADING) degrees in some place on the spreadsheet…then make a turn to lets say left heading (or track?!?!?) 120 degrees and fly that track for 30 seconds then record GPS speed and track …then fly a track of 240 degrees…

Got it....this compares GPS speed to ground speed and the three legs cancel out for the effects of the wind...some people say to do four legs so I presume that would be mo-betta...

So now is the part(S) I don’t get…

What I want to do is to ensure that my Calibrated Air Speed is close to my Indicated Air Speed and that my Indicated altitude is close to True Altitude (MSL) not just at the one speed and altitude I just did these three/ four legs but I want to ensure that the CAS and True Altitude (MSL) is accurate for speeds from VS+5 all the way up to whatever my engine can get the airplane speed and I want to do this from 1000 MSL up to about 17500 MSL so I should do this same test at the various speeds (VS+5, 80, 100, 120, 140, 180, 200) at 2000 MSL and then go up to higher altitudes (5000, 10000, 15000) and then do the same steps again…

Then I need to plug and chug this data into a spreadsheet and then that spreadsheet will squirt out the data so I can compare my Dynon altimeter and IAS and then make whatever adjustments I need to make the Dynon match what it really should be.

Different air speeds may really affect the static source pressure and that might screw around with both IAS and True Altitude. Example: I do these three legs at 70 KIAS and the GPS altitude shows 2000 feet and then when I do the 180 KIAS legs the GPS altitude might be showing 2800 feet due to poor static source location

The only way to ensure that the indicated altitude when set to the local altimeter setting is close to True Altitude MSL is to actually compare the Indicated altitude to the GPS altitude at the various airspeeds.

So I cant imagine that I am the first person to want to check this data so someone out there in Cyberspace has most likely come up with a spreadsheet that has lots of data blocks to record data such as the local altimeter setting for the location I am doing my triangles over, the OAT for where I am triangulating…and the spreadsheet that will do this fancy Greek math for me and work the E6B backwards to come up with all of this stuff...

Then I presume the Dynon will have places to make these calibration changes and I will study the manual for that however the first step is to determine what procedure to use, what data to collect, and then what spreadsheet to use...

Anyone out there that can help...Please???

THANKS!!!

 

[DBRV10]

Should have asked me earlier ;-)

Check ya email

 

[GalinHdz]

There are several ways of doing this check. Here is a link to a .pdf file of how I did mine: Airspeed Calibration

 

[kellym]

Should have asked me earlier ;-)

Check ya email

It would be useful information for the rest of the community. I would also check with EAA for their amateur built test flying handbook, which is intended to guide one through verifying the performance of an aircraft during its Phase 1 flight test period. I will check with a friend who has the test flight book and see if they have a card for specifically verifying CAS and altitude.
Kelly

 

[Dynon101]

Hi everyone. Thanks for the help!!!

I am surprised that Dynon does not have a procedure for this because as Galin pointed out in the write up you can have the altimeter test done and come out perfect however if the static source is not reliable there could be severe altimeter changes with airspeed.

So if you start off the first triangle leg 1000 feet above an uncontrolled airport that has ASOS you set your baro to the ASOS setting and then engage alt hold mode of the autopilot and then fly the Vs +5 triangle and then you do a number of increased IAS steps (80/ 100/ 120) all while maintaining that original 2000 MSL alt hold mode and when you get to the max speed you verify that your GPS altitude did not change due to static source error then you are good however if your GPS altitude changes significantly after speeding up to 100 KIAS then stop the test and try to shim the static ports because any other testing will be a waste of time due to GIGO.

If the static source is deemed to be good at various speeds because that is the primary step to accomplish…then the TAS to CAS math is just a backwards E6B problem and with this fancy EFIS stuff should be very accurate.


If the static source is accurate then Indicated Altitude should be accurate as well so I am rethinking the need to conduct the triangles at many different altitudes because GPS altitude will almost never equal MSL unless the atmosphere is exactly standard both in temperature and pressure.

If you are doing the triangles on a very cold day you will be much lower than GPS altitude if these were being done on a very hot day and that error is amplified by altitude…so…


Thinking and typing out loud…I think the best way to do this is to fly over an uncontrolled airport that advertises the altimeter setting and fly at 1000 AGL to do the triangles because that altimeter setting will be close enough (unless here is a huge lapse rate change or an inversion) and as long as the GPS altitude does not change significantly from Vs +5 to the max IAS that the engine can produce then you know that the static source is not affected by airflow over the airplane surfaces or compressed by the wing and this airflow will be the same regardless of density altitude.

The pitot should be a reliable indication because they seem to be rather forgiving to installation angle errors so I think the IAS will be really close with this fancy new electronic stuff…but there is only one way to find out so maybe I will do the triangles at 1000 AGL over a known altimeter setting and then do a 10000, and 15000 triangle and just watch the GPS altitude…I’ve been really wanting to do some fuel consumption performance tests at different altitudes and speeds anyway so this is a good opportunity.

 

[GalinHdz]

FWIW; Dynon can't have a procedure for this since they can't control static source location(s) on every airplane. Only the builder can control this.

 

[brianreid]

Two things. Is the Gps datum the same as the MSL datum? And please don't fly 1000 feet above an airport; you'll be flying right through the traffic pattern while concentrating on other things.

 

[Michael Jackson]

I am FINALLY getting around to doing the airspeed and altimeter calibration in my airplane.

Does anyone have a procedure and an associated spreadsheet that I can use to do this test?

I have seen procedures where you (I) fly one leg on a given TRACK (and some use a HEADING) lets say use 000 (Due North at level altitude and do not make changes in power and let the IAS stabilize (at about the 30 second point should do it) and then I record the GPS Speed and the TRACK (or HEADING) degrees in some place on the spreadsheet…then make a turn to lets say left heading (or track?!?!?) 120 degrees and fly that track for 30 seconds then record GPS speed and track …then fly a track of 240 degrees…

Got it....this compares GPS speed to ground speed and the three legs cancel out for the effects of the wind...some people say to do four legs so I presume that would be mo-betta...

So now is the part(S) I don’t get…

What I want to do is to ensure that my Calibrated Air Speed is close to my Indicated Air Speed and that my Indicated altitude is close to True Altitude (MSL) not just at the one speed and altitude I just did these three/ four legs but I want to ensure that the CAS and True Altitude (MSL) is accurate for speeds from VS+5 all the way up to whatever my engine can get the airplane speed and I want to do this from 1000 MSL up to about 17500 MSL so I should do this same test at the various speeds (VS+5, 80, 100, 120, 140, 180, 200) at 2000 MSL and then go up to higher altitudes (5000, 10000, 15000) and then do the same steps again…

Then I need to plug and chug this data into a spreadsheet and then that spreadsheet will squirt out the data so I can compare my Dynon altimeter and IAS and then make whatever adjustments I need to make the Dynon match what it really should be.

Different air speeds may really affect the static source pressure and that might screw around with both IAS and True Altitude. Example: I do these three legs at 70 KIAS and the GPS altitude shows 2000 feet and then when I do the 180 KIAS legs the GPS altitude might be showing 2800 feet due to poor static source location

The only way to ensure that the indicated altitude when set to the local altimeter setting is close to True Altitude MSL is to actually compare the Indicated altitude to the GPS altitude at the various airspeeds.

So I cant imagine that I am the first person to want to check this data so someone out there in Cyberspace has most likely come up with a spreadsheet that has lots of data blocks to record data such as the local altimeter setting for the location I am doing my triangles over, the OAT for where I am triangulating…and the spreadsheet that will do this fancy Greek math for me and work the E6B backwards to come up with all of this stuff...

Then I presume the Dynon will have places to make these calibration changes and I will study the manual for that however the first step is to determine what procedure to use, what data to collect, and then what spreadsheet to use...

Anyone out there that can help...Please???

THANKS!!!

The Flight Data Viewer will be releasing a new tool on September 30th which does all these calculations for you. It will be released with a video showing how to run the tests.

In the meantime take a look at the application. https://www.JASFlyer.com. It currently has tools to measure take off performance, climb & glide performance, and compare cylinder temperatures. There are many other useful features.

 

[Dynon101]

> And please don't fly 1000 feet above an airport; you'll be flying right through the traffic pattern while concentrating on other things.

Good advice...thank you.

I was not implying that I would fly "through" the traffic pattern. Of course a pilot can fly "down the runway centerline at 1000 feet HAT" (as in doing a non-precision instrument approach" and can even depart and fly a "downwind departure" and can even fly "near" the airport without flying "through the traffic pattern"...there are many uncontrolled airports that have very little traffic so a prudent and courteous pilot can do this procedure without causing any kind of inconvienience.

 

[Dynon101]

> Is the Gps datum the same as the MSL datum?

The GPS Altitude is almost never the MSL altitude because there are any factors that make the two different. The GPS altitude is supposed to be accurate within about 10 meters so if you are on the runway surface and have set the altimeter to the advertised ASOS/ ATIS altimeter setting then that is about the accuracy you can expect. After takeoff your altimeter reading in MSL would be the same 10 meter accuracy if the day you are flying is a standard temperature and also a standard lapse rate of temperature decrease as you climb. Of course that will not ever be "standard".

The whole purpose of using GPS altitude is to compare that as the airplane increases in speed that the static ports are sensing a "clean" pressure reading and not influenced by the surface of the airplane or a wing or strut. So if you start the test at VS+5 KIAS and the airplane is flying in ALT HOLD mode on the autopilot and it is set for 1000 MSL and the GPS altitude just happens to be 1030 feet and you begin to add power the airplane will accelerate and of course the autopilot will hold 1000 MSL so that part is fixed and when the airplane gets to the max speed that the engine can provide, lets say 180 KIAS then you would compare the fixed MSL to the GPS altitude and if the GPS altitude is still showing 1030 feet then the static ports on your airplane are "clean" and they are not affected by the airplane surfaces of the pressure inducing parts like wings or tail.

 

[kellym]

> Is the Gps datum the same as the MSL datum?

The GPS Altitude is almost never the MSL altitude because there are any factors that make the two different. The GPS altitude is supposed to be accurate within about 10 meters so if you are on the runway surface and have set the altimeter to the advertised ASOS/ ATIS altimeter setting then that is about the accuracy you can expect. After takeoff your altimeter reading in MSL would be the same 10 meter accuracy if the day you are flying is a standard temperature and also a standard lapse rate of temperature decrease as you climb. Of course that will not ever be "standard".

The whole purpose of using GPS altitude is to compare that as the airplane increases in speed that the static ports are sensing a "clean" pressure reading and not influenced by the surface of the airplane or a wing or strut. So if you start the test at VS+5 KIAS and the airplane is flying in ALT HOLD mode on the autopilot and it is set for 1000 MSL and the GPS altitude just happens to be 1030 feet and you begin to add power the airplane will accelerate and of course the autopilot will hold 1000 MSL so that part is fixed and when the airplane gets to the max speed that the engine can provide, lets say 180 KIAS then you would compare the fixed MSL to the GPS altitude and if the GPS altitude is still showing 1030 feet then the static ports on your airplane are "clean" and they are not affected by the airplane surfaces of the pressure inducing parts like wings or tail.

Suggest using 1000 AGL. Or even better select an even number just above 1000 AGL. For many locations 1000MSL would have to be in a tunnel. (My hangar is at 1230 MSL.)

 

[GalinHdz]

FWIW: GPS altitude is best for not hitting objects that are on the Earth's surface. Baro Altitude (Altimeter) is best for not hitting other flying aircraft.