Question and suggestion. Calibrating ADAHRS

[sunfish]

I am now trying to calibrate the System. My new build has flown for three hours. I am trying to deal with multiple issues in getting the aircraft to fly straight and for the Skyview to indicate its straight. A "first flights" guide would be useful to try and establish the order to tackle things.



problem 1 - the ball is outside the bars and to the left when I'm flying the aircraft straight and level by eye. The horizon display shows me as level and thats referenced to the aircraft structure, as is the pitch access which was zeroed to a level airframe. The manual says"shim the ADAHRS" to get the ball centred. I don't understand. Which axis am I trying to shim? The left to right roll axis? The Pitch longtitudinal access? The vertical access? Referenced to what? The ADAHRS was installed level with everything I thought.I could easily waste a few hours of flight time because I don't understand the adjustment.

I have to fix this immediately because its dangerously confusing and it is important on that base to final turn. I've avoided the issue by straight in landings so far.

problem2: pitch offset. Do I want that referenced to the structure or the flying attitude of the aircraft at what speed?

problem 3 - Calibrating IAS. I think I've got this roughly done and the error is about 2 - 3 knots. However do I have other errors that affect it? Is that good enough?

Problem 4 Compass calibration, is a figure of 126 OK?

Apart from these issues, the system is great!

 

[sunfish]

Problem 1 fixed - fuel sloshing. One tank 40 lbs heavier than the other. Must remember fuel management.

problem 3 calibrating IAS - not fixed - i should have compared it to TAS the error is about 8 knots. Work needed.

 

[Raymo]

If the ball is off center in straight flight, it means the ARHRS is not aligned with the aircraft. It will need to be slightly rotated to correct this. Compass calibration is done roughly on the ground, then fine tuned in flight by following the instructions.

 

[sunfish]

Yes Raymo, it may need to be aligned, but which axis is off? Pitch (for and aft)? Heading? Roll?

 

[GalinHdz]

problem2: pitch offset. Do I want that referenced to the structure or the flying attitude of the aircraft at what speed?

problem 3 - Calibrating IAS. I think I've got this roughly done and the error is about 2 - 3 knots. However do I have other errors that affect it? Is that good enough?

Problem 4 Compass calibration, is a figure of 126 OK?

Problem 2 - When the airplane is completely level in pitch and roll ON THE GROUND, then you level the ADAHRS by shimming it until it is very close to perfectly level. If close to correct, the "ball" should be in the middle between the two lines. If not then something is wrong. After the ADAHRS/airplane are both level and while still on the ground, you make minor adjustments via the EFIS. Remember, your pitch attitude will change as you fly depending on weight and airspeed so you want your ADAHRS to accurately show your aircraft attitude regardless of weight and airspeed. Your "eye" will never be accurate enough.

Problem 3 - Is the TAS, as displayed on your SkyView, accurate as verified by a 3 heading GPS test flight? If so then 2-3Kts is fine. If not then you will have some work ahead since this error might be affecting your altimeter indication, which can be dangerous.

Problem 4 - According to the installation manual, anything greater than 100 is fine.

 

[jakej]

Sunfish - just some added info to Galins post -
On the ground aircraft lateral leveling is essential, then use a short digital level on the ADAHRS (I aim for .1 of a degree).
Also have the ADAHRS perpendicular to the longitudinal Axis of the aircraft.
AFTER that is done then
1. make sure the skidball is centred in level flight at normal cruise settings - if not then use a rudder trim tab of sorts to get the ball in the centre (you may find the aircraft may tend to roll one way.
2. If there is a heavy wing with Roll Trim centred then you can 'pick it up' by several methods including lowering 1 flap slightly, or use an aileron Trim Tab etc. These or other airframe rigging methods should be used & do not level the plane with the electrical or mechanical trim system, as you end up with not enough trim capability in 1 direction. It's best that the roll trim system is centred first with the fuel levels approx equal & then 'adjust' the airframe/rigging.
Why do all this ? - 1. You'll end up with a well balanced aircraft aerodynamically & 2. IF you have an auto pilot - its job will be a lot easier without loading it up & causing extra strain & wear due to unnecessary loads on it.
Problem 4 - as said more than 100 is ok however I always go as high as I can & have seen up to 135 .

 

[sunfish]

Thank you all for your advice. I am going to test some more before I do anything. I have a biggger speed error than i thought because I wasn't comparing TAS with TAS but with IAS.

Much analysis to do and then probably some work with aluminium shims and duct tape.

 

[GalinHdz]

Having a big airspeed error when you first fly with a new EFIS is not that uncommon, especially with an experimental airplane. It happened to me so BTDT! Unfortunately it just takes time to get it correct. The Good thing is that it is rarely the EFIS' fault. BTW, for these tests I recommend you use Gorilla tape since it is much thicker than Duct tape.

Check out my SkyView TAS Calibration write up in the MAINTENANCE section of my web page www.puertoricoflyer.com

 

[Bill Putney]

The ADAHRS pitch, roll and yaw alignment has to be within 1º of flight level. Dynon suggests that a bubble level isn't accurate enough. The yaw axis is along the center line of the aircraft. The ADAHRS can be offset horizontally from the centerline (somewhat) but it still has to be in line. You could attach a straight edge to the side of the ADAHRS case and line it up exactly with the airframe center line. The longer you can make this straight edge the more exact you can make this alignment. If your ADAHRS is in a really tight spot and you can't just push it up against a piece of airframe structure that you are sure is aligned exactly with the yaw axis, this could present a problem.

To get the ADAHRS in line in pitch and roll is a little harder. First you have to put your airplane on jacks to get it in the designed flight level condition. The maintenance manual for my airplane gives details for doing this and I'm sure most aircraft designs call this out as well. Next you have to use something like a digital level (inclinometer) attached to the top of and aligned with the sides of the ADAHRS case to be able to make these measurements. I bought a pair of Bluetooth digital inclinometers from Amazon for this purpose that will read down to 0.01º. One attaches to a flight level platform on the airframe and one is on a jig I 3D printed to attach it to the ADAHRS case so that it is aligned in pitch roll and yaw with the ADAHRS. I used two inclinometers because I assumed that my being inside the airplane while i'm working on the ADAHRS mounting would effect level.

By comparing the two inclinometers on my Android tablet, I can shim the ADAHRS and remote magnetometer to be the same as the inclinometer attached to the airframe level platform.

Dynon says that the remote magnetometer has to be aligned this way as well. My jig works for both cases.

This may be way to crazy complicated but it seems to be what Dynon's installation manual directs in not so many words and it wasn't too expensive to do.

- Bill

P.S. The link for the inclinometers on Amazon is https://smile.amazon.com/gp/product/B07T2C97WN/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

 

[sunfish]

GalinHdz, I've done a roughly averaged GPS/TAS 3 leg calculation and my computed error is 6.3 knots. I will try for a more accurate figure next flight and I'm also going to try an adjustment using double sided foam tape.

Memory is a problem as we get older. I'm sure I did level the ADAHRS in all 3 axis as required when I installed it. I think I have a fuel sloshing issue that I'm going to address before i start shimming anything.

 

[GalinHdz]

GalinHdz, I've done a roughly averaged GPS/TAS 3 leg calculation and my computed error is 6.3 knots. I will try for a more accurate figure next flight and I'm also going to try an adjustment using double sided foam tape.

Memory is a problem as we get older. I'm sure I did level the ADAHRS in all 3 axis as required when I installed it. I think I have a fuel sloshing issue that I'm going to address before i start shimming anything.

How did you come up with the 6.3Kt error? You have to be super accurate in your flight testing procedures since this will greatly affect your results which can be deadly.

The "gold standard" is to fly three legs, about 120 degrees apart without changing altitude or engine power settings during smooth air conditions. After your altitude and speed have stabilized, jot down the GPS heading and ground speed. Then turn to the next heading and repeat the process until you have all three readings. Make sure you write down the TAS as displayed on your SkyView system during the flight. If you have an autopilot use it since it makes everything more stable and easier to do.

Then use those GPS ground speeds and headings to calculate your real TAS using some trigonometry. Here is a link to an EXCEL spreadsheet that does the calculations so you don't have to be a math wiz to do this. Compare the calculated TAS against the TAS displayed on your SkyView system. Then you will know if you have an error or not and how to proceed.

SkyView uses IAS for TAS calculations so what you are doing is making your IAS and CAS the same. When your IAS and CAS are the same then both your Pitot and Static systems are providing accurate data. This insures all SkyView displayed values, based on Pitot and/or Static data, are correct. It isn't super hard to do just time consuming, especially if you have to make multiple flights to get the error fixed.

BTW make sure where the error is before you change things. The easiest way is to verify both airspeed and altitude. If only airspeed is wrong but altitude is correct then your error is only in the Pitot system. DO NOT USE GPS ALTITUDE FOR THIS VERIFICATION. If both airspeed and altitude are wrong, then more than likely your static system is the culprit.

 

[sunfish]

And an update.....

‘’The ball problem turned out to be me flying lopsided. Almost all fuel drained from the right tank to the left - making a 30lb+ imbalance.

The speed error is because I installed the wrong sort of static port. It stuck out like a pipe end about 0.07 into the airstrram and Mr Bernoulli did the rest - creating a partial vacuum. The ports are bonded in so I made aluminium washers for each port that produced a Cessna like smooth surface with a hole in the middle. This is something like what dynon is selling in their pitot/static kit now.

The result - after flying a triangle and using gps and spreadsheet, is an error of +- 1 knot TAS.

 

[GalinHdz]

Great news. When you get a chance, verify your altitude display. With a correct static port installation the displayed altitude should be very accurate. Again, DO NOT USE GPS ALTITUDE FOR THIS VERIFICATION.

 

[sunfish]

Wilco GalinHdz.