Pitot Position Error

[paul.mcallister]

Hi, does the D100 have the ability to create a calibration table to correct position errors of the pitot system ?

If not is this something that is being considered for a future software release ?

Paul

 

[dynonsupport]

The EFIS does need to receive "true" pitot pressure in order to operate properly. One issue with calibrating airspeed in the field is that you need extreme sensitivity at lower airspeeds to be accurate. You would also need to calibrate at many speed points, and then repeat those over different temperatures.

 

[paul.mcallister]

Hi,

Can I assume from your reply that this isn't a feature you offer, nor intend to ?

Thanks, Paul

 

[dynonsupport]

No, not really. Pitot systems should use a standard pitot tube, and thus give standard pitot pressure. We calibrate our units here to that standard, just like all ASI's are calibrated.

Pressure versus speed is not a linear relationship, so creating a "table" of corrections would be complex and may not be accurate between points.

We are always listening to customer interest however, and we could be wrong about the demand for this. Can we ask why your pitot system is non-standard? Most pitots are insensitive to angle errors up to about 15 degrees, so I assume it's your static that is off. This is an even trickier problem to solve since it changes altitude and airspeed.

 

[khorton]

I agree that any error is almost certainly in the static source, unless the pitot source is very non-standard, or poorly located, or poorly angled.

I would also be interested in the ability to input a table of static source error corrections (SSEC).  The ground and flight testing to determine these errors is fairly painstaking, but requires no special equipment other than a water manometer (to determine EFIS ASI instrument error via a ground test), GPS (to determine TAS via analysis of GPS data on several tracks) and OAT (required input to get CAS from TAS).  More info here: http://www.kilohotel.com/rv8/rvlinks/ssec.html.

SSEC test results are traditionally plotted as a curve of error in the static pressure vs the dynamic pressure.  I envision the capability to input several points on this curve, and the EFIS would do linear interpolation between the points, applying the correction to the static pressure before calculating the airspeed and altitude.  Obviously the default would be to apply no correction.

One small complication is that the SSEC curve normally varies with flap angle, so the user would have to pick one flap angle (usually flaps up), and optimize the corrections there.

 

[paul.mcallister]

Dynon support,

I think Kevin stated the problem / solution much better than I have. I do know that my ASI reads a few knots low below 60 and about 9 knots fast by the time I get do 140 knots. The only thing I can state for sure is that I don't have any leaks, and given that the ASI is a high quality unit of German origin, that the instrument is probably okay, (mind you this isn't a safe assumption to make).

What I really desire to have is a CAS and TAS that are fairly accurate, and yes, it took me many, many hours of flight hours to quantify the errors, so I appreciate that it wouldn't be a trivial exercise to create the calibration curves.

Paul

 

[dynonsupport]

Paul,
Are you saying the Dyon doesn't match a mechanical ASI? If this is true, one of them is off, and if it's us, we'll happily re-calibrate your unit, and apologize thouroughly for letting a unit get that far out of calibration.

The best way to find your error is with a water manometer on the ground. Run this into your pitot system, and you should be able to see if the mechanical and Dynon ASI's agree at a variety of airspeeds. If they don't, taking measurements of the water height will allow you to easily figure out which one is wrong. This test doesn't verify that your static is correct however, but one way to do that is to hold a constant GPS altitude over a variety of airspeeds and see if your altimeter moves much.

 

[paul.mcallister]

Dynon Support,

I must admit to being very impressed with the quality and promptness of your responses. I need to clarify my situation. I am about to purchase an EFIS to replace my vacuum system and "6 pack" in my Europa. I am trying to decide between the GRT Sport (which has just started shipping) and your product. A couple of things interest me with the GRT unit, namely an SL30 interface for VOR/LOC/ILS and the ability to null out pitot static system errors, (for TAS only).

My situation is that from flight testing I know my ASI reads low under 60 knots and 9 knots high at 140 knots. Other than check for leaks I haven't tried to debug this, but my fear is that I might find the same issue with an EFIS and the only solution may be that I will have to calibrate it out in the EFIS, hence my questions.

I am finding the decision difficult, on one hand you have a proven product with several thousand units in the field, whereas the GRT product is a similar price but offering more features with an upgrade path.

I apologize if I misled you into thinking I am an existing customer and I do appreciate the answers you have given me.

Thanks again, Paul

 

[dynonsupport]

Paul,
A manometer is a pretty easy thing to set up. You may want to do this first and see if the error is in your ASI or your pitot-static system. This seems like a good idea no matter what, since correcting TAS only on the GRT unit won't really help you if the error is in your static and messes with your altimeter or if you care about IAS at all.

 

[funflying]

Where can I get the information on setting up a manometer?

Thanks, Pat

 

[dynonsupport]

Here's a basic manometer setup:

http://www.iflyez.com/manometer.htm

It's literally just a "U" of tubing. Fill it halfway up with water, hook one side to your pitot system, and apply pressure. The difference in height of the water is your pressure in InH2O. You can convert this to PSI and/or to airspeed.