Position Error Correction

[nigelspeedy]

G'day Dynon,

I would like to request a new feature, the ability to correct for pitot static position errors. If my static port does not read the real ambient pressure I get errors in airspeed and altitude. It is relatively easy to test for these. One method is the 4 leg GPS method, a spreadsheet to do this is available at the National Test Pilot School website www.ntps.edu under the download section. At the minute I have up to 5 knots delta Vpc which makes my IAS and TAS optimistic, which is initially good for morale, but this is soon offset by the depression that results from always having a 5 knot headwind. We have the ability to correct the magnetic sensor and define engine sensors so at least conceptually this request seems possible. The only hidden catch I can see is limiting application of errors based on a curve fit of data outside the range of speeds from which the original data was collected. I'd also rather apply a correction electronically than have to move the static ports.
For your consideration
Nigel Speedy

 

[dynonsupport]

Nigel,
Static source correction is problematic.

Your table would be a static offset vs. airspeed.

But airspeed uses static pressure! So you get into a catch-22. You adjust your static, which adjusts your airspeed, which adjusts your static....

It can be done, but it requires a lot of testing and calculating to get a good, reliable table. And then what if you have an AOA dependence?

You generally don't need to move static ports to get good performance. You can build up a "dam" in front of or behind the port to adjust it. Since you have an RV-8, head over to VAF and they have all sorts of discussions on the right kind of static port shapes to use, and how to correct them if they aren't perfect. A lot of RV's deviate from Vans' plans on static ports which generally seem to cause issues, especially if they are flush.

 

[nigelspeedy]

The table would be airspeed and altitude position error correction v indicated airspeed. You could do the corrections as a lookup table or calculated from curve fit coefficients.
The addition of a position error correction to an indicated airspeed converts it to a calibrated airspeed. You measure the indicated, add the correction then display the calibrated value. There is no readjusting of the static pressure, no catch 22, no endless loop.
The testing for position errors is pretty quick and easy, you could do the whole speed range of an aircraft using the GPS method in a 1 hour flight. The data reduction using a spreadsheet takes about 20 minutes.
There could be AoA and flap setting dependencies but I don't see that as a reason to not make the level flight accuracy better, which is where the aircraft spends most of its life.