ASI over read

[flyingforfun2004]

Guys,
Posted in off topic.

Forgive me for been thick how do i work out my ASI error as im sure from the below its over reading. I flew down wind at 115 KTS indicated with a GS of 121 KTS and in to wind with 115 KTS indicated and a GS of 90 KTS what would the over read be on my ASI. Im thinking if i have 6 kts on the tail I should have 6 kts on the nose and therefore its way out. Any advice appriciated.

 

[flyingforfun2004]

Sorry I meant to post in GA off topic not the Skyview area but these results are from my twin SV1000 set up

 

[nigelspeedy]

Hi Flyingforfun2004,

At the risk of making this answer more complicated than you may enjoy there are several possible places for errors in the airspeed indicating system. The first we can call instrument errors. These arise when the instrument does not show the airspeed that corresponds to the pressures that are being applied. In the case of Skyview these would come from the ADAHRS. I checked both of mine using a calibrated pitot static test box and the instrument errors were less than 1 knot from 30 - 300 knots. If you take what you see on the gage, 'indicated airspeed' and add the instrument correction you get 'instrument corrected airspeed'. The next place errors arise is the static port. Due to airflow over the airframe the pressure at the static port may not be the same as the free stream static pressure, this is called 'position error'. More than likely this is the major cause of your potential problem and can be in the order of 0 to 10 knots depending on how good the installation is. Adding the position error correction to the instrument corrected airspeed will result in 'calibrated airspeed'. It is normal in fixed wing aircraft to assume that there is no error in reading the pitot pressure, just the static pressure. As long as the pitot tube is not behind a prop or some such foolishness this is a safe assumption. If you are flying high and fast (above 20,000' and 200 knots) you need to account for compressibility to convert calibrated airspeed into 'equivalent airspeed'. Given the speeds you quote these errors will be zero for all practical purposes. To turn your calibrated airspeed, which is the same as the equivalent airspeed in this case, into 'true airspeed' you need to divide by the square root of the density ratio. You really need to be comparing true airspeed with the ground speed. The best way to do this is to use the three or four leg GPS method. You can download instructions and a spreadsheet from www.ntps.edu look under info/downloads/Using GPS to determine Pressure Error Corrections. If you have any questions on how to do the flight test or use the spreadsheet please feel free to ask. It is a bit hard to explain in words but I also have a nice flow chart showing the equations and relationships.
Cheers
Nigel

 

[dynonsupport]

First, it appears you checked IAS vs GS. You need to check TAS vs GS, unless you happen to be at sea level on a standard day. Just being 3,000' up will cause about 6 knots of error! You also need to make sure your OAT isn't way off or your TAS is wrong too.

Then, in order to check airspeed against GPS speed, you need to fly a triangle, not just up and down wind, unless you have some way of being 100% sure you are directly into the wind. You can't use the wind arrow on SkyView since that's based on TAS which is based on IAS!

As we mentioned in the other thread, most errors are Static errors. Here's some quick ways to check for static error:

1) Fly at a fixed GPS altitude (as shown on the GPS map). Speed up and slow down by at least 50 knots. Your indicated altitude should not change. If it does, you have static error.

2) Much more fun: Do a low pass over a runway, once slow, and once fast. Your indicated altitude should be the same both times.

If you do have static error, there are lots of threads around on how to build small "dams" in front of or behind your static ports to reduce the errors.

 

[flyingforfun2004]

Hi Nigel and Dynon Support,

There was me thinking this was going to be an easy query! I really appreciate your real comprehensive reply. Im not too concerned with it been a knot or 2 out and i was thinking of flying alongside a friend with a comparable aircraft and see what ASI he was getting but ill do the slow and fast pass first and see what that says. Ill also have a look at the spread sheet too.
Thanks guys
Kev
Loving the Skyview only had 2 flights total one when i bought the plane and flew it home then one since upgrading to version 7.