Wrong winds aloft reading

[Camps]

Hi Dynon

I have a Remos G3, D100, OAT, EDC10A, AP74, 296 GPS.

I  installed  the OAT under the wing, far from engine heat. The temp reading seems to be quite reliable.

I did the EDC-10a calibration, it is not perfect but when on the ground I compare the readings of the D100 and the GPS 296 and they are almost the same. It varies up to a maximum of 4 degrees.

When flying, the winds aloft reading is always showing something around 10/12 kt head wind.  :-[

Any ideas? Thank's

Roberto Camps

 

[dynonsupport]

Have you verified your pitot/static system? This sounds like your airspeed is reading high, which is common if you have a static system error.

 

[Camps]

Some data readings

OAT Temp 24 23

OAT alt dens 5.350 5.300

GPS Altitude 3.490 3.586

Altimeter Altit 3.540  3.540

Efis Altitude 3.520  3.530

GPS GS         80 97

Efis IAS         93 98

Speedmeter IAS 92 98

OAT TAS        101 106

Efis Head wind  23    3

Compass headi 274      87

:-/

 

[khorton]

Check for a static system leak.  Static leaks are quite common, and the pressure in the fuselage is usually less than ambient pressure, as the air has to accelerate to go around the fuselage. Bernoulli tells us that this accelerated air has a pressure that is lower than ambient pressure, and any gaps around doors, windows or canopy allow the pressure in the fuselage to equalize with this low pressure that is flowing around the fuselage. Thus static leaks lead to ASIs that read too high.

To do a complete end-to-end test, put a piece of electrical tape over one static port. Find a way to apply controlled suction to the other static port. I use a piece of surgical tubing - push it hard against the fuselage side to make a seal over the static port, then suck on the other end. Suck hard enough to cause the altimeter to increase more than 1000 ft, then put your tongue over the end of the tube to hold the pressure. The altimeter will jump around a bit until all the pressures equalize, then it shouldn't move more than 100 ft in a minute, starting from 1000 ft above field elevation. Any more than that is an unacceptable leak.

When you are done, release the vacuum slowly, to avoid any risk of damaging the instruments.
__________________

 

[dynonsupport]

The other thing which is probable - and this is hard for you to test for - is that the indicated airspeed that is coming from the pitot/static system is not very close to calibrated airspeed. This is a function of the pitot/static locations on the aircraft. Unfortunately there's not much you can do about it on an SLSA. You might touch base with Remos to see if they have known differences between IAS and CAS in the aircraft.

 

[DBRV10]

Static ports can make big errors, here is a little story. Brand new Vans RV-10, instead of using the horrible Vans rivet head static port we elected to fit nice robust disc type vents like you see on many aircraft. result was a TAS that was about 9 knots slow. Glued a rivet head over the disc and we went to 6 knots fast..... a 15knot variation. All tests were done at full bore at 3000' and around 175knots from memory. Errors like this also give altitude errors of up to 140 feet and for IFR ops this is not acceptable. In the end I have a filed down rivet head and a piece of TIG wire (alum) rolled into an arc glued to the face of teh staic port. :-?

So unless you fly a GPS box and verify your TAS and IAS you will not know.

Next observation, given the numbers you posted, the IAS and a TAS as posted the wind vector looks correct!!

So the only thing left to do is verify that your IAS is correct. If the indicated is wrong due to static errors, how on earth with the TAS be correct!

Statics.....check for leaks, then correct any, then fly the GPS box. If you need the spreadsheet program and instructions on how to use it PM me and I will send it to you. You can search for them on the net also.

Cheers
David

 

[dynonsupport]

You can do a static check even without a GPS. Fly over a runway at 10-20 feet off the ground. Then change the speed of the plane by a fair amount and re-fly. If your altitude changes, your static is bad and is affected by airspeed.

 

[Camps]

Today I've donne a test.

I was keeping the altitude and track constant.

When flying at 200Km/h and the D100 was showing wind aloft as 18Kt

then changing speed to 95Km/h wind aloft changed to 12Kt.

I 've donne it three times with the same result.

 

[dynonsupport]

That can be a static error leading to airspeed errors, or it could be a heading error. Unfortunately that test isn't conclusive.

 

[DBRV10]

I will bet its a static error for sure.........And DS, some LSA manufacturers would have never even considered it I dare say!

CAMPS.....you must fly the GPS box test. This will tell you definately there is a static error.

PM me your email address and I will send the program and instructions on how to do the test.

The result of your last one suggests it is a static error. The problem is at speed your altimeter will be wrong also. At one point I had an induced error of 130' so it can be quite significant.

I think Dynon could add a section in their manual and a spreadsheet program for people to test their installation first before harassing them with complaints.

EVERYONE SHOULD DO THIS TEST.

 

[Camps]

Hi David

Thank you for your kind suport.

My email is camps@uol.com.br

By the way : what's GPS box?

Regards

Roberto Camps

 

[DBRV10]

Email is on its way.

The GPS box is just you flying a GPS TRACK of 0degrees, noting the GS, 90 degrees note the GS 180 &270 degrees, so you fly a box in the sky noting the ground speeds. The spreadsheet calculates your TAS.

You then compare this to the displayed TAS on your Dynon, or your calculated TAS if you have to do it the hard way. Any errors more than 2 knots I would chase down with small mods to the static ports.

If Kevin Horton frequents this site....which I think he does he could post a text book on the topic!

 

[Camps]

David

You were right. I had an altitude error plus 130ft comparing to my GPS.

I put a half wash behind each whole of the static. Then I got an altitude error minus 200ft, and the IAS was VERY slowly.

So the job is to trim the thickness of the wash to match the GPS.

Regards

Camps

 

[khorton]

David

You were right. I had an altitude error plus 130ft comparing to my GPS.

I put a half wash behind each whole of the static. Then I got an altitude error minus 200ft, and the IAS was VERY slowly.

So the job is to trim the thickness of the wash to match the GPS.

ERROR - GPS altitude and barometric altitude are different things, and you shouldn't expect them to be the same.  The GPS altitude should be fairly close to the actual altitude above sea level, plus or minus various errors in the GPS system.  But, barometric altitude only agrees with the actual altitude if the air temperature matches the standard atmosphere air temperature at all altitudes between the source of the altimeter setting and the altitude of the aircraft.  

If the air temperature differs from standard temperature, the difference between barometric altitude and true altitude may be quite large.  This difference may easily be more than 1000 ft in the winter.  

Info on cold temperature effects on altimeters.

Info on the laws of physics behind the cold temperature effects on barometric altimeters.

See the diagram at the bottom of the page showing how the pressure levels are scrunched closer together in cold temperatures.

Info on an incident where an airliner missed terrain by about 150 ft due to cold temperatures.

===========
How to check for errors in your airspeed system:

Step 1 - Check for static and pitot system leaks (you've already done this).

Step 2 - Confirm EFIS ASI and OAT instrument errors are low.  The EFIS ASI can either be checked at an instrument shop, or you can make a water manometer to check it against.  The OAT probe can be checked in some ice water, and you can compare it against the reported temperatures at your airport.  Be aware that some OAT systems may have errors due to poor placement of the OAT probe - it may be heated by the exhaust or by warm air coming from the cowling, or the back side of the probe may be heated by warm air in the cockpit.

Step 3 - Do a flight test.  Set the altimeter to 29.92 (or 1013.25 mb).  Fly a GPS box pattern with four legs, being sure to hold exactly the same IAS and altitude on each leg.  Record the GPS ground speed and track, IAS, pressure altitude and OAT on each leg.

Step 4 - On the ground, crunch the data, using this Excel spreadsheet I created for a Kitplanes article.

I wrote two articles for Kitplanes magazine earlier this year.  They go into much more detail on how to check the accuracy of your static system.