No matter what my direction I always show a headwind which translates to a slower than accurate groundspeed. When I change direction the indicator will change to about 45 degrees then flip to a headwind. What is going on? HDX1100 new install on RV14.
Always shows a headwind
- Thread starter cwood2000
- Start date
RV8JD
Active Member
- Joined
- Dec 17, 2017
- Messages
- 378
Re: "... which translates to a slower than accurate groundspeed".
The winds as calculated may well be in error, but the groundspeed is probably not, since the groundspeed is GPS derived.
The wind speed and direction are calculated by subtracting the true air speed vector from the ground speed vector. The winds are calculated using the groundspeed and track from the GPS, the airplane's heading from the magnetometer, and the true airspeed. The true airspeed is calculated using the indicated airspeed, altitude, and OAT.
There are a lot of possible sources of error in the winds calculation, but groundspeed is probably not one of them.
The winds as calculated may well be in error, but the groundspeed is probably not, since the groundspeed is GPS derived.
The wind speed and direction are calculated by subtracting the true air speed vector from the ground speed vector. The winds are calculated using the groundspeed and track from the GPS, the airplane's heading from the magnetometer, and the true airspeed. The true airspeed is calculated using the indicated airspeed, altitude, and OAT.
There are a lot of possible sources of error in the winds calculation, but groundspeed is probably not one of them.
Last edited:
DBRV10
Active Member
You have possibly one of or all of the following that need correction ASAP. Especially if you operate in busy or controlled airspace.
1. OAT error. I see this ALL the time. The manual needs to get more demanding on people. If your OAT probe is not out in clear air, move it there. The wing inspection panels are ideal. punch a hole through one of them. Use the Pitot tube or A/P servo position. The inspection panel on the tail under the horizontal stab works also, but the wing is best. Never in or near the wing root.
2. Compass must be calibrated on the ground accurately, and not with an iPhone compass. Must be done right, then the inflight calibration performed.
3. Static error. This is a big one like OAT, and people will argue with me (despite being wrong) that it checked out perfect with the A&P/LAME test. The static port is a dynamic thing, and in flight you can have an error either high pressure or low pressure at the port. Post pictures of your static port please. Oddly enough, Dynon flukes getting the static ports right for RV10's and possibly the 14 also, but you have to test.
The test is to fly a 4 way GPS box, flown in smooth air and accurately and making sure you het the same IAS in each direction, then the calculated TAS needs to be compared to the TAS shown on the EFIS. It needs to be within 2 knots. If not you have to start experimenting to fix it.
Why is this so important? Because if you have a big errr in TAS, based on static error you also have bad altimeter results and I have seen results around 200' out. For IFR ops a discrepancy of only 60' is not acceptable. So this matters. ATC only see what your transponder transmits, and it transmits your altitude on static alone.
This is serious stuff. Do not just accept it because someone says who cares mate, you only want to know your ground speed.
Hope this helps.
1. OAT error. I see this ALL the time. The manual needs to get more demanding on people. If your OAT probe is not out in clear air, move it there. The wing inspection panels are ideal. punch a hole through one of them. Use the Pitot tube or A/P servo position. The inspection panel on the tail under the horizontal stab works also, but the wing is best. Never in or near the wing root.
2. Compass must be calibrated on the ground accurately, and not with an iPhone compass. Must be done right, then the inflight calibration performed.
3. Static error. This is a big one like OAT, and people will argue with me (despite being wrong) that it checked out perfect with the A&P/LAME test. The static port is a dynamic thing, and in flight you can have an error either high pressure or low pressure at the port. Post pictures of your static port please. Oddly enough, Dynon flukes getting the static ports right for RV10's and possibly the 14 also, but you have to test.
The test is to fly a 4 way GPS box, flown in smooth air and accurately and making sure you het the same IAS in each direction, then the calculated TAS needs to be compared to the TAS shown on the EFIS. It needs to be within 2 knots. If not you have to start experimenting to fix it.
Why is this so important? Because if you have a big errr in TAS, based on static error you also have bad altimeter results and I have seen results around 200' out. For IFR ops a discrepancy of only 60' is not acceptable. So this matters. ATC only see what your transponder transmits, and it transmits your altitude on static alone.
This is serious stuff. Do not just accept it because someone says who cares mate, you only want to know your ground speed.
Hope this helps.
Thanks, I read this earlier and flew today with a 10kt wind right out of the north. First I confirmed my OAT with AWOS and it was within 1degree C. The probe is mid wing. I then calculated my TAS using the formula and IAS, it came out with 3kts. Next I re-configured my compass which was showing a 104 quality (in the green) through several attempts I got it to 117. So, going 360 degrees it showed a head wind of 17kts (4,000 msl) as I turned to 180 degrees the arrow correctly followed until about the 160 degree mark then snapped back, now showing a wind of 17kts out of the south. The GS was affected so it was always 17kts less than TAS. I also have a iPad that is wifi to the HDX and it showed the same numbers as the HDX in GS. The unit and plane has 27 hours on it and I believe it was showing good numbers at first, I started to notice the forever headwind at about 10 hours. So I guess the GS is not calculated by the GPS, but by TAS +- the wind X component, but will confirm with mother ship tomorrow.
RV8JD
Active Member
- Joined
- Dec 17, 2017
- Messages
- 378
Last edited:
DBRV10
Active Member
Maybe you have not explained it well enough, but I think you are missing my points.
1. Mid wing is good, and I assume it is poking out of the wing and not inside the wing.
2. Calculating your TAS using IAS and the formula for density height WILL NOT prove anything. If your IAS is wrong due to a false static, then your TAS is wrong, despite you calculating a similar result. You must do a GPS box and compare that result to the TAS on your EFIS. Otherwise you are getting conformation bias on wrong things.
3. Your GS is the actual GPS Ground speed.
I can email you the GPS box spreadsheet, or fly the 4 leg box and post the results here. Once this is sorted you will be happy!
Here is what you need to do. Find some smooth air. Fly at a constant indicated altitude and IAS, using the Auto Pilot is best. Fly a box pattern with the legs roughly 90 degrees apart 360/90/180/270, and record:
- IAS (should be the same on all legs)
- pressure altitude (i.e. with altimeter set to 29.92) or the actual QNH and your reported altitude
- OAT,
- GPS groundspeed and
- GPS track (360/090/180/270) Note this is Ground Track not Heading, as heading could be anything.
On each leg, use power as required to achieve exactly the same IAS, as closely as you can do it. Or, just wide open throttle/2500 RPM/LOP or whatever you want, just keep it constant. Stabilize on a constant IAS, altitude and heading long enough for the GPS data to stabilize before recording the GPS groundspeed and track.
Thanks, good information. I'll call in today and see what mothership says, there must be a answer. I'll fly again this morning and collect more data.
Thanks for your assistance, my email is cwood2000@gmail.com Here is what I did today. I re ran the static test, then recalibrated the compass and got the quality to 118. Problem still persisted as I would turn from a headwind to tailwind the arrow would follow my track correctly until I leveled the wings and it would revert to a headwind abruptly. Later I flew with another RV and our IAS, TAS etc were accurate to one another. We did notice a difference in altitude with the same setting of a couple hundred feet. The GS indication seemed yo be correct, but the winds were not that great. In any case I'll do the box tomorrow morning when the winds generally calm.
Thanks again!
You can run the calibration on the ground and do static checks a million times but if it is a dynamic airflow issue due to static port location, nothing will change. If it is a dynamic airflow issue, you have to do the GPS check in order to confirm it. You can also fly a high speed "missed approach" just above the runway, download the EMS data if you have an EMS and check the results. Either one of them will quickly show if you have a dynamic airflow issue which is probably a static location problem. Honestly, that is what it sounds like. You have to fly to find and correct this, no amount of ground checks will work.
FWIW I had a similar situation when I installed my SkyView system. You can read about how I figured it out and fixed the problem with the help of DYNON Tech Support on the Maintenance section of our WebPage under SkyView TAS Calibration. Good luck.
Last edited:
DBRV10
Active Member
Sounds to me like you have two RV's with problems.
I never trust flying on someone else plane. I have available to me a couple of "known to be good" RV's. One is ours, one is a friends. Comparing against an unknown bad example is not helping you and is likely confusing to you. Please ignore them until yours is right. Then help him.
Email on its way.
Steveden
Member
David, you've provided a lot of good info and I have a question also. In a tube and fabric airplane like a cub, how important is it to have the static vents to the outside of the aircraft or can they exit into the fuse or wing?
Thanks, very helpful! I’ll try the gorilla tape method to see if addresses the issue.
Just to update, I did the box again, several times and using both the spreadsheet method and the old E6B they came out accurate. Mothership said they have seen this before and I need to do a zero pressure recalibration. In the set up menu>Hardware calibration>Adahrs Calibration>Zero Pressure test. Said in the hangar, close the pitot and static ports and run the test, it recalibrated the system. So, stay tuned I'll let you know how it works.
DBRV10
Active Member
Nobody will ever know.....cabin/wing/fuse pressure can be very different to true static. And vary from plane to plane. In practise however, it may not be much different.
DBRV10
Active Member
Actually I forgot to mention that, I usually do that as a matter of course with peoples installations. I look forward to the results, as that would be a spectacular error, of a magnitude I have not seen before.
Here is the current status. I mis-interpreted section 5-13 that says a warning alert is on screen until a on ground or in flight calibration. I thought that meant either would suffice for calibration, but it only pertains to the warning message. Sooooo, yesterday I flew to a airport that has a compass rose and did the on-ground calibration successfully and followed by a in-flight calibration, quality level in the green at 122. Did some maneuvers and the problem still exist. The indicators always shows a headwind. It will follow the plane around correctly in a 180degree turn, but when I level wings the indicator will snap back and show a headwind. I sent a video to mothership and will report their findings. All other static-pitot systems seem ok, but I must be missing something. I also taped over one stain port but no effect.
FWIW: make sure when you use the tape method, if you have two static ports tape both static ports the same way for testing. Don't tape OVER the entire static port hole, just as close behind, or in front of the holes depending on your error, as possible.
I have dual static ports so here is a photo of how the "wedges" wound up looking like on both of my static port. The photo makes the "wedges" look thicker than they really are.
Last edited:
Steveden
Member
Galin, when you say "Don't tape OVER the entire static port hole, just as close behind, or in front of the holes depending on your error, as possible." Could you explain what side should be taped for what error?
Janekom
I love flying!
I realy think the OP has a static port error. The easiest way to test this it to go out to your local runway and make sure that you set your altimeter while stationary. Take off and do a low level fly by over the runway at a reasonable speed. Be careful. Observe the altimeter reading while a few feet off the deck over the runway. If it differs from your stationary reading you have a static port problem.
Now start off with cutting a thick metal or plastic washer in two halves. Then use thin double sided tape to glue one half that you have cut to each forward side of the two static ports and test again. It might be that you end up with it on the aft sides of the ports.
And to answer Steveden - I know of an experimental aircraft where the static is inside the aircraft. One pilot reported about a 5 kts change of IAS when he opened the storm window. Clearly one needs the static ports to be outside in my opinion.
Now start off with cutting a thick metal or plastic washer in two halves. Then use thin double sided tape to glue one half that you have cut to each forward side of the two static ports and test again. It might be that you end up with it on the aft sides of the ports.
And to answer Steveden - I know of an experimental aircraft where the static is inside the aircraft. One pilot reported about a 5 kts change of IAS when he opened the storm window. Clearly one needs the static ports to be outside in my opinion.