Airspeed vs Groundspeed

[skyboy999]

On recent flights, I gradually came to realise that I always seemed to have a 'headwind' i.e. groundspeed was always lower than IAS. Once alerted, I flew reciprocal tracks to prove. I considered:
- reported groundspeed could be incorrect- Unlikely and, regardless, validated by other GPS devices in cockpit
- static blockage/partial- unlikely as altimeter ok as is VSI
- pitot blockage/partial- again seems to report consistent with aircraft behaviour but unable to confirm accuracy

Yesterday, after parking up by hangar in nil wind, I noticed IAS was showing 15kts. I did a 'Zero pressure IAS calibration' which was successful and I thought that the ADAHRS had had a hiccup and the problem would be down to that and fixed.

Unfortunately, flying again today the problem is still there. I tried disconnecting from the instruments and back-blowing the pitot/static/AoA- only by mouth but no evidence of any blockage/constriction.

Suggestions welcome. Given the issue last night cleared by the calibration, I suspect something awry with the ADAHRS but stumped as to where to go next.
Kit: SV-D1000T, SV-ADAHRS-200, s/w 15.4.7

Thanks

 

[Snoho3]

I'll be following this one... Just yesterday I noticed that my GS was a few kts faster than my airspeed, and this with a headwind component. I plan to do some test flights today to validate/troubleshoot.

 

[Raymo]

What was your TAS? Was the OAT reading correctly? (required to accurately calculate density altitude, which is included in the TAS calculation)

 

[GalinHdz]

Check your static port installation. I had the same issue. You can go to my web page (www.puertoricoflyer.com) and in the MAINTENANCE section you can read how I corrected the problem.

 

[Snoho3]

Check your static port installation. I had the same issue. You can go to my web page (www.puertoricoflyer.com) and in the MAINTENANCE section you can read how I corrected the problem.

That’s a very helpful write-up. Thanks for the link!

 

[Stevec]

As mentioned previously check the static ports are clear and you are not using static air from inside the aircraft. I discovered to my horror after an avionics check that a T fitting was broken by the engineer and once airborne discovered the pressure inside the cockpit reduces and causes a considerable error.

 

[GalinHdz]

As mentioned previously check the static ports are clear and you are not using static air from inside the aircraft. I discovered to my horror after an avionics check that a T fitting was broken by the engineer and once airborne discovered the pressure inside the cockpit reduces and causes a considerable error.

This situation happens more times than you think and it gives the exact same indications as my static port installation error. If you have an "Alternate Static Source", activate it while in cruise flight. Your IAS and Altitude should change. If they do not, then you have an open static line somewhere inside your airplane.

 

[skyboy999]

Thanks for helpful comments. I omitted to mention that the installation has been working fine for about six years so can rule out any issues with component design/placement.

My gut feel is that the problem lies with the ADAHRS on the basis of this, clear tubes, and the circumstances leading to the zero pressure calibration. Are there any tests I can run to firm up?

 

[MTozoni]

Thanks for helpful comments. I omitted to mention that the installation has been working fine for about six years so can rule out any issues with component design/placement.

My gut feel is that the problem lies with the ADAHRS on the basis of this, clear tubes, and the circumstances leading to the zero pressure calibration. Are there any tests I can run to firm up?

Fly slow at a fixed pressure altitude, look at your GPS altitude. Do the same course and fly as fast as you can at the same pressure altitude, and compare your GPS altitude. If they're different, it's your static port issue, where it's creating more suction at increased speeds, either due to using air from within the cockpit, or due to laminar flow over the static port. I have a Velocity where static ports are flush with the fuselage and had the same "always headwinds" problem, so I had to put a small ridge behind the static port holes to disturb and compress the air into them (about the thickness of one sheet of Gorilla tape). That fixed the issue.

 

[skyboy999]

Hi everyone. I've still not managed to resolve my problem but have managed to check various components to narrow the field. Hope someone out there can help me finally sort it!

Just to recap, the problem is that airspeed is always over-reading (hence 'headwind' as regardless of direction IAS always > GS). I have SV1000T and (analogue) backup ASI and Alt. Findings:
1) the two ASI's agree which rules out any fundamental problem with either instrument.
2) the 'excess' airspeed variance is greater at speed (stalls at very similar IAS, cruise maybe over-reads 10kts).
3) changing static source on one instrument does not make any difference, ruling out static as the problem.
4) I've disconnected instruments and back-blown all the tubes.

By process of elimination, it seems the only component I can't check is the pitot head assembly itself (Dynon combined pitot/AoA unheated probe). I can't get my head around how this could give rise to 'excess' airspeed but wondered:
a) would some constriction somewhere cause this? My understand is not as system is closed and a constriction would, if anything, lessen air pressure. Also, I've back blown the tubes and airflow appears ok.
b) drain holes? As far as I can tell, these are clear and I can poke fine wire in to prove. However, I don't have a schematic/cutaway so can't tell if the channel(s) leading to the drain hole(s) could have a blockage.
c) (really thinking outside the box here) could AoA pressure somehow be bleeding into pitot channel?

It would be great if anyone/Dynon could help with advice and/or schematic/cutaway of the pitot head assembly and/or instructions on whether/how to dismantle the assembly to check. Going nuts here!

Thanks

 

[Raymo]

Is your stall vane on the leading edge in-line with the pitot tube?
Is the pitot tube mounted 6 inches below the wing? (must be on Vans aircraft).

 

[skyboy999]

Thanks. I should have reiterated that, until recently, everything has been working fine for seven years so can discount any potential design issues.

 

[My2008CTLS]

I had a similar problem late Friday afternoon with my Dynon D-100 showing a variable 17 to 21 knot airspeed after I finished run-up, while parked with the brakes on at about a 45 degree angle to the 8 knot wind reported by the tower. I taxied back to my hangar which was in the middle of parallel rows of hangars perpendicular to the runway, and the airspeed immediately dropped to 000 as I turned between the hangars. We cycled the power to the Dynon, and taxied back to the run-up ramp, and the Dynon was again showing showing 17 to 21 kias. We scrubbed the flight.

I returned to the airport Saturday morning, with the tower reporting a 3 kias, and the Dynon did not show any airspeed during the run-up, which was normal, I flew 4 trips around the pattern, and airspeeds and ground speeds appeared to be reasonable with the tower reported winds. This appears to be a new problem in my 2008 Flight Design CTLS. I don't remember ever seeing the Dynon report an airspeed during run-up before. The airplane had been flown 7 times in the previous week, without seeing any strange airspeeds, which I continually monitor during takeoff roll. I also keep a cover on the pitot tube when the aircraft is in the hangar.

 

[skyboy999]

I've had similar to this on my Skyview and it's resolved by doing a 'zero ;pressure IAS calibration' (don't know if D-100 has a similar function). Unfortunately, doesn't explain/resolve my issue.

Hoping Dynon can add something, if only schematic/cutaway etc of the pitot head assembly as all roads- rather bizarrely- seem to lead to that.

 

[Dynon]

So a few thoughts:

First, the pitot can't be disassembled. That will destroy it.

But beyond that, yeah, there's not much that can ADD pressure, I wouldn't think, even if the AOA were mixed up (it's clearly not, this isn't a new install) or if there was some gross leak.

So admittedly, I'm sort of stumped here. You might connect with our support team to see if they have any other ideas (contact info in sig)

 

[skyboy999]

As I thought but good that you guys confirm it can't be disassembled.

After trying everything else, I took the assembly off in order to get an airline on with strong pressure to back blow- had previously only used an airbed pump- and that seems to have cured it. As said, I had discounted everything else which meant the problem had to be in the pitot assembly.
I'm sure I read somewhere about airspeed problems if the drain hole(s) get blocked with polish or something and that seems the most likely scenario as I had given everything a thorough clean as part of return to flight post-lockdown.

So, my gut feel is something to do with the drain hole but can't rationalise why this should be so. Only that as designed to drain moisture, they must also leak air so the assembly must be designed to compensate somehow and therefore- if blocked- would increase pressure???

Perhaps I'm overthinking. Happy that sorted but frustrated not being able to explain why.

Thanks to all who tried to help.

 

[swatson999]

As I thought but good that you guys confirm it can't be disassembled.

After trying everything else, I took the assembly off in order to get an airline on with strong pressure to back blow- had previously only used an airbed pump- and that seems to have cured it. As said, I had discounted everything else which meant the problem had to be in the pitot assembly.
I'm sure I read somewhere about airspeed problems if the drain hole(s) get blocked with polish or something and that seems the most likely scenario as I had given everything a thorough clean as part of return to flight post-lockdown.

So, my gut feel is something to do with the drain hole but can't rationalise why this should be so. Only that as designed to drain moisture, they must also leak air so the assembly must be designed to compensate somehow and therefore- if blocked- would increase pressure???

Perhaps I'm overthinking. Happy that sorted but frustrated not being able to explain why.

Thanks to all who tried to help.

Why don't you test it to confirm that was the problem? Tape over the drain hole with something that will seal it up nice and tight (packing tape, perhaps?) and go fly, see if the problem re-appears.