An extraordinarily bizarre airspeed mystery that no A&P can solve

I just flew another test flight and took it up to 12,600' DA.

Previously, at 12,600 DA I was flying:

120 KTAS
105 KIAS
5400 RPM
3.8 GPH

Now at 12,600 DA I’m flying:

110 KTAS
91 KIAS
5430 RPM
3.7 GPH

Fire-walling it doesn’t accomplish much:

112 KTAS
92 KIAS
5550 RPM
4.8 GPH

I confirmed I have both a decrease in cruise speed *and* climb performance.
  • Previously 6 minute climb to 6,600’ DA, now 8.5 minutes (+2.5m)
  • Previously 9m to 8,900’ DA, now 12m (+3m)
  • Previously 12.5m to 10,900’ DA, now 15m (+2.5m)
  • Previously 17m to 12,400’ DA, now 18.5m (+1.5m)
EGTs are bang-on with how they've always been before and after this performance problem. They rise ~100 degrees when Eco Mode activates (leaned out) and decrease by as much when full power is reapplied (and the ECU goes full rich).

Also confirmed engine RPM is accurate, or at least during idle it is. On the ground, the iPhone app is amazing and told me it was within 10rpm of what my EMS was showing. In the air, it wouldn't give a read and I think that's because there's too many other noises, including the prop. Still going to test the prop speed as soon as the TruTachII arrives.
 
kicktireslightfires said:
Airspeed.png
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What were the dates and OAT’s on each of the TAS data points?

Summer vs winter temps? 2 knots for colder temps, 2 knots for the rudder, 2 knots for a new prop, 1-2 knots for the static leak, and these begin to add up.
 
kicktireslightfires said:
Previously, at 12,600 DA I was flying:

120 KTAS
105 KIAS
5400 RPM
3.8 GPH

Now at 12,600 DA I’m flying:

110 KTAS
91 KIAS
5430 RPM
3.7 GPH
Click to expand...
What I notice is the bigger difference between indicated and true airspeed between the tests. 15 knots before and 19 knots now. Is the TAS you posted here a calculation based on a ground triangle or a calculation based on indicated airspeed and air pressure and temperature?
 
iamtheari said:
What I notice is the bigger difference between indicated and true airspeed between the tests. 15 knots before and 19 knots now. Is the TAS you posted here a calculation based on a ground triangle or a calculation based on indicated airspeed and air pressure and temperature?
Click to expand...
I noticed that, too. The TAS here is from my ADAHRS and Air Data Computer, so the latter.
 
When the GPS antenna was changed (or about the same time) was there any need to access, update, or verify any type of setting in the Skyview ADAHRS setup? It’s involved in a lot related calculations. Could its OAT sensor be providing bad input?

Have you flown formation with a few different aircraft types to compare IAS/TAS airspeed indications? (You know, an unscientific, fun, “why not?” type of pitot/static test).

If all the pitot static related leak issues or and ADAHRS / OAT calculations and information are all correct, I’m leaning towards the rigging/drag theory. Maybe something occurred about the same time the antenna was changed.
 
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Where is the oat sensor located? Has it been relocated? Was it replaced? Has the exhaust or cowling changed in any way that it could be effecting the OAT sensor?
 
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kicktireslightfires said:
I just flew another test flight and took it up to 12,600' DA.

Previously, at 12,600 DA I was flying:

120 KTAS
105 KIAS
5400 RPM
3.8 GPH

Now at 12,600 DA I’m flying:

110 KTAS
91 KIAS
5430 RPM
3.7 GPH
Click to expand...
Have you revisited your rudder trim? You changed it and it certainly had an impact on you flying out of balance. That is extra drag, which is going to slow you down. I wouldn’t rule out an 5-6% drop.

Manual trim tabs are only ever necessary to correct (or cause) an out of balance condition.

Is it possible your indicators are off and you’re chasing getting them “right” at the price of putting your plane out of balance?
 
He should be able to tell if the aircraft is yawing by feel but if not simply tape a 12 inch string to the cowl in front of the windshield. I am still betting on the nose gear cocking.
 
Are you 100% certain that it you were going faster before? Perhaps you had an erroneous indication before, so you weren't going as fast as you thought?

You cruise at 5400 and max is 5500... you're cruising at 95% power???

Everything I've seen indicates that you should be around 5600 or a bit more at WOT in level flight, indicating that you may have too much pitch... but while reducing pitch would make your climb better it will make your level flight speed slower at a given rpm.
 
Ever slip to get down? That should answer the question.
 
kicktireslightfires said:
I noticed that, too. The TAS here is from my ADAHRS and Air Data Computer, so the latter.
Click to expand...
IMO, you can’t use the TAS computed from the ADAHRS data that use your IAS, or technically Calibrated AS, since both of your ASIs are plumbed to the same static/pitot system, unless you are sure it is accurate.

That is why to know what your authentic TAS is, you may need to perform the testing using an approved mathematical method not utilizing your instrument indications, but using GPS using 3 or 4 vectors at altitude keeping heading/courses without deviation, keeping power setting , attitude and altitude constant and allowing the gps GS to stabilize in the same DA airspace and time to eliminating errors from changing winds aloft over time. Depending on your % power used and aircraft loading, this will create a data point for those parameters. You can then compare that with POH.
 
Dana said:
Are you 100% certain that it you were going faster before? Perhaps you had an erroneous indication before, so you weren't going as fast as you thought?

You cruise at 5400 and max is 5500... you're cruising at 95% power???
Click to expand...
Depending on the altitude, the above could be true. I like to do my testing at WOT which requires a high enough altitude to allow leaning for max power at 75% or less.
 
Dana said:
You cruise at 5400 and max is 5500... you're cruising at 95% power???
Click to expand...
LOL. At one point, I thought my Archer was much faster than book. After investigation, I found my tach reading 175-200 rpm less than actual ( not uncommon). So until that discovery, when I used the erroneous tach reading for 75% power, I was actually using more power which gave me more speed than POH for 75%. An inaccurate tach in the opposite direction would cause your opposite problem.
 
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MajorTurbulence said:
IMO, you can’t use the TAS computed from the ADAHRS data that use your IAS, or technically Calibrated AS, since both of your ASIs are plumbed to the same static/pitot system, unless you are sure it is accurate.

That is why to know what your authentic TAS is, you may need to perform the testing using an approved mathematical method not utilizing your instrument indications, but using GPS using 3 or 4 vectors at altitude keeping heading/courses without deviation, keeping power setting , attitude and altitude constant and allowing the gps GS to stabilize in the same DA airspace and time to eliminating errors from changing winds aloft over time. Depending on your % power used and aircraft loading, this will create a data point for those parameters. You can then compare that with POH.
Click to expand...
Throw in some reverse courses, too.
 
Just have a guy come out and do a pitot static check. He will be able to relay to you, whether it is correct or not.
 
Question: Is the Pitot system side of the Pitot Static system checked when the biannual certification is done?
I am probably missing something but I ask this because in all the times I watched the procedure, the device over the Pitot/Static system seemed to only be used to check the integrity with regards to the Static side and the performance of my both my altimeters and I think I remember that the VSI moved as a change in pressure was induced. The encoding transponder reading was also assessed.
If so, what proves the integrity of the Pitot side?
 
It's not required so they don't normally do it. But the ones I had done I paid the extra $50 to verify the ASI was correct.
 
MajorTurbulence said:
If so, what proves the integrity of the Pitot side?
Click to expand...
Nothing. Unless you perform a separate pitot system only test you don't know. Most of the test boxes used to perform the recurring pitot/static system tests connect both the pitot system and the static system so they can crossfeed to keep the airspeed from pegging out when they test above 1000-1500 feet. Whats interesting in a previous post it was stated there was a 200ft static leak which wasn't corrected and exceeded the normal limits. That static leak could have easily been at the airspeed indicator internal diaphragm which could affect the pitot side at higher airspeeds. ANd just to add there is no required regular testing of the standalone pitot system.
 
The OP did a proper TAS check running a triangle using a TAS calculator for the results. He confirmed the GS with multiple GPS’s. His current TAS is accurate.
 
How does your airspeed performance compare to other examples of the airplane?

During your most recent flights since you last commented, have you been able to increase your airspeed by using small inputs to your rudder and ailerons (to trim it out)?
 
TrueCourse said:
How does your airspeed performance compare to other examples of the airplane?

During your most recent flights since you last commented, have you been able to increase your airspeed by using small inputs to your rudder and ailerons (to trim it out)?
Click to expand...
There are only two other examples in the USA so hard to answer that question.

No, was not able to increase speed by flying coordinated.
 
Jeff767 said:
The OP did a proper TAS check running a triangle using a TAS calculator for the results. He confirmed the GS with multiple GPS’s. His current TAS is accurate.
Click to expand...
What about his prior TAS, was it accurate?
 
Just removed the nose pant and torqued the nose wheel bolt so it requires exactly 13 lbs of force to move in either direction. Previously, it was moving with just 3-4 lbs and the maintenance manual says it should be 13 lbs. What's fascinating is that particular bolt is among the most sensitive I've ever seen. I probably turned it just 1/32nd of a turn to increase the force from 3-4 lbs to 13 lbs. Also marked it with security paint to ensure it doesn't move. I don't think it's going to make a difference in cruise speed, but we'll see!
 
That’s very sensitive. You may want to check that all the proper washers ect.. are present. Your next flight will be interesting. At a minimum you may have fixed the trim issues.
 
I haven't gone through 6 pages of thread so forgive me if this has been asked and/or answered. Since you say TAS was accurate before and after the issue in question (1) were both of these calculated from wind triangles (or any method independent of the pitot-static system), and if so (2) if you calculate calibrated airspeed from these TASs how does it compare to your indicated airspeed at those TASs? If it's within a knot or two in both cases (assuming something close to cruise speeds) you've actually slowed down (so drag if MAP and RPM haven't changed), and if it's off by something close to the airspeed difference you're investigating it's a pitot-static issue.

If you don't have wind triangle data from the previous configuration you can still compare calculated CAS to cockpit IAS for new config test data and see if calibrated is close to indicated. If it's off by more than a small amount in cruise you still have an air data issue that's probably affecting altitude as well. You just won't know if you had sthe same issue before.

Nauga,
and a lot of tower flyby data
 
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kicktireslightfires said:
I just flew another test flight and took it up to 12,600' DA.

Previously, at 12,600 DA I was flying:

120 KTAS
105 KIAS
5400 RPM
3.8 GPH

Now at 12,600 DA I’m flying:

110 KTAS
91 KIAS
5430 RPM
3.7 GPH
Click to expand...
120 ktas / 105 kias = 1.143

110 ktas / 91 kias = 1.208

Your true airspeed calculation from EFIS seems to be quite different between the original flights and current flights. Something not quite right here.

My WAG... Either your EFIS is on the fritz, or possibly some moisture/water in the pitot lines.

Like others before me have pointed out... take the time and money to at least troubleshoot the static/pitot system first.
 
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Let'sgoflying! said:
$8.99 - ok if it works as advertised.
Click to expand...
I was a beta tester for the android version and found it to work very well. On the ground it was very stable, in the air you have to set the "hints" a little tighter to avoid it picking up extraneous noise in my open cockpit.

I can't vouch for its accuracy, but as a digital device counting exhaust pulses it should be as accurate as the device's built in clock chip. If it's not adjusted right you'll get some obviously screwy numbers. Basically you tell it how many cylinders you have and whether it's 2- or 4-stroke, then set upper and lower limits for an rpm "hint" so it doesn't latch on to some other harmonics.
 
Dana said:
You cruise at 5400 and max is 5500... you're cruising at 95% power???
Click to expand...
Both RPM and torque figure into power, along with density altitude. Where are the cruise charts for that engine or airplane?

For example, we can look at the chart for a Cessna 172N:

1704411797506.png

That engine redlines at 2700. There are cruise RPMs up to 2650 at, for example, 8000 feet and standard temperature for that altitude (around 0°C). It makes only 75% power there, even though the RPM is 98% of redline.
 
Looks like we have been ghosted by the OP.
 
6 pages.....and an A&P still couldn't fix it?

Bell and Doc got nuth'n....;)
 
I sure hope they took his money before they told him they couldn't help him fly his plane correctly..... :cool:
 
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