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IAS accuracy

Veetail88

Well Known Member
Seeking knowledge and wisdom regarding the importance of indicated airspeed accuracy.

I recently installed a heated pitot on my 8 having flown it for the past year and a half with the standard Vans bent aluminum tube. During phase I testing, I verified it?s accuracy with the usual GPS triangulation method and established stall speeds and whatnot.

I fly airspeed in the pattern and my normal speeds have been 90 downwind, 80 crosswind and 70 on final and this has been working out fine, although probably a bit faster than required, but I?m good with that. I also have AOA, and in the pattern I check it here and there if I feel I?m getting a bit slow or if I?m cranking a turn to final to avoid overshooting, a scenario I keep a close eye on. The AOA (AFS flavor) is calibrated a bit conservative, so I don?t get the aural warning ?angle angle push push? in normal patterns until I?m actually right about in the flare. Of course the chevrons give me visual feedback along the entire range but the aural part comes in when I still have maybe 4 red chevrons left.

Due to the length of the pitot mast I have, the distance from the bottom of the wing to the center of the pitot inlet is shorter, like around 4 inches, and if I recall the standard one is 6 or more. I figured I?d install it with what I had, fly it to see how things were and adjust as required. Feeling safe with this approach since the AOA is present.
I?ve only had time to make a couple of flights with it so far, but those had purpose and I didn?t have time to run it through a stall sequence. It appears the pitot reads accurately at higher speeds, but when the angle of attack increases I?m pretty certain it?s reading higher than the old one, probably quite a bit. On downwind at 90 knots, I?m getting the AOA aural warning, so of course I carried quite a bit extra indicated air speed to keep her quiet and me safe. I?ve been using the AOA as a backup check so I never actually tracked its indications during normal pattern work otherwise.

I?ll do more testing when I can get to it, but my real question here is, does it really matter if it?s actually right? If it?s consistent and I fly the new numbers instead I don?t really see a problem with it, other than when I let another RV guy fly the airplane and he might expect the more normal numbers.
With multiple GPS units on board, I really don?t see any use for IAS en route. I get ground speed and time to waypoint and that?s what really matters for trip and fuel planning. Am I missing some point as to the need for accuracy here other than just to be correct? Like I said, I?ll likely order a new mast and redo the installation, I?m just pondering?..
 
I recently installed the Dynon heated pitot tube (not flying yet) but it said that the mast should be at least 6 inches long for accurate readings.

As for the importance of inaccuracy, it may have a negative affect on other calculations that depend on it, such as wind speed. I believe ADS-B uses GPS data for speed, not IAS.
 
One pretty important reason to verify TAS (and thus IAS) accuracy during a test program is to verify that the static system is designed, installed, and operating properly. Usually there will be more potential for error in the static system than the pitot. (Remember that your airspeed indicator requires both dynamic (pitot) and static pressure.)

If the static system has gross errors, then not only will your airspeed be off, but more importantly your altitude will as well. That could mean a world of difference when shooting that VOR approach to minimums.
 
Double check you have the pitot and AoA lines connected to the correct orrifice.

Do not assume anything........I have seen this before when test flying another persons RV7. We wondered why things were weird in IAS and AoA.

No amount of me describing the evidence would convince the builder, until we started testing which hole was pressurising the ASI. :cool:
 
If the static system has gross errors, then not only will your airspeed be off, but more importantly your altitude will as well. That could mean a world of difference when shooting that VOR approach to minimums.

Such an error would be caught by the IFR Pitot/Static/Transponder check so unless you're doing the approach without the required certifications/tests I think this is a remote possibility.
 
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Thanks for the responses folks, but to be clear, I only changed the pitot. Didn't mess with any other tubes or fittings and there's no reason to think there's anything wrong with any other component. My static line and AOA tubing are entirely separate.
 
Originally Posted by krw5927 View Post

If the static system has gross errors, then not only will your airspeed be off, but more importantly your altitude will as well. That could mean a world of difference when shooting that VOR approach to minimums.

Such an error would be caught by the IFR Pitot/Static/Transponder check so unless you're doing the approach without the required certifications/tests I think this is a remote possibility.

Not really true, the static system check on an "IFR" cert is merely a system leak check, this has nothing to do with the dynamics of the static system during flight. Airspeed/altitude errors from misplaced or non-standard static ports cannot be checked during ground testing.
 
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Not really true, the static system check on an "IFR" cert is merely a system leak check, this has nothing to do with the dynamics of the static system during flight. Airspeed/altitude errors from misplaced or non-standard static ports cannot be checked during ground testing.

The key point is gross errors. An error which indicates 50' high or low - nope, might not be caught. A static error that results in a 1000' deviation for test altitudes of (for example) 5000' MSL would be problematic and I'd submit that if the pitot/static check didn't catch this then there is no point in doing one.
 
The key point is gross errors. An error which indicates 50' high or low - nope, might not be caught. A static error that results in a 1000' deviation for test altitudes of (for example) 5000' MSL would be problematic and I'd submit that if the pitot/static check didn't catch this then there is no point in doing one.

Maybe we are talking about 2 different things, an IFR cert indeed checks the accuracy of the altimeter plus static system integrity. The FAR's "assume" that the static system design criteria have already been met by the manufacturer.

The OP was asking about indicated airspeed errors, airspeed errors can be introduced thru the static system but those type errors (generally static port installation) cannot be checked on the ground.
 
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IMHO I think people are saying the same thing but with different baselines. There is only one way to insure everything is correct in an EAB aircraft.

1 - Have a complete pitot/static ground check to include system leaks.
2 - Do an in flight verification of IAS using the GPS method.
3 - Adjust pitot/static locations if needed until all indications are correct.

:cool:
 
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Guys,

While I appreciate any response, the thread drift here is not addressing the question posed at all. Due to the lack of substantive response addressing reasons IAC accuracy is critical, I guess I'm going to go with my gut in that it isn't for the purposes cited.

Thanks
 
Guys,

While I appreciate any response, the thread drift here is not addressing the question posed at all. Due to the lack of substantive response addressing reasons IAC accuracy is critical, I guess I'm going to go with my gut in that it isn't for the purposes cited.

Thanks

To clarify, as long as you are 100% certain that your static pressure source is accurate in flight, and therefore is not the cause of your erroneous indicated airspeed, then your position seems reasonable to me.
 
Jesse,

To your question, yes - accurate IAS is important. If you are flying with IAS that you do not trust, why would you not think this is an indication of potential future failures?

To why your think your IAS is not accurate, you are assuming the short mast is the issue. This assumption may or may not be correct. You can do some quick tests to further isolate the issue:
1. Connect a simple manometer to verify IAS readout accuracy. Here is a link: http://www.iflyez.com/Manometer.pdf I found this simple rig to be amazingly accurate. It will also quickly tell you if you have any leaks in the pitot system.
2. Use the same manometer to check for static leaks.
3. Do a test flight with the static system open to the cockpit and compare.

Carl
 
Jesse,

To your question, yes - accurate IAS is important. If you are flying with IAS that you do not trust, why would you not think this is an indication of potential future failures?

IAS accuracy is very important since it is an early indicator of other more serious issues, especially if you fly actual IMC. Only doing an in flight verification check will tell for sure if your IAS is accurate or not. No simple ground test will accomplish this since the airflow around your aircraft while in flight can significantly change the pressures felt by your equipment. This can easily skew your IAS indication in one direction or the other.

The easiest and most accurate in flight IAS verification is done using the GPS method explained in much detail in the National Test Pilot School others have referred to. Do this in flight check and, depending on you results, go from there. You might not have a pitot location problem but a more serious static location problem. They are both interrelated. Besides, it is a very good excuse to go flying.

http://www.ntps.edu/information/downloads.html

My 2 cents.

:cool:
 
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There is no reason to believe that the change in airspeed indication has anything to do with the static system or any other components. The pitot pressure line only was disconnected from the old 6" tube and reconnected to to the new 4" tube. Prior to that all was fine.

At higher airspeed, I am getting the same readings I used to get. (i.e. right around 172 knots at 4500' 30 degrees, WOT and around 9 GPM fuel flow.) No problem here. When I slow down and hold altitude (higher angle of attack), the airspeed reads higher than before as indicated by the AOA warning.

I'm no aerodynamicsist, but it seems the pressure goes up closer to the wing skin at higher angles of attack, and maybe that would explain the higher pitot readings.
 
Such an error would be caught by the IFR Pitot/Static/Transponder check so unless you're doing the approach without the required certifications/tests I think this is a remote possibility.

Not really, the IFR Static/Transponder checks for leak and Transponder operation and not for any error that can be as a result of incorrect static port or location.
 
There is no reason to believe that the change in airspeed indication has anything to do with the static system or any other components. The pitot pressure line only was disconnected from the old 6" tube and reconnected to to the new 4" tube. Prior to that all was fine.

At higher airspeed, I am getting the same readings I used to get. (i.e. right around 172 knots at 4500' 30 degrees, WOT and around 9 GPM fuel flow.) No problem here. When I slow down and hold altitude (higher angle of attack), the airspeed reads higher than before as indicated by the AOA warning.

I'm no aerodynamicsist, but it seems the pressure goes up closer to the wing skin at higher angles of attack, and maybe that would explain the higher pitot readings.

Makes sense but the only way to confirm your IAS is correct is doing the in flight GPS check at both high cruise speed configuration and approach speed configuration. IMHO approach speed configuration accuracy is more important since you will be getting closer to the ground and this is where you want your instrument(s) to be most accurate. Showing a significantly faster IAS than what you are actually doing will have you significantly closer to a stall than you might want. But since you have a properly calibrated AOA, it "should" warn you with enough time. But FWIW, I want to have both things accurate especially when low and slow.

:cool:
 
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Simple question: In the up-down direction, does the new tube have the same orientation as the old one? I mean in angle, pointing up or down or level when in cruise flight.
 
Showing a significantly faster IAS than what you are actually doing will have you significantly closer to a stall than you might want. But since you have a properly calibrated AOA, it "should" warn you with enough time.

Assuming the AOA is calibrated for the current configuration, the actual values of IAS should be completely irrelevant to AOA indication, no?
 
Assuming the AOA is calibrated for the current configuration, the actual values of IAS should be completely irrelevant to AOA indication, no?

Yes and AOA is a better indication for stall warning that IAS.

:cool:
 
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My only concern would be whether or not the airspeed error is simply due to the mast. If the static is on the mast then being closer to the wing is going to give you more error, so it could be just that. If the static ports are on the aft fuse I don't see why you should be getting a significant error. Pitot is not prone to error (pitot is NOT dynamic by the way, it is total pressure, dynamic is total - static). Pitot should not vary even +/- 10 deg of angle difference in pitch or yaw.

So if it is just position, then fine, it doesn't matter as long as it is consistent. You are using airspeed indication to indirectly tell you what the AOA is. But if it all of a sudden changes, because you fix a leak, or water that is in the system evaporates etc etc and things change, then you lose your correlation between AOA and airspeed.

If you really know your airplane well then you can determine correct AOA/airspeed on approach by the sight picture. An experienced pilot can land his airplane without AOA or airspeed just by flying attitude and knowing how things should look out of the window. I had to do it as part of my training and it was not difficult. It scares some people but it shouldn't. After I got the hang of it every time I was on short final and the instructor pulled away the cover hiding the ASI I was right on speed. I personally think that training is a better risk mitigation than AOA indication, but everyone has their own preference.
 
I'm with you Scott, and use that method as well. I really like the AOA though when I'm banking. Low and slow is one thing, add turning and the pucker factor goes up.
 
I took off one time last year with the pitot cover not removed, GPS ground speed works well on final if you factor in the wind. :)
 
The AFS AOA system relies on a pitot input also. I am not quite sure how it all ties together but I do know if you lose the pitot tube you lose the AOA. I would keep in mind you may now have a AOA error or a airspeed error or both. I would redo the AOA calibration.
G
 
AOA is sensed from two little holes in the opposite wing. No pitot input at all.

The AFS AOA does indeed have two little holes in the wing, however both the pitot and the static lines are plumbed into the AOA box and are used in the calculation of AOA. I have found leaks where these inputs are teed into the the main lines. Not suggesting this is your issue, just keeping the facts straight.

From the installation manual:

The Angle of Attack Instrument (AOA Instrument) utilizes pressures from two pressure ports located on your aircraft?s upper and lower airfoil or probe, and pressures from your aircraft?s pitot and static ports. The result of dividing the airfoil or probe differential pressure Pw by the pitot static differential pressure Pp is a coefficient of pressure (CP). There is a unique variation of CP with angle of attack. This variation is very linear over most of the airfoil's AOA.
 
The AFS AOA does indeed have two little holes in the wing, however both the pitot and the static lines are plumbed into the AOA box and are used in the calculation of AOA. I have found leaks where these inputs are teed into the the main lines. Not suggesting this is your issue, just keeping the facts straight.

From the installation manual:

The Angle of Attack Instrument (AOA Instrument) utilizes pressures from two pressure ports located on your aircraft’s upper and lower airfoil or probe, and pressures from your aircraft’s pitot and static ports. The result of dividing the airfoil or probe differential pressure Pw by the pitot static differential pressure Pp is a coefficient of pressure (CP). There is a unique variation of CP with angle of attack. This variation is very linear over most of the airfoil's AOA.

Ahhh, thank you for that info Mark!

The fact that I was getting AOA warning doing 90 knots on downwind really doesn't play well in my mind as that would indicate a difference in reading in excess of 25 knots! That is what hasn't seemed feasible to me.

With the pitot pressure being part of the overall AOA equation, it may be possible that a very slight mismatch in the values between the small AOA ports and the pitot could shift the values used in the calculation by a lot!

So MAYBE the IAS isn't much different and the true culprit is the AOA! MAYBE, all that is really needed is to re-calibrate the AOA! Now that I replay the last landing in my head, it does seem like I was smoking right along on final.

Chomping at the bit now to take the RV up for some stalls and slow flight to check this all out. Unfortunately, between work and the weather, I may not get a chance until Sunday. :(

I really hate when my day job gets in the way of the rest of my life.:(
 
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