Scott Will

Scott Will

Well Known Member
Ever since first flight my indicated airspeeds have been off. It gets worse as velocity increases. I have the Dynon AOA probe hooked up to my mechanical and GRT display (they agree pretty well). Static system checks out via an IFR check and flying in formation with other aircraft.

I'm wondering if the pitot tube itself isn't tilted up or down. That would definitely throw things off (I remember that from college days! we did experiments mounting pitot tubes at different angles in the wind tunnel).

I have the Gretz Aero pitot mast. Did any of you have to shim it at all? I just ordered a new one and noticed in the directions about adding a washer.

Wonder if there is a way to determine the correct angle... maybe with longerons level?
 
Scott,

I just poked Tim Olson who had similar issues on an RV-10 and resolved them. A couple of notes tho.

Mathematically these may not make any sense. But I'm not pitot genius.

a) while static could pass the IFR cert, the ASI uses "differential" between pitot and static to determine where it should read. Do some looking for how to build a "mamometer" I think a pitot system can be checked with specific inches of water, etc.

b) I'm pretty sure that Tim looked in to the "angle" topic and determined that it really didn't make that much difference. However, what he did find was that his static ports (which also passed an IFR cert) and simple altimeter comparisons, we most of the effect. I can't remember if he used a kind that protruded and then went to flush mounted ones, or vice-a-versa, but they ultimately solved his low reading.

Like I said, I poked him, so maybe he'll show up and educate us all.

Have fun, I'm jealous!
 
It's very true and simple Bernoulli to determine that airspeed uses the difference between total and static (not the true difference, there's some math involved, but the main variables are those two items).

While I have the flush mounted ports and I know there have been posts about them, I believe my static system is fine because you and I flew side by side in your fancy G1000 182 and I matched you within a few feet. That tells me that the static pressure is OK.

And few flights ago I put a raised strip right in front of the static ports held on by aluminum tape... that didn't help at all.
 
pitot angle

Scott,

From the tunnel experiments you did, there should have been negligle affects up to about 20 or 30 degrees from freestream velocity. Most pitot tubes are "good" to 20-30 degrees from V infinity depending on who's book you read. I worked for an aircraft manufacturer and it took about 8 months to get the pitot-static system right on a prototype aircraft. It sounds suspiciously like a static problem. My 2 cents.

Brett
 
If it's a static problem, then my altitude and vertical speed should be off. As far as I can tell, they are OK.
 
Scott Will said:
It's very true and simple Bernoulli to determine that airspeed uses the difference between total and static (not the true difference, there's some math involved, but the main variables are those two items).

While I have the flush mounted ports and I know there have been posts about them, I believe my static system is fine because you and I flew side by side in your fancy G1000 182 and I matched you within a few feet. That tells me that the static pressure is OK.

And few flights ago I put a raised strip right in front of the static ports held on by aluminum tape... that didn't help at all.
Click to expand...


I guess what I'm just not trusting, is the altimeter as the indicator that the static system is "aeronautically" correct...but this math stuff is too hard for us normal guys anyway :)...

So, let me ask.... How hard would it be to change your static ports to the vans type that poke out a little? If simple, I can't believe the ports are very expensive. Only way to *really* know ... :)
 
Scott Will said:
If it's a static problem, then my altitude and vertical speed should be off. As far as I can tell, they are OK.
Click to expand...
The airflow under the wing is roughly aligned with the wing skin. The pitot tube should be accurate as long as it is aligned within about 15 degrees of the airflow (info from a NACA or NASA report). It is quite unlikely that pitot tube misalignment is the cause of this problem, assuming your pitot tube is pointing straight ahead, and is roughly aligned with the bottom wing skin. A pitot leak would cause a low ASI reading. Have you done a leak check?

I've seen more than one report of flush static ports causing airspeed errors on the order of 10 kt at cruise speed, with the error being less below that. This would lead to about a 100 to 150 ft altitude error, but would have essentially no effect on vertical speed, as the vertical speed reads altitude changes, not absolute altitudes. The altitude error would be hard to see by comparison with another aircraft, as each aircraft has an altimeter error of 50 to 75 ft.

Supposedly one of the flush port manufacturers changed his port design to make it stick out above the skin to address this problem. I haven't seen any test results that prove or disprove this claim.

How are you determining your airspeed error, and how much error do you have? Some people try to use inaccurate methods, such as averaging GPS ground speeds to get TAS. This method is only accurate if there is zero wind.

Have you checked the ASI for instrument error? Even EFIS ASIs can have errors.

For lots more info see the info on pitot-static system calibration on my web page.

I'm on the road right now visiting relatives over Christmas, and have very infrequent web access. I'll check back in a day or so, I hope.
 
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Scott,

Make a simple little washer from aluminum. Make it about 1/2" in diameter and about .040 thick. Drill a hole in it the same size as the static port hole. Round the edges of the washer. Temporarily afix it over your flush static port (you must be sure the holes line up and that it will stay put!!). Go fly--use your gps to capture ground speeds and Kevins spread sheet to convert it to TAS. I will bet you a beer that a large portion of your error goes away!!

Pitot tubes can be significantly out of alignment with the aircraft and still be accurate. Dynon's pitot has a significant range--don't remember what it is exactly but it is on their web site.

I also had NO NOTICEABLE error in my altitimeter while my airspeed was 7+% off. It was the flush static port! As mentioned in your other thread on this issue I put in 2 months and many tests to correct this situation. You have identical equipment, an identical main airframe, and the same symptoms.

Isn't VAF a great resource!

Cheers,

db
RV9a/ECI0360/James Cowl/Catto prop--90 hrs and a permanent smile!!
 
OAT error?

One more thing to think about - OAT accuracy. You need to know the OAT to convert between CAS and TAS. Some people put the OAT probe in a NACA scoop on the forward fuselage. Many people have found that OAT probes in this location over read - if you used a too high OAT value to convert from TAS to CAS, if could lead you to believe your CAS was reading too high. 3 deg error in OAT is equal to about 1 kt at RV cruise speeds.

Also, there may be three or hour degrees C of ram temperature rise on the OAT indication at RV cruise speeds, depending on the recovery factor of your probe. You can check for this by flying at as high a speed as possible at a constant altitude until the OAT indication stabilizes. Then slow down to as slow a speed as possible at the same altitude, and let the OAT stabilze. If the two OAT readings are the same, then there is either no ram temperture rise, or it is being corrected for by the EFIS. If the OAT indication varies with speed, then gather enough data to know how the indicated OAT varies with speed, and figure out what recovery factor you need to correct for it.

IOAT = OAT + K * (TAS ^ 2) / 7592

IOAT is indicated OAT in deg C
OAT is the actual OAT in deg C
K is the OAT probe's recovery factor. Usually somewhere between 0.8 and 0.95, but it could be lower in some cases
TAS is true airspeed in knots

Plot IOAT vs (TAS ^ 2) / 7592. The slope of the line is K, the probe's recovery factor.
 
db1yg said:
Scott,

Make a simple little washer from aluminum. Make it about 1/2" in diameter and about .040 thick. Drill a hole in it the same size as the static port hole. Round the edges of the washer. Temporarily afix it over your flush static port (you must be sure the holes line up and that it will stay put!!). Go fly--use your gps to capture ground speeds and Kevins spread sheet to convert it to TAS. I will bet you a beer that a large portion of your error goes away!!

Pitot tubes can be significantly out of alignment with the aircraft and still be accurate. Dynon's pitot has a significant range--don't remember what it is exactly but it is on their web site.

I also had NO NOTICEABLE error in my altitimeter while my airspeed was 7+% off. It was the flush static port! As mentioned in your other thread on this issue I put in 2 months and many tests to correct this situation. You have identical equipment, an identical main airframe, and the same symptoms.

Isn't VAF a great resource!

Cheers,

db
RV9a/ECI0360/James Cowl/Catto prop--90 hrs and a permanent smile!!
Click to expand...

I agree, db.

At one point with a previous homebuilt, I had 3 ASI's hooked up to the same pitot tube but with different static sources, plastic tubes and instruments laying across the right seat.

Finally, with much "fence" experiementing at the static port, I got the primary ASI reasonably accurate. As I recall, it was a .035 dam aft of the port that worked best.

It's a different challenge with every airplane, but I bet some experiementing with the static port, as you suggest, will make it better. It doesn't make sense that this works with a good altimeter reading but there's more going on here than we know about, so it is probably true.

dd
 
OK Guys! I concede!!

Today I drilled out my flush static port to 1/8 dia and JB Weld'd the stock Van's pop rivet in there (with the stem removed, obviously). Drizzle and low clouds kept me on the ground. Will report back ASAP!

BTW - my OAT probe is on the underside of my right wing, on my first inspection panel. Seems pretty accurate but yes, I do know that OAT affects TAS.

At work we deal with IAS, CAS, EAS, TAS, Mach, you name it.
 
Hey Scott, How about just vent static to the cabin for some tests?
While the pressure dif outside to in is slight, as long as it relative, which it is, this would be a good test.
 
Kahuna said:
Hey Scott, How about just vent static to the cabin for some tests?
While the pressure dif outside to in is slight, as long as it relative, which it is, this would be a good test.
Click to expand...
If you do this test, try opening and closing the fresh air vents and cockpit heat while in level flight in smooth air. Note the effect on the airspeed and altitude indications. This should give a good idea why cockpit static sources are not a good idea on fast aircraft.
 
Well I am back after a field test with my modified static ports. I drilled out my SafeAir1 ports to 1/8 (to about a 1/4" dept) to accept the Van's pop rivet meant for baffles.

I could tell on climbout that my mods seemed to help a lot. My IAS/TAS readings were a lot closer together in the climb.

At 5500' on 3 different headings I came up with a GPS verified TAS of 172.3 kts and a GRT displayed TAS of 167.6 (averaged on the runs). This gives a difference of 4.7 kts, or 2.7% error. My IAS was 151.6 at an OAT of 13C/56F (about 9C above standard). With the altitude and temp correction, my corrected IAS to TAS is 166.9 - pretty darn close to the average TAS from the GRT. This tells me the algorithms to compute TAS in the GRT units is good.

My RPM was 2650 at 24.9" MAP.

Looks like I can stand to increase the IAS a tad more to get my TAS closer to actual. Didn't get a chance to check altitudes because I didnt have a chase aircraft.

Merry Christmas to all! Now I have my own sleigh I can chase Santa around with!
 
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Scott,

Congratulations--it looks like you have taken at least 1/2 the error out of your system. I bet a small 040 washer under the domed port would eliminate 1/2 of the remaining error.

Best Wishes and Merry Christmas,

Cheers,

db
 
Scott - Congrats on some significant progress.

Have you done a ground test to check the accuracy of the EFIS ASI? My EFIS has ASI errors of up to three knots at some speeds, and it is quite likely that most EFIS have some errors. If any of the remaining error is due to EFIS ASI instrument error, then you wouldn't want to try to correct it by messing around with the static port, as now you would be creating static system error which would affect the altimeter.

Have you verified that the EFIS correctly calculates the TAS? E.g., if you start from the EFIS IAS, OAT and altitude, do you calculate the same TAS that the EFIS displays? If is probably correct, but it would be good to confirm before you do any more static port surgery.
 
Kevin...

While I am waiting to get my autopilot servo back so I can do more accurate testing, from what I can tell today... yes the EFIS correctly takes IAS to TAS. Used this website as quick calculator since I have all my stuff at work.

http://www.flightplan.za.net/trueAirspeed.php

IAS = 151.67
OAT = 13C
ALT = 5500
Theoretical TAS = 167
Tested TAS = 167.67 (probably right at 167 but one of my readings was slightly higher so that raised the average)

I did an IFR check but we had lots of problems finding a fitting to properly test the ASI. Besides he just took it to 105 kts and that was it. He focused on the static system after that.
 
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