Matronics Email Lists :: View topic - Dynon Skyview Airspeed Calibration...

Matronics Email Lists
Web Forum Interface to the Matronics Email Lists

Get Email Distribution Too! FAQ Search Memberlist Usergroups Register

Profile Log in to check your private messages Log in

Dynon Skyview Airspeed Calibration...
Goto page 1, 2 Next

Matronics Email Lists Forum Index -> RV-List

View previous topic :: View next topic

Author Message

Matt Dralle
Site Admin

Joined: 08 Nov 2005
Posts: 25781
Location: Livermore CA USA

Posted: Mon Dec 26, 2011 7:43 pm Post subject: Dynon Skyview Airspeed Calibration...

Dear Listers,



I've been flying the new 10" Dynon Skyview in the RV-6 for a few weeks now and it seems like the airspeed is reading maybe 10mph fast.  The GS always reads 10mph or more slower than the True airspeed, no matter which way I fly with respect to the current wind.  Looking through the configuration options on the Skyview, I'm not seeing parameters to calibrate the airspeed.  If the airspeed were *slow* compared to the GS, I could envision making adjustments to the Pitot tube to get it in better alignment with slipstream.  But *fast* is a head scratcher.  If there's no electronic configuration parameters to adjust, what do you do?  Are there Pitot line "attenuators" like for RF in coax? 



Matt



-

Matt "Red Dawg" Dralle



RV-8 #82880 N998RV "Ruby Vixen"

http://www.mattsrv8.com - Matt's Complete RV-8 Construction Log

http://www.mattsrv8.com/Mishap - Landing Mishap Rebuild Log

http://www.youtube.com/MattsRV8 - Matt's RV-8 HDTV YouTube Channel

Status: 170+ Hours TTSN - Rebuilding Fuselage After Landing Mishap...



RV-6 #20916 N360EM "The Flyer"

http://www.mattsrv6.com - Matt's RV-6 Revitalization Log

Status: 42+ Hours Since Purchase - Upgrades Complete; Now In Full Flyer Mode

_________________
Matt Dralle
Matronics Email List Administrator

Kellym

Joined: 10 Jan 2006
Posts: 1705
Location: Sun Lakes AZ

Posted: Mon Dec 26, 2011 7:54 pm Post subject: Dynon Skyview Airspeed Calibration...

On the static port, it is critical that there be a lip. Whether you used 

Van's pop-rivet port or a machined static port, it must not be flush 

with the aircraft skin. It needs to stick out about 1/16".



On 12/26/2011 8:39 PM, Matt Dralle wrote:

 	  Quote: 	
		   



 Dear Listers,



 I've been flying the new 10" Dynon Skyview in the RV-6 for a few weeks now and it seems like the airspeed is reading maybe 10mph fast.  The GS always reads 10mph or more slower than the True airspeed, no matter which way I fly with respect to the current wind.  Looking through the configuration options on the Skyview, I'm not seeing parameters to calibrate the airspeed.  If the airspeed were *slow* compared to the GS, I could envision making adjustments to the Pitot tube to get it in better alignment with slipstream.  But *fast* is a head scratcher.  If there's no electronic configuration parameters to adjust, what do you do?  Are there Pitot line "attenuators" like for RF in coax? 



 Matt



 -

 Matt "Red Dawg" Dralle



 RV-8 #82880 N998RV "Ruby Vixen"

 http://www.mattsrv8.com - Matt's Complete RV-8 Construction Log

 http://www.mattsrv8.com/Mishap - Landing Mishap Rebuild Log

 http://www.youtube.com/MattsRV8 - Matt's RV-8 HDTV YouTube Channel

 Status: 170+ Hours TTSN - Rebuilding Fuselage After Landing Mishap...



 RV-6 #20916 N360EM "The Flyer"

 http://www.mattsrv6.com - Matt's RV-6 Revitalization Log

 Status: 42+ Hours Since Purchase - Upgrades Complete; Now In Full Flyer Mode

_________________
Kelly McMullen
A&P/IA, EAA Tech Counselor # 5286
KCHD

dmaib(at)mac.com
Guest

Posted: Mon Dec 26, 2011 8:03 pm Post subject: Dynon Skyview Airspeed Calibration...

Maybe a static port error? Check out some of Kevin Horton's articles  

in Kitplanes. Static port errors can, and do,  cause "fast" errors.  

IIRC, Kevin says that pitot alignment does not have to be very  

precise to be pretty accurate.



David Maib

Rv-10 40559

Flying.

On Dec 26, 2011, at 10:39 PM, Matt Dralle wrote:



 	  Quote: 	
		   



 Dear Listers,



 I've been flying the new 10" Dynon Skyview in the RV-6 for a few  

 weeks now and it seems like the airspeed is reading maybe 10mph  

 fast.  The GS always reads 10mph or more slower than the True  

 airspeed, no matter which way I fly with respect to the current  

 wind.  Looking through the configuration options on the Skyview,  

 I'm not seeing parameters to calibrate the airspeed.  If the  

 airspeed were *slow* compared to the GS, I could envision making  

 adjustments to the Pitot tube to get it in better alignment with  

 slipstream.  But *fast* is a head scratcher.  If there's no  

 electronic configuration parameters to adjust, what do you do?  Are  

 there Pitot line "attenuators" like for RF in coax? 



 Matt



 -

 Matt "Red Dawg" Dralle



 RV-8 #82880 N998RV "Ruby Vixen"

 http://www.mattsrv8.com - Matt's Complete RV-8 Construction Log

 http://www.mattsrv8.com/Mishap - Landing Mishap Rebuild Log

 http://www.youtube.com/MattsRV8 - Matt's RV-8 HDTV YouTube Channel

 Status: 170+ Hours TTSN - Rebuilding Fuselage After Landing Mishap...



 RV-6 #20916 N360EM "The Flyer"

 http://www.mattsrv6.com - Matt's RV-6 Revitalization Log

 Status: 42+ Hours Since Purchase - Upgrades Complete; Now In Full  

 Flyer Mode

pitts_pilot(at)bellsouth.
Guest

Posted: Mon Dec 26, 2011 8:05 pm Post subject: Dynon Skyview Airspeed Calibration...

I think you're on the right track ..... airspeed is the measurement of 

the difference in pressure between the pitot and static.  So, it appears 

(to me anyway) that the static pressure is in a lower pressure area than 

it 'should be'.  You should have two static ports, one on each side of 

the fuselage about 1/2 way between wing trailing edge and horizontal 

stab leading edge.  This should be the area of least disturbed air going 

by.  Having the ports on each side balances the pressures caused by 

slipping and sliding.  The port should stick out a little to get out of 

the laminar air flow along the fuselage.



Since there's no airflow in either the pitot or static lines, a 

restricter won't have any effect.  The angle of the pitot to relative 

airflow will have some effect on the airspeed reading too.



One of our RV-4 pilots (he bought it, not built it) was complaining 

about inaccurate airspeed and we found the single static port above and 

forward of the wing leading edge on the left fuselage side.  It's a 

'washer' about 1" dia and 1/16" thick with a small hole in the center.  

It's located right where prop wash will really hit it with turbulent 

air.  He plans on moving it and installing two ports as above.

Linn

On 12/26/2011 10:39 PM, Matt Dralle wrote:

 	  Quote: 	
		   



 Dear Listers,



 I've been flying the new 10" Dynon Skyview in the RV-6 for a few weeks now and it seems like the airspeed is reading maybe 10mph fast.  The GS always reads 10mph or more slower than the True airspeed, no matter which way I fly with respect to the current wind.  Looking through the configuration options on the Skyview, I'm not seeing parameters to calibrate the airspeed.  If the airspeed were *slow* compared to the GS, I could envision making adjustments to the Pitot tube to get it in better alignment with slipstream.  But *fast* is a head scratcher.  If there's no electronic configuration parameters to adjust, what do you do?  Are there Pitot line "attenuators" like for RF in coax? 



 Matt



 -

 Matt "Red Dawg" Dralle



 RV-8 #82880 N998RV "Ruby Vixen"

 http://www.mattsrv8.com - Matt's Complete RV-8 Construction Log

 http://www.mattsrv8.com/Mishap - Landing Mishap Rebuild Log

 http://www.youtube.com/MattsRV8 - Matt's RV-8 HDTV YouTube Channel

 Status: 170+ Hours TTSN - Rebuilding Fuselage After Landing Mishap...



 RV-6 #20916 N360EM "The Flyer"

 http://www.mattsrv6.com - Matt's RV-6 Revitalization Log

 Status: 42+ Hours Since Purchase - Upgrades Complete; Now In Full Flyer Mode

mr.sun

Joined: 30 Jan 2007
Posts: 85

Posted: Mon Dec 26, 2011 9:20 pm Post subject: Dynon Skyview Airspeed Calibration...

Is it a true vs indicated issue? 

Greg

_________________
Greg RV-7

lessdragprod(at)aol.com
Guest

Posted: Tue Dec 27, 2011 8:51 am Post subject: Dynon Skyview Airspeed Calibration...

An easy way to check the static side of the airspeed indicator is to use your altimeter.

    

   I flew along a ridge line at cruise speed that has a 2200 feet peak as shown on my sectional.

    

   I may not have been exactly at the same level.  Could have been 10 or 20 feet off in hieght.  My altimeter read 2,210 feet.

   So, at that time, my static system was pretty close.

    

   Jim Ayers

 Less Drag Special sn 1 (Modified HR2)

 

 

   --

khorton01(at)rogers.com
Guest

Posted: Tue Dec 27, 2011 11:49 am Post subject: Dynon Skyview Airspeed Calibration...

Checking altimeter accuracy against tops of ridge lines only works if the temperature is close to standard temperature.  



If the temperature differs from standard, the error is about 4 ft per 1000 ft per degree C of difference from standard temperature.  E.g., if the top of the ridge line is 2000 ft above the airport where the altimeter setting came from, and the temperature is 15 deg C warmer than standard temperature, the altimeter will read about 4/1000 * 2000 * 15 = 120 ft lower than the actual altitude.



See:



http://bathursted.ccnb.nb.ca/vatcan/fir/moncton/WeeklyTopics/Archives/20040104/CurrentTopic.html

http://44rf.com/misc/USAF_AIS_Cold_WX_Altimeter.ppt



Kevin Horton



On 2011-12-27, at 12:44 , Jim Ayers wrote:



[quote] An easy way to check the static side of the airspeed indicator is to use your altimeter.

  

 I flew along a ridge line at cruise speed that has a 2200 feet peak as shown on my sectional.

  

 I may not have been exactly at the same level.  Could have been 10 or 20 feet off in hieght.  My altimeter read 2,210 feet.

 So, at that time, my static system was pretty close.

  

 Jim Ayers

 Less Drag Special sn 1 (Modified HR2)

 

 --

Matt Dralle
Site Admin

Joined: 08 Nov 2005
Posts: 25781
Location: Livermore CA USA

Posted: Tue Dec 27, 2011 9:16 pm Post subject: Dynon Skyview Airspeed Calibration...

At 07:39 PM 12/26/2011 Monday, Matt Dralle wrote:

 	  Quote: 	
		  Dear Listers,



I've been flying the new 10" Dynon Skyview in the RV-6 for a few weeks now and it seems like the airspeed is reading maybe 10mph fast.  The GS always reads 10mph or more slower than the True airspeed, no matter which way I fly with respect to the current wind.  Looking through the configuration options on the Skyview, I'm not seeing parameters to calibrate the airspeed.  If the airspeed were *slow* compared to the GS, I could envision making adjustments to the Pitot tube to get it in better alignment with slipstream.  But *fast* is a head scratcher.  If there's no electronic configuration parameters to adjust, what do you do?  Are there Pitot line "attenuators" like for RF in coax? 



Matt


	




Listers,



Thank you for all the great suggestions on resolving this airspeed issue!  I took a look at the static ports on the RV-6 today and found that they are done using a flush head screw with a hole drilled in the center on either side of the fuselage.  They do *not* protrude from the side of the plane at all.  They are totally flush (see attached picture).  Just for fun, I whipped up a couple of quick test deals using a 3/16" washer and some electrical tape.  I poked a 1/16" hole in the center of the tape and then put the washer centered over the static ports on both sides of the fuselage (see attached pictures).  Then, I went flying on this beautiful December 27 day in California!



I didn't have time to do any real scientific multi-leg testing, but I was amazed that the True Airspeed is now falling in a much more believable range compared to the GPS-derived ground speed.  On one cross-country leg, I had an exactly 90 degree crosswind component and the True Airspeed and GPS Ground Speed were tracking exactly the same.  Yahoo!



Obviously I need to do some additional, more scientific measurements and probably come up with a slightly more "permanent" washer arrangement, but the early returns are very promising! 



Thanks again for all the great feedback!



Matt 



-

Matt "Red Dawg" Dralle



RV-8 #82880 N998RV "Ruby Vixen"

http://www.mattsrv8.com - Matt's Complete RV-8 Construction Log

http://www.mattsrv8.com/Mishap - Landing Mishap Rebuild Log

http://www.youtube.com/MattsRV8 - Matt's RV-8 HDTV YouTube Channel

Status: 170+ Hours TTSN - Rebuilding Fuselage After Landing Mishap...



RV-6 #20916 N360EM "The Flyer"

http://www.mattsrv6.com - Matt's RV-6 Revitalization Log

Status: 42+ Hours Since Purchase - Upgrades Complete; Now In Full Flyer Mode

2011.12.27_-_RV-6_-_Static_Air_Ports_(1)_-_Original_Port.jpg

Description:

Filesize:

57.45 KB

Viewed:

21535 Time(s)

2011.12.27_-_RV-6_-_Static_Air_Ports_(1)_-_Original_Port.jpg

2011.12.27_-_RV-6_-_Static_Air_Ports_(2)_-_#10_Washer_&_Some_Electrical_Tape.jpg

Description:

Filesize:

136.56 KB

Viewed:

21535 Time(s)

2011.12.27_-_RV-6_-_Static_Air_Ports_(2)_-_#10_Washer_&_Some_Electrical_Tape.jpg

2011.12.27_-_RV-6_-_Static_Air_Ports_(3)_-_Static_Port_Test_Modification.jpg

Description:

Filesize:

124.68 KB

Viewed:

21535 Time(s)

2011.12.27_-_RV-6_-_Static_Air_Ports_(3)_-_Static_Port_Test_Modification.jpg

2011.12.27_-_RV-6_-_Static_Air_Ports_(4)_-_Static_Port_Test_Modification.jpg

Description:

Filesize:

68.35 KB

Viewed:

21535 Time(s)

2011.12.27_-_RV-6_-_Static_Air_Ports_(4)_-_Static_Port_Test_Modification.jpg

_________________
Matt Dralle
Matronics Email List Administrator

rvmills(at)sbcglobal.net
Guest

Posted: Tue Dec 27, 2011 11:42 pm Post subject: Dynon Skyview Airspeed Calibration...

Matt,



Been trying to shoot a note out while on the road, with no success. In the

meantime, you've gotten some good info, and sounds like you're making

progress with testing. Here's what I wrote...just a little more data from

the field:



As the others said, sounds like a possible static port issue. Also agree it

could be OAT, depending on whether the error is there at all speeds or just

high speeds. Or it could be a bit of both.



On the static port...that rivet fix sounds like a great idea. A buddy did it

another way (though I like that rivet idea). My bud used AN-960 washers to

build up the lip on the static ports on his F1 Rocket. In his case, he

actually cut the washer in half, and glued it around the forward half of his

port. He played with thickness until his TAS was very close to his GPS

speeds (multi-leg test). You could probably experiment with -6, -8 and -10

washers, thin and thick washers, sanding or grinding the edges down a bit to

soften the corner, and full or half washers, to see what gets you the

closest. Painting it afterwards could have a slight impact too, so consider

that as well.



On the OAT, I've found that my TAS error increases the faster I

go...typically between 2.5-4 knots high. I did a test once, slowing from 170

to 70 and then accelerating back to 170 (KIAS, level flight). My OAT

decreased 5 degrees as I slowed, then increased back to the original temp as

I accelerated. So my OAT during max speed tests is inducing an error from

Ram Rise (small, but there). Mine is under the H-Stab on the side of the

fuse. Another Rocket gent saw the same effect with his OAT probe on the

aileron bellcrank inspection panel. He built a bracket and mounted it inside

the same panel (left the hole there, just "retracted" the probe inside it,

if you will, and found the probe still worked and the Ram Rise error went

away. I may try that on my tail-mounted probe as well.



Good luck chasing it!



Cheers,

Bob (Nasty)



--

Matt Dralle
Site Admin

Joined: 08 Nov 2005
Posts: 25781
Location: Livermore CA USA

Posted: Thu Dec 29, 2011 7:36 pm Post subject: Dynon Skyview Airspeed Calibration...

At 07:39 PM 12/26/2011  Monday, you wrote:

  	  Quote: 	
		  --> RV-List message posted by: Matt Dralle <dralle(at)matronics.com>



 Dear Listers,



 I've been flying the new 10" Dynon Skyview in the RV-6 for a few weeks now and it seems like the airspeed is reading maybe 10mph fast.  The GS always reads 10mph or more slower than the True airspeed, no matter which way I fly with respect to the current wind.  Looking through the configuration options on the Skyview, I'm not seeing parameters to calibrate the airspeed.  If the airspeed were *slow* compared to the GS, I could envision making adjustments to the Pitot tube to get it in better alignment with slipstream.  But *fast* is a head scratcher.  If there's no electronic configuration parameters to adjust, what do you do?  Are there Pitot line "attenuators" like for RF in coax? 



 Matt 	




 Listers,



 I went flying tonight in the RV-6 around sunset and took some airspeed numbers flying the four points of the compass.  I let each direction normalize for about 2 minutes before taking the readings.  I've still got my little black-tape washer deals on the static ports that I detailed in a previous email.  



 Do these numbers make any sense?

 

 I set the altimeter to 30.15 (KLVK ATIS).  I compared the Dynon altitude with the Garmin 696 GPS altitude.  The Dynon was high by 100 ft.  I used the Dynon altitude adjustment to set it to match the GPS (-100ft).  The readings above were after this adjustment.



 

 -

 Matt "Red Dawg" Dralle



 RV-8 #82880 N998RV "Ruby Vixen"

  http://www.mattsrv8.com - Matt's Complete RV-8 Construction Log

  http://www.mattsrv8.com/Mishap - Landing Mishap Rebuild Log

  http://www.youtube.com/MattsRV8 - Matt's RV-8 HDTV YouTube Channel

 Status: 172+ Hours TTSN - Rebuilding Fuselage After Landing Mishap...



 RV-6 #20916 N360EM "The Flyer"

  http://www.mattsrv6.com - Matt's RV-6 Revitalization Log

 Status: 48+ Hours Since Purchase - Upgrades Complete; Now In Full Flyer Mode

4d5f3059.jpg

Description:

Filesize:

36.65 KB

Viewed:

21524 Time(s)

_________________
Matt Dralle
Matronics Email List Administrator

m.l.marcotte(at)sympatico
Guest

Posted: Fri Dec 30, 2011 11:10 am Post subject: Dynon Skyview Airspeed Calibration...

I think one can calculate from the GPS numbers that you  had a 14 MPH (approx) wind from the WNW and you were doing 182 TAS.   Your IAS would appear to be 12 MPH too high.

  

  



  From: Matt Dralle (dralle(at)matronics.com) 

 Sent: Thursday, December 29, 2011 10:33 PM

 To: rv-list(at)matronics.com (rv-list(at)matronics.com) ; rv8-list(at)matronics.com (rv8-list(at)matronics.com) ; rv7-list(at)matronics.com (rv7-list(at)matronics.com) ; rv10-list(at)matronics.com (rv10-list(at)matronics.com) 

 Subject: Re: Dynon Skyview Airspeed  Calibration...

 



At 07:39 PM 12/26/2011  Monday, you wrote:

  	  Quote: 	
		  --> RV-List message posted by:    Matt Dralle <dralle(at)matronics.com (dralle(at)matronics.com)>



Dear    Listers,



I've been flying the new 10" Dynon Skyview in the RV-6 for a    few weeks now and it seems like the airspeed is reading maybe 10mph    fast.  The GS always reads 10mph or more slower than the True airspeed,    no matter which way I fly with respect to the current wind.  Looking    through the configuration options on the Skyview, I'm not seeing parameters to    calibrate the airspeed.  If the airspeed were *slow* compared to the GS,    I could envision making adjustments to the Pitot tube to get it in better    alignment with slipstream.  But *fast* is a head scratcher.  If    there's no electronic configuration parameters to adjust, what do you    do?  Are there Pitot line "attenuators" like for RF in coax?    



Matt 	




Listers,



I went flying tonight  in the RV-6 around sunset and took some airspeed numbers flying the four points  of the compass.  I let each direction normalize for about 2 minutes before  taking the readings.  I've still got my little black-tape washer deals on  the static ports that I detailed in a previous email.  



Do these  numbers make any sense?

[img]cid:22D7DA0DB537424EAE369E728B14E060(at)MobilePC[/img]

I  set the altimeter to 30.15 (KLVK ATIS).  I compared the Dynon altitude with  the Garmin 696 GPS altitude.  The Dynon was high by 100 ft.  I used  the Dynon altitude adjustment to set it to match the GPS (-100ft).  The  readings above were after this adjustment.



 

-

Matt "Red Dawg" Dralle



RV-8 #82880 N998RV  "Ruby Vixen"

http://www.mattsrv8.com - Matt's Complete RV-8 Construction  Log

http://www.mattsrv8.com/Mishap - Landing Mishap Rebuild  Log

http://www.youtube.com/MattsRV8 - Matt's RV-8 HDTV YouTube  Channel

Status: 172+ Hours TTSN - Rebuilding Fuselage After Landing  Mishap...



RV-6 #20916 N360EM "The Flyer"

http://www.mattsrv6.com -  Matt's RV-6 Revitalization Log

Status: 48+ Hours Since Purchase - Upgrades  Complete; Now In Full Flyer Mode

4d5f3059.jpg

Description:

Filesize:

36.65 KB

Viewed:

21518 Time(s)

dcarter(at)datarecall.net
Guest

Posted: Fri Dec 30, 2011 11:11 am Post subject: Dynon Skyview Airspeed Calibration...

My ears perk up when I read of people comparing  "barometric altitude readings" with "GPS altitude".

 . . .  In winter the atmosphere shrinks and in  summer it expands.  Worded differently, the "pressure lapse rate" is not  standard in hot or cold weather, so your baro altimeter isn't going  to accurately show "altitude above MSL".

 . . . Flying F-100s in the 132nd TFW at Des Moines,  Iowa, in the 1970s we used gun camera film extensively to measure  our actual dive angles on various dive bomb events (low, medium, and high  angle).

 . . . We modified  standard Air Force charts and added horizontal lines to show release  altitude.  Then we began noticing that we were releasing below preplanned  release altitude in winter (bombs hit long) and were releasing higher in summer  (bombs hit short of aim point).  We worked with Offutt AFB Weather and  learned to use a "D Value chart" to plot actual amount of deviation (D value) of  a "baro altitude" (pressure level) from where it would be on a standard day  (with standard day pressure lapse rate).

 . . . It will be difficult to give a tutorial via  e-mail to properly treat this issue.  Perhaps if there were interest, I  could add something to a small web page I have.

  

 GPS altitude is based on a mathematical model of  the theoretical shape of the earth (ellipsoid), which is not a round ball, i.e.,  the earths radius is larger at the equater than at the poles.

 . . . I'm not sure but suspect GPS math models  use actual "digital terrain database" values to refine the local "ground"  elevation (MSL).. Kevin Horton??

  

 Bottom line, be aware that GPS altitude will  probably be closer to "real altitude" above Mean Sea Level than "baro  altitude" in winter and in summer (i.e., on non-standard days of temperature and  pressure lapse rates).  You'd have to have a D Value chart and call a local  weather station to get data points from their latest weather balloon releases to  plot the error between "baro" altitude (what you read with correct local  "SURFACE" altimeter setting) and actual (like with a radar altimeter)  altitude.

  

 David Carter

cell 409-718-2268

 RV-6A N164RS (great airplane built by Ron Smith of  Goodyear, AZ)

 

 

   [quote][b]

chriswelsh(at)sbcglobal.n
Guest

Posted: Fri Dec 30, 2011 11:59 am Post subject: Dynon Skyview Airspeed Calibration...

Here's a link explaining in more detail what David just pointed out.



http://rogallo.co.uk/tutorials:differences-between-pressure-and-gps-altitude



-chris

From: David Carter <dcarter(at)datarecall.net>

To: RV-list <rv-list(at)matronics.com>

Sent: Fri, December 30, 2011 11:07:19 AM

Subject: Re: Dynon Skyview Airspeed Calibration...



        My ears perk up when I read of people comparing  "barometric altitude readings" with "GPS altitude".

 . . .  In winter the atmosphere shrinks and in  summer it expands.  Worded differently, the "pressure lapse rate" is not  standard in hot or cold weather, so your baro altimeter isn't going  to accurately show "altitude above MSL".

 . . . Flying F-100s in the 132nd TFW at Des Moines,  Iowa, in the 1970s we used gun camera film extensively to measure  our actual dive angles on various dive bomb events (low, medium, and high  angle).

 . . . We modified  standard Air Force charts and added horizontal lines to show release  altitude.  Then we began noticing that we were releasing below preplanned  release altitude in winter (bombs hit long) and were releasing higher in summer  (bombs hit short of aim point).  We worked with Offutt AFB Weather and  learned to use a "D Value chart" to plot actual amount of deviation (D value) of  a "baro altitude" (pressure level) from where it would be on a standard day  (with standard day pressure lapse rate).

 . . . It will be difficult to give a tutorial via  e-mail to properly treat this issue.  Perhaps if there were interest, I  could add something to a small web page I have.

  

 GPS altitude is based on a mathematical model of  the theoretical shape of the earth (ellipsoid), which is not a round ball, i.e.,  the earths radius is larger at the equater than at the poles.

 . . . I'm not sure but suspect GPS math models  use actual "digital terrain database" values to refine the local "ground"  elevation (MSL).. Kevin Horton??

  

 Bottom line, be aware that GPS altitude will  probably be closer to "real altitude" above Mean Sea Level than "baro  altitude" in winter and in summer (i.e., on non-standard days of temperature and  pressure lapse rates).  You'd have to have a D Value chart and call a local  weather station to get data points from their latest weather balloon releases to  plot the error between "baro" altitude (what you read with correct local  "SURFACE" altimeter setting) and actual (like with a radar altimeter)  altitude.

  

 David Carter

cell 409-718-2268

 RV-6A N164RS (great airplane built by Ron Smith of  Goodyear, AZ)

 

 

   [quote]

[b]

Bubblehead

Joined: 26 Oct 2007
Posts: 48
Location: N. Richland Hills, TX

Posted: Mon Jan 02, 2012 6:07 am Post subject: Re: Dynon Skyview Airspeed Calibration...

One big question:



Does changing the geometry of the static port change indicated altitude when flying?



It seems like if we change the static pressure by applying washers or lips we would change indicated altitude along with IAS.

_________________
John
Keller, TX
RV-8 N247TD

pitts_pilot(at)bellsouth.
Guest

Posted: Mon Jan 02, 2012 6:28 am Post subject: Dynon Skyview Airspeed Calibration...

On 1/2/2012 9:07 AM, Bubblehead wrote:

 	  Quote: 	
		   



 One big question:



 Does changing the geometry of the static port change indicated altitude when flying?

Yes, it surely could.  If the static system isn't truly 'neutral' ..... 
	


no pressure or suction on it ....  it really messes things up.

 	  Quote: 	
		   It seems like if we change the static pressure by applying washers or lips we would change indicated altitude along with IAS.

The reason for the 'washer' or 'rivet head' (whatever you use) is to get 
	


the port out of the laminar flow down the fuselage into undisturbed 

(relatively) air.

That makes location pretty important.    It also means that port 

location/thickness/shape/ .......  is the ONLY thing you can change to 

fix airspeed/altitude errors in flight AFAIK.

It also means you need two ports, one on each side to cancel induced 

errors due to uncoordinated flight.

Linn.

 	  Quote: 	
		  

 --------

 John

 Keller, TX

 RV-8 N247TD





 Read this topic online here:



 http://forums.matronics.com/viewtopic.php?p=362222#362222

lessdragprod(at)aol.com
Guest

Posted: Mon Jan 02, 2012 7:39 am Post subject: Dynon Skyview Airspeed Calibration...

Do you have a test  method that we can all use to calibrate the alimeter at cruise speed?



The altimeter only has the static system for its input.



Calibrating the static vents in cruise conditions for the altimeter could provide a correction for airspeed readout errors.



Correcting airspeed readouts errors by changing the static vent geometery leaves your altitude readings at cruise speeds as a total unknown.



Jim Ayers



--

khorton01(at)rogers.com
Guest

Posted: Mon Jan 02, 2012 9:25 am Post subject: Dynon Skyview Airspeed Calibration...

On 2012-01-02, at 09:07 , Bubblehead wrote:



 	  Quote: 	
		   

 

 One big question:

 

 Does changing the geometry of the static port change indicated altitude when flying?

 

 It seems like if we change the static pressure by applying washers or lips we would change indicated altitude along with IAS.

 

 --------

 John

====================
	




If the original IAS error was due to a poorly located or shaped static port (i.e. static system position error), then the original errors would have affected both IAS and altitude indications.  In this case the correct fix is whatever it takes to get the static port to sense a pressure that is as close as possible to true ambient pressure.  This will correct both the IAS and altitude indications.



If the source of the error is static source position error, then any "fix" that only addressed the IAS (such as the adjustments that some EFIS provide) will only correct the IAS.  The indicated altitude will still be wrong, and this error could easily be much more than 100 ft.

--

Kevin Horton

RV-8

Ottawa, Canada

http://www.kilohotel.com/rv8

khorton01(at)rogers.com
Guest

Posted: Mon Jan 02, 2012 9:41 am Post subject: Dynon Skyview Airspeed Calibration...

The classical test method that directly compares actual vs indicated altitude is the "tower fly-by".   Typically a camera is placed on top of a tower about 100 ft high, with the tower abeam the middle of a long runway.  The camera is very accurately aimed, and the geometry of the tower, camera lens, etc is very well known so that the height of the aircraft above the runway can be determined from the images, assuming the aircraft accurately tracks down the runway centreline.  The aircraft instrumentation system records the altitude, with a very accurate time stamp.  Ground instrumentation records the altitude and OAT at ground level, and the camera images have an accurate time stamp.  Post flight analysis is used to determine the actual vs indicated altitude as the aircraft passes by the tower at a range of airspeeds. This is used by some major test centres, but it is not practical for us as it requires too much specialized equipment to achieve reasonable accuracy.



The method that is most practical for us is the speed course method (sometimes called ground course method).  It assumes that the pitot pressure is accurate, which is a reasonable assumption, and there are no static or pitot leaks, which can be confirmed by ground test.  The ASI instrument error must be measured.  The TAS is measured using one of several possible methods (GPS data from a four course box pattern is the current best method), and the CAS is back calculated from the TAS, indicated altitude and OAT.  The difference between CAS and IAS (corrected for instrument error) must be due to static source position error.



More info:



http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAdvisoryCircular.nsf/list/AC%2023-8B/$FILE/Final-Part8.pdf



http://www.kilohotel.com/rv8/rvlinks/ssec.html



Kevin Horton



On 2012-01-02, at 10:35 , Jim Ayers wrote:



[quote] Do you have a test  method that we can all use to calibrate the alimeter at cruise speed?

  

 The altimeter only has the static system for its input.

  

 Calibrating the static vents in cruise conditions for the altimeter could provide a correction for airspeed readout errors.

  

 Correcting airspeed readouts errors by changing the static vent geometery leaves your altitude readings at cruise speeds as a total unknown.

  

 Jim Ayers

 --

Bubblehead

Joined: 26 Oct 2007
Posts: 48
Location: N. Richland Hills, TX

Posted: Mon Jan 02, 2012 12:36 pm Post subject: Re: Dynon Skyview Airspeed Calibration...

Kevin,



I've been on your site a lot today reading about this.  Next weekend if weather permits I will collect a lot of Numbers to plug into your speadsheet to see about IAS and TAS. I suspect the TAS on my Skyview is significantly off because I am getting wind vectors that disagree significantly with forecast winds aloft and what I observe for a correction angle (crab angle) while flying. 



In your experience if I correct my IAS and TAS by modifying the static port will I cause large changes and induce possible error on "indicated" altitude?



As info i have the SafeAir1 system on my 8 including static ports and the static leak test was passed with flying colors just a month ago.



I appreciate all the info you've share on this and other topics.



John

_________________
John
Keller, TX
RV-8 N247TD

rvmills(at)sbcglobal.net
Guest

Posted: Mon Jan 02, 2012 3:14 pm Post subject: Dynon Skyview Airspeed Calibration...

John,



As you prepare to make your airspeed calibration runs, I'd also recommend

you ensure your compass is well calibrated also. I have a legacy Dynon

panel, but have also found that I saw very odd wind readouts before I ran a

compass cal (or re-ran the cal after maintenance). Heading and speed errors

will both wreak havoc with the GPS wind readouts, so eliminating both is

important. Recommend running the compass cal with engine running, avionics

on, and canopy closed...as close as you can get to normal in-flight

conditions.



I've used the NTPS spreadsheet from Kevin's site a lot (thanks Kevin), and

it's a great tool! My TAS readout is consistently 2-4 knots faster than GPS

readouts...lower when the standard deviation block on the spreadsheet is

lower (meaning the data is better). My static ports are the standard rivet,

so I think the shape is good. I actually think I have a bit of an issue with

OAT (actually a touch of Ram Rise), since I always test at top speed (for

speed mods), and I can see a change in OAT just by slowing or accelerating.

Before I add thickness to the static port, I'm going to run some tests at

slower speeds to see if the TAS-GPS speed delta decreases, then try moving

the OAT probe from under the H-stab to inside the fuselage back there. We'll

see.



Just mentioned all this, as there are a few inputs to consider, and nulling

out the others before making static port changes may be a good idea. I'd

also like to hear Kevin's (and others') thoughts on the speed error versus

altitude error question, but I would think that with all inputs calibrated,

making a physical change to the static ports to reduce IAS/TAS errors would

also reduce other static system output errors (like altitude).



Good discussion!



Cheers,

Bob

RV-6 N600SS

Reno, NV 



--

Display posts from previous:

	Matronics Email Lists Forum Index -> RV-List	All times are GMT - 8 Hours Goto page 1, 2 Next
Page 1 of 2

Jump to:

You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
You cannot attach files in this forum
You can download files in this forum