REFLECTOR: TAS calibration?

Bob Jackson (Jax Tech) bobj at JaxTechLLC.com
Wed Oct 2 10:21:34 CDT 2013


Geoff, 

 

Yes, I think you'll likely need a bunch of layers if you're using AL tape.
Mine took four or five layers of blue masking tape, directly behind the
static port, to make the correction.  And blue masking tape is a lot thicker
than AL tape, I think.  

 

You want to add enough layers to over-correct the problem, then you'll know
exactly how many layers to remove from that to make the right correction (by
interpolation).

 

Bob Jackson

N2XF

  _____  

From: reflector-bounces at tvbf.org [mailto:reflector-bounces at tvbf.org] On
Behalf Of Geoff Gerhardt
Sent: Wednesday, October 02, 2013 10:13 AM
To: Velocity Aircraft Owners and Builders list
Subject: Re: REFLECTOR: TAS calibration?

 

Last night, I put a 1/4" strip of aluminum tape ~1mm ahead of the static
ports on my pitot-static tube and took it out for a spin.  Before taking
off, I adjusted the barometric pressure on my altimeters (analog and Dynon
Skyview) so the indicated altitude exactly matched the GPS (field) altitude.

 

On takeoff, as I lifted of and accelerated, I could already see my indicated
altitude dip as my speed increased.  I climbed to 7000', leveled off and
established a cruise speed of 160kts.  My GPS altitude was reading 6990ft
and my Dynon altimeter was reading 6620ft (at cruise speed).  I did two
passes with headings 180deg apart.  My GPS ground speeds in the two passes
were 168kts/182kts while holding a steady 158ktas airspeed.  Essentially no
change from readings I got with no aluminum tape ahead of the static port.

 

So, it doesn't seem like the one thin bit of aluminum tape was enough.  I'll
put a couple more layers on and see if it moves the needle in the right
direction.

 

Geoff

 

On Tue, Oct 1, 2013 at 3:22 PM, Laurence Coen <lwcoen at hotmail.com> wrote:

Geoff,

 

Yes and yes.  In a previous post I showed that with a 10knt wind you would
have to miss the actual wind direction by 28 deg. to cause an error of 1knt.
On a 180 heading you would also get a 1knt error but in the opposite
direction thus canceling the original error.  A winds aloft report is more
than enough to select your initial heading.  Most of the other stuff that
was done was to determine the source of the TAS error.  The temperature
error due to speed is less than 1 C at speeds less than 200MPH.  I have an
RMI IAS,TAS, OAT, VSI, ALT, PALT, DALT that also corrects for OAT speed
error.  I have rechecked my TAS reading against the above and it is within
1knt.  My altitude matches my WAAS GPS within +- 10 feet typically.

 

Larry Coen

N136LC

 

From: Geoff Gerhardt <mailto:geoff.gerhardt at gmail.com>  

Sent: Tuesday, October 01, 2013 11:35 AM

To: Velocity <mailto:reflector at tvbf.org>  Aircraft Owners and Builders list 

Subject: Re: REFLECTOR: TAS calibration?

 

Thanks for the great article...the guy looks like George Carlin, tho. 

 

So, rather than cruising down the runway at low altitude, can't I just fly
my plane at a constant GPS altitude and make sure my indicated altitude does
not change with air speed?  Or, that my TAS matches my GPS average ground
speed after two runs with headings 180deg apart?

 

Might head out to the hangar after work and give it a go.

 

Geoff

 

 

On Tue, Oct 1, 2013 at 12:16 PM, Laurence Coen <lwcoen at hotmail.com> wrote:

That is an excellent article.  Since I don't think anyone actually believed
me in a previous discussion when I said that to determine your TAS, Fly into
the wind, record GPS ground speed, fly 180 degrees and take the ground speed
and average the two.  Read the last sentence in the second paragraph.

 

Larry Coen

N136LC

 

From: Henk <mailto:hjvooijen at yahoo.com>  van Ooijen 

Sent: Tuesday, October 01, 2013 9:37 AM

To: Velocity <mailto:reflector at tvbf.org>  Aircraft Owners and Builders list 

Subject: Re: REFLECTOR: TAS calibration?

 

A nice reading about this from Paul Lipps


  _____  


Regards,

Henk van Ooijen

 

 

Op 1 okt. 2013, om 11:40 heeft Geoff Gerhardt het volgende geschreven:





Dave, 

 

I have a Dynon Skyview.  It does the TAS calculation (using OAT) and reports
GPS speed in knots.

 

Geoff

 

On Tue, Oct 1, 2013 at 12:23 AM, Dave Philipsen <velocity at davebiz.com>
wrote:

I'm just taking another wild stab...are you sure your GPS is reading
nautical miles and not statute miles?  How is your TAS being calculated?  Do
you have an EFIS that does it for you or....?  Are you estimating OAT or
actually measuring it?




Dave Philipsen
Velocity STD FG
N83DP

On 9/30/2013 4:29 PM, Geoff Gerhardt wrote:

Mine is a combination pitot static tube.  I wonder if I could calibrate it
in a similar fashion by putting tape around the pitot tube before or after
the static holes. 

 

Dave - yes, I realize that IAS changes with altitude.  I am observing an
offset between my TAS and GPS ground speed (after accounting for wind).

 

Geoff

 

On Mon, Sep 30, 2013 at 4:24 PM, Scott Derrick <scott at tnstaafl.net> wrote:

I have a static port(3 vertical holes, 1/4 inch apart)  under the carnard,
about a foot, that works well. However it does need to be calibrated. An
uncalibrated static port will cause erroneous altitude and IAS indications.

You can calibrate the port by putting a 1/4 inch wide strip of aluminum tape
either right ahead of the the hole(s) or right behind them.  You can add
layers of the tape to increase the effect.



To determine if you need less pressure(tape before the hole) or more
pressure(tape after the hole). 

    Set your altimeter to field elevation, just before departure.  Fly at
cruise speed, ball in the center, 15-20 ft above the runway.  The people on
the ground love this!  Note your altitude. 

    If your indicating higher than 20 ft above field elevation, the pressure
is too low and you need tape behind the port. If your indicating lower than
20ft above filed elevation, the pressure is too high and you need tape ahead
of the port. 

    If your within 50 ft, your good to go.  I did this after I bought my
plane, it was off 300ft. and after I painted, again off 300ft. The tape
lasts forever and looks like an embedded metal strip.

my $.002, works for me.

Scott

-------- Original Message --------
Subject: Re: REFLECTOR: TAS calibration?

From: Geoff Gerhardt  <mailto:geoff.gerhardt at gmail.com>
<geoff.gerhardt at gmail.com>
To: Velocity Aircraft Owners and Builders list  <mailto:reflector at tvbf.org>
<reflector at tvbf.org>
Date: 09/30/2013 01:56 PM

Guys, 

 

My pitot/static is positioned under the canard - I suspect it is seeing
increased pressure at high speeds thus giving me a low indicated airspeed.
This weekend, I thought I was smart and put a small static port right ahead
of the dog house thinking that was a great place.  After a quick flight
where my air speeds were through the roof, I realized that that spot was
actually doing the opposite and was creating a low pressure region as the
air bends over the nose.  I see now that I need to find a place on the flat
section of the fuselage to put the static port. 

 

Where does everyone else have their static port?

 

Thanks.

 

Geoff

 

On Sun, Sep 29, 2013 at 11:43 AM, Kurt Winker <nmflyer1 at aol.com> wrote:

Sounds like a trip-strip may be in order.

Kurt

Sent from my iPhone


> On Sep 29, 2013, at 6:47, Scott Baker <scottb33333 at gmail.com> wrote:
>
> I would assume that you are factoring altitude and temperature when
calculating TAS. If so, airspeed error might be related to a pressurization
of the static system (a common problem in Velocity aircraft that have their
static port located forward of the entry door).
>
> Scott Baker
>
>> On Sep 28, 2013, at 8:29 PM, Geoff Gerhardt <geoff.gerhardt at gmail.com>
wrote:
>>
>> Guys,
>>
>> Flying today I realized that my ground speed seems to be always greater
than my TAS.  I figured at least some of the time it should be less (i.e.
into a headwind).  But it always seemed to be greater.  So, today, I flew in
a box, E, W, N, S and recorded my TAS and GS.  My TAS was constant at
159ktas on each leg, and my GS (GPS) was 185, 179, 171, 175 for the W, N, E,
S legs.  If I add all those up and divide by four, I get 177.5kts, yet my
TAS registered 158ktas.  Am I missing something?  Is my airspeed indicator
reading too low?
>>
>> Thanks.
>>
>> Geoff
>>
>> _______________________________________________
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>
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-- 
Even if you're on the right track, you'll get run over if you just sit
there.
Will Rogers


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  _____  


A nice reading about this from Paul Lipps

Regards,
Henk van Ooijen


Op 1 okt. 2013, om 11:40 heeft Geoff Gerhardt het volgende geschreven: 



> Dave,
> 
> I have a Dynon Skyview.  It does the TAS calculation (using OAT) and
reports GPS speed in knots.
> 
> Geoff
> 
> 
> On Tue, Oct 1, 2013 at 12:23 AM, Dave Philipsen <velocity at davebiz.com>
wrote:
> I'm just taking another wild stab...are you sure your GPS is reading
nautical miles and not statute miles?  How is your TAS being calculated?  Do
you have an EFIS that does it for you or....?  Are you estimating OAT or
actually measuring it?
> 
> Dave Philipsen
> Velocity STD FG
> N83DP
> On 9/30/2013 4:29 PM, Geoff Gerhardt wrote:
>> Mine is a combination pitot static tube.  I wonder if I could calibrate
it in a similar fashion by putting tape around the pitot tube before or
after the static holes.
>> 
>> Dave - yes, I realize that IAS changes with altitude.  I am observing an
offset between my TAS and GPS ground speed (after accounting for wind).
>> 
>> Geoff
>> 
>> 
>> On Mon, Sep 30, 2013 at 4:24 PM, Scott Derrick <scott at tnstaafl.net>
wrote:

>> I have a static port(3 vertical holes, 1/4 inch apart)  under the
carnard, about a foot, that works well. However it does need to be
calibrated. An uncalibrated static port will cause erroneous altitude and
IAS indications.
>> 
>> You can calibrate the port by putting a 1/4 inch wide strip of aluminum
tape either right ahead of the the hole(s) or right behind them.  You can
add layers of the tape to increase the effect.


>> 
>> To determine if you need less pressure(tape before the hole) or more
pressure(tape after the hole). 
>> 
>>     Set your altimeter to field elevation, just before departure.  Fly at
cruise speed, ball in the center, 15-20 ft above the runway.  The people on
the ground love this!  Note your altitude. 
>> 
>>     If your indicating higher than 20 ft above field elevation, the
pressure is too low and you need tape behind the port. If your indicating
lower than 20ft above filed elevation, the pressure is too high and you need
tape ahead of the port. 
>> 
>>     If your within 50 ft, your good to go.  I did this after I bought my
plane, it was off 300ft. and after I painted, again off 300ft. The tape
lasts forever and looks like an embedded metal strip.
>> 
>> my $.002, works for me.
>> 
>> Scott
>> 
>> -------- Original Message --------
>> Subject: Re: REFLECTOR: TAS calibration?
>> From: Geoff Gerhardt <geoff.gerhardt at gmail.com>
>> To: Velocity Aircraft Owners and Builders list <reflector at tvbf.org>
>> Date: 09/30/2013 01:56 PM
>>> Guys,
>>> 
>>> My pitot/static is positioned under the canard - I suspect it is seeing
increased pressure at high speeds thus giving me a low indicated airspeed.
This weekend, I thought I was smart and put a small static port right ahead
of the dog house thinking that was a great place.  After a quick flight
where my air speeds were through the roof, I realized that that spot was
actually doing the opposite and was creating a low pressure region as the
air bends over the nose.  I see now that I need to find a place on the flat
section of the fuselage to put the static port.
>>> 
>>> Where does everyone else have their static port?
>>> 
>>> Thanks.
>>> 
>>> Geoff
>>> 
>>> 
>>> On Sun, Sep 29, 2013 at 11:43 AM, Kurt Winker <nmflyer1 at aol.com> wrote:
>>> Sounds like a trip-strip may be in order.
>>> 
>>> Kurt
>>> 
>>> Sent from my iPhone
>>> 
>>> > On Sep 29, 2013, at 6:47, Scott Baker <scottb33333 at gmail.com> wrote:
>>> >
>>> > I would assume that you are factoring altitude and temperature when
calculating TAS. If so, airspeed error might be related to a pressurization
of the static system (a common problem in Velocity aircraft that have their
static port located forward of the entry door).
>>> >
>>> > Scott Baker
>>> >
>>> >> On Sep 28, 2013, at 8:29 PM, Geoff Gerhardt
<geoff.gerhardt at gmail.com> wrote:
>>> >>
>>> >> Guys,
>>> >>
>>> >> Flying today I realized that my ground speed seems to be always
greater than my TAS.  I figured at least some of the time it should be less
(i.e. into a headwind).  But it always seemed to be greater.  So, today, I
flew in a box, E, W, N, S and recorded my TAS and GS.  My TAS was constant
at 159ktas on each leg, and my GS (GPS) was 185, 179, 171, 175 for the W, N,
E, S legs.  If I add all those up and divide by four, I get 177.5kts, yet my
TAS registered 158ktas.  Am I missing something?  Is my airspeed indicator
reading too low?
>>> >>
>>> >> Thanks.
>>> >>
>>> >> Geoff
>>> >>
>>> >> _______________________________________________
>>> >> To change your email address, visit
http://www.tvbf.org/mailman/listinfo/reflector
>>> >>
>>> >> Visit the gallery!  www.tvbf.org/gallery
>>> >> user:pw = tvbf:jamaicangoose
>>> >> Check new archives: www.tvbf.org/pipermail
>>> >> Check old archives:
http://www.tvbf.org/archives/velocity/maillist.html
>>> >
>>> > _______________________________________________
>>> > To change your email address, visit
http://www.tvbf.org/mailman/listinfo/reflector
>>> >
>>> > Visit the gallery!  www.tvbf.org/gallery
>>> > user:pw = tvbf:jamaicangoose
>>> > Check new archives: www.tvbf.org/pipermail
>>> > Check old archives:
http://www.tvbf.org/archives/velocity/maillist.html
>>> 
>>> _______________________________________________
>>> To change your email address, visit
http://www.tvbf.org/mailman/listinfo/reflector
>>> 
>>> Visit the gallery!  www.tvbf.org/gallery
>>> user:pw = tvbf:jamaicangoose
>>> Check new archives: www.tvbf.org/pipermail
>>> Check old archives: http://www.tvbf.org/archives/velocity/maillist.html
>>> 
>>> 
>>> 
>>> _______________________________________________
>>> To change your email address, visit
http://www.tvbf.org/mailman/listinfo/reflector
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>>> Visit the gallery!  www.tvbf.org/gallery
>>> user:pw = tvbf:jamaicangoose
>>> Check new archives: www.tvbf.org/pipermail
>>> Check old archives: http://www.tvbf.org/archives/velocity/maillist.html
>> 
>>  -- 
>> Even if you're on the right track, you'll get run over if you just sit
there.
>> Will Rogers
>> 
>> _______________________________________________
>> To change your email address, visit
http://www.tvbf.org/mailman/listinfo/reflector
>> 
>> Visit the gallery!  www.tvbf.org/gallery
>> user:pw = tvbf:jamaicangoose
>> Check new archives: www.tvbf.org/pipermail
>> Check old archives: http://www.tvbf.org/archives/velocity/maillist.html
>> 
>> 
>> 
>> _______________________________________________
>> To change your email address, visit
http://www.tvbf.org/mailman/listinfo/reflector
>> 
>> Visit the gallery!  www.tvbf.org/gallery
>> user:pw = tvbf:jamaicangoose
>> Check new archives: www.tvbf.org/pipermail
>> Check old archives: http://www.tvbf.org/archives/velocity/maillist.html
> 
> 
> _______________________________________________
> To change your email address, visit
http://www.tvbf.org/mailman/listinfo/reflector
> 
> Visit the gallery!  www.tvbf.org/gallery
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  _____  


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