REFLECTOR: TAS calibration?

Dave Philipsen velocity at davebiz.com
Mon Sep 30 23:23:27 CDT 2013


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 
> <mailto: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
>     altitute and IAS indications.
>
>     You can calibrate the port by putting a 1/4 inch wide strip of
>     aluminin 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>
>     <mailto:geoff.gerhardt at gmail.com>
>     To: Velocity Aircraft Owners and Builders list
>     <reflector at tvbf.org> <mailto: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
>>     <mailto: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 <mailto: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 <mailto: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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