REFLECTOR: Calibrating the static port

[Scott Derrick]

Airspeed deviation is most common as your speed increases, because the
static pressure port is providing a faulty  high or low relative pressure. 

There is a simple test for your static port. 

At the runway, Set your altimeter to field elevation. 

Now fly the airplane at cruise speed as close to the runway as you are
comfortable.  50 ft is a good safe altitude.  Set your elevator trim up
so it takes a bit of down pressure on the stick to maintain level
flight. If you get distracted and let up on the stick you will naturally
climb away from the ground. You don't want to do a John Denver!

Note the altitude.  Your altimeter should indicate  50 ft above field
elevation. 

IF NOT:

If your altimeter shows a lower altitude this means the static port is
in a low pressure zone relative to the real or unaffected pressure. 
Usually this requires a ridge behind the port to raise the pressure. Put
a thin piece of aluminum  tape right behind the port.  Fly again.   It
may require  multiple layers, it may require  tape in front of the hole.
Its a trial and error process.

If it shows a higher altitude, the the same as above but start with the
tape strip ahead of the port to lower the relative pressure.

Mine was off 400 ft(high) at cruise.  No wonder the ground seemed so low
in the pattern! 3 layers of aluminum tape ahead of the  static holes got
it spot on, and my indicated airspeed now agreed with my GPS's computed
airspeed(not ground speed!).

Flying at cruise speed just above the runway requires calm attentive
piloting.  Don't do this  as part of a known low pass with  people 
watching and waving.   Too distracting.  Don't do it.  This is flight
testing and serious business.   Have one person  near the runway to
estimate how high you are as you make the pass to verify your altitude.

Scott


LAURA WALKER wrote:
> Al, 
>
> My pitot tube is in the factory location. Before my first flight, I checked for leaks with a manometer and all was well. My airspeeds and groundspeeds are pretty close to around 140 - 150 kts, but higher than that they become grossly inaccurate. Haven't really thought about other issues, but your similar location dictates maybe a little more scientific research. 
>
> Here is one thing: I have a alternate static source toggle "valve" in my panel. It does not really switch between outside and cabin sources, but merely opens the complete static side pressure to both outside and inside values. When I open this valve, my indicated speeds drop from 215 to about 202 kts indicated, while my groundspeed says more like 187 kts. I thought that opening this should increase my indicated speeds, and since mine decrease, I am still about 20 kts to the bad. Could a gross static side leak have this kind of effect while leaving the speed range from 70 - 140 kts mostly unaffected? 
>
> In addition to the GRT system, I also have an analog airspeed indicator and it matches the GRT to within 2 kts at 200 indicated.
>
> John Abraham's superfast speeds suggest I have had this problem from the very beginning.
>
> Thanks,
>
> Bill
> ----- Original Message ----
> From: "reflector-request at tvbf.org" <reflector-request at tvbf.org>
> To: reflector at tvbf.org
> Sent: Sunday, February 22, 2009 1:00:17 PM
> Subject: Reflector Digest, Vol 57, Issue 68
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>   1. Re:  N36 LV not so fast....Darn it !!! (Al Gietzen)
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> ----------------------------------------------------------------------
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> Message: 1
> Date: Sat, 21 Feb 2009 21:26:29 -0800
> From: "Al Gietzen" <ALVentures at cox.net>
> Subject: Re: REFLECTOR: N36 LV not so fast....Darn it !!!
> To: "'Velocity Aircraft Owners and Builders list'"
>     <reflector at tvbf.org>
> Message-ID: <1A00D38F41B9466291AAED3E9B3D6CC5 at BigAl>
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>
> Yep, you guessed it. I put my static port behind the lower rear corner of
> the passenger door and smack dab in a nice low pressurre spot. 
>
> Bill; 
> That's where I have my static port, and my indicated airspeeds are right on.
> Where is your pitot tube?
>
> Al
>
>
>
>
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