X-Virus-Scanned: clean according to Sophos on Logan.com From: "Charlie England" Received: from mail-pa0-f46.google.com ([209.85.220.46] verified) by logan.com (CommuniGate Pro SMTP 6.1.5) with ESMTPS id 8015724 for flyrotary@lancaironline.net; Thu, 24 Sep 2015 20:06:19 -0400 Received-SPF: pass receiver=logan.com; client-ip=209.85.220.46; envelope-from=ceengland7@gmail.com Received: by pacgz1 with SMTP id gz1so4641403pac.3 for ; Thu, 24 Sep 2015 17:05:45 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=subject:to:references:from:message-id:date:user-agent:mime-version :in-reply-to:content-type; bh=gEQFtS/Alx+JQw9LN13WkqnAeZ/F0J5Uhdf3gxZsEXI=; b=La5GLrVYYTyAoCi7NljfO4hPKHLI4K+s03D/Za7F6wVl8WhoCOxbcbakkNG3giOPqG 4u5OMAJ9kPEce2FKEQBpXiYQvRzym6os2lwu5ss2SF6Z4du0ZlM5blR/DaAl3RLrk+up GqasA/3BTa1+zYJVINq5qH5rwfsDhW/rgWPpCxZyDjXLt1++by+QmdwFSK5mxbZZqWGn 0TazkDQVSWDMplm7fk+5Gm5OwmgKMvRMtdr5dYI9kjhHulT16sCeX7ArhuZJteFrw8Ox qq8ar0nEcFmuQhJgdP79X3Jmsv5Qfl/W9N1cXExTBcbWNx6/KesyemZ83vJDRVFtYpA5 lrAQ== X-Received: by 10.68.131.6 with SMTP id oi6mr3059874pbb.3.1443139545124; Thu, 24 Sep 2015 17:05:45 -0700 (PDT) Return-Path: Received: from ?IPv6:2602:306:25fa:a9a9:a1bf:f766:c61:f53f? ([2602:306:25fa:a9a9:a1bf:f766:c61:f53f]) by smtp.googlemail.com with ESMTPSA id of1sm557522pbc.11.2015.09.24.17.05.43 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 24 Sep 2015 17:05:44 -0700 (PDT) Subject: Re: [FlyRotary] Re: OT: Calibrating altimeter and alt encoder To: Rotary motors in aircraft References: Message-ID: <56049093.8070207@gmail.com> Date: Thu, 24 Sep 2015 19:08:51 -0500 User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.2.0 MIME-Version: 1.0 In-Reply-To: Content-Type: multipart/alternative; boundary="------------000804080406070703030102" This is a multi-part message in MIME format. --------------000804080406070703030102 Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Probably a good idea. On 9/24/2015 1:49 PM, kenpowell wrote: > Great idea Charlie but... How about having a camera on the altimeter > as you perform the low pass rather than stealing a look at the > instruments. > > ------------------------------------------------------------------------ > *From: *"Charlie England" > *To: *"Rotary motors in aircraft" > *Sent: *Thursday, September 24, 2015 11:50:02 AM > *Subject: *[FlyRotary] Re: OT: Calibrating altimeter and alt encoder > > > > On Thu, Sep 24, 2015 at 8:57 AM, Finn Lassen > > wrote: > > Off topic, except it's in a 13B Van's RV-3 ;-) > > Again, after flying home from Sun'n'Fun, ATC saw me several > hundred feet below what I saw on my altimeter. > > Last week I finally got around to removing all the screws that > holds the fuselage top over the instruments. > > I hooked LEDs across the data lines that run from the altitude > encoder to the transponder (gray code). > (note that 1 is actually 0 volts -- active low). > > I checked METARs at nearby airports -- CTY, GNV... and I know my > elevation exactly. > That matched what my Dynon D10A and steam altimeter showed. > > I then spent several hours adjusting the high and low pots on the > alt encoder until the codes changed within 10 to 20 feet of the 50 > foot points when applying vacuum to the static system with a > syringe. Can't get better that that, I though. > > Alas, after replacing the fuselage top and multitude of screws, > yesterday I hopped over to Cross City (CTY) a mere 15 miles away. > I had to set the altimeter to 0.03 or 0.04 below what their METAR > reported in order to get altimeter to show their field elevation > -- 42 feet. Basically shows 30 to 50 feet too high altitude if I > set the Dynon to the reported pressure. All that work for nothing! > > Now I'm beginning to suspect that the pressures reported by the > METARs are not all that precise. > Checking METARS right now at nearby airports, they range from > 29.98 to 30.02. > > Any suggestions on how to obtain an accurate air pressure reference? > > Finn > > Fly to a field that has atc & radar on the field, land, & get them to > check your reporting while on the ground. (Fixed, known altitude) > > Transponders are really finicky critters, and ATC's radars are even > worse. Dirty contacts in the coax path from xponder to antenna, > dirty/oily antenna, etc can all cause transmit errors, and atc is > always a crap shoot, no matter what they tell you. Memphis Approach > used to consistently report 'errors' of various types to a/c that had > no issues in other atc areas. > > Another issue is static port accuracy while in flight. It can > introduce asignificant error at speed, if not set up right. Easiest > way to see the error is to set your altimeter to your field elevation > (or zero, if you prefer, but don't forget you did that), and fly a > high speed low pass over the field. Steal a quick look at the > altimeter while down low and fast. If it says you're lower (or higher) > than you'd expect for your 10-20 foot altitude above field altitude, > you have a static error problem. If it indicates lower altitude than > expected, your airspeed indication is likely low as well. Correct the > static and increase your airspeed by 10 knots, with no additional fuel > burn. :-) > > Charlie > > --------------000804080406070703030102 Content-Type: text/html; charset=utf-8 Content-Transfer-Encoding: 8bit
Probably a good idea.

On 9/24/2015 1:49 PM, kenpowell wrote:
Great idea Charlie but...   How about having a camera on the altimeter as you perform the low pass rather than stealing a look at the instruments.

From: "Charlie England" <flyrotary@lancaironline.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Thursday, September 24, 2015 11:50:02 AM
Subject: [FlyRotary] Re: OT: Calibrating altimeter and alt encoder



On Thu, Sep 24, 2015 at 8:57 AM, Finn Lassen <flyrotary@lancaironline.net> wrote:
Off topic, except it's in a 13B Van's RV-3 ;-)

Again, after flying home from Sun'n'Fun, ATC saw me several hundred feet below what I saw on my altimeter.

Last week I finally got around to removing all the screws that holds the fuselage top over the instruments.

I hooked LEDs across the data lines that run from the altitude encoder to the transponder (gray code).
(note that 1 is actually 0 volts -- active low).

I checked METARs at nearby airports -- CTY, GNV... and I know my elevation exactly.
That matched what my Dynon D10A and steam altimeter showed.

I then spent several hours adjusting the high and low pots on the alt encoder until the codes changed within 10 to 20 feet of the 50 foot points when applying vacuum to the static system with a syringe. Can't get better that that, I though.

Alas, after replacing the fuselage top and multitude of screws, yesterday I hopped over to Cross City (CTY) a mere 15 miles away.  I had to set the altimeter to 0.03 or 0.04 below what their METAR reported in order to get altimeter to show their field elevation -- 42 feet. Basically shows 30 to 50 feet too high altitude if I set the Dynon to the reported pressure. All that work for nothing!

Now I'm beginning to suspect that the pressures reported by the METARs are not all that precise.
Checking METARS right now at nearby airports, they range from 29.98 to 30.02.

Any suggestions on how to obtain an accurate air pressure reference?

Finn

Fly to a field that has atc & radar on the field, land, & get them to check your reporting while on the ground. (Fixed, known altitude)

Transponders are really finicky critters, and ATC's radars are even worse. Dirty contacts in the coax path from xponder to antenna, dirty/oily antenna, etc can all cause transmit errors, and atc is always a crap shoot, no matter what they tell you. Memphis Approach used to consistently report 'errors' of various types to a/c that had no issues in other atc areas.

Another issue is static port accuracy while in flight. It can introduce asignificant error at speed, if not set up right. Easiest way to see the error is to set your altimeter to your field elevation (or zero, if you prefer, but don't forget you did that), and fly a high speed low pass over the field. Steal a quick look at the altimeter while down low and fast. If it says you're lower (or higher) than you'd expect for your 10-20 foot altitude above field altitude, you have a static error problem. If it indicates lower altitude than expected, your airspeed indication is likely low as well. Correct the static and increase your airspeed by 10 knots, with no additional fuel burn. :-)

Charlie 



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