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Subject: Measuring true airspeed
From: Craig Jimenez <jimenez.craig@gmail.com>
X-Original-To: Lancair List <lml@lancaironline.net>
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Speed measurement of high & fast aircraft would likely be an area of
expertise and interest for you guys?

Yesterday I flew a Mooney Rocket.  They do some nice STCs (e.g. JetProp),
and claimed that 31x2400 with 20-21 gph (ROP TIT ~1600) would get you 215
KTAS at 18,000' and 203 KTAS at 12,000' (
http://www.rocketengineering.com/content/rocket-specifications).

It was a windy day (~50 knots at 17.5), so I used a GPS groundspeed method
described by Williams that looks pretty robust (
http://williams.best.vwh.net/avform.html#3gs).  You just fly 3 headings
separated by 120-deg, measure the GPS groundspeeds and use my spreadsheet to
remove the wind effect and get true airspeed.

I wouldn't expect to hit book numbers, particularly since this plane was
heavier than standard (2228 vs 2068#), had two pilots (190 + 180 lb), and
has TKS.  I was surprised though to measure true airspeeds as much as 20
knots below book and probably a 9-knot error in indicated airspeed.  What am
I missing?

My spreadsheet with data and formulas is on (
http://AwlBiz.com/fly/True_Airspeed.xls<http://awlbiz.com/fly/True_Airspeed.xls>).
Parameters for the 18,000' test given below:

 31  MP (in)
 24  RPM
 20.3  FF (gph)
 1,570  TIT (deg F)

 17,500 Indicated altitude
 30.30 Altimeter setting
 -8 OAT (deg C)  --> Density altitude 18,455

 162  GPS GS1 (on arbitrary HDG1)
 247  GPS GS2 (on HDG1 + 120)
 188  GPS GS3 (on HDG1 +240)
 155  KIAS

 195  KTAS based on GPS groundspeeds
 207  KTAS based on KIAS and DA
 217  KTAS expected from book

--
Craig "wanna go faster than my 160 KTAS Mooney M20J" Jimenez

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<div>Speed measurement of high &amp; fast aircraft would likely be an area =
of expertise and interest=A0for you guys?</div>
<div>=A0</div>
<div>Yesterday I flew a Mooney Rocket.=A0 They do=A0some nice STCs (e.g. Je=
tProp), and claimed that 31x2400 with 20-21 gph (ROP TIT ~1600) would get y=
ou 215 KTAS at 18,000&#39; and 203 KTAS at 12,000&#39; (<a href=3D"http://w=
ww.rocketengineering.com/content/rocket-specifications">http://www.rocketen=
gineering.com/content/rocket-specifications</a>).</div>

<div>=A0</div>
<div>It was a windy day (~50 knots at 17.5), so I used a GPS groundspeed me=
thod described by Williams that looks pretty robust (<a href=3D"http://will=
iams.best.vwh.net/avform.html#3gs">http://williams.best.vwh.net/avform.html=
#3gs</a>).=A0 You just fly 3 headings separated by 120-deg, measure the GPS=
 groundspeeds and use my spreadsheet to remove the wind effect and get true=
 airspeed.=A0 </div>

<div>=A0</div>
<div>I wouldn&#39;t expect to hit book numbers, particularly since this pla=
ne was heavier than standard (2228 vs 2068#), had two pilots (190 + 180 lb)=
, and has TKS.=A0 I was surprised though to measure true airspeeds as much =
as 20 knots below book and probably a 9-knot error in indicated airspeed.=
=A0 What am I missing?=A0 </div>

<div>=A0</div>
<div>My=A0spreadsheet with data and formulas=A0is on (<a href=3D"http://awl=
biz.com/fly/True_Airspeed.xls">http://AwlBiz.com/fly/True_Airspeed.xls</a>)=
.=A0 Parameters for the 18,000&#39; test given=A0below:</div>
<div>=A0</div>
<div>=A031 =A0MP (in)<br>=A024 =A0RPM<br>=A020.3 =A0FF (gph)<br>=A01,570 =
=A0TIT (deg F)</div>
<div>=A0</div>
<div>=A017,500 Indicated altitude<br>=A030.30=A0Altimeter setting<br>=A0-8=
=A0OAT (deg C)=A0 --&gt; Density altitude 18,455</div>
<div>=A0</div>
<div>=A0162 =A0GPS GS1 (on arbitrary HDG1)<br>=A0247 =A0GPS GS2 (on HDG1 + =
120)<br>=A0188 =A0GPS GS3 (on HDG1 +240)<br>=A0155 =A0KIAS<br>=A0=A0<br>=A0=
195 =A0KTAS based on GPS groundspeeds <br>=A0207 =A0KTAS based on KIAS and =
DA <br>=A0217 =A0KTAS expected from book </div>

<div>=A0</div>
<div>--</div>
<div>Craig &quot;wanna go faster than my 160 KTAS Mooney M20J&quot; Jimenez=
</div>
<div>=A0</div>
<div>=A0</div>
<div>=A0</div>
<div>=A0</div>

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